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Primary scintillation characteristics of Ar+CF4 gas mixtures excited by proton and alpha particles
Nuclear Instruments and Methods in Physics Research A 694 (2012) 157–161
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Primary scintillation characteristics of Ar þCF4 gas mixtures excited by proton and alpha particles Jinliang Liu a,b,n, Xiaoping Ouyang a,b,nn, Liang Chen b, Xianpeng Zhang b,c, Jun Liu b, Zhongbing Zhang b, Jinlu Ruan b a
Department of Engineering Physics, Tsinghua University, Beijing 100084, People’s Republic of China Radiation Detection Research Center, Northwest Institute of Nuclear Technology, P.O. Box 69–9, Xi’an Shaanxi 710024, People’s Republic of China c School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an Shaanxi 710049, People’s Republic of China b
a r t i c l e i n f o
abstract
Article history: Received 7 May 2012 Received in revised form 1 August 2012 Accepted 7 August 2012 Available online 16 August 2012
In this paper, we report how the concentration of carbon-tetrafluoride (CF4) affects the primary scintillation from Ar þ CF4 gas mixtures excited by proton and alpha particles. The single photon counting method was used to measure the time spectra of the primary scintillation from Ar, CF4 and their mixtures at atmospheric pressure. Pure Ar exhibits a fast decay time constant, which is approximately 6 ns, and a slow decay time tail. Initially, increases in the concentration of CF4 increase the decay time of the Ar þ CF4 gas mixture. However, when the concentration of CF4 exceeds a certain threshold, the decay time decreases to that of pure CF4, approximately 8–9 ns. We also report the photon emission spectra of Ar þ CF4 mixtures excited by protons. The emission spectrum of the primary scintillation from Ar is improved by CF4 in both the ultraviolet region and the visible to near-infrared region. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Time spectra Emission spectra CF4 concentration Arþ CF4 Proton and alpha excitation
1. Introduction In the field of nuclear technology, the use of noble gases as gas scintillators to detect both neutral and charged particles has attracted a lot of attention [1–3]. Noble gases have unique physical properties, such as high scintillation yields [4], relatively fast decay times [5], near linear responses to charged particles over a wide range of dE/dx [6,7] and large absorption cross sections to thermal neutrons (3He). It is easy to build a good 2p or 4p geometry detector using noble gas scintillators, and the stopping power can be controlled by varying the gas pressure. Nitrogen, oxygen, and the noble gas argon (Ar) are the three most abundant elements in the atmosphere. Ar is inexpensive and available in large quantities. In pulsed neutron detection, it is inevitable that induced g-ray will interfere with the neutron detector. The neutron detector must have either high neutron sensitivity and relatively low gamma
n Corresponding author at: Radiation Detection Research Center, Northwest Institute of Nuclear Technology, P.O. Box 69–9, Xi’an Shaanxi 710024, People’s Republic of China. Tel.: þ 86 29 84767212; fax: þ86 29 83366333. nn Corresponding author at: Radiation Detection Research Center, Northwest Institute of Nuclear Technology, P.O. Box 69–9, Xi’an Shaanxi 710024, People’s Republic of China. Tel.: þ 86 29 84765632; fax: þ86 29 83366333. E-mail addresses: [email protected] (J. Liu), [email protected] (X. Ouyang).
sensitivity, or a good n/g discrimination ratio. We have built a noble gas (Ar and He) scintillation fission chamber detector [8] for pulsed neutron detection. A UO2 foil and noble gas scintillator were used as detection media. The primary scintillation photons excited by heavy-charged fission fragments, which are generated by fission reactions, were registered using a Photomultiplier Tube (PMT). Our scintillation fission chamber detector has an excellent n/g discrimination ratio. However, the detector responds poorly to the ultraviolet emissions of noble gases, and thus is not sensitive enough to detect low-intensity neutron radiation. The emissions from noble gases are mainly in the ultraviolet region [9]. It has been reported that adding CF4 to the noble gas can improve the secondary scintillation characteristics of the noble gases. Carbon-tetrafluoride (CF4) is a heavy molecule that contains only low-Z atoms, and its sensitivity to gamma radiation is low [10]. To improve the sensitivity of the chamber detector while maintaining the n/g discrimination ability, we propose using Arþ CF4 gas mixtures as detection media. In addition, CF4 is a fascinating gas scintillator, i.e., its emission spectrum ranges from the far-ultraviolet to the nearinfrared with a significant fraction in the visible region [10–12]. It has been reported that the secondary emission spectra from CF4 based noble gas mixtures, e.g., ArþCF4 or HeþCF4, are similar to that of pure CF4 under X-ray and electron excitation in MicroPattern Gas Detectors (MPGDs) [13–17]. Based on our literature survey, there are few studies of the primary scintillation characteristics from ArþCF4 gas mixtures
0168-9002/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.08.018
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under heavy-charged particle excitation. In this paper, we report how the concentration of CF4 affects the decay time and emission spectrum of the primary scintillation from an ArþCF4 mixture at 1 bar total pressure. The time spectra are recorded using the single photon counting method by analyzing the time delay between the detection of an excitation particle (alpha or proton) and the detection of a scintillation photon. We also report the ultraviolet to near-infrared emission spectra of ArþCF4 gas mixtures excited by a proton beam.
2. Experimental techniques Fig. 1(a) shows the experimental setup used to record the time spectrum of each gas mixture under alpha particle excitation. The following procedure was performed twice before each measurement: the stainless steel gas cell was filled with Ar and CF4, evacuated to 10 6 bar and refilled with fresh gases. All the gases used in the experiments were 99.999% pure. In the center of the gas cell, an a-particle source (239Pu, which emits 1500 a-particles per second) was installed parallel to a solid state detector (PIN, j20 0.39 mm3) at a distance of 3 mm. The energies of the alpha particles were 5.156 MeV, 5.144 MeV, and 5.105 MeV with branching ratios 73.3%, 15.1%, and 11.5%, respectively. After depositing a part of their energy in the gas mixture, the a-particles were registered by the PIN detector and provided the ‘‘start’’ signal. After passing through the quartz window (synthetic quartz, 10 mm thick), the primary scintillation photons were recorded using a
micro-channel plate (MCP) PMT and provided the ‘‘stop’’ signal. The very fast MCP-PMT R3809U-50, with a time jitter of 25 ps [18], was used as a single photon detector. Fig. 1(b) shows the experimental setup used to record the time and emission spectra of each gas mixture under proton excitation. The experiment was performed using the EN-6 tandem accelerator at the Institute of Heavy Ion Physics, Peking University. The energy of the proton beam was 9 MeV. For the time spectrum measurement, an organic scintillation film (EJ228, j20 0.5 mm, ELJEN Technology, U.S.) was installed along the path before the protons entered the gas cell. Scintillation photons from the organic film were registered using PMT1 (ETL 9813SB, U.K.) and provided the ‘‘start’’ signal. To reduce the dispersion of the time-of-flight, two small tubes were placed in the cell along the proton beam’s incident direction with a gap of approximately 5 mm. Scintillation photons not emitted from the gap were blocked. Photons emitted from the gap were detected using PMT2 (R3809U-50) and provided the ‘‘stop’’ signal. For the emission spectrum measurement, the organic scintillation film and the small tubes were removed, and PMT2 was replaced by a monochromator. To reduce the waiting time of the measurement system, we used the start–stop inversion arrangement. The time spectrum experiment was performed using a Fast-Preamplifier (FP), ORTEC 9306, and a Constant Fraction Discriminator (CFD), ORTEC 935. The decay time curve was measured using an ORTEC 567 Time-toAmplitude Converter (TAC) and recorded by a portable multichannel analyzer (MCA8000A, AmpTek, U.S.). The timing resolution of the
Fig. 1. Schematic drawings of the experimental setups used for spectral studies under (a) alpha particle and (b) proton excitations. Note: 1) The incident direction of proton beam is from up to down along the tube in Fig. 1(b). 2) The entrance windows of PMT in Fig. 1(a) and PMT2 in Fig. 1(b) are masked by dark papers with a 2 mm hole in center.
Fig. 2. Black line: the effective decay curve of the primary scintillation from (a) Ar under proton excitation and (b) CF4 excited by alpha particles. Red line: the exponential fit to the data in a limited time range: (a) 13–38 ns and (b) 20–98 ns. R-square: the coefficient of determination. As the fit to the data improves, R-square approaches 1. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
J. Liu et al. / Nuclear Instruments and Methods in Physics Research A 694 (2012) 157–161
system was approximately 0.2 ns (note: the system time resolution was measured by coincident experiment with 511 keV annihilation quanta from 22Na gamma source), including the electronic timing uncertainties and timing jitters of the PMTs and the PIN detector. The time calibration of the TACþMCA was performed using a precise signal source generator, Agilent M81110A. A monochromator (Omni-l300, Zolix, China) and a side window photomultiplier (PMTH-S1-CR131, Zolix) were used to record the emission spectrum of the primary scintillation. There were two optical gratings in the monochromator: a 1200 grooves/mm grating blazed at 300 nm and a 600 grooves/mm grating blazed at 750 nm. The wavelength position was calibrated using a mercury lamp. The wavelength resolution of the monochromator was approximately 3 nm (FWHM).
3. Results and discussions 3.1. Time spectra The effective decay times of the primary scintillation from Ar, CF4 and their mixtures were measured for up to 400 ns. Fig. 2 shows typical decay curves for Ar and CF4 excited by proton and alpha particles, respectively. The falling part of the obtained time spectrum was fitted with an exponential decay plus a constant term to take into account possible background. In the time spectrum of Ar, two decay time components are observed. The fast component is nearly exponential and has a typical decay time constant of approximately 6.4 ns. The slow decay time ‘‘tail’’ is
159
visible at a time delay above 50 ns. The tail part in Fig. 2(a) is not caused by constant background, it surely caused by slow decay components, we will further clarify this issue in Fig. 3. The time spectrum of pure CF4 under alpha particle excitation is nearly mono-exponential, and the typical decay time constant is approximately 8.5 ns. Since the emission spectrum of CF4 contains many emit components, different components may have quite different population and depopulation dynamics due to energy transfer reactions: when one of the components is in the pure decay phase, another one could still be in the excitation phase when the emission intensity is increasing. So the rise time of the effective decay time curve measured in our experiment is somewhat slow (see Fig. 2(b)). In Fig. 3, the decay curve of the primary scintillation from an Ar þCF4 mixture excited by protons is shown for several concentrations of CF4. As the CF4 concentration increases, the fast-to-slow component ratio of the ArþCF4 gas mixture increases, and the slow decay component gradually diminishes. In addition, the fast decay component of the primary scintillation from ArþCF4 gas mixtures increases as the CF4 concentration increases. The fast decay time components are 7.0 ns and 13.7 ns for Arþ(0.5%)CF4 and Arþ (1.4%)CF4, respectively. As the concentration of CF4 increases beyond a certain value, the decay time constant slowly decreases to 9.2 ns for Arþ(20%)CF4. Decay time curves of Fig. 3(a)–(d) and Fig. 2(a) were obtained in the same measurement condition with almost the same measurement time, so the backgrounds due to random coincidence should be almost the same. We can see that the background counts are really rare in Fig. 3(d), this clarify that the slow tail in Fig. 2(a) is
Fig. 3. Black line: the effective decay curve of the primary scintillation from Arþ CF4 under proton excitation. Each graph represents a different concentration of CF4 in the mixture. Red line: the exponential fit to the data in a limited time range: (a) 10–50 ns, (b) 12–100 ns, (c) 12–75 ns and (d) 12–70 ns. R-square: the coefficient of determination. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
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not caused by constant background. The decay time constant of the primary scintillation from ArþCF4 shows a similar tendency under both alpha and proton excitations, as shown in Fig. 4.
3.2. Emission spectra Using two optical gratings, the emission spectra were recorded for wavelengths ranging from 200 to 550 nm and from 500 to 1100 nm. The spectra were corrected for the spectral response of the detection system, which was estimated by taking into account the transmission efficiency of the quartz window, the relative grating efficiency of the optical gratings and the quantum efficiency of the PMT detector. The emission spectra of Ar excited by a proton beam are shown in Fig. 5. In the visible and near-infrared regions, there are many Ar atomic lines that correspond to the transitions between the (3p54p) excited state of Ar and the (3p54 s) electronic configurations [19]. The peak that is centered at 309 nm is generally identified as the OH emission due to the dissociative excitation of water vapor, which is an impurity in the gas scintillator [20]. The broad band in the ultraviolet region (centered at 230 nm) is identified as the emission from excited dimers or molecular ions of Ar. In our previous study, this emission continuum could only be observed for high pressured Ar
(430 kPa) under the excitation of X-rays. The small narrow bands that are centered at 337 nm, 358 nm and 381 nm are attributed to the transitions of the second positive system of N2 impurities present in the Ar. Because the second order of the monochromator was not suppressed in the experiment, the small continuum above 400 nm (centered at 450 nm) might be the second order of the band that is centered at 230 nm. Fig. 6 shows the emission spectra of several ArþCF4 gas mixtures under proton excitation. In the ultraviolet region, the OH emission (309 nm) attributed to the water vapor impurity significantly decreases as the concentration of CF4 increases. For high concentrations of CF4, there is a broad continuum (centered at 290 nm) that extends from 220 to 400 nm and a shoulder at 240 nm. This emission is mainly attributed to CFn3 ð2A10 -1A20 Þ transitions of excited CF3 molecules and CF4þ ðC -X , A , B Þ transitions of excited CF4 ions [21]. At high concentrations of CF4, the atomic lines of Ar disappear in the visible and near-infrared regions. Instead, two continuum bands appear. One band, centered at 620 nm, extends from 520 to 750 nm. The second band, centered at 860 nm, extends from 780 to 1000 nm. The continua are mainly attributed to CFn3 ð1E10 , 2A200 -1A10 Þ transitions. The spectral differences observed for ArþCF4 gas mixtures with different concentrations of CF4 indicate that the direct excitation of the CF4 molecule is not the only channel that leads to CFn3 and CF4þ n emission and that energy is transferred between the excited atomic state of Ar and the excited state of CF4. Pure Ar exhibits at least two decay components. The excited states of the slow emission component of Ar, which has a longer mean lifetime, have a greater probability of transferring their energy to the CF4 through molecular collisions. As the concentration of CF4 increases, the slow decay time ‘‘tail’’ of Ar þCF4 diminishes, and the effective decay time constant increases. When the CF4 concentration is greater than a certain value, more CF4 is directly excited by proton interactions. As the concentration of CF4 continues to increase, the measured decay time constant slightly decreases to that of pure CF4.
4. Conclusions
Fig. 4. Decay time constant of the primary scintillation from Ar þCF4 under proton excitation (solid squares) and alpha particle excitation (open circles) as a function of the CF4 concentration.
We have studied how the concentration of CF4 affects the primary scintillation from an ArþCF4 gas mixture by recording the time and emission spectra under proton and alpha particle excitations at 1 bar total pressure. The experimental results show that as the CF4 concentration increases, the decay time constant of the ArþCF4 gas mixture increases. However, when the CF4 concentration exceeds a certain value, the decay time constant slightly decreases. This tendency is observed for Ar þCF4 gas
Fig. 5. Emission spectra of the proton induced primary scintillation from Ar in the ultraviolet (a) and the visible to near-infrared (b) regions.
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Fig. 6. Emission spectra of the proton induced primary scintillation from Arþ CF4 gas mixtures in (a) the ultraviolet and (b) the visible to near-infrared regions. The concentration of CF4 ranges from 1% to 20%.
mixtures under either alpha or proton excitation. The emission spectrum from Arþ CF4 is also strongly affected by the CF4 concentration. The emission spectrum of the primary scintillation from Ar is improved by CF4 in both the ultraviolet region and the visible to near-infrared region.
Acknowledgments The authors are grateful to Prof. Xiaotang REN and Prof. Zhengyuan JIANG of the Institute of Heavy Ion Physics, Peking University for the operation of the EN-6 tandem accelerator to provide the proton source. The authors also thank Mr. Tao BAI of the Northwest Institute of Nuclear Technology for providing the alpha particle source. In addition, we appreciate the language editing provided by Prof. Bin LIU of the School of Nuclear and Engineering, North China Electric Power University. References [1] R.Lee Aamodt, Leon J. Brown, Gordon M. Smith, Review of Scientific Instruments 37 (1966) 1338. [2] Elena Aprile, Aleksey E. Bolotnikov, Alexander I. Bolozdynya, et al., Noble gas detectors, Wiley-VCH Verlag GmbH & Co. KGaA, 2006. [3] W. Tornow, J.H. Esterline, C.A. Leckey, et al., Nuclear Instruments and Methods A 647 (2011) 86. [4] S. Kobayashi, N. Hasebe, T. Igarashi, et al., Nuclear Instruments and Methods A 531 (2004) 327.
[5] H.A. Koehler, L.J. Ferderber, D.L. Redhead, et al., Physical Review A 9 (1974) 768. [6] J.B. Birks, The theory and practice of scintillation counting, Pergamon Press, Oxford, London, 1964. [7] Tadayoshi Doke, Kimiaki Masuda, Nuclear Instruments and Methods A 420 (1999) 62. [8] Zhang Xiaodong, Qiu Mengtong, Zhang Jianfu, et al., High Power Laser and Particle Beams 24 (2012) 1151 in Chinese. [9] Joshua Jortner, Lothar Meyer, Stuart A. Rice, et al., The Journal of chemical physics 42 (1965) 4250. [10] A. Morozov, M.M.F.R. Fraga, L. Pereira, et al., Nuclear Instruments and Methods A 628 (2010) 360. [11] R. Gernhauser, B. Bauer, J. Friese, et al., Nuclear Instruments and Methods A 371 (1996) 300. [12] A. Morozov, M.M.F.R. Fraga, L. Pereira, et al., Nuclear Instruments and Methods B 268 (2010) 1456. [13] F.A.F. Fraga, L.M.S. Margato, S.T.G. Fetal, et al., Nuclear Instruments and Methods A 478 (2002) 357. [14] F.A.F. Fraga, L.M.S. Margato, S.T.G. Fetal, et al., Nuclear Instruments and Methods A 513 (2003) 379. [15] Fuyuki Tokanai, Suzuka Motegi, Hirohisa Sakurai, et al., Nuclear Instruments and Methods A 529 (2004) 332. [16] S.T.G. Fetal, F.A.F. Fraga, L.M.S. Margato, et al., Nuclear Instruments and Methods A 581 (2007) 202. [17] F. Tokanai, H. Sakurai, S. Gunji, et al., Nuclear Instruments and Methods A 581 (2007) 236. [18] Hamamatsu Photonics K.K., Microchannel plate photomultiplier tube (MCP-PMTs) R3809U-50 Series, 2001. [19] K. Wagatsuma, K. Hirokawa, Analytical chemistry 57 (1985) 2901. [20] M.M.F.R. Fraga, F.A.F. Fraga, S.T.G. Fetal, et al., Nuclear Instruments and Methods A 504 (2003) 88. [21] Nobuaki Washida, Masako Suto, Shigeru Nagase, et al., The Journal of chemical physics 78 (1983) 1025.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Primary scintillation characteristics of Ar þCF4 gas mixtures excited by proton and alpha particles Jinliang Liu a,b,n, Xiaoping Ouyang a,b,nn, Liang Chen b, Xianpeng Zhang b,c, Jun Liu b, Zhongbing Zhang b, Jinlu Ruan b a
Department of Engineering Physics, Tsinghua University, Beijing 100084, People’s Republic of China Radiation Detection Research Center, Northwest Institute of Nuclear Technology, P.O. Box 69–9, Xi’an Shaanxi 710024, People’s Republic of China c School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an Shaanxi 710049, People’s Republic of China b
a r t i c l e i n f o
abstract
Article history: Received 7 May 2012 Received in revised form 1 August 2012 Accepted 7 August 2012 Available online 16 August 2012
In this paper, we report how the concentration of carbon-tetrafluoride (CF4) affects the primary scintillation from Ar þ CF4 gas mixtures excited by proton and alpha particles. The single photon counting method was used to measure the time spectra of the primary scintillation from Ar, CF4 and their mixtures at atmospheric pressure. Pure Ar exhibits a fast decay time constant, which is approximately 6 ns, and a slow decay time tail. Initially, increases in the concentration of CF4 increase the decay time of the Ar þ CF4 gas mixture. However, when the concentration of CF4 exceeds a certain threshold, the decay time decreases to that of pure CF4, approximately 8–9 ns. We also report the photon emission spectra of Ar þ CF4 mixtures excited by protons. The emission spectrum of the primary scintillation from Ar is improved by CF4 in both the ultraviolet region and the visible to near-infrared region. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Time spectra Emission spectra CF4 concentration Arþ CF4 Proton and alpha excitation
1. Introduction In the field of nuclear technology, the use of noble gases as gas scintillators to detect both neutral and charged particles has attracted a lot of attention [1–3]. Noble gases have unique physical properties, such as high scintillation yields [4], relatively fast decay times [5], near linear responses to charged particles over a wide range of dE/dx [6,7] and large absorption cross sections to thermal neutrons (3He). It is easy to build a good 2p or 4p geometry detector using noble gas scintillators, and the stopping power can be controlled by varying the gas pressure. Nitrogen, oxygen, and the noble gas argon (Ar) are the three most abundant elements in the atmosphere. Ar is inexpensive and available in large quantities. In pulsed neutron detection, it is inevitable that induced g-ray will interfere with the neutron detector. The neutron detector must have either high neutron sensitivity and relatively low gamma
n Corresponding author at: Radiation Detection Research Center, Northwest Institute of Nuclear Technology, P.O. Box 69–9, Xi’an Shaanxi 710024, People’s Republic of China. Tel.: þ 86 29 84767212; fax: þ86 29 83366333. nn Corresponding author at: Radiation Detection Research Center, Northwest Institute of Nuclear Technology, P.O. Box 69–9, Xi’an Shaanxi 710024, People’s Republic of China. Tel.: þ 86 29 84765632; fax: þ86 29 83366333. E-mail addresses: [email protected] (J. Liu), [email protected] (X. Ouyang).
sensitivity, or a good n/g discrimination ratio. We have built a noble gas (Ar and He) scintillation fission chamber detector [8] for pulsed neutron detection. A UO2 foil and noble gas scintillator were used as detection media. The primary scintillation photons excited by heavy-charged fission fragments, which are generated by fission reactions, were registered using a Photomultiplier Tube (PMT). Our scintillation fission chamber detector has an excellent n/g discrimination ratio. However, the detector responds poorly to the ultraviolet emissions of noble gases, and thus is not sensitive enough to detect low-intensity neutron radiation. The emissions from noble gases are mainly in the ultraviolet region [9]. It has been reported that adding CF4 to the noble gas can improve the secondary scintillation characteristics of the noble gases. Carbon-tetrafluoride (CF4) is a heavy molecule that contains only low-Z atoms, and its sensitivity to gamma radiation is low [10]. To improve the sensitivity of the chamber detector while maintaining the n/g discrimination ability, we propose using Arþ CF4 gas mixtures as detection media. In addition, CF4 is a fascinating gas scintillator, i.e., its emission spectrum ranges from the far-ultraviolet to the nearinfrared with a significant fraction in the visible region [10–12]. It has been reported that the secondary emission spectra from CF4 based noble gas mixtures, e.g., ArþCF4 or HeþCF4, are similar to that of pure CF4 under X-ray and electron excitation in MicroPattern Gas Detectors (MPGDs) [13–17]. Based on our literature survey, there are few studies of the primary scintillation characteristics from ArþCF4 gas mixtures
0168-9002/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.08.018
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J. Liu et al. / Nuclear Instruments and Methods in Physics Research A 694 (2012) 157–161
under heavy-charged particle excitation. In this paper, we report how the concentration of CF4 affects the decay time and emission spectrum of the primary scintillation from an ArþCF4 mixture at 1 bar total pressure. The time spectra are recorded using the single photon counting method by analyzing the time delay between the detection of an excitation particle (alpha or proton) and the detection of a scintillation photon. We also report the ultraviolet to near-infrared emission spectra of ArþCF4 gas mixtures excited by a proton beam.
2. Experimental techniques Fig. 1(a) shows the experimental setup used to record the time spectrum of each gas mixture under alpha particle excitation. The following procedure was performed twice before each measurement: the stainless steel gas cell was filled with Ar and CF4, evacuated to 10 6 bar and refilled with fresh gases. All the gases used in the experiments were 99.999% pure. In the center of the gas cell, an a-particle source (239Pu, which emits 1500 a-particles per second) was installed parallel to a solid state detector (PIN, j20 0.39 mm3) at a distance of 3 mm. The energies of the alpha particles were 5.156 MeV, 5.144 MeV, and 5.105 MeV with branching ratios 73.3%, 15.1%, and 11.5%, respectively. After depositing a part of their energy in the gas mixture, the a-particles were registered by the PIN detector and provided the ‘‘start’’ signal. After passing through the quartz window (synthetic quartz, 10 mm thick), the primary scintillation photons were recorded using a
micro-channel plate (MCP) PMT and provided the ‘‘stop’’ signal. The very fast MCP-PMT R3809U-50, with a time jitter of 25 ps [18], was used as a single photon detector. Fig. 1(b) shows the experimental setup used to record the time and emission spectra of each gas mixture under proton excitation. The experiment was performed using the EN-6 tandem accelerator at the Institute of Heavy Ion Physics, Peking University. The energy of the proton beam was 9 MeV. For the time spectrum measurement, an organic scintillation film (EJ228, j20 0.5 mm, ELJEN Technology, U.S.) was installed along the path before the protons entered the gas cell. Scintillation photons from the organic film were registered using PMT1 (ETL 9813SB, U.K.) and provided the ‘‘start’’ signal. To reduce the dispersion of the time-of-flight, two small tubes were placed in the cell along the proton beam’s incident direction with a gap of approximately 5 mm. Scintillation photons not emitted from the gap were blocked. Photons emitted from the gap were detected using PMT2 (R3809U-50) and provided the ‘‘stop’’ signal. For the emission spectrum measurement, the organic scintillation film and the small tubes were removed, and PMT2 was replaced by a monochromator. To reduce the waiting time of the measurement system, we used the start–stop inversion arrangement. The time spectrum experiment was performed using a Fast-Preamplifier (FP), ORTEC 9306, and a Constant Fraction Discriminator (CFD), ORTEC 935. The decay time curve was measured using an ORTEC 567 Time-toAmplitude Converter (TAC) and recorded by a portable multichannel analyzer (MCA8000A, AmpTek, U.S.). The timing resolution of the
Fig. 1. Schematic drawings of the experimental setups used for spectral studies under (a) alpha particle and (b) proton excitations. Note: 1) The incident direction of proton beam is from up to down along the tube in Fig. 1(b). 2) The entrance windows of PMT in Fig. 1(a) and PMT2 in Fig. 1(b) are masked by dark papers with a 2 mm hole in center.
Fig. 2. Black line: the effective decay curve of the primary scintillation from (a) Ar under proton excitation and (b) CF4 excited by alpha particles. Red line: the exponential fit to the data in a limited time range: (a) 13–38 ns and (b) 20–98 ns. R-square: the coefficient of determination. As the fit to the data improves, R-square approaches 1. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
J. Liu et al. / Nuclear Instruments and Methods in Physics Research A 694 (2012) 157–161
system was approximately 0.2 ns (note: the system time resolution was measured by coincident experiment with 511 keV annihilation quanta from 22Na gamma source), including the electronic timing uncertainties and timing jitters of the PMTs and the PIN detector. The time calibration of the TACþMCA was performed using a precise signal source generator, Agilent M81110A. A monochromator (Omni-l300, Zolix, China) and a side window photomultiplier (PMTH-S1-CR131, Zolix) were used to record the emission spectrum of the primary scintillation. There were two optical gratings in the monochromator: a 1200 grooves/mm grating blazed at 300 nm and a 600 grooves/mm grating blazed at 750 nm. The wavelength position was calibrated using a mercury lamp. The wavelength resolution of the monochromator was approximately 3 nm (FWHM).
3. Results and discussions 3.1. Time spectra The effective decay times of the primary scintillation from Ar, CF4 and their mixtures were measured for up to 400 ns. Fig. 2 shows typical decay curves for Ar and CF4 excited by proton and alpha particles, respectively. The falling part of the obtained time spectrum was fitted with an exponential decay plus a constant term to take into account possible background. In the time spectrum of Ar, two decay time components are observed. The fast component is nearly exponential and has a typical decay time constant of approximately 6.4 ns. The slow decay time ‘‘tail’’ is
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visible at a time delay above 50 ns. The tail part in Fig. 2(a) is not caused by constant background, it surely caused by slow decay components, we will further clarify this issue in Fig. 3. The time spectrum of pure CF4 under alpha particle excitation is nearly mono-exponential, and the typical decay time constant is approximately 8.5 ns. Since the emission spectrum of CF4 contains many emit components, different components may have quite different population and depopulation dynamics due to energy transfer reactions: when one of the components is in the pure decay phase, another one could still be in the excitation phase when the emission intensity is increasing. So the rise time of the effective decay time curve measured in our experiment is somewhat slow (see Fig. 2(b)). In Fig. 3, the decay curve of the primary scintillation from an Ar þCF4 mixture excited by protons is shown for several concentrations of CF4. As the CF4 concentration increases, the fast-to-slow component ratio of the ArþCF4 gas mixture increases, and the slow decay component gradually diminishes. In addition, the fast decay component of the primary scintillation from ArþCF4 gas mixtures increases as the CF4 concentration increases. The fast decay time components are 7.0 ns and 13.7 ns for Arþ(0.5%)CF4 and Arþ (1.4%)CF4, respectively. As the concentration of CF4 increases beyond a certain value, the decay time constant slowly decreases to 9.2 ns for Arþ(20%)CF4. Decay time curves of Fig. 3(a)–(d) and Fig. 2(a) were obtained in the same measurement condition with almost the same measurement time, so the backgrounds due to random coincidence should be almost the same. We can see that the background counts are really rare in Fig. 3(d), this clarify that the slow tail in Fig. 2(a) is
Fig. 3. Black line: the effective decay curve of the primary scintillation from Arþ CF4 under proton excitation. Each graph represents a different concentration of CF4 in the mixture. Red line: the exponential fit to the data in a limited time range: (a) 10–50 ns, (b) 12–100 ns, (c) 12–75 ns and (d) 12–70 ns. R-square: the coefficient of determination. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
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not caused by constant background. The decay time constant of the primary scintillation from ArþCF4 shows a similar tendency under both alpha and proton excitations, as shown in Fig. 4.
3.2. Emission spectra Using two optical gratings, the emission spectra were recorded for wavelengths ranging from 200 to 550 nm and from 500 to 1100 nm. The spectra were corrected for the spectral response of the detection system, which was estimated by taking into account the transmission efficiency of the quartz window, the relative grating efficiency of the optical gratings and the quantum efficiency of the PMT detector. The emission spectra of Ar excited by a proton beam are shown in Fig. 5. In the visible and near-infrared regions, there are many Ar atomic lines that correspond to the transitions between the (3p54p) excited state of Ar and the (3p54 s) electronic configurations [19]. The peak that is centered at 309 nm is generally identified as the OH emission due to the dissociative excitation of water vapor, which is an impurity in the gas scintillator [20]. The broad band in the ultraviolet region (centered at 230 nm) is identified as the emission from excited dimers or molecular ions of Ar. In our previous study, this emission continuum could only be observed for high pressured Ar
(430 kPa) under the excitation of X-rays. The small narrow bands that are centered at 337 nm, 358 nm and 381 nm are attributed to the transitions of the second positive system of N2 impurities present in the Ar. Because the second order of the monochromator was not suppressed in the experiment, the small continuum above 400 nm (centered at 450 nm) might be the second order of the band that is centered at 230 nm. Fig. 6 shows the emission spectra of several ArþCF4 gas mixtures under proton excitation. In the ultraviolet region, the OH emission (309 nm) attributed to the water vapor impurity significantly decreases as the concentration of CF4 increases. For high concentrations of CF4, there is a broad continuum (centered at 290 nm) that extends from 220 to 400 nm and a shoulder at 240 nm. This emission is mainly attributed to CFn3 ð2A10 -1A20 Þ transitions of excited CF3 molecules and CF4þ ðC -X , A , B Þ transitions of excited CF4 ions [21]. At high concentrations of CF4, the atomic lines of Ar disappear in the visible and near-infrared regions. Instead, two continuum bands appear. One band, centered at 620 nm, extends from 520 to 750 nm. The second band, centered at 860 nm, extends from 780 to 1000 nm. The continua are mainly attributed to CFn3 ð1E10 , 2A200 -1A10 Þ transitions. The spectral differences observed for ArþCF4 gas mixtures with different concentrations of CF4 indicate that the direct excitation of the CF4 molecule is not the only channel that leads to CFn3 and CF4þ n emission and that energy is transferred between the excited atomic state of Ar and the excited state of CF4. Pure Ar exhibits at least two decay components. The excited states of the slow emission component of Ar, which has a longer mean lifetime, have a greater probability of transferring their energy to the CF4 through molecular collisions. As the concentration of CF4 increases, the slow decay time ‘‘tail’’ of Ar þCF4 diminishes, and the effective decay time constant increases. When the CF4 concentration is greater than a certain value, more CF4 is directly excited by proton interactions. As the concentration of CF4 continues to increase, the measured decay time constant slightly decreases to that of pure CF4.
4. Conclusions
Fig. 4. Decay time constant of the primary scintillation from Ar þCF4 under proton excitation (solid squares) and alpha particle excitation (open circles) as a function of the CF4 concentration.
We have studied how the concentration of CF4 affects the primary scintillation from an ArþCF4 gas mixture by recording the time and emission spectra under proton and alpha particle excitations at 1 bar total pressure. The experimental results show that as the CF4 concentration increases, the decay time constant of the ArþCF4 gas mixture increases. However, when the CF4 concentration exceeds a certain value, the decay time constant slightly decreases. This tendency is observed for Ar þCF4 gas
Fig. 5. Emission spectra of the proton induced primary scintillation from Ar in the ultraviolet (a) and the visible to near-infrared (b) regions.
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Fig. 6. Emission spectra of the proton induced primary scintillation from Arþ CF4 gas mixtures in (a) the ultraviolet and (b) the visible to near-infrared regions. The concentration of CF4 ranges from 1% to 20%.
mixtures under either alpha or proton excitation. The emission spectrum from Arþ CF4 is also strongly affected by the CF4 concentration. The emission spectrum of the primary scintillation from Ar is improved by CF4 in both the ultraviolet region and the visible to near-infrared region.
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