E. Ariesanti

A Comparison Between Spectroscopic Performance of HgI2 and CdZnTe Frisch Collar Detectors

Abstract In this study, the charge collection efficiencies (CCEs) of a 7.8- × 7.8- × 15.6-mm3 CdZnTe Frisch collar detector and a 2.1- × 2.1- × 4.1-mm3 HgI2 Frisch collar detector were measured and compared. Two Frisch collar devices were designed and fabricated to have identical aspect ratios of 2.0 to maintain similar weighting potential distributions. Pulse-height spectra were acquired from both Frisch collar devices with a standard calibration gamma-ray source of 137Cs, and the results are presented. As known, the Frisch collar alters the weighting potential within the planar device and enhances the CCE distributions. Thus, the parameters affecting these distributions have great impact on the pulse-height spectrum. The device length and mobility-lifetime product μτ have great impacts on CCE. Primarily, crystal (device) length L directly affects CCE because more charge carriers are trapped in longer devices with longer traveling distances. Alternatively, the better mobility-lifetime product of the charge carriers enhances CCE of the fabricated device. It is shown in this study that as a result of similarity in shape for both devices (equal aspect ratio), the weighting potential distributions resemble each other. However, as a result of the trapping effect (due to both length and μτ), the CCE profiles are not the same, and the CdZnTe detector shows more uniform response to gamma rays and, therefore, better spectroscopic performance (even with a longer device length), which is confirmed through CCE simulations. Finally, by applying the CCE model to the HgI2 Frisch collar device, the mobility-lifetime products μe, h τe, h of electrons and holes were estimated to be 0.0008 and 0.00003 cm2·V–1, respectively, for the HgI2 crystal.

Tl 2 LiYCl 6 :Ce: A New Elpasolite Scintillator

Tl2LiYCl6:Ce (TLYC), a new cerium doped-thallium based, dual mode gamma and neutron elpasolite scintillation crystal, has been grown and evaluated at RMD. Energy resolution of 4.2% at 662 keV (FWHM) is measured for samples of this material. From comparison with a 137Cs spectrum collected with NaI:Tl, a gamma-ray induced light yield of 26,000 ph/MeV is estimated for TLYC. The material also shows better proportionality of response than both LaBr3:Ce and NaI:Tl in the energy range between 32 keV to 1275 keV. Single thermal neutron interactions produce a peak measured at a gamma equivalent energy of 1.9 MeVee, corresponding to a (neutron induced) light yield of approximately 47,000 ph/n. Decay times obtained from gamma-ray interactions in TLYC are measured at about 57 ns, 431 ns, and 1055 ns, with slightly shorter values measured for neutron interactions. These differences allow for gamma-neutron pulse shape discrimination (PSD) and a PSD Figure-of-Merit (FOM) of 2 is measured with TLYC.

Mercuric Iodide Crystal Growth and Frisch Collar Detector Fabrication

Abstract Being a high-Z material, mercuric iodide (HgI2) has a relatively high gamma-ray absorption coefficient. Its low charge carrier mobilities, however, have somewhat hampered the interest in using this material as a room-temperature gamma-ray spectrometer. By using the Frisch collar technology, the influence of the low charge carrier can be significantly reduced. The growth of HgI2 by the Faile method in a horizontal furnace fortuitously produces tetragonal prismatic crystals. These crystals with appropriate dimensions can be fabricated into Frisch collar spectrometers. With the Frisch collar technology, 1.8% energy resolution for 662-keV gamma rays has been achieved.

Vapor growth of tetragonal prismatic mercuric iodide crystals

Tetragonal prismatic HgI2 crystals are obtained from growth with low Mw polyethylene. Thermal desorption studies coupled with gas chromatography/mass spectroscopy (TDGC/MS) reveal the existence of normal alkanes and ketones in the growth ampoules with low molecular weight polyethylene. Polar bonds, such as carbonyl bonds found in ketones, may form weak bonds with Hg ions in the {220} faces, thus inhibiting the growth rate of the {220} faces. A tetragonal prismatic HgI2 crystal grown with this method has been fabricated into a room-temperature Frisch collar γ-ray spectrometer with an energy resolution of 1.8% at 662 keV.

A systematic study of mercuric iodide platelet growth in horizontal furnaces

Growth of mercuric iodide platelets in horizontal furnaces with the addition of polyethylene powder has been analyzed with a 23 full factorial analysis. The factors investigated were the temperature gradient between the zones, the source/hot zone temperature, and the amount of polyethylene powder. The crystallization zone length was chosen as the observable. Normal probability and Lenths plots were used to analyze the effects. Both plots show that the temperature gradient is an active effect.

Tl 2 LiLaBr 6 :Ce and Tl 2 LiYCl 6 :Ce: New elpasolite scintillators

TlLiLaBr6:Ce (TLLB) and Tl2LiYCl6:Ce (TLYC), new cerium doped-thallium based, dual mode gamma and neutron elpasolite scintillation crystals, were discovered by and grown at RMD. Energy resolutions at 662 keV of 5.7% and 4.2% have been measured for TLLB:Ce and TLYC:Ce, respectively. Gamma-neutron pulse shape discrimination (PSD) with TLYC:Ce produces a result with a figure-of-merit (FOM) of 2.

High-performance large diameter SrI 2 :Eu 2+ crystals

– In this presentation, we will discuss crystal growth and scintillation properties of large diameter SrI<inf>2</inf>:Eu²⁺ single transparent crystals. The crystals were grown successfully using the vertical Bridgman technique. Crystals with different diameters from 1 inch to 2 inches are discussed. SrI<inf>2</inf>:Eu²⁺, a scintillator discovered a half century ago by Hofstadter, was recently re-discovered as an outstanding material for gamma ray-spectroscopy. Its key properties include good non-proportionality, high light yield (from 85,000 to 120,000 photons/MeV) and excellent energy resolution (2.8 % at 662 keV). Due to its good properties our immediate focus has been on scaling up and development of this material for commercial use. Other properties of SrI<inf>2</inf>:Eu include Z<inf>eff</inf> of 50, a mono-exponential decay with a time constant of 1μs to 5 μs (depending on crystal size and dopant concentration), an emission band at 410-450 nm (2.7-3.0 eV). SrI<inf>2</inf> also shows no intrinsic radioactivity unlike lanthanide compositions.

Effects of cerium concentration in Tl 2 LiYCl 6 scintillation detectors

Cerium doped Tl 2 LiYCl 6 (TLYC) is a new, Tl-based elpasolite scintillation detector. TLYC crystals with 0%, 0.5%, 3%, 5%, and 9% cerium concentrations have been grown. Emission spectra of TLYC with these different Ce concentrations peak between 430 and 445 nm. Energy resolution values of 6.8%, 5.2%, 4.2%, 3.9%, and 4.4% at 662 keV (FWHM) are measured for TLYC doped with 0%Ce, 0.5%Ce, 3%Ce, 5%Ce, and 9%Ce concentrations, respectively. Comparison with a 137Cs spectrum collected with a NaI:Tl results in gamma-ray induced light yields of 18,000 ph/MeV for 0% Ce, 27,000 ph/MeV for 0.5%Ce, 27,000 ph/MeV for 3%Ce, 30,000 ph/MeV for 5%Ce, and 28,000 ph/MeV for 9%Ce.

Prismatic platelet growth of HgI 2 with organic additives

The Faile growth method, when used with purified mercuric iodide (HgI2), along with low molecular weight polyethylene, has yielded tetragonal prismatic HgI2 crystals. Thermal desorption-gas chromatography/mass spectroscopy (TD-GC/MS) studies reveal the existence of alkanes, alkynes, and ketones in the growth ampoules along with low molecular weight polyethylene, presumably decomposition byproducts from the polyethylene. The influence of these desorbed byproducts on the HgI2 crystal morphology is compared to the basic Bravais-Friedel-Donnay-Harker (BFDH) crystal model. From these comparisons, it is apparent that ketones assert the most influence on the HgI2 crystal morphology by slowing the growth of the {110} faces, thereby, causing the prismatic crystal shapes to grow. The growth method has been used to produce HgI2 crystals with proper dimensions for room-temperature-operated Frisch collar gamma-ray spectrometers. Presently, these HgI2 devices have yielded a best energy resolution of 1.8% at 662 keV.

Horizontal Ampoule Growth and Characterization of Mercuric Iodide at Controlled Gas Pressures for X-Ray and Gamma Ray Spectrometers

The project developed a new method for producing high quality mercuric iodide crystals of x-ray and gamma spectrometers. Included are characterization of mercuric iodide crystal properties as a function of growth environment and fabrication and demonstration of room-temperature-operated high-resolution mercuric iodide spectrometers.