E.V.D. van Loef

High-energy-resolution scintillator: Ce3+ activated LaBr3

The scintillation properties of LaCl3 doped with 10% Ce3+ are presented. Under optical and gamma ray excitation, Ce3+ emission is observed to peak at 330 and 352 nm. The scintillation light output is 46 000±1000 photons/MeV at 662 keV. Forty percent is emitted with a decay time of 26 ns, 30% with 210 ns, and 30% with about 1000 ns. An energy resolution (full width at half maximum over the peak position) of 3.3±0.3% was observed for the 662 keV full absorption peak.

Scintillators With Potential to Supersede Lanthanum Bromide

New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI2, CaI2, SrBr2, BaI2 and BaBr2-SrI2 is the most promising. SrI2(Eu) emits into the Eu2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.

Crystal Growth and Scintillation Properties of Strontium Iodide Scintillators

Single crystals of SrI₂:Eu and SrI₂:Ce/Na were grown from anhydrous iodides by the vertical Bridgman technique in evacuated silica ampoules. Growth rates were of the order of 5-30 mm/day. Radioluminescence spectra of SrI₂:Eu and SrI₂:Ce/Na exhibit a broad band due to Eu²⁺ and Ce³⁺ emission, respectively. The maximum in the luminescence spectrum of SrI₂:Eu is found at 435 nm. The spectrum of SrI₂:Ce/Na exhibits a doublet peaking at 404 and 435 nm attributed to Ce³⁺ emission, while additional impurity-or defect-related emission is present at approximately 525 nm. The strontium iodide scintillators show very high light yields of up to 120 000 photons/MeV, have energy resolutions down to 3% at 662 keV (Full Width Half Maximum) and exhibit excellent light yield proportionality with a standard deviation of less than 5% between 6 and 460 keV.

CeBr/sub 3/ scintillators for gamma-ray spectroscopy

In this paper, we report on a new scintillator, cerium bromide (CeBr3), for gamma-ray spectroscopy. Crystals of this scintillator have been grown using Bridgman process. In this material Ce 3+ is an intrinsic constituent as well as a luminescence center for the scintillation process. Samples of CeBr3 showed high light output (~68000 photons/MeV) and fast decay constant (~17 ns). Furthermore, they exhibited excellent energy resolution for gamma-ray detection. For example, energy resolution of <4% full width at half maximum (FWHM) has been achieved using this scintillator for 662 keV photons (137Cs source) at room temperature. High timing resolution (<200 ps-FWHM) has been recorded with CeBr3-photomultiplier (PMT) and BaF2-PMT detectors operating in coincidence using 511 keV positron annihilation gamma-ray pairs

Scintillation and spectroscopy of the pure and Ce3+-doped elpasolites: Cs2LiYX6 (X = Cl, Br)

The optical and scintillation properties of pure and Ce3+-doped Cs2LiYX6 (X = Cl, Br) are presented. X-ray-excited optical luminescence spectra, optical excitation and emission spectra, time-resolved excitation and emission spectra, scintillation pulse height spectra and scintillation decay time spectra of Ce3+-doped Cs2LiYX6 (X = Cl, Br) crystals measured from 10 to 300 K are presented. Factors influencing the scintillation mechanism and the presence of core?valence luminescence are discussed.

Scintillation properties of LaCl/sub 3/:Ce/sup 3+/ crystals: fast, efficient, and high-energy resolution scintillators

The scintillation properties of LaCl/sub 3/ doped with different Ce/sup 3+/ concentrations studied by means of optical, X-ray, and /spl gamma/-ray excitation are presented. Under optical and /spl gamma/-ray excitation, Ce/sup 3+/ emission is observed peaking at 330 and 352 nm. For LaCl/sub 3/ doped with 2%, 4%, 10%, and 30% Ce/sup 3+/ and pure CeCl/sub 3/, we measured a light yield of 46 000/spl plusmn/3000 photons per MeV of absorbed /spl gamma/-ray energy. The scintillation decay curve can be described by three decay components: short (/spl tau/=25 ns), intermediate (/spl tau/=200-800 ns), and long (/spl tau/=0.8-14 /spl mu/s). The contribution of the short decay component to the total light yield increases with Ce/sup 3+/ concentration: ranging from 10% for LaCl/sub 3/: 2% Ce/sup 3+/ to 69% for pure CeCl/sub 3/. An energy resolution (full-width half-maximum over peak position) for the 662-keV full energy peak of 3.5/spl plusmn/0.4%, 3.5/spl plusmn/0.4%, 3.1/spl plusmn/0.3%, 3.3/spl plusmn/0.3%, and 3.4/spl plusmn/ 0.3%, respectively, was observed for LaCl/sub 3/: 2%, 4%, 10%, 30% Ce/sup 3+/ and pure CeCl/sub 3/.

Scintillation Properties of 1 Inch

We have grown and investigated 1 inch diameter (CLYC) crystals for gamma and neutron detection. The samples provided excellent results. For example, 5.1plusmn0.1% energy resolution was obtained at 662 keV (5.5% at 511 keV). The light output of 4500plusmn350 photoelectrons/MeV (PMT, Hamamatsu R6233) was measured. The samples also showed excellent plusmn1.2% non-proportionality in the 14.4 to 1274 keV range. This suggests a possibility for even better energy resolution with a superior photodetector. The intrinsic energy resolution of investigated crystals was estimated to be 2.1%. The neutron detection was also confirmed. The neutron peak was observed at about 3.2 MeV (gamma equivalent) and its resolution was 2.9plusmn0.1%. Gamma-neutron pulse shape discrimination was also achieved.

{\rm Cs}_{2}{\rm LiYCl}_{6}{:}{\rm Ce}

In this communication we report on our investigation of scintillation properties of LaBr/sub 3/:Ce as a function of Ce concentration. We have studied crystals nominally doped with 0.5, 5, 10, 20, and 30% Ce (by mole). Reports published so far suggested that as the Ce content increases, there is a decrease in light output and little or no change in decay time constants. Our results show that the light output does not change with Ce concentration up to 30% and depends mostly on the crystal and measurement quality. On the other hand we have found timing properties to be a strong function of concentration. As the Ce content increases the principal decay time constant of scintillation decreases from /spl sim/26 ns for 0.5% Ce to /spl sim/17 ns for crystals with >5% Ce. Moreover, there is also a significant change in rise time constants. The rise time measured for a sample doped with 0.5% Ce is up to 9 ns, whereas for samples doped with >10% Ce it reduces to less than 0.5 ns. The change of rise time has a major effect on the timing properties of this scintillator, with timing resolution improving from 390 ps to less than 200 ps (FWHM).

Crystals

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.

Effects of Ce concentration on scintillation properties of LaBr/sub 3/:Ce

We have characterized the gamma and X-ray scintillation properties of Ce/sup 3+/ doped Lu/sub 2/Si/sub 2/O/sub 7/, a lutetium pyro-silicate (LPS) material. The rare earth ions are located in an octahedral site with C/sub 2/ symmetry. The compound exhibits chemical stability, transparency in a wide optical range and congruent melting, which allows growing of single crystals of good optical quality. The melting temperature is about 2000/spl deg/C, slightly lower than that of Lu/sub 2/SiO/sub 5/ (LSO). Two maxima at 305 nm and 355 nm are observed in the optical absorption spectrum. The emission spectrum of Ce/sup 3+/ in LPS shows a broad band peaking at 380 nm. For a nominal Ce/sup 3+/ atomic concentration of /spl sim/0.01, under gamma-ray excitation we observe light yields in the range 13000-23000 photons/MeV, depending on the preparation atmosphere. The actual cerium concentration inside the crystal still needs to be determined. The scintillation decay time is around 30 ns and, in the first studies, neither a long component nor afterglow was observed.