- Email: [email protected]
Cryogenic THGEM–GPM for the readout of scintillation light from liquid argon
Nuclear Instruments and Methods in Physics Research A 774 (2015) 120–126
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Cryogenic THGEM–GPM for the readout of scintillation light from liquid argon Wenqing Xie a, Yidong Fu a, Yulan Li a,n, Jin Li b, Yuanjing Li a, Qian Yue a a b
Department of Engineering Physics, Tsinghua University, Key Laboratory of Particle & Radiation Imaging, Ministry of Education, Beijing 100084, China Institute of High Energy Physics Chinese Academy of Sciences, Beijing 100084, China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 June 2014 Received in revised form 18 November 2014 Accepted 26 November 2014 Available online 4 December 2014
A GPM (Gaseous Photo Multiplier) based on GEMs (Gas Electron Multipliers) and THGEMs (Thick Gas Electron Multipliers) is a promising detector for VUV (Vacuum Ultra Violet) photon readouts in rare event experiments which use cryogenic two-phase detectors with detection media of Ar and Xe. A GPM based on THGEM made of PTFE (herein named PTFE-THGEM) was developed inspired by the wide use of PTFE (polytetrafluoroethene) boards as low radioactive background PCB in rare event experiments. The efficiencies of the THGEM, a CsI photocathode, and finally a GPM are presented here. At low temperature (113 K) and 1.1 atm, the quantum efficiency of the GPM for VUV photons from liquid Ar in a two-phase detector is estimated to be 8.1% and the low threshold of the detector system for initial electrons prior to multiplication is 12 using 5 N purity Ar (0.99999). & 2014 Elsevier B.V. All rights reserved.
Keywords: GPM THGEM Liquid argon Scintillation light Cryogenic
1. Introduction In recent years, the cryogenic liquid scintillation detectors, such as the LAr (liquid argon) detector and LXe (liquid xenon) detector, have been widely used in rare event experiments, including experiments study of neutrinos and dark matter [1–7]. The liquid argon (xenon) scintillation detector works at a very low temperature of 87 K (165 K) with the scintillation light having very short wavelengths peaking at 128 nm (178 nm). Therefore, suitable photoelectric conversion devices are needed, which are sensitive to short wavelength light, have low radiation backgrounds, and work well at low temperatures. The generally used PMT (Photo Multiplier Tube) barely meets these demands only with a WLS (Wave Length Shifter). However, the radioactive 40K contained in the glass tube makes the PMT not the best option. As a potential alternative of the PMT, GPM based on the MPGD (Micro Pattern Gas Detector) have attracted more attention [8–12] due to their advantages, such as low radioactive background, simple structure, large active area, low cost, robustness, low working temperatures and insensitivity to magnetic fields. The THGEM is a MPGD with more robustness and higher gain [13–15]. Much effort has been devoted to investigating GPM based on THGEM (THGEM–GPM) with much progress, but there are still many problems to be resolved.
n
Corresponding author. Tel.: þ 86 1062781327; fax: þ 86 1062782967. E-mail address: [email protected] (Y. Li).
http://dx.doi.org/10.1016/j.nima.2014.11.092 0168-9002/& 2014 Elsevier B.V. All rights reserved.
Currently, the THGEM are mainly made of common PCB materials (G10/FR-4) which have relatively high radioactivity due to the glass fibers. However, PTFE is comprised of carbon and fluorine and has extremely low radioactivity mainly from pollution during the production process. Calculations show that the specific activity of 40K in the PTFE used here is only about 0.54 10 3 mBq/g, while this value is 15.41 mBq/g in FR-4 and 1.86 mBq/g in PMT (model 9537KB). Thus, PTFE based PCB are widely used in rare-event experiments. Inspired by this usage, this study investigates a PTFE based cryogenic THGEM–GPM. The THGEM, CsI photocathode, and GPM studies are described in Sections 2–4, and Section 5 gives the conclusions and discusses the application of the GPM in CDEX.
2. The gain of THGEM In this paper, the THGEM gain is defined as by the Budker INP and Weizmann Institute groups [16] as the ratio of the output charge to the input initial charge prior to multiplication. The initial charge prior to multiplication is calculated from the total deposited energy and the ionization energy in the working gas, while the output charge is obtained from the readout (MCA). 2.1. Experimental setup The experimental setup and procedure for the studies of PTFETHGEM at room temperature and low temperature were described elsewhere [17]. The PTFE-THGEM can work in two modes at low
W. Xie et al. / Nuclear Instruments and Methods in Physics Research A 774 (2015) 120–126
temperatures as the gas-flowing mode and the sealed mode. In the gas-flowing mode, the working gas in the detector is supplied from outside the system. In the sealed mode, the PTFE-THGEM detector is placed in the gas phase above the LAr (purity: 5 N) in a sealed insulated tank. In this study, the THGEM–GPM system was set up based on the sealed mode. The GAr pressure was kept at 1.1 atm through an auto vent valve. As shown in Fig. 1, the 8.09 keV characteristic X ray of Cu, which was generated by striking the Cu sheet with an X ray from the Mini-X (Amptek), passed through an aluminum window on the end cap of the tank as the radiation source. 2.2. Test result Fig. 2 shows the typical energy spectra of the double PTFE-THGEM detector at different low temperatures. The spectra peaks shift left with the decrease of the working temperature, indicating a decrease of the maximum gain of the detector, as shown in Fig. 3. The maximum detector gain exceeds 1500 at 99 K.
3. Quantum efficiency and aging of CsI photocathode CsI films are widely used as photocathodes since they have the largest quantum efficiency (QE) of the alkali halides for VUV photons
Fig. 1. Schematic of the gain measurement experiment.
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[18]. In this section, the QE and the aging of CsI films coated on the PTFE-THGEM are tested at low temperatures.
3.1. Experimental setup A CsI film was coated on a PTFE-THGEM by vacuum evaporation. During evaporation process, the evaporation chamber pressure was kept lower than 2 10 3 Pa. The rate of CsI evaporation was 9–21 Å/s. The CsI films deposited on the THGEM were from 130 nm to 1280 nm thick. The quantum efficiency (QE) and aging of the CsI photocathode was studied by coating a PTFE PCB board with a CsI film (about 450 nm thick) which was placed in an aluminum chamber above LN2 in an insulated tank, as shown in Fig. 4. The temperature near the CsI film changes according to the distance between the aluminum chamber and the LN2 surface. The aluminum chamber was filled with GAr (5 N) in the gas-flowing mode. A deuterium lamp (HAMAMATSU X2D2 L9841) with a main wavelength of 160 nm is used as the VUV source. A positive voltage was applied on the metal mesh to provide the electric field. The QE of the CsI photocathode was calculated from the photocurrent in the film and the photon flux data from the deuterium lamp handbook. The aging of CsI photocathode induced
Fig.3. The maximum gain of the double PTFE-THGEM detector (sealed mode) as a function of temperature. The working gas pressure was 1.1 atm. The voltage across the PTFE-THGEM increases with decreasing temperature, reaching 1470 V at 99 K.
Fig. 2. Energy spectra of the 8.09 keV X ray recorded by the double PTFE-THGEM detector at different temperatures. The detector worked in the sealed mode and the working gas pressure at 1.1 atm.
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Fig. 4. Schematic of the QE measurement experiment.
Fig. 7. Aging curve of a CsI photocathode at low temperature (Q: charge accumulated on the CsI with continuous VUV irradiation). The D2 lamp was turned off for a 10 min after working for 5 min. The working conditions are the same as in Fig. 6.
Fig. 5. QE of a CsI film coated on a PTFE PCB board in GAr (5 N, gas-flow mode) in various temperatures and electric fields. The CsI film was 450 nm thick. The data at 113 K is an interpolated result.
Fig. 8. QE of CsI photocathodes in GAr (5 N) on PTFE and FR-4 at room temperature. The wavelengths of the VUV photons were mainly 160 nm.
Fig. 6. QE of a CsI film coated on a PTFE PCB board in GAr (5 N, gas-flow mode) at various temperatures. The CsI film was 450 nm thick.
at 103 K. The photocurrent at 115 K falls to 30% of the initial value when the accumulated charge increases to 0.065 μC/mm2. The results indicate that CsI rapidly ages at low temperature. In Section 4, the GPM experiment used 128 nm scintillation light from LAr as the VUV source. Results by Xie et al. show that the QE of CsI increases with decreasing of the wavelength [19,20]. QE for 128 nm scintillation light is 2.5–4 times that for the 160 nm light from the deuterium light. Therefore, in this study the QE of the CsI photocathode for 128 nm light should be at least 16.8% at 103 K. The QE of CsI films deposited on PTFE and FR-4 substrates were compared for room temperature conditions. The results in Fig. 8 indicate that the QE of CsI on PTFE are a little higher than that on FR-4.
by the VUV photons was studied by irradiating the CsI film for an extended period of time. 4. GPM efficiency 3.2. Test results 4.1. Photoelectron extraction efficiency The efficiencies of the CsI photocathode at low temperatures are shown in Figs. 5–7. The QE of the photocathode increases with increasing electric field at its surface up to a plateau. The QE significantly decreases with temperature, from 11.9% at 169 K to 6.7%
The photoelectron extraction efficiency, ηex , was defined as the ratio of the photoelectrons extracted into the holes of the THGEM to all of the photoelectrons escaping from the CsI photocathode.
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4.1.1. Experimental setup The X-ray experiment to test the GPM shown in Fig. 9 was mainly designed to measure the photoelectron extraction efficiency. The GPM was placed in the gas phase of argon with a purity of 5 N. The GPM had a drift mesh, two PTFE-THGEMs with the side facing the LAr of the first THGEM coated with CsI and a PCB readout pad. The PTFE-THEGM parameters were carefully studied and set as a thickness of 0.38 mm, a hole diameter of 0.3 mm, a hole pitch of 0.7 mm, a rim width of 40 μm and an active area of 5 cm 5 cm. Both sides of the PTFE-THGEM were coated with 18 μm thick Cu electrodes. During the test, the temperature near the GPM was kept at 113 K. A HAMAMATSU L9631 X-ray machine, with tube voltages from 40 kV to 110 kV and a maximum power of 50 W, was put next to the sealed insulated tank. The X-rays were collimated by a cylindrical lead column and entered into the LAr in the tank. The photocurrent was recorded by an electrometer (KEITHLEY 6517A) from the bottom of the THGEM2. 4.1.2. Test result The photoelectron extraction efficiency ηex was calculated as: (1) The top THGEM1 surface (the CsI side) was connected to ground with a positive high voltage on the mesh. The X-ray machine was turned on (100 kV/500 μA), with the current on the top THGEM1 surface recorded as I1; (2) The bottom THGEM1 surface was connected to ground with the same negative high voltage ( 700 V) applied to the top THGEM1 surface and the mesh. The X-ray machine was turned on (100 kV/500 μA), with the current on the bottom THGEM1 surface recorded as I2; (3) ηex was calculated as I 2 =I 1 .
Fig. 10. electric field on the surface of CsI film.
For a voltage across the THGEM of 700 V, the photoelectron extraction efficiency, ηex , was 65.9% at 113 K. 4.2. Total quantum efficiency The total quantum efficiency of the GPM for incidence photons, ηqe , was defined as
Fig. 11. Schematic of the GPM experiments with an α source.
ηqe ¼ Aef f ηex Q E; where Aef f is the fraction of the CsI-deposited area to the total THGEM active area, which is 73.3% in this study. ηex was measured as in the previous section. Edrift was set to zero in the THGEM–GPM system, but the electric field on the CsI film surface was not uniformly zero. The voltage distribution was calculated using MAXWELL by dividing the CsI surface into many elements (1 μm 1 μm), as shown in Fig. 10. Then the QE of the entire CsI film was calculated from the weighted average of the QE for each small element using the data shown in
Fig. 9. Schematic of the GPM experiment with an X-ray source.
Fig. 5 for the local electric field. The QE was 6.7% for 160 nm VUV and 16.8% for 128 nm VUV. The ηqe was then calculated to be 8.1%. 4.3. Detection threshold 4.3.1. Experimental setup Fig. 11 shows a scheme of the GPM experiment with an α source. The setup was almost identical to that used in Section 4.1 with the same mesh and THGEMs, but different sources and readout systems. The signal was recorded through an ORTEC 142AH preamplifier followed by an ORTEC 572 amplifier and an ORTEC EASY-MCA-8 K. The corresponding electric fields in the GPM were fixed as Edrift ¼0, VTHGEM1 ¼1200 V, VTHGEM2 ¼1300 V, Etran ¼2 kV/cm (region between the two THGEMs) and Eind ¼3 kV/cm (region between THGEM2 and the readout pad). The temperature was 113 K in the experiment. Fig. 12 shows an assembly drawing of the α source experiment. An 241 Am α source was placed in the LAr in a cylindrical 502 ml chamber (chamber 1). The source was 1 cm from the LAr surface. The GPM was placed in a PTFE chamber (chamber 2). The two chambers were connected by a PTFE barrel to create an interconnected chamber. The drift mesh of the GPM was 8 cm from the 241Am source. Chamber 1 was surrounded by LN2 (liquid nitrogen) in the outer tank to reduce the low temperature and to slow the evaporation of the LAr. Two pt100 RTD were used to monitor the temperatures, with #1 placed
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Fig. 12. Assembly drawing of the GPM radiation experiment with the α source (left) and the internal structure of the interconnected chamber (right).
Table 1 Impurities in the liquid argon used in this study.
Table 3 Liquid argon scintillation light components [25].
Impurity
H2O
O2
THC
N2
H2
Components
Decay time (ns)
Ratio (%)
Content (ppm)
2
1.5
1
4
0.5
Fast Middle Slow
4.9 7 0.2 347 3 1260 7 30
18.8 7.4 73.8
Table 2 Effects of the impurities on the scintillation light. Impurity Attenuation coefficient of the scintillation light yield (ppm/μs) O2[22] N2[23] H2O
0.54 7 0.03 0.11 70.01 4O2
Transmission attenuation coefficient (ppm/cm) 0.034 7 0.016 Negligible 0.1897 0.004[24]
near the GPM and #2 in the middle of chamber 1. The main steps of the experiment were as follows: 1) Inject LN2 into the outer insulated tank to a height of 50 cm. 2) Evacuate the interconnected chamber to 4 10 3 Pa. 3) Slowly place the interconnected chamber into the outer insulated tank until chamber 1 is submerged into the LN2. 4) Slowly inject LAr into chamber 1 until the liquidometer float reaches the designed level where chamber 1 is filled with LAr and the α source is about 1 cm under the LAr surface. 5) The pressure inside the interconnected chamber was kept at 1.1 atm during the entire experiment.
4.3.2. Generation and transmission of scintillation light in the test system In pure LAr, the ideal yield of scintillation light is np0 ¼ 3:54 104 =MeV [21]. However, impurities reduce this value and cause photons to be lost during transmission in the LAr. The impurities of
the LAr (5 N) and their effects are listed in Tables 1 and 2. Table 3 shows that the scintillation light from the LAr includes three components with different decay times. The data in Tables 1–3 gives the total attenuation coefficient for the scintillation light yield, ηatt , as 29.3% and the scintillation light absorption length in liquid argon as 2.44 cm. The Monte Carlo program Geant4 was used to simulate the loss of the scintillation light during its transmission in the test system. The influence of the geometry, which comes from the different surface areas on the THGEM and the liquid argon, was also taken into account in the Monte Carlo calculation. The result shows that the transmission efficiency of VUV photons, ηtra , is 0.83%. The wires in the mesh block part of the scintillation light which leads to a photon transmission efficiency, ηm , of 60.6%. In summary, for the α source experiment, the average number of electrons after multiplication, ne0 , can be calculated as ne0 ¼ E0 np0 ηatt ηtra ηm ηqe G;
ð1Þ
here, E0 is the energy of the α particles from 241Am, 4.516 MeV, ηqe is the total quantum efficiency of the GPM for the incident VUV photons, and G is the gain of the THGEM detector, which is 1600 at 113 K. The standard deviation of the number of electrons after multiplication, Δne0 , can be estimated as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 ; ð2Þ Δne0 ¼ ne0 η2α þ η2e0 1 þ G where ηα is the broadening of the energy spectrum of the 241Am source, 8.36%, (the 241Am powder was held between a gold film
W. Xie et al. / Nuclear Instruments and Methods in Physics Research A 774 (2015) 120–126
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and a stainless steel substrate, with the gold film leading to the energy broadening of the α particles), and ηe0 is the statistical pffiffiffiffiffiffiffi fluctuation which is 1= ne0 . Using Eqs. (1) and (2), ne0 and Δne0 from the α source were calculated to be 3.05 104 and 7.15 103.
Fig. 13. Energy spectrum of 241Am recorded by the THGEM–GPM and a curve fit. The working gas pressure was 1.1 atm. The gain of the double PTFE-THGEM detector was about 1600.
Table 4 The thresholds of the GPM in different stages. Threshold The number of electrons after multiplication The number of electrons prior to multiplication The number of incidence VUV photons (128 nm)
1.85 104 12 143
4.3.3. Test result The α source spectrum is shown in Fig. 13. Its energy resolution η was fit to be 30.93% (standard deviation). Since the broadening is related to the number of initial electrons prior to multiplication and the energy of the α source and electronic noise, the spectrum standard deviation, Δn, can be calculated using Eq. (3) where ENE is the equivalent noise electrons in the readout electronics. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Δn ¼ Δn2e0 þ ENE2 ð3Þ Δn ¼ n η
ð4Þ
Since the average value of the electronic noise is zero, the average value of the number of electrons after multiplication in the measured spectrum, n, should be equal to ne0 , 3.05 104. The standard deviation, Δn, can be calculated using Eq. (4). Then, ENE was calculated to be 6.16 103 from Eqs. (3) and (4). Assume the SNR (Signal to Noise Ratio) is not less than 3, and then the threshold of the GPM is summarized in Table 4.
5. Conclusions and discussion
Fig. 14. Background spectrum of CDEX-10 recorded by a HPGe detector (simulated result).
A GPM was built based on a PTFE-THGEM working in twophase Ar. The double PTFE-THGEM detector works steadily at low temperature and has a gain of 1508 at 99 K and 1.1 atm. A CsI photocathode deposited on the PTFE was sensitive to the 128 nm scintillation light from the LAr with a QE of 16.8% for 128 nm VUV at 113 K. The total quantum efficiency of the GPM system for VUV photons from LAr was estimated to be 8.1%. The average number of initial electrons prior to multiplication for an 241Am α source was about 19. The lower threshold of the detector system for the initial electrons prior to multiplication was 12 for Ar with a purity of 5 N. The CDEX (China Dark matter Experiment), a rare event experiment, is designed to search for dark matter in a low mass region using direct detection through a point-contact HPGe detector [5]. In CDEX-10, the HPGe detector is immersed in liquid Ar which serves as a cooling and veto detector. The GPM studied here can be used as the readout of the scintillation light from the LAr detector. The simulated background spectrum in Fig. 14 for the CDEX-10 as recorded by a HPGe detector
Fig. 15. Monte Carlo simulation model of the photon transmissions in CDEX-10 (left) and the simulation result (right).
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shows that the count rate in the low energy region is so high that a veto detector with a low energy threshold is needed. The energy threshold of the detector with a PMT readout is aimed to be 10 keV. The simulated result of the photon transmission in the two-phase argon detector in CDEX-10 is shown in Fig. 15. The photon transmission is affected by the absorption in the liquid argon and the reflection on the inner copper wall. When the Ar purity is 6 N (0.999999) and the wall reflectivity is 100%, the efficiency of photons escaping from the liquid argon and further entering into the gas phase is 16%. When the active area of the PTFE-THGEM detector is enlarged to cover the whole surface area of the liquid argon, the lower threshold of the THGEM–GPM system for detecting incidence VUV photons is still 143 if the electronic noise does not change. Then, the energy threshold of the veto system is 22 keV, a promising result that may lead to a 10 keV system in the following work. The low gain of the THGEM at low temperatures is still a problem which limits the use of the detector. Two ways to increase the gain are to increase the number of multipliers and to add some impurities into the working gas. The placement of a GEM after two THGEMs has been shown to efficiently increase the detector gain [10], but it leads to a higher voltage and larger discharge probability. The introduction of impurities also has some disadvantages because it reduces the scintillation yield and influences the photon transmission. A small fraction of N2 has been shown to not improve the detector gain [10]. Noble gases, such as Ne and Xe, may be more effective. Acknowledgments We sincerely thank Professor Yi-Gang Xie of the University of Chinese Academy of Sciences for useful advice on the THGEM and also Professor Amos Breskin of the Weizmann Institute of Science for discussions on the experiments. References [1] C Rubbia, M Antonello, P Aprili, et al., Journal of Instrumentation 6 (07) (2011) P07011.
[2] M Moe, P. Vogel, Annual Review of Nuclear and Particle Science 44 (1) (1994) 247. [3] Heider M B. Heidelberg, Germany: Combined Faculties of the Natural Sciences and Mathematics of the Ruperto-Carola University, 2009. [4] A. Buzulutskov, Journal of Instrumentation 7 (2012) C02025. [5] V Chepel, H. Araujo, Journal of Instrumentation 8 (2013) R04001. [6] K H Ackermann, M Agostini, M Allardt, et al., European Physical Journal C: Particles and Fields 73 (3) (2013) 1. [7] Y Wu, X Hao, Q Yue, et al., Chinese Physics C 37 (2013) 086001. [8] Pereiale L, Peskov V, Iacobaeus C, et al. ArXiv Physics e-prints, 2004. [9] A Breskin, V Peskov, M Cortesi, et al., Nuclear Instruments & Methods in Physics Research Section A—Accelerators Spectrometers Detectors and Associated Equipment 639 (1) (2011) 117. [10] S Duval, A Breskin, R Budnik, et al., Journal of Instrumentation 6 (04) (2011) P04007. [11] A Bondar, A Buzulutskov, A Grebenuk, et al., Nuclear Instruments & Methods in Physics Research Section A—Accelerators Spectrometers Detectors and Associated Equipment 581 (1–2) (2007) 241. [12] A Bondar, A Buzulutskov, A Dolgov, et al., Journal of Instrumentation 8 (2013) P02008. [13] R Chechik, A Breskin, C Shalem, et al., Nuclear Instruments & Methods in Physics Research Section A—Accelerators Spectrometers Detectors and Associated Equipment 535 (1–2) (2004) 303. [14] A Breskin, R Alon, M Cortesi, et al., Nuclear Instruments & Methods in Physics Research Section A—Accelerators Spectrometers Detectors and Associated Equipment 598 (1) (2009) 107. [15] C Shalem, R Chechik, A Breskin, et al., Nuclear Instruments & Methods in Physics Research Section A—Accelerators Spectrometers Detectors and Associated Equipment 558 (2) (2006) 475. [16] A Bondar, A Buzulutskov, A Grebenuk, et al., Journal of Instrumentation 6 (07) (2011) P07008. [17] W Xie, Y Li, et al., Chinese Physics C 37 (11) (2013) 116001. [18] A. Breskin, Nuclear Instruments & Methods in Physics Research Section A—Accelerators Spectrometers Detectors and Associated Equipment 371 (1-2) (1996) 116. [19] Y Xie, H Liu, A Zhang, et al., Nuclear Instruments & Methods in Physics Research Section A—Accelerators Spectrometers Detectors and Associated Equipment 664 (1) (2012) 310. [20] Y Xie, J Lu, A Zhang, et al., Nuclear Instruments & Methods in Physics Research Section A—Accelerators Spectrometers Detectors and Associated Equipment 729 (0) (2013) 809. [21] T Doke, A Hitachi, J Kikuchi, et al., Journal of Applied Physics 41 (3R) (2002) 1538. [22] R Acciarri, M Antonello, B Baibussinov, et al., Journal of Instrumentation 5 (05) (2010) P05003. [23] R Acciarri, M Antonello, B Baibussinov, et al., Journal of Instrumentation 5 (06) (2010) P06003. [24] 〈https://www.phenix.bnl.gov/WWW/TPCHBD/Transmittance_H2O_O2.pdf〉. [25] R Acciarri, M Antonello, B Baibussinov, Nuclear Physics B: Proceedings Supplements 197 (1) (2009) 70.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Cryogenic THGEM–GPM for the readout of scintillation light from liquid argon Wenqing Xie a, Yidong Fu a, Yulan Li a,n, Jin Li b, Yuanjing Li a, Qian Yue a a b
Department of Engineering Physics, Tsinghua University, Key Laboratory of Particle & Radiation Imaging, Ministry of Education, Beijing 100084, China Institute of High Energy Physics Chinese Academy of Sciences, Beijing 100084, China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 June 2014 Received in revised form 18 November 2014 Accepted 26 November 2014 Available online 4 December 2014
A GPM (Gaseous Photo Multiplier) based on GEMs (Gas Electron Multipliers) and THGEMs (Thick Gas Electron Multipliers) is a promising detector for VUV (Vacuum Ultra Violet) photon readouts in rare event experiments which use cryogenic two-phase detectors with detection media of Ar and Xe. A GPM based on THGEM made of PTFE (herein named PTFE-THGEM) was developed inspired by the wide use of PTFE (polytetrafluoroethene) boards as low radioactive background PCB in rare event experiments. The efficiencies of the THGEM, a CsI photocathode, and finally a GPM are presented here. At low temperature (113 K) and 1.1 atm, the quantum efficiency of the GPM for VUV photons from liquid Ar in a two-phase detector is estimated to be 8.1% and the low threshold of the detector system for initial electrons prior to multiplication is 12 using 5 N purity Ar (0.99999). & 2014 Elsevier B.V. All rights reserved.
Keywords: GPM THGEM Liquid argon Scintillation light Cryogenic
1. Introduction In recent years, the cryogenic liquid scintillation detectors, such as the LAr (liquid argon) detector and LXe (liquid xenon) detector, have been widely used in rare event experiments, including experiments study of neutrinos and dark matter [1–7]. The liquid argon (xenon) scintillation detector works at a very low temperature of 87 K (165 K) with the scintillation light having very short wavelengths peaking at 128 nm (178 nm). Therefore, suitable photoelectric conversion devices are needed, which are sensitive to short wavelength light, have low radiation backgrounds, and work well at low temperatures. The generally used PMT (Photo Multiplier Tube) barely meets these demands only with a WLS (Wave Length Shifter). However, the radioactive 40K contained in the glass tube makes the PMT not the best option. As a potential alternative of the PMT, GPM based on the MPGD (Micro Pattern Gas Detector) have attracted more attention [8–12] due to their advantages, such as low radioactive background, simple structure, large active area, low cost, robustness, low working temperatures and insensitivity to magnetic fields. The THGEM is a MPGD with more robustness and higher gain [13–15]. Much effort has been devoted to investigating GPM based on THGEM (THGEM–GPM) with much progress, but there are still many problems to be resolved.
n
Corresponding author. Tel.: þ 86 1062781327; fax: þ 86 1062782967. E-mail address: [email protected] (Y. Li).
http://dx.doi.org/10.1016/j.nima.2014.11.092 0168-9002/& 2014 Elsevier B.V. All rights reserved.
Currently, the THGEM are mainly made of common PCB materials (G10/FR-4) which have relatively high radioactivity due to the glass fibers. However, PTFE is comprised of carbon and fluorine and has extremely low radioactivity mainly from pollution during the production process. Calculations show that the specific activity of 40K in the PTFE used here is only about 0.54 10 3 mBq/g, while this value is 15.41 mBq/g in FR-4 and 1.86 mBq/g in PMT (model 9537KB). Thus, PTFE based PCB are widely used in rare-event experiments. Inspired by this usage, this study investigates a PTFE based cryogenic THGEM–GPM. The THGEM, CsI photocathode, and GPM studies are described in Sections 2–4, and Section 5 gives the conclusions and discusses the application of the GPM in CDEX.
2. The gain of THGEM In this paper, the THGEM gain is defined as by the Budker INP and Weizmann Institute groups [16] as the ratio of the output charge to the input initial charge prior to multiplication. The initial charge prior to multiplication is calculated from the total deposited energy and the ionization energy in the working gas, while the output charge is obtained from the readout (MCA). 2.1. Experimental setup The experimental setup and procedure for the studies of PTFETHGEM at room temperature and low temperature were described elsewhere [17]. The PTFE-THGEM can work in two modes at low
W. Xie et al. / Nuclear Instruments and Methods in Physics Research A 774 (2015) 120–126
temperatures as the gas-flowing mode and the sealed mode. In the gas-flowing mode, the working gas in the detector is supplied from outside the system. In the sealed mode, the PTFE-THGEM detector is placed in the gas phase above the LAr (purity: 5 N) in a sealed insulated tank. In this study, the THGEM–GPM system was set up based on the sealed mode. The GAr pressure was kept at 1.1 atm through an auto vent valve. As shown in Fig. 1, the 8.09 keV characteristic X ray of Cu, which was generated by striking the Cu sheet with an X ray from the Mini-X (Amptek), passed through an aluminum window on the end cap of the tank as the radiation source. 2.2. Test result Fig. 2 shows the typical energy spectra of the double PTFE-THGEM detector at different low temperatures. The spectra peaks shift left with the decrease of the working temperature, indicating a decrease of the maximum gain of the detector, as shown in Fig. 3. The maximum detector gain exceeds 1500 at 99 K.
3. Quantum efficiency and aging of CsI photocathode CsI films are widely used as photocathodes since they have the largest quantum efficiency (QE) of the alkali halides for VUV photons
Fig. 1. Schematic of the gain measurement experiment.
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[18]. In this section, the QE and the aging of CsI films coated on the PTFE-THGEM are tested at low temperatures.
3.1. Experimental setup A CsI film was coated on a PTFE-THGEM by vacuum evaporation. During evaporation process, the evaporation chamber pressure was kept lower than 2 10 3 Pa. The rate of CsI evaporation was 9–21 Å/s. The CsI films deposited on the THGEM were from 130 nm to 1280 nm thick. The quantum efficiency (QE) and aging of the CsI photocathode was studied by coating a PTFE PCB board with a CsI film (about 450 nm thick) which was placed in an aluminum chamber above LN2 in an insulated tank, as shown in Fig. 4. The temperature near the CsI film changes according to the distance between the aluminum chamber and the LN2 surface. The aluminum chamber was filled with GAr (5 N) in the gas-flowing mode. A deuterium lamp (HAMAMATSU X2D2 L9841) with a main wavelength of 160 nm is used as the VUV source. A positive voltage was applied on the metal mesh to provide the electric field. The QE of the CsI photocathode was calculated from the photocurrent in the film and the photon flux data from the deuterium lamp handbook. The aging of CsI photocathode induced
Fig.3. The maximum gain of the double PTFE-THGEM detector (sealed mode) as a function of temperature. The working gas pressure was 1.1 atm. The voltage across the PTFE-THGEM increases with decreasing temperature, reaching 1470 V at 99 K.
Fig. 2. Energy spectra of the 8.09 keV X ray recorded by the double PTFE-THGEM detector at different temperatures. The detector worked in the sealed mode and the working gas pressure at 1.1 atm.
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Fig. 4. Schematic of the QE measurement experiment.
Fig. 7. Aging curve of a CsI photocathode at low temperature (Q: charge accumulated on the CsI with continuous VUV irradiation). The D2 lamp was turned off for a 10 min after working for 5 min. The working conditions are the same as in Fig. 6.
Fig. 5. QE of a CsI film coated on a PTFE PCB board in GAr (5 N, gas-flow mode) in various temperatures and electric fields. The CsI film was 450 nm thick. The data at 113 K is an interpolated result.
Fig. 8. QE of CsI photocathodes in GAr (5 N) on PTFE and FR-4 at room temperature. The wavelengths of the VUV photons were mainly 160 nm.
Fig. 6. QE of a CsI film coated on a PTFE PCB board in GAr (5 N, gas-flow mode) at various temperatures. The CsI film was 450 nm thick.
at 103 K. The photocurrent at 115 K falls to 30% of the initial value when the accumulated charge increases to 0.065 μC/mm2. The results indicate that CsI rapidly ages at low temperature. In Section 4, the GPM experiment used 128 nm scintillation light from LAr as the VUV source. Results by Xie et al. show that the QE of CsI increases with decreasing of the wavelength [19,20]. QE for 128 nm scintillation light is 2.5–4 times that for the 160 nm light from the deuterium light. Therefore, in this study the QE of the CsI photocathode for 128 nm light should be at least 16.8% at 103 K. The QE of CsI films deposited on PTFE and FR-4 substrates were compared for room temperature conditions. The results in Fig. 8 indicate that the QE of CsI on PTFE are a little higher than that on FR-4.
by the VUV photons was studied by irradiating the CsI film for an extended period of time. 4. GPM efficiency 3.2. Test results 4.1. Photoelectron extraction efficiency The efficiencies of the CsI photocathode at low temperatures are shown in Figs. 5–7. The QE of the photocathode increases with increasing electric field at its surface up to a plateau. The QE significantly decreases with temperature, from 11.9% at 169 K to 6.7%
The photoelectron extraction efficiency, ηex , was defined as the ratio of the photoelectrons extracted into the holes of the THGEM to all of the photoelectrons escaping from the CsI photocathode.
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4.1.1. Experimental setup The X-ray experiment to test the GPM shown in Fig. 9 was mainly designed to measure the photoelectron extraction efficiency. The GPM was placed in the gas phase of argon with a purity of 5 N. The GPM had a drift mesh, two PTFE-THGEMs with the side facing the LAr of the first THGEM coated with CsI and a PCB readout pad. The PTFE-THEGM parameters were carefully studied and set as a thickness of 0.38 mm, a hole diameter of 0.3 mm, a hole pitch of 0.7 mm, a rim width of 40 μm and an active area of 5 cm 5 cm. Both sides of the PTFE-THGEM were coated with 18 μm thick Cu electrodes. During the test, the temperature near the GPM was kept at 113 K. A HAMAMATSU L9631 X-ray machine, with tube voltages from 40 kV to 110 kV and a maximum power of 50 W, was put next to the sealed insulated tank. The X-rays were collimated by a cylindrical lead column and entered into the LAr in the tank. The photocurrent was recorded by an electrometer (KEITHLEY 6517A) from the bottom of the THGEM2. 4.1.2. Test result The photoelectron extraction efficiency ηex was calculated as: (1) The top THGEM1 surface (the CsI side) was connected to ground with a positive high voltage on the mesh. The X-ray machine was turned on (100 kV/500 μA), with the current on the top THGEM1 surface recorded as I1; (2) The bottom THGEM1 surface was connected to ground with the same negative high voltage ( 700 V) applied to the top THGEM1 surface and the mesh. The X-ray machine was turned on (100 kV/500 μA), with the current on the bottom THGEM1 surface recorded as I2; (3) ηex was calculated as I 2 =I 1 .
Fig. 10. electric field on the surface of CsI film.
For a voltage across the THGEM of 700 V, the photoelectron extraction efficiency, ηex , was 65.9% at 113 K. 4.2. Total quantum efficiency The total quantum efficiency of the GPM for incidence photons, ηqe , was defined as
Fig. 11. Schematic of the GPM experiments with an α source.
ηqe ¼ Aef f ηex Q E; where Aef f is the fraction of the CsI-deposited area to the total THGEM active area, which is 73.3% in this study. ηex was measured as in the previous section. Edrift was set to zero in the THGEM–GPM system, but the electric field on the CsI film surface was not uniformly zero. The voltage distribution was calculated using MAXWELL by dividing the CsI surface into many elements (1 μm 1 μm), as shown in Fig. 10. Then the QE of the entire CsI film was calculated from the weighted average of the QE for each small element using the data shown in
Fig. 9. Schematic of the GPM experiment with an X-ray source.
Fig. 5 for the local electric field. The QE was 6.7% for 160 nm VUV and 16.8% for 128 nm VUV. The ηqe was then calculated to be 8.1%. 4.3. Detection threshold 4.3.1. Experimental setup Fig. 11 shows a scheme of the GPM experiment with an α source. The setup was almost identical to that used in Section 4.1 with the same mesh and THGEMs, but different sources and readout systems. The signal was recorded through an ORTEC 142AH preamplifier followed by an ORTEC 572 amplifier and an ORTEC EASY-MCA-8 K. The corresponding electric fields in the GPM were fixed as Edrift ¼0, VTHGEM1 ¼1200 V, VTHGEM2 ¼1300 V, Etran ¼2 kV/cm (region between the two THGEMs) and Eind ¼3 kV/cm (region between THGEM2 and the readout pad). The temperature was 113 K in the experiment. Fig. 12 shows an assembly drawing of the α source experiment. An 241 Am α source was placed in the LAr in a cylindrical 502 ml chamber (chamber 1). The source was 1 cm from the LAr surface. The GPM was placed in a PTFE chamber (chamber 2). The two chambers were connected by a PTFE barrel to create an interconnected chamber. The drift mesh of the GPM was 8 cm from the 241Am source. Chamber 1 was surrounded by LN2 (liquid nitrogen) in the outer tank to reduce the low temperature and to slow the evaporation of the LAr. Two pt100 RTD were used to monitor the temperatures, with #1 placed
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Fig. 12. Assembly drawing of the GPM radiation experiment with the α source (left) and the internal structure of the interconnected chamber (right).
Table 1 Impurities in the liquid argon used in this study.
Table 3 Liquid argon scintillation light components [25].
Impurity
H2O
O2
THC
N2
H2
Components
Decay time (ns)
Ratio (%)
Content (ppm)
2
1.5
1
4
0.5
Fast Middle Slow
4.9 7 0.2 347 3 1260 7 30
18.8 7.4 73.8
Table 2 Effects of the impurities on the scintillation light. Impurity Attenuation coefficient of the scintillation light yield (ppm/μs) O2[22] N2[23] H2O
0.54 7 0.03 0.11 70.01 4O2
Transmission attenuation coefficient (ppm/cm) 0.034 7 0.016 Negligible 0.1897 0.004[24]
near the GPM and #2 in the middle of chamber 1. The main steps of the experiment were as follows: 1) Inject LN2 into the outer insulated tank to a height of 50 cm. 2) Evacuate the interconnected chamber to 4 10 3 Pa. 3) Slowly place the interconnected chamber into the outer insulated tank until chamber 1 is submerged into the LN2. 4) Slowly inject LAr into chamber 1 until the liquidometer float reaches the designed level where chamber 1 is filled with LAr and the α source is about 1 cm under the LAr surface. 5) The pressure inside the interconnected chamber was kept at 1.1 atm during the entire experiment.
4.3.2. Generation and transmission of scintillation light in the test system In pure LAr, the ideal yield of scintillation light is np0 ¼ 3:54 104 =MeV [21]. However, impurities reduce this value and cause photons to be lost during transmission in the LAr. The impurities of
the LAr (5 N) and their effects are listed in Tables 1 and 2. Table 3 shows that the scintillation light from the LAr includes three components with different decay times. The data in Tables 1–3 gives the total attenuation coefficient for the scintillation light yield, ηatt , as 29.3% and the scintillation light absorption length in liquid argon as 2.44 cm. The Monte Carlo program Geant4 was used to simulate the loss of the scintillation light during its transmission in the test system. The influence of the geometry, which comes from the different surface areas on the THGEM and the liquid argon, was also taken into account in the Monte Carlo calculation. The result shows that the transmission efficiency of VUV photons, ηtra , is 0.83%. The wires in the mesh block part of the scintillation light which leads to a photon transmission efficiency, ηm , of 60.6%. In summary, for the α source experiment, the average number of electrons after multiplication, ne0 , can be calculated as ne0 ¼ E0 np0 ηatt ηtra ηm ηqe G;
ð1Þ
here, E0 is the energy of the α particles from 241Am, 4.516 MeV, ηqe is the total quantum efficiency of the GPM for the incident VUV photons, and G is the gain of the THGEM detector, which is 1600 at 113 K. The standard deviation of the number of electrons after multiplication, Δne0 , can be estimated as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 ; ð2Þ Δne0 ¼ ne0 η2α þ η2e0 1 þ G where ηα is the broadening of the energy spectrum of the 241Am source, 8.36%, (the 241Am powder was held between a gold film
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and a stainless steel substrate, with the gold film leading to the energy broadening of the α particles), and ηe0 is the statistical pffiffiffiffiffiffiffi fluctuation which is 1= ne0 . Using Eqs. (1) and (2), ne0 and Δne0 from the α source were calculated to be 3.05 104 and 7.15 103.
Fig. 13. Energy spectrum of 241Am recorded by the THGEM–GPM and a curve fit. The working gas pressure was 1.1 atm. The gain of the double PTFE-THGEM detector was about 1600.
Table 4 The thresholds of the GPM in different stages. Threshold The number of electrons after multiplication The number of electrons prior to multiplication The number of incidence VUV photons (128 nm)
1.85 104 12 143
4.3.3. Test result The α source spectrum is shown in Fig. 13. Its energy resolution η was fit to be 30.93% (standard deviation). Since the broadening is related to the number of initial electrons prior to multiplication and the energy of the α source and electronic noise, the spectrum standard deviation, Δn, can be calculated using Eq. (3) where ENE is the equivalent noise electrons in the readout electronics. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Δn ¼ Δn2e0 þ ENE2 ð3Þ Δn ¼ n η
ð4Þ
Since the average value of the electronic noise is zero, the average value of the number of electrons after multiplication in the measured spectrum, n, should be equal to ne0 , 3.05 104. The standard deviation, Δn, can be calculated using Eq. (4). Then, ENE was calculated to be 6.16 103 from Eqs. (3) and (4). Assume the SNR (Signal to Noise Ratio) is not less than 3, and then the threshold of the GPM is summarized in Table 4.
5. Conclusions and discussion
Fig. 14. Background spectrum of CDEX-10 recorded by a HPGe detector (simulated result).
A GPM was built based on a PTFE-THGEM working in twophase Ar. The double PTFE-THGEM detector works steadily at low temperature and has a gain of 1508 at 99 K and 1.1 atm. A CsI photocathode deposited on the PTFE was sensitive to the 128 nm scintillation light from the LAr with a QE of 16.8% for 128 nm VUV at 113 K. The total quantum efficiency of the GPM system for VUV photons from LAr was estimated to be 8.1%. The average number of initial electrons prior to multiplication for an 241Am α source was about 19. The lower threshold of the detector system for the initial electrons prior to multiplication was 12 for Ar with a purity of 5 N. The CDEX (China Dark matter Experiment), a rare event experiment, is designed to search for dark matter in a low mass region using direct detection through a point-contact HPGe detector [5]. In CDEX-10, the HPGe detector is immersed in liquid Ar which serves as a cooling and veto detector. The GPM studied here can be used as the readout of the scintillation light from the LAr detector. The simulated background spectrum in Fig. 14 for the CDEX-10 as recorded by a HPGe detector
Fig. 15. Monte Carlo simulation model of the photon transmissions in CDEX-10 (left) and the simulation result (right).
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shows that the count rate in the low energy region is so high that a veto detector with a low energy threshold is needed. The energy threshold of the detector with a PMT readout is aimed to be 10 keV. The simulated result of the photon transmission in the two-phase argon detector in CDEX-10 is shown in Fig. 15. The photon transmission is affected by the absorption in the liquid argon and the reflection on the inner copper wall. When the Ar purity is 6 N (0.999999) and the wall reflectivity is 100%, the efficiency of photons escaping from the liquid argon and further entering into the gas phase is 16%. When the active area of the PTFE-THGEM detector is enlarged to cover the whole surface area of the liquid argon, the lower threshold of the THGEM–GPM system for detecting incidence VUV photons is still 143 if the electronic noise does not change. Then, the energy threshold of the veto system is 22 keV, a promising result that may lead to a 10 keV system in the following work. The low gain of the THGEM at low temperatures is still a problem which limits the use of the detector. Two ways to increase the gain are to increase the number of multipliers and to add some impurities into the working gas. The placement of a GEM after two THGEMs has been shown to efficiently increase the detector gain [10], but it leads to a higher voltage and larger discharge probability. The introduction of impurities also has some disadvantages because it reduces the scintillation yield and influences the photon transmission. A small fraction of N2 has been shown to not improve the detector gain [10]. Noble gases, such as Ne and Xe, may be more effective. Acknowledgments We sincerely thank Professor Yi-Gang Xie of the University of Chinese Academy of Sciences for useful advice on the THGEM and also Professor Amos Breskin of the Weizmann Institute of Science for discussions on the experiments. References [1] C Rubbia, M Antonello, P Aprili, et al., Journal of Instrumentation 6 (07) (2011) P07011.
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