Scintillation Properties of
LuI
3
:Ce
3+
-High Light
Yield Scintillators
M. D. Birowosuto, P. Dorenbos, C. W. E. van Eijk, Member, IEEE, K. W. Krämer, and H. U. Güdel
Abstract—The scintillation properties of LuI3:Ce3+(pure, 0.5%, 2%, and 5%Ce3+), a new member of the generation of
Ce3+doped lanthanide trihalide scintillators, are presented. This
material has a calculated density of 5.6 g cm3 and an atomic number of 60.2. Under optical and X-ray excitation,Ce3+ emission is observed to peak at 472 and 535 nm. A high light yield of 76 000 photons per MeV (ph/MeV) was measured for
LuI3:2%Ce3+.LuI3:Ce3+has a nonexponential decay time.
The contribution of short decay components (within 50 ns) to the total light yield is around 50%. An energy resolution (full-width half maximum over peak position) for the 662 keV full energy peak of4 7 0 5% was observed for LuI3:0.5%Ce3+.
Index Terms—Ce3+, decay time, lanthanide trihalide, light yield,LuI3, nonproportionality, scintillator.
I. INTRODUCTION
I
N the search for better scintillators, we have directed the search toward lanthanide trihalide scintillators. The com-bination of high light yield, fast response and good energyresolution of and were reported by
Van Loef et al. [1], [2]. scintillates efficiently with a light output of . It has the excellent energy resolution of 3.1% for 662 keV quanta. has the even higher light output of , in addition a better energy resolution was reported. , which in theory can have a higher light output than , does not emit scintillation photons at room temperature [3].
From the scintillation properties of doped oxides, it is known that lutetium based compounds often show good lumi-nescence properties when doped with . Lutetium based compounds are also attractive for their high density. On the other hand appears to emit inefficiently in most lanthanum based oxide compound [4].
Scintillation properties and mechanisms of and were previously reported [5]. Regarding their low
light yields, and are less attractive
scintillators compared to and .
Re-cently, the scintillation properties of were published
Manuscript received November 2, 2004; revised February 11, 2005. This work was supported by Netherlands Technology Foundation (STW), the Swiss National Science Foundation and Saint Gobain, division crystals and detectors, Nemours, France.
M. D. Birowosuto, P. Dorenbos, and C. W. E. van Eijk are with the Radia-tion Technology Group, Interfaculty Reactor Institute, Delft University of Tech-nology, Delft 2629 JB, The Netherland.
K. W. Krämer and H. U. Güdel are with the Department of Chemistry and Biochemistry, University of Bern, Bern 3000, Switzerland.
Digital Object Identifier 10.1109/TNS.2005.852630
by Glodo et al. [6] and Shah et al. [7]. Light yield, energy res-olution, decay time, proportionality response, and time resolu-tion were parts of the investigaresolu-tion. Scintillaresolu-tion properties of pure and :2% were not discussed in detail. In this paper, the scintillation properties of pure, 0.5%, 2%, and 5% doped under X-ray and -ray excitation are pre-sented.
II. EXPERIMENTALTECHNIQUES
The crystals of 8 mm diameter and 1 cm length were grown by the Bridgman technique. Starting materials were prepared by iodinating the lutetium metal with iodine vapor [8]. ME71-4N grade lutetium (99.99% pure according to factory certificate) and “special purity” grade iodine powder (Merck, suprapur) were used. Sublimation was necessary for purification of . The amount of in the feed material was adjusted in order to produce . Since crystals are sensitive to moisture, the crystals were sealed in quartz ampoules. For this experiment, crystals of pure, 0.5%, 2%, and 5% (mol%) doped were grown.
crystallizes in type structure in the hexagonal space group R-3 (no. 148) [9]. Due to its structure and lattice parame-ters, has a calculated density of 5.6 and an effec-tive atomic number of 60.2. The probability of photoelec-tric effect and the total attenuation length for 511 keV quanta are 28% and 1.82 cm, respectively.
Radioluminescence spectra were recorded in reflection using an X-ray tube with a Cu-anode. The anode was operated at 35 kV and 25 mA. The emission of the sample was dispersed by an Acton Research Company (ARC) VM-504 monochro-mator and detected by Hamamatsu R934-04 Photomultiplier Tube (PMT). The ARC VM-504 monochromator has a 1200 grooves/mm grating blazed at 300 nm. The spectra presented in this study were corrected for the transmission of the system and for the quantum efficiency of the PMT. Temperature de-pendent radioluminescence measurements were performed be-tween 80 and 600 K using a JANIS VPF-700 Cryostat operated with Model 331 LakeShore Temperature Controller.
Pulse height spectra were obtained with a 1.6 cm diameter Advanced Photonic 630-70-73-500 Avalanche Photodiode (APD). APD was operated with a bias voltage of 1700 V. To avoid gain drift, APD was stabilized at 288 K. The crystals were mounted without optical coupling fluid on a quartz window on top of the APD and put inside an M-Braun UNILAB dry box. The crystals were covered with Teflon tape and a Teflon dome for better collection of scintillation light. The number of elec-tron hole pairs created after detection of scintillation light (e-h 0018-9499/$20.00 © 2005 IEEE
pairs/MeV) was obtained through comparison of the photopeak position relative to the 17.8 keV peak of characteristic X-rays from the Np decay product of directly detected by the APD. The effective quantum efficiency of the APD is around 95–100% due to the reflecting covering around the scintillator [10].
Other pulse height spectra were obtained with a Hamamatsu R1791 PMT with a box type dynode structure connected to a pre-amplifier and an Ortec 672 spectroscopic amplifier. The crystals were mounted with an optical grease (Viscasil 60 000 cSt) to the window of the PMT and covered with several Teflon layers. The yield, expressed in photoelectrons per MeV of ab-sorbed -ray energy (phe/MeV), was determined by comparison of the peak position of the 662-keV photopeak relative to the po-sition of single photoelectron peak. The light yield expressed in photons per MeV (ph/MeV) is determined using the quantum efficiency and reflectivity of the PMT [10].
Scintillation decay time spectra were recorded using two methods. The first method is the single-photon counting tech-nique described by Bollinger and Thomas [11]. For this method, scintillation decay time spectra were recorded at time ranges up to 200 with XP2020Q PMT’s, Ortec 934 Constant Fraction Discriminators, Ortec 567 Time to Amplitude Converter (TAC) and AD413A CAMAC Analog to Digital Converter (ADC).
The second method is the multi-hit method [12]. The TAC and ADC in single-photon counting technique were replaced by a Lecroy 4208 Time to Digital Converter having a channel width of 1 ns. By this method, the short decay component and its contribution to the total light yield are less accurately obtained than those by the single-photon counting technique.
Decay time spectrum of emission was measured at the SUPERLUMI vacuum ultraviolet station of HASYLAB at the DESY synchrotron facility in Hamburg, Germany. Details about this experiment were described elsewhere [13].
Small (2–4 ) pieces of transparent crystals were cut from the original crystal boules. Samples were sealed in 5 mm di-ameter quartz ampoules for radioluminescence and decay time studies outside the dry box. Other samples of the same dimen-sion were used for the light yield studies with the APD and the PMT inside the dry box.
III. RESULTS ANDDISCUSSION
A. Radioluminescence
Radioluminescence spectra of pure , :0.5%, 2%, and 5% are shown in Fig. 1. Spectra were normalized in such a way that the integral of each spectrum over all wave-lengths is equal to the value for the absolute light yield as found from the pulse height measurement with a 10 shaping time.
For pure , the spectrum consists of a broad band peaking at 460 nm. It is attributed to self trapped exciton (STE) emission [6]. The sharp peaks observed in the range of 550- to 800-nm for the pure and 0.5% doped samples are due to the unwanted rare earth impurities and . The emission consists of two main overlapping bands at 472 nm and 535 nm. They correspond to transitions from the lowest 5d level to
and levels on ions. :0.5% and 2%
spectra are dominated by the transition
Fig. 1. Radioluminescence spectra at room temperature of pure LuI , LuI :0.5%, 2%, and 5% Ce . The y-axis has been calibrated using light yields derived from pulse height spectra.
Fig. 2. Radioluminescence spectra between 80 and 600 K of (left)LuI : 2% and (right) 5%Ce . The spectra were recorded at 80, 150, 240, 330, 420, 510, and 600 K. The zero intensity for each spectrum has been vertically shifted with respect to each other for clarity.
(472 nm) while :5% spectrum is dominated by the transition (535 nm). The average of the emission is shifted to longer wavelengths with increasing concentration.
Temperature-dependent-radioluminescence spectra of :2% and 5% measured between 80 and 600 K are shown in Fig. 2. The intensity of the band centered at 640 nm increases toward the higher concentration. This band might be related to the emission of a second center.
Temperature dependence of light yields of pure , :2%, and 5% were derived from the integral of each spectrum measured between 80 and 600 K (see Fig. 3). From 80 to 600 K, light yields of pure , :2%, and 5% decrease to 9, 28, and 18%, respectively. The decrease of light yield of pure is related to quenching of the STE luminescence (Fig. 3). In this compound, STE luminescence starts already to quench at very low temperature. At tempera-tures between 80 and 400 K, the light yield of :2% decreases slowly. At temperatures between 400 and 600 K, the
Fig. 3. Temperature dependence of the light yields of pureLuI , LuI :2%, and 5%Ce derived from radioluminescence spectra.
Fig. 4. Pulse height spectra of (a)LuI :0.5%, (b) 2%, and (c) 5% Ce for 662 keV rays. Spectra were measured with a Photonic 630-70-73-500 APD. The high energy sides of the full energy peaks were positioned at the same channel number. The inset shows the pulse height spectrum ofLuI :0.5% Ce on a linear scale.
light yield of :2% decreases faster than that of pure
and :5% .
B. Pulse Height Experiments
Pulse height spectra of radiation from a source recorded with :0.5%, 2%, and 5% are presented in Fig. 4. Usually in small crystals, the photopeak is accompanied by satellite peaks at lower energy due to escape of the char-acteristic and X-rays of lutetium [14]. However, this cannot explain the structure in the full energy peaks of curves (b) and (c). Presumably the crystals are inhomogeneous. This means that different parts of the crystal produce different light yields at the same absorbed energy.
Electron-hole pair yields derived from pulse height spectra measured with a Photonic 630-70-73-500 APD are presented in Table I. The yield of pure
-obtained with the shaping time of 10 is the lowest. Yields are much higher for doped . These yields vary from
to - . The energy
resolution for the 662 keV full energy peak of curve (a) in
TABLE I
NUMBER OFELECTRON-HOLEPAIRS ANDENERGYRESOLUTIONDERIVED
FROMPULSEHEIGHTSPECTRA OFPURELuI , LuI :0.5%, 2%,AND5%Ce UNDER662 KEV -RAYEXCITATIONMEASUREDWITH APHOTONIC
630-70-73-500 APD. THEYIELD OFCSI:TLISADDED FORCOMPARISON
TABLE II
LIGHTYIELD ANDENERGYRESOLUTIONDERIVEDFROMPULSEHEIGHT
SPECTRA OFPURELuI , LuI :0.5%, 2%,AND5%Ce UNDER662 KEV -RAYEXCITATIONMEASUREDWITH AHAMAMATSUR1791 PMT. LIGHT
YIELD OFCSI:TLISADDED FORCOMPARISON
Fig. 5. The proportionality curves forLuI :0.5%, 2%, and 5% Ce . LuI with 0.5% and 2%Ce crystals have the same size (2mm ) while LuI :5% Ce is a thick crystal (30mm ).
Fig. 4 was obtained by fitting a photopeak and an escape peak. A value of 4.7% was obtained.
Light yields derived from pulse height spectra measured with a Hamamatsu R1791 PMT are presented in Table II. Consid-ering the 95–100% effective quantum efficiency of the APD, the light yield in Table II is slightly larger than the number of electron-hole pairs in Table I. Energy resolution measured with the PMT is poorer than that measured with the APD.
Using different and X-ray energy sources, the proportion-ality of the response of has been determined. The yield as function of or X-ray energy relative to the yield at 662 keV is shown in Fig. 5. In this figure, light yields were de-rived from the pulse height spectra recorded with a Hamamatsu R1791 PMT. For pulse height spectra of :2% and :5% , the full energy peaks have less structure than
Fig. 6. Scintillation decay curves at room temperature of (a) pureLuI , (b)LuI :0.5% Ce , (c) LuI :2% Ce , and (d) LuI :5% Ce . These spectra were recorded using the multi-hit method. The dotted lines are the corresponding background levels from each spectrum (same order).
Fig. 7. Scintillation decay curves at room temperature of (a) LuI :0.5% Ce , (b) LuI :2% Ce , and (c) LuI :5% Ce . Decay curve of (d) Ce emission ofLuI :0.5% Ce was measured at 310-nm excitation and 495-nm emission. Decay curves of (a)–(c) were recorded using single photon counting technique while (d) was recorded using SUPERLUMI experimental set-up of HASYLAB at the DESY synchrotron facility in Hamburg, Germany. The dotted lines are the corresponding background levels from each spectrum (same order).
those measured with the APD due to the poorer energy resolu-tion of the PMT. They were fitted with the same procedure as that of :0.5% .
The light yield of :0.5% in Fig. 5 is constant within at energies between 40 keV and 1.5 MeV. On the low energy side from 40 to 13 keV, the yield drops by 10%. A poor proportionality response is observed for :5% . Light yields drop by 25% from energy 662 to 13 keV.
C. Time Profile
Scintillation decay time spectra of pure , :0.5%, 2%, and 5% recorded at room temperature under -ray excitation using the multi-hit method are shown in Fig. 6. The decay curves are not exponential and have a long decay component extending into the microsecond region.
Short-time scale decay time spectra, measured by the single photon counting method, are shown in Fig. 7. In these spectra, a time scale of 200 ns was used. The fast decay component is also nonexponential. If the first few nanoseconds are consid-ered, the decay time decreases when the concentration in-creases. For comparison, the decay curve of emission of :0.5% , measured at 310-nm excitation and 495-nm emission at 10 K, is added. This curve has been fitted with a single exponential decay of .
TABLE III
THERATIOS OFLIGHTYIELDS AT0.05, 0.5AND3s TIMERANGESRELATIVE TO THELIGHTYIELD AT10-s TIMERANGEDERIVEDFROM THETIME
INTEGRAL OFLuI :0.5%, 2%,AND5%Ce
It is not possible to fit the other decay curves with one expo-nential. The time integrals of the decay curves were calculated for the time ranges of 0.05, 0.5, 3, and 10 . The thus obtained light yields normalized to that in 10 are shown in Table III. The 0.5 and 3 values correspond well to the ratios of the light yields which were obtained from the pulse height spectra. The contribution of short decay components (within 50 ns) to the total light yield is around 50%.
IV. CONCLUSION
is a lanthanide halide scintillator with high light yield, fast response and the potentiality of good energy reso-lution. A good proportionality response was observed for the :0.5% crystal. The location of the emission bands of in the 450- to 600-nm region is highly advan-tageous since it fits the peak-sensitivity wavelength range of the APD. Poorer scintillation properties were observed for thick and high concentration crystals. Inhomogeneities in the crystal appear to be the principal shortcoming. To give a more definitive judgment, investigation on photospectroscopy and temperature-dependent decay time measurements have to be performed, and better quality high concentration crys-tals are needed.
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