- Email: [email protected]
Scintillation properties of Cs2LiLaBr6 (CLLB) crystals with varying Ce3+ concentration
Nuclear Instruments and Methods in Physics Research A 652 (2011) 268–270
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Scintillation properties of Cs2LiLaBr6 (CLLB) crystals with varying Ce3 + concentration Urmila Shirwadkar n, Jarek Glodo, Edgar V. van Loef, Rastgo Hawrami, Sharmishtha Mukhopadhyay, Alexei Churilov, William M. Higgins, Kanai S. Shah Radiation Monitoring Devices, Watertown, MA 02472, USA
a r t i c l e in f o
a b s t r a c t
Available online 7 September 2010
Investigations of Ce3 + -doped Cs2LiLaBr6 (CLLB) crystals show a systematic trend in their scintillation properties with varying Ce concentrations. The concentration studies provide input in the optimization of growth of the CLLB crystals. Scintillation properties viz. radioluminescence, energy resolution, light yield, decay times, and non-proportionality are discussed for samples from 0% to 20% Ce concentration. & 2010 Elsevier B.V. All rights reserved.
Keywords: Crystal growth Scintillation properties Elpasolites Cs2LiLaBr6 Ce concentration Light yield
1. Introduction Recent investigations of Ce3 + -doped Cs2LiLaBr6 (CLLB) scintillators (and other elpasolites, e.g. Cs2LiYCl6 and Cs2LiLaCl6) show very promising results for gamma ray and thermal neutron detection [1–4]. CLLB has a cubic elpasolite structure with a density of 4.2 g/cm3, and can be grown by melt-based processes. Crystals presented in this study were grown using two-zone vertical Bridgman furnaces. High quality CLLB crystals up to 1 in. in diameter and 2 in. in length have been grown. Examples of such crystals are shown in Fig. 1. The objective of this project is to study the scintillation properties of CLLB as a function of Ce concentration. The concentration studies will enable the optimization of cost-effective growth of high quality CLLB crystals. The following sections discuss the scintillation properties of CLLB crystals with Ce concentrations varying from 0 to 10 mol%.
2. Scintillation properties 2.1. Emission Optical emission spectra of CLLB crystals were recorded using X-ray radiation from a Philips X-ray generator operated at 40 KVp and 20 mA. The scintillation light was passed through a McPherson 0.2-m monochromator and detected with a C31034 photomultiplier tube (PMT). Fig. 2 (left) shows the normalized emission spectra for different CLLB samples with varying Ce n
Corresponding author. E-mail address: [email protected] (U. Shirwadkar).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.08.050
concentration. The undoped sample has a broad response, whereas the Ce-doped samples show two distinct peaks corresponding to the 5d-4f transitions of Ce3 + . The wavelengths corresponding to the two peaks are 390 and 420 nm. The emission peak at 420 nm shows a systematic increase in its intensity relative to that of the 390 nm peak as the amount of Ce is increased in the samples (Fig. 2, right). This is attributed to the absorption of 390 nm light, which is re-emitted proportionately at 420 nm with increasing Ce concentration in the material. The emission wavelength of Ce-doped CLLB samples matches well with the response function of PMTs and silicon-based photodiodes. 2.2. Light output Light yield measurements were performed by coupling the samples to a super bialkali R6233-100 PMT, operating at a voltage of 650 V. In order to estimate light yield, first, the peak position for thermally generated single photoelectrons in the PMT was measured. This position was assumed to be due to a single photon. By comparing the photopeak positions measured under gamma excitation to that of a single photon, light output of the CLLB crystal was estimated. A correction for quantum efficiency of the PMT for CLLB emission (33%) was included as well. Using this approach, we estimated the light output of CLLB to be around 50,000 photons/MeV. Relative light yield measurements were conducted with the samples placed in a quartz cup containing oil. The cup was wrapped with Teflon tape and covered with a highly reflective material to enhance the light output. The use of the cup was to maintain uniformity between measurements for each sample. The results show that light yield is relatively low for the
U. Shirwadkar et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 268–270
269
Fig. 1. 10-mm-diameter CLLB crystals grown using vertical Bridgman method (left, middle) and 1 in. diameter and at least 2 in. length crystal of CLLB in the ampoule (right).
Fig. 2. X-ray induced emission spectra measured for CLLB samples with varying Ce concentration (left). Ratio of the intensities of the 390 to 420 nm peak is plotted as a function of Ce concentration (right).
Fig. 3. Energy spectra measured under 137Cs excitation for selected CLLB samples, where the light yield is proportional to the photopeak position (left). The relative light yield as a function of Ce concentration (right).
samples containing 0% and 0.2% Ce and is significantly higher for the samples with 0.5% and higher Ce concentration. This trend is clearly observed in Fig. 3 (right), where the relative light yield is plotted as a function of Ce concentration. It should be noted that the average sample size was about a few mm. 2.3. Energy resolution and non-proportionality Using the same set-up described in Section 2.2, pulse height spectra were recorded for a CLLB:10% Ce sample under 662 keV g-ray excitation from a 137Cs source at room temperature. An excellent energy resolution of 2.9% has been achieved, which is more than two times better than the energy resolution of 7.4% obtained using a commercially available NaI(Tl) scintillator under identical operating conditions (see Fig. 4, left). The results are consistently good at 122 and 136 keV, which yield an energy resolution of 5.96% and 5.6%, respectively (Fig. 4, right). A spectrum was also measured under 60 keV gamma-ray excitation from an 241 Am source and a very good resolution of 8.3% has been achieved. 22 Na energy spectrum was also recorded with the same CLLB:10% Ce
sample and its energy resolution was 3.3% (FWHM) and 2.3% (FWHM), respectively, at 511 and 1275 keV. The results are amongst the best energy resolution values recorded for any scintillator-based system. High light yield and good energy resolution of CLLB also allow for the efficient detection of low energy photons, which is indicated in Fig. 4 (right) by the presence of a 14 keV gamma-ray peak. Using the above set-up, the non-proportionality of CLLB samples was tested under g-ray excitation using different radioactive isotopes covering a wide range of energies from 14.4 up to 1274 keV. As observed in Fig. 5 (left), deviation from linearity for the CLLB samples is less than 2% and is independent of doping concentration. CLLB is significantly more linear than LaBr3 at lower energies. 2.4. Decay times Scintillation decay time spectra were measured under 511 keV
g-ray excitation (22Na source). Decay curves were fitted with a multi-exponential decay function. The undoped sample shows a relatively slow decay time of 2300 ns. For samples with Ce3 +
270
U. Shirwadkar et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 268–270
Fig. 4. Energy spectra measured under 662 keV g-ray excitation (137Cs source) for a standard NaI scintillator and CLLB:10% Ce (left), and under 122 keV g-ray excitation (57Co source) for the same CLLB sample.
Fig. 5. Non-proportionality plot showing data for different CLLB samples, normalized to 662 keV g-ray (left). Decay time spectra for CLLB crystals, undoped as well as those with varying Ce3 + concentration (right).
doping, a 55 ns decay component was observed, arising due to Ce3 + luminescence (Fig. 5, right). Additionally, longer component(s) with Z270 ns time constants has also been observed. The relative contribution of the 55 ns component (in comparison with the longer ones) increases with increase in Ce3 + concentration.
3. Summary Scintillation properties of CLLB have been studied as a function of Ce concentration. Some of the outstanding properties of CLLB crystals with Ce concentrations of 2%, 5%, and 10% are high light output, excellent energy resolution of 2.9% at 662 keV gamma-ray energy (137Cs source), reasonably fast decay component of 55 ns, and extremely linear response over a wide range of
energies. These studies provide crucial input in optimizing Ce concentration levels to grow high quality CLLB crystals. References [1] J. Glodo, R. Hawrami, E.V.D. van Loef, W.M. Higgins, U. Shirwadkar, K.S. Shah, Dual Gamma Neutron detection with Cs2LiLaCl6, Proc. SPIE 7449 (2009) 74490E. ¨ ¨ [2] A. Bessiere, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H.U. Gudel, Luminescence and scintillation properties of Cs2LiYCl6:Ce for Y and neutron detection, Nucl. Instr. and Meth. A 537 (2005) 242. [3] J. Glodo, W.M. Higgins, E.V.D. Van Loef, K.S. Shah, Cs2LiYCl6 : Ce scintillator for nuclear monitoring applications, IEEE Trans. Nucl. Sci. 56 (3) (2009) 1257–1261. [4] E.V.D. Van Loef, J. Glodo, W.M. Higgins, K.S. Shah, Optical and scintillation properties of Cs2LiYCl6:Ce3+ and Cs2LiYCl6:Pr3+ crystals, IEEE Trans. Nucl. Sci. 52 (2005) 1819–1822.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Scintillation properties of Cs2LiLaBr6 (CLLB) crystals with varying Ce3 + concentration Urmila Shirwadkar n, Jarek Glodo, Edgar V. van Loef, Rastgo Hawrami, Sharmishtha Mukhopadhyay, Alexei Churilov, William M. Higgins, Kanai S. Shah Radiation Monitoring Devices, Watertown, MA 02472, USA
a r t i c l e in f o
a b s t r a c t
Available online 7 September 2010
Investigations of Ce3 + -doped Cs2LiLaBr6 (CLLB) crystals show a systematic trend in their scintillation properties with varying Ce concentrations. The concentration studies provide input in the optimization of growth of the CLLB crystals. Scintillation properties viz. radioluminescence, energy resolution, light yield, decay times, and non-proportionality are discussed for samples from 0% to 20% Ce concentration. & 2010 Elsevier B.V. All rights reserved.
Keywords: Crystal growth Scintillation properties Elpasolites Cs2LiLaBr6 Ce concentration Light yield
1. Introduction Recent investigations of Ce3 + -doped Cs2LiLaBr6 (CLLB) scintillators (and other elpasolites, e.g. Cs2LiYCl6 and Cs2LiLaCl6) show very promising results for gamma ray and thermal neutron detection [1–4]. CLLB has a cubic elpasolite structure with a density of 4.2 g/cm3, and can be grown by melt-based processes. Crystals presented in this study were grown using two-zone vertical Bridgman furnaces. High quality CLLB crystals up to 1 in. in diameter and 2 in. in length have been grown. Examples of such crystals are shown in Fig. 1. The objective of this project is to study the scintillation properties of CLLB as a function of Ce concentration. The concentration studies will enable the optimization of cost-effective growth of high quality CLLB crystals. The following sections discuss the scintillation properties of CLLB crystals with Ce concentrations varying from 0 to 10 mol%.
2. Scintillation properties 2.1. Emission Optical emission spectra of CLLB crystals were recorded using X-ray radiation from a Philips X-ray generator operated at 40 KVp and 20 mA. The scintillation light was passed through a McPherson 0.2-m monochromator and detected with a C31034 photomultiplier tube (PMT). Fig. 2 (left) shows the normalized emission spectra for different CLLB samples with varying Ce n
Corresponding author. E-mail address: [email protected] (U. Shirwadkar).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.08.050
concentration. The undoped sample has a broad response, whereas the Ce-doped samples show two distinct peaks corresponding to the 5d-4f transitions of Ce3 + . The wavelengths corresponding to the two peaks are 390 and 420 nm. The emission peak at 420 nm shows a systematic increase in its intensity relative to that of the 390 nm peak as the amount of Ce is increased in the samples (Fig. 2, right). This is attributed to the absorption of 390 nm light, which is re-emitted proportionately at 420 nm with increasing Ce concentration in the material. The emission wavelength of Ce-doped CLLB samples matches well with the response function of PMTs and silicon-based photodiodes. 2.2. Light output Light yield measurements were performed by coupling the samples to a super bialkali R6233-100 PMT, operating at a voltage of 650 V. In order to estimate light yield, first, the peak position for thermally generated single photoelectrons in the PMT was measured. This position was assumed to be due to a single photon. By comparing the photopeak positions measured under gamma excitation to that of a single photon, light output of the CLLB crystal was estimated. A correction for quantum efficiency of the PMT for CLLB emission (33%) was included as well. Using this approach, we estimated the light output of CLLB to be around 50,000 photons/MeV. Relative light yield measurements were conducted with the samples placed in a quartz cup containing oil. The cup was wrapped with Teflon tape and covered with a highly reflective material to enhance the light output. The use of the cup was to maintain uniformity between measurements for each sample. The results show that light yield is relatively low for the
U. Shirwadkar et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 268–270
269
Fig. 1. 10-mm-diameter CLLB crystals grown using vertical Bridgman method (left, middle) and 1 in. diameter and at least 2 in. length crystal of CLLB in the ampoule (right).
Fig. 2. X-ray induced emission spectra measured for CLLB samples with varying Ce concentration (left). Ratio of the intensities of the 390 to 420 nm peak is plotted as a function of Ce concentration (right).
Fig. 3. Energy spectra measured under 137Cs excitation for selected CLLB samples, where the light yield is proportional to the photopeak position (left). The relative light yield as a function of Ce concentration (right).
samples containing 0% and 0.2% Ce and is significantly higher for the samples with 0.5% and higher Ce concentration. This trend is clearly observed in Fig. 3 (right), where the relative light yield is plotted as a function of Ce concentration. It should be noted that the average sample size was about a few mm. 2.3. Energy resolution and non-proportionality Using the same set-up described in Section 2.2, pulse height spectra were recorded for a CLLB:10% Ce sample under 662 keV g-ray excitation from a 137Cs source at room temperature. An excellent energy resolution of 2.9% has been achieved, which is more than two times better than the energy resolution of 7.4% obtained using a commercially available NaI(Tl) scintillator under identical operating conditions (see Fig. 4, left). The results are consistently good at 122 and 136 keV, which yield an energy resolution of 5.96% and 5.6%, respectively (Fig. 4, right). A spectrum was also measured under 60 keV gamma-ray excitation from an 241 Am source and a very good resolution of 8.3% has been achieved. 22 Na energy spectrum was also recorded with the same CLLB:10% Ce
sample and its energy resolution was 3.3% (FWHM) and 2.3% (FWHM), respectively, at 511 and 1275 keV. The results are amongst the best energy resolution values recorded for any scintillator-based system. High light yield and good energy resolution of CLLB also allow for the efficient detection of low energy photons, which is indicated in Fig. 4 (right) by the presence of a 14 keV gamma-ray peak. Using the above set-up, the non-proportionality of CLLB samples was tested under g-ray excitation using different radioactive isotopes covering a wide range of energies from 14.4 up to 1274 keV. As observed in Fig. 5 (left), deviation from linearity for the CLLB samples is less than 2% and is independent of doping concentration. CLLB is significantly more linear than LaBr3 at lower energies. 2.4. Decay times Scintillation decay time spectra were measured under 511 keV
g-ray excitation (22Na source). Decay curves were fitted with a multi-exponential decay function. The undoped sample shows a relatively slow decay time of 2300 ns. For samples with Ce3 +
270
U. Shirwadkar et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 268–270
Fig. 4. Energy spectra measured under 662 keV g-ray excitation (137Cs source) for a standard NaI scintillator and CLLB:10% Ce (left), and under 122 keV g-ray excitation (57Co source) for the same CLLB sample.
Fig. 5. Non-proportionality plot showing data for different CLLB samples, normalized to 662 keV g-ray (left). Decay time spectra for CLLB crystals, undoped as well as those with varying Ce3 + concentration (right).
doping, a 55 ns decay component was observed, arising due to Ce3 + luminescence (Fig. 5, right). Additionally, longer component(s) with Z270 ns time constants has also been observed. The relative contribution of the 55 ns component (in comparison with the longer ones) increases with increase in Ce3 + concentration.
3. Summary Scintillation properties of CLLB have been studied as a function of Ce concentration. Some of the outstanding properties of CLLB crystals with Ce concentrations of 2%, 5%, and 10% are high light output, excellent energy resolution of 2.9% at 662 keV gamma-ray energy (137Cs source), reasonably fast decay component of 55 ns, and extremely linear response over a wide range of
energies. These studies provide crucial input in optimizing Ce concentration levels to grow high quality CLLB crystals. References [1] J. Glodo, R. Hawrami, E.V.D. van Loef, W.M. Higgins, U. Shirwadkar, K.S. Shah, Dual Gamma Neutron detection with Cs2LiLaCl6, Proc. SPIE 7449 (2009) 74490E. ¨ ¨ [2] A. Bessiere, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H.U. Gudel, Luminescence and scintillation properties of Cs2LiYCl6:Ce for Y and neutron detection, Nucl. Instr. and Meth. A 537 (2005) 242. [3] J. Glodo, W.M. Higgins, E.V.D. Van Loef, K.S. Shah, Cs2LiYCl6 : Ce scintillator for nuclear monitoring applications, IEEE Trans. Nucl. Sci. 56 (3) (2009) 1257–1261. [4] E.V.D. Van Loef, J. Glodo, W.M. Higgins, K.S. Shah, Optical and scintillation properties of Cs2LiYCl6:Ce3+ and Cs2LiYCl6:Pr3+ crystals, IEEE Trans. Nucl. Sci. 52 (2005) 1819–1822.