S.E. Derenzo

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

Scintillation Properties of SrHfO

In this paper we report on the scintillation properties of cerium doped strontium - and barium hafnate. Radioluminescence, pulse height, scintillation decay and timing spectra are presented. Radioluminescence spectra of SrHfO3:Ce3+ and BaHfO3:Ce3+ consist of a broad band due to Ce3+ emission peaking at 410 nm and 400 nm, respectively. The light yield of BaHfO3:Ce3+ and SrHfO3:Ce3+ is approximately 40 000 photons/MeV when compared to a crystal of BGO. The principal decay time constant for SrHfO3:Ce3+ and BaHfO3:Ce3+ is 42 and 25 ns, respectively. A timing resolution of 276 ps (FWHM) was obtained with transparent optical ceramic of SrHfO3:Ce3+.

_{3}

This paper reports on the scintillation and optical properties of the recently discovered europium doped BaBrI and CsBa₂I₅ scintillators. Single crystals of BaBrI and CsBa₂I₅ were grown from the melt by the vertical Bridgman technique. Crystals were doped with 8% and 4% Eu²⁺ for BaBrI and CsBa₂I₅ , respectively. These crystals have higher optical quality and present improved scintillation performance compared to the previously reported values. An estimated light yield of 97,000±5,000 and 102,000±6,000 photons per MeV (ph/MeV) of absorbed gamma-ray energy is now measured for BaBrI:8%Eu²⁺ and CsBa₂I₅:4%Eu²⁺ , respectively. A better energy resolution has also been observed. BaBrI energy resolution for the 662 keV full absorption peak (FWHM over peak position) decreased to 3.4±0.2% and CsBa₂I₅:4%Eu²⁺ energy resolution decreased to 2.55±0.2% . Additional measurements show both scintillators with an excellent light yield proportionality. Finally, based on optical and x-ray excited emission spectra and decay curves, energy transfer mechanisms in BaBrI and CsBa₂I₅ scintillators are briefly discussed.

:Ce

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 CeBr3, Ce3+ is an intrinsic constituent as well as a luminescence center for the scintillation process, has high light output (~68,000 photons/MeV) and fast decay constant (~17 ns). Furthermore, it shows excellent energy resolution for gamma-ray detection. For example, energy resolution of <4% (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-PMT (photomultiplier tubes) and BaF2-PMT detectors operating in coincidence using 511 keV positron annihilation gamma-ray pairs. Potential applications of this material are addressed

^{3+}

Lanthanum and yttrium compounds with composition RE2 M2 O7 (REu2009=u2009Y, La; Mu2009=u2009Ti, Zr, Hf) have high density and high Z and can be doped with Ce onto the La and Y sites. This makes these compounds good candidates for Ce-activated scintillator γ-ray detectors particularly for the hafnate systems which have a very high density. There is disagreement in the literature concerning La2 Hf2 O7:Ce as it has been reported to show both bright as well as no Ce-activated luminescence by different experimental groups. We have performed first-principles electronic structure calculations of these compounds doped with Ce using the pseudopotential method based on the generalized gradient approximation in density functional theory. The positions of the Ce 4f states relative to the valence band maximum and the position of the Ce 5d states relative to the conduction band minimum (CBM) of the host material are determined. We find, unlike Ce-activated La and Y compounds where the CBM is typically of La 5d or Y 4d character, that ...

and BaHfO

In this paper we report on our investigation of the CeBr3 timing properties for Time-of-Flight (TOF) Positron Emission Tomography (PET). The TOF detection can significantly improve the signal-to-noise ratio in a reconstructed PET image and reduce the number of events necessary to do the reconstruction. In order to achieve TOF PET, a scintillator must have good timing resolution and good light output. We have investigated seven CeBr3 samples of different height, ranging from 3 mm to 37 mm. Two Hamamatsu R4998 fast photomultipliers were used for most experiments. Each sample was measured against a common standard (LaBr3:5%Ce) and the average timing resolution was found to be about 166 ps (fwhm). The two reference (LaBr3:5%Ce) crystals measured against each other gave the timing resolution of 170 ps. The best time resolution of 108 ps (fwhm) was measured for two CeBr3 samples (3 and 12 mm height). The longest sample (37 mm) provided 215 ps (fwhm). Two such samples working in coincidence would give about 252 ps timing resolution (fwhm, calculated). We have also measured TOF, where the source was moved between the two detectors and the change in a timing peak position was registered. The results show that a shift of 5 cm could be easily resolved by such a system.

_{3}

Radionuclide techniques will be increasingly important for the development of biofuels, since they can both uniquely characterize metabolic pathways and image large model systems. We are investigating the application of nuclear medical imaging tools and techniques to biofuel development, using high-sensitivity chromatographic radiation detectors and compounds radiolabeled with short-lived, cyclotron-produced, positron-emitting isotopes (e.g., 11C). The application of high-sensitivity radiation detectors to standard chromatographic techniques should allow for the measurement of numerous pathway constituents whose levels normally fall below detection limits of conventional instrumentation. In this paper, we describe a proposed parallel-plane PET camera designed for HPLC (high performance liquid chromatography) with over 40 times greater sensitivity than a conventional HPLC radiation detector; this PET camera could be used to image radioactivity in the HPLC exit tube or column. We also present results using a high-sensitivity HPLC radiation detector comprised of 8 HR+ PET detector modules arranged into a parallel plane and read out with HRRT electronics. This high-sensitivity radiation detector was placed in line after a conventional HPLC radiation detector (a small CsI:Tl scintillator crystal coupled to a PIN photodiode). If we inject 9.3 μCi of [18F]FDG into the HPLC system, we see similarly shaped peaks with an excellent signal-to-noise ratio from both radiation detectors. If we inject only 4.1 nCi of [18F]FDG, we measure a signal-to-noise ratio of 27:1 with the high-sensitivity radiation detector and about 3:1 for the conventional radiation detector. We have therefore demonstrated that a high-sensitivity radiation detector, using a parallel-plane PET camera, could become an important tool for fundamental biofuel research.

:Ce

This paper is the first report of n-type GaAs as a cryogenic scintillation radiation detector for the detection of electron recoils from interacting dark matter (DM) particles in the poorly explored MeV/c2 mass range. Seven GaAs samples from two commercial suppliers and with different silicon and boron concentrations were studied for their low temperature optical and scintillation properties. All samples are n-type even at low temperatures and exhibit emission between silicon donors and boron acceptors that peaks at 1.33 eV (930 nm). The lowest excitation band peaks at 1.44 eV (860 nm) and the overlap between the emission and excitation bands is small. The X-ray excited luminosities range from 7 to 43 photons/keV. Thermally stimulated luminescence measurements show that n-type GaAs does not accumulate metastable radiative states that could cause afterglow. Further development and use with cryogenic photodetectors promises a remarkable combination of large target size, ultra-low backgrounds, and a sensitivity to electron recoils of a few eV that would be produced by DM particles as light as a few MeV/c2.