- Email: [email protected]
Characterization of LFS-3 scintillator in comparison with LSO
Nuclear Instruments and Methods in Physics Research A 652 (2011) 226–230
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Characterization of LFS-3 scintillator in comparison with LSO M. Grodzicka a,n, M. Moszyn´ski a, T. Szcz˛es´niak a, A. Syntfeld-Kaz˙uch a, Ł. S´widerski a, A.F. Zerrouk b, J. Owczarczyk c Soltan Institute for Nuclear Studies, PL 05-400 Otwock-S´wierk, Poland Zecotek Photonics Inc; 20 Science Park Road, #01-23/25, Teletech Park, Science Park II, Singapore 117674, Singapore c Compart Medical Systems, PL 04-703 Warsaw, Poz˙aryskiego 28, Poland a
b
a r t i c l e i n f o
a b s t r a c t
Available online 25 December 2010
Performance of Lutetium Fine Silicate (LFS-3) scintillator in gamma-ray spectrometry has been investigated in comparison with the well known LSO crystal. The tests covered measurements of light output in terms of number of photoelectrons, energy resolution and non-proportionality. Time resolution was measured in coincidence experiments with 511 keV annihilation quanta from a 22Na gamma source. Decay time constants of the light pulse were calculated on the basis of timing spectra obtained using Thomas–Bollinger single photon method. Afterglow was measured about 30 ms after the crystal was irradiated by a strong 13.9 GBq 241Am source. Improvement in the energy resolution for 662 keV g-rays from 137Cs, shorter decay time constants and better time resolution were observed in case of LFS-3, when compared with LSO. For LFS-3 energy resolution for 662 keV from 137Cs, decay time and time resolution were equal to about 7.66 7 0.23%, 40.5 7 1.2 ns and 161 7 5 ps, respectively, whereas for LSO the same parameters were equal to 8.13 70.23%, 43.9 7 1.3 ns and 173 7 5 ps, respectively. The study showed that LFS-3 crystal is an excellent substitute for LSO crystal. & 2010 Elsevier B.V. All rights reserved.
Keywords: LFS-3 scintillator LSO crystal Non-proportionality Number of photoelectrons Energy resolution Time resolution Decay time constants Afterglow
1. Introduction Performance of positron emission tomography (PET) scanners strongly depends on the quality of scintillators in the detector modules. Application of LSO scintillator [1] with a high light output of about 30,000 photons/MeV and a fast light pulse with the decay time constant of 40 ns improved dramatically the performance of a new generation of PET scanners. It was possible due to a much better statistical accuracy of the LSO signal, a much better coincidence time resolution and high counting rate capabilities of modern LSO-based block detectors. In the last years, a number of other scintillators were developed like LYSO, LGSO, LuAG:Pr and LFS-3. The LFS-3 (Lutetium Fine Silicate) has been developed by the Zecotek Photonics Inc and its composition is covered by patents worldwide (US patent no. 7,132,060). The crystal is characterized by a high density of 7.34 g/cm3 and wavelength emission spectrum peak at 435 nm. Its good properties for PET detectors were reported in Ref. [2]. The Zecotek Photonics offers different versions of LFS scintillator. At present available for sale are marked as LFS-3 and LFS-7. They have different densities and maximum emission peaks. Zecotek provided our laboratory with three samples of LFS-3 crystal. Our laboratory was not informed about their chemical composition.
The aim of the present study is an evaluation of the properties of LFS-3 in comparison with LSO in gamma spectrometry. The tests covered measurements of light output in terms of number of photoelectrons [3,4], energy resolution and non-proportionality. Also timing characteristics and an intensity of afterglow of the LFS-3 crystals were studied.
2. Experimental details 2.1. Scintillators and photomultipliers Two samples of LFS-3 crystals with 10 10 5 mm3 size (LFS-3 no. 1 and no. 2) and a finger-like pixel crystal with 3 3 10 mm3 size (LFS-3 no. 3) were tested. For comparison the 10 10 5 mm3 selected LSO crystal was used. This selected crystal was put at our disposal by Chuck Melcher in 2005 to use in fast timing and it is characterized by a high light output and a very good time resolution. All the tested crystals were polished on all surfaces and coated by Teflon tape. The measurements were done with a Photonis XP20D0 no. 2025 photomultiplier (PMT) characterized by a high blue sensitivity of 13.2 mA/lmF. Main parameters of the PMT are presented in Table 1. 2.2. Experimental methods
n
Corresponding author. E-mail address: [email protected] (M. Grodzicka).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.12.094
The number of photoelectrons per unit energy (phe/MeV) was measured for all the crystals using the Bertolaccini method [3,4],
M. Grodzicka et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 226–230
which compares the peak position from single photoelectrons (which determines the gain of the photomultiplier) with the position of the 662 keV full energy peak from a Cs-137 g-source. The energy resolution and peak position were determined by the Gaussian fit to the full energy peaks using procedures included in the Multi Channel Analyzer (Tukan 8k) [5]. The Gaussian fit was used also in case of analysis of complex double peaks characteristic for K X-rays and those exhibiting an escape peak. The non-proportionality of the light yield was defined here as the photoelectron yield measured at specific gamma ray energy relative to the photoelectron yield at 662 keV gamma peak and it was measured for the energy range of 14.4 up to 1.332 MeV.
Table 1 Main parameters of the used photomultiplier. XP20D0 no. 2025 Diameter Photocathode Window Blue sensitivity (mA/lmF)a White sensitivity (mA/lm) Time jitter (ps) Screening grid No. of dynodes a
52 mm Bialkali Lime glass 13.2 150 610 7 30 Yes 8
F– where F stands for filtered for blue light.
Fig. 1. Slow–fast arrangement for timing measurements. In the fast part, related to the anode signal, the time spectrum of the response difference of the detectors is taken. In the slow part, formatted using dynode signals, the gate is generated to choose the energy range of interest.
227
The following radioactive sources were used in the tests of energy resolution and non-proportionality: 57Co (14.4, 122.1 keV), 93 Mo (16.6 K X-rays), 109Cd (22.1 K X-rays, 88 keV), 133Ba (30.9 K X-rays, 80.9 keV), 137Cs (32.1 K X-rays, 661.6 keV), 145Pm (37.2 K X-rays), 241Am (59.5 keV), 51Cr (320.1 keV), 22Na (511, 1274.5 keV), 207 Bi (569.7, 1063.7 keV), 54Mn (834.8 keV) and 60Co (1173.2, 1332.5 keV). The time resolution was measured in coincidence experiments with 511 keV annihilation quanta from a 22Na source. As the reference detector (in the start channel) a truncated cone, 25 and 20 mm in diameter and 15 mm high BaF2 crystal was used coupled to the XP20Y0Q/DA PMT. Its time resolution for 22Na was equal to 143 75 ps [6]. The decay time constant measurements were made using a Thomas–Bollinger single photon method [7,8]. A 137Cs g-source was used to excite a scintillator. Single photons were detected by a very fast photomultiplier R5320 from Hamamatsu, which is characterized by time jitter of 140 ps. Each tested crystal was wrapped with Teflon but only on the sides leaving one surface opened to the Hamamatsu PMT. Such configuration assured detection of single photons from a scintillator induced by a gamma source. The afterglow was measured at room temperature following the method of Ref. [9]. Before the measurement each crystal was left to recover natural level of background spectrum from exposure to room light. Next, each measured crystal was irradiated by a 13.9 GBq 241Am source for about 1830 s and then an electromagnetic shutter was closed to block radiation from the source. The closing time of the shutter was equal to about 30 ms. Each afterglow spectrum was measured for about 9 h with MCS (Multichannel Scalers) with a dwell time set to 5 s. Additional measurements of the afterglow were made at 10 ms dwell time to estimate the shortest decay time components of the afterglow [9]. The detailed diagram of the experimental set-up is presented in Fig. 2.
Fig. 2. Block diagram of the set-up used for measurements of afterglow.
228
M. Grodzicka et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 226–230
2.3. Electronics Fig. 1 presents the block diagram of the experimental set-up used in timing experiments. During of the decay time constant measurements the energy window was set at the single photoelectron peak in the Hamamatsu R5320 and at the 662 keV full energy peak in XP20D0 coupled with the tested crystal. In case of the time resolution measurements the energy window was set at the 511 keV full energy peak in both photomultiplier XP20D0 and reference XP2020Q. A block diagram of experimental set-up used for measurements of afterglow is presented in Fig. 2. During the measurement of afterglow, a signal from the PMT anode, corresponding to single photoelectrons was used. In the case of afterglow additional Fast Spectroscopy Amplifier, Canbera 2024, was used working with a lower gain to observe well 59.5 keV gamma ray. The threshold was placed on its ICR (Incoming Count Rate) output signal at above 20 keV. Dual counter and timer, Ortec 994, provided the average number of 59.5 keV photons absorbed in the crystal per 1 s. The spectra were recorded by a PC-based multichannel analyzer (MCA) and multichannel scaler (MCS), Tukan8k [5].
equal to about 7800 phe/MeV and the light output is close to 30,000 ph/MeV. A comparison of the non-proportionality curves for all the tested crystals is shown in Fig. 3. Non-proportionality characteristic are common for all the tested crystal. The results of the measured number of photoelectrons and nonproportionality curves did not show differences between LFS-3 and LSO.
Table 3 Energy resolution of tested crystals for 662 keV g-rays, as measured with the XP 20D0 PMT. Crystal
Size (mm3)
Energy resolution
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
10 10 5 10 10 5 10 10 5 3 3 10
8.13% 7 0.26 7.66% 7 0.23 7.61% 7 0.23 7.56% 7 0.23
3. Results and discussion 3.1. Number of photoelectrons and non-proportionality The number of photoelectrons was measured for three LFS-3 samples and one LSO sample. All the results of the photoelectron number for the tasted crystals are presented in Table 2. The value of the number of photoelectrons per unit energy (phe/MeV) for all the tested crystal, within the margin of errors, is Table 2 Number of the photoelectrons of tested crystals for 662 keV g-rays measured with the XP 20D0 PMT and the light output calculated in relation to the LSO 2005a. Crystal
Size (mm3)
N phe/Mev (phe/Mev)
Light output (ph/MeV)
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
10 10 5 10 10 5 10 10 5 3 3 10
7760 7 230 7720 7 230 7850 7 230 8160 7 230
28,8007 1500a 28,7007 1500 29,1007 1500 20,3007 1500
a
Fig. 4. Measured energy resolution (DE/E) of all the tested crystals versus g-rays energy (shaping time¼ 0.25 ms). Error bars are within the size of the points.
See Ref. [10]
Fig. 3. Comparison of non-proportionality characteristics of the tested crystals.
Fig. 5. Measured (DE/E) energy resolution of tested crystal about size 10 10 5 mm3 versus g-rays energy (shaping time¼ 0.25 ms). Error bars are within the size of the points.
M. Grodzicka et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 226–230
229
3.2. Energy resolution Energy resolution was determined in the way described in Section 2. The results for the tested crystals at 662 keV g- rays from a 137Cs are presented in Table 3. Energy resolution for 662 keV g-rays in case of all LFS-3 crystals was approximately the same, irrespective of the size of the tested sample, and was equal to about 7.66 70.23%. While for LSO sample it was equal to about 8.1370.23%. Radioactive sources that were used for measurements of energy resolution are presented in Section 2. The results for all the tested crystals are presented in Fig. 4. In order to better show differences between LFS-3 crystal and LSO crystal in Fig. 5, only the results of the crystals with size 10 10 5 mm3 are presented. In the case of LFS-3 no. 1 and no. 2 crystals, energy resolution in high g-rays energy range was slightly improved comparing to LSO crystal. Fig. 7. Time spectra for 511 keV from 22Na g-source measured with the tested crystals coupled to the Photonis XP20D0 no. 2025 photomultiplier.
3.3. Decay time constants and time resolution The decay time constant of the light pulses was measured using the Thomas–Bollinger single photon method. Experimental set-up is described in Fig.1b in Section 2. Example of the decay time spectrum for LFS-3 no. 1 together with fits of single exponential decay is presented in Fig. 6. All the results of the analysis of the decay time constants measured with the tested crystals are collected in Table 4. Decay time constants for the samples of LFS-3 no. 1 and no. 2 were equal to 40.5 and 40.9 ns, respectively, whereas for the LSO sample decay time was equal to 43.9 ns. The observed difference is within the known spread for LSO crystals. Time resolution was measured using experimental set-up described in Fig. 1a in Section 2. Examples of the timing spectra for LSF-3 no. 1 and LSO crystals are presented in Fig. 7.
Table 5 Time resolution measured with tested crystal coupled to the Photonis XP20D0 no. 2025 photomultiplier. Crystal
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
Time resolution at FWHM, Dt (ps) Measured
Correcteda
224 77 216 77 215 77 214 77
173 7 5 161 7 5 161 7 5 159 7 5
pffiffiffiffi pffiffiffiffiffiffiffiffiffiffi
N phe for 511 keV
pffiffiffiffiffiffiffiffiffi ðps pheÞ103
39657118 39447118 4011 7120 4069 7125
10.97 0.5 10.17 0.5 10.27 0.5 10.17 0.5
Dt N = ENF
a Corrected for the contribution of the BaF2 reference detector of 143 7 4 ps. The largest differences between the measured crystals were on the level of afterglow. The small afterglow was observed in case of the LSO crystal.
Fig. 6. Light pulse shape of the LFS-3 no. 1 crystal. Fig. 8. Afterglow of the tested scintillators measured for dwell time¼ 5 s in the MCS. The crystals were excited with g-rays from a 13.9 GBq 241Am source. Background level was subtracted for all the presented spectra.
Table 4 Decay time of the tested crystals. Crystal
Size (mm3)
Decay time constant (ns)
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
10 10 5 10 10 5 10 10 5 3 3 10
43.9 71.3 40.5 71.2 40.9 71.2 39.7 71.2
All the results of the analysis of the time resolution spectra measured with the tested crystals are collected in Table 5. The measured time resolution is presented in the second column. Then, the corrected values for the contribution of the reference BaF2 detector are collected in the third column. Next column is the number of photoelectrons for 511 keV full energy peak. The last
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Table 6 Integrated afterglow, effective decay time, dose of energy for afterglow and total afterglow for the tested crystals. Crystal
Size (mm3)
seff (s)
Integrated afterglow (phe)
D0 (MeV)
Atot (phe/MeV)
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
10 10 5 10 10 5 10 10 5 3 3 10
1638 7100 5612 7340 5575 7330 6794 7410
1.10 109 3.87 109 4.31 109 6.11 109
6.59 106 5.82 106 5.02 106 1.28 106
1707 10 6507 30 8607 40 4740 7 240
column shows the time resolution normalized to the number of photoelectrons and excess noise factor (ENF), following Ref. [6]. Time resolution is equal to 161 ps for LFS-3 no. 1 and no. 2 crystals, whereas for the LSO crystal is equal to 173 ps. A better time resolution of LFS-3, in comparison with LSO, is probably related to the faster decay times of the tested crystals.
4. Conclusions
3.4. Afterglow The Afterglow was measured using experimental set-up described in Fig. 2 in Section 2. Fig. 8. shows the afterglow decays. The afterglow decay curves were fitted with multi-exponential functions. The best fits for LSO and LFS-3 were obtained with twoexponential function. The results of the analysis of the afterglow decay spectra measured with the tested crystals are collected in Table 6. The effective decay time was calculated as a weight value of decay times and is presented in the second column. Then, integrated afterglow is collected in the third column. Dose of the energy (in MeV), D0, which is converted to afterglow during the X-ray irradiation is presented in the fourth column. It was estimated according to Eq. (1), following Ref. [9]: D0 ¼
PEg NP
ltot
ð1eltot tirr Þ
The afterglow originates, to a large extent, from defects in the crystal structure reflected in deep and shallow traps [11,12]. Thus, the number of defects can be observed randomly in different samples.
ð1Þ
where P, equal to about 1 105 ph/s, is the average count rate of 0.0595 MeV gamma quanta from 241Am source and NP is the nonproportionality of given crystal at 59,5 keV, ltot (s 1) is the total decay constant of afterglow (l ¼1/t) and tirr ¼1830 s is the irradiation time. The last column shows the total afterglow per MeV, Atot, i.e. integrated afterglow normalized to the dose of energy (in MeV) for afterglow. The details of the calculation of individual variables are included in work [9]. The lowest intensity of the total afterglow, equal to about 170 phe/MeV, was observed for the selected LSO, while for the LFS-3 no. 1 and no. 2 crystals, it was equal to 650 and 860 phe/MeV, respectively. All the above numbers are comparable to the lowest values reported in Ref. [9].
The study showed that LFS-3 crystal is an excellent substitute for LSO crystal. Its light output, energy resolution and time resolution are systematically slightly better than those measured with the LSO crystal. The largest differences between the measured crystals were observed on the level of afterglow. The smallest afterglow was observed in case of the LSO crystal.
Acknowledgements This study was supported in part by the International Atomic Energy Agency, Research Contract no. 14360, the Polish Committee for Scientific Research, grant no. 657/W-IAEA/2010/0 and the EU Structural Funds Project no. POIG.01.01.02-14-012/08-00.
References [1] C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. NS-39 (1992) 502. [2] T.K. Lewellen, et al., IEEE Nucl. Sci. Symp. Conference Record, vol. 5, 2004, pp. 2915–2918. [3] M. Bertolaccini, S. Cova, C. Bussolati , in: Proceedings of the Nuclear Electronics Symposium, Versalles, France, 1968. [4] M. Moszyn´ski, et al., IEEE Trans. Nucl. Sci. NS-44 (1997) 1052. [5] Z. Guzik, et al., IEEE Trans. Nucl. Sci. NS-53 (2006) 231. [6] T. Szcz˛es´niak, et al., IEEE Trans. Nucl. Sci. NS-56 (2009) 173. [7] L.M. Bollinger, G.E. Thomas, Rev. Sci. Instr. 32 (1961) 1044. [8] M. Moszyn´ski, B. Bengtson, Nucl. Instr. and Meth. 142 (1977). [9] A. Syntfeld-Kaz˙uch, et al., IEEE Trans. Nucl. Sci. NS-56 (2009) 2972. [10] M. Moszyn´ski, et al., IEEE Trans. Nucl. Sci., NS-57 (2010) 2886. [11] R.H. Bartram, et al., J. Lumin. 75 (1997) 183. [12] P. Dorenbos, et al., J. Phys.: Condens. Matter 6 (1994) 4180.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Characterization of LFS-3 scintillator in comparison with LSO M. Grodzicka a,n, M. Moszyn´ski a, T. Szcz˛es´niak a, A. Syntfeld-Kaz˙uch a, Ł. S´widerski a, A.F. Zerrouk b, J. Owczarczyk c Soltan Institute for Nuclear Studies, PL 05-400 Otwock-S´wierk, Poland Zecotek Photonics Inc; 20 Science Park Road, #01-23/25, Teletech Park, Science Park II, Singapore 117674, Singapore c Compart Medical Systems, PL 04-703 Warsaw, Poz˙aryskiego 28, Poland a
b
a r t i c l e i n f o
a b s t r a c t
Available online 25 December 2010
Performance of Lutetium Fine Silicate (LFS-3) scintillator in gamma-ray spectrometry has been investigated in comparison with the well known LSO crystal. The tests covered measurements of light output in terms of number of photoelectrons, energy resolution and non-proportionality. Time resolution was measured in coincidence experiments with 511 keV annihilation quanta from a 22Na gamma source. Decay time constants of the light pulse were calculated on the basis of timing spectra obtained using Thomas–Bollinger single photon method. Afterglow was measured about 30 ms after the crystal was irradiated by a strong 13.9 GBq 241Am source. Improvement in the energy resolution for 662 keV g-rays from 137Cs, shorter decay time constants and better time resolution were observed in case of LFS-3, when compared with LSO. For LFS-3 energy resolution for 662 keV from 137Cs, decay time and time resolution were equal to about 7.66 7 0.23%, 40.5 7 1.2 ns and 161 7 5 ps, respectively, whereas for LSO the same parameters were equal to 8.13 70.23%, 43.9 7 1.3 ns and 173 7 5 ps, respectively. The study showed that LFS-3 crystal is an excellent substitute for LSO crystal. & 2010 Elsevier B.V. All rights reserved.
Keywords: LFS-3 scintillator LSO crystal Non-proportionality Number of photoelectrons Energy resolution Time resolution Decay time constants Afterglow
1. Introduction Performance of positron emission tomography (PET) scanners strongly depends on the quality of scintillators in the detector modules. Application of LSO scintillator [1] with a high light output of about 30,000 photons/MeV and a fast light pulse with the decay time constant of 40 ns improved dramatically the performance of a new generation of PET scanners. It was possible due to a much better statistical accuracy of the LSO signal, a much better coincidence time resolution and high counting rate capabilities of modern LSO-based block detectors. In the last years, a number of other scintillators were developed like LYSO, LGSO, LuAG:Pr and LFS-3. The LFS-3 (Lutetium Fine Silicate) has been developed by the Zecotek Photonics Inc and its composition is covered by patents worldwide (US patent no. 7,132,060). The crystal is characterized by a high density of 7.34 g/cm3 and wavelength emission spectrum peak at 435 nm. Its good properties for PET detectors were reported in Ref. [2]. The Zecotek Photonics offers different versions of LFS scintillator. At present available for sale are marked as LFS-3 and LFS-7. They have different densities and maximum emission peaks. Zecotek provided our laboratory with three samples of LFS-3 crystal. Our laboratory was not informed about their chemical composition.
The aim of the present study is an evaluation of the properties of LFS-3 in comparison with LSO in gamma spectrometry. The tests covered measurements of light output in terms of number of photoelectrons [3,4], energy resolution and non-proportionality. Also timing characteristics and an intensity of afterglow of the LFS-3 crystals were studied.
2. Experimental details 2.1. Scintillators and photomultipliers Two samples of LFS-3 crystals with 10 10 5 mm3 size (LFS-3 no. 1 and no. 2) and a finger-like pixel crystal with 3 3 10 mm3 size (LFS-3 no. 3) were tested. For comparison the 10 10 5 mm3 selected LSO crystal was used. This selected crystal was put at our disposal by Chuck Melcher in 2005 to use in fast timing and it is characterized by a high light output and a very good time resolution. All the tested crystals were polished on all surfaces and coated by Teflon tape. The measurements were done with a Photonis XP20D0 no. 2025 photomultiplier (PMT) characterized by a high blue sensitivity of 13.2 mA/lmF. Main parameters of the PMT are presented in Table 1. 2.2. Experimental methods
n
Corresponding author. E-mail address: [email protected] (M. Grodzicka).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.12.094
The number of photoelectrons per unit energy (phe/MeV) was measured for all the crystals using the Bertolaccini method [3,4],
M. Grodzicka et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 226–230
which compares the peak position from single photoelectrons (which determines the gain of the photomultiplier) with the position of the 662 keV full energy peak from a Cs-137 g-source. The energy resolution and peak position were determined by the Gaussian fit to the full energy peaks using procedures included in the Multi Channel Analyzer (Tukan 8k) [5]. The Gaussian fit was used also in case of analysis of complex double peaks characteristic for K X-rays and those exhibiting an escape peak. The non-proportionality of the light yield was defined here as the photoelectron yield measured at specific gamma ray energy relative to the photoelectron yield at 662 keV gamma peak and it was measured for the energy range of 14.4 up to 1.332 MeV.
Table 1 Main parameters of the used photomultiplier. XP20D0 no. 2025 Diameter Photocathode Window Blue sensitivity (mA/lmF)a White sensitivity (mA/lm) Time jitter (ps) Screening grid No. of dynodes a
52 mm Bialkali Lime glass 13.2 150 610 7 30 Yes 8
F– where F stands for filtered for blue light.
Fig. 1. Slow–fast arrangement for timing measurements. In the fast part, related to the anode signal, the time spectrum of the response difference of the detectors is taken. In the slow part, formatted using dynode signals, the gate is generated to choose the energy range of interest.
227
The following radioactive sources were used in the tests of energy resolution and non-proportionality: 57Co (14.4, 122.1 keV), 93 Mo (16.6 K X-rays), 109Cd (22.1 K X-rays, 88 keV), 133Ba (30.9 K X-rays, 80.9 keV), 137Cs (32.1 K X-rays, 661.6 keV), 145Pm (37.2 K X-rays), 241Am (59.5 keV), 51Cr (320.1 keV), 22Na (511, 1274.5 keV), 207 Bi (569.7, 1063.7 keV), 54Mn (834.8 keV) and 60Co (1173.2, 1332.5 keV). The time resolution was measured in coincidence experiments with 511 keV annihilation quanta from a 22Na source. As the reference detector (in the start channel) a truncated cone, 25 and 20 mm in diameter and 15 mm high BaF2 crystal was used coupled to the XP20Y0Q/DA PMT. Its time resolution for 22Na was equal to 143 75 ps [6]. The decay time constant measurements were made using a Thomas–Bollinger single photon method [7,8]. A 137Cs g-source was used to excite a scintillator. Single photons were detected by a very fast photomultiplier R5320 from Hamamatsu, which is characterized by time jitter of 140 ps. Each tested crystal was wrapped with Teflon but only on the sides leaving one surface opened to the Hamamatsu PMT. Such configuration assured detection of single photons from a scintillator induced by a gamma source. The afterglow was measured at room temperature following the method of Ref. [9]. Before the measurement each crystal was left to recover natural level of background spectrum from exposure to room light. Next, each measured crystal was irradiated by a 13.9 GBq 241Am source for about 1830 s and then an electromagnetic shutter was closed to block radiation from the source. The closing time of the shutter was equal to about 30 ms. Each afterglow spectrum was measured for about 9 h with MCS (Multichannel Scalers) with a dwell time set to 5 s. Additional measurements of the afterglow were made at 10 ms dwell time to estimate the shortest decay time components of the afterglow [9]. The detailed diagram of the experimental set-up is presented in Fig. 2.
Fig. 2. Block diagram of the set-up used for measurements of afterglow.
228
M. Grodzicka et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 226–230
2.3. Electronics Fig. 1 presents the block diagram of the experimental set-up used in timing experiments. During of the decay time constant measurements the energy window was set at the single photoelectron peak in the Hamamatsu R5320 and at the 662 keV full energy peak in XP20D0 coupled with the tested crystal. In case of the time resolution measurements the energy window was set at the 511 keV full energy peak in both photomultiplier XP20D0 and reference XP2020Q. A block diagram of experimental set-up used for measurements of afterglow is presented in Fig. 2. During the measurement of afterglow, a signal from the PMT anode, corresponding to single photoelectrons was used. In the case of afterglow additional Fast Spectroscopy Amplifier, Canbera 2024, was used working with a lower gain to observe well 59.5 keV gamma ray. The threshold was placed on its ICR (Incoming Count Rate) output signal at above 20 keV. Dual counter and timer, Ortec 994, provided the average number of 59.5 keV photons absorbed in the crystal per 1 s. The spectra were recorded by a PC-based multichannel analyzer (MCA) and multichannel scaler (MCS), Tukan8k [5].
equal to about 7800 phe/MeV and the light output is close to 30,000 ph/MeV. A comparison of the non-proportionality curves for all the tested crystals is shown in Fig. 3. Non-proportionality characteristic are common for all the tested crystal. The results of the measured number of photoelectrons and nonproportionality curves did not show differences between LFS-3 and LSO.
Table 3 Energy resolution of tested crystals for 662 keV g-rays, as measured with the XP 20D0 PMT. Crystal
Size (mm3)
Energy resolution
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
10 10 5 10 10 5 10 10 5 3 3 10
8.13% 7 0.26 7.66% 7 0.23 7.61% 7 0.23 7.56% 7 0.23
3. Results and discussion 3.1. Number of photoelectrons and non-proportionality The number of photoelectrons was measured for three LFS-3 samples and one LSO sample. All the results of the photoelectron number for the tasted crystals are presented in Table 2. The value of the number of photoelectrons per unit energy (phe/MeV) for all the tested crystal, within the margin of errors, is Table 2 Number of the photoelectrons of tested crystals for 662 keV g-rays measured with the XP 20D0 PMT and the light output calculated in relation to the LSO 2005a. Crystal
Size (mm3)
N phe/Mev (phe/Mev)
Light output (ph/MeV)
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
10 10 5 10 10 5 10 10 5 3 3 10
7760 7 230 7720 7 230 7850 7 230 8160 7 230
28,8007 1500a 28,7007 1500 29,1007 1500 20,3007 1500
a
Fig. 4. Measured energy resolution (DE/E) of all the tested crystals versus g-rays energy (shaping time¼ 0.25 ms). Error bars are within the size of the points.
See Ref. [10]
Fig. 3. Comparison of non-proportionality characteristics of the tested crystals.
Fig. 5. Measured (DE/E) energy resolution of tested crystal about size 10 10 5 mm3 versus g-rays energy (shaping time¼ 0.25 ms). Error bars are within the size of the points.
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3.2. Energy resolution Energy resolution was determined in the way described in Section 2. The results for the tested crystals at 662 keV g- rays from a 137Cs are presented in Table 3. Energy resolution for 662 keV g-rays in case of all LFS-3 crystals was approximately the same, irrespective of the size of the tested sample, and was equal to about 7.66 70.23%. While for LSO sample it was equal to about 8.1370.23%. Radioactive sources that were used for measurements of energy resolution are presented in Section 2. The results for all the tested crystals are presented in Fig. 4. In order to better show differences between LFS-3 crystal and LSO crystal in Fig. 5, only the results of the crystals with size 10 10 5 mm3 are presented. In the case of LFS-3 no. 1 and no. 2 crystals, energy resolution in high g-rays energy range was slightly improved comparing to LSO crystal. Fig. 7. Time spectra for 511 keV from 22Na g-source measured with the tested crystals coupled to the Photonis XP20D0 no. 2025 photomultiplier.
3.3. Decay time constants and time resolution The decay time constant of the light pulses was measured using the Thomas–Bollinger single photon method. Experimental set-up is described in Fig.1b in Section 2. Example of the decay time spectrum for LFS-3 no. 1 together with fits of single exponential decay is presented in Fig. 6. All the results of the analysis of the decay time constants measured with the tested crystals are collected in Table 4. Decay time constants for the samples of LFS-3 no. 1 and no. 2 were equal to 40.5 and 40.9 ns, respectively, whereas for the LSO sample decay time was equal to 43.9 ns. The observed difference is within the known spread for LSO crystals. Time resolution was measured using experimental set-up described in Fig. 1a in Section 2. Examples of the timing spectra for LSF-3 no. 1 and LSO crystals are presented in Fig. 7.
Table 5 Time resolution measured with tested crystal coupled to the Photonis XP20D0 no. 2025 photomultiplier. Crystal
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
Time resolution at FWHM, Dt (ps) Measured
Correcteda
224 77 216 77 215 77 214 77
173 7 5 161 7 5 161 7 5 159 7 5
pffiffiffiffi pffiffiffiffiffiffiffiffiffiffi
N phe for 511 keV
pffiffiffiffiffiffiffiffiffi ðps pheÞ103
39657118 39447118 4011 7120 4069 7125
10.97 0.5 10.17 0.5 10.27 0.5 10.17 0.5
Dt N = ENF
a Corrected for the contribution of the BaF2 reference detector of 143 7 4 ps. The largest differences between the measured crystals were on the level of afterglow. The small afterglow was observed in case of the LSO crystal.
Fig. 6. Light pulse shape of the LFS-3 no. 1 crystal. Fig. 8. Afterglow of the tested scintillators measured for dwell time¼ 5 s in the MCS. The crystals were excited with g-rays from a 13.9 GBq 241Am source. Background level was subtracted for all the presented spectra.
Table 4 Decay time of the tested crystals. Crystal
Size (mm3)
Decay time constant (ns)
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
10 10 5 10 10 5 10 10 5 3 3 10
43.9 71.3 40.5 71.2 40.9 71.2 39.7 71.2
All the results of the analysis of the time resolution spectra measured with the tested crystals are collected in Table 5. The measured time resolution is presented in the second column. Then, the corrected values for the contribution of the reference BaF2 detector are collected in the third column. Next column is the number of photoelectrons for 511 keV full energy peak. The last
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Table 6 Integrated afterglow, effective decay time, dose of energy for afterglow and total afterglow for the tested crystals. Crystal
Size (mm3)
seff (s)
Integrated afterglow (phe)
D0 (MeV)
Atot (phe/MeV)
LSO (selected) LFS-3 no. 1 LFS-3 no. 2 LFS-3 no. 3
10 10 5 10 10 5 10 10 5 3 3 10
1638 7100 5612 7340 5575 7330 6794 7410
1.10 109 3.87 109 4.31 109 6.11 109
6.59 106 5.82 106 5.02 106 1.28 106
1707 10 6507 30 8607 40 4740 7 240
column shows the time resolution normalized to the number of photoelectrons and excess noise factor (ENF), following Ref. [6]. Time resolution is equal to 161 ps for LFS-3 no. 1 and no. 2 crystals, whereas for the LSO crystal is equal to 173 ps. A better time resolution of LFS-3, in comparison with LSO, is probably related to the faster decay times of the tested crystals.
4. Conclusions
3.4. Afterglow The Afterglow was measured using experimental set-up described in Fig. 2 in Section 2. Fig. 8. shows the afterglow decays. The afterglow decay curves were fitted with multi-exponential functions. The best fits for LSO and LFS-3 were obtained with twoexponential function. The results of the analysis of the afterglow decay spectra measured with the tested crystals are collected in Table 6. The effective decay time was calculated as a weight value of decay times and is presented in the second column. Then, integrated afterglow is collected in the third column. Dose of the energy (in MeV), D0, which is converted to afterglow during the X-ray irradiation is presented in the fourth column. It was estimated according to Eq. (1), following Ref. [9]: D0 ¼
PEg NP
ltot
ð1eltot tirr Þ
The afterglow originates, to a large extent, from defects in the crystal structure reflected in deep and shallow traps [11,12]. Thus, the number of defects can be observed randomly in different samples.
ð1Þ
where P, equal to about 1 105 ph/s, is the average count rate of 0.0595 MeV gamma quanta from 241Am source and NP is the nonproportionality of given crystal at 59,5 keV, ltot (s 1) is the total decay constant of afterglow (l ¼1/t) and tirr ¼1830 s is the irradiation time. The last column shows the total afterglow per MeV, Atot, i.e. integrated afterglow normalized to the dose of energy (in MeV) for afterglow. The details of the calculation of individual variables are included in work [9]. The lowest intensity of the total afterglow, equal to about 170 phe/MeV, was observed for the selected LSO, while for the LFS-3 no. 1 and no. 2 crystals, it was equal to 650 and 860 phe/MeV, respectively. All the above numbers are comparable to the lowest values reported in Ref. [9].
The study showed that LFS-3 crystal is an excellent substitute for LSO crystal. Its light output, energy resolution and time resolution are systematically slightly better than those measured with the LSO crystal. The largest differences between the measured crystals were observed on the level of afterglow. The smallest afterglow was observed in case of the LSO crystal.
Acknowledgements This study was supported in part by the International Atomic Energy Agency, Research Contract no. 14360, the Polish Committee for Scientific Research, grant no. 657/W-IAEA/2010/0 and the EU Structural Funds Project no. POIG.01.01.02-14-012/08-00.
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