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Ultrasensitive resonance ionization mass spectrometer for evaluating krypton contamination in xenon dark matter detectors
Nuclear Instruments and Methods in Physics Research A 797 (2015) 64–69
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Ultrasensitive resonance ionization mass spectrometer for evaluating krypton contamination in xenon dark matter detectors Y. Iwata a,n, H. Sekiya b,c, C. Ito a a
Experimental Fast Reactor Department, Oarai Research and Development Center, Japan Atomic Energy Agency, 4002 Narita, Oarai, Ibaraki 311-1393, Japan Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Higashi-Mozumi, Kamioka, Hida, Gifu 506-1205, Japan c Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa, Chiba 277-8582, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 March 2015 Received in revised form 5 June 2015 Accepted 24 June 2015 Available online 2 July 2015
An ultrasensitive resonance ionization mass spectrometer that can be applied to evaluate krypton (Kr) contamination in xenon (Xe) dark matter detectors has been developed for measuring Kr at the partsper-trillion (ppt) or sub-ppt level in Xe. The gas sample is introduced without any condensation into a time-of-flight mass spectrometer through a pulsed supersonic valve. Using a nanosecond pulsed laser at 212.6 nm, 84Kr atoms in the sample are resonantly ionized along with other Kr isotopes. 84Kr ions are then mass separated and detected by the mass spectrometer in order to measure the Kr impurity concentration. With our current setup, approximately 0.4 ppt of Kr impurities contained in pure argon (Ar) gas are detectable with a measurement time of 1000 s. Although Kr detection sensitivity in Xe is expected to be approximately half of that in Ar, our spectrometer can evaluate Kr contamination in Xe to the sub-ppt level. & 2015 Elsevier B.V. All rights reserved.
Keywords: Resonance ionization mass spectrometry Krypton in xenon Optical parametric generation
1. Introduction Recently, liquid xenon (LXe) has been used widely in many underground experiments aiming to achieve the direct detection of dark matter, such as XMASS [1], XENON100 [2], and LUX [3]. With its high scintillation yields, self-shielding capability, and potential for scaling up, LXe is considered a promising target material for detecting rare events, which requires an extremely low background environment. The long-lived radioactive isotope krypton (85Kr) (with a half-life of 10.756 years) in Xe is a major background source because the main decay process (with a branching ratio of 99.563%) involves the emission of a single β particle with a maximum energy of 687 keV (Fig. 1). A large portion of 85Kr is released into the air through nuclear fission processes, accounting for the isotopic abundance of 85 Kr/Kr to be approximately 10 11. High-purity Xe gas commercially obtained through the distillation of liquid air contains Kr traces at the parts-per-billion (ppb) level. A distillation system has been developed to further reduce Kr impurity concentration to the parts-per-trillion (ppt) level [1,4], which is equivalent to a single 85 Kr atom in 1023 Xe atoms (approximately 22 g). Fig. 2 shows background event rates of 85Kr under impurity concentrations of 200 ppb and 2 ppt compared to the signal rate
n
Corresponding author. Tel.: þ 81 29 267 4141; fax: þ 81 29 267 7481. E-mail address: [email protected] (Y. Iwata).
http://dx.doi.org/10.1016/j.nima.2015.06.037 0168-9002/& 2015 Elsevier B.V. All rights reserved.
from weakly interacting massive particle (WIMP) dark matter in Xe target experiments. Here, a WIMP mass of 100 GeV/c2 and a WIMP-nucleon cross-section of 10 45 cm2 are assumed. Nextgeneration experiments are expected to achieve a cross-section sensitivity of 10 46–10 47 cm2 [6] with Kr impurity concentration reduced to the sub-ppt level. To ensure that the detector meets the sensitivity requirements, it is necessary to develop an analytical tool capable of quantifying the abundance of 85Kr. There are two possible approaches: direct counting of 85Kr decay events, and mass analysis of Kr stable isotopes such as 84Kr and 86Kr. In the former method, a delayed coincidence technique is used for identifying the minor decay of 85Kr, which involves the emission of a single β particle with a maximum energy of 173 keV, followed by that of a 514 keV γ-ray (Fig. 1). However, as an example, for detecting 1 ppt of Kr in the XMASS 835 kg LXe detector, the event rate is calculated to be only approximately 0.03 day 1, assuming 85Kr/Kr ¼1 10 11. This value is very low to obtain a statistically significant number of 85Kr decay events even with a few months of life time. The latter method, i.e., mass analysis of 84Kr or 86Kr, enables high sensitivity measurements in a short time and thus is an effective method for evaluating 85Kr contamination if its isotopic abundance can be determined with high accuracy. Gas chromatography is often used in advance of mass analysis to separate Kr from Xe. Coupled with a gas chromatographic system, an atmospheric pressure ionization mass spectrometer and a sector field
Y. Iwata et al. / Nuclear Instruments and Methods in Physics Research A 797 (2015) 64–69
85
Kr
65
10.756 y
Q
0.434% - = 173 keV
1.015
s
514 keV Q
99.563% - = 687 keV
85
Rate [events/kg/day/keVee]
Fig. 1. Decay diagram of
102 101 100 10-1 10-2 10-3 10-4
stable
Rb 85
Kr.
Kr 200 ppb Fig. 3. Resonance ionization scheme of Kr.
WIMP signal mass: 100 GeV/c2 cross section: 10-45 cm2
2. Experimental setup
Kr 2 ppt
10-5 10-6 0 10
101 102 Energy [keVee]
103
Fig. 2. Background event rates of 85Kr compared to the signal rate from WIMP dark matter in liquid Xe dark matter detectors. No background and no effect of detector geometry are taken into account in this figure. The scintillation efficiency assumed in the calculation is based on [5]. See text for the calculation conditions.
mass spectrometer have been developed independently to be applied to the XMASS [1] and XENON100 [7] detectors, respectively. With the help of a condensation process before gas introduction into the mass spectrometer, a detection limit of 8 parts-per-quadrillion (ppq) has been achieved [7]. However, the systematic uncertainty of this process would be a possible drawback in the measurements of Kr concentration. Another promising method, i.e., atom trap trace analysis, has high elemental and isotopic selectivity for detecting a single atom [8]. However, due to the limited number of atoms that can be trapped simultaneously, the overall detection efficiency is expected to be relatively low compared to that of other spectrometers. Herein, we present a resonance ionization mass spectrometer developed for precise and accurate Kr measurement at the ppt or sub-ppt level in Xe. A narrow-linewidth nanosecond pulsed laser at 212.6 nm is used to resonantly ionize Kr atoms in the gas sample. Two 212.6 nm photons excite Kr atoms from the ground state (4p6 1S0) to the 5p½1=20 state, and a third photon of the same wavelength leads to the ionization (Fig. 3). 84Kr ions are mass separated and then detected by a time-of-flight mass spectrometer (TOF-MS). Along with ppt level detection sensitivity, resonance ionization mass spectrometry (RIMS) realizes high elemental selectivity without requiring any condensation procedures. In view of these advantages, isotopic analysis of Kr and Xe using RIMS has already been developed for identifying the failed fuel assembly location in nuclear reactors [9,10]. Next, we describe the detailed experimental setup, including a pulsed laser system and a commercial TOF-MS. The gas sample preparation is also mentioned. In Section 3, we discuss the characterization of our spectrometer based on the measurement results. Finally, a brief summary and future prospects are given in Section 4. Some promising techniques are also presented to further improve the detection limit of the spectrometer.
Fig. 4 shows a schematic of our resonance ionization mass spectrometer. It can be divided into three parts: a laser system, a sample preparation system, and a mass spectrometer. The first two have been newly established in this study, as described in detail below, while the mass spectrometer is the same as that used in another study [9,10].
2.1. Laser system Fig. 3 shows that a pulsed laser at 212.6 nm can be used for the resonance ionization of Kr atoms. In this laser system, the third harmonic (354.8 nm) of a Continuum Powerlite Precision II 9010 Nd:YAG laser is used as a pump source to generate laser light at 530.2 and 212.6 nm. The entire system is operated at a repetition rate of 10 Hz with approximately 5 ns pulse width. For precise and accurate Kr measurements, the wavelength and output energy of the pulsed laser at 212.6 nm have to be stabilized during operation. We have developed an optical system that utilizes optical parametric generation (OPG) without resonator mirrors in order to reduce the linewidth of 530.2 nm light to a level comparable to that of the pump laser (approximately 90 MHz or approximately 0.04 pm at 354.8 nm). The linewidth is sufficiently smaller than the Doppler width of approximately 2 GHz of 84 Kr atoms. Fig. 5 shows a detailed schematic of the OPG system. A Sacher Lasertechnik model TEC-100-1080-060 continuous-wave externalcavity diode laser (ECDL) with a linewidth of o 1 MHz (1 ms) was used as a seed source for efficient wavelength conversion. Its wavelength was fine-tuned to 1072.8 nm, and then the ECDL light was overlapped with a fraction (approximately 30 mJ/pulse) of the pump light to produce a 3–4 mJ/pulse of 530.2 nm light: 354:8 nm-530:2 nm þ 1072:8 nm. The diameter of each laser beam was 2–3 mm. To achieve sufficient conversion efficiency without resonator mirrors, four BBO (β-BaB2O4) crystals were arranged in a row, two with a length of 20 mm and the other two with a length of 15 mm. Each crystal was mounted on a mirror holder to rotate it for phase matching in the OPG and for other nonlinear optical processes also described below. A Bristol Instruments model 821B-VIS wavelength meter operated from 350 to 1100 nm was used for monitoring the ECDL wavelength. The generated 530.2 nm light was separated from the incident laser beams, and then overlapped with a fraction (approximately 200 mJ/pulse) of the pump light to increase the 530.2 nm laser
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Y. Iwata et al. / Nuclear Instruments and Methods in Physics Research A 797 (2015) 64–69
Fig. 4. Schematic of the resonance ionization mass spectrometer.
Fig. 5. Detailed schematic of the optical parametric generation (OPG) system.
output energy to 40–50 mJ/pulse. This optical parametric amplification (OPA) process (Fig. 4) was realized by two BBO crystals with lengths of 10 mm. The diameter of each laser beam was enlarged to approximately 6 mm to avoid damage to optical components. The amplified 530.2 nm light was separated from the incident pump light, and then sum-frequency mixed with the residual (approximately 100 mJ/pulse) pump light to produce approximately 10 mJ/ pulse of 212:6 nm light : 530:2 nm þ354:8 nm-212:6 nm. A 7-mm length BBO crystal was used for this sum frequency generation (SFG) process (Fig. 4). To evaluate the wavelength stability of 530.2 nm light, a fraction of the transmitted 530.2 nm laser beam was diverted to another wavelength meter (Bristol Instruments model 821B-VIS). The generated 212.6 nm light was focused into the ionization region of the spectrometer. An Ophir Optronics model PE50-DIF pyroelectric energy meter was used to measure the transmitted pulse energy of the 212.6 nm light. 2.2. Sample preparation Since commercially available pure Xe gas is expensive and still contains a high concentration (approximately 10 ppb) of Kr impurities, it is hard to prepare Xe gas samples containing Kr at the ppt or sub-ppt level. First of all, in the sample preparation, gas samples of argon (Ar) containing approximately 5, 10, and 20 ppt of Kr were prepared and used instead of Xe samples to evaluate Kr detection sensitivity at the ppt level in Ar. Two pairs of mass flow controllers (MFCs) were used to mix the commercially available Ar containing 9.31 parts-per-million (ppm) of Kr and pure Ar gases in order to dilute Kr concentration to the ppt level. In this study, pure Ar gas was
obtained from a 175 L ELF tank filled with liquid Ar (Z 99:999% (5N), purchased from Tomoe Shokai Co., Ltd.) to decrease the concentration of Kr impurities. The other commercially available Ar and Xe gases (all purchased from Tomoe Shokai Co., Ltd.) described in this section were supplied from an individual gas cylinder. Next, a pure Xe gas sample and gas samples of Xe and Ar each containing approximately 100, 200, and 400 ppb of Kr (seven samples in total) were prepared to compare Kr detection sensitivity in Xe with that in Ar. The concentrations of Kr in the latter six samples were set larger than the Kr impurity concentration (10 ppb) in pure Xe gas. From these two measurements, Kr detection sensitivity at the ppt level in Xe can be estimated. For Xe preparation containing a certain concentration of Kr, the high cost of Xe gas makes it difficult to use MFCs because of the large amount of Xe gas consumed in the creation of the gas mixture. The setup shown in Fig. 6 was constructed for preparing the seven samples. After evacuating the entire region (V A , V B , V C , and V D ), a small quantity of the commercially available Ar gas containing 50.2 ppm of Kr was supplied to V B at a pressure P of 0.02–0.1 MPa, and then diffused to V A through the valve V3. After evacuating V B , a certain amount of pure Xe or Ar gas was supplied to V B , V C , and V D at a pressure P0 of 0.38–0.39 MPa. It was then diffused to V A to dilute the concentration C Kr of Kr as follows: C Kr ¼
50; 200 P V B =ðV A þ V B Þ ½ppb: P 0 ðV B þ V C þ V D Þ=ðV A þ V B þ V C þ V D Þ
ð1Þ
Eq. (1) gives the concentration C Kr of approximately 100 ppb when the initial pressure P is adjusted to approximately 0.024 MPa. This equation was based on the assumption that the Ar gas containing 50.2 ppm of Kr (at a pressure of P V B =ðV A þ V B Þ) hardly flowed
Y. Iwata et al. / Nuclear Instruments and Methods in Physics Research A 797 (2015) 64–69
67
Fig. 6. Experimental setup for preparing Xe and Ar gas samples each containing approximately 100, 200, and 400 ppb of Kr.
away from V A , but it was diluted by pure Xe or Ar gas, due to the large difference in gas pressures. In order to verify this assumption, Kr concentration in the region V B , V C , and V D was measured when an Ar gas sample containing approximately 400 ppb of Kr (P C 0:096 MPa) was prepared. The rate of Ar gas containing 50.2 ppm of Kr flowing away from V A was estimated to be 0.2% or less, which is negligibly small compared to the accuracy of pressures P and P0 measured each by a different digital pressure gauge (Nagano Keiki model ZT64). The display accuracy of the pressure gauge is 7 0:007 MPa, or approximately 7% if the measured value P is approximately 0.096 MPa. After evacuating V B and V C , the diluted gas sample was diffused to that region, and then its pressure was reduced to approximately 0.1 MPa (1 atm). Each gas sample at approximately 1 atm, prepared using either MFCs or the setup shown in Fig. 6, was introduced into the mass spectrometer through an R.M. Jordan C-211 pulsed supersonic valve (PSV). 2.3. Mass spectrometer An R.M. Jordan reflectron-type TOF-MS was used for mass analysis. In this study, a positive pulse VY of approximately 230 V with a width of approximately 250 ns was applied to the deflector electrode while 84Kr ions passed through the deflector. After mass separation, the generated 84Kr ions were detected by a microchannel plate (MCP). For detecting Kr at the ppt level in Ar, the MCP voltage was set to 2500 V, and the detector signal was further amplified by a Hamamatsu Photonics amplifier unit C6438-01 with a gain of 500. For detecting 100–400 ppb of Kr in Xe or Ar, the amplifier unit was removed, and the MCP voltage was reduced to 2400 V (with the total gain reduced to approximately 1/1500). The output pulses were then averaged and stored in an EG&G Instruments FASTFLIGHT digital signal averager.
3. Characterization of the spectrometer 3.1. Wavelength and output energy stability of 212.6 nm laser pulses Before Kr measurement, the wavelength and output energy stability of the generated 212.6 nm laser pulses must be evaluated. Since the wavelength meter does not cover the wavelength region below 300 nm, as already mentioned in Section 2.1, the wavelength of the 530.2 nm laser pulses was measured instead. Fig. 7 (A) shows the results over 1000 s (10,000 pulses), where the data for the 530.2 nm laser pulses generated by OPG are denoted as “OPG.” The other data set denoted as optical parametric oscillator “OPO” in the same figure are those for the laser pulses generated by a Continuum Sunlite EX OPO with a pair of resonator mirrors. A
slight difference in the average wavelength of the two data sets is due to the difference in the pump laser wavelength used for the OPG and OPO processes. Each laser wavelength at approximately 530 nm was fine-tuned so that the wavelength of the sum frequency radiation at 212.6 nm was matched to the two-photon resonance excitation of Kr. It was clearly observed that the wavelength fluctuation (approximately 0.0001 nm or approximately 100 MHz at 530.1504 nm) of the OPG output was significantly lower than that (approximately 0.001 nm or approximately 1 GHz at 530.1425 nm) of the OPO output. With the help of a narrow linewidth seed source, the linewidth of the 530.2 nm laser pulses was reduced to nearly that (approximately 90 MHz) of the pump laser, which is comparable to the Fourier-transform-limited linewidth of approximately 88 MHz with a 5-ns Gaussian pulse. Consequently, the wavelength of 212.6 nm laser pulses was stable enough for the resonance ionization of 84Kr atoms. Fig. 7(B) shows the measured output energy of 212.6 nm laser pulses taken over 10 s (100 pulses). Here, it was also observed that the short-term fluctuation of the laser pulse energy generated using the OPG process was smaller than that generated using the OPO process. A temporary drop in the output energy observed at the 40–45th laser pulses is mainly due to the thermal instability of the BBO crystal used for generating the 212.6 nm light. The influence of the crystal temperature will be investigated and temperature control of the crystal will be performed in future. 3.2. Kr detection sensitivity at the ppt level in Ar To evaluate Kr detection sensitivity at the ppt level in Ar, the pure Ar gas sample and Ar gas samples containing 5, 10, and 20 ppt of Kr were measured over 1000 s. Fig. 8 shows the mass spectra obtained with the samples of (A) Ar gas containing 5 ppt of Kr and (B) pure Ar gas. From each of the obtained spectra, the 84Kr signal intensity was calculated as the net output voltage integrated over the 84Kr TOF region of 44.10–44.30 μs. The background level was determined by a linear fit of the voltage data in adjacent time regions: 44.05–44.10 μs and 44.30–44.35 μs. To diminish laser power fluctuation effect, the measured 84Kr signal intensity Smeas was then divided by the average pulse energy Emeas (mJ/pulse) of the transmitted 212.6 nm light to calculate the assumed signal intensity S7mJ obtained with the transmitted pulse energy of 7 mJ/ pulse: S7mJ ¼ Smeas 7=Emeas . Fig. 9 shows a linear fit to the data of the calculated 84Kr signal intensity S7mJ as a function of Kr concentration in Ar. The horizontal axis is expressed as the values with the Kr impurity concentration in pure Ar gas subtracted. Each data measured over 1000 s was divided into 10 equal time intervals (100 s) to estimate the statistical uncertainty, which is shown as an error bar in Fig. 9. A good linearity can be observed across the Kr concentration from
Y. Iwata et al. / Nuclear Instruments and Methods in Physics Research A 797 (2015) 64–69
OPG OPO
530.1510 OPG (left axis)
530.1505
530.1455
45
530.1450
40
530.1445
530.1500
530.1440
530.1495
530.1435
530.1490
530.1430
530.1485
530.1425
530.1480
530.1420 OPO (right axis)
530.1475 530.1470
0
2000
4000
6000
530.1415
20 15
43.0
43.5
44.0
10
10 8 6 4 2 20
30
40
50
60
70
80
45.0
pure Ar 40Ar
8
10
44.5
Flight Time [µs]
OPG OPO
0
86Kr
83Kr
5
Voltage [mV]
Output Energy at 212.6 nm [mJ/pulse]
84Kr
10
0 42.5
12
2
25
Laser Pulse
0
40Ar
Kr in Ar, MFC
30
530.1410 10000
8000
5 ppt
35 Voltage [mV]
Signal Wavelength (OPG) [nm]
530.1515
Signal Wavelength (OPO) [nm]
68
Laser Pulse
43.0
43.5 44.0 Flight Time [µs]
44.5
45.0
Fig. 8. Mass spectra obtained with the samples of (A) Ar gas containing 5 ppt of Kr and (B) pure Ar gas.
1800
fit Kr in Ar (MFC), pure Ar
Kr Signal Intensity [mV ns] (7 mJ/pulse equiv.)
1600
84
The 84Kr signal intensity can vary with different main components (i.e., Xe and Ar). To compare Kr detection sensitivity in Xe with that in Ar, each of seven gas samples, a pure Xe gas sample and Xe and Ar gas samples each containing 100, 200, and 400 ppb of Kr, was measured over 100 s. From each of the obtained mass spectra, the 84Kr signal intensity S7mJ was calculated in the same way as described in Section 3.2. Fig. 10 shows linear fits to the data of the calculated 84Kr signal intensity S7mJ as a function of Kr concentration in Xe (straight line) and in Ar (dashed line through the origin). The horizontal axis for the data of Kr in Xe is expressed as the values with the Kr impurity concentration in pure Xe gas subtracted. Here, the data of Kr in Ar was fitted with a line through the origin because the Kr impurity (approximately 0.4 ppt) contained in pure Ar gas is negligible compared to the 100–400 ppb of Kr contained in the Ar gas samples. The statistical uncertainty of each data in Fig. 10 could be ignored due to the large concentration
84Kr
4
0 42.5
Fig. 7. Measured stability of the wavelength and output energy of the laser pulses generated by optical parametric generation (OPG) and optical parametric oscillation (OPO).
3.3. Estimated Kr detection sensitivity in Xe
6
2
90 100
0 to 20 ppt. The slope of the fitted line in Fig. 9 corresponds to Kr detection sensitivity at the ppt level in Ar. To evaluate the Kr impurity concentration in pure Ar gas, the 84Kr signal intensity obtained with pure Ar gas (denoted as “0 ppt” of Kr) was divided by the slope of the fitted line. It indicates that approximately 0.4 ppt of Kr impurities were contained in the pure Ar gas. As shown in Fig. 8(B), the presence of Ar2þ ions (m=z ¼ 80) can interfere with 84Kr detection (m=z ¼ 84) at the sub-ppt level. This interference is attributed to the multiphoton and photoelectric ionization of Ar atoms or Ar2 molecules. For detecting Kr in Xe, the peaks of Xe2 þ (m=z ¼62–68), Xe þ (m=z ¼124–136), and Xe2þ (m=z ¼ 248–272) ions generated by the same phenomenon have little effect on Kr detection due to the large mass difference.
2
1400 1200 1000 800 600 400 200 0
0
5
10
15
20
25
Kr Concentration [ppt] Fig. 9. Linear fit to the data of the 84Kr signal intensity (7 mJ/pulse equiv.) as a function of Kr concentration in Ar. The horizontal axis is expressed as the values with the Kr impurity concentration in pure Ar gas subtracted. See Section 2.2 for detailed sample preparation.
of Kr in Xe or Ar. The display accuracy of the pressure gauge described in Section 2.2 was the main source of systematic uncertainty, which is shown as an error bar in Fig. 10. The signal intensity obtained with “0 ppt” of Kr in Xe is considered to be the Kr impurity contained in pure Xe gas. The slopes of the fitted lines indicate that Kr detection sensitivity in Xe was reduced to approximately 54% of that in Ar. The decrease in detection sensitivity can be attributed to the difference in the density distribution of the gas samples with different main components, which would depend on the individual characteristics of the PSV.
Y. Iwata et al. / Nuclear Instruments and Methods in Physics Research A 797 (2015) 64–69
20000 ns] (7 mJ/pulse equiv.)
Kr Signal Intensity [mV 84
Kr in Xe Kr in Ar
18000 16000 14000 12000 10000 8000 6000 4000 2000 0
0
100 200 300 400 Kr Concentration CKr [ppb]
500
Fig. 10. Linear fits to the data of the 84Kr signal intensity (7 mJ/pulse equiv.) as a function of Kr concentration in Xe (straight line) and in Ar (dashed line through the origin). The horizontal axis for the data of Kr in Xe is expressed as the values with the Kr impurity concentration in pure Xe gas subtracted. See Section 2.2 for detailed sample preparation.
69
of 1000 s. Due to the difference in gas density, Kr detection sensitivity in Xe was reduced to approximately half of that in Ar. Nonetheless, our spectrometer is sensitive to Kr impurities in Xe to the sub-ppt level (approximately 0.8 ppt). It is sufficiently sensitive for evaluating Kr contamination in current Xe dark matter detectors. The ultrapure Xe gas used in the XMASS detector will be evaluated with this spectrometer. To search for WIMPs with a cross-section sensitivity below 10 46 cm2, Kr contamination at the ppq level in Xe should be evaluated. There are some promising techniques for further improving the detection efficiency, such as the collection of Kr atoms on a cold finger surface and single-photon excitation of Kr atoms using a pulsed vacuum ultraviolet laser at 116.5 nm. By combining these two techniques, the detection limit of o 106 atoms of Kr in air has been reported [11]. The entire setup would be much more complicated, but RIMS has a strong potential for detecting Kr at levels far below the ppt level in Xe.
Acknowledgments Although Kr detection sensitivity in Xe is expected to be approximately half of that in Ar, our spectrometer can evaluate Kr contamination in Xe to the sub-ppt level (approximately 0.8 ppt). The detection limit for measuring Kr in Xe will be comprehensively evaluated with the ultrapure Xe gas used in the XMASS detector.
The authors are grateful to T. Arima for his technical assistance during the experiments including the laser system operation. This study was partly supported by JSPS KAKENHI Grant no. 25707020.
References 4. Conclusion and future prospects We have developed an ultrasensitive resonance ionization mass spectrometer for measuring Kr at the ppt or sub-ppt level in Xe. With the help of an OPG system without resonator mirrors, the wavelength of the generated 212.6 nm laser pulses proved to be sufficiently stable for the resonance ionization of 84Kr atoms. A temperature control of the BBO crystal for SFG is expected to be effective for further stabilizing the 212.6 nm laser pulse energy. The results show that approximately 0.4 ppt of Kr impurities contained in pure Ar gas were detected with a measurement time
[1] K. Abe, et al., Nuclear Instruments and Methods in Physics Research Section A 716 (2013) 78. [2] E. Aprile, et al., Physical Review Letters 109 (2012) 181301. [3] D.S. Akerib, et al., Physical Review Letters 112 (2014) 091303. [4] K. Abe, et al., Astroparticle Physics 31 (2009) 290. [5] E. Aprile, et al., Physical Review Letters 107 (2011) 131302. [6] P. Cushman, et al., Snowmass CF1 Summary: WIMP Dark Matter Direct Detection arXiv:1310.8327v2, 2013. [7] S. Lindemann, H. Simgen, European Physical Journal C 74 (2014) 2746. [8] E. Aprile, et al., Review of Scientific Instruments 84 (2013) 093105. [9] Y. Iwata, et al., Journal of Nuclear Science and Technology 51 (2014) 465. [10] Y. Iwata, et al., International Journal of Mass Spectrometry 296 (2010) 15. [11] I. Strashnov, et al., Optics Communications 282 (2009) 966.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Ultrasensitive resonance ionization mass spectrometer for evaluating krypton contamination in xenon dark matter detectors Y. Iwata a,n, H. Sekiya b,c, C. Ito a a
Experimental Fast Reactor Department, Oarai Research and Development Center, Japan Atomic Energy Agency, 4002 Narita, Oarai, Ibaraki 311-1393, Japan Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Higashi-Mozumi, Kamioka, Hida, Gifu 506-1205, Japan c Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa, Chiba 277-8582, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 March 2015 Received in revised form 5 June 2015 Accepted 24 June 2015 Available online 2 July 2015
An ultrasensitive resonance ionization mass spectrometer that can be applied to evaluate krypton (Kr) contamination in xenon (Xe) dark matter detectors has been developed for measuring Kr at the partsper-trillion (ppt) or sub-ppt level in Xe. The gas sample is introduced without any condensation into a time-of-flight mass spectrometer through a pulsed supersonic valve. Using a nanosecond pulsed laser at 212.6 nm, 84Kr atoms in the sample are resonantly ionized along with other Kr isotopes. 84Kr ions are then mass separated and detected by the mass spectrometer in order to measure the Kr impurity concentration. With our current setup, approximately 0.4 ppt of Kr impurities contained in pure argon (Ar) gas are detectable with a measurement time of 1000 s. Although Kr detection sensitivity in Xe is expected to be approximately half of that in Ar, our spectrometer can evaluate Kr contamination in Xe to the sub-ppt level. & 2015 Elsevier B.V. All rights reserved.
Keywords: Resonance ionization mass spectrometry Krypton in xenon Optical parametric generation
1. Introduction Recently, liquid xenon (LXe) has been used widely in many underground experiments aiming to achieve the direct detection of dark matter, such as XMASS [1], XENON100 [2], and LUX [3]. With its high scintillation yields, self-shielding capability, and potential for scaling up, LXe is considered a promising target material for detecting rare events, which requires an extremely low background environment. The long-lived radioactive isotope krypton (85Kr) (with a half-life of 10.756 years) in Xe is a major background source because the main decay process (with a branching ratio of 99.563%) involves the emission of a single β particle with a maximum energy of 687 keV (Fig. 1). A large portion of 85Kr is released into the air through nuclear fission processes, accounting for the isotopic abundance of 85 Kr/Kr to be approximately 10 11. High-purity Xe gas commercially obtained through the distillation of liquid air contains Kr traces at the parts-per-billion (ppb) level. A distillation system has been developed to further reduce Kr impurity concentration to the parts-per-trillion (ppt) level [1,4], which is equivalent to a single 85 Kr atom in 1023 Xe atoms (approximately 22 g). Fig. 2 shows background event rates of 85Kr under impurity concentrations of 200 ppb and 2 ppt compared to the signal rate
n
Corresponding author. Tel.: þ 81 29 267 4141; fax: þ 81 29 267 7481. E-mail address: [email protected] (Y. Iwata).
http://dx.doi.org/10.1016/j.nima.2015.06.037 0168-9002/& 2015 Elsevier B.V. All rights reserved.
from weakly interacting massive particle (WIMP) dark matter in Xe target experiments. Here, a WIMP mass of 100 GeV/c2 and a WIMP-nucleon cross-section of 10 45 cm2 are assumed. Nextgeneration experiments are expected to achieve a cross-section sensitivity of 10 46–10 47 cm2 [6] with Kr impurity concentration reduced to the sub-ppt level. To ensure that the detector meets the sensitivity requirements, it is necessary to develop an analytical tool capable of quantifying the abundance of 85Kr. There are two possible approaches: direct counting of 85Kr decay events, and mass analysis of Kr stable isotopes such as 84Kr and 86Kr. In the former method, a delayed coincidence technique is used for identifying the minor decay of 85Kr, which involves the emission of a single β particle with a maximum energy of 173 keV, followed by that of a 514 keV γ-ray (Fig. 1). However, as an example, for detecting 1 ppt of Kr in the XMASS 835 kg LXe detector, the event rate is calculated to be only approximately 0.03 day 1, assuming 85Kr/Kr ¼1 10 11. This value is very low to obtain a statistically significant number of 85Kr decay events even with a few months of life time. The latter method, i.e., mass analysis of 84Kr or 86Kr, enables high sensitivity measurements in a short time and thus is an effective method for evaluating 85Kr contamination if its isotopic abundance can be determined with high accuracy. Gas chromatography is often used in advance of mass analysis to separate Kr from Xe. Coupled with a gas chromatographic system, an atmospheric pressure ionization mass spectrometer and a sector field
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85
Kr
65
10.756 y
Q
0.434% - = 173 keV
1.015
s
514 keV Q
99.563% - = 687 keV
85
Rate [events/kg/day/keVee]
Fig. 1. Decay diagram of
102 101 100 10-1 10-2 10-3 10-4
stable
Rb 85
Kr.
Kr 200 ppb Fig. 3. Resonance ionization scheme of Kr.
WIMP signal mass: 100 GeV/c2 cross section: 10-45 cm2
2. Experimental setup
Kr 2 ppt
10-5 10-6 0 10
101 102 Energy [keVee]
103
Fig. 2. Background event rates of 85Kr compared to the signal rate from WIMP dark matter in liquid Xe dark matter detectors. No background and no effect of detector geometry are taken into account in this figure. The scintillation efficiency assumed in the calculation is based on [5]. See text for the calculation conditions.
mass spectrometer have been developed independently to be applied to the XMASS [1] and XENON100 [7] detectors, respectively. With the help of a condensation process before gas introduction into the mass spectrometer, a detection limit of 8 parts-per-quadrillion (ppq) has been achieved [7]. However, the systematic uncertainty of this process would be a possible drawback in the measurements of Kr concentration. Another promising method, i.e., atom trap trace analysis, has high elemental and isotopic selectivity for detecting a single atom [8]. However, due to the limited number of atoms that can be trapped simultaneously, the overall detection efficiency is expected to be relatively low compared to that of other spectrometers. Herein, we present a resonance ionization mass spectrometer developed for precise and accurate Kr measurement at the ppt or sub-ppt level in Xe. A narrow-linewidth nanosecond pulsed laser at 212.6 nm is used to resonantly ionize Kr atoms in the gas sample. Two 212.6 nm photons excite Kr atoms from the ground state (4p6 1S0) to the 5p½1=20 state, and a third photon of the same wavelength leads to the ionization (Fig. 3). 84Kr ions are mass separated and then detected by a time-of-flight mass spectrometer (TOF-MS). Along with ppt level detection sensitivity, resonance ionization mass spectrometry (RIMS) realizes high elemental selectivity without requiring any condensation procedures. In view of these advantages, isotopic analysis of Kr and Xe using RIMS has already been developed for identifying the failed fuel assembly location in nuclear reactors [9,10]. Next, we describe the detailed experimental setup, including a pulsed laser system and a commercial TOF-MS. The gas sample preparation is also mentioned. In Section 3, we discuss the characterization of our spectrometer based on the measurement results. Finally, a brief summary and future prospects are given in Section 4. Some promising techniques are also presented to further improve the detection limit of the spectrometer.
Fig. 4 shows a schematic of our resonance ionization mass spectrometer. It can be divided into three parts: a laser system, a sample preparation system, and a mass spectrometer. The first two have been newly established in this study, as described in detail below, while the mass spectrometer is the same as that used in another study [9,10].
2.1. Laser system Fig. 3 shows that a pulsed laser at 212.6 nm can be used for the resonance ionization of Kr atoms. In this laser system, the third harmonic (354.8 nm) of a Continuum Powerlite Precision II 9010 Nd:YAG laser is used as a pump source to generate laser light at 530.2 and 212.6 nm. The entire system is operated at a repetition rate of 10 Hz with approximately 5 ns pulse width. For precise and accurate Kr measurements, the wavelength and output energy of the pulsed laser at 212.6 nm have to be stabilized during operation. We have developed an optical system that utilizes optical parametric generation (OPG) without resonator mirrors in order to reduce the linewidth of 530.2 nm light to a level comparable to that of the pump laser (approximately 90 MHz or approximately 0.04 pm at 354.8 nm). The linewidth is sufficiently smaller than the Doppler width of approximately 2 GHz of 84 Kr atoms. Fig. 5 shows a detailed schematic of the OPG system. A Sacher Lasertechnik model TEC-100-1080-060 continuous-wave externalcavity diode laser (ECDL) with a linewidth of o 1 MHz (1 ms) was used as a seed source for efficient wavelength conversion. Its wavelength was fine-tuned to 1072.8 nm, and then the ECDL light was overlapped with a fraction (approximately 30 mJ/pulse) of the pump light to produce a 3–4 mJ/pulse of 530.2 nm light: 354:8 nm-530:2 nm þ 1072:8 nm. The diameter of each laser beam was 2–3 mm. To achieve sufficient conversion efficiency without resonator mirrors, four BBO (β-BaB2O4) crystals were arranged in a row, two with a length of 20 mm and the other two with a length of 15 mm. Each crystal was mounted on a mirror holder to rotate it for phase matching in the OPG and for other nonlinear optical processes also described below. A Bristol Instruments model 821B-VIS wavelength meter operated from 350 to 1100 nm was used for monitoring the ECDL wavelength. The generated 530.2 nm light was separated from the incident laser beams, and then overlapped with a fraction (approximately 200 mJ/pulse) of the pump light to increase the 530.2 nm laser
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Fig. 4. Schematic of the resonance ionization mass spectrometer.
Fig. 5. Detailed schematic of the optical parametric generation (OPG) system.
output energy to 40–50 mJ/pulse. This optical parametric amplification (OPA) process (Fig. 4) was realized by two BBO crystals with lengths of 10 mm. The diameter of each laser beam was enlarged to approximately 6 mm to avoid damage to optical components. The amplified 530.2 nm light was separated from the incident pump light, and then sum-frequency mixed with the residual (approximately 100 mJ/pulse) pump light to produce approximately 10 mJ/ pulse of 212:6 nm light : 530:2 nm þ354:8 nm-212:6 nm. A 7-mm length BBO crystal was used for this sum frequency generation (SFG) process (Fig. 4). To evaluate the wavelength stability of 530.2 nm light, a fraction of the transmitted 530.2 nm laser beam was diverted to another wavelength meter (Bristol Instruments model 821B-VIS). The generated 212.6 nm light was focused into the ionization region of the spectrometer. An Ophir Optronics model PE50-DIF pyroelectric energy meter was used to measure the transmitted pulse energy of the 212.6 nm light. 2.2. Sample preparation Since commercially available pure Xe gas is expensive and still contains a high concentration (approximately 10 ppb) of Kr impurities, it is hard to prepare Xe gas samples containing Kr at the ppt or sub-ppt level. First of all, in the sample preparation, gas samples of argon (Ar) containing approximately 5, 10, and 20 ppt of Kr were prepared and used instead of Xe samples to evaluate Kr detection sensitivity at the ppt level in Ar. Two pairs of mass flow controllers (MFCs) were used to mix the commercially available Ar containing 9.31 parts-per-million (ppm) of Kr and pure Ar gases in order to dilute Kr concentration to the ppt level. In this study, pure Ar gas was
obtained from a 175 L ELF tank filled with liquid Ar (Z 99:999% (5N), purchased from Tomoe Shokai Co., Ltd.) to decrease the concentration of Kr impurities. The other commercially available Ar and Xe gases (all purchased from Tomoe Shokai Co., Ltd.) described in this section were supplied from an individual gas cylinder. Next, a pure Xe gas sample and gas samples of Xe and Ar each containing approximately 100, 200, and 400 ppb of Kr (seven samples in total) were prepared to compare Kr detection sensitivity in Xe with that in Ar. The concentrations of Kr in the latter six samples were set larger than the Kr impurity concentration (10 ppb) in pure Xe gas. From these two measurements, Kr detection sensitivity at the ppt level in Xe can be estimated. For Xe preparation containing a certain concentration of Kr, the high cost of Xe gas makes it difficult to use MFCs because of the large amount of Xe gas consumed in the creation of the gas mixture. The setup shown in Fig. 6 was constructed for preparing the seven samples. After evacuating the entire region (V A , V B , V C , and V D ), a small quantity of the commercially available Ar gas containing 50.2 ppm of Kr was supplied to V B at a pressure P of 0.02–0.1 MPa, and then diffused to V A through the valve V3. After evacuating V B , a certain amount of pure Xe or Ar gas was supplied to V B , V C , and V D at a pressure P0 of 0.38–0.39 MPa. It was then diffused to V A to dilute the concentration C Kr of Kr as follows: C Kr ¼
50; 200 P V B =ðV A þ V B Þ ½ppb: P 0 ðV B þ V C þ V D Þ=ðV A þ V B þ V C þ V D Þ
ð1Þ
Eq. (1) gives the concentration C Kr of approximately 100 ppb when the initial pressure P is adjusted to approximately 0.024 MPa. This equation was based on the assumption that the Ar gas containing 50.2 ppm of Kr (at a pressure of P V B =ðV A þ V B Þ) hardly flowed
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67
Fig. 6. Experimental setup for preparing Xe and Ar gas samples each containing approximately 100, 200, and 400 ppb of Kr.
away from V A , but it was diluted by pure Xe or Ar gas, due to the large difference in gas pressures. In order to verify this assumption, Kr concentration in the region V B , V C , and V D was measured when an Ar gas sample containing approximately 400 ppb of Kr (P C 0:096 MPa) was prepared. The rate of Ar gas containing 50.2 ppm of Kr flowing away from V A was estimated to be 0.2% or less, which is negligibly small compared to the accuracy of pressures P and P0 measured each by a different digital pressure gauge (Nagano Keiki model ZT64). The display accuracy of the pressure gauge is 7 0:007 MPa, or approximately 7% if the measured value P is approximately 0.096 MPa. After evacuating V B and V C , the diluted gas sample was diffused to that region, and then its pressure was reduced to approximately 0.1 MPa (1 atm). Each gas sample at approximately 1 atm, prepared using either MFCs or the setup shown in Fig. 6, was introduced into the mass spectrometer through an R.M. Jordan C-211 pulsed supersonic valve (PSV). 2.3. Mass spectrometer An R.M. Jordan reflectron-type TOF-MS was used for mass analysis. In this study, a positive pulse VY of approximately 230 V with a width of approximately 250 ns was applied to the deflector electrode while 84Kr ions passed through the deflector. After mass separation, the generated 84Kr ions were detected by a microchannel plate (MCP). For detecting Kr at the ppt level in Ar, the MCP voltage was set to 2500 V, and the detector signal was further amplified by a Hamamatsu Photonics amplifier unit C6438-01 with a gain of 500. For detecting 100–400 ppb of Kr in Xe or Ar, the amplifier unit was removed, and the MCP voltage was reduced to 2400 V (with the total gain reduced to approximately 1/1500). The output pulses were then averaged and stored in an EG&G Instruments FASTFLIGHT digital signal averager.
3. Characterization of the spectrometer 3.1. Wavelength and output energy stability of 212.6 nm laser pulses Before Kr measurement, the wavelength and output energy stability of the generated 212.6 nm laser pulses must be evaluated. Since the wavelength meter does not cover the wavelength region below 300 nm, as already mentioned in Section 2.1, the wavelength of the 530.2 nm laser pulses was measured instead. Fig. 7 (A) shows the results over 1000 s (10,000 pulses), where the data for the 530.2 nm laser pulses generated by OPG are denoted as “OPG.” The other data set denoted as optical parametric oscillator “OPO” in the same figure are those for the laser pulses generated by a Continuum Sunlite EX OPO with a pair of resonator mirrors. A
slight difference in the average wavelength of the two data sets is due to the difference in the pump laser wavelength used for the OPG and OPO processes. Each laser wavelength at approximately 530 nm was fine-tuned so that the wavelength of the sum frequency radiation at 212.6 nm was matched to the two-photon resonance excitation of Kr. It was clearly observed that the wavelength fluctuation (approximately 0.0001 nm or approximately 100 MHz at 530.1504 nm) of the OPG output was significantly lower than that (approximately 0.001 nm or approximately 1 GHz at 530.1425 nm) of the OPO output. With the help of a narrow linewidth seed source, the linewidth of the 530.2 nm laser pulses was reduced to nearly that (approximately 90 MHz) of the pump laser, which is comparable to the Fourier-transform-limited linewidth of approximately 88 MHz with a 5-ns Gaussian pulse. Consequently, the wavelength of 212.6 nm laser pulses was stable enough for the resonance ionization of 84Kr atoms. Fig. 7(B) shows the measured output energy of 212.6 nm laser pulses taken over 10 s (100 pulses). Here, it was also observed that the short-term fluctuation of the laser pulse energy generated using the OPG process was smaller than that generated using the OPO process. A temporary drop in the output energy observed at the 40–45th laser pulses is mainly due to the thermal instability of the BBO crystal used for generating the 212.6 nm light. The influence of the crystal temperature will be investigated and temperature control of the crystal will be performed in future. 3.2. Kr detection sensitivity at the ppt level in Ar To evaluate Kr detection sensitivity at the ppt level in Ar, the pure Ar gas sample and Ar gas samples containing 5, 10, and 20 ppt of Kr were measured over 1000 s. Fig. 8 shows the mass spectra obtained with the samples of (A) Ar gas containing 5 ppt of Kr and (B) pure Ar gas. From each of the obtained spectra, the 84Kr signal intensity was calculated as the net output voltage integrated over the 84Kr TOF region of 44.10–44.30 μs. The background level was determined by a linear fit of the voltage data in adjacent time regions: 44.05–44.10 μs and 44.30–44.35 μs. To diminish laser power fluctuation effect, the measured 84Kr signal intensity Smeas was then divided by the average pulse energy Emeas (mJ/pulse) of the transmitted 212.6 nm light to calculate the assumed signal intensity S7mJ obtained with the transmitted pulse energy of 7 mJ/ pulse: S7mJ ¼ Smeas 7=Emeas . Fig. 9 shows a linear fit to the data of the calculated 84Kr signal intensity S7mJ as a function of Kr concentration in Ar. The horizontal axis is expressed as the values with the Kr impurity concentration in pure Ar gas subtracted. Each data measured over 1000 s was divided into 10 equal time intervals (100 s) to estimate the statistical uncertainty, which is shown as an error bar in Fig. 9. A good linearity can be observed across the Kr concentration from
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OPG OPO
530.1510 OPG (left axis)
530.1505
530.1455
45
530.1450
40
530.1445
530.1500
530.1440
530.1495
530.1435
530.1490
530.1430
530.1485
530.1425
530.1480
530.1420 OPO (right axis)
530.1475 530.1470
0
2000
4000
6000
530.1415
20 15
43.0
43.5
44.0
10
10 8 6 4 2 20
30
40
50
60
70
80
45.0
pure Ar 40Ar
8
10
44.5
Flight Time [µs]
OPG OPO
0
86Kr
83Kr
5
Voltage [mV]
Output Energy at 212.6 nm [mJ/pulse]
84Kr
10
0 42.5
12
2
25
Laser Pulse
0
40Ar
Kr in Ar, MFC
30
530.1410 10000
8000
5 ppt
35 Voltage [mV]
Signal Wavelength (OPG) [nm]
530.1515
Signal Wavelength (OPO) [nm]
68
Laser Pulse
43.0
43.5 44.0 Flight Time [µs]
44.5
45.0
Fig. 8. Mass spectra obtained with the samples of (A) Ar gas containing 5 ppt of Kr and (B) pure Ar gas.
1800
fit Kr in Ar (MFC), pure Ar
Kr Signal Intensity [mV ns] (7 mJ/pulse equiv.)
1600
84
The 84Kr signal intensity can vary with different main components (i.e., Xe and Ar). To compare Kr detection sensitivity in Xe with that in Ar, each of seven gas samples, a pure Xe gas sample and Xe and Ar gas samples each containing 100, 200, and 400 ppb of Kr, was measured over 100 s. From each of the obtained mass spectra, the 84Kr signal intensity S7mJ was calculated in the same way as described in Section 3.2. Fig. 10 shows linear fits to the data of the calculated 84Kr signal intensity S7mJ as a function of Kr concentration in Xe (straight line) and in Ar (dashed line through the origin). The horizontal axis for the data of Kr in Xe is expressed as the values with the Kr impurity concentration in pure Xe gas subtracted. Here, the data of Kr in Ar was fitted with a line through the origin because the Kr impurity (approximately 0.4 ppt) contained in pure Ar gas is negligible compared to the 100–400 ppb of Kr contained in the Ar gas samples. The statistical uncertainty of each data in Fig. 10 could be ignored due to the large concentration
84Kr
4
0 42.5
Fig. 7. Measured stability of the wavelength and output energy of the laser pulses generated by optical parametric generation (OPG) and optical parametric oscillation (OPO).
3.3. Estimated Kr detection sensitivity in Xe
6
2
90 100
0 to 20 ppt. The slope of the fitted line in Fig. 9 corresponds to Kr detection sensitivity at the ppt level in Ar. To evaluate the Kr impurity concentration in pure Ar gas, the 84Kr signal intensity obtained with pure Ar gas (denoted as “0 ppt” of Kr) was divided by the slope of the fitted line. It indicates that approximately 0.4 ppt of Kr impurities were contained in the pure Ar gas. As shown in Fig. 8(B), the presence of Ar2þ ions (m=z ¼ 80) can interfere with 84Kr detection (m=z ¼ 84) at the sub-ppt level. This interference is attributed to the multiphoton and photoelectric ionization of Ar atoms or Ar2 molecules. For detecting Kr in Xe, the peaks of Xe2 þ (m=z ¼62–68), Xe þ (m=z ¼124–136), and Xe2þ (m=z ¼ 248–272) ions generated by the same phenomenon have little effect on Kr detection due to the large mass difference.
2
1400 1200 1000 800 600 400 200 0
0
5
10
15
20
25
Kr Concentration [ppt] Fig. 9. Linear fit to the data of the 84Kr signal intensity (7 mJ/pulse equiv.) as a function of Kr concentration in Ar. The horizontal axis is expressed as the values with the Kr impurity concentration in pure Ar gas subtracted. See Section 2.2 for detailed sample preparation.
of Kr in Xe or Ar. The display accuracy of the pressure gauge described in Section 2.2 was the main source of systematic uncertainty, which is shown as an error bar in Fig. 10. The signal intensity obtained with “0 ppt” of Kr in Xe is considered to be the Kr impurity contained in pure Xe gas. The slopes of the fitted lines indicate that Kr detection sensitivity in Xe was reduced to approximately 54% of that in Ar. The decrease in detection sensitivity can be attributed to the difference in the density distribution of the gas samples with different main components, which would depend on the individual characteristics of the PSV.
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20000 ns] (7 mJ/pulse equiv.)
Kr Signal Intensity [mV 84
Kr in Xe Kr in Ar
18000 16000 14000 12000 10000 8000 6000 4000 2000 0
0
100 200 300 400 Kr Concentration CKr [ppb]
500
Fig. 10. Linear fits to the data of the 84Kr signal intensity (7 mJ/pulse equiv.) as a function of Kr concentration in Xe (straight line) and in Ar (dashed line through the origin). The horizontal axis for the data of Kr in Xe is expressed as the values with the Kr impurity concentration in pure Xe gas subtracted. See Section 2.2 for detailed sample preparation.
69
of 1000 s. Due to the difference in gas density, Kr detection sensitivity in Xe was reduced to approximately half of that in Ar. Nonetheless, our spectrometer is sensitive to Kr impurities in Xe to the sub-ppt level (approximately 0.8 ppt). It is sufficiently sensitive for evaluating Kr contamination in current Xe dark matter detectors. The ultrapure Xe gas used in the XMASS detector will be evaluated with this spectrometer. To search for WIMPs with a cross-section sensitivity below 10 46 cm2, Kr contamination at the ppq level in Xe should be evaluated. There are some promising techniques for further improving the detection efficiency, such as the collection of Kr atoms on a cold finger surface and single-photon excitation of Kr atoms using a pulsed vacuum ultraviolet laser at 116.5 nm. By combining these two techniques, the detection limit of o 106 atoms of Kr in air has been reported [11]. The entire setup would be much more complicated, but RIMS has a strong potential for detecting Kr at levels far below the ppt level in Xe.
Acknowledgments Although Kr detection sensitivity in Xe is expected to be approximately half of that in Ar, our spectrometer can evaluate Kr contamination in Xe to the sub-ppt level (approximately 0.8 ppt). The detection limit for measuring Kr in Xe will be comprehensively evaluated with the ultrapure Xe gas used in the XMASS detector.
The authors are grateful to T. Arima for his technical assistance during the experiments including the laser system operation. This study was partly supported by JSPS KAKENHI Grant no. 25707020.
References 4. Conclusion and future prospects We have developed an ultrasensitive resonance ionization mass spectrometer for measuring Kr at the ppt or sub-ppt level in Xe. With the help of an OPG system without resonator mirrors, the wavelength of the generated 212.6 nm laser pulses proved to be sufficiently stable for the resonance ionization of 84Kr atoms. A temperature control of the BBO crystal for SFG is expected to be effective for further stabilizing the 212.6 nm laser pulse energy. The results show that approximately 0.4 ppt of Kr impurities contained in pure Ar gas were detected with a measurement time
[1] K. Abe, et al., Nuclear Instruments and Methods in Physics Research Section A 716 (2013) 78. [2] E. Aprile, et al., Physical Review Letters 109 (2012) 181301. [3] D.S. Akerib, et al., Physical Review Letters 112 (2014) 091303. [4] K. Abe, et al., Astroparticle Physics 31 (2009) 290. [5] E. Aprile, et al., Physical Review Letters 107 (2011) 131302. [6] P. Cushman, et al., Snowmass CF1 Summary: WIMP Dark Matter Direct Detection arXiv:1310.8327v2, 2013. [7] S. Lindemann, H. Simgen, European Physical Journal C 74 (2014) 2746. [8] E. Aprile, et al., Review of Scientific Instruments 84 (2013) 093105. [9] Y. Iwata, et al., Journal of Nuclear Science and Technology 51 (2014) 465. [10] Y. Iwata, et al., International Journal of Mass Spectrometry 296 (2010) 15. [11] I. Strashnov, et al., Optics Communications 282 (2009) 966.