- Email: [email protected]
Time profile of the scintillation from liquid and gaseous xenon
Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Time profile of the scintillation from liquid and gaseous xenon Ikuko Murayama n, Shogo Nakamura Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama, Kanagawa 240-8501, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2014 Received in revised form 1 July 2014 Accepted 2 July 2014 Available online 10 July 2014
The decay time profile of vacuum ultraviolet scintillation induced by electronic recoils has been studied for liquid and gaseous xenon. The scintillation light from xenon excited by a gamma source was measured by using two vacuum ultraviolet sensitive photomultipliers, one for detecting scintillation and the other for counting photons of weak monochromatic light. The analysis results based on the timecorrelated single photon counting method show that the time profile in the 176 nm scintillation decay curve for liquid xenon is consistent with a single exponential component and the decay time constant is 31.5 71.3 ns. This constant does not change significantly for pressure ranges between 90 kPa and 130 kPa. There is no emission wavelength dependence of the decay constant. The result corresponds to an average on electronic recoil energies up to 1.3 MeV. & 2014 Elsevier B.V. All rights reserved.
Keywords: Scintillation Liquid xenon Photon counting Decay constant
Xen þ Xe-Xen2 ;
1. Introduction Xenon scintillation has been readily observed in gaseous xenon (Gas.Xe) and liquid xenon (Liq.Xe), and has been applied to various modern experiments for radiation detection. The scintillation time profile is a valuable property for understanding the mechanism of scintillation. In liquid xenon, and also in gaseous xenon above a few hundred Torr, the scintillation photon from an excited state is produced by two mechanisms [1] as follows. One is the scintillation process “excitation” that result because of excited atoms directly produced by an ionizing charged particle. Xen þ Xe-Xen2 Xen2 -2Xe þhν
ð1Þ ðVUVÞ
ð2Þ
The excited state Xe forms the excited metastable excimer Xen2 in its lowest singlet or triplet states. The scintillation photons are emitted in a transition from one of the two lowest electronic excited states to the ground state. Their lifetimes are distinct. The other scintillation process known as “recombination” is attributed to electron-ion pairs produced by ionizing charged particles. n
Xe þ þ Xe-Xe2þ ;
ð3Þ
Xe2þ þ e -Xenn þ Xe;
ð4Þ
Xenn -Xen þ heat;
ð5Þ
n
Corresponding author. E-mail address: [email protected] (I. Murayama).
http://dx.doi.org/10.1016/j.nima.2014.07.003 0168-9002/& 2014 Elsevier B.V. All rights reserved.
Xen2 -2Xe þ hν
ð6Þ ðV UV Þ
ð7Þ
A xenon atomic ion and a xenon atom form the molecular ion Xe2þ which recombines with a thermalized electron and forms Xen2 . Subsequently, processes lead to the production of a vacuum ultraviolet (VUV) scintillation photon from an excited state. In the past, these processes have been studied for liquid xenon by excitation with electrons, alpha particles and fission-fragments. Different lifetimes and relative intensities of these processes were reported depending on the type of radiation source. Therefore, the time profile of scintillation from liquid xenon helps to identify incident particles. According to Hitachi et al. [2], two lifetime components were observed under alpha particle (singlet: 4 ns, triplet: 22 ns) and fission-fragment (singlet: 4 ns, triplet: 21 ns) excitation, and only a single component 45 ns was observed under electron excitation. However, different lifetime values were reported for electron excitation depending on the experiment [2,3,4,5,6]. The results are listed in Table 1. Akimov et al. indicated that the decay time values vary for low energy recoils [5]. Dawson et al. studied the dependence with energy [6], and a recent simulation study on liquid xenon scintillation processes suggested that the excitation lifetime varies depending on the energy of gamma rays at zero applied electric field [7]. In the present work, we have excited liquid and gaseous xenon by a 60Co gamma source and measured the scintillation time profile at several wavelengths in the emission spectrum at zero electric field. We then measured the decay constant of liquid xenon and gaseous xenon. We also studied, for liquid xenon, the emission wavelength dependence and the pressure dependence of the decay constant for the peak of the emission spectrum. In this
534
I. Murayama, S. Nakamura / Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
Table 1 Decay time for the singlet τs and the triplet τt components from liquid xenon. The experimental conditions for incident source, energies and electric field are also shown. Energy uses measurement value themselves and “Unknown” for it is not measured. Source
τs (ns)
Electron 207 Bi, e 2.2 7 0.3 Electron beam 207 Bi, e 60 Co 60 Co 60 Co, γ
60
Co, γ
τt (ns)
Energy
Electric field
Reference
347 2 27 71 327 2 45 29.17 0.6 34.0 7 0.6 46.17 0.1 25.2 7 0.1 22.6 7 0.1 31.5 7 1.3
Unknown Unknown 200 keV Unknown 13.5 keV 37.5 keV Unknown
Null 4 kV/cm Null Null Null Null Null 0.5 kV/cm 3.7 kV/cm null
Kubota et al. [3]
Unknown
Keto et al. [4] Hitachi et al. [2] Akimov et al. [5] Dawson et al. [6]
This work
α, 252
Cf
Fission-fragment Cf
252
4.3 7 0.6
227 1.5
Null
Hitachi et al. [2]
4.3 7 0.5
217 2
Null
Hitachi et al. [2]
first step, we were not able to study the energy dependence. Consequently, results presented in this paper correspond to an average on electric recoil energies up to 1.3 MeV.
2. Experiment 2.1. Apparatus A schematic diagram of the experimental apparatus is shown in Fig. 1. A stainless-steel vacuum chamber is connected to a vacuum monochromator (Acton Research Corp.VM-502-S). The wavelength resolution of the monochromator is 0.1 nm. The xenon cell is a cylindrical vessel made by SUS304 (16 mm in diameter, 56.6 mm in length) and installed in the vacuum chamber. This xenon cell is designed to minimize the effect of scattering or absorption of light. Both ends of the cell are equipped with MgF2 windows (10 mm in diameter and 1 mm thick) with one end facing the entrance slit of the monochromator. The other end faces the 18 mm square VUVsensitive photomultiplier tube (PMT) (R7600-06: HAMAMATSU) in the vacuum chamber, which has a quartz window and a bialkali photocathode. Another VUV-sensitive 28 mm diameter PMT (R6836: HAMAMATSU) is set at the exit slit of the monochromator. Xenon was excited by a 1.8 MBq 60Co gamma source, which was set besides the chamber. The cell was filled with research-grade xenon gases (Japan Air Gases Co. with a purity of 4 99.999%) through a purifier. The purifier contained about 5000 pieces of St707 getters (4 mm in diameter and 2 mm thick) supplied by SAES Getters. The getters were activated at approximately 250 1C for 15 min. Prior to filling, the chamber and filling systems were evacuated down to 10 4 Pa. The purified gas was cooled and liquefied by a pulsetube refrigerator, which was set at the top of the vacuum chamber.
Fig. 1. Schematic view of the experimental apparatus (vacuum chamber and vacuum monochromator).
2.2. Measurements Fig. 2 shows a block diagram of the electronic system. A charge integrating analog-to-digital converter (ADC) was used to measure the luminosity. The ADC measured the total charge signal from PMT2 (R6836) detecting the monochromatic photon. The gate pulse for the ADC was issued by the constant fraction discriminator (CFD) output from PMT1 (R7600) detecting scintillation occurrence. The width of the gate pulse for the charge integration is 250 ns and it is located 20 ns prior to the PMT2 signal. 500 μs
Fig. 2. Block diagram of the electronic system used to measure the time profile of scintillation from liquid and gaseous xenon. The ADC measured the total charge signal of the monochromatic photon. VETO input occurring during the time from the Gate start up to 500 ms.
I. Murayama, S. Nakamura / Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
width veto signal is used to inhibit gate inputs during a digitizing time (200 μs) by the ADC. The number of photoelectrons obtained from the charge distribution data was proportional to the intensity of the scintillation. The time profile at a particular wavelength was determined by measuring the time distribution of the monochromatic photon that arrives at the photomultiplier after the detection of scintillation by another photomultiplier. A Lecroy2228A time-to digital converter (TDC) is started by a trigger pulse from PMT1 that detects scintillation luminescence through a CFD, and is stopped by a trigger pulse from PMT2 through another CFD and a delay unit. Only one time can be measured per trigger pulse on PMT1. The counting rate of PMT2 is 0.4 s 1, compared with 500 s 1 for the rate of PMT1. The proportion of coincidence events to triggered events on PMT1 was 0.0006. The threshold levels of the CFDs were both set low enough to detect a single photon. PMT1 signal had small fluctuations at GND level but the threshold level was higher than fluctuations. The intensity and time profile were analyzed by the time correlated single photon counting (TCSPC) method [8].
535
Fig. 4. Transmittance of the MgF2, the grating efficiency of the monochromator, the reflectance of the mirror on the monochromator, the quantum efficiency of R6836 PMT and the transmission efficiency of liquid xenon plotted as wavelength.
2.3. Analysis Fig. 3 shows the typical charge distribution for liquid xenon. Peaks at 70 ch and 90 ch are respectively the pedestal peak and the dynodes pulse peaks. The spectrum above 120 ch is dominated by a Gaussian distribution of single photoelectrons. The number of events was calculated and regarded as the photon intensity at each wavelength. Photoelectron events under 0.8 p.e. were excluded from the analysis to cut off the pedestal, noise and the dynode pulse. Photoelectron level was checked by ADC calibration before and after this experiment. The photon intensity C(λ) is corrected by the following efficiencies: the spectral transmittance of the MgF2 window Tw(λ), the grating efficiency of the monochromator Ge(λ), the reflectance of the mirror on the monochromator Rm(λ), the quantum efficiency of PMT Qe(λ) and the transmission efficiency of liquid xenon Txe (λ). Tw(λ), Ge(λ), Rm(λ) and Qe(λ) were
measured by each manufacturers, and the Txe (λ) was calculated from refractive index [9]. The individual efficiency curves are shown in Fig. 4. The intensity of the emission spectrum I(λ) is obtained from the following equation: IðλÞ ¼
CðλÞ TwðλÞ GeðλÞ RmðλÞ Q eðλÞ TxeðλÞ
ð8Þ
The wavelength resolution depends on the slit width of the monochromator. The slit width and wavelength resolution were 0.8 nm and 1.2 nm for liquid xenon measurements, 2.4 nm and 3.5 nm for gaseous xenon measurements respectively. Gaseous xenon measurement used wide slit due to low count rate. The resolutions determined by the monochromator calibration at the end of experiments. The mercury calibration light source unit was directly attached to monochromator entrance slit and measured intensity of 184.9 nm emission line by R6836 PMT. The time profile data was fitted by a functional form composed of exponential terms, a constant term and the instrumental resolution [10]: " ! # t t0 σ2 t t 0 σ CðtÞ ¼ ∑Ai exp þ 2 erfc pffiffiffi þ pffiffiffi ð9Þ τi 2τ i 2σ 2τ i where, C(t) is the photon count, τi is the decay constant, t0 is the offset, Ai is the intensity and σ is the instrumental time resolution. The instrument response function was measured with a laser diode pulsed light source and obtained an instrument time resolution σ ¼ 1.0 ns (typ). The decay constant is to be 0.1 ns longer when not considering a finite σ for the liquid xenon. 3. Result and discussion 3.1. Liquid xenon
Fig. 3. Typical ADC charge distribution at 110 kPa for 176 nm scintillation. Peaks at 70 ch and 90 ch are respectively the pedestal peak and the dynodes pulse peaks. The spectrum above 120 ch is dominated by a Gaussian distribution of single photoelectrons.
The typical emission spectrum for liquid xenon is shown in Fig. 5. The emission central peak obtained by Gaussian fitting is 175.9 70.8 nm and the full width at half-maximum (FWHM) is 11.070.3 nm. The emission central peak is shorter than 178.0 nm reported by Jortner et al. [11] but agree with 175.0 71.0 nm measured by Fujii et al. [12]. The time profile of scintillation was examined for peak emission spectrum at different pressures of 90 kPa, 110 kPa and 130 kPa, under vapor–liquid equilibrium conditions. Typical decay curves of the 176 nm emission from liquid xenon are shown in Fig. 6 for 90 kPa, 110 kPa and 130 kPa. The
536
I. Murayama, S. Nakamura / Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
Fig. 5. Typical liquid and gaseous xenon emission spectrum. The intensity is corrected by the efficiency of the spectral transmittance of MgF2, the grating efficiency of the monochromator, the reflectance of mirror on the monochromator, and the quantum efficiency of PMT.
The time profiles were scrutinized with changing a fitting range. The narrow range (o130 ns) fitting results were 30.67 5.5 ns, 31.676.4 ns and 40.7 79.7 ns for 90 kPa, 110 kPa and 130 kPa, respectively. Fig. 7 shows the decay constant as a function of wavelength at the 110 kPa condition. The decay constant for 173 and 179 nm are all in 31.571.3 ns, the decay constant at 176 nm. The decay constant for 170 nm scintillation (wavelength at half of the FWHM shorter than the peak intensity) is slightly smaller, but the difference is not significant. In principle, the TCSPC counts the photon arriving first and does not count the photon arriving later, so it is possible that the multi-photon event affects the decay curve and makes the decay constant smaller. However, we can select a single photon event on the ADC charge distribution data, and the decay constant obtained is free from the effect of multiphoton event. Applying the Poisson distribution to counting number of photon per event, the probability of multi-photon event is very low. The effect of the position dependence of scintillation in the xenon cell must be minimized to the time profile. Considering the size of our xenon cell, the time fluctuation because of the difference in optical path length from the position of scintillation event to both MgF2 windows in the xenon is sufficiently small, thus, the effect is negligible. Kubota et al. [3] reported that the recombination process was suppressed by the electric field, and obtained two lifetime components (singlet: 2.2 ns, triplet: 27 ns) from the measurement with an electric field of 4 kV/cm. The time profile of scintillation from the entire region of 168–181 nm suggests two components with a double exponential form fit even without electric field. However the instrument time resolution is not good enough and this should be improved to discern multiple components. Furthermore, the requirement of more event statistics is indisputable. We are planning to replace the PMT for monochromatic photon counting with another PMT or a photon counting device with a good time resolution to distinguish a nanosecond decay constant in the next experiment. We compared the relative light yield of the scintillation at 110 kPa with the 130 kPa measurement data. A slight variation in the count rate was observed along with increasing and decreasing pressure. The relative light yield is calculated as a function of xenon density and scintillation wavelength. The transmission efficiency of liquid xenon Txe was taken into account as a function of the liquid xenon density and scintillation wavelength when the photon intensity C(λ) was corrected by efficiencies. The typical refractive index of the MgF2 is 1.44 and the liquid xenon is 1.632 at the 176 nm wavelength [9], so the transmission efficiency increases with the decreasing the refractive index of the liquid xenon. The relative light yield Y(λ) is obtained from the following: YðλÞ ¼
Fig. 6. Decay curves of 176 nm scintillation from liquid xenon at 90 kPa, 110 kPa, and 130 kPa. The solid lines show the fitted curves with Eq. (9). The errors are statistical only.
decay constant of the time profile was obtained by fitting Eq. (9). Even after trying to fit the data with sums of up to three independent exponential functions, the time profile was well fitted with a single exponential decay curve. Obtained well fitted decay constant of 176 nm profiles are 29.0 71.7 ns, 31.571.3 ns and 32.2 71.4 ns for 90 kPa, 110 kPa and 130 kPa, respectively. The χ2/dof value of these fit are close to 1. These decay constants agree within the margin of error and significant differences not shown.
IðλÞ T xe ðρÞ
ð10Þ
The density is 2.93 g/cm3 and 2.91 g/cm3 at 110 kPa and 130 kPa measurement, respectively [13]. The ratio of scintillation light yield at 110 kPa to at 130 kPa were 0.91 70.05, 1.007 0.05, 0.967 0.05 and 1.01 70.05 for 170, 173, 176 and 179 nm respectively. There are no significant differences in the relative light intensities and decay constant by pressure differences in this study.
3.2. Gaseous xenon The measurements for the scintillation of gaseous xenon were performed at 138 kPa and 174 K condition. The typical spectrum is shown in Fig. 4. The emission central peak is 173.4 7 0.3 nm and the FWHM is 14.4 nm. The spectrum shape and peak position
I. Murayama, S. Nakamura / Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
537
4. Conclusions
Fig. 7. Decay constant as a function of wavelength for 110 kPa liquid xenon.
The time profile of scintillation from liquid and gaseous xenon has been studied. Analysis results based on the time-correlated single photon counting method indicate that the time profile in the decay curve of scintillation from liquid xenon excited by a 60Co gamma source was well fitted with a single exponential function. In addition, the dependence of the decay constant on emission wavelength was studied. The decay constants for 173, 176 and 179 nm at the 110 kPa were consistent within errors with 31.57 1.3 ns. The scintillation decay constant and the intensity of emission peak were not significantly changed by pressure rising from 90 kPa to 130 kPa.. There is some disagreement with other experiments. Therefore, our result provides new data for discussion about the potential for predicting the decay constant and the scintillation pulse shape by the incident energy of the source. More event statistical data and further investigation are required. The time profile of gaseous xenon showed a decay curve of two exponential components and the decay constants almost agree with previous works.
Acknowledgment We would like to thank H.Tawara, S. Mihara, K. Saito, K. Kasami and S.Sasaki for their technical support and useful comments during this study. We would also like to thanks to M.Fujita, S.Oyama and K.Fujii for their assistance in data taking. This work was partially supported by JSPS KAKENHI Grant no. 22540307. References
Fig. 8. Typical decay curves obtained for the time profile of scintillation from gaseous xenon at 138 kPa and 174 K..
Table 2 Decay time for slow τ1 and fast τ2 components from gaseous xenon. τf (ns)
τs (ns)
Reference
4.6 70.3
997 2 997 10 967 5 100 (20us) 90.0 7 9.5
Bonifield et al. [15] Leichner et al. [16] Keto et al. [17] Suzuki et al. [18] (Recombination decay) This work
5.5 71.0 4 7.0 7 1.6
obtained in this work agree with those reported by Saito et al. [14] (172.4 nm for the peak and 13.3 nm for the FWHM) within error. The time profile of the scintillation for 164–182 nm wavelengths is shown in Fig. 8. The curve shows two exponential decay components. The fast and slow decay constants are 7.07 1.6 ns and 90.0 79.5 ns, respectively (χ2/dof¼ 1.22). Table 2 presents the values of this work together with values reported by other measurements. The values in this work agree with measurements by other researchers [15–18]. Suzuki et al. [18] has reported that the luminescence from the recombination process had non-exponential recombination decay much longer than that in the triplet lifetime, but this was not observed in our study.
[1] S. Kubota, M. Hishida, M. Suzuki, J.Z. Ruan, Physical Review B 20 (1979) 3486. [2] A. Hitachi, T. Takahashi, N. Funayama, K. Masuda, J. Kikuchi, T. Doke, Physical Review B 27 (1983) 5279. [3] S. Kubota, M. Hishida, J.Z Raun, Journal of Physics C: Solid State Physics 11 (1978) 2645. [4] J.W. Keto, R.E. Gleason, F.K. Soley, The Journal of Chemical Physics 71 (1979) 2676. [5] D. Akimov, A. Bewick, D. Davidge, J. Dawson, A.S. Howard, I. Ivaniouchenkov, W.G. Jones, J. Joshi, V.A. Kudryavtsev, T.B. Lawson, V. Lebedenko, M.J. Lehner, P.K. Lightfoot, I. Liubarsky, R. Lüscher, J.E. McMillan, C.D. Peak, J.J. Quenby, N.J.C. Spooner, T.J. Sumner, D.R. Tovey, C.K. Ward, Physics Letters B 524 (2002) 245. [6] J.V. Dawson, A.S. Howard, D. Akimov, H. Araujo, A. Bewick, D.C.R. Davidge, W.G. Jones, M. Joshi, V.N. Lebedenko, I. Liubarsky, J.J. Quenby, G. Rochester, D. Shaul, T.J. Sumner, R.J. Walker, Detectors and Associated Equipment 545 (2005) 690. [7] J. Mock, N. Barry, K. Kazkaz, M. Szydagis, M. Tripathi, S. Uvarov, M. Woods, N. Walsh, Journal of Instrumentation 9 (2014) T04002. [8] W. Becker, Advanced Time-Correlated Single Photon Counting Techniques, Springer, Berlin - Heidelberg, 2005. [9] S. Nakamura, et al., in: Proceedings of the Workshop on Ionization and Scintillation Counters and their Uses, (2007) p. 27. [10] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, US, New York, 2006. [11] J. Jortner, L. Meyer, S.A. Rice, E.G. Wilson, The Journal of Chemical Physics 42 (1965) 4250. [12] K. Fujii, Y. Torigoe, Y. Endo, S. Nakamura, H. Tawara, S. Sasaki, K. Kasami, S. Mihara, K. Saito, T. Haruyama, in: Proceedings of the 25th Workshop on Radiation Detectors and their Uses, 2011, pp. 84–89. [13] E.W. Lemmon, M.O. McLinden, D.G. Friend, in: P.J. Linstrom, W.G. Mallard (Eds.), Thermophysical Properties of Fluid Systems” in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, vol. 20899, National Institute of Standards and Technology, Gaithersburg MD, 2014 (retrieved February 7). [14] K. Saito, et al., in: Proceedings of the 68th Autumn Meeting of the Japan Society of Applied Physics, 2007, 6p-ZC-14. [15] T.D. Bonifield, F.H.K. Rambow, G.K. Walters, M.V. McCusker, D.C. Lorents, R.A. Gutcheck, The Journal of Chemical Physics 72 (1980) 2914. [16] P.K. Leichner, K.F. Palmer, J.D. Cook, M. Thieneman, Physical Review A 13 (1976) 1787. [17] J.W. Keto, R.E. Gleason, G.K. Walters, Physical Review Letters 33 (1974) 1365. [18] M. Suzuki, J.Z. Ruan, S. Kubota, Nuclear Instruments and Methods in Physics Research 192 (1982) 565.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Time profile of the scintillation from liquid and gaseous xenon Ikuko Murayama n, Shogo Nakamura Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama, Kanagawa 240-8501, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2014 Received in revised form 1 July 2014 Accepted 2 July 2014 Available online 10 July 2014
The decay time profile of vacuum ultraviolet scintillation induced by electronic recoils has been studied for liquid and gaseous xenon. The scintillation light from xenon excited by a gamma source was measured by using two vacuum ultraviolet sensitive photomultipliers, one for detecting scintillation and the other for counting photons of weak monochromatic light. The analysis results based on the timecorrelated single photon counting method show that the time profile in the 176 nm scintillation decay curve for liquid xenon is consistent with a single exponential component and the decay time constant is 31.5 71.3 ns. This constant does not change significantly for pressure ranges between 90 kPa and 130 kPa. There is no emission wavelength dependence of the decay constant. The result corresponds to an average on electronic recoil energies up to 1.3 MeV. & 2014 Elsevier B.V. All rights reserved.
Keywords: Scintillation Liquid xenon Photon counting Decay constant
Xen þ Xe-Xen2 ;
1. Introduction Xenon scintillation has been readily observed in gaseous xenon (Gas.Xe) and liquid xenon (Liq.Xe), and has been applied to various modern experiments for radiation detection. The scintillation time profile is a valuable property for understanding the mechanism of scintillation. In liquid xenon, and also in gaseous xenon above a few hundred Torr, the scintillation photon from an excited state is produced by two mechanisms [1] as follows. One is the scintillation process “excitation” that result because of excited atoms directly produced by an ionizing charged particle. Xen þ Xe-Xen2 Xen2 -2Xe þhν
ð1Þ ðVUVÞ
ð2Þ
The excited state Xe forms the excited metastable excimer Xen2 in its lowest singlet or triplet states. The scintillation photons are emitted in a transition from one of the two lowest electronic excited states to the ground state. Their lifetimes are distinct. The other scintillation process known as “recombination” is attributed to electron-ion pairs produced by ionizing charged particles. n
Xe þ þ Xe-Xe2þ ;
ð3Þ
Xe2þ þ e -Xenn þ Xe;
ð4Þ
Xenn -Xen þ heat;
ð5Þ
n
Corresponding author. E-mail address: [email protected] (I. Murayama).
http://dx.doi.org/10.1016/j.nima.2014.07.003 0168-9002/& 2014 Elsevier B.V. All rights reserved.
Xen2 -2Xe þ hν
ð6Þ ðV UV Þ
ð7Þ
A xenon atomic ion and a xenon atom form the molecular ion Xe2þ which recombines with a thermalized electron and forms Xen2 . Subsequently, processes lead to the production of a vacuum ultraviolet (VUV) scintillation photon from an excited state. In the past, these processes have been studied for liquid xenon by excitation with electrons, alpha particles and fission-fragments. Different lifetimes and relative intensities of these processes were reported depending on the type of radiation source. Therefore, the time profile of scintillation from liquid xenon helps to identify incident particles. According to Hitachi et al. [2], two lifetime components were observed under alpha particle (singlet: 4 ns, triplet: 22 ns) and fission-fragment (singlet: 4 ns, triplet: 21 ns) excitation, and only a single component 45 ns was observed under electron excitation. However, different lifetime values were reported for electron excitation depending on the experiment [2,3,4,5,6]. The results are listed in Table 1. Akimov et al. indicated that the decay time values vary for low energy recoils [5]. Dawson et al. studied the dependence with energy [6], and a recent simulation study on liquid xenon scintillation processes suggested that the excitation lifetime varies depending on the energy of gamma rays at zero applied electric field [7]. In the present work, we have excited liquid and gaseous xenon by a 60Co gamma source and measured the scintillation time profile at several wavelengths in the emission spectrum at zero electric field. We then measured the decay constant of liquid xenon and gaseous xenon. We also studied, for liquid xenon, the emission wavelength dependence and the pressure dependence of the decay constant for the peak of the emission spectrum. In this
534
I. Murayama, S. Nakamura / Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
Table 1 Decay time for the singlet τs and the triplet τt components from liquid xenon. The experimental conditions for incident source, energies and electric field are also shown. Energy uses measurement value themselves and “Unknown” for it is not measured. Source
τs (ns)
Electron 207 Bi, e 2.2 7 0.3 Electron beam 207 Bi, e 60 Co 60 Co 60 Co, γ
60
Co, γ
τt (ns)
Energy
Electric field
Reference
347 2 27 71 327 2 45 29.17 0.6 34.0 7 0.6 46.17 0.1 25.2 7 0.1 22.6 7 0.1 31.5 7 1.3
Unknown Unknown 200 keV Unknown 13.5 keV 37.5 keV Unknown
Null 4 kV/cm Null Null Null Null Null 0.5 kV/cm 3.7 kV/cm null
Kubota et al. [3]
Unknown
Keto et al. [4] Hitachi et al. [2] Akimov et al. [5] Dawson et al. [6]
This work
α, 252
Cf
Fission-fragment Cf
252
4.3 7 0.6
227 1.5
Null
Hitachi et al. [2]
4.3 7 0.5
217 2
Null
Hitachi et al. [2]
first step, we were not able to study the energy dependence. Consequently, results presented in this paper correspond to an average on electric recoil energies up to 1.3 MeV.
2. Experiment 2.1. Apparatus A schematic diagram of the experimental apparatus is shown in Fig. 1. A stainless-steel vacuum chamber is connected to a vacuum monochromator (Acton Research Corp.VM-502-S). The wavelength resolution of the monochromator is 0.1 nm. The xenon cell is a cylindrical vessel made by SUS304 (16 mm in diameter, 56.6 mm in length) and installed in the vacuum chamber. This xenon cell is designed to minimize the effect of scattering or absorption of light. Both ends of the cell are equipped with MgF2 windows (10 mm in diameter and 1 mm thick) with one end facing the entrance slit of the monochromator. The other end faces the 18 mm square VUVsensitive photomultiplier tube (PMT) (R7600-06: HAMAMATSU) in the vacuum chamber, which has a quartz window and a bialkali photocathode. Another VUV-sensitive 28 mm diameter PMT (R6836: HAMAMATSU) is set at the exit slit of the monochromator. Xenon was excited by a 1.8 MBq 60Co gamma source, which was set besides the chamber. The cell was filled with research-grade xenon gases (Japan Air Gases Co. with a purity of 4 99.999%) through a purifier. The purifier contained about 5000 pieces of St707 getters (4 mm in diameter and 2 mm thick) supplied by SAES Getters. The getters were activated at approximately 250 1C for 15 min. Prior to filling, the chamber and filling systems were evacuated down to 10 4 Pa. The purified gas was cooled and liquefied by a pulsetube refrigerator, which was set at the top of the vacuum chamber.
Fig. 1. Schematic view of the experimental apparatus (vacuum chamber and vacuum monochromator).
2.2. Measurements Fig. 2 shows a block diagram of the electronic system. A charge integrating analog-to-digital converter (ADC) was used to measure the luminosity. The ADC measured the total charge signal from PMT2 (R6836) detecting the monochromatic photon. The gate pulse for the ADC was issued by the constant fraction discriminator (CFD) output from PMT1 (R7600) detecting scintillation occurrence. The width of the gate pulse for the charge integration is 250 ns and it is located 20 ns prior to the PMT2 signal. 500 μs
Fig. 2. Block diagram of the electronic system used to measure the time profile of scintillation from liquid and gaseous xenon. The ADC measured the total charge signal of the monochromatic photon. VETO input occurring during the time from the Gate start up to 500 ms.
I. Murayama, S. Nakamura / Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
width veto signal is used to inhibit gate inputs during a digitizing time (200 μs) by the ADC. The number of photoelectrons obtained from the charge distribution data was proportional to the intensity of the scintillation. The time profile at a particular wavelength was determined by measuring the time distribution of the monochromatic photon that arrives at the photomultiplier after the detection of scintillation by another photomultiplier. A Lecroy2228A time-to digital converter (TDC) is started by a trigger pulse from PMT1 that detects scintillation luminescence through a CFD, and is stopped by a trigger pulse from PMT2 through another CFD and a delay unit. Only one time can be measured per trigger pulse on PMT1. The counting rate of PMT2 is 0.4 s 1, compared with 500 s 1 for the rate of PMT1. The proportion of coincidence events to triggered events on PMT1 was 0.0006. The threshold levels of the CFDs were both set low enough to detect a single photon. PMT1 signal had small fluctuations at GND level but the threshold level was higher than fluctuations. The intensity and time profile were analyzed by the time correlated single photon counting (TCSPC) method [8].
535
Fig. 4. Transmittance of the MgF2, the grating efficiency of the monochromator, the reflectance of the mirror on the monochromator, the quantum efficiency of R6836 PMT and the transmission efficiency of liquid xenon plotted as wavelength.
2.3. Analysis Fig. 3 shows the typical charge distribution for liquid xenon. Peaks at 70 ch and 90 ch are respectively the pedestal peak and the dynodes pulse peaks. The spectrum above 120 ch is dominated by a Gaussian distribution of single photoelectrons. The number of events was calculated and regarded as the photon intensity at each wavelength. Photoelectron events under 0.8 p.e. were excluded from the analysis to cut off the pedestal, noise and the dynode pulse. Photoelectron level was checked by ADC calibration before and after this experiment. The photon intensity C(λ) is corrected by the following efficiencies: the spectral transmittance of the MgF2 window Tw(λ), the grating efficiency of the monochromator Ge(λ), the reflectance of the mirror on the monochromator Rm(λ), the quantum efficiency of PMT Qe(λ) and the transmission efficiency of liquid xenon Txe (λ). Tw(λ), Ge(λ), Rm(λ) and Qe(λ) were
measured by each manufacturers, and the Txe (λ) was calculated from refractive index [9]. The individual efficiency curves are shown in Fig. 4. The intensity of the emission spectrum I(λ) is obtained from the following equation: IðλÞ ¼
CðλÞ TwðλÞ GeðλÞ RmðλÞ Q eðλÞ TxeðλÞ
ð8Þ
The wavelength resolution depends on the slit width of the monochromator. The slit width and wavelength resolution were 0.8 nm and 1.2 nm for liquid xenon measurements, 2.4 nm and 3.5 nm for gaseous xenon measurements respectively. Gaseous xenon measurement used wide slit due to low count rate. The resolutions determined by the monochromator calibration at the end of experiments. The mercury calibration light source unit was directly attached to monochromator entrance slit and measured intensity of 184.9 nm emission line by R6836 PMT. The time profile data was fitted by a functional form composed of exponential terms, a constant term and the instrumental resolution [10]: " ! # t t0 σ2 t t 0 σ CðtÞ ¼ ∑Ai exp þ 2 erfc pffiffiffi þ pffiffiffi ð9Þ τi 2τ i 2σ 2τ i where, C(t) is the photon count, τi is the decay constant, t0 is the offset, Ai is the intensity and σ is the instrumental time resolution. The instrument response function was measured with a laser diode pulsed light source and obtained an instrument time resolution σ ¼ 1.0 ns (typ). The decay constant is to be 0.1 ns longer when not considering a finite σ for the liquid xenon. 3. Result and discussion 3.1. Liquid xenon
Fig. 3. Typical ADC charge distribution at 110 kPa for 176 nm scintillation. Peaks at 70 ch and 90 ch are respectively the pedestal peak and the dynodes pulse peaks. The spectrum above 120 ch is dominated by a Gaussian distribution of single photoelectrons.
The typical emission spectrum for liquid xenon is shown in Fig. 5. The emission central peak obtained by Gaussian fitting is 175.9 70.8 nm and the full width at half-maximum (FWHM) is 11.070.3 nm. The emission central peak is shorter than 178.0 nm reported by Jortner et al. [11] but agree with 175.0 71.0 nm measured by Fujii et al. [12]. The time profile of scintillation was examined for peak emission spectrum at different pressures of 90 kPa, 110 kPa and 130 kPa, under vapor–liquid equilibrium conditions. Typical decay curves of the 176 nm emission from liquid xenon are shown in Fig. 6 for 90 kPa, 110 kPa and 130 kPa. The
536
I. Murayama, S. Nakamura / Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
Fig. 5. Typical liquid and gaseous xenon emission spectrum. The intensity is corrected by the efficiency of the spectral transmittance of MgF2, the grating efficiency of the monochromator, the reflectance of mirror on the monochromator, and the quantum efficiency of PMT.
The time profiles were scrutinized with changing a fitting range. The narrow range (o130 ns) fitting results were 30.67 5.5 ns, 31.676.4 ns and 40.7 79.7 ns for 90 kPa, 110 kPa and 130 kPa, respectively. Fig. 7 shows the decay constant as a function of wavelength at the 110 kPa condition. The decay constant for 173 and 179 nm are all in 31.571.3 ns, the decay constant at 176 nm. The decay constant for 170 nm scintillation (wavelength at half of the FWHM shorter than the peak intensity) is slightly smaller, but the difference is not significant. In principle, the TCSPC counts the photon arriving first and does not count the photon arriving later, so it is possible that the multi-photon event affects the decay curve and makes the decay constant smaller. However, we can select a single photon event on the ADC charge distribution data, and the decay constant obtained is free from the effect of multiphoton event. Applying the Poisson distribution to counting number of photon per event, the probability of multi-photon event is very low. The effect of the position dependence of scintillation in the xenon cell must be minimized to the time profile. Considering the size of our xenon cell, the time fluctuation because of the difference in optical path length from the position of scintillation event to both MgF2 windows in the xenon is sufficiently small, thus, the effect is negligible. Kubota et al. [3] reported that the recombination process was suppressed by the electric field, and obtained two lifetime components (singlet: 2.2 ns, triplet: 27 ns) from the measurement with an electric field of 4 kV/cm. The time profile of scintillation from the entire region of 168–181 nm suggests two components with a double exponential form fit even without electric field. However the instrument time resolution is not good enough and this should be improved to discern multiple components. Furthermore, the requirement of more event statistics is indisputable. We are planning to replace the PMT for monochromatic photon counting with another PMT or a photon counting device with a good time resolution to distinguish a nanosecond decay constant in the next experiment. We compared the relative light yield of the scintillation at 110 kPa with the 130 kPa measurement data. A slight variation in the count rate was observed along with increasing and decreasing pressure. The relative light yield is calculated as a function of xenon density and scintillation wavelength. The transmission efficiency of liquid xenon Txe was taken into account as a function of the liquid xenon density and scintillation wavelength when the photon intensity C(λ) was corrected by efficiencies. The typical refractive index of the MgF2 is 1.44 and the liquid xenon is 1.632 at the 176 nm wavelength [9], so the transmission efficiency increases with the decreasing the refractive index of the liquid xenon. The relative light yield Y(λ) is obtained from the following: YðλÞ ¼
Fig. 6. Decay curves of 176 nm scintillation from liquid xenon at 90 kPa, 110 kPa, and 130 kPa. The solid lines show the fitted curves with Eq. (9). The errors are statistical only.
decay constant of the time profile was obtained by fitting Eq. (9). Even after trying to fit the data with sums of up to three independent exponential functions, the time profile was well fitted with a single exponential decay curve. Obtained well fitted decay constant of 176 nm profiles are 29.0 71.7 ns, 31.571.3 ns and 32.2 71.4 ns for 90 kPa, 110 kPa and 130 kPa, respectively. The χ2/dof value of these fit are close to 1. These decay constants agree within the margin of error and significant differences not shown.
IðλÞ T xe ðρÞ
ð10Þ
The density is 2.93 g/cm3 and 2.91 g/cm3 at 110 kPa and 130 kPa measurement, respectively [13]. The ratio of scintillation light yield at 110 kPa to at 130 kPa were 0.91 70.05, 1.007 0.05, 0.967 0.05 and 1.01 70.05 for 170, 173, 176 and 179 nm respectively. There are no significant differences in the relative light intensities and decay constant by pressure differences in this study.
3.2. Gaseous xenon The measurements for the scintillation of gaseous xenon were performed at 138 kPa and 174 K condition. The typical spectrum is shown in Fig. 4. The emission central peak is 173.4 7 0.3 nm and the FWHM is 14.4 nm. The spectrum shape and peak position
I. Murayama, S. Nakamura / Nuclear Instruments and Methods in Physics Research A 763 (2014) 533–537
537
4. Conclusions
Fig. 7. Decay constant as a function of wavelength for 110 kPa liquid xenon.
The time profile of scintillation from liquid and gaseous xenon has been studied. Analysis results based on the time-correlated single photon counting method indicate that the time profile in the decay curve of scintillation from liquid xenon excited by a 60Co gamma source was well fitted with a single exponential function. In addition, the dependence of the decay constant on emission wavelength was studied. The decay constants for 173, 176 and 179 nm at the 110 kPa were consistent within errors with 31.57 1.3 ns. The scintillation decay constant and the intensity of emission peak were not significantly changed by pressure rising from 90 kPa to 130 kPa.. There is some disagreement with other experiments. Therefore, our result provides new data for discussion about the potential for predicting the decay constant and the scintillation pulse shape by the incident energy of the source. More event statistical data and further investigation are required. The time profile of gaseous xenon showed a decay curve of two exponential components and the decay constants almost agree with previous works.
Acknowledgment We would like to thank H.Tawara, S. Mihara, K. Saito, K. Kasami and S.Sasaki for their technical support and useful comments during this study. We would also like to thanks to M.Fujita, S.Oyama and K.Fujii for their assistance in data taking. This work was partially supported by JSPS KAKENHI Grant no. 22540307. References
Fig. 8. Typical decay curves obtained for the time profile of scintillation from gaseous xenon at 138 kPa and 174 K..
Table 2 Decay time for slow τ1 and fast τ2 components from gaseous xenon. τf (ns)
τs (ns)
Reference
4.6 70.3
997 2 997 10 967 5 100 (20us) 90.0 7 9.5
Bonifield et al. [15] Leichner et al. [16] Keto et al. [17] Suzuki et al. [18] (Recombination decay) This work
5.5 71.0 4 7.0 7 1.6
obtained in this work agree with those reported by Saito et al. [14] (172.4 nm for the peak and 13.3 nm for the FWHM) within error. The time profile of the scintillation for 164–182 nm wavelengths is shown in Fig. 8. The curve shows two exponential decay components. The fast and slow decay constants are 7.07 1.6 ns and 90.0 79.5 ns, respectively (χ2/dof¼ 1.22). Table 2 presents the values of this work together with values reported by other measurements. The values in this work agree with measurements by other researchers [15–18]. Suzuki et al. [18] has reported that the luminescence from the recombination process had non-exponential recombination decay much longer than that in the triplet lifetime, but this was not observed in our study.
[1] S. Kubota, M. Hishida, M. Suzuki, J.Z. Ruan, Physical Review B 20 (1979) 3486. [2] A. Hitachi, T. Takahashi, N. Funayama, K. Masuda, J. Kikuchi, T. Doke, Physical Review B 27 (1983) 5279. [3] S. Kubota, M. Hishida, J.Z Raun, Journal of Physics C: Solid State Physics 11 (1978) 2645. [4] J.W. Keto, R.E. Gleason, F.K. Soley, The Journal of Chemical Physics 71 (1979) 2676. [5] D. Akimov, A. Bewick, D. Davidge, J. Dawson, A.S. Howard, I. Ivaniouchenkov, W.G. Jones, J. Joshi, V.A. Kudryavtsev, T.B. Lawson, V. Lebedenko, M.J. Lehner, P.K. Lightfoot, I. Liubarsky, R. Lüscher, J.E. McMillan, C.D. Peak, J.J. Quenby, N.J.C. Spooner, T.J. Sumner, D.R. Tovey, C.K. Ward, Physics Letters B 524 (2002) 245. [6] J.V. Dawson, A.S. Howard, D. Akimov, H. Araujo, A. Bewick, D.C.R. Davidge, W.G. Jones, M. Joshi, V.N. Lebedenko, I. Liubarsky, J.J. Quenby, G. Rochester, D. Shaul, T.J. Sumner, R.J. Walker, Detectors and Associated Equipment 545 (2005) 690. [7] J. Mock, N. Barry, K. Kazkaz, M. Szydagis, M. Tripathi, S. Uvarov, M. Woods, N. Walsh, Journal of Instrumentation 9 (2014) T04002. [8] W. Becker, Advanced Time-Correlated Single Photon Counting Techniques, Springer, Berlin - Heidelberg, 2005. [9] S. Nakamura, et al., in: Proceedings of the Workshop on Ionization and Scintillation Counters and their Uses, (2007) p. 27. [10] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, US, New York, 2006. [11] J. Jortner, L. Meyer, S.A. Rice, E.G. Wilson, The Journal of Chemical Physics 42 (1965) 4250. [12] K. Fujii, Y. Torigoe, Y. Endo, S. Nakamura, H. Tawara, S. Sasaki, K. Kasami, S. Mihara, K. Saito, T. Haruyama, in: Proceedings of the 25th Workshop on Radiation Detectors and their Uses, 2011, pp. 84–89. [13] E.W. Lemmon, M.O. McLinden, D.G. Friend, in: P.J. Linstrom, W.G. Mallard (Eds.), Thermophysical Properties of Fluid Systems” in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, vol. 20899, National Institute of Standards and Technology, Gaithersburg MD, 2014 (retrieved February 7). [14] K. Saito, et al., in: Proceedings of the 68th Autumn Meeting of the Japan Society of Applied Physics, 2007, 6p-ZC-14. [15] T.D. Bonifield, F.H.K. Rambow, G.K. Walters, M.V. McCusker, D.C. Lorents, R.A. Gutcheck, The Journal of Chemical Physics 72 (1980) 2914. [16] P.K. Leichner, K.F. Palmer, J.D. Cook, M. Thieneman, Physical Review A 13 (1976) 1787. [17] J.W. Keto, R.E. Gleason, G.K. Walters, Physical Review Letters 33 (1974) 1365. [18] M. Suzuki, J.Z. Ruan, S. Kubota, Nuclear Instruments and Methods in Physics Research 192 (1982) 565.