Europium doped caesium bromo iodide scintillator and detectors thereof with
improved conversion efficiency.
Description
Technical Field
[0001] The present invention relates to a scintillator comprising CsBrxl(i-X) doped
with europium (CsBrxl( -X) :Eu). The scintillator shows a high conversion
efficiency for high energy radiation detection and a low afterglow.
Therefore, the invention also relates to digital radiography flat panel
detectors (FPDs) and high energy radiation detectors comprising the
CsBrxl(i -X) :Eu scintillator with high conversion efficiency. The present
invention is useful in the X-ray imaging field, in particular where a high
quality image is important and for high energy radiation detection
applications as well.
Background Art
[0002] Inorganic scintillators are employed in most of the current medical
diagnostic imaging modalities using X-rays or gamma rays.
[0003] In digital radiography (DR) flat panel detectors (FPDs), which are X-ray
detectors that capture images from objects during inspection procedures
or from body parts of patients to be examined, scintillators are used to
convert X-rays into light. This light interacts with an amorphous silicon (a-
Si) semiconductor sensor layer, where electrical charges are created. The
charges are collected by thin film transistors (TFT's) which are arranged in
an array. The transistors are switched-on row by row and column by
column to read out and the collected charges are transformed into a
voltage, which is transformed into a digital number and stored in a
computer to make up a digital image corresponding with the shadow
image of the irradiated object. This way of conversion of X-rays into
electrical charges is called indirect conversion direct radiography (ICDR).
Typical scintillating material for use in ICDR is caesium iodide doped with
thallium (Csl:TI).
[0004] Scintillators are also particularly useful for the detection of high energy
radiation in combination with a photomultiplier tube (PMT). When high
energy radiation interacts with the scintillator material, photons are created
that can activate the PMT. The scintillation light is emitted isotropically; so
the scintillator is typically surrounded with reflective material to minimize
the loss of light and then is optically coupled to the photocathode of the
PMT. Scintillation photons incident on the photocathode liberate electrons
through the photoelectric effect, and these photoelectrons are then
accelerated by a strong electric field in the PMT. The output signal
produced is proportional to the energy of the gamma ray in the scintillator.
High energy radiation detectors based on a scintillator and a PMT are
useful in detection of radiation in gamma ray cameras and positron
scanners (Positron Emission Tomography and single-photon emission
computed tomography). High energy radiation detectors are also used in
scintillation counting mode for measuring radiation in activation analysis,
X-ray fluorescent analysis, Transmission Electron Microscopy (TEM), Time
of Flight Mass Spectrometry (TOF MS), high energy physics collision
detection and detection of cosmic rays. Scintillators which are coupled with
a photomultiplier tube (PMT) are used in survey meters to detect
radioactive contamination, monitoring and testing nuclear material.
[0005] Scintillators are also used in security, baggage cargo and personal
screening. Another important application of scintillators is in dosimetry for
personal safety dosimeters. Dosimeters are used to measure the radiation
dose received by body, tissue and matter, received from indirect or direct
ionizing radiation. It is very important that the dosimeter or other detection
radiation device has a high sensitivity and can detect any level of radiation.
[0006] The scintillation conversion is a relatively complicated process that can be
generally divided in three sub-processes: conversion, transport and
luminescence. These three steps determine the emission efficiency of the
scintillator material. In this respect, structured scintillators made of
inorganic materials crystals doped with an activator element, such as
sodium iodide doped with thallium (Nal:TI) or caesium iodide doped with
thallium (Csl:TI), were developed to allow detecting and monitoring higher
energy X- or g -rays (high energy: below ~ 1 MeV) and are employed in the
(X- or g -rays) photon counting regime.
[0007] The mechanism of luminescence of scintillators consists in accumulating
the generated light arriving soon after the initial conversion stage is
accomplished and the most important parameters determining the
conversion efficiency are: ( 1 ) the light yield or conversion efficiency; (2) the
X-ray stopping power; (3) the decay time; (4) the spectral match between
the scintillator emission spectrum and the sensitivity spectrum of the
photosensitive detector; (5) the chemical stability and radiation resistance
of the scintillator; and (6) the energy resolution. The conversion efficiency
of a scintillator can be measured in a relative way, i.e. by measuring the
light emission of the scintillator under study and comparing the results with
the measurements of a known standard scintillator, as a reference. By
improving the conversion efficiency of scintillators better image quality and
shorter image acquisition time can be obtained.
[0008] Despite the acknowledged advantages of CshTI in many scintillator
applications with respect to conversion efficiency, a characteristic property
that undermines its use in high-speed radiographic and radionuclide
imaging is the presence of a strong afterglow component in its scintillation
decay. This causes pulse pile up in high count-rate applications, reduced
energy resolution in radionuclide imaging, and reconstruction artefacts in
computed tomography applications. Another disadvantage of CshTI is the
presence of very toxic Tl. The very toxic Tl represents an important safety
issue in production of CshTI based scintillators. With regard to the high
energy radiation detectors based on a combination of a scintillator with a
PMT, the spectrum of the emitted light of Csl(TI) with its maximum at
550nm does not match very well the sensitivity spectrum of the
photocathode of the PMT having a maximum between 400 and 450 nm.
[0009] Europium doped caesium bromo iodide (CsBrxl(i -X) :Eu) based scintillators,
with a high content of iodide (x<0.5) show a very low afterglow level, do
not include a highly toxic Tl activator and their emission spectrum matches
well the sensitivity spectrum of the photocathode of a PMT. CsBrxl(i -X) :Eu
based scintillators however do not show a high conversion efficiency. It is
thus desirable to increase the conversion efficiency of CsBrxl(i -X) :Eu in an
easy and reliable manner. Therefore, research has been done to improve
the conversion efficiency while maintaining the advantage of a low
afterglow level of europium doped Csl material.
[0010] US7560046 relates to a scintillator material that increases the conversion
efficiency by avoiding the production of radiation damages that can lead to
the occurrence of ghost images. Therefore, this document discloses an
annealed scintillator composition with a formula of A3B2C3O12, where A is
at least one member of the group consisting of Tb, Ce, and Lu, or
combinations thereof; B is an octahedral site (Al), and C is a tetrahedral
site (also Al). One or more substitutions are included. These materials do
not include alkali halide compounds doped with at least one activator
compound.
[001 1] Cherginets et al. (Luminescent properties of Csl single crystals grown from
the melt treated with Eu - V. L. Cherginets, T. P. Rebrova, Yu. N. Datsko,
V. F. Goncharenko, N. N. Kosinov, R. P. Yavetsky, and V. Yu. Pedash
Cryst. Res. Technol. 47, No. 6 , 684-688. 2012) studied the scavenger
properties of super-pure alkaline earth halides, namely Csl single crystals
compositions doped with different concentrations of europium in the form
of Eul2, from 10 4 to 10 2 mol.kg 1. The luminescent properties of Cs Eu
crystals are attributed to the distortion of the crystal lattice and not
necessarily to the doping of Eu. The proven improvement of the Eu dopant
is the reduction of afterglow by suppression of the slow components of the
scintillator pulse, which occurs at E concentration in Csl melt equal to
0.01 mol.kg- 1.
[0012] Thacker et al. (Low-Afterglow Csl:TI microcolumnar films for small animal
high-speed microCT - S.C. Thacker, B. Singh, V. Gaysinskiy, E.E.
Ovechkina, S.R. Miller, C. Brecher, and V.V. Nagarkar, Nucl. Instrum.
Methods Phys. Res. A . 2009 June 1, 604(1 ) , 89-92) discovered that
adding a second dopant Eu2+ to CshTI reduces afterglow with a factor of
40 at 2 ms after a short excitation pulse of 20 ns, and with a factor of 15 at
2 ms after a long excitation pulse of 00 ms. The Eu is added to reduce
the afterglow, and it is not used as a scintillator activator.
[001 3] EP1 113290 relates to the improvement in output decrease over time of the
scintillator, by adding a light absorbing layer between the scintillator and a
light sensitive imaging array. This is to reduce the rate at which the light
sensitive imaging array saturates, to reduce light incident on the switching
devices, and/or to compensate for the aging of a scintillator. The invention
is related to the improvement of the efficiency after degradation due to
operation.
[0014] US7558412 discloses a method for detecting the potential of an X-ray
imaging system to create images with scintillator hysteresis artefacts. The
method comprises measuring signal levels for different areas of interest
and comparing all measurements with a given threshold to determine if
scintillator hysteresis artefacts may be produced by a certain scintillator in
result from a large x-ray flux dose. Said effect can occur even in
scintillators including Csl doped with Tl (Csl:TI). US7558412 further
discloses that the method may optionally include exposing the scintillator
to an x-ray flux if the difference between the two signals obtained is
greater than a given threshold and thus detecting the potential of said
scintillator to produce image artefacts. The method is connected to "
bleaching" of the scintillator to the original level of efficiency (before
irradiation) and not adding it.
[0015] In Gektin et al. (Radiation damage of Csl:Eu crystals. Functional Materials,
20; n.2 (2013) - STC "Institute for Single Crystals" of National Academy of
Sciences of Ukraine) a study is presented on the radiation damage and
afterglow nature for CshEu crystals concluding that the luminescence
parameters depend on the X-ray irradiation conditions and that irradiation
leads to emission suppression at doses less than 100 Gy when the
induced absorption was not observed yet.
[0016] In another document, Gektin et al. (Europium emission centers in CshEu
crystal. Optical Materials 35 (2013), 2613 - 2617) the absorption,
excitation and luminescence spectra of pure and Eu doped single crystals
were studied depending on the activator content, the excitation energy, the
heat treatment and the X-ray radiation. It is shown that the structure and
concentration of the complex centres changes at heat treatment. Only an
increase of annealing temperature from 300°C to 405°C, followed by
quenching, has a marked influence on the spectral composition and
intensity of luminescence.
[0017] Giokaris et al. (Nuclear Instruments and Methods in Physics Research A
569 (2006) 185-187) compared scintillators based on Csl:TI and CshNa
crystals, coupled with Position Sensitive Photomultiplier Tubes (PSPMTs)
for gamma-ray detection with respect to their performance in terms of
sensitivity, spatial and energy resolution. CshNa based scintillators are
very hygroscopic and hence difficult to coat via a dispersion. Csl:TI based
scintillators have a light emission spectrum which matches less good with
the sensitivity spectrum of the PMT than Csl:Eu and Cs TI crystals are
obtained after a long and hence expensive production process.
[0018] Document EP1944350A2 discloses the method of optimizing speed of
storage phosphor needle image plates (NIP), particularly europium doped
caesium bromide (CsBr:Eu), by annealing. The object of this patent is
realised with the marker of stable Eu-ligand complexes measured with
electron paramagnetic resonance (EPR). The peaks of the EPR signal are
measured at the frequency of 34 GHz and the following flux density of
magnetic filed are specified: 880, 1220, 1380 and 1420 mT. Europium
doped caesium bromide (CsBnEu) is very hygroscopic and the methods of
applying this material onto a substrate are therefore not compatible with a
coating process from a coating dispersion.
[0019] EP2067841 discloses a phosphor storage plate based on a binderless
needle-shaped Cs(X,X'), X representing Br and X' representing F, CI, Br, I
but no specific combination of X being Br with X' being I is disclosed.
[0020] However, none of these documents discloses a method of production that
increases the conversion efficiency of CsBrxl(i -X) :Eu as a scintillator and
maintaining the advantage of a low afterglow level of europium doped
CsBrxl(i -X) material.
Summary of invention
[0021] The above stated problem is solved by the invention as defined in claim 1.
In the invention is provided, europium doped CsBrxl( -X) material with x<0.5
which is annealed and which is optionally exposed to electromagnetic
radiation, leading to a scintillator having a high conversion efficiency for Xrays
and high energy radiation and showing a low level of afterglow.
[0022] It is further an object of the invention to provide a high energy radiation
detection apparatus according to claim 8.
[0023] Another object of the present invention is to provide a radiography flat
panel detector according to claim 9 .
[0024] Another object of the present invention is to provide methods of preparing
europium doped CsBrxl(i-X) scintillators in an easy and straightforward way
according to claim 10.
[0025] Further advantages and embodiments of the present invention will become
apparent from the following description and the dependent claims.
Brief description of drawings
[0026] Figure 1: EPR spectrum of a CsBrxl(i -X):Eu annealed scintillator measured
at room temperature (25°C) and at a frequency of 34 GHz, with the
magnetic field scanned from 500 to 600 mT. Maxima are visual at
magnetic fields: 1090mT, 1140mT, 1200mT, 1220mT, and minima at 1250
mT and 1350mT. The signal with a maximum at 1200 mT has the largest
peak height.
[0027] Figure 2: EPR spectrum of a CsBrxl(i -X):Eu scintillator which is not
annealed, measured at room temperature (25°C) and at a frequency of 34
GHz, with the magnetic field scanned from 500 to 1600 mT.
[0028] Figure 3: Light conversion signal (at 450nm) of a CsBrxl( -X):Eu annealed
scintillator with a coating weight of 49.2 mg/cm2, in function of the
absorbed X-ray dose (D).
[0029] Figure 4: Comparison of afterglow, measured after X-ray exposure in a
range of 0 to 14 s of different scintillators, based on CshTI, GOS:Tb and
CsBrxl( -X):Eu. The CsBrxl(i -X):Eu scintillator shows the lowest afterglow and
is 10 times lower than CshTI.
[0030] Figure 5: EPR spectrum of a CshEu annealed scintillator measured at
room temperature (25°C) and at a frequency of 34 GHz, with the magnetic
field scanned from 500 to 1600 mT. A maximum is visual at a magnetic
field of 1200mT and minima at 1240 mT and 1350mT. The signal with a
maximum at 1200 mT has the largest peak height.
[0031] Figure 6 : EPR spectrum of a CsBro.osl(o.92):Eu annealed scintillator
measured at room temperature (25°C) and at a frequency of 34 GHz, with
the magnetic field scanned from 500 to 1600 mT. A maximum is visual at a
magnetic field of 1200mT and minima at 1240 mT and 1340mT. The signal
with a maximum at 1200 mT has the largest peak height.
Description of embodiments
. Raw materials.
[0032] Caesium bromo iodide (CsBr xl(i -X)) doped with Europium (Eu) wherein
x<0.5, according to the present invention can be obtained starting from
Csl, optionally CsBr and Eu containing compounds as raw materials. Eu
containing compounds can be: pure Europium, europium oxides (EU2O3,
EU3O4) , europium halides (EU F2, EUF3, EuC , EuC , EUCI2.2H2O,
EUCI2.6H2O, EuBr3, Eu , Eu ), europium oxyhalides (Eu On X3m-2n, where
X = F, CI, Br or I), europium chalcogenides (EuS, EuSe, EuTe), europium
nitrides (EuN) or other europium complexes (CsEuBr3, Eu(C5Hr02)3,
EuBaTiO, CsEuh, etc.). The most preferred ones are EuC , EuBr3, Eul2
CsEuBr3, due to the close proximity of melting point to the melting point of
Csl (621 °C), and europium oxyhalides, because they decompose into
europium oxides and europium halides upon heating.
[0033] The ratio of Csl and CsBr is chosen as to obtain a CsBrxl(i-X) material with
x<0.5, preferably <0.1 , most preferably x< 0.05. Throughout the text when
specifying CsBrx l(i -X) without mentioning the value of x, x is then assumed
to be < 0.5. Both Csl and optionally CsBr can be mixed to achieve a
homogeneous composition in powder form or in liquid solution but they
can also be provided separately.
[0034] The molar ratio of the CsBrxl(i-X) and Eu is preferably in the range of
99.9/0.1 to 98/2, and more preferably from 99.5/0.5 to 99/1 . To obtain
these molar ratios, the Csl and optionally CsBr and the Eu compound(s)
can be mixed to achieve a homogeneous composition in powder form or in
liquid solution but they can also be provided separately. Preferably the
Csl, and optionally CsBr are mixed with the Eu compounds in the gas
phase.
[0035] Optionally, the raw materials can be purified before the scintillator is
prepared, and pre-heated to evaporate the present water.
2. Preparation of the CsBrxl(i -X):Eu material
2.1 . Vapour deposition of the CsBrxl(i -X):Eu material
[0036] In a preferred embodiment, the CsBr l( -X):Eu material can be produced
with a physical vapour deposition process (PVD) of Csl, optionally CsBr
and Eu compounds (EuC , EuBr3, Eu , CsEuBr3, Eu OnX3m-2n) onto a
substrate in a vacuum deposition chamber, where at least one crucible is
filled with powder of the selected compounds and heated to a temperature
not higher than 750°C. During the physical vapour deposition process, the
Csl, CsBr and Eu compounds are mixed in the gas phase. Another
embodiment is to use mixed crystals such as CsBryl(i -y ) wherein y<1 . In a
more preferred embodiment, only Csl and Eu containing compounds are
used as raw material in the preparation of the scintillator. This so obtained
scintillator is assumed to essentially consist of Csl and Eu, hence x=0, and
is denoted as Csl:Eu.
[0037] More preferably, the compounds are evaporated from at least 2 crucibles
at the same time. Eu is evaporated from at least one separate crucible
comprising only the Eu containing compound, in order to get a more
constant evaporation rate and, hence, a more homogeneous Eu
distribution in function of the thickness of the deposited layer.
[0038] The vapour deposited CsBrxl( -X):Eu material comprises crystals which can
have a needle like structure or can be non-needle like, depending on the
circumstances of deposition and the thickness of the deposited layer.
[0039] The time of the evaporation in a vacuum deposition chamber is preferably
between 30 and 360 min. After the evaporation the CsBrxl(i -X):Eu material
is cooled down and removed from the vacuum deposition chamber.
[0040] The substrate for the deposition of the CsBrxl( -X):Eu material, can be either
rigid or flexible, such as an aluminium plate, an aluminium foil, a film of
polyethylene terephthalate (PET), polyethylene naphthalate (PEN),
polyimide (PI), polyethersulphone (PES), polyphenylsulfone (PPSU),
polyphenylene sulphide (PPS), polyether etherketon (PEEK), polybutylene
terephthalate (PBT), polyetherimide (PEI), a metal foil, a carbon fibre
reinforced plastic (CFRP) sheet, glass, flexible glass, triacetate and a
combination thereof or laminates thereof. More preferable substrates are
flexible substrates: metal foils, especially of aluminium, foils of
polyethylene terephthalate (PET), polyethylene naphthalate (PEN),
polyimide (PI), polyethersulphone (PES), polyphenylsulfone (PPSU),
polyphenylene sulphide (PPS) and combinations thereof or laminates
thereof. The substrates can have a thickness between 20 and 800 mih ,
preferably between 70 and 300 m.
[0041] Optionally the substrate, prior to the deposition of the CsBrxl(i -X):Eu
material can be coated with additional functional layer(s), for example to
improve the adhesion, to cover the visible pattern of the surface of the
substrate, to protect from moisture or chemicals, etc. Examples of
additional functional layers are disclosed in the unpublished application
EP141661 5 1.2, [0032] and [0037]. The combination of a CsBrxl(i -x):Eu
material applied on a substrate is hereafter denoted as a scintillator
screen.
[0042] In a preferred embodiment of the invention, the CsBrxl(i -X):Eu material
can be directly deposited onto the detector of a FPD. The detector
comprises a plurality of photosensitive elements which can convert light
into an electric signal. Examples of photosensitive elements are
amorphous Si, CMOS and CCD.
[0043] In another preferred embodiment, the CsBrxl( -X):Eu material can be
directly deposited onto the radiation entrance window of a PMT.
[0044] When producing a scintillator for FPD applications, CsBrxl( -X):Eu material
with a needle like crystal structure is preferred. The preferred thickness for
these layers varies in the range from 50 to 500 mih , the coating weight
varies in the range from 40 to 250 mg/cm2 and the needle diameter varies
in the range from 2 to 10 m h . The preferred concentration of Eu in the
deposited CsBrxl( -X):Eu material is from 1 to 10000 ppm.
2.2. Alternative methods of preparation of the CsBrxl( -X):Eu material
[0045] In an embodiment of the invention, the CsBrxl( -X):Eu material can also be
prepared by mixing Csl, optionally CsBr with one or more Eu containing
compounds and by heating the obtained powder mixture in an oven. A
temperature of at least equal to the melting point of one of the
components, preferably higher than 620 °C is required. The obtained
CsBrxl( -X):Eu material is denoted hereafter as fired CsBrxl( -X):Eu. The
obtained CsBr l( -X):Eu material which has usually the form of a brick, can
be pulverised before the annealing step.
[0046] In another embodiment, the CsBrxl( -X):Eu material can be produced by
mixing powders of Csl; optionally CsBr and one or more Eu containing
compounds. The powders are then pulverized. This can be done by any
suitable pulverization method.
[0047] In another embodiment, the CsBrxl(i -X):Eu material can be produced by a
crystal growth method as Czochralski, Bridgman-Stockbarger, Kyropoulos
or any other known crystal growth techniques. In those cases the crystal is
grown from the melt of Csl and one or more Eu containing compounds or
from a melt of CsBrxl(i -X) and one or more Eu containing compounds. The
crystals can be pulverized to obtain small particles.
2.3. Deposition of CsBrxl( -X) material from a dispersion
[0048] According to the present invention, the CsBrxl( -X):Eu material with x< 0.5
as obtained in the previous sections can be applied on a substrate from a
dispersion. Therefore, the CsBrxl(i -X):Eu material is pulverized to obtain
small particles. Preferably the median particle size of the CsBrxl( -X):Eu
particles to be applied as a dispersion is between about 0.5 miti and about
40 miti . A median particle size of between I m and about 20 pm is more
preferred for optimizing properties, such as speed, image sharpness and
noise.
[0049] In a highly preferred embodiment of the invention, the fired CsBrxl(i -X):Eu
material is first prepared by mixing Csl with optionally CsBr and with one
or more Eu containing compounds, heating the obtained powder mixture in
an oven at a temperature of at least equal to the melting point of one of the
compounds. After heating, the mixture is cooled down to obtain a brick of
CsBrxl(i -X):Eu which is then pulverized. The pulverized CsBrxl( -X):Eu
material can then be added to a solution comprising a binder to obtain a
dispersion of CsBrxl(i -X):Eu particles.
[0050] In another preferred embodiment of the invention, the CsBrxl(i -X):Eu is first
prepared by the PVD technique as described above, then removed from
the substrate and pulverized to obtain small particles. The pulverized
CsBrxl(i - ):Eu material can then be added to a solution comprising a binder
to obtain a dispersion of CsBrxl( -X):Eu particles.
[0051] A preferred method of applying the CsBrxl( -X):Eu particles to a substrate is
via a coating process in which a layer is applied on the substrate from the
dispersion of CsBrxl( -X):Eu particles. The dispersion can be applied on a
substrate using a coating knife, preferably a doctor blade and is hereafter
denoted as a coating dispersion. The coating is dried, preferably in an
oven or with hot air to obtain a scintillator screen.
[0052] In another embodiment of the invention, CsBrxl(i -X):Eu is deposited from a
dispersion with preferably a high viscosity, meaning a viscosity higher than
2 Pa*s. The dispersion is applied on a substrate using a mould and is
dried to obtain a scintillator screen. Examples of suitable substrates are
described above.
[0053] In an embodiment of the invention, the CsBrxl( -X):Eu material is produced
by mixing Csl, CsBr and Eu compounds together in a solvent to obtain a
dispersion.
[0054] Suitable solvents for the preparation of the dispersion can be acetone,
hexane, methyl acetate, ethyl acetate, isopropanol, methoxy propanol,
isobutyl acetate, ethanol, methanol, methylene chloride and water.
[0055] Binders suitable for the preparation of a coating dispersion can be
inorganic binders or organic binders. Examples of organic polymers are
cellulose acetate butyrate, polyalkyl (meth)acrylates, polyvinyl-n-butyral,
poly(vinylacetate-co-vinylchloride), poly(acrylonitrile-co-butadiene-costyrene),
polyvinyl chloride-co-vinyl acetate-co-vinylalcohol), poly(butyl
acrylate), poly(ethyl acrylate), poly(methacrylic acid), polyvinyl butyral),
trimellitic acid, butenedioic anhydride, phthalic anhydride, polyisoprene
and/or a mixture thereof. Preferably, the binder comprises one or more
styrene- hydrogenated diene block copolymers, having a saturated rubber
block from polybutadiene or polyisoprene, as rubbery and/or elastomeric
polymers. Particularly suitable thermoplastic rubbers, which can be used
as block-copolymeric binders, in accordance with this invention, are the
KRATON™ G rubbers, KRATON™ being a trade name from SHELL.
[0056] Examples of suitable inorganic binders are alumina, silica or alumina
nanoparticles, aluminium phosphate, sodium borate, barium phosphate,
phosphoric acid, barium nitrate. Inorganic binders have the advantage of a
higher resistance to the annealing step, especially if the annealing step is
performed at temperatures in the high temperature range.
[0057] The annealing step of the CsBrxl(i -X):Eu particles (see below) can be
performed before the preparation of the coating dispersion or after the
coating and drying of the deposited layer. Preferably, the annealing step of
the CsBrxl(i -X):Eu particles is done before the preparation of the coating
dispersion because a higher temperature of annealing is possible due to
the absence of binder in the material to be annealed.
3. Annealing
[0058] The CsBrxl(i -X):Eu material as obtained by any of the above described
methods is annealed to obtain a scintillator according to the present
invention. It has been found that by annealing the CsBrxl( -X):Eu, the
conversion from X-rays into light by the scintillator is greatly enhanced. It
has been found that the CsBrxl( -X):Eu scintillator after annealing within a
specific temperature range shows a very characteristic electron
paramagnetic resonance (EPR) spectrum.
The annealing step can be performed on deposited CsBrxl( -X):Eu material,
e.g. vapour deposited needle like crystals or on CsBrxl(i -X):Eu material
which has been pulverized. The pulverized material can be obtained by
removing and pulverizing the deposited layer, or by pulverizing a brick of
fired material or by pulverizing crystals grown via crystal growth
techniques such as Czochralski, Bridgman-Stockbarger or Kyropoulos
methods. Alternatives methods for pulverizing are milling or grinding. All of
these methods can be performed by apparatus known in the art. By
pulverizing, the specific surface of the CsBrxl(i -X):Eu material is enhanced.
Also in case of layers comprising needle like crystals the specific surface
is comparable with pulverized material. The specific surface of the
CsBrxl( -X):Eu material to be annealed is preferably more than 1 cm2/g,
more preferably between 2 and 600 cm2/g, most preferably between 20
and 200 cm2/g. The enhancement of the annealing effect with CsBrxl( -
X):Eu material having an increased specific surface is thought to be that
upon annealing Eu2+ monomer centres must be stabilized by water
molecules and which can be incorporated into the material. The higher the
specific surface of the CsBrxl(i -X):Eu material is, the easier water can
diffuse throughout the volume of the material. The presence of stabilized
Eu2+ monomers seems to be essential to enhance the conversion
efficiency from X-rays into light. The specific surface of the needle like
crystals is calculated based on following equation: 1 / r.d (r being the
radius of the needle which is considered to be a cylinder; d being the
density of the CsBrxl( -X):Eu material and is equal to 4.5 g/cm3) . For the
calculation of the specific surface of the non-needle like crystals, these
crystals are considered to be spheres and the specific surface is according
to the following equation: 3 / r.d (r being the radius of the sphere; d being
the density of the CsBrxl(i -X):Eu material and is equal to 4.5 g/cm3) .
[0060] In case of preparation of a CsBrxl( -X):Eu scintillator screen from a
dispersion, the annealing step can be performed before the preparation of
the dispersion or after the drying step which followed the coating or
deposition step.
[0061] By annealing the CsBrxl(i -X):Eu material, activator centres are probably
produced which are responsible for the luminescence as can be shown via
electron paramagnetic resonance (EPR). The EPR spectra can be
measured by any suitable Q band EPR spectrometer operating at
frequency of ~ 34GHz. EPR detects stable Eu-ligand-complexes. The
measured EPR spectra are usually presented as the slope of the
measured EPR signal in function of the magnetic field. If in the text,
reference is made to an EPR spectrum, spectra are meant wherein the
slope of the measured EPR signal is plotted in function of the magnetic
field. The EPR spectra of annealed CsBrxl( -X):Eu material measured at a
frequency of 34 GHz at room temperature (25°C) shows at least a
maximum in signal height at a magnetic field of 1200 mT (see Figure 1,
Figure 5 and Figure 6). The maximum in signal height at 1200 mT showing
the largest peak height from all maxima. The peak height is defined as the
difference between the absolute value of the signal at its maximum and
the value of the signal at the minimum situated at the lower side in
magnetic field of the peak. The maxima signal height at 1090 mT and
1140 mT does not exceed 40%, wherein the normalised signal height
percentage at 1200 mT is being calculated as 100%. The above
mentioned values in magnetic field, at which the maxima and minima in
signal height occur, are not to be seen as absolute values but as values
having a range of ± 15 mT due to the experimental error of the measuring
method or the variation in composition of the CsBrxl( -X).
[0062] Annealing of the CsBrxl(i -X) material can be performed by exposing the
material to heat so as to obtain a temperature of the material in a range
from 50 to 280°C, preferably from 100 to 230°C, more preferably from 130
°C to 200°C and for a time period of at least 5 min, preferably at least 30
min. The temperature and time period of annealing are very closely related
to each other. This is the case for the conversion efficiency for high energy
electromagnetic irradiation of the scintillator but also for the stability of this
conversion efficiency. Exposing the CsBrxl( -X):Eu material to heat so as to
obtain a temperature which is higher than 280°C, e.g. 300°C, is called
over-annealing. Over-annealing the CsBrxl( -X):Eu material does not lead to
a scintillator showing an increase in conversion from X-rays into light, or at
least an increase in conversion from X-rays into light which is significantly
lower than by annealing at a temperature between 50 to 280°C. Moreover,
after over-annealing, the obtained scintillator does not show an EPR
spectrum having a maximum in signal height at a magnetic field of 1200
mT showing the largest peak of all maxima.
[0063] In a preferred embodiment the annealing step is performed in an oven with
air circulation. This oven can operate in ambient conditions. The CsBrxl(i -X)
material can be put in the oven before the oven is starting up heating. The
material is then heated in the oven while the temperature in the oven is
increased to the target temperature at which the annealing has to be
performed. Another way of annealing is to put the CsBrxl(i -X) material in the
oven when the oven is already at the target temperature. After the required
time period of exposure to heat has lapsed, the CsBr l(i-X) material is
cooled down to room temperature. This cooling down can be done by
removing the material from the oven or by turning off the heating of the
oven and allow the material to cool in the oven. Optionally, annealing can
be performed in oven under vacuum, oxygen, nitrogen, argon or dry air
flow.
[0064] Most preferably, the annealing step can be performed at a temperature in
the range from 50 to 200°C during 15 to 120 min, in the oven with air
circulation without ventilation.
[0065] In case of evaporated or coated scintillator layers, if after preparation of
said scintillator the layer is yellow, for example due to an excess of l2 ions,
annealing can strongly reduce staining by which the light emission by the
scintillator is also further increased.
4. Irradiation
[0066] In a preferred embodiment of the invention, the annealed CsBrxl( -X):Eu
scintillator can be exposed to electromagnetic radiation having a
wavelength between 1pm and 800 nm, including X-rays, high energy
electromagnetic radiation, UV light and visible light to further increase the
conversion efficiency. X-rays is an electromagnetic radiation having a
wavelength in the range of 0.01 to 10 nanometres, corresponding to
frequencies in the range of 30 petahertz to 30 exahertz (3 <1016 Hz to 3
1019 Hz) and energies in the range 100 eV to 100 keV.
[0067] In case of X-ray exposure the total irradiation dose is between 0.1 and
1200 Gy, preferable between 1.5 and 150 Gy, and most preferably
between 3 and 60 Gy. The dose can be delivered in one step or in multiple
steps. The total dose delivered to the scintillator is in the range of 0.01 to
100 Gy/min, preferably the total dose delivered is in the range of 0.1 to 10
Gy/min, and most preferably in delivered doses of 0.2 to 2 Gy/min. The
total time of exposure is 0.01 min to 2800 min, preferably between 60 min
and 360 min.
[0068] In case of exposure to UV light, having a wavelength between 100 and
400 nm, the total electromagnetic radiation dose is between 0 and
300000 J/m2. Preferably the dose is between 200 and 35000 J/m2, and
most preferably between 900 and 10000 J/m2.
[0069] In case of exposure with visible light having a wavelength of 300 to 800
nm, the total light dose is between 10 and 400000 J/m2. Preferably the
dose is between 2000 and 200000 J/m2, and most preferably between
8000 and 40000 J/m2. Within the visible light irradiation, a range of 300 to
600nm, more preferably between 300 nm and 500 nm is preferred.
[0070] By using any of the above described irradiation methods, it is possible to
obtain a CsBrxl(i -X):Eu scintillator having increased conversion efficiency
with respect to the non irradiated but annealed CsBrxl( -X):Eu scintillator.
The X-ray and UV radiation lead to an increase at least of 2 relative to non
irradiated CsBrxl(i -X):Eu scintillators, while for the irradiation with visible
light, the increase is smaller and on average of a factor 2 , meaning that
scintillators are obtained with a very high conversion efficiency for Xconversion.
[0071] The CsBrxl(i -X):Eu scintillator after annealing and after the exposure to
electromagnetic radiation shows the same characteristic EPR spectra as
before the exposure to the electromagnetic radiation.
[0072] The exposure to the electromagnetic radiation can be done on the
CsBrxl(i -X):Eu scintillator, but preferably on the CsBr l( -X):Eu scintillator
screen.
5. Radiological image and detection apparatus
[0073] The scintillators of the present invention are suitable as scintillators in
radiological image detection apparatus such as FPDs and in high energy
radiation detection apparatus.
[0074] When producing FPDs with scintillators according to the present invention,
the scintillator can be applied on a substrate as described above and
coupled to a detector comprising a plurality of photosensitive elements
which can convert light into an electric signal (for example: amorphous Si,
CMOS or CCD) with an intermediate adhesive layer. Suitable ways to
couple the scintillator screen of the invention to the detector are described
in US20140014843A, US20130313438A, US201 30 343 2A, and
US701 9304. In another embodiment, the CsBrxl( - ):Eu scintillator can be
directly deposited onto the detector comprising a plurality of photosensitive
elements which can convert light into an electric signal .
[0075] The FPDs based on CsBrxl( -X):Eu scintillators according to the present
invention can be obtained after or before annealing the CsBrxl( -X):Eu
material. The annealing is preferably performed after the scintillator was
applied onto a substrate (scintillator screen) and before the coupling is
done with the detector of the FPD.
[0076] In a preferred embodiment, the CsBrxl( - ):Eu scintillator is first applied on a
substrate followed by an annealing step. After the annealing step, the
exposure of the scintillator screen to electromagnetic radiation can be
performed as described in §4, followed by the step of coupling the
scintillator to the detector of the FPD.
[0077] In another preferred embodiment, the FPD comprising an annealed
CsBrxl( -X):Eu scintillator is exposed to electromagnetic radiation as
described above in §4.
[0078] In another preferred embodiment, a FPD comprising a CsBrxl(i -X):Eu
scintillator is first annealed and then exposed to electromagnetic radiation
as described above in § 4.
[0079] In another preferred embodiment, the CsBrx !(i -X):Eu material can be
annealed before it is coated or deposited on a substrate. After the
annealing step, the scintillator screen is coupled to the detector.
[0080] The scintillator of the invention is also suitable to be combined with a
photomultiplier tube for high energy radiation detection such as gamma
ray detection or image detection in for example gamma scanners.
Because the light is emitted in all directions, all photons which do not
directly reach the photo cathode of the PMT have to be directed to reach
the photo cathode by internal reflection. For many applications, truncatedcone
and parabolic shapes of scintillators give the most satisfactory
results, but simple shapes like rods and blocks, which are less expensive
to fabricate, are often good enough. To promote internal reflection the
parallel surfaces of the scintillator should be in some cases, reflectively
coated. The most suitable coatings consist of MgO, T1O2 or Al.
[0081] The coupling between the scintillator and the photo cathode of the PMT
can be done via direct coupling or light guided coupling. In the case of
direct coupling, the scintillator surface is butted together with the cathode
window. Use of a mating compound such as silicone grease, with a
refractive index close to the refractive indices of the scintillator and the
glass, is necessary to minimize interface losses. In the case of light-guide
coupling a light guide is used between the scintillator surface and the
window of the cathode. The usual materials are glass, fused silica,
polystyrene, polyvinyltoluene, and especially polymethyl methacrylate.
[0082] In a FPD or radiation detection apparatus, a wavelength shifting material
can be included to better match the wavelength of the emitted light by the
scintillator with the sensitivity spectrum of the detector. The wavelength
shifting material is optically coupled to the scintillator and optically coupled
to the detector. Examples of wavelength shifting material which are
suitable for the invention are disclosed in Table I of WO201 2 04798.
[0083] While the present invention will hereinafter in the examples be described
in connection with preferred embodiments thereof, it will be understood
that it is not intended to limit the invention to those embodiments.
EXAMPLES
. Materials
[0084] Most materials used in the following examples are readily available from
standard sources such as ALDRICH CHEMICAL Co. (Belgium), ACROS
(Belgium), VWR (Belgium) and BASF (Belgium) unless otherwise
specified. All materials are used without further purification unless
otherwise specified.
• Caesium Iodide (Csl): (CAS 7789-1 7-5) 99.999%; from Rockwood
Lithium;
• Ceasium Bromide (CsBr): (CAS 7787-69-1 ) 99.999%; from Rockwood
Lithium;
CEBLA: Caesium Europium Bromide (CsEuBr3) from Agfa -
Healthcare;
• Thallium Iodide (Til): 99.999%; from Rockwood Lithium;
T1O2TR-52 from Huntsman;
• CAB 381 -2: 20(wt.)% of Cellulose Acetate Butyrate (CAB-381 -2) from
Eastman in MEK;
Baysilon: Baysilon Paint additive MA from Bayer;
Ebecryl: 20(wt.)% of Ebecryl 1290 in MEK, a hexafunctional aliphatic
urethane acrylate oligomer from Allnex in MEK;
• Filter AU09E1 1NG with pore size of 20 from 3M
A-G CP-Bu: Radiological film from Agfa Healthcare;
SE4 CAWO: Powder scintillator screen based on calcium tungstate
(CaW0 4) from CAWO;
CAWO Superfine 115 SW: Powder scintillator based on gadolinium
oxysulphide (GOS:Tb) from CAWO;
Black polyethylene bag: PE, Type B, 260x369 mm, 0,19mm thickness,
from Cornells Plastic;
Stann JF-95B: dispersant from Sakyo;
Disperse Ayd™ 9100: anionic surfactant/Fatty Ester dispersant from
Daniel Produkts Company;
• Kraton™ FG1910X: (new name = Kraton™ FG1901 GT), a clear,
linear triblock copolymer based on styrene and ethylene/butylene with
a polystyrene content of 30%, from Shell Chemicals;
Black PET substrate: polyethylene terephthalate (PET) film with a
thickness of 0.188 mm, obtained from Toray, trade name Lumirror
X30;
• Aluminium 3 18G: plate from Alanod having a thickness of 0.3 mm.
• Eul2 : from SAFC Hitech 99.9% CAS 22015-35-6.
2. Measuring methods
2.1 Conversion efficiency with scintillator screen / radiological film set-up.
The conversion efficiency of the scintillator screens was determined in a
scintillator screen / radiological film (S/F) set-up. To guarantee optimal
contact between scintillator screen and radiological film, a vacuum
packaging was used. The scintillator screen with its scintillators side was
brought in contact with an A-G CP-Bu radiological film and both
components were placed in a black polyethylene bag. The whole package
was light tight and vacuum packaged. Exposure was through the
scintillator, in front of the radiological film. The exposure for measuring the
conversion efficiency was performed by a Philips Super 80 CP X-ray
source with following conditions: 72 kVp, E-filter, distance range over 6 15
cm, 2 1 steps with dlogH of 0.10, 10 mAs, small focus. The substrate side
of the scintillator screen was directed towards the X-ray source. After
exposure, the radiological film was developed in G138i (Agfa Healthcare)
at 33°C for 90s and placed in a MacBeth densitometer, type TR-924 to
measure the optical density of the developed film. The conversion
efficiency was determined by means of the density graph in the region
wherein the density is linear with the irradiation dose and was calculated
relative to a reference scintillator screen. Unless otherwise specified, the
reference scintillator screen is SE4 CAWO, and has been chosen due to
its stable performance under irradiation and blue colour emission under Xray
irradiation.
2.2 Afterglow
[0086] The afterglow was measured with an X-ray source Pantak-Seifert Isovolt
16-M2/0.4-1 .5 working in continuous radiation mode, together with a
shutter FPS900M of the company Cedrat, to generate X-ray pulses with
very high radiation dose of 0.56 Gy/s. The shutter was mounted at a
distance of 25 cm from the X-ray source and had an open diaphragm of 2
mm diameter. The rise time of the shutter was 30 ms. To avoid scattering
of X-rays two diaphragms of lead were installed, on both sides of the
shutter. The position of the shutter (open-closed) was changed by applying
a voltage to the digital input of the electronics of the shutter. To create a
pulse series with a chosen frequency and pulse length, a pulse generator
HP 8 1 6A was used. The distance between the scintillator screen and the
shutter was fixed at 8 cm. The scintillator was exposed to the X-ray source
having following conditions: 70 kVp, 20 mAs, large focus, no external
filters. The generator parameters were fixed: HIL = 2.5 Volt, LOL = 0 Volt,
Frequency: 30 mHz.
[0087] The light generated by the scintillator was captured with optical fibres and
was further guided into a spectrofluorometer (Fluorlog from Jobin Yvon)
and measured. The measured wavelength was fixed at the maximum of
the light emission of the scintillator and it was continuously measured
within the time between 2 cycles (on/off) in a range from 0.1 to 1 s. After
all data was collected, each cycle was separated and the curves
measured between cycles were analysed. To reduce the noise level all
cycles were averaged and a spectrum was provided.
2.3 Conversion efficiency of the scintillator screen by measurement of the
light emission.
2.3.a. Dynamic mode
[0088] Measurement of the conversion efficiency in dynamic mode was done by
recording the light emission spectra of the scintillator screens with a
spectrofluorometer (Fluorlog from the company Jobin Yvon) in a
wavelength range between 370 and 700 nm. Exposure was done by
placing the scintillator under the X-ray source Pantak-Seifert. The
exposure conditions of the X-ray source were: 70 kVp, 10 mAs, a distance
from X-ray to scintillator of 90 cm and a 2 1 mm Al filter. The X-ray beamquality
corresponds with the norm RQA5 (RQA X-ray beam qualities as
defined in IEC standard 61267, 1s Ed. (1994)). The emitted light was
captured with an optical fibre and transferred to the spectrofluorometer
where it was stored on its computer and the light emission spectrum was
generated. The conversion efficiency was calculated as an integral of the
spectrum between 370 and 700 nm.
2.3.b. Kinetic mode
[0089] Measurement of the conversion efficiency in kinetic mode was done by
recording the light emission from scintillator screens in a continuous mode,
i.e. in function of the X-ray radiation dose, with the same
spectrofluorometer used for the dynamic mode. The spectrofluorometer
was set to the detection at a fixed wavelength of 450 nm corresponding to
the maximum of the emission of the CsBrxl( -X):Eu scintillator, unless
specified otherwise. The conversion efficiency was taken at the maximum
value of detected light emission in function of X-ray dose. The X-ray
source conditions were set to 70 kVp, 20 mAs, the distance from the X-ray
source to the scintillator was 80 cm, and no external filters were used. The
light emission was integrated with 0.1 7 Gy radiation intervals.
3. Preparation of scintillator screens
3.1 . Preparation of the substrate of the scintillator screens
[0090] For the preparation of the scintillator screens, 2 types of substrates were
used: Al plates without a pre-coat and Al plates with a pre-coat.
[0091] To prepare a pre-coat, a coating dispersion was prepared by mixing 0.2 g
of CAB 381-2 with 1 g of Ti0 2 TR-52, 0.001 g of Baysilon and 2.6 g of MEK
in a horizontal agitator bead mill. Ebecryl was added to achieve a weight
ratio CAB 381-2 : Ebecryl of 1 : 1. The solution was filtered with Filter
AU09E1 NG. The solid content of T1O2TR-52 in the coating dispersion is
35(wt.)%. The coating dispersion was coated with a doctor blade at a
coating speed of 2 m/min on the aluminium 3 8G plate with a size of
18x24 cm. The wet layer thickness was 150 miti as to obtain a dry layer
thickness of 17 m. The drying of the pre-coat was done at room
temperature for at least 15 min, followed by drying in an oven for 30 min at
60°C and 20 min at 90°C.
3.2. Preparation of needle based CsBrxl( -X):Eu scintillator screens.
[0092] CsBrxl( -X):Eu needle scintillator screens according to the present invention
(Inv. Scr. 1 - 17) were obtained via physical vapour deposition (PVD) of
Csl and CsEuBr3 on Al plates with or without a pre-coat (see § 3.1 ) . A
mixture containing 210 g of Csl and 2.1 g of CsEuBr3 was placed in a
crucible in a vacuum deposition chamber. As the content of bromide in the
obtained scintillator is only originating from the CsEuBr3, the CsBrxl(i -X):Eu
scintillators (Inv. Scr. 1-1 7) consist essentially of Csl:Eu. The crucible was
subsequently heated to a temperature of 680-690°C and the vaporized
compounds were deposited onto the substrate.
[0093] CsBro.o8lo.92:Eu needle scintillator screens (Inv. Scr. 18) were obtained via
physical vapour deposition (PVD) of Csl, CsBr and CsEuBr3 on Al plates
without a pre-coat. A mixture containing 189 g of Csl, 2 1 g of CsBr and 2.1
g of CsEuBr3 was placed in a crucible in a vacuum deposition chamber.
The crucible was subsequently heated to a temperature of 680-690°C and
the vaporized compounds were deposited onto the substrate. The bromide
to iodide ratio in the deposited scintillator was determined by means of
potentiometric titration.
[0094] The distance between the crucible and the substrate was fixed at 20 cm.
During evaporation, the substrate was rotated at 2 r.p.m. and kept at a
temperature of 140°C. Before the start of the evaporation, the chamber
was evacuated to a pressure of 5x1 0 5 mbar and during the evaporation
process argon gas was introduced into the chamber. The process took
from 5 till 160 min, depending on the coating weight.
[0095] After the evaporation process the scintillator screen and the chamber were
cooled down to room temperature and the scintillator was removed from
the vacuum chamber. Each scintillator screen was weighed and the
coating weight was obtained by applying formula 1. The coating weights of
the thus obtained scintillator screens Inv. Scr. 1 to Inv. Scr. 18 are
reported in Table 1.
(WF-WS) / As
Formula 1
Where:
F = weight of the scintillator screen,
Ws = weight of the substrate,
As = surface area of the substrate.
3.3. Preparation of needle based Csl:Eu scintillator screen.
This scintillator was prepared in the same way as described in § 3.2.
(Inv.Scr. 1-17) but CsEuBrswas replaced by E . The coating weight was
determined according to formula 1 and is reported in Table 1.
3.4. Preparation of an undoped Csl scintillator screen (Comp. Scr. 1).
[0096] This scintillator was prepared in the same way as described in § 3.2.
(Inv.Scr. 1-17) but no CsEuBr3 was used. The coating weight was
determined according to formula 1 and is reported in Table 1.
3.5. Preparation of needle CsBrxl( -X):Eu scintillator screens which will not
be annealed (Comp. Scr. 2 / 3 / 4).
[0097] Needle CsBrxl(i -X):Eu scintillator screens which will not be annealed (see
below) were prepared in the same way as described in § 3.2 (Inv.Scr. 1-
17) and the coating weight is reported in Table 1.
3.6. Preparation of a Csl:TI scintillator screen (Comp. Scr. 5).
[0098] A CshTI scintillator was prepared in the same way as described in § 3.2
(Inv.Scr. 1-17) but the CsEuBr3 was replaced by 2.6 g of thallium iodide.
The coating weight of the obtained screen Comp. Scr. 5 as determined by
formula 1 is reported in Table 1.
Table 1
Scintillator screen Substrate Composition Coating weight
(mg/cm 2)
Inv. Scr. 1 Al plate + pre-coat CsBrxl( - ):Eu 50.2
Inv. Scr. 2 Al plate + pre-coat CsBrxl( - ):Eu 49.2
Inv. Scr. 3 Al plate + pre-coat CsBrxl( -X):Eu 116.6
Inv. Scr. 4 Al plate + pre-coat CsBrxl(i - ):Eu 114.8
Inv. Scr. 5 Al plate + pre-coat CsBrxl( -X):Eu 115.1
Inv. Scr. 6 Al plate + pre-coat CsBrxl( -X):Eu 52.8
Inv. Scr. 7 Al plate + pre-coat CsBr l( -X):Eu 52.7
Inv. Scr. 8 Al plate + pre-coat CsBrxl( -X):Eu 52.6
Inv. Scr. 9 Al plate + pre-coat CsBrxl( - ):Eu 45.9
Inv. Scr. 10 Al plate + pre-coat CsBrxl(i -X):Eu 88.1
Inv. Scr. 1 Al plate + pre-coat CsBrxl(i -X):Eu 116.6
Inv. Scr. 12 Al plate + pre-coat CsBrxl( -X):Eu 5 1.6
Inv. Scr. 13 Al-plate CsBrxl( -X):Eu 4 1.7
Inv. Scr. 14 Al-plate CsBrxl( -X):Eu 45.9
Inv. Scr. 15 Al-plate CsBrxl( -X):Eu 42.9
Inv. Scr. 16 Al-plate CsBrxl( -X):Eu 42.6
Inv. Scr. 17 Al-plate CsBrxl( -X):Eu 42.7
Inv. Scr. 18 Al-plate CsBro.o8lo.92 :E u 4 1.1
Inv. Scr. 19 Al plate + pre-coat Csl:Eu 42.1
Comp. Scr. 1 Al plate + pre-coat Csl 48.7
Comp. Scr. 2 Al plate + pre-coat CsBrxl( -X):Eu 49.3
Comp. Scr. 3 Al plate + pre-coat CsBrxl( -X):Eu 118.5
Comp. Scr. 4 Al plate + pre-coat CsBrxl( -X):Eu 5 1.6
Comp. Scr. 5 Al plate + pre-coat Csl:TI 130.0
Ref. Scr. 1 - SE4 CAWO 46.9
3.7. Preparation of powder based scintillator screens
[0099] 0.013 g of Stann JF-95B and 0.009 g of Disperse AYD™ 9 00 were mixed
in 0.153 g of toluene and 0. 03 g of butyl acetate. The mixture was mixed
in a Turbula ® shaker-mixer T2F for 30 min. To that mixture, 0.256 g of
methylcyclohexane was added and again mixed in the Turbula ® shakermixer
T2F for 30 min. Finally, a mixture containing 0.087 g of Kraton™
FG1901 GT, 0.145 g of methylcyclohexane, 0.087 g of toluene and 0.058
g of butyl acetate was added and mixed for another 30 min. To 0.91 g of
the so obtained binder solution, 0.5 g of Csl material or mixtures thereof
(see below) was added and mixed for another 30 min.
[00100] Inv. Scr. 10 was prepared with CsBrxl( -X):Eu material which was obtained
by removing the annealed Csl:Eu needle scintillator layer from a scintillator
layer obtained as described in §3.2 and §4, and having a coating weight of
12 mg/cm2. The annealing of the CsBrxl( -X):Eu needle scintillator was
done as described in § 4 for the Inv. Scr. 1 - 12. After removal from the
substrate, the material was pulverized in a mortar before adding it to the
binder solution.
[01 01] Inv. Scr. 2 1 was prepared with CsBrxl( -X):Eu material obtained by mixing
0.49 g of Csl and 0.1 g of CEBLA as powders. The mixture was placed in
the oven and heated at 1°C/min until a temperature of 660°C, where it was
kept for 1 h. Hereafter the oven was switched off and the material was let
to cool down. The obtained material, also called the fired material was
subsequently pulverized and annealed for 1h at 160°C in an oven. After
cooling down it was added to the binder solution.
[0102] Inv. Scr. 22 was prepared with CsBrxl( -X):Eu obtained by mixing 0.49 g of
Csl and 0.1 g of CEBLA. The material was pulverized in a mortar and
annealed for 1h at 160°C in an oven and after cooling down, added to the
binder solution.
[0103] Comp. Scr. 6 is prepared by mixing 0.49 g Csl and 0.1 g of CEBLA and
added to the binder solution.
[0104] Comp. Scr. 7 is prepared by mixing 0.49 g Csl and 0.1 g of CEBLA,
pulverized and added to the binder solution.
[0105] The dispersions obtained by adding Csl:Eu or a mixture of Csl + CEBLA,
as obtained above, to the binder solution as described above, were poured
into a stainless steal mould having inside dimensions of 3.0 x 2.5 cm and
1.0 cm height wherein the black PET substrate was placed. The
dispersions in the mould were dried in an oven at 50°C for 10 h. Hereafter,
the obtained powder based scintillator screen was removed from the
mould and was further dried in the air for 1 day.
4. Process of annealing the Csl:Eu and CsBrxl( -X):Eu scintillator screens
[0106] The needle based scintillator screens Inv. Scr. 1 to Inv. Scr. 12, Inv. Scr.
18, Inv. Scr. 19 and Comp. Scr. 1 were placed in an oven with air
circulation and the temperature was set to 170°C. The scintillator was kept
at that temperature for a time period of 1h. The CsBrxl( -X):Eu needle
based scintillator screens Inv. Scr. 13 to Inv. Scr. 17 were annealed in the
same oven at temperatures in a range between 100°C and 300°C and
kept at these temperatures for a time period between 0.5h and 24h. After
the time period has lapsed, the CsBrxl(i -X):Eu needle based scintillator
screens were allowed to cool down.
5. Exposure of an annealed CsBrxl( -X):Eu needle scintillator to
electromagnetic radiation .
5.1 X-rays
[0107] Inv. Scr. 1 to Inv. Scr.6 were exposed to X-ray radiation with an X-ray
source Pantak-Seifert Isovolt 16-M2/0.4-1 .5 in continuous radiation mode,
in such a way that half of each scintillator screen was exposed to an X-ray
dose of 17 Gy with a dose rate of 0.3 Gy/min. The X-ray source conditions
were: 70 kVp, 5 mAs, no external filter, distance to the X-ray source 80 cm
unless otherwise specified. The other half of each scintillator screen was
covered with a lead plate having a thickness of 1mm to avoid any
exposure.
5.2 Xenon Lamp irradiation
[0108] Inv. Scr. 8 and 9 were exposed with a Xenon lamp (Suntest XLS+, Atlas)
emitting light from 300 nm to 800 nm. The exposure was performed in 2
ways: a) a half of the scintillator screen was exposed with a total light
intensity of 765 W/m2 for 2 h with the second part covered; and b) the
scintillator screen was exposed with a circular beam of 11 mm diameter at
a fixed dose rate of 0.483 J/s.m2 for 0.75-24h.
5.3 UV light irradiation
[01 09] Inv. Scr. 10 was exposed with an OMT-lamp-unit build-in Xenon-lamp
(Hamamatsu) of 75 W with a narrow-band-transmission filter having a
central wavelength of 365 nm. The scintillator screens were exposed with
a circular beam of mm diameter and at a fixed dose rate of 0.1 76
J/s.m2 for 0.5 to 24h.
6 . EPR spectra
[01 0] The EPR spectra of the CsBrxl(i -X):Eu material were measured on a
Bruker® ELEXSYS E500, Q-band EPR/ENDOR spectrometer at Ghent
University, at room temperature (25°C), with the following settings:
microwave frequency of 34 GHz, microwave power of 20 mW, field
modulation frequency of 100 kHz, field modulation amplitude of 0.5 mT,
receiver gain of 60 dB, scan time of 20 min and time constant of 80 ms.
The CsBrxl( -X):Eu material was removed from the substrate with a cleaving
knife and pulverized. The pulverized material was introduced into Q-band
quartz tubes with outer diameter of 2 mm and inner diameter of 1.4mm, to
a height of 5 mm and the spectra were collected.
[01 11] The EPR spectrum of the annealed needle CsBrxl(i -X):Eu scintillator Inv.
Scr. 1 is shown in Figure 1 and of the non-annealed needle CsBrxl(i -X):Eu
scintillator Comp. Scr. 3 is shown in Figure 2.
[01 12] The EPR spectrum of the annealed needle CsBrxl(i -X):Eu scintillator Inv.
Scr. 1 showed maxima in signal height at magnetic fields of 1090 mT,
1140 mT and 1200 mT, and minima at 1250 mT and 1350 mT. The
maximum in signal height at 1200mT showing the largest peak height from
all maxima. The peak height is defined as the difference between the
absolute value of the signal at its maximum and the value of the signal at
the minimum situated at the lower side in magnetic field of the peak. The
signal height at 1090 mT and 1140 does not exceed normalised signal
height of 40% wherein the normalised signal height percentage at 1200
mT is being calculated as 100%. The absolute height of the signals can
differ depending on parameters such as coating weight and measuring
circumstances.
[01 13] The EPR spectrum of the non-annealed needle CsBrxl( -X):Eu scintillator
Comp. Scr. 3 is characterised by a broad spectrum, clearly different from
the spectrum of the annealed sample and is further characterised by a
peak in signal height at 1200mT being not the highest. Sometimes,
measured EPR spectra of non-annealed CsBr xl( i -X):Eu material showed
very small maxima at 1090 mT and 1140 m. However the normalised
signal height of at least one of these maxima exceeds 40%, whereby the
normalised signal height percentage at 1200 mT is being calculated as
100%.
[01 14] The EPR spectrum of the annealed needle Csl:Eu scintillator Inv. Scr. 19
is shown in Figure 5 and of the annealed needle CsBro.o8l (o.92):Eu
scintillator Inv. Scr. 18 is shown in Figure 6 .
[01 15] The EPR spectrum of the annealed needle Csl:Eu scintillator Inv. Scr. 19
showed a maximum in signal height at a magnetic field of 1200 mT. The
peak height is defined as the difference between the absolute value of the
signal at its maximum and the value of the signal at the minimum situated
at the lower side in magnetic field of the peak. The signal height at 1090
mT and 1140 mT do not exceed normalised signal height of 40% wherein
the normalised signal height percentage at 1200 mT is being calculated as
00%. This EPR spectrum is very characteristic for the Csl:Eu after
annealing and only the absolute height of the signals can differ depending
on parameters such as coating weight and measuring circumstances.
[01 16] The EPR spectrum of the annealed needle CsBro.oel(o.92):Eu scintillator
Inv. Scr. 18 showed a maxima in signal height at a magnetic field of 1200
mT. The peak height is defined as the difference between the absolute
value of the signal at its maximum and the value of the signal at the
minimum situated at the lower side in magnetic field of the peak. The
signal height at 1090 mT and 1140 mT do not exceed normalised signal
height of 40% wherein the normalised signal height percentage at 1200
mT is being calculated as 100%. This EPR spectrum is very characteristic
for the CsBro.oel(o.92):Eu after annealing and only the absolute height of the
signals can differ depending on parameters such as coating weight and
measuring circumstances.
7 . Results of conversion efficiency measurements
7.1 Conversion efficiency of an annealed and a non annealed scintillator
screen.
The conversion efficiency of the scintillator screens Inv. Scr. 12 and Comp.
Scr. 4 with Ref. Scr. 1 as a reference (conversion efficiency = 1), hereafter
denoted as relative conversion efficiency, were measured according to the
method described in §2.1 . The results are included in Table 2.
Table 2
[01 18] The annealing step clearly increased the conversion efficiency of the
scintillator screen based on CsBrxl( -X):Eu scintillators comprising of
crystals having a needle like structure.
7.2 Conversion efficiency measurement in dynamic mode after X-ray
irradiation.
[01 19] Light emission under X-ray excitation was measured of Inv. Scr. 1, Comp.
Scr. 1 and Comp. Scr.2 as described in § 2 .3.a. before and after X-ray
exposure. The X-ray exposure is performed as described in § 5.1 . The
emission spectra of parts of the scintillator screen which were not exposed
to electromagnetic radiation, have a maximum at 450 nm, except undoped
Csl which has a maximum at a much lower wavelength, and having a
rather low signal. The spectra measured after exposure to X-ray irradiation
showed a change in conversion efficiency signal. No shifts in wavelength
were observed for both Inv. Scr. 1 and Comp. Scr. 1-2, but in both cases
the intensity of the spectra changed. Due to the exposure to X-rays, the
conversion efficiency increased of the Eu-doped and annealed CsBrxl(i -X)
screen (Inv. Scr. 1), while the conversion efficiency decreased of the
undoped Csl scintillator screen (Comp. Scr. 1) and the Eu doped CsBr l( -
x) scintillator screen which had not been annealed (Comp. Scr. 2). The
conversion efficiency of the screens after X-ray exposure, relative to the
conversion efficiency before X-ray exposure is reported in Table 3. The
conversion efficiency before X-ray exposure is calculated as 1 for all the
screens.
Table 3
7.3 Conversion efficiency in kinetic mode at 450 nm after X-ray radiation
The conversion efficiency of the scintillator screen (Inv. Scr. 2) was
measured as described in §2.3.b. The increase of the conversion
efficiency started immediately after even a very low dose of X-ray
irradiation according to §5. , such as 0.1 Gy and increased to a maximum
until the total dose of around 12 Gy is achieved (see Figure 3) . At the total
dose of about 12 Gy the conversion efficiency saturates even when higher
dose of irradiation were provided. From an X-ray irradiation dose of about
12 Gy to a dose of about 28 Gy, the conversion efficiency levels off at
500% of the conversion efficiency when no irradiation was applied yet. The
results of the same measurement as described in Gektin et al. (Radiation
damage of Csl:Eu crystals. Functional Materials, 20; n.2 (2013) - STC "
Institute for Single Crystals" of National Academy of Sciences of Ukraine)
do show on the contrary that irradiation leads to emission suppression at
X-ray doses of less than 100 Gy.
7.4 Conversion efficiency of scintillator screens after exposure to X-rays
for different coating weights
The conversion efficiency of the scintillator screens, Inv. Scr. 3 to Inv. Scr.
6 was measured in S/F set-up as described in §2.1 before and after X-ray
exposure according to §5.1 . The conversion efficiency of the non-exposed
scintillator screens is taken as a reference and equal to . The conversion
efficiency increased by a factor of about 3 for high coating weights, and a
factor of about 2 for low coating weights with respect to the non-exposed
parts of the scintillator screens (see Table 4) .
Table 4
[0122] The conversion efficiency, measured in a S/F set-up as described in §2.1 .,
of Inv. Scr. 7 was compared with Ref. Scr. 1. Ref. Scr. 1 did not show an
increase in conversion efficiency after X-ray exposure. Half of the Inv. Scr.
7 was exposed to a total X-ray dose of 748 Gy (70 kVp, 5 i A , 80 cm
distance, no external filter), with a dose rate of 0.3 Gy/min and a total time
of irradiation of 4 h 48 min. The other half of the scintillator screen was
covered with a lead plate having a thickness of mm to avoid any
exposure of the screen. The conversion efficiencies of both halves of the
screen were measured and compared to Ref. Scr. 1 to obtain relative
conversion efficiencies. The Inv. Scr. 7 showed a relative conversion
efficiency of a factor of 3.5 higher than the relative conversion efficiency of
the non exposed part of the scintillator screen. The relative conversion
efficiencies of non-exposed and exposed parts of scintillator screen Inv.
Scr. 7 are shown in Table 5. The results show that although the
conversion efficiency of the Inv. Scr. 7, after annealing was lower than Ref.
Scr. 1, the irradiation with X-rays increased the conversion efficiency of the
screen to a higher value than the reference screen Ref. Scr. 1.
Table 5
7.5. Conversion efficiency measurements and EPR spectra of scintillator
screens annealed at different temperatures and in different time periods.
The conversion efficiency of Inv. Scr. 13 to 19, which were annealed at
different temperatures for different time periods, were measured in the
kinetic mode as described in § 2.3.b. The value of the relative conversion
efficiency at maximum value in function of X-ray dose with the efficiency of
the non-annealed screen as a reference (= 1) is reported in Table 6.
Table 6
Scintillator T(°C) of t (h) of Relative conversion
Screen annealing annealing efficiency
Inv. Scr. 13 00 0.5 1.3
Inv. Scr. 13 00 1 1.4
Inv. Scr. 13 100 3 1.7
Inv. Scr. 13 00 24 4.7
Inv. Scr. 14 135 0.5 2.4
Inv. Scr. 14 35 1 3.1
Inv. Scr. 14 35 3 6.1
Inv. Scr. 14 35 24 14,5
Inv. Scr. 15 200 0.5 10.5
Inv. Scr. 15 200 1 8.1
Inv. Scr. 15 200 3 6.7
Inv. Scr. 15 200 24 3.5
Inv. Scr. 16 250 0.5 3.5
Inv. Scr. 16 250 1 3.0
Inv. Scr. 16 250 3 2.1
Inv. Scr. 17 300 0.5 0.85
Inv. Scr. 17 300 1 0.28
Inv. Scr. 17 300 3 0.1 5
Inv. Scr. 17 300 24 0.12
Inv. Scr. 18 135 1 4.50
Inv. Scr. 18 170 1 8.50
Inv. Scr. 18 200 1 4.65
Inv. Scr. 19 170 1 10.10
[0124] From the results reported in Table 6 it can be seen that annealing of the
scintillator screens based on CsBrxl(i -X):Eu crystals having a needle like
structure at temperatures below 300°C show a higher conversion
efficiency with respect to the non-annealed scintillator screens.
[0125] The EPR spectra of the CsBrxl( - ):Eu crystals having a needle like
structure and which were over-annealed at 300°C are characterised by a
broad spectrum, clearly different from the spectra of the annealed sample
and are further characterised by the absence of maxima at 1090 mT and
40 mT and of a minimum at 1350 mT.
7.6 Conversion efficiency of scintillator screens after exposure to light
Half of the scintillator screens Inv. Scr. 8 and Ref. Scr. 1 were exposed to
the Xe lamp as described in §5.2.a. The relative conversion efficiency was
measured in dynamic mode according to §2.3 .a for both the exposed and
not exposed areas with respect to Ref. Scr. 1. The results are reported in
Table 7. The increase in relative conversion efficiency due to the exposure
to light is 2.7.
Table 7
[01 27] The scintillator screen Inv. Scr. 9 was exposed with circular beams of the
Xe lamp as in §5.2.b and a half of the scintillator screen Inv. Scr. 0 was
exposed with circular beams of UV light as in §5.3. The relative conversion
efficiency was measured in dynamic mode according to §2.3.a for all
exposed areas with reference to the non-exposed area and the results are
summarised in Table 8.
Table 8
[0128] An increase in conversion efficiency due to exposure to light was
observed. The increase in conversion efficiency due to UV irradiation was
higher than the increase due to visible light irradiation.
7.7. Conversion efficiency measurements of powder based scintillator
screens.
Each of the screens was measured in dynamic and kinetic mode. The
signal to noise ratio and the maximum emission was determined of the
light emission spectra while the conversion efficiency, relative to the
conversion efficiency before X-ray irradiation was determined in the kinetic
mode (see §2.3.b). The results are reported in Table 9.
Table 9
[0130] All the scintillator screens obtained by annealed CsBrxl( -X):Eu material
showed an increase in conversion efficiency due to X-ray exposure and a
higher conversion efficiency than scintillator screens obtained with nonannealed
Csl:Eu material. The best results were obtained for the
scintillator screen obtained by pulverizing annealed CsBrxl( -X):Eu needles.
The conversion efficiency of the non-annealed screens was within the
experimental error and it is not to be considered as an increase in
conversion efficiency.
8 . Afterglow measurements
[01 31] The afterglow of Inv. Scr. 1 after annealing as described in § 4, was
measured according to §2.2 in comparison with 2 scintillator screens:
Comp. Scr. 5 and CAWO Superfine 115 SW. The results are shown in
Figure 4. As can be seen, the afterglow of a scintillator based on CsBrxl(ix):
Eu after annealing, is much lower than the afterglow of a Csl:TI based
scintillator.
These results prove that annealing the scintillator based on CsBrxl( -X):Eu
increases the conversion efficiency for X-rays into light while maintaining
the afterglow to a very low level, making the invention very suitable for
high speed radiographic imaging and high energy radiation detection.
Moreover, the replacement of Tl by Eu represents less safety issues for
operators during the production of CsBrxl( -X) based scintillators.
Claims
Claim 1. A scintillator comprising CsBrxl( -X) doped with Europium (CsBrxl(-ix):
Eu ) wherein x< 0.5, obtainable by annealing CsBrxl( -X):Eu material at a
temperature from 50 °C to 280 °C characterised in that the EPR spectrum of
the so obtained scintillator measured at room temperature at a frequency of
34 GHz, shows a maximum in signal height at a magnetic field of 1200 mT the
signal height at 1090 mT and 1140 mT not exceeding 40%, wherein the
normalised signal height percentage at 1200 mT is calculated to be 100%.
Claim 2. The scintillator according to claim 1 wherein the EPR spectrum,
shows maxima in signal height at magnetic fields of 1090 mT, 40 mT and
1200 mT, and minima at 1250 mT and 1350 mT, the maxima at 1090 mT and
140 mT exceeding normalised signal height percentages of 10% but not
exceeding 40%, wherein the normalised signal height percentage at 1200 mT
is calculated to be 100%.
Claim 3. The scintillator according to claim 1 wherein the CsBrxl(i -X):Eu
material has a needle like crystal structure.
Claim 4 . The scintillator according to any of the preceding claims wherein the
scintillator consists essentially of CshEu.
Claim 5. The scintillator according to any of the preceding claims wherein the
scintillator after having been annealed is exposed to electromagnetic radiation
having a wavelength between 1pm and 800 nm.
Claim 6. The scintillator according to claim 5 wherein the electromagnetic
radiation consists of X-rays.
Claim 7. A scintillator screen comprising a scintillator and a substrate, wherein
the scintillator is defined as in any of the preceding claims.
Claim 8. A high energy radiation detection apparatus comprising a scintillator
and a photomultiplier tube, wherein the scintillator is as defined in any of the
preceding claims.
Claim 9 . A radiography flat panel detector comprising a scintillator and a
detector which comprises a plurality of photosensitive elements which can
convert light into an electric signal wherein the scintillator is as defined in
claims 1 to 5 .
Claim 10. A method of preparing a scintillator as defined in claim 1 comprising
the steps of:
a) providing CsBrxl( -X):Eu material by mixing Csl optionally with a Br containing
compound together with an Eu containing compound; and
b) annealing by exposing the CsBrxl( -X):Eu material to heat so as to obtain a
temperature from 50°C to 280 °C for at least 5 min.; and
c) optionally expose the annealed CsBrxl(i -X):Eu to electromagnetic radiation
having a wavelength between 1pm and 800 nm.