CROSS-REFERENCE TO RELATED APPUCATION
[0001] This application claims the benefit of priority ofU.S. provisional patent
application no. 62/033,691, titled "HIGH EFFICIENCY NEUTRON CAPTtJRE PRODUCT
PRODUCTION," filed on August 6, 2014, which is incorporated herein in its entirety by this
reference.
TECHNICAL FIELD
[0002] The present disclosure relates to neutron capture and to neutron efficient
reaction assemblies and methods. More particularly, this disclosure relates to producing a
nuclear rnaterial, such as a nuclear imaging material, through neutron capture by a reactant
nuclear material.
BACKGROUND
[0003] In nuclear medicine, radioactive substances are used in diagnostic and
therapeutic medical procedures. Elemental radionuclides are often combined with other
elements to form chemical compounds, or combined with existing pharmaceutical
compounds to form radiopharmaceuticals. These radiopharmaceuticals, once administered to
a patient, can localize to specific organs or cellular receptors. This property of
radiopharmaceuticals allows the imaging of the extent of a disease process in the body, based
on cellular function and physiology, rather than relying on physical changes in tissue
anatomy. Additionally, in some diseases, procedures in nuclear medicine can identify
medical problems at an earlier stage than other diagnostic tests.
[0004] An important aspect of nuclear medicine is the use of radioactive tracers. A
radioactive tracer (also known as a radioactive label) is a substance containing a radioisotope
(RI) that is used to measure the speed of a biochemical processes and to track the movement
of a substance through a natural system such as a cell or tissue. An important radioactive
tracer is technetium-99m (symbolically represented as 99mTc), which can be readily detected
in the body by medical imaging equiprnent.
[0005] 99111Tc is a metastable nuclear isomer of technetium-99. The "m" indicates that
this is a metastable nuclear isomer, the half· life of which is 6 hours. This is considerably
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longer (by many orders of mag11itude) than most nuclear isomers that undergo gamma decay.
Thus, the half-life of 99mTc is very long in terms of average de-excitation, yet short in
comparison \vith many commonly observed radioactive decay half-lives, and in comparison
with radionuclides used in many kinds of nuclear medicine tests.
[0006] 99mTc is used in radioactive isotope medical applications. It is a radioactive
tracer that can be detected vvithin the human body by medical imaging equipment. Tt is well
suited to this role because it emits readily detectable photons at energies convenient for
medical imaging. Additionally, 99mTc also dissolves in aqua regia, nitric acid, and
concentrated sulfuric acid, but is not soluble in hydrochloric acid of any strength.
Radiopharmaceuticals based on 99mTc are used for imaging and functional studies of the
brain, bone, myocardium, thyroid, lungs, liver, gallbladder, kidneys, skeleton, blood, and
turnors.
[0007] 99mTc is used in about 20 million diagnostic nuclear medical procedures every
year. Approximately 85 percent of diagnostic imaging procedures in nuclear medicine use
this isotope. Depending on the type of nuclear medicine procedure, the 99mTc is tagged or
bound to a pharmaceutical that transports the 99111Tc to an intended location.
[0008] An imp01iant advantage of99mTc is that, due to its half-life of 6.0058 hours for
gamma emission, 93.7 percent of 99mTc decays to technetium-99 in 24 hours. Thus, the short
half-life of the metastable nuclear isorner, in terms of human-activity and metabolism, allows
for scanning procedures that collect data rapidly but keep total patient radiation exposure low.
The resulting technetium-99 ground state, which has a half.- life of 211,000 years decaying to
stable ruthenium-99, emits soft beta particles (electrons of nuclear origin) without causing
significant gamma-ray exposure. All of these characteristics ensure that the use of 99111Tc
represents minimal radiation burden on the body, providing significant medical imaging
benefits.
[0009] Due to its short half-life, the 99mTc used in nuclear rnedicine is typically
extracted from 99111Tc generators containing molybdenum-99 C9J\/fo), which has a half-life of
2. 75 days and is the usual parent nuclide for 99mTc. Unfortunately, a significant amount of
99Mo produced for 99mTc medical use comes frorn the fission of highly enriched uranium from
only five reactors around the world: NRU in Canada; BR2 in Belgium; SAFARI-1 in South
Africa; HFR (Petten) in the Netherlands; and the OSIRIS reactor in Saclay, France.
[00010] A known induced single fission incident is diagrammatically represented in
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FIG. l. A neutron 100 collides with a uranium-235 (235lJ) nucleus 102, which absorbs the
neutron 100 as represented by the uranium-236 nucleus 104 undergoing fission and
producing neutrons 106, gamma radiation 110, a 99Mo nucleus 112 (roughly 6% of the time),
and other fission products 114. The 99Mo nucleus 1 1 2 decays to produce a 99"'Tc nucleus 1 16,
and other radiations 120 including a beta particle (an electron of nuclear origin) and
antineutrinos 118.
[00011] A small amount of 99Mo is also produced from low··enriched uranium at the
new OPAL reactor in Australia, as well as a few other sites in the world. An even smaller
amount of 99Mo is produced by neutron activation of molybdenurn-98 using an acceleratorbased
method of neutron production. More commonly, a uranium target with highly enriched
13 '\J (up to 90 percent 13'\J) or !ow enriched uranium (less than 20 percent 13 '\J) is irradiated
with neutrons to form 99Mo as a fission product, which is then separated from other fission
products in a hot cell.
[00012] A neutron activation process is sho\v11 in FIG. 2. In FIG. 2, a neutron 200
collides with a nucleus 202 of 98Mo that absorbs the neutron 200 and produces a nucleus 204
of 99Mo. The 99Mo nucleus 204 then decays to produce a nucleus 206 of 99111Tc and other
radiations 210 including an electron (beta radiation) and antineutrinos. The neutron generator
may be a deuterium/tritium accelerator ("D··T accelerator") or may operate by using an
altemate neutron producing nuclear reaction, for exarnple, spallation.
[00013] Unfortunately, current accelerator-based methods of neutron production tend
to create 991\tlo and 99mTc uneconomically or at low neutron efficiency, with no more than a
few particles of 99Mo created for every l 000 neutrons produced. Additionally, for political
and safety reasons, two aging nuclear reactors (NRU and HFR) have shut down repeatedly
for extended maintenance periods. These tvv·o reactors produce a large prop01iion oftbe
world supply of 99Mo. The resulting global shortages of 99"'Tc have suggested the need for
additional production capability.
[000 14] Therefore, there is a need for systems and methods for using a neutron source
to produce isotopes or radioisotopes (such as 99mTc) at a high efficiency so as to obviate the
need fc:r current nuclear reactor production schemes. More generally, a scheme for making
most isotopes without the use of nuclear reactors or subcritical assemblies is needed.
SUMMARY
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[00015] This summary is provided to introduce in a simplified form concepts that are
further described in the follmving detailed descriptions. This summary is not intended to
identify key features or essential features of the claimed subject matter, nor is it to be
constmed as limiting the scope of the claimed subject matter.
[00016] According to at least one embodiment, an apparatus for producing reactionproduct
nuclei from reactant nuclei includes a plurality of reactant nuclei and a plurality of
moderating nuclei. The moderating nuclei include nuclei of atoms that are chosen from a
group consisting of deuterium, tritium, helium-4, lithium-7, beryllium, boron-11, carbon,
nitrogen-15, oxygen, fluorine, neon-20 and neon-22. A neutron source that is neither a
nuclear reactor nor a subcritical assembly is in proximity to the reactant nuclei sufficient to
produce reaction·-product nuclei by neutron capture. The reactant nuclei include
molybdenum-98. The rate of mo!ybdenum-98 nuclei neutron capture divided by the rate of
the neutron source's neutron production is greater than approximately l %,. The mass of
molybdenum-98 is less than approximately 1000 kg. The mass of moderating nuclei is at
least 1 kg. For purposes of this disclosure, the term subcritical assembly is any assembly with
a neutron multiplication factor of greater than 0.6 and less than L
[000 17] In at least one example, the rate of reactant nuclei neutron capture divided by
the rate of neutron source production is greater than approximately 5%.
[000 18] In at least one example, temperature control capable of maintaining at least 2
different regions of the apparatus at different temperatures is used, and at least one region is
cooled to a temperature below· 250 degrees Kelvin. The temperature control may include the
use of a cryogenic fluid.
[000 19] In at least one example, at least one neutron ref1ector at least partially
surrounds the pluralities of reactant nuclei and moderating nuclei. The reflector includes
moderating nuclei. The ret1ector thickness is greater than approximately 20 centimeters, and
in may be principle be limitlessly thick, although in practice, an economically designed
reflector can be less than approximately 15 rneters in thickness, and rnore preferably less than
3 meters in thickness. Within the description of this disclosure, rei1ectors vvill be described as
between 20 centimeters and 15 rneters, with the understanding that thicker reflectors are
possible.
[00020] In at least one example, both an outer and an inner neutron reflector reflect
neutrons tow·ards regions containing higher densities of reactant nuclei. In some
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embodiments, at least one reflector will cause backscattering of neutrons in a single collision,
while in other embodiments, several collisions are necessary to cause a full or nearly full
reversal in the direction of a neutron's motion.
[00021] In at least one example, the pluralities are arranged in one or more
approximately parallel layers, at least one layer being distinct frorn another layer on the basis
of elemental composition, concentration of chemical species, density or temperature.
[00022] In at least one example, a target is configured to emit neutrons when impacted
by accelerated particles. The target includes atoms chosen from a group consisting of
deuterium, tritimn, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20, neon-22, tantalum, tungsten, lead, mercury, thallium, thorium, uranium,
neptunium and other transuranics. The accelerated particles enter the system via an access
channel configured to accept greater than 50 percent of the accelerated particles that impinge
upon the access channeL The access channel here is any etTective path for the particle of
interest to enter the apparatus and impinge upon a target
[00023] According to at least one embodiment, an apparatus for producing reactionproduct
nuclei frorn reactant nuclei includes a plurality of reactant nuclei having a first
average microscopic thermal neutron capture cross-section and a plurality of moderating
nucleL For the purposes of this disclosure, average in this context means the weighted
average of all nuclei of interest; for example, the average microscopic cross-section of 10
light water molecules and 2 almninum atoms would be approximately the sum of
10*2*(hydrogen-1 cross--section)+ 10*1 *(oxygen--16 cross-section) +2*1 *(aluminum-27
cross-section), collectively divided by (20+ 10 +2). Hereby defined is a collection of isotopes
consisting of those isotopes w·hose nuclei species capture at least l% of all emitted neutrons
from the neutron source and which are not reactant nuclei. Also defined is a second plurality
of nuclei consisting of all nuclei from the collection of isotopes, wherein at least
approximately 90% of the nuclei have microscopic thermal neutron capture cross-sections
that are lower than the microscopic thermal neutron capture cross-section of any of the
reactant nuclei. For the purposes of this disclosure, nuclei species in this context means the
nuclei of a particular isotope, such as hydrogen-2, oxygen-16, etc.
[00024] For the purpose of interpreting, the "collection of isotopes consisting of those
isotopes whose nuclei species capture at least 1% of all emitted neutrons from the neutron
source ... " should be understood to imply "for the neutron source configuration being
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utilized." There rnay be many ways to configure a neutron source so as to alter the isotopes
within the collection of isotopes, but the particular configuration being used to produce
reaction-product nuclei is the configuration to vvhich the claims apply.
[00025] The total mass of moderating nuclei is greater than approximately 1 kilogram
A neutron source that is neither a nuclear reactor nor a subcritical assernbly is in proximity to
the reactant nuclei sufficient to produce reaction-product nuclei by neutron capture.
[00026] In at least one example, the reactant nuclei comprise molybdenum-98, and the
rate of reactant nuclei neutron capture divided by the rate of the neutron source's neutron
production is greater than approximately 1 !).;,~.
[00027] In at least one example, the reactant nuclei comprise molybdenum-98, and the
rate of reactant nuclei neutron capture divided by the rate of neutron source production is
greater than approximately 5%.
[00028] In at least one example, the apparatus includes molybdenum-99 reactionproduct
nuclei and technetium--99m decay-product nuclei.
[00029] In at least one example, the moderating nuclei comprise nuclei of atoms that
are chosen from a group consisting of deuterium, tritium, heiium-4, lithium-7, beryllium,
boron-11, carbon, nitrogen-15, oxygen, t1uorine, neon-20 and neon-22.
[00030] In at least one example, temperature control is used to maintain at least two
different regions of the apparatus at diflerent temperatures, and least one region is cooled to a
temperature below approximately 250 degrees Kelvin. The ternperature control may include
the use of a cryogenic fluid. At least one neutron reflector may at least partially sun-ound the
pluralities of reactant nuclei and rnoderating nuclei, the reflector including moderating nuclei
and having a thickness that is greater than approximately 20 centirneters and less than
approximately 15 meters. The apparatus may include both an outer and an inner neutron
reflector that ret1ect neutrons towards regions of the pluralities containing higher densities of
reactant nuclei.
[00031] In at least one example, a target is configured to emit neutrons when impacted
by accelerated particles, and the target includes atoms chosen from a group consisting of
deuteriurn, tritium, helium A, lithiurn-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20, neon-22, tantalum, tungsten, lead, rnercury, thallium, thorium, uranium,
neptunium and other transuranics. The accelerated particles enter the system via an access
channel configured to accept greater than 50 percent of the accelerated particles that impinge
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upon the access channel.
[00032] According to at least one ernbodirnent, a method for producing decay-product
nuclei from a reactant isotope using a neutron source includes preparing a plurality of
reactant nuclei having a first average microscopic thermal neutron capture cross-section and
a plurality of moderating nuclei_ Hereby defined is a collection of isotopes consisting of
those isotopes whose nuclei species capture at least 1% of all emitted neutrons from the
neutron source and which are not reactant nuclei. Also defined is a second plurality of nuclei
consisting of all nuclei from the collection of isotopes, wherein at least approximately 90% of
the nuclei have microscopic thermal neutron capture cross-sections that are lower than the
microscopic thermal neutron capture cross-section of any of the reactant nuclei. The total
mass of moderating nuclei is greater than approximately l kilogram, The method includes
generating neutrons and irradiating the plurality with the neutrons such that a reaction
product is generated when the neutrons are captured by the reactant nuclei. The method
includes extracting from the plurality a decay product that is generated by radioactive decay
of the reaction product isotope.
[00033] In at least one example, the neutrons are generated by a nuclear reactor or a
subcritical assembly.
[00034] In at least one example, the neutrons are generated by a source that is neither a
reactor nor a subcritica! assembly.
[00035] The reactant nuclei may include molybdenurn-98, in which case the decay
product includes teclmetium .. 99m. ln that case, the rate of reactant nuclei neutron capture
divided by the rate of neutron production is greater than approximately l %.
[00036] In at least one example, the rnodemting nuclei include nuclei of atoms that are
chosen from a group consisting of deuterium, tritium, helium-4, lithium-7, beryllium, boron-
11, carbon, nitrogen-15, oxygen, fluorine, neon-20 and neon .. 22.
[00037] The pluralities of reactant and moderating nuclei may be at least partially
surrounded vvith at least one neutron reflector including moderating nuclei and whose
thickness is greater than approximately 20 centimeters and less than approximately 15
meters. Both an outer and an inner neutron reflector may reflect neutrons towards regions
containing higher densities of reactant nuclei.
[00038] In at least one example, temperature control is capable of maintaining at least
2 different regions of the apparatus at different temperatures, and at least one region is cooled
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to a temperature below 250 degrees Kelvin. The temperature control may use a cryogenic
fluid.
[00039] In at least one example, a target is configured to emit neutrons vvhen impacted
by accelerated particles, the target including atoms chosen from a group consisting of
deuterium, tritium, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20, neon-22, tantalum, tungsten, lead, mercury, thallium, thorium, uranium,
neptunium and other transuranics. The accelerated particles enter the system via an access
channel configured to accept greater than 50 percent of the accelerated particles that impinge
upon the access channel.
[00040] According to at least one embodiment, a system for producing a decay product
from a reactant using a neutron source includes a plurality of reactant nuclei, a plurality of
moderating nuclei, and a neutron source. The moderating nuclei include nuclei of atoms that
are chosen from a group consisting of deuterium, tritium, helium-4, litbium-7, beryllium,
boron--11, carbon, nitrogen--15, oxygen, i1uorine, neon--20 and neon--22. The reactant nuclei
include molybdenurn-98. The rate of rnolybdenum-98 nuclei neutron capture divided by the
rate of neutron source production is greater than approximately l %. The mass of
molybdenum-98 is less than approximately 100 kg, and the mass of moderating nuclei is at
least 1 kg.
[00041]
assembly.
[00042]
In at least one exarnple, the neutron source is a nuclear reactor or subcritical
In at least one example, the neutrons are generated by a source that is neither a
reactor nor a subcritical assembly.
[00043] In at least one example, the rnodemting nuclei include nuclei of atoms that are
chosen from a group consisting of deuterium, tritium, helium-4, lithium-7, beryllium, boron-
11, carbon, nitrogen-15, oxygen, fluorine, neon-20 and neon--22.
[00044] In at least one exarnple, a target is configured to emit neutrons when impacted
by accelerated particles. The target includes atoms chosen from a group consisting of
deuterium, tritium, helium-4, lithium-7, beryllium, boron--11, carbon, nitrogen--15, oxygen,
i1uorine, neon-20, neon-22, tantalum, tungsten, lead, mercury, thalliurn, thorium, uranium,
neptunium and other transuranics.
[00045] The accelerated particles may enter the system via an access channel
configured to accept greater than 50 percent of the accelerated particles that impinge upon the
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access channel.
[00046] In at least one example, at least one neutron reflector includes moderating
nuclei, and the neutron reflector at least partially surrounds the layers of the pluralities of
reactant and moderating nuclei. The thickness of the neutron rd1ector is greater than
approximately 20 centimeters and less than approximately 15 meters.
[00047] In at least one example, both an outer and an inner neutron reflector ret1ect
neutrons towards regions of the pluralities containing higher densities of reactant nuclei.
[00048] In at least one example, temperature control is capable of rnaintaining at !east
two different regions of the apparatus at different temperatures, \vherein at least one region is
cooled to a temperature below 250 degrees Kelvin. The temperature control may include a
cryogenic fluid.
[00049] According to at least one embodiment, a process includes combining a
plurality of reactant nuclei and a plurality of moderating nuclei. The moderating nuclei
include nuclei of atoms that are chosen from a group consisting of deuterium, tritium, helium·
4, lithiurn-7, beryllium, boron-11, carbon, nitrogen-15, oxygen, fluorine, neon-20 and neon-
22. A neutron source that is neither a nuclear reactor nor a subcritical assembly is placed in
proximity to the reactant nuclei sufficient to produce reaction-product nuclei by neutron
capture. The reactant nuclei include molybdenum--98. The rate of molybdenum-98 nuclei
neutron capture divided by the rate of neutron production is greater than approximately 1 °;(;.
The n1ass of molybdenurn-98 is less than approximately l 00 kg, and the mass of moderating
nuclei is at least 1 kg. The combination is irradiated with neutrons such that radioactive
reaction-product nuclei are generated when the neutrons are captured by the reactant nuclei.
[00050] In at least one example, the reactant nuclei include molybdenum-98 and the
rate of reactant nuclei neutron capture divided by the rate of the neutron source's neutron
production is greater than approximately 1 ~~- i\t least one of the reaction· product nuclei and
decay-product nuclei may be extracted from the irradiated combination. The decay-product
nuclei may include technetium-99m
[00051] According to at least one embodiment, a system for producing reaction··
product nuclei includes reactant nuclei, a neutron source in proximity to the reactant nuclei
sufficient to produce reaction-product nuclei by neutron capture, and temperature control
capable of cooling at least 1 kg of the system to a temperature at or below approximately 250
Kelvin.
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[00052] In at least one example, the rate ofmolybdenurn-98 nuclei neutron capture
divided by the rate of the neutron source's neutron production is greater than approximately
1%. ]\/foderating nuclei may include nuclei of atoms that are chosen from a group consisting
of deuterium, tritium, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20 and nemi-22. The reactant nuclei may include rnolybdenum-98.
BRIEF' DESCRIPTION OF" THE DRI\\VINGS
[00053] The previous summary and the fc:llowing detailed descriptions are to be read
in view of the dravvings, vvhich illustrate particular exemplary embodiments and features as
briefly described below. The summary and detailed descriptions, however, are not limited to
only those embodiments and features explicitly illustrated.
[00054] FIG. 1 illustrates a known induced fission process of producing molybdenum-
99 and technetium-99m
[00055] FlG. 2 illustrates a known neutron activation process of producing
mo!ybdenum-99 and technetium-99rn.
[00056] FTG. 3 is a cross-sectional view of an apparatus for producing reaction-product
nuclei from reactant nuclei according to at least one embodiment.
[00057] FIG. 4 is a perspective view of another apparatus for producing reaction··
product nuclei from reactant nuclei according to at least one other embodiment.
[00058] FIG. 5 is a diagrammatic view of the interior of the apparatus of FIG. 4.
[00059] FlG. 6 is a diagrammatic view of the interior of an apparatus for producing
reaction-product nuclei from reactant nuclei according to yet another embodiment.
[00060] FTG. 7 is a cross-sectional view of a system for the production of desired
nuclear species according to still another embodiment.
[00061] FIG. 8 illustrates a particle accelerator directing a high-energy beam of
particles into the system of FIG. 7 according to at !east one embodiment.
[00062] FIG. 9 illustrates a thennal control system in use with a layered shell vessel.
DETAlLED DESCRIPHONS
[00063] These descriptions are presented w·ith sufficient details to provide an
understanding of one or more particular embodiments of broader inventive subject matters.
These descriptions expound upon and exemplify particular features of those particular
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embodiments without limiting the inventive su~ject matters to the explicitly described
embodiments and features. Considerations in view of these descriptions will likely give rise
to additional and similar embodiments and features without departing from the scope of the
inventive subject matters. Although steps may be implied relating to features of processes or
methods, no implication is made of any particular order or sequence among expressed or
implied steps unless an order or sequence is explicitly stated.
[00064] To promote an understanding of the below descriptions of particular
exemplary embodiments, and to clarity that the full scope of the descriptions extends beyond
any particularly described ernbodiment, several underlying principles may be considered
without imposing limitations on the exemplary embodiments. According to these underlying
principles, isotope production can be implemented by:
1) Using a neutron source that is neither a nuclear reactor nor a subcritical
assembly;
2) Achieving a high likelihood of neutron capture by intended reactant nuclei by
choice of moderating nuclei;
3) Returning escaping or leaking neutrons to a reaction chamber by use of a
neutron reflector; and/or
4) Optionally using temperature enhancement of neutron capture, preferably low
temperature enhancement of neutron capture.
[00065] That is, neutrons can be provided for neutron capture reactions without the use
of a nuclear fission reactor or a subcritical assembly, in which naturally fissile material in a
subcritical amount or arrangement undergoes some degree of induced flssion without
reaching criticality. For the purposes of this disclosure, the term subcritical assernbly shall be
understood to imply the possibility of a subcritical reactor. In the folknving descriptions,
specially designed structures are implemented to cause entering neutrons to be moderated and
reflected in such a way as to greatly increase their chances of being captured by a given
intended reactant nucleus such as molybdenum-98 C8Mo). Vessels and housings described
below are configured, generally, to minimize neutron leakage and to maximize internal
neutron scattering, lik:e a f1ux trap, but not in the context or confines of a nuclear reactor.
Optionally, cooling a volume containing reactant nuclei, for exarnple by using a cryogenic
fluid like liquid helium, oxygen, nitrogen, or deuterium, increases the likelihood of neutron
capture by an intended reactant nucleus. For the purposes of this description, a reaction
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chamber is any volume in which reactant nuclei capture neutrons.
[00066] An exception to underlying principle 1 described above, encountered in some
of the succeeding embodiments, is when the source of neutrons could be a nuclear reactor or
subcritical assembly. For example, a nuclear reactor or subcritical assembly might be
configured to leak neutrons so as to provide a source of neutrons for some of the
embodiments described below. For example, in one embodiment, a nuclear reactor or
subcritical assembly is located in proximity to the reactant nuclei sufficient to produce
reaction-product nuclei by neutron capture at a rate exceeding l 07 neutron captures/second.
In these examples, the geometry of either the nuclear reactor or the apparatus might be
modified so as to facilitate the transport of neutrons fi·om the nuclear reactor or subcritical
assembly to the volumes of apparatus containing reactant nuclei.
[00067] Modeling simulations indicate that a system including a 1-3 meter diameter
vessel (spheres and concentric spherical shells) that implements at least to some degree some
of the above principles can produce many hard-to--manufacture radioisotopes ("RI''s),
including 99Mo, at neutron efficiencies exceeding the current state of the art, where neutron
efficiency is defined as:
·Jn.".--,,' eu t.r on rt·r· · (production rate of reaction-rJroduct nuclei) ; D _ 1cwncv = · 1 . " (neutron productiOn rate)
For this definition, it is understood that the production rates described are those observed
during periods of operation, more specifically during the period of operation when neutrons
are being produced. For this definition, it is also understood that the neutrons in the
denominator's "neutron production rate" refer to the initially produced neutrons (e.g. by
nuclear spallation, DT reactions, from fission) rather than neutrons subsequently produced
due to secondary reactions (e.g. such as from (n,xn) reactions). Initially produced neutrons,
for example, would include both neutrons originating external to the apparatus and then
incident upon the apparatus, and also neutrons produced within the apparatus by the action of
a charged particle reaction such as a DT reaction or a spallation reaction. Subsequent neutron
multiplication, e.g. (n,xn) reactions, do not contribute towards increasing the denominator. lt
is also understood, for this definition, that the rates mentioned in the numerator and
denominator are considered to be averaged over short time intervals, preferably minutes and
more preferably seconds, rather than over long periods of time such as hours, days or more.
The rates described should be considered in the context of instantaneous rates rather than
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averaged rates considered over long tirne spans such as hours.
[00068] Sirnulations indicate that neutron efficiency values of up to 25 percent are
achievable upon implementation of one or more of the embodiments described below.
Higher efficiencies might be realized upon improvement upon one or more of the
embodiments by certain improvements in reflector geometry, temperature and/or elemental
composition. Several embodiments of a system for the production of at least one isotope are
described in the follow·ing detailed descriptions and are represented in the drawings. In at
least one such embodiment, a system fc:r the production of isotopes includes a reaction
charnber in which reactant nuclei are present, and in which moderating nuclei may be present.
Neutrons for neutron capture reactions are introduced into the reaction chamber from an
external neutron source, or are produced locally, for example via spallation when a spallation
target inside of or nearby the reaction chamber is impacted by accelerated particles. At least
some of the neutrons that initially avoid capture and escape regions vvith high concentrations
of reactant nuclei are subsequently ret1ected back towards those regions, to increase the
likelihood that each neutron will be captured by the intended reactant nuclei. Single and
multiple reflector ammgements are described in the following. The reflector may double in
function as a physical wall of the reaction chamber. The reflector may be formed using
moderating nuclei within the reaction chamber.
[00069] The following descriptions refer to reactant nuclei, reaction product nuclei,
decay-product nuclei, and moderating nuclei. Unless othenvise expressly stated or implied,
such references are made without regard to whether electrons are bound in electron shells
about the nuclei. These descriptions relate therefore to both ionized and charge-balanced
atornic arrangernents of the described nuclei, such that the described nuclei may be that of
uncharged atoms, ionized atoms, free atoms, and atoms bound in molecular bonds including
ionic and covalent bonds. The described nuclei may be present in solid, gas, gel, liquid, or
other forms. The reactant and moderating species may be combined as disordered mixtures,
regular matrices, and molecular compounds prior to neutron exposure and such arrangements
may be maintained, altered, or lost upon, for example, neutron capture reactions leading to
subsequent decays. The reactant nuclei may be concentrated in a single location or dispersed
throughout the apparatus. The reactant nuclei may, if in solid form, be concentrated into one
or more pellets, or into foils with large surfr:tce areas, or into other shapes. The reactant nuclei
may be in solution within a solvent, or may be a component in a liquid, or may be in gaseous
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form.
[00070] An apparatus 300 for producing reaction-product nuclei from reactant nuclei is
shown in cross-sectional view in FIG. 3. The apparatus 300 shaped in cross-sectional view as
a circular sector defines a reaction chamber 304. Radial sidewalls 306 diverge from a
proximal end wall 310 to a distal end wall 312 such that the chamber 304 expands from the
proximal to distal end wall. An access channel 314 formed through the proximal end \vall
310 permits neutrons to enter the reaction chamber 304 from a neutron source 316, which in
the illustrated ernbodiment is not a nuclear reactor or a subcritical assembly. The neutron
source 316 is in proximity to the reactant nuclei sufficient to produce reaction-product nuclei
by neutron capture. The element 318 in FIG. 3 represents a channel or process by vvbich, for
example, reactants, products, or other chemical or nuclear species are entered into or
extracted frorn the charnber 304.
[00071] The walls of the reaction chamber or chambers optionally serve as neutron
re:l:1ectors, surrounding the reaction chamber 304, retuming at least some of the neutrons that
reach and/or enter the walls to the reaction chamber 304 to increase the likelihood that each
neutron \vill be captured by the intended reactant nuclei. The walls include moderating
nuclei, such that the walls are composed of high moderating ratio material having a low
microscopic thermal neutron capture cross section. For example, the walls may include
beryllium and/or carbon. The walls in terms of thickness, in at !east one embodiment, are
greater than approximately 20 centirneters and less than approxirnately 15 meters.
[00072] Within the reaction chamber 304, neutrons 320 are preferably captured by
reactant nuclei 322 to produce desired reaction product nuclei 324. Moderating nuclei 326
are also present in the reaction chamber 304 in the illustrated embodiment, the moderating
nuclei also optionally serving as reflecting nuclei. A sufficient thickness ofretlecting nuclei
may also serve as a reflector in a variety of geometric embodiments.
[00073] In at least one embodiment, the plurality of reactant nuclei 322 has a first
average microscopic thermal neutron capture cross-section. Also included and hereby defined
is a collection of isotopes consisting of those isotopes whose nuclei species capture at least
1 (1;;, of all emitted neutrons from the neutron source and which are not reactant nuclei. Also
defined is a second plurality of nuclei consisting of all nuclei from the collection of isotopes,
\vherein at least approximately 90% of the nuclei have microscopic thermal neutron capture
cross··sections that are lower than the microscopic thermal neutron capture cross·section of
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any ofthe reactant nuclei.
[00074] In at least one ernbodirnent, the total mass of moderating nuclei is greater than
approximately 1 kilogram. In at least one embodiment, the apparatus 300 includes
temperature control capable of cooling at least 0.1 kg of the system to a temperature at or
below approxirnately 250 degrees Kelvin.
[00075] In at least one embodiment, the reactant nuclei 322 include molybdenum-98
nuclei, and the rate of reactant nuclei neutron capture divided by the rate of neutron source
production is greater than approximately l 0
;(;. In at least one ernbodirnent, the rate of reactant
nuclei neutron capture divided by the rate of neutron source production is greater than
approximately 5~~.
[00076] In at least one embodiment, the reaction product nuclei 324 include
molybdenum-99 nuclei produced from molybdenum-98 reaction product nuclei 324 by
neutron capture reactions. Due to the decay ofmolybdenum-99 nuclei, technetium-99m
decay-·product nuclei may also be present.
[00077] In at least one ernbodiment, the moderating nuclei 326 include nuclei of atoms
that are chosen from a group consisting of deuterium, tritium, heliurn-4, lithium-7, beryllium,
boron-11, carbon, nitrogen-15, oxygen, t1uorine, neon-20 and neon-22.
[00078] The neutron source 316 diagrammatically represents many types of neutron
sources that are not nuclear reactors or subcritica! assemblies. Suitable examples include
neutron emitters, neutron generators and neutron production devices. Tn at least one
embodiment, the illustrated neutron source 316 represents a neutron generator with a neutron
emission that is greater than l "1 014 neutrons per second. The neutron generator may include
a proton, deuteron, or helium-ion accelerator with a projectile energy greater than 8 MeV
(much lower for !JT neutron-producing reactions) and beam current typically in the range of
a milliamp or higher, although systems may be designed with beam current in the range of
mlcroamps to hundreds of mlcroamps. Thus, in various embodiments, neutrons 320 are
provided by the neutron source 3 l 6, and the neutrons irradiate the reactant nuclei 322 such
that the reaction product nuclei 324 are generated vvhen the neutrons 320 are captured by the
reactant nuclei 322.
[00079] The system 300 in at least one embodiment is utilized to produce a particular
nuclear species in a staged process that includes induced neutron capture followed by one or
more stages of radioactive decay resulting in production of the particular nuclear species. In
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at least one example, reactant nuclei 322 are exposed to neutrons 320 to produce, through
neutron capture, reaction-product nuclei 324. Natural radioactive decays of the reactionproduct
nuclei 324 then subsequently produce decay-product nuclei 328 of a desired
particular nuclear species. In a particular example, the reactant nuclei 322 in FIG. 3 represent
98Mo nuclei that capture neutrons 320 to produce 99Mo nuclei, w·hich are represented by
reaction-product nuclei 324. Continuing that particular example, the decay-product nuclei
328 represent a desired nuclear species, 99mTc, produced by the radioactive decay of99Mo. It
should be understood that FIG. 3 represents many other particular examples in which a
desired nuclear species is populated in a decay chain in w·hich one or more radioactive decays
occur following the production of the reaction-product nuclei 324 by neutron capture
reactions induced by irradiating reactant nuclei 322 vvith neutrons 320. While 99Mo produces
99mTc in a single stage of radioactive decay following neutron capture by 98Mo, these
descriptions relate as \vell to various other decay sequences in which multiple stages of decay
occur.
[00080] Generally, various solvents or fluids into which the 98Mo reactant isotope 322
may be dissolved or suspended may be used as the moderator 326 or a part thereof, as long as
the atoms within each solvent or fluid constituent have a low microscopic thermal neutron
capture cross--section relative to that of the 98Mo reactant 322. Generally, these solvents or
t1uids may be composed of elements like hydrogen, helium, beryllium, carbon, oxygen,
fluorine, and a few isotopes of other elements. The 981v1o reactant 322 may be suspended or
dissolved either in reg11lar or liquid form into these solvents or f1uids.
[00081] For example, the microscopic thermal neutron capture cross-section of the
98Mo of the reactant 322 is much higher than the microscopic thermal neutron cross-section
of deuterium oxide 332 nuclei in liquid. As an example, the microscopic thermal neutron
cross--section of 98Mo reactant 322 is about 130 millibams versus less than one miHibam for
either of those of the two hydrogen or one oxygen atoms. As such, the per nucleus
probability of a neutron 320 being captured by the 98Mo reactant 322 nucleus is much higher
than the probability of a neutron being captured by deuterium oxide. Once a neutron 320 is
captured by the 98Mo reactant nucleus 322, it forms the 99Mo reaction-product 324. The 99Mo
reaction-product 324 then decays to create the 99mTc decay-product 328, which may be
directly and continuously extracted from the chamber 304 via a channel 318, which may
include a valve or other mechanical and chemical structures meant to perform radiochemical
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separation to isolate the desired reaction or decay product.
[00082] Alternatively, instead of suspending or dissolving the 98Mo reactant 322 in a
separate liquid or t1uid, the 98Mo reactant 322 may itself be part of a liquid compound. Nonlimiting
examples include: difluoromolybdenum C8MoF2); molybdenum fluoride C8MoF3);
molybdenum tetraf1uoride (98MoF4); molybdenum hexaf1uoride (98MoF6); compounds of
molybdenum, oxygen, and/or fluorine (lVio011Fn,).
[00083] Generally, the apparatus 300 directs neutrons 320 into a volume where
neutrons are captured by 98Mo instead of by other nuclei, in a ratio that is significantly higher
than in systems that lack strong thermalization and/or high moderating ratios, even though
98Mo has a low microscopic thermal neutron capture cross-sl~ction in isolation (of about 130
mlllibarns). As such, the apparatus 300 allows the neutrons 320 ernitted from the neutron
source to be captured by the desired reactant nuclei without the majority of neutrons 320
leaking out or being captured by nuclei other than 98.!',/fo, and thus being \vasted. The result is
that the efficiency of capturing neutrons on the desired reactant nuclei in the apparatus 300 is
high (between 1 and 30%) and generally described by the following previously described
relationship:
"n'e'.u t . ron E·~f.' f'"1 C·t encv = (production rate ofreaction-rJroduct nuclei). 1; · · (neutron productiOn rate)
where the reaction-product nuclei are 99Mo in situations where the reactant nuclei comprise
98Mo. However, certain models have shovm that by altering the temperature of the reactant
nuclei, higher efiiciencies than 30% may be attainable.
[00084] A system 400 for producing reaction-product nuclei from reactant nuclei
according to at least one embodiment is represented in FIGS. 4-5. The system includes a
spherical wall402. An access channel ·404 defined through the wall 402 permits access for
accelerated particles to a central spherical target 406. A spherical shell .. shaped reaction
chamber 410 (FIG. 5) at least partially surrounds the target 406. The surrounding chamber
may surround the target concentrically. The target 406 is configured to emit neutrons when
impacted by accelerated particles that reach the target through the access channel404. The
target 406 may be constructed of such rnaterials as deuterium, tritium, helium-4, lithium-7,
beryllium, boron-11, carbon, nitrogen-15, oxygen, fluorine, neon-20, neon-22, tantalurn,
tungsten, lead, mercury, thallium, thorium, uranium, neptunium and other transuranics. The
access channel configured to accept greater than 50 percent of the accelerated particles that
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impinge upon the access channeL
[00085] Like the walls of the reaction chamber 304 ofFIG. 3, the wal1402 ofFIGS. 4-
5 serves as a neutron reflector, surrounding the reaction chamber 410 and retuming at least
some of the neutrons that reach the wall to the reaction chamber 410, which contains higher
densities of reactant nuclei, to increase the likelihood that each neutron will be captured by
reactant nuclei. The wall includes moderating nuclei, such that the \vall is composed of high
moderating ratio material having a low neutron capture cross section. For example, the wall
402 rnay include beryllium and/or carbon. The wall in terrns of thickness, in at !east one
embodiment, is greater than approximately 20 centimeters and less than approximately 15
meters. Tn some embodiments, thickness values between one and three meters are used.
[00086] In the illustrated embodiment, the access channel 404 is defined by a radially
extending tubular wall 412 that connects an outer surface 414 of the vessel wall402 to the
target 406. As shovvn in FIG. 5, the spherical shell shaped reaction chamber 410 is isolated
from the access channel 404 by the spherical target 406 and radially extending tubular wall
412. The spherical housing 400 may also include a channel 416 for directly extracting
reaction or decay products from the reaction chamber 41 0. The channel 416 rnay include a
valve and other similar mechanical stmctures.
[00087] A particle accelerator 420 directs a high .. energy beam 422 of particles into the
access channel404 of the system 400 (FIG. 7.) The target 406 within the path of the beam
422 produces neutrons as the beam of particles strike the target. The particle accelerator 420
may provide, for example, a high-energy beam of protons, deuterons, tritons, helium, or other
particles. The particle accelerator 420 and target 406 together constitute a neutron source that
is neither a reactor nor a subcritical assernbly.
[00088] In at least one embodiment, neutrons produced at the target 406 are emitted in
a fully or partially isotropic fashion. Thus, putting the neutron emitting target at the center of
the approximately spherical reaction chamber facilitates a relatively uniform distribution of
neutrons in that volume of the reaction charnber 410 where intended reactant nuclei await the
emitted nuclei. Nonetheless, in embodiments vvhere neutrons are provided or emitted
anisotropically or directionally, the target 406 may be constructed and placed, for example
non-concentrically vvith the wall 402, at any location within or relative to the reaction
chamber 410 to maximize neutron efficiency with regard to capture by intended reactant
nuclei in the chamber.
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[00089] Within the reaction chamber 410 (FIG. 5), as already described with reference
to the reaction chamber 304 of FIG. 3, neutrons 320 are captured by reactant nuclei 322 to
produce desired reaction product nuclei 324. As such, the reaction product nuclei 324 may
include molybdenum--99 nuclei produced from molybdenum-98 reaction product nuclei 324
by neutron capture reactions. Due to the decay of molybdenum-99 nuclei, technetium-99m
decay-product nuclei 328 may also be present Moderating nuclei 326 may also present in
the reaction chamber 410, the moderating nuclei also optionally serving as reflecting nuclei.
In at least one embodiment, and as already described with reference to FIG. 3, the moderating
nuclei 326 include nuclei of atoms that are chosen frorn a group consisting of deuterium,
tritium, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen, fluorine,
neon-20 and neon--22.
[00090] A system 500 for producing reaction-product nuclei from reactant nuclei
according to at least one other embodiment is represented in FIG. 6. The system 500 dift(.~rs
from the system 400 of FIG. 5 in that a neutron source 424 is used in lieu of the accelerator
420 and target 406. In FIG. 6, a central chamber 408 is defined within the reaction chamber
410 to receive the neutron source 424 through the access channel 404. The neutron source
424 may be a neutron generator device capable of emitting neutrons in an isotropic fashion,
or a radioactive decay source that emits neutrons, such as Californium--252. The system 500
is otherwise similar to the system 400 to such extent that above descriptions referring to fiG.
5 are applicable as well to FTG. 6, particularly where like reference number refer to like
elements.
[00091] In an operational example relating to FIGS. 3, 5 and 6, the reactant 322
intended for neutron capture is 98Mo, the reaction product 324 is 99Mo, and the decay product
isotope is 99mTc. Neutrons 320 are emitted into the reaction chamber 410. When a neutron
320 is captured by a 98Mo reactant nucleus 322, a 99Mo reaction--product 324 nucleus is
produced. The 99Mo reaction-product nucleus 324 ultimately decays to form a 99mTc decayproduct
nucleus 328 through beta decay. In this example, the reactant nuclei 322 and
moderating nuclei 326 may be present in a liquid, gas, or gel as non-limiting examples.
[00092] In this operational example, the 98Mo reactant 322 may be combined with the
moderator 326 in a solution, or suspension. The moderating nuclei 326 may include
deuterium, tritium, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
:l:1uorine, neon--20 and neon-22 as non--limiting examples. Further moderator examples
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include deuterated cornpounds such as deuteriurn oxide (D20), deuterated hydrogen peroxide
(D20 2) and deuterated organic compounds (DnCmOp) . Other moderator exarnples include
oxygen (02), carbon dioxide (C02), deuterated methanol, deuterated ethanol, and t1uorine.
[00093] Various solvents or fluids in which the 98Mo reactant may be suspended may
be used as long as the solvent or fluid constituents have a lovv microscopic thermal neutron
capture cross-section relative to the 98 ~!',/fo reactant Generally, these solvents or fluids may be
composed of elements like hydrogen, helium, carbon, oxygen, fluorine, and a few isotopes of
other elements. The 98Mo reactant may be suspended or dissolved into these i1uids or
solvents.
[00094] Alternatively, instead of suspending or dissolving the 981v1o reactant in a
separate liquid or i1uid, the 98Mo reactant may be a constituent of a liquid or solid compound
such as, for non-limiting examples, dit1uoromolybdenum, molybdenum fluoride,
molybdenum tetratluoride, molybdenum hexatluoride, molybdenum oxides, and compounds
of molybdenum, oxygen, and f1uorine (Mo011Fm).
[00095] The decay-product or reaction-product nuclei may be directly and
continuously extracted from the reaction chamber via the element 318 (FTG. 3) or channel
416 (FTGS. 5 and 6), each of which may include a valve or other mechanical and chemical
structures meant to perform radiochemical separation to isolate the desired product.
Processing may proceed continuously or in batch mode.
[00096] A system 700 for the production of desired nuclear species is represented in
FIG. 7 according to at least one embodiment The system 700 includes an at least partially
spherical assembly within an outer waH 702. The system 700 is supported by a base 704,
represented as a trapezoidal pedestal for exemplary purposes. The base 704 supports the
weight of the system 700 without interfering vvith practical operation. Within the outer wall
702, multiple spherical shell layers surround a central target 706 in a concentric arrangement.
The central target 706 ernits neutrons when a beam 750 of particles is incident upon the
target. The neutrons are emitted into the spherical shell layers surrounding the central target
706, at least some of the neutrons passing from the inner·-most layer 710 to more outer layers.
At least one of the spherical shell layers serves as a neutron capture reaction volume, having
nuclei intended for neutron capture reactions present. Other layers serve as moderating
and/or reflective layers to increase the likelihood that neutrons emitted from the central target
706 are captured by intended nuclei.
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[00097] Although five spherical shell layers are illustrated in FIG. 7, these descriptions
relate to layered structures having less than and more than five spherical shell layers. For
PUl1XJses of example, the spherical shell reaction chamber layers illustrated in FIG. 7 are
described herein, in increasing radial size from the central spherical target 706 to the outer
wall 702, as: a first layer 710; a second layer 712; a third layer 714; a fourth layer 716; and a
fifth layer 718, in which their numbered order corresponds to their radially ordered positions
in the layered structure. ln at least one embodiment, the central spherical target is replaced
by a chamber defining an inner-most layer.
[00098] Like the walls of the reaction chamber 304 ofFIG. 3, the wall 702 of FIGS. 7
may serve as a neutron reflector, surrounding the reaction chambers and retlecting at least
some of the neutrons that reach the wall to increase the likelihood that each neutron will be
captured by an intended reactant nucleus. The wall includes moderating nuclei, such that the
wall is composed of high moderating ratio material having a low neutron capture cross
section. For example, the wall 702 may include beryllium, oxygen and/or carbon.
Moderating nuclei may be present in the spherical shell layers interior to the wails of the
reaction chamber, and these moderating nuclei may also serve as neutron reflectors. The wall
in terms of thickness, in at least one embodiment, is greater than approximately 20
centimeters and less than approximately 15 meters. Thicknesses close to 1-2 meters are used
in some embodiments.
[00099] In FIG. 7, four spherical radially intermediary boundaries are illustrated in
cross-section as: a first boundary 720 bet\veen the first layer 710 and second layer 712; a
second boundary 722 between the second layer 712 and third layer 714; a third boundary 724
between the third layer 714 and fourth layer 716; and a fourth boundary 726 between the
fourth layer 716 and fifth layer 718.
[000 100] The spherical boundaries represent, in various embodiments, either: structural
materials supporting and separating the adjacent !ayers; or the interface where !ayers meet
vvithout additional structural materials maintaining their separation. That is, the central
spherical target 706 and ordered layers 710, 712, 714, 716 and 718 are distinct in various
embodiments by their positions, contents and other physical properties such as temperatures
with or w·ithout intervening material between them at the radially intermediary boundaries.
Additional structural members may be used to connect and/or support each layer and
boundary. Exemplary radially arranged beams 730, extending like spokes, are illustrated in
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FIG. 7 as interconnecting the boundaries 720, 722, 724 and 726.
[0001011 Structural materials by \vhich the spherical boundaries may be constmcted, in
at least one embodiment of the system 700 of FIG. 7, include knv microscopic thermal
neutron capture cross··section materials. For example silicon carbide, beryllium carbide,
carbon, and zirconium may be used. The optirnum thickness for structural material layers
and their compositions vary among embodiments. Structural layers in various embodiment
are thick enough to impart stability but not so thick as to excessively capture neutrons and
lower neutron ei1lciency. In some embodiments, structural materials with average
microscopic thermal neutron capture cross sections of less than 300 millibams, such as
zirconium, may be used. In other embodiments, structural material with average microscopic
thermal neutron capture cross sections of less than 30 millibarns may be used, such as
polymers or plastics containing carbon, deuterium and oxygen. In some embodiments,
materials providing structural support may double as moderating nuclei.
[000102] Upon emission of neutrons from the target 706, neutron capture processes
occur between the target and an outer wall 702 of the systern 700 within one or more of the
surrounding layers 710, 712, 714, 716, and 718 to facilitate production of a desired nuclear
species. The outer wall 702 is illustrated as a spherical boundary for convenience but may
take other form in some embodiments.
[000103] An access channel 732 is represented as a tapered bore that diminishes in size
from the outer wall 702 toward the central spherical target 706. The access channel 732
permits a beam 750 of particles to reach the target 706. A radially extending wall734
defining the access channel732 is illustrated in fiG. 7 as conically shaped to match the
tapered bore through the layered stmcture. In various other examples, the \vall 734 and bore
have other shapes, for example matching cylindrical shapes without taper, to define the
access channel 732. The radially extending wall 734 in at least one embodiment connects the
outer waH 702 to the central target 706, isolating the layers 710, 712, 714, 716 and 718 from
each other, and from the exterior of the outer \vall 702 while permitting access to the central
target 706.
[000 1 04] The access channel 732 is illustrated as sub tending s solid angle 736, which is
shown as an acute angle for exemplary illustrative purpose. In other ernbodirnents, the angle
736 is obtuse. In at least one embodiment, the solid angle 736 is approximately 2rr steradians
such that the layers 710-718 are hemisphericaL Thus, in various embodiments, the solid
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angle 730 can be of any value and the access channel 732 can be of any size and shape.
[000 l 051 The access channel 732 can serve two or more purposes. Tt allmvs a particle
beam to reach the center of the system 700, where incoming particles such as protons,
deuterons, helium nuclei, and other projectiles can produce neutrons, for example by
inducing nuclear reactions at a central target. For exarnple, if beryllium is used as the central
target 706, incoming high energy particles like protons can make neutrons by a 9Be(p,n)
reaction. The access channel 732 also allows for the entry and exit of cooling f1uid to control
temperature in the layers of the system 700. For example, liquid heliurn, oxygen, and/or
other coolants might be used to rnaintain the temperatures of the central spherical target 706
and layers 710-718, each at a particular respective temperature. In some embodiments of the
system 700, temperatures are maintained to preferentially control the moderating ratio and/or
to increase the microscopic neutron capture cross-section of a desired reactant isotope. In
some embodiments, temperatures are lowered in some layers to less than 1 00 degrees K, and
even to as low as the boiling point of helium, and even lower stilL
[000106] One or both of these described purposes might be served by the access channel
732 in various embodiments. For example, a neutron generation mechanism might be
entirely contained inside the vessel, and cooling or heating may or may not be needed.
Furthermore, more than one bore may be present. The shapes, sizes, locations and numbers
of bores can vary without changing the principles above. Some ernbodirnents might not use
any bores.
[000107] In at least one embodiment, the neutron--emitting target comprises at least 10
grams of nuclei that possess a microscopic thermal neutron capture cross-section greater than
that of the reactant nuclei, in a volurne where neutron energy is much higher than thermal
enerr:,ry. In at least one embodiment, the neutron-emitting target comprises at least 10 grams of
nuclei that possess a microscopic thermal neutron capture cross·-section greater than that of
the reactant nuclei, and where the neutron-emitting target is configured (e.g. geometrically,
thermally) to absorb as few neutrons as possible, for example less than 1, 5, or 1 0~1,, of all
neutrons produced at the target
[000108] In at least one embodirnent, an externally delivered beam 750 of particles
enters the access channel to create neutrons through nuclear reactions at the target 706. In
another example, an RI decay source of neutrons is placed at the target location, for example
AmBe, 252 C:l:~ PuBe, and other sources may be used.
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[000109] Because the system is designed to moderate neutrons of even high energy (for
exarnple, even greater than 8-14 MeV), the system can handle a \vide variety of neutron
enerr:,ry input \vithout loss of function .. For example, DT neutrons and spallation sources
may be used. Various neutron intensities or input rates are also acceptable. An underlying
design principle implemented by one or more embodiments described herein is directed to
increasing the probability that any one neutron gets captured by a given intended reactant. As
a result, the intensity of the neutrons delivered into the vessel should not greatly vary the
average neutron efficiency.
[000 ll 01 There already exist commercial accelerators, for example, that can reach the
energies and beam currents necessary to produce these neutrons intensities. For example,
particles including protons, deuterium, tritium, helium, and other examples, vvhen incident
upon a neutron-producing target, can make between 0.1 and 5 neutrons per incident particle
at energies of tens to a few hundreds of ]\/feV. Different targets yield different neutron yields
in a process called nuclear spallation.
[000111] In at least one example, a beam ofhigh energy (tens to hundreds MeV)
particles are incident on a neutron rich target, generating sufficient neutrons for practical
operation. Assuming approximately one neutron is generated per incident nuclear particle, a
beam of about 1016 particles per second, or a few milliamps of beam current, is necessary.
Beams that can provide a few tens or hundreds of MeV at milliamp beam cunents or higher
are available in industry research implementations, for example in proton therapy. Higher or
lower beam energies and beam currents may be warranted to reduce cost or to change
production rate over time to alter the rate of neutron-producing-target heating, for example
due to heating as the high energy particles decelerate, in which some collisions generate heat
instead of (or in addition to) neutrons.
[000 112] In FIG. 8, a particle accelerator 800 directs a high .. energy beam 802 of
particles into the access channel 732 of the system 700 (FIG. 7.) In FIG. 8, only the central
spherical target 706 is expressly illustrated to represent the layered shell structure more
expressly illustrated in FIG. 7. As with FIG. 7, FIG. 8 represents layered structures having
any number of approximately spherical shell !ayers. FIG. 8 illustrates a configuration in
which an intended reactant nuclide may be present in any or all layers, and may additionally
serve as structural materiaL Target nuclei 804 \vithin the path of the beam 802 produce
neutrons 806 as the beam particles strike. ln this diagrammatic representation, a particle
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accelerator 800, emitting for example protons, deuterons, tritons, helium, or other particles,
makes a high energy beam of incident particles or nuclei that enter the system 700 through
the access channel 732, hit the target 804, causing neutrons 806 to be emitted. FIGS. 7-8
represent many configurations of materials and geometries that are characterized by a high
moderating ratio volume within the system 700. The arrangements represented can be varied
within the scope of these descriptions without compromising the high neutron efficiency.
[000113] Many forms of neutron production according to embodiments within the scope
of these descriptions cause neutrons to be emitted in a rnostly or partially isotropic fashion.
Putting the neutron ernitting target close to the center of the volume can help distribute
isotropically emitted neutrons uniformly into zones rich with nuclei intended for neutron
capture to help maximize neutron efficiency. Alternatively, in situations with anisotropy in
neutron emission, the neutron emitter location and shape may be varied or optimally selected
to improve neutron efficiency.
[000114] Heating and/or cooling is provided in various embodiments to maintain a
neutron-producing target at a selected stable temperature. Cooling may be used to facilitate
enhancement of the neutron capture rate of reactant nuclei by reducing neutron energy and
thereby enhancing the microscopic neutron capture cross-section of the intended reactant
nuclei, such as 98Mo nuclei. Cooling may include cryogenic cooling. Heating may also be
used to reduce neutron capture of non-reactant nuclei by increasing neutron energy. The
coolant and any tubing, piping, and casing that can)' the coolant within the housings
described herein preferably also have small microscopic neutron capture cross-sections, but
are able to handle colder-than-room-temperature or cryogenic temperatures without
compromising functional integrity. Tubing materials can include, for exarnple: any polymer
constructed with carbon, deuterium, oxygen, beryllium, t1uorine, and other knv microscopic
neutron capture cross-section materials; or metals like, for example, zirconium. Sufficient
coolant should be applied to remove waste heat created during neutron creation, and also to
cool any layers of the volurne down to temperatures below room ternperature, for example,
down to 100 degrees K, 30 degrees K, 10 degrees K, or below the boiling point ofhelium. ln
sorne embodiments, the intentional use of heating to raise temperatures above room
ternperature might also be employed in order to increase neutron energy and thereby reduce
microscopic neutron capture cross-sections.
[000115] Stmctural material used in constructions should be able to operate at low·er--
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than-room-temperature and cryogenic temperatures, and also to withstand cycles of
ternperature between room temperature and lower-than-roorn-temperature or cryogenic
temperature, if lower temperatures are used. Coolant, if used, may enter and leave interior
volumes at more than one place. For example, in addition to entering and/or exiting at the
bore, coolant might enter or leave at various conduits. Specialty low rnicroscopic thermal
neutron capture cross-section variants of commercially available structural materials, pipes,
electronic components, heat exchangers, etc. may be used or specifically manufactured for
use in this apparatus.
[0001161 In FIG. 9, an exernplary thermal control system 900 is represented for use vvith
a layered shell vessel 902 that represents layered structures having any number of spherical
shell layers 904 concentrically arranged around a central spherical target or cavity 906. A
high-energy beam 910 enters the vessel902 through a bore 912 and a target nucleus 914
within the path of the beam 910 produces neutrons as the beam particles strike the target.
FIG. 9 illustrates a configuration in which an intended reactant nuclide may be present in any
or ali layers, and thus diagrammatically represents thermally maintained implementations of
at least the systems illustrated in FIGS. 5 and 7-8.
[000117] The thermal control system 900 includes any number of primary conduits 920
and sub--conduits 922 and 924 defining send and return fluid paths constituting a branched
fluid distribution network implemented in the layered structure of the layered shell vessel 902
such that the shell layers 904 can be independently or together thermally maintained. In at
least one embodiment, a standard cryogenic Huid producer 926 is used to cycle low
microscopic neutron capture cross-section coolant fluid that is sui1lciently free of higher
microscopic neutron capture cross-section contaminants. In another embodiment, cooling
fluid is used to keep reactant nuclei between 200 and 250 K. The cooling, whether cryogenic
or otherwise, should be done in such a way so as not to interfere vvith the beam 910 entering
the bore.
[000118] Particularly within the vessel 902, conduit lines, tubing, enclosures and the
distributed coolant should have low microscopic neutron capture cross-sections so as to
minimize neutrons being captured by materia! within the vessel other than the intended
reactant. Various cooling systems and arrangements meeting these conditions are within the
scope of these descriptions, as are various methods of lower-than-room-temperature or
cryogenic fluid delivery, storage, and production.
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[000119] In some embodiments, removal of a produced radioisotope may proceed by
allowing the various layers to cool or heat up to room temperature naturally, or may occur by
removing frozen material, if material is present in frozen form (for example 98lVio or 99Mo in
solid D20, oxygen, nitrogen-15, etc.). Removal may also involve allowing liquid to
evaporate, leaving only Mo, for example from Mo in liquid oxygen or helium.
[000 120] In some embodiments, to extract and ship radioisotopes such as 991v1o, existing
radiochemical methods and existing or modified supply chain procedures may be followed.
In situations where 99Mo may not be easily extracted from 98Mo precursor or where such
extraction is not \varranted or necessary, the mass ofMo may be shipped together, used in
Technetium-99m generators available commercially today, and retumed to have the
molybdenum extracted for re-use. In some embodiments, altered or improved Technetium-·
99m generators that can successfully use lower specific radioactivity levels than those used
by the cuJTent commercial state of the art technetium generators may be used. Because using
enriched molybdenum is helpful to high neutron efficiency operation, using a method of
shipping and returning the vessel which minimizes the loss of enriched (expensive) 98Mo
might be desirable. Apparatuses according to these descriptions are constructed in such a
way that removing or adding reactant nuclei material, such as 98Mo, is fast and easy. For
example, it may be constructed in such a way that the layer or layers containing reactant
nuclei material are easily removed, pumped out, or added back.
[000 121] The volume or volumes with the intended reactant nuclei for neutron
absorption/capture is preferably specially constructed for convenient removal of the activated
material after irradiation. Further, enriched material (for example 98Mo at 80 percent or more
enrichment) may be used.
[000122] Several exemplary configurations are specified in further detail below in
Tables 1-4, which specifies densities, materials, temperatures, dimensions are specified for
the central target 706 and layers 710 (first layer), 712 (second layer), 714 (third layer), 716
(fourth layer) and 718 (fifth layer) fiJr the system 700 of FIG. 7. It should be understood that
these configurations are provided as examples without limiting these descriptions to those
examples. Not ail conceivable configurations will have a spherical centra! target and five
concentric layers. These are just examples.
[000123] These exemplary configurations are derived from computer modeling using
neutron transpmt codes. Modeling was performed using MCNP5, vvhich is knovvn and often
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used by those of skill in the art. For the sake of simplicity, modeling did not include
stmctural materials such as those shovvn at radial positions conesponding to boundaries 720
722, 724, 726 in FIG. 7. In one instance of computer modeling of the system, summarized
below, input neutrons were assumed to be emitted isotropically from the target 706 at 8 MeV.
in other instances, computer modeling of the system assumed that a substantially larger
spectrum of input neutron energies was used, with similar results. Note that the various
materials in the below· configurations may be in gas, liquid or solid form depending on
temperature. In each, the molybdenum is assurned to be uniformly distributed in its indicated
layer for simplicity, though it need not be for operation.
Table 1 ··· Configuration 1
! Charnber
I
Density
(' gi' cm~ )
Material Temperature Radial Range
(degrees K) (em)
! Central Target L85 Beryllium 300 0-20 em
t··i~~y~~--i······················· ··a············ ··v~;i-~i······················································ ·N7.:\··········· ···2o~3r1··~~~················
!i La.v. er 2
! Layer 3
i
l.l
1.86
DzO 300 30-35 em
51 parts 0, 1 part 98Mo 300 35--38 em
! Layer 4 l. J D20 300 38-58 em
~--········································ ............................................................................................... ·--------------------------------- ---------------------------------------
! Layer 5 l.l D20 300 58-199 ern
L _________________________________________ -------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Tabk 2- Configuration 2
~--(j~-~;;·b-e~------------------- --n~~;-;;i-ty _______ --~-i~te~~i-~i"--------------------------------------------- --T~~:~r~~:~1~-;~e--- --R-~-di-~i"-R-~~~g~-----
~~~~~
!i La.v. er 1 0 Void N/A 20-30 em
! Layer 2
i
1.1 30 30--35 em
!' Layer 3 1.86 51 parts 0, 1 part '18'M o 30 35-38 em
~------------------------------------------ --------------------------- -------------------------------------------------------------------- ---------------------------------- ---------------------------------------
! Layer 4 L l D20 30 38-58 ern
~---L~y-e~:--5 ...................... ""TT ................. .. 5;1:5 ....................................................... --"3·6·6 ....................... --·5·8·~-199--~;~------------
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Table 3 -Configuration 3
! Chamber Density Material Ternperature Radial Range
[____ ______________________________________ ---~-~~~-~~~~------------------------------------------------------------------------------~-~~-:~~-=~--:~~-------~-:~-~-~--------------------------- Central Target 1.85 Berylliurn 300 0-20 em
Layer 1 0 Void N/A 20-30 em
Layer 2 Ll D20
.,
_) 30--35 em
! Layer 3 1.86 51 parts 0, 1 part "0Mo 3 35-38 em
~------------------------------------------ --------------------------- -------------------------------------------------------------------- ---------------------------------- ---------------------------------------
! Layer 4 L l D20 3 38-58 ern
i
! Layer 5
i
Ll 300 58-199cm
[000124] Neutron efficiency is predicted to go from about 2% to 15% as one goes from
configuration l to configuration 3, representing production yields that would be
commercially competitive 99Mo production rates in machine implementation.
Table 4 --- Configuration 4
! Chamber Density
(. gi' cm~ )
Material Temperature Radial Range
I (degrees K) (em)
! Central Target 1.85 Beryllium 300 0-10 em
~~~~~ i
! Layer 3
i
! Laver 4--5 ! ..
[000125]
[000126]
Ll 3 20-40 em
1.1 3 40--199 em
Configuration 4 has an estimated neutron efficiency of about 20~~.
Extra layers could be added, or layers removed, or these principles modified,
or geometries or materials added or changed or altered without changing the premise of these
descriptions.
[000127] Particular embodiments and features have been described with reference to the
drawings. it is to be understood that these descriptions are not limited to any single
embodiment or any particular set of features, and that similar embodiments and features may
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arise or modifications and additions may be made without departing from the scope of these
descriptions and the spirit of the appended claims.

CLAIMS
\Vhat is claimed is:
1. An apparatus for producing reaction-product nuclei frorn reactant nuclei, the
apparatus comprising:
a plurality of reactant nuclei and a plurality of moderating nuclei wherein the
moderating nuclei comprise nuclei of atoms that are chosen from a group consisting of
deuterium, tritiurn, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20 and neon-22; and
a neutron source that is neither a nuclear reactor nor a subcritical assembly, in
proximity to the reactant nuclei sufficient to produce reaction-product nuclei by neutron
capture;
wherein the reactant nuclei comprise molybdenum--98;
wherein the rate of molybdenum-98 nuclei neutron capture divided by the rate of the
neutron source's neutron production is greater than approximately l %;
wherein the mass of molybdenum-98 is less than approximately 1000 kg; and
wherein the mass of moderating nuclei is at least 1 kg.
2. The apparatus of claim l, w·herein the mass ofmolybdenum-98 is less than
approximately l 00 kg.
3. The apparatus of claim 1, wherein the mass of molybdenum-98 is less than
approximately 25 kg.
4. The apparatus of claim 2, wherein the rate of reactant nuclei neutron capture divided
by the rate of the neutron source's neutron production is greater than approxirnately 5~S.
5. The apparatus of claim 1, further comprising the use of temperature control capable of
maintaining at least 2 different regions of the apparatus at different temperatures;
wherein at least one region is cooled to a temperature below 250 degrees Kelvin.
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6. The apparatus of claim 5, wherein the temperature control comprises the use of a
cryogenic fluid.
7. The apparatus of claim 1, fu1iher comprising at least one neutron re:l:1ector at least
partially surrounding the pluralities of reactant nuclei and rnoderating nuclei, wherein the
reflector comprises moderating nuclei and wherein the reflector thickness is greater than
approximately 20 centimeters and less than approximately 15 meters.
8. The apparatus of claim 1, further comprising both an outer and an inner neutron
reflector that reflect neutrons towards regions containing higher densities of reactant nuclei.
9. The apparatus of claim l, wherein the pluralities are arranged in one or more
approximately parallel layers, and wherein at least one layer is distinct from another layer on
the basis of elemental composition, concentration of chemical species, density or
temperature.
10. The system of claim 1, further comprising a target configured to emit neutrons when
impacted by accelerated particles;
wherein the target is comprised of atoms chosen from a group consisting of
deuterium, tritium, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20, neon-22, tantalum, tungsten, lead, mercury, thallium, thorium, uranium,
neptunium and other transuranics,
w·herein the accelerated particles enter the system via an access channel configured to
accept greater than 50 percent of the accelerated particles that impinge upon the access
channel.
11. An apparatus for producing reaction-product nuclei from reactant nuclei, the
apparatus comprising:
a neutron source that is neither a nuclear reactor nor a subcritical assembly;
a first plurality of reactant nuclei having a first average microscopic thermal neutron
capture cross section;
a collection of isotopes consisting of those isotopes whose nuclei species capture at
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least 1% of ail emitted neutrons from a neutron source and which are not reactant nuclei;
a second plurality of nuclei consisting of all nuclei from the collection of isotopes,
\vherein at least approximately 90% of the nuclei have microscopic thermal neutron capture
cross--sections that are lower than the microscopic thermal neutron capture cross-section of
any of the reactant nuclei, and wherein the total mass ofthe second plurality of nuclei is
greater than approximately 1 kilogram; and
wherein the neutron source is in proximity to the reactant nuclei sufiicient to produce
reaction-product nuclei by neutron capture.
12. The apparatus of claim 11, wherein tbe reactant nuclei comprise Molybdenum-98,
wherein the rate of reactant nuclei neutron capture divided by the rate of the neutron source's
neutron production is greater than approximately l 0
;(;.
13. The apparatus of claim 11, wherein the reactant nuclei comprise Molybdenum-98,
wherein the rate of reactant nuclei neutron capture divided by the rate of the neutron source's
neutron production is greater than approxirnately 5~S.
14. The apparatus of claim 11, wherein the apparatus further comprises molybdenum-99
reaction-product nuclei and technetium-99m decay-product nuclei.
15. The method of claim 11, wherein the second plurality of nuclei comprise nuclei of
atoms that are chosen fl·om a group consisting of deuterium, tritiurn, helium-4, lithiurn-7,
beryllium, boron-11, carbon, nitrogen-15, oxygen, fluorine, neon-20 and neon-22.
16. The apparatus of claim 11, further comprising the use of temperature control to
maintain at least two difierent regions of the apparatus at different temperatures, wherein at
least one region is cooled to a ternperature below approxirnately 250 degrees Kelvin.
17. The apparatus of claim 16, wherein the temperature control comprises the use of a
cryogenic fluid.
18. The apparatus of claim 15, fmiher comprising at least one neutron re:l:1ector at least
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partially surrounding the first and second pluralities of nuclei, wherein the retlector
comprises nuclei of the second plurality of nuclei and wherein the reflector thickness is
greater than approximately 20 centimeters and less than approximately 15 meters.
19. The apparatus of claim 15, further cornprising both an outer and an inner neutron
reflector that reflect neutrons to\vards regions of the pluralities containing higher densities of
reactant nuclei.
20. The apparatus of claim 11, further comprising a target configured to emit neutrons
when impacted by accelerated particles, wherein the target is comprised of atoms chosen
from a group consisting of deuterium, tritium, helium A, lithium·7, beryllium, boron··ll,
carbon, nitrogen-15, oxygen, fluorine, neon-20, neon-22, tantalum, tungsten, lead, mercury,
thallium, thorium, uranium, neptunium and other transuranics; and
wherein the accelerated particles enter the system via an access channel configured to
accept greater than 50 percent of the accelerated particles that impinge upon the access
channel.
21. A method for producing decay·product nuclei from a reactant isotope using a neutron
source, the method comprising:
generating neutrons;
preparing a first plurality of reactant nuclei having a :first average microscopic thermal
neutron capture cross section;
preparing a collection of isotopes consisting ofthose isotopes whose nuclei species
capture at least 1% of all emitted neutrons from a neutron source and which are not reactant
nuclei;
preparing a second plurality of nuclei consisting of all nuclei from the collection of
isotopes, wherein at least approximately 90% of the nuclei have rnicroscopic thermal neutron
capture cross·sections that are lovver than the microscopic thermal neutron capture cross·
section of any of the reactant nuclei, and wherein the total mass of the second plurality of
nuclei is greater than approximately 1 kilograrn; and
inadiating the plurality vvith the neutrons such that a reaction product is generated
when the neutrons are captured by the reactant nuclei; and
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extracting frorn the plurality a decay product that is generated by radioactive decay of
the reaction product isotope.
22. The method of claim 21, wherein the neutrons are generated by a nuclear reactor or a
subcritical assernbly.
The method of claim 21, vvherein the neutrons are generated by a source that is neither
a reactor nor a subcritical assembly.
24. The method of claim 21, wherein:
the reactant nuclei comprise Molybdenum--98; and
the decay product comprises technetiurn-99m; and
wherein the rate of reactant nuclei neutron capture divided by the rate of the neutron
source's neutron production is greater than approximately 1%.
25. The method of claim 21, wherein the second plurality of nuclei comprise nuclei of
atoms that are chosen from a group consisting of deuterium, tritium, helium-4, lithium-7,
beryllium, boron-11, carbon, nitrogen--15, oxygen, Huorine, neon--20 and neon--22.
26. The method of claim 25, further comprising at least partially summnding the first and
second pluralities of nuclei with at least one neutron rei1ector comprising nuclei of the second
plurality of nuclei and whose thickness is greater than approximately 20 centimeters and less
than approximately 15 meters.
27. The method of claim 25, further comprising both an outer and an inner neutron
reflector that reflect neutrons towards regions containing higher densities of reactant nuclei.
28. The method of claim 21, further comprising the use of temperature control capable of
maintaining at least 2 different regions of the apparatus at different temperatures;
vvherein at least one region is cooled to a ternperature below 250 degrees Kelvin.
29. The method of claim 28, wherein the temperature control comprises the use of a
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cryogenic ibid.
30. The method of claim 21, further comprising configuring a target to emit neutrons
when impacted by accelerated particles, wherein the target is comprised of atoms chosen
from a group consisting of deuterium, tritium, he!ium-4, lithium-7, beryllium, boron-11,
carbon, nitrogen-15, oxygen, fluorine, neon-20, neon-22, tantalum, tungsten, lead, mercury,
thallium, thorium, uranium, neptunium and other transuranics, and
wherein the accelerated particles enter the system via an access channel configured to
accept greater than 50 percent of the accelerated particles that irnpinge upon the access
channel.
31. A system for producing a decay product frorn a reactant using a neutron source,
compnsmg:
a plurality of reactant nuclei and a plurality of moderating nuclei wherein the
moderating nuclei comprise nuclei of atoms that are chosen from a group consisting of
deuterium, tritiurn, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20 and neon-22; and
a neutron source;
wherein the reactant nuclei comprise mo!ybdenum-98;
wherein the rate ofmolybdenum-98 nuclei neutron capture divided by the rate of the
neutron source's neutron production is greater than approximately 1 %;
wherein the mass of molybdenum-98 is less than approximately l 00 kg; and
\vherein the mass of moderating nuclei is at least l kg.
32. The system of claim 31, wherein the neutron source is a nuclear reactor or subcritical
assernbly.
3 3. The system of claim 31, wherein the neutrons are generated by a source that is neither
a reactor nor a subcritical assembly.
34. The system of claim 31, further comprising a target configured to emit neutrons when
impacted by accelerated particles;
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wherein the target is comprised of atoms chosen fl·om a group consisting of
deuterium, tritimn, helium-4, lithimn-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20, neon-22, tantalum, tungsten, lead, mercury, thallium, thorium, uranium,
neptunium and other transuranics, and
wherein the accelerated particles enter the system via an access channel configured to
accept greater than 50 percent of the accelerated particles that impinge upon the access
channel.
35. The system of claim 31, further comprising at least one neutron reflector cornprising
moderating nuclei, wherein the neutron reflector at least partially surrounds the layers of the
pluralities of reactant and moderating nuclei, and wherein the thickness of the neutron
reflector is greater than approximately 20 centimeters and less than approximately 15 meters.
36. The system of claim 31, further comprising both an outer and an inner neutron
ref1ector that reflect neutrons towards regions of the pluralities containing higher densities of
reactant nuclei.
3 7. The system of claim 31, further comprising temperature control capable of
maintaining at least 2 different regions of the apparatus at different temperatures, wherein at
least one region is cooled to a ternperature below 250 degrees Kelvin.
38. The system of claim 37, wherein the temperature control comprises a cryogenic fluid.
39. A process comprising:
combining a plurality of reactant nuclei and a plurality of moderating nuclei wherein
the moderating nuclei comprise nuclei of atoms that are chosen from a group consisting of
deuterium, tritium, helium-4, lithium-7, beryllium, boron-11, carbon, nitrogen-15, oxygen,
fluorine, neon-20 and neon--22, together with
placing a neutron source that is neither a nuclear reactor nor a subcritical assembly in
proximity to the reactant nuclei sufficient to produce reaction-product nuclei by neutron
capture;
wherein the reactant nuclei comprise molybdenum-98;
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wherein the rate of rnolybdenum-98 nuclei neutron capture divided by the rate of the
neutron source's neutron production is greater than approximately l %;
wherein the mass ofmolybdenum-98 is less than approximately 100 kg;
wherein the mass of moderating nuclei is at least 1 kg; and
irradiating the combination with neutrons such that radioactive reaction-product
nuclei are generated vvben the neutrons are captured by the reactant nuclei.
40. The process of claim 39, wherein the reactant nuclei comprise Molybdenum-98,
wherein the rate of reactant nuclei neutron capture divided by the rate of the neutron source's
neutron production is greater than approximately 1%1.
41. A radioactive materia! according to claim 40, further comprising extracting at least
one of the reaction-product nuclei and decay-product nuclei from the irradiated combination.
41 A radioactive material according to clairn 41, wherein the decay-product nuclei are
technetium-99m.
43. A system for producing reaction··product nuclei, comprising reactant nuclei, a neutron
source in proximity to the reactant nuclei sui1lcient to produce reaction-product nuclei by
neutron capture and wherein at least l 00 g of the system is cooled to a temperature at or
below approximately 250 Kelvin.
44. A system according to claim 43, wherein the reactant nuclei cornprise molybdenum-
98.
45. A system according to clairn 44, wherein the rate ofmolybdenurn-98 nuclei neutron
capture divided by the rate of the neutron source's neutron production is greater than
approximately l %.
46. A system according to claim 45, further comprising rnoderating nuclei wherein the
moderating nuclei comprise nuclei of atoms that are chosen from a group consisting of
deuterium, tritium, helium--4, lithium··7, beryllium, boron··ll, carbon, nitrogen··15, oxygen,
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fluorine, neon-20 and neon-22.
47 An apparatus for producing reaction-product nuclei from reactant nuclei, the
apparatus comprising:
a neutron source;
a first plurality of reactant nuclei having a first average microscopic thermal neutron
capture cross section;
a collection of isotopes consisting ofthose isotopes whose nuclei species capture at
least 1 ~,~ of all emitted neutrons from a neutron source and vvhich are not reactant nuclei;
a second plurality of nuclei consisting of all nuclei from the collection of isotopes,
wherein at least approximately 90% of the nuclei have microscopic therrnal neutron capture
cross-sections that are lower than the microscopic thermal neutron capture cross-section of
any of the reactant nuclei, and wherein the total mass of the second plurality of nuclei is
greater than approximately l kilogram; and
wherein the neutron source is in proximity to the reactant nuclei sufficient to produce
reaction-product nuclei by neutron capture.
48. A rnethod for producing decay-product nuclei from a reactant isotope using a neutron
source, the method cornprising:
generating neutrons;
preparing a first plurality of reactant nuclei having a first average rnicroscopic thermal
neutron capture cross section;
preparing a collection of isotopes consisting of those isotopes whose nuclei species
capture at least 1% of all emitted neutrons from a neutron source and which are not reactant
nuclei;
preparing a second plurality of nuclei consisting of all nuclei frorn the collection of
isotopes, wherein at least approximately 90% of the nuclei have microscopic thermal neutron
capture cross-sections that are lower than the microscopic thermal neutron capture crosssection
of any of the reactant nuclei, and wherein the total mass of the second plurality of
nuclei is greater than approximately 1 kilogram; and
in-adiating the plurality vvith the neutrons such that a reaction product is generated