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Scintillation Crystals and its General Characteristics
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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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   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 80C. 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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