| Light
output (LO)
is the coefficient of conversion of ionizing radiation into light
energy. Having the highest LO, NaI(Tl) crystal is the most popular
scintillation material. Therefore, LO of NaI(Tl) is taken to be
100%. Light output of other scintillators is determined relative
to that of NaI(Tl) (%). LO (Photon/MeV) is the number of visible
photons produced in the bulk of scintillator under gamma radiation.
Scintillation Decay time is the time
required for scintillation emission to decrease to e-1
of its maximum.
Energy resolution is the full width
of distribution, measured at half of its maximum (FWHM), divided
by the number of peak channel, and multiplied by 100. Usually
Energy resolution is determined by using a 137Cs source.
The above description is illustrated in Fig. 1. Energy resolution
shows the ability of a detector to distinguish gamma-sources with
slightly different energies, which is of great importance for
gamma-spectroscopy.
Emission spectrum is the relative number
of photons emitted by scintillator as a function of wavelength.
The Emission spectrum is shown in Fig. 2. The intensity maximum
corresponds to the Imax wavelength shown in the table.
For coefficient detection of emitted photons, the maximum of PMT
quantum efficiency should coincide with Imax.
Background is a quantity determined as
a number of luminescent pulses emitted by radioactive substance
within 1 second in the bulk of the scintillator with the weight
of 1 kg.
Most scintillation crystals reveal a
number of luminescent components. The main component corresponds
to Decay time, however less intense and slower ones also exist.
Commonly, the strength of these components is estimated by using
the intensity of a scintillator's glow, measured at specified
time after the Decay time. Afterglow is the ratio of the intensity
measured at this specified time (usually, after 6 ms) to the intensity
of the main component measured at Decay time. |
Fig. 1
Fig. 2 |
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MolTech, Scintillation Single Crystals:
| Complex oxide crystals Gadolinium
Silicate doped with Cerium (Gd2SiO5(Ce)
or GSO), BGO, CWO and PWO have a number of advantages
over alkali halide crystals: high effective atomic number, high
density, good energy resolution in the energy region over 5 MeV,
low afterglow, and non-hygroscopicity. Due to these features,
detectors with oxide crystals are fail-safer, have no need of
hermetization, and have mass and volume several times less than
Alkali Halide analogues at the same detection efficiency. Yet
oxide scintillators are characterized by lower light output and
somewhat lower energy resolution at energies less than 5 MeV.
|
Bismuth Germanate (Bi4Ge3012
or BGO) is one of the most widely used scintillation materials
of the oxide type. It has high atomic number and density values.
Detectors based on BGO have volume 10 - 4 times and mass 5 - 7
times less than those with Alkali Halide scintillators. BGO is
mechanically strong enough, rugged, non-hygroscopic, and has no
cleavage .BGO has an extreme high density of 7.13 g /cm3 and has
a high Z value which makes these crystals very suitable for the
detection of natural radioactivity (U, Th, K), for high energy
physics applications (high photofraction) or in compact Compton
suppression spectrometers.
BGO detectors are characterized by high energy resolution in the
energy range 5 - 20 MeV, a relatively short decay time; its parameters
remain stable up to the doses of 5 x 104 Gy; large-size
single crystals are possible to obtain. Due to these features,
BGO crystals are used in high-energy physics (scintillators for
electromagnetic calorimeters and detecting assemblies of accelerators),
spectrometry and radiometry of gamma-radiation, positron tomography |
| Cadmium Tungstate (CdWO4
or CWO) has high density and atomic number values. Therefore,
for CWO, the light output is 2.5 - 3 times higher than that of
Bismuth Germanate. Due to low intrinsic background and afterglow
and to rather high light output of CWO, the most suitable areas
of its application are spectrometry and radiometry of radionuclides
in extra-low activities. CWO is the most widely used scintillator
for computer tomography. A rather great decay time value (3 -
5 Cls) is a significant feature of CWO which restricts the possibilities
of its application in many cases. |
| Lead Tungstate (PbWO4
or PWO) is a heavy (density = 8.28 g/cm3, Z = 73)
and fast (decay time = 3 - 5 ns) scintillation material. It has
the least radiation length and Moliere radius values (0.9 and
2.19, respectively) among all known scintillators. Radiation damage
appears at doses exceeding 105 Gy. Yet the light output of PWO
is as low as about 1% of Csl(TI), so that the material can be
used in high-energy physics only. |
| Double Natrium-Bismuth Tungstate
(NaBi(WO4)2 or NBWO) is a new material
that can be used as a Cherenkov radiator for particle detection.
This crystal has scheelite structure of the space group C64h.
Na+ and Bi3+ cations are statistically distributed
among structural 4a positions (structural-statistic disorder).
The unit cell contains two formula units. The unit cell parameters
according to x-ray data are: a=5,281±0,001 Å; c=11,510±0,002
Å; the density r=7,588±0,004 g/cm3. Optical
and luminescent properties of NBW are scarcely studied because
this crystal can hardly be used as a scintillator due to low quantum
yield of its luminescence. It was reported that X-ray luminescence
spectrum has maximum ~520 nm and the luminescence intensity is
about 5% of BGO intensity. |
| Thallium doped Sodium Iodide
NaI(Tl) is the most widely used scintillation material. NaI(TI)
is used traditionally in nuclear medicine, environmental measurements,
geophysics, medium-energy physics, etc. The fact of its great
light output among scintillators, convenient emission range (in
coincidence with maximum efficiency region of photomultiplier
(PMT) with bialkali photocatodes), the possibility of large-size
crystals production, and their low prices compared to other scintillation
materials compensate to a great extent for the main Nal(TI) disadvantage.
Which is namely the hygroscopicity, on account of which NaI(TI)
can be used only in hermetically sealed assemblies. Varying of
crystal growth conditions, dopant concentration, raw material
quality, etc. makes it possible to improve specific parameters,
e.g., to enhance the radiation resistance, to increase the transparency,
and to reduce the afterglow. For specific applications, low-background
crystals can be grown. NaI(TI) crystals with increased dopant
concentration are used to manufacture X-ray detectors of high
spectrometric quality. NaI(TI) is produced in two forms: single
crystals and polycrystals. The optical and scintillating characteristics
of the material are the same in both states. In some cases of
application, however, the use of the polycrystalline material
allows coping with a number of additional problems. First, a press
forging makes it possible to obtain crystals with linear dimensions
exceeding significantly than those of grown single crystals. Second,
the polycrystals are ruggedized, which is important in some cases.
Moreover, NaI(TI) polycrystals do not possess the perfect cleavage,
so the probability of their destruction in the course of the use
is reduced. The use of extrusion in converting NaI(TI) into the
polycrystalline state makes it also possible to obtain complex-shaped
parts without additional expensive machining. |
| The most important feature of
Cesium Iodide crystals doped with Thallium CsI(Tl) is their
emission spectrum having the maximum at 550 nm, which allows photodiodes
to be used to detect the emission. The use of a scintillator-photodiode
pair makes it possible to diminish significantly the size of the
detecting system (due to the use of photodiode instead of PMT),
to do without high-voltage supply source, and to use detecting
systems in magnetic fields. The high radiation resistance (up
to 102 Gy) allows CsI(TI) to be used in nuclear, medium
and high-energy physics. Special treatment ensures obtaining of
CsI(TI) scintillators with a low afterglow (less than 0.1% after
5 ms) for the use in tomographic systems. |
| Cesium Iodide doped with Sodium
CsI(Na) is a widely used material nowadays. High light output
(85% of that of NaI(TI)), emission in the blue spectral region
(in coincidence with the maximum sensitivity range of the most
popular PMT with bialkali photocatodes), and substantially lower
hygroscopicity in comparison with that of NaI(TI) makes this material
a good alternative for NaI(TI) in many standard applications.
The temperature dependence of light output has its maximum at
80°C. This makes it possible to use CsI(Na) the scintillation
material at elevated temperatures. The decay time of CsI(Na) depends
on the dopant concentration and varies in the range of 500 - 700
ns. |
| Zinc Selenide ZnSe(Te)
scintillation material was created especially for matching with
photodiode, which its emission maximum is at 640 nm. Matching
coefficient between scintillator and photodiode is up to 0.9.
ZnSe scintillators are sharply different from ZnS. "Fast" ZnSe
has the time decay of 3 - 5 µs, "slow" - 30 - 50 µs. These
are used preferably for X-rays and gamma-particle registration.
Crystals ZnSe(Te) do not have very good transparency, therefore
we don't recommend it with the use of more than 3 - 4 mm thickness.
Relative to CsI(Tl), light output for X-rays with E<100 keV (CsI(Tl)=100%)
is up to 170% at 2 mm thickness. Non-uniformity is usually less
than 1%. Crystals ZnSe(Te) are non-higroscopic and good enough
for mechanical treatment without any cleavage. Standard ZnSe(Te)
boules have a diameter of 24 mm. Diameters up to 40 mm are available
on request. |
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