Scintillation Properties of RbGd Br :Ce
Advantages and Limitations
O. Guillot-No¨el, J. C. van’t Spijker, J. T. M de Haas, P. Dorenbos,
C. W. E. van Eijk, K. W. Kr¨amer, and H. U. G¨udel
Abstract— The scintillation properties of RbGd2Br7 crystals, doped with Ce3+concentrations of 0.02, 0.11, 0.88, 2.05, 4.1, and 9.8%, are studied under X-ray and -quanta excitations.
For the RbGd2Br7 sample doped with 9.8% Ce, the authors measured a light yield of 56 000 6 6000 photons per MeV of absorbed -ray energy with a main decay time of 43 6 1 ns, using a Hamamatsu R1791 photomultipler (PMT), a137Cs radioactive source, and a shaping time of 10s. A time resolution of 790
6 10 ps was measured for the RbGd2Br7: 9.8% Ce compound, using Baf2 as second scintillator, two XP2020Q PMT’s, a 22Na source, and an energy threshold set atE 511 keV.
With the R1791 PMT, an energy resolution of 4.1% (FWHM over peak position) for the 662-keV full absorption peak has been observed for two crystals of 72 4 2 2 mm3 and 152 5 2 1 mm3 with 4.1 and 9.8% Ce content, respectively. Moreover, the non-proportional responses of three RbGd2Br7:Ce compounds with different concentrations (0.11, 2.05, and 9.8%) were studied revealing an almost-constant light output response from 17.4 keV to 1 MeV.
These properties are compared to three other well-known scintillators: NaI:Tl, CsI:Tl, and Lu2SiO5:Ce.
Index Terms— Energy resolution, -ray detection, nonpropor-tionality, rare-earth halides, scintillation crystals, time resolution.
I. INTRODUCTION
S
CINTILLATORS have to meet many criteria like high light yield, a fast and linear response, a high density, a high effective atomic number, and more practical concerns such as easy crystal growth and radiation hardness. Research into scintillators has been going on since the 1930’s and the ideal scintillator has not been found yet. In the last few years, a lot of studies have been devoted to oxide scintillators doped with Ce ions. The Ce ion is used for its potential to yield fast scintillation in the 300–500-nm wavelength range due to electric dipole allowed 5d–4f transitions. Many oxide scintillators using Ce doping have been discovered like Lu SiO :Ce [1], [2], Gd SiO :Ce [3], YAlO :Ce [4], [5], LuAlO :Ce [6]. They show high light yields and a fast scintillation response. For example, Lu SiO :Ce shows aManuscript received March 1, 1999; revised June 9, 1999. This work was supported by the Netherlands Technology Foundation (STW) and by the Swiss National Science Foundation.
O. Guillot-No¨el, J. C. van’t Spijker, J. T. M. de Haas, P. Dorenbos, and C. W. E. van Eijk are with the Radiation Technology Group, Interfaculty Reactor Institute, Delft University of Technology, 2629 JB Delft, The Nether-lands.
K. W. Kr¨amer and H. U. G¨udel are with the Department of Chemistry and Biochemistry, University of Bern, 3000 Bern 9, Switzerland.
Publisher Item Identifier S 0018-9499(99)08142-3.
decay time of 40 ns and a light yield of 26 000 photons per megaelectronvolt (ph/MeV) [7].
Surprisingly, studies on halide compounds other than fluo-rides and iodides are rarely reported. For example, Tl -doped NaI and CsI are widely used as scintillators. These materials, which were discovered in 1948 [8] and 1950 [9], respectively, show light yield higher than 43 000 ph/MeV [10]–[12] under -ray excitation. Such high photon yields have not been reported for other scintillators. A couple of years ago, we reported results on K LaCl :Ce [13], [14]. This compound scintillates efficiently with a photon yield of 28 000 ph/MeV and shows an excellent energy resolution of 5.1 0.2% for 662-keV -quanta. To our knowledge, apart from this chloride compound and the alkali halides, hardly any research has been done on other cerium-doped chlorides, bromides, or iodide inorganic crystals. Probably, the reason is that these materials are often hygroscopic and therefore special crystal growth techniques and special methods are required to study their properties.
In the present study, we investigated the scintillation prop-erties of RbGd Br crystals doped with Ce concentrations of 0.02, 0.11, 0.88, 2.05, 4.1, and 9.8% under X-ray and -quantum excitation. The different measurements performed in this study are compared with the results obtained on the three following scintillators: NaI:Tl, CsI:Tl, and Lu SiO :Ce.
Several reasons made us decide to work on the RbGd Br compound.
1) In order to obtain high light yields, the number of thermalized electron–hole (e–h) pairs created under ex-citation by a -quantum, should be high. In first approx-imation [15], this number is inversely proportional to the bandgap energy of the compound. Usually, chlorides and bromides have smaller bandgap energies than fluorides and thus larger numbers of e–h pairs are created [16]. 2) After excitation, the created e–h pairs have to transfer
their energy in a fast and very efficient way to the luminescence centers. Previous studies have shown that for some Gd-based scintillators, the energy of electrons and holes can be fast and efficiently transferred via the Gd-sublattice to the Ce centers [17]–[19]. Therefore, Gd–bromide compounds are possible candidates for high light yield scintillators with efficient and fast energy transfer. Another advantage to having a Gd-compound is that Gd ions have a high atomic number
which contributes to a high -quantum attenuation coefficient for the scintillator. Moreover, Ce can be
easily incorporated on a Gd site because these two ions have the same charge valency and nearly the same ionic radius.
II. EXPERIMENTAL PROCEDURE
The crystals of RbGd Br with 0.02, 0.11, 0.88, 2.05, 4.1, and 9.8% Ce concentration, investigated in this study, were grown by the Bridgman technique using a moving furnace and a static vertical ampoule. The preparation has already been described in [20]. RbGd Br crystallizes in the RbDy Cl structure in the orthorhombic space group Pnma [21]. The 1-2-7 crystals grow quite fast along the and axes but slowly along the axis. Therefore, all the crystals have a plate-like shape. Due to the slow growth along , the crystals tend to twin along this axis. After the crystal growth, we split off these twins with a sharp knife until only one single crystal remained. The Ce concentrations in the crystals were determined by Induction Coupled Plasma Spectroscopy (ICPS), for which a small part of the sample was dissolved, sprayed into an argon plasma, and the cerium emission was detected. In this work, we express the determined Ce content in the crystals as
in RbGd Ce Br .
RbGd Br crystals are rather hygroscopic. Some of them were exposed to air and after a couple of months, the quality of these crystals worsened slightly due to hydration of the surface. Therefore, some of the studied crystals were sealed into small quartz ampoules with a 5 40 mm size under nitrogen atmosphere to avoid such deterioration.
X-ray-induced emission spectra were recorded with an X-ray tube with a Cu anode operating at 35 kV and 25 mA. The emission spectra were measured with an ARC VM504 monochromator (blazed at 300 nm, 1200 grooves/mm) and an EMI 9462 PMT. The spectra presented in this study were not corrected for transmission of the system and for quantum efficiency (QE) of the photomultiplier (PMT). In our setup, the detection efficiency increases linearly from 160 to 400 nm and decreases drastically at higher wavelengths until zero near 500 nm.
Pulse-height spectra were recorded with a Hamamatsu R1791 PMT. This tube has a box-type dynode structure assuring an optimal photoelectron charge collection efficiency. Occasionally, a Phillips XP2020Q PMT with linear focused dynodes was used. A home-made preamplifier and an Ortec 672 spectroscopic amplifier were used. The bare crystal or the quartz ampoule containing a crystal was mounted with an optical coupling compound (Viscasil 60 000St from General Electric) to the window of the PMT. To optimize the light collection, the crystal or ampoule was covered with several layers of 0.1-mm-thick UV reflecting Teflon tape.
Light yields, expressed in collected photoelectrons per megaelectronvolt of absorbed -ray energy (phe/MeV), were obtained by comparing the peak position of the photopeak with that of the single-electron spectrum [22]. We used
Am, Co, Ba, Cs, Na, Co -ray sources to
excite the compounds at energies between 59 keV and 1.332 MeV. An Amershan (code AMC.2084) variable X-ray source was employed to excite the samples at energies between
8 and 44.5 keV. In this source, -rays of Am produce characteristic K and K X-rays from Cu, Rb, Mo, Ag, Ba, and Tb targets. Depending on the type of excitation source used, the photopeak was accompanied by satellite peaks. These peaks are either due to escape of characteristic X-rays from the crystal or because the source emits more than one type of X-ray, i.e., K and K X-rays. Therefore, in order to determine the location of the photopeak, we fitted, when necessary, the composite photopeak with a sum of several Gaussian shaped peaks. In general, when the photopeak was not symmetrical in shape, due to the presence of unresolved X-ray peaks, an additional error in the position was taken into account. The error made in the photopeak position, caused by errors in the determination of the relative gain of the PMT and the position of the photopeak, is typically 3%.
The absolute quantum efficiency of the R1791 PMT as function of wavelength was determined from the quan-tum efficiency curve supplied by the manufacturer and the Corning blue sensitivity. We refer to [23] for more details. The estimated relative accuracy is 10%. The charge-collection efficiency of PMT’s with box-type dynodes is close to optimal
and in this work we assume . The light
generated in the crystal that reaches the photocathode, the light collection efficiency , is estimated to be 0.95 0.05. Knowing the overall detection efficiencies, , the light yields, expressed in photons per megaelectronvolt of absorbed -ray energy (ph/MeV), were determined. Photoelec-tron yields measured with the XP2020Q photomultiplier tube are usually 25% lower than the number measured with the R1791 photomultiplier tube. It is attributed to a combination of a lower charge-collection efficiency estimated to be
and a lower quantum efficiency.
To study the light yield dependence on the -ray energy, the so-called nonproportionality curve, we mounted quartz-sealed samples, wrapped in several Teflon layers, onto the PMT window and performed the measurements as described before. For nonproportional response, all measurement conditions were kept equal, i.e., the crystal was not dismounted. Due to the absorption of the quartz ampoule, the pulse-height spectra were only recorded between 17.4 keV and 1.332 MeV.
Scintillation decay-time spectra at time scales up to 200 s were recorded using the multihit method [24] with XP2020Q PMT’s, Ortec 934 Constant Fraction Discriminators, and a Lecroy 4208 Time to Digital Convertor having a channel width of 1 ns. Two measurement configurations were used. In the first method, called in this paper the conventional multihit start–stop method, the sample under study is mounted on the so-called “start” PMT and is excited by -rays of a Cs source. The starting moment is derived from the rising edge of the scintillation flash detected in the “start” PMT. Stop moments are derived from the so-called “stop” PMT which observes via a narrow slit at most eight single photons (the so-called multihits) from the scintillation event. In the other method, called in this paper the coincidence method, we use the two 511-keV photons emitted collinearly when a positron of a Na source annihilates with an electron. A mm BaF crystal is mounted on the start PMT. The sample under study is mounted between the start tube and the slit in front of
TABLE I
PHYSICAL ANDOPTICAL PROPERTIES OFSOMESCINTILLATORS
the stop PMT. Between these two crystals, the Na source is placed. The BaF crystal detects one of the 511-keV photons, giving the start signal. The other 511-keV photon excites the crystal under study and some photons of the scintillation flash are detected by the “stop” PMT. With this method, the “start” moment is better defined than when the conventional method is employed, which avoids time fluctuations in its determination. Time-resolution experiments were carried out by using the
setup of the coincidence method. A mm fast
scintillating BaF crystal was coupled to the “start” XP2020Q PMT. The sample under study is mounted on the “stop” tube and the Na source is placed near the stop tube between the two crystals. The Na source emits two 511-keV -ray quanta resulting from positron annihilation within the source, in coincidence with a 1275-keV -ray. The time differences between the moments of detection of the coincident -ray quanta in the two PMT are accumulated in the time-resolution spectrum. Only events from -ray quanta with energy larger than about 500 keV were accepted. The time resolution is defined as the width (FWHM) of the time resolution peak. In all measurements Al discs with thickness of 3 mm were placed on both sides of the source. It appears that this improves significantly the time resolution.
III. RESULTS AND DISCUSSION
A. X-Ray-Excited Emission Experiments
The X-ray-excited luminescence spectra of RbGd Br :Ce with 0.02, 0.11, 0.88, 2.05, 4.1, and 9.8% cerium concentration are shown in Fig. 1 together with the spectra of NaI:Tl and Lu SiO :Ce crystals. The spectral shape is not corrected for the grating efficiency and the photodetector quantum efficiency. Each spectrum is normalized such that its integral over all wavelengths is equal to the value for the light output as found from pulse-height spectra recorded at 10- s shaping time (see Section III-C). The RbGd Br : Ce crystals show a Ce 5d–4f emission band between 350 and 550 nm with a maximum peaking at 420 nm (see Table I and Fig. 1), which is in the same wavelength range as the emission of NaI : Tl and Lu SiO : Ce. This broad band is composed of the two emissions from the lowest level of the 5d configuration to the two F , F multiplets of the 4f configuration. At very low concentration, 0.02% Ce, the broad emission band is composed of the two Ce emissions but probably also of
Fig. 1. X-ray-induced emission spectra of RbGd2Br7 doped with 0.02 (inset), 0.11, 0.88, 2.05, 4.1, and 9.8% Ce3+. The spectra of NaI : Tl and Lu2SiO5: Ce are also shown for comparison. They-axis has been calibrated using the light yields derived from the pulse height measurements. The star indicates the 4f7–4f7Gd3+ 6P7=2!8S7=2transition.
the self-trapped exciton (STE) luminescence peaking around 450 nm (see inset in Fig. 1). For other bromides, such as RbBr and CsBr, similar peaks attributed to STE have been observed in this spectral range [25]. For higher Ce concentrations, the 5d–4f emissions are dominant compared to the STE luminescence and increase with the cerium concentration (Fig. 1). The narrow emission band seen around 313 nm for all the RbGd Br : Ce compounds (indicated by a star in Fig. 1) is due to the 4f –4f Gd P S transition.
B. Scintillation Decay, Time Resolution
All the decay time measurements performed under -ray excitation on the RbGd Br : Ce crystals are gathered in Fig. 2. Fig. 2(a) shows the decays of RbGd Br : Ce for different Ce concentrations, together with those of Lu SiO : Ce in the first 1000-ns range studied by the conventional method (under Cs excitation). NaI : Tl and CsI : Tl data are also shown [20], [26]. The distortion near the arrow in Fig. 2(a) is caused by afterpulses in the XP2020Q tube. Fig. 2(b) represents long-time-scale decay spectra for only three cerium concentrations: 0.02, 4.1, and 9.8%. Fig. 2(c) shows the rising part of the decay curve for RbGd Br with 0.02% Ce obtained with the conventional ( Cs source) and coincidence ( Na source) methods.
Fig. 2(a) and (b) shows that the decay curves of RbGd Br : Ce are not single-exponential. The decay curves
(a)
(b)
(c)
Fig. 2. (a) Scintillation decay time spectra of (1) Lu2SiO5: Ce, RbGd2Br7: (2) 0.02% Ce, (3) 0.11% Ce, (4) 0.88% Ce, (5) 2.05% Ce, (6) 4.1% Ce, (7) 9.8% Ce, (8) NaI : Tl from [20], and (9) CsI : Tl from [26]. The distortion near the arrow is caused by after pulses in the XP2020Q tube. (b) Scintillation decay time spectra on a longer time scale for RbGd2Br7with 0.02, 4.1, and 9.8% Ce3+. (c) Leading edge of the decay curve for RbGd2Br7: 0.02% Ce obtained with (1) the coincidence and (2) the conventional method.
were fitted by assuming three exponential decay components. This assumption has no physical meaning but is used to characterize the decay curves. In Table II, the three decay time components are presented as well as their contribution to the total light yield. The last column of Table II shows the time, , in which 90% of the total scintillation output has been emitted. The first component (see Table II) decreases from 94 1 ns to 43 1 ns when the cerium concentration increases from 0.02 to 9.8%. This short component represents the major contribution to the total light yield (Table II). This contribution decreases in relative measure but increases in absolute measure with Ce concentration (see also Table III).
Fig. 3. Integrated light yield as function of time for Lu2SiO5: Ce, NaI : Tl, CsI : Tl, and RbGd2Br7with 0.02, 0.11, 0.88, and 9.8% Ce3+.
Another contribution, with a value around 450 ns, is also present for each concentration and represents between 20 and 40% of the total light yield for the RbGd Br samples with 0.88 to 9.8% Ce content. For the same concentrations, a long component appears Fig. 2(b) with a value around 20 to 25 s. This long component represents only 3–4% of the total light yield (Table II).
For comparison, the decay spectra of Lu SiO : Ce, NaI : Tl and CsI : Tl are also presented in Fig. 2(a). The Lu SiO : Ce spectrum is dominated by a very fast component of 40 ns [7]. For NaI : Tl, the decay spectrum is dominated by a 230-ns decay component [27] and for CsI : Tl, two components of 600 ns [28] (800 ns [12]) and 3.4 s [28] (6 s [12]) have been determined.
Fig. 2(c) represents the rising part of the decay curve of the RbGd Br compound doped with 0.02% cerium recorded with the two experimental methods. Using the conventional method with a Cs radioactive source, a rising slope of 100 ns over two decades is seen. This rising part is also seen for the other concentrations but over a shorter time range. This characteristic of the decay curve is not measured in the coincidence method, performed with a Na radioactive source. Contrary to what we reported in [20], this rising slope does not represent a genuine part of the scintillation pulse but is due to an experimental artifact. It is caused by the poor coupling of the quartz-sealed samples to the PMT window, resulting in a low number of photoelectrons present at the start and giving rise to a jitter in the starting moment. In the coincidence method, it is the BaF pulse which defines the starting moment. Because of the high initial scintillation intensity of the BaF crystal, the jitter in the starting moment is much smaller than the 1-ns TDC time resolution.
Fig. 3 shows the integral light yield as a function of time for some RbGd Br : Ce samples together with the Lu SiO : Ce, NaI : Tl and CsI : Tl compounds. As we see in this figure, the RbGd Br samples have roughly the same characteristics as the NaI : Tl compound and are between the Lu SiO : Ce matrix, which possesses the fastest decay curve, and the CsI : Tl matrix, which has the slowest decay curve. All these characteristics are well represented by the parameter (Table II), which is equal to 100, 760, between 890–1350, and 7700 ns for Lu SiO : Ce, NaI : Tl, RbGd Br : Ce, and CsI : Tl, respectively.
TABLE II
CHARACTERISTICCOMPONENTS OF THESCINTILLATIONDECAYTIMEUNDER662-keV -RAYEXCITATION AND THETIME 90%nsINWHICH90%
OF THETOTALLIGHTYIELDHASBEENEMITTED FORRbGd2Br7: Ce, COMPARED TONaI : Tl, CsI : TlANDLu2SiO5: Ce
TABLE III
LIGHTYIELDS OFRbGd2Br7: Ce, DERIVED FROMPULSEHEIGHTSPECTRA,COMPARED TONaI : Tl, CsI : TlANDLu2SiO5: Ce. FOR THE
RbGd2Br7COMPOUNDSCOUPLED TO THEXP2020QAND THER1791 PMT’s,THEDETECTIONEFFICIENCYIS0.180AND0.240, RESPECTIVELY
Increasing the Ce concentration in the bromide compounds has hardly any effect on the integrated light yield (Fig. 3). From 0.88 to 9.8% Ce, the curves are nearly overlapping, which means probably that above 0.88% Ce, the extra cerium has no important influence on the mechanisms responsible for the scintillation process.
Fig. 4 presents the comparison of time resolution spectra measured with 15 5 1 mm RbGd Br : 9.8% Ce, 10 10 2.5 mm Lu SiO : Ce, and 12 2 mm BaF crystals
for an energy threshold set at 511 keV. The measured values (FWHM) are 850 10 610 3, and 420 4 ps for RbGd Br : 9.8% Ce, Lu SiO : Ce, and BaF , respectively. They represent the combined time resolution of the crystal under study and the reference crystal BaF . To compare the crystals under study, the values have to be corrected for the contribution of the time resolution from the reference BaF crystal itself. After correction, we obtained 790 10, 530 3, and 300 4 ps for RbGd Br : 9.8% Ce, Lu SiO : Ce, and
Fig. 4. Comparison of the time spectra measured with the RbGd2Br7: 9.8% Ce, Lu2SiO5: Ce, and BaF2 crystals for22Na -rays and with the energy threshold set atE 511 keV. The inset in Fig. 4 gathers time-resolution spectra performed with the Lu2SiO5: Ce compound with and without Al foils around the22Na radioactive source.
BaF , respectively. For Lu SiO : Ce, a time resolution of 410 ps was reported in the literature by Ludziejewski et al. [29] using an XP2020Q PMT, a Co source, and an energy threshold at 100 keV. For NaI : Tl, 800 ps was reported by Moszynski et al. [30] with an XP2020 PMT, a Co source, and an energy threshold set at 100 keV. The RbGd Br : 9.8% Ce compound has a good time resolution in the same range as that of NaI : Tl and 1.5 times higher than that of Lu SiO : Ce, which makes this compound suitable for fast timing applications.
The inset in Fig. 4 shows time resolution spectra obtained with the Lu SiO : Ce compound with and without Al discs around the Na radioactive source. Without Al, the time resolution is enhanced from 530 to 610 3 ps. The discs block the positrons emitted from the source which otherwise may annihilate in either the crystals or the surrounding material. The poorer time resolution without Al discs is now attributed to a much longer positron lifetime than the lifetime of positrons annihilating in the source or in the Al discs. We estimate a lifetime of about 300 ps, possibly due to annihilations in Lu SiO : Ce.
C. Pulse-Height Experiments and Nonproportional Response Light yields derived from pulse height spectra, expressed in photons per megaelectronvolt of absorbed -ray energy, are compiled in Table III. Some results of Table III (indicated by star) are probably overestimated by 5–10%. This error is due to a malfunctioning of the manual pole-zero of the spectroscopic amplifier. Unfortunately, these measurements could only be performed once because the samples were exposed to air and thus a strong hydration of the surface occurred. The surface of the material became nontransparent and transformed into powder resulting in a lower light yield.
The light yield increases with increasing cerium content in the bromide compounds (Table III). RbGd Br : 9.8% Ce shows the highest light yield of 56 000 6000 ph/MeV, obtained with a Cs source, a Hamamatsu R1791 photo-multiplier, and using a shaping time of 10 s. This number is higher than the 43 000 ph/MeV of NaI : Tl [11]. Only CsI : Tl shows a higher light yield with 64 000 ph/MeV, reported by Valentine et al. in [12].
(a)
(b)
Fig. 5. Pulse-height spectra of RbGd2Br7: 9.8% Ce recorded with -rays. (a) 137Cs and (b) 241Am. A Hamamatsu R1791 PMT is used for these measurements. The fits of the photopeak are indicated by dashed lines. In (a), the 87Rb background is also shown; this spectrum has been recorded without the 137Cs radioactive source, in the same conditions as the above spectrum. The X-ray “escape” peak in (b) represents the contributions due to backscattering and some escape of the characteristic Gd L X-rays and of the characteristic Rb and Br X-rays.
For the RbGd Br : Ce compounds, most of the scintillation light is emitted within 0.5 s. Only an increase of 10–20% in light yield is obtained for a larger shaping time (Table III). These results are in total agreement with expectations based on the previous decay measurements.
In order to calculate the energy transfer efficiency in the RbGd Br : Ce compounds, it is necessary to know the number of thermalized e–h pairs per megaelectronvolt of absorbed -ray energy. For halide compounds, the energy needed for the creation of an e–h pair is assumed to be 2.5 times the bandgap energy [15]. For RbGd Br , we estimate from the low-temperature absorption spectrum a bandgap of 5.6 eV, which is in the range of values reported for other lanthanide bromides [31]. The number of created e–h pairs per mega-electronvolt of absorbed -quantum energy is thus 71 000 7000 e–h/MeV. This implies that 3, 11, 48, 66, 75, and 79% of the e–h pairs result in Ce luminescence for RbGd Br doped with 0.02, 0.11, 0.88, 2.05, 4.1, and 9.8% Ce, respectively. Therefore, the transfer efficiency to Ce and the luminescence efficiency of Ce must be close to unity for the higher Ce concentrations.
For three different concentrations: 0.11, 4.1, and 9.8%, pulse-height spectra were recorded for several -ray energies. Fig. 5(a) and (b) presents some of these spectra recorded for
the 9.8% doped sample with the Cs and Am radioactive sources. Due to the presence of the radioactive isotope Rb, scintillation pulses are generated intrinsically. The background spectrum shown in Fig. 5(a) has been recorded without a radioactive source under the same conditions as the spectrum with the Cs source. The Rb isotope, with a natural abun-dance of 27%, emits -particles with an end-point energy of 275 keV. Per a cubic centimeter of RbGd Br , the background represents nearly 350 counts per second.
The nonproportional responses of RbGd Br with 0.11, 4.1, and 9.8% Ce are shown Fig. 6(a)–(c). The curves represent the relative light yield normalized to the light yield obtained at 662 keV with the Cs source. The error bars are due only to the fitting of the photopeak. All experiments were performed under the same conditions. For comparison, the nonproportionality of Lu SiO : Ce [32], NaI : Tl [33] and CsI : Tl [33] are given in Fig. 6(d)–(f), respectively. More recent results on nonproportionality can be found in [34] for Lu SiO : Ce and NaI : Tl and in [35] for CsI : Tl. These new results confirm the previous one [32], [33].
The nonproportional responses are nearly the same for the three cerium concentrations (0.11, 2.05, 9.8%). These curves show an almost constant light yield per megaelectronvolt for -ray energies higher than 60 keV. The variation in the light yield is only 2–5% in the range between 60 keV and 1.332 MeV. Between 60 to 30 keV, small and barely significant drops of 2–3% are observed near the gadolinium K-edge (50.2 keV). Then, below 30 keV, each curve of Fig. 6(a)–(c) shows a decrease of 5%. In total, a variation of 15% in the light yield is measured from 17.4 keV to 1.332 MeV. For Lu SiO : Ce (in Fig. 6(d)), the response remains approximately constant down to 80 keV. It shows a small drop near the K-edge of Lu ions (63.3 keV) followed by a decrease to 55% near the Lu L absorption edge. For NaI : Tl and CsI : Tl, the nonproportional responses are nearly identical with a 15–20% gradually increasing light yield between 1 MeV and 50 keV. A drop of 10 and 5% is, respectively, observed for these two compounds near the iodine and cesium K-shell energies. Then, a decrease of 5 and 10% occurs between 25 and 10 keV for NaI : Tl and CsI : Tl, respectively. In total, the response of NaI : Tl and CsI : Tl shows a nonproportionality of 20% from 10 keV to 1 MeV. Recently, the nonproportional response of YAP : Ce crystals was studied [36] revealing a constant light output response from 14.4 keV to 1.275 MeV.
D. Energy Resolution
To determine the energy resolution of the RbGd Br : Ce compounds for different -ray energies, part of the photopeaks has been fitted by Gaussian curves. Some of the fits are shown in Fig. 5(a) and (b) by dashed lines. The photopeak, obtained with a Cs source [Fig. 5(a)], is accompanied by a satellite peak at lower energy due to escape of the characteristic K X-rays of gadolinium. In addition to the photopeaks obtained with -ray and X-rays coming from a Am source [Fig. 5(b)], a broadband peaking at 53 keV is observed. This band contains contributions of backscattering (at 48 keV) and the escape of characteristic L X-rays of Gd and
probably also the escape of characteristic X-rays of Rb and Br. Due to the poor resolution of this peak, all these contributions could not be separated.
With the R1791 photomultiplier, an energy resolution of 7.0 0.1% (FWHM over peak position) for the 662-keV full absorption peak has been observed for the RBGd Br : 0.11% Ce compound having a 5 2 1.5 mm size (Table IV). By increasing the Ce content in the bromide compounds, energy resolutions of 4.10 0.02% and 4.10 0.04% are reached for RbGd Br with 2.05 and 9.8% Ce concentrations, with a 7 4 2 mm and 15 5 1 mm size, respectively [see Fig. 5(a)]. Such measurements have been performed on quartz-sealed crystals. To our knowledge, this energy resolution has never been achieved with any other scintillator detectors before. The 4.1% value is considerably better than the 5.6% [37] and 7.5% [38] of NaI : Tl and Lu SiO : Ce crystals, respectively. The best energy resolution ever reported is 4.34% [39] for a CsI : Tl coupled to a silicon drift detector. A very good energy resolution of 4.38 0.11% has also been reported for YAP : Ce crystals using an XP2020Q PMT [36]. In an earlier paper [20], we reported an energy resolution of 3.8% for a RbGd Br compound coupled to an XP2020Q PMT, doped with 4.1% Ce and with a 16.7 7.6 1.15 mm size. The accuracy of this value was not too good due to a poor statistics in the pulse-height spectrum.
Usually, four separate contributions to are distinguished according to
(1) where is the contribution of the nonproportional response of the scintillator, is connected with inhomogeneities in the scintillation crystals, which can cause local variations in the scintillation light output. The transfer resolution is connected to fluctuations in the transfer efficiency, which represents the probability that a photon from the scintillator results in the arrival of a photoelectron at the first dynode, which subsequently undergoes the full multiplication in the PMT. A lot of factors determine this transfer efficiency. For example: 1) the wavelength of the photon and then the quantum efficiency of the PMT at this wavelength; 2) a nonuniform reflectivity of the reflecting covering of the PMT; and 3) nonuniform properties of the photocathode. is the photomultiplier resolution which can be written as [32], [40]
(2)
where is the fractional variance in the multiplication in the PMT, and is the photoelectron yield. For multi-stage linear focused photomultiplier tubes with an enhanced first dynode and a bialkali photocathode, is typically 0.08–0.2 [32]. The combined effects of and are usually named scintillator resolution and is defined by
(3) For an ideal scintillator, the three and resolutions
(a) (d)
(b) (e)
(c) (f)
Fig. 6. Nonproportional response of RbGd2Br7with (a) 0.11% Ce3+, (b) 2.1% Ce3+, and (c) 9.8% Ce3+. For comparison, Fig. 6(d)–(f) represents the nonproportional response of Lu2SiO5: Ce from [32], NaI : Tl from [33], and CsI : Tl from [33].
According to (2), can be calculated experimentally by the determination of the photoelectron yields and the value of . is determined from the width of the single electron pulse-height spectrum using the relation
(FWHM) . The single photoelectron peak
is due to photoelectrons which have experienced the full multiplication in the electron multiplier and thus the width of this peak is linked to fluctuations in this multiplication process, i.e., the fractional variance . For the XP2020Q
and R1791 PMT’s, one obtains and ,
respectively.
With the value obtained for the R1791 tube, the pho-tomultiplier resolution was calculated and the scintillator
resolution was obtained from . All the data
are given in Table IV. For comparison, we show also these values for NaI : Tl, CsI : Tl and Lu SiO : Ce. Fig. 7 represents the two resolutions and of the bromide compounds as a function of the inverse square root of the number of photoelectron for three different Ce contents: 0.11, 2.05, and 9.8%.
From Table IV and Fig. 7(a)–(c), we learn that when the -ray excitation energy decreases the energy resolution increases and becomes much larger than one would expect based on the statistical variation in the number of detected photoelectrons . Apparently there is a significant contribu-tions of . This effect seems to be identical for the different RbGd Br compounds and, therefore, reflects probably an intrinsic property of the RbGd Br matrix. For the compounds
TABLE IV
ENERGYRESOLUTIONRIN%OFRbGd2Br7: 0.02, 0.11, 0.88, 2.05, 4.1, AND9.8% Ce3+, COMPARED TONaI : Tl, CsI : Tl,ANDLu2SiO5: Ce. THEPHOTOMULTIPLIERRM AND THESCINTILLATORRESOLUTIONRS AREALSOPRESENTED
doped with 2.05 and 9.8% Ce, and for a given -ray excitation energy, the scintillator resolution is nearly identical.
As (3) shows, several parameters may explain the increase of the scintillator resolution when the -ray excitation energy decreases. First, can be influenced by which is linked to the nonproportional response of the studied com-pound. Indeed, for low -ray excitation energy, the RbGd Br compounds present a nonproportional response [Fig. 6(a)–(c)] that implies an increase in the nonproportional resolution and consequently an increase of the energy resolution observed in Fig. 7(a)–(c). These results appear to be confirmed by the nonproportionality study of the light output of YAP : Ce crystals versus energy [36]. In this study, the observed good proportionality of YAP : Ce to energy seems to be directly cor-related with the very good energy resolution of this scintillator. Second, can be due to an increase of the resolution . For a 662-keV photon, the absorption length in the bromide compounds is 2.8 cm. For this energy, the -rays are uniformly absorbed in the crystal bulk. For lower energies, the absorption length is considerably shorter: at 59.5 keV, 0.36 mm; at 32.1 keV, 0.15 mm. These low energetic photons are only absorbed in the surface layer of the crystal. If the scintillation or the light-collection properties at the surface are somewhat different from the bulk, this will result in large dispersion of the energy resolution. Therefore, for low energetic -rays, increases leading to an increase of the scintillator resolution and thus to an increase of the energy resolution .
IV. CONCLUSION
The study of the scintillator properties of RbGd Br com-pounds doped with cerium ions, under X-ray and -quanta
excitations, has shown very good results and this matrix presents a lot of advantages.
1) With a Hamamatsu R1791 photomultiplier, a high pho-ton yield of 56 000 6000 ph/MeV has been measured for the compound with 9.8% Ce content, under a 662-keV gamma-ray excitation with a shaping time of 10 s. This value is close to the theoretically maximal light yield of RbGd Br (71 000 ph/MeV) and, conse-quently, this matrix is a very efficient scintillator. The light output of this compound is higher than those of NaI : Tl and Lu SiO : Ce and approximately identical to that of CsI : Tl.
2) RbGd Br presents a good linearity in the energy response, the photon yield per megaelectronvolt being almost constant down to 20 keV with only 15% varia-tion. Its proportionality is better than the proportionality of NaI : Tl, CsI : Tl and Lu SiO : Ce.
3) This very important behavior may be one of the main causes for the other desirable scintillator characteristic: a very good energy resolution. With a Hamamatsu R1791 photomultiplier, two samples with 4.1 and 9.8% Ce content show a resolution of 4.1% which has never been obtained before.
4) The -ray excited decay curve shows a main fast exponential component of 43 1 ns for the RbGd Br : 9.8%Ce sample. About 55% of the total light yield, i.e., 31 000 ph/MeV, is emitted by this fast component. The time response of the bromide compounds is faster than for CsI : Tl, in the same range as for NaI : Tl, and slower compared to Lu SiO : Ce matrix.
(a)
(b)
(c)
Fig. 7. Energy resolutionR of RbGd2Br7with: (a) 0.11% Ce3+, (b) 2.1% Ce3+, and (c) 9.8% Ce3+as a function of the -ray excitation energy. The photomultiplier resolutionRM is also presented. The deviation ofR from RM is by definition the scintillator resolutionRS.
5) A time resolution of 790 10 ps is obtained for the RbGd Br : 9.8% Ce sample which is in the same range as that of NaI : Tl and 1.5 times higher than that of Lu SiO : Ce. This crystal can be easily used in coincidence experiments.
6) Other advantages of the RbGd Br matrix are its con-gruent melting at a rather low melting point of 590 C and a density higher than that of NaI : Tl and CsI : Tl, providing an attenuation coefficient for gamma-rays higher than that of NaI : Tl.
This scintillator has also drawbacks. The RbGd Br com-pound is hygroscopic, quite brittle, and cleaves easily. More-over, the presence of an intrinsic -continuum with an end point of 275 keV, due to the Rb isotope, can be a problem for application at low energies. Finally, until now, only small samples ( mm ) were studied and we do not know whether larger, homogeneous crystals can be grown of good quality, and if for these larger crystals all the good reported properties will remain.
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