R. Hawrami

Selected Properties of Cs

Homeland security applications often require detection of both neutron and gamma radiation. A combination of two detectors registering neutrons and gammas separately is typically used. Recently, a number of scintillators from the elpasolite crystal family were proposed, that provide detection of both types of radiation. The most promising are Cs2LiYCl6, Cs2LiLaCl6, and Cs2LiLaBr6. All are doped with Ce3+. They are capable of providing very high energy resolution. The best values achieved for each material are 3.9%, 3.4%, and 2.9% at 662 keV (FWHM), respectively. Since 6Li has an acceptable cross-section for thermal neutron capture, these materials also detect thermal neutrons. In the energy spectra, the full energy thermal neutron peak typically appears above 3 MeV gamma equivalent energy. Thus very effective pulse height discrimination can be implemented with these materials. The CLLC and CLYC emissions consist of two main components: Core-to-Valence Luminescence (CVL; 220 nm to 320 nm) and Ce emission (350 to 500 nm). The former is of particular interest since it appears only under gamma excitation. It is also very fast and decays with less than 2 ns time constant. The CVL provides a significant difference to temporal responses under gamma and neutron excitation thus it may be used for effective pulse shape discrimination.

_{2}

In recent years, a number of materials from the elpasolite crystal family have been under development for either or both gamma ray and neutron detection. The scintillators show good energy resolution and thermal neutron detection efficiency. The latter is achieved due to the fact, that the selected compositions contain Li-6 ions. In order to effectively and reliably register both types of radiation, it is necessary to separate them through particle identification schemes. This can be accomplished using either pulse height or/and pulse shape discrimination, with the latter being more reliable. In this paper, we summarize our work and provide current status of pulse shape discrimination in the selected elpasolite scintillators. These include Cs2LiYCl6 (CLYC), Cs2LiLaCl6(CLLC), Cs2LiLaBr6(CLLB), and Cs2LiYBr6(CLYB).

LiYCl

Both Te inclusions and point defects can trap the charge carriers generated by ionizing particles in CdZnTe (CZT) detectors. The amount of charge trapped by point defects is proportional to the carriers drift time and can be corrected electronically. In the case of Te inclusions, the charge loss depends upon their random locations with respect to the electron cloud. Consequently, inclusions introduce fluctuations in the charge signals, which cannot be easily corrected. In this paper, we describe direct measurements of the cumulative effect of Te inclusions and its influence on the response of CZT detectors of different thicknesses and different sizes and concentrations of Te inclusions. We also discuss a means of partially correcting their adverse effects.

_{6}

Up to 1-in-diameter, Eu-doped CsBa₂I₅ and BaBrI crystals have been grown using the vertical Bridgman method. Scintillation properties of the crystals up to ~ 1 × 1 × 1 cm³ have been studied under gamma-ray irradiation. Systematic concentration studies have been performed for both these alkaline earth halides with Eu²⁺ doping levels ranging from 0% to 10% to optimize growth of large crystals. We have achieved excellent energy resolution of ~ 2.6% at 662 keV for a 2 × 2 × 1-mm³ sample of CsBa₂I₅:2%Eu. However, in this paper, the emphasis is on the larger crystals that can be used practically in nuclear nonproliferation applications. Our measurements of a 1.0 × 1.0 × 1.0-cm³ sample of CsBa₂I₅:3%Eu, processed from a 1-in-diameter crystal, shows very good energy resolution of 3.9% at 662 keV, high light output of ~ 80 000 photons/MeV, and a principal decay time of ~ 900 ns. The second investigated composition, BaBrI:5%Eu, 1.0-cm-diameter × 1.0-cm-thick sample shows high light output of 71 000 photons/MeV and energy resolution of 4.3% at 662 keV. A decay time of 480 ns was measured for this sample, which is shorter than most Eu²⁺ doped materials of this size. The emission in these materials is due to d → f transitions of Eu²⁺ and occurs over a single band between 400-500 nm. It peaks at ~ 430 nm for CsBa₂I₅:Eu and at ~ 413 nm for BaBrI:Eu. Since both compositions show high proportionality over a wide range of energies, we expect to see improvements in the energy resolution of large crystals, as better quality of 1-in-diameter crystals are grown.

, Cs

This paper describes detectors comprised of Cs2LiYCl6:Ce (CLYC) scintillators coupled to silicon photomultipliers (MPPC, Hamamatsu). CLYC has been developed for dual gamma ray and thermal neutron detection. MPPCs are compact detectors with a very thin profile (~2 mm), but also a small active area (6 mm × 6 mm). The combination of both can create a compact replacement of a He-3 tube. Three different crystal sizes were tested: a 5 mm × 5 mm × 5 mm cube, a 1 cm × 1 cm × 4 cm cuboid, and a Ø1 in × 1 in cylinder. All three detectors showed a well-resolved thermal neutron peak with energy resolution of 5.4%, 7%, and 11% (FWHM), respectively. The degradation in larger crystals is due to a mismatch between the coupled crystal face and the active area of the light detector. All detectors showed the 662 keV gamma ray peak from a Cs-137 source, indicating capability for limited gamma ray spectroscopy, with energy resolutions of 10%, 17%, and 26%, respectively. The capability for pulse shape discrimination was shown as well. The efficiency of the detectors was measured against a 10 atm He-3 tube (9 cm3 volume). The best absolute efficiency was shown by the cylinder detector, next by the He-3 tube (almost the same as the cylinder detector), and finally by the cuboid detector. The latter showed the best neutron count per cm3, twice as high as the two other detectors.

_{2}

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.

LiLaCl

New scintillators belonging to Cs(Ba,Sr)(Br,I)3 crystal family have been grown and investigated at RMD. The crystals were grown using the melt-based vertical Bridgman method. The iodide compositions were brighter compared to the bromides. Scintillation properties of these crystals have been studied under gamma irradiation, particularly CsBaI3:Eu in more details. CsBaI3:Eu has a tetragonal structure, and a density of 4.8 g/cc. The primary investigations of CsBaI3:Eu show an excellent energy resolution of 3.8% at 662 keV, high light yield of ∼55,000 photons/MeV, and a primary decay time of 760 ns. For CsBaI3:Eu, the emission occurs over a single band (400–500 nm), which is due to d → ƒ transition of Eu2+, with the emission wavelength peaking at 429 nm. The crystal shows high proportionality over a wide range of energies from 14 keV to up to 1.2 MeV.

_{6}

Recently a number of alkaline earth halide scintillators doped with Eu2+ have been reported on. They are characterized by very good proportionality, high up to 100,000 photons/MeV light yield, and very good energy resolution, as low as 2.8% at 662 keV. Yet, one of the issues facing these materials is radiation trapping. Radiation trapping results from a small Stokes shift that creates considerable overlap between emission and absorption bands. As a result the scintillation light is absorbed and emitted multiple times, leading to a prolongation of the scintillation decay, potential light losses and degradation of energy resolution. Spectroscopic properties of various Eu2+ doped alkaline earth halides are presented. Materials studied include: SrI2, BaI2, BaBrI and other compounds. It appears that some compositions are less affected by radiation trapping.

, and Cs

We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of SrI2(Eu)-based gamma ray spectrometers. SrI2(Eu) offers a light yield in excess of 100,000 photons/MeV and light yield proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of SrI2 and EuI2 indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy. We have demonstrated energy resolution with SrI2(4-6%Eu) of 2.6% at 662 keV and 7.6% at 60 keV with small crystals, while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed and re-emitted in large and/or highly Eu2+ -doped crystals. The light yield proportionality of SrI2(Eu) is found to be superior to that of other known scintillator materials, such as LaBr3(Ce) and NaI(Tl).

_{2}

A new scintillation material CS2LiLaBr6-xCIx:Ce (CLLBC:Ce) belonging to the elpasolite crystal family has been discovered and investigated at RMD. A Cs2LiLaBr4.8Cl1.2: 2% Ce crystal was grown using the melt-based, vertical Bridgman method. The first grown crystal of CLLBC:Ce is 1.5 cm in diameter and a few inches in length. This material has a cubic structure, therefore, potentially can be grown in large sizes. Its scintillation properties have been studied under gamma and neutron irradiation. The presence of 6Li and 35Cl in CLLBC:Ce allows for thermal and fast neutron detection, respectively. CLLBCs response to fast neutrons via 35Cl(n,p)35S reaction was tested at different energies of mono-energetic fast neutron beam, acquired at the University of Massachusetts Lowell 5.5 MV Van de Graaff generator. A fast neutron peak was identified, and its centroid was observed to move linearly with the beam energy. The differences in the decay times of neutrons and gammas allowed pulse shape discrimination to differentiate between the two types of radiation. The material also shows an excellent response to gamma-ray irradiation, with gamma-ray energy resolution of ~3.0 % at 662 keV, and light output of 40,000 photons/MeV. These properties make CLLBC:Ce an excellent candidate for combined gamma-ray and neutron (thermal and fast) spectroscopy.