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. 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 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.
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.