Discovery of Underground Argon with a Low Level of Radioactive 39Ar And

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OPEN ACCESS Recent citations Discovery of underground argon with a low level of - A Review of Basic Energy Reconstruction 39 Techniques in Liquid Xenon and Argon radioactive Ar and possible applications to Detectors for Dark Matter and Neutrino Physics Using NEST WIMP dark matter detectors Matthew Szydagis et al - Sensitivity of the COHERENT experiment To cite this article: C Galbiati et al 2008 J. Phys.: Conf. Ser. 120 042015 to accelerator-produced dark matter D. Akimov et al

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This content was downloaded from IP address 170.106.202.126 on 27/09/2021 at 00:06 10th Int. Conf. on Topics in Astroparticle and Underground Physics (TAUP2007) IOP Publishing Journal of Physics: Conference Series 120 (2008) 042015 doi:10.1088/1742-6596/120/4/042015

Discovery of underground argon with low level of radioactive 39Ar and possible applications to WIMP dark matter detectors

C Galbiati∗,1, D Acosta-Kane1, R Acciarri2, O Amaize1, M Antonello2, B Baibussinov3, M Baldo Ceolin3, C J Ballentine4, R Bansal5, L Basgall6, A Bazarko7, P Benetti8, J Benziger9, A Burgers1, F Calaprice1, E Calligarich8, M Cambiaghi8, N Canci2, F Carbonara10, M Cassidy11, F Cavanna2, S Centro3, A Chavarria1, D Cheng1, A G Cocco10, P Collon12, F Dalnoki-Veress1, E de Haas1, F Di Pompeo2, G Fiorillo10, F Fitch13, V Gallo10, M Gaull1, S Gazzana14, L Grandi14, A Goretti1, R Highfill5, T Highfill5, T Hohman1, Al Ianni14, An Ianni1, A LaCava15, M Laubenstein14, H Y Lee16, M Leung1, B Loer1, H. H Loosli17, B Lyons1, D Marks1, K McCarty1, G Meng3, C Montanari8, S Mukhopadhyay18, A Nelson1, O Palamara14, L Pandola14, F Pietropaolo3, T Pivonka5, A Pocar19, R Purtschert17, A Rappoldi8, G Raselli8, F Resnati20, D Robertson12, M Roncadelli8, M Rossella8, C Rubbia14, J Ruderman1, R Saldanha1, C Schmitt12, R Scott16, E Segreto14, A Shirley21, A M Szelc22,2, R Tartaglia14, T Tesileanu1, S Ventura3, C Vignoli8, C Visnjic1, R Vondrasek16 and A Yushkov10 ∗ Contribution Presented by C. Galbiati at TAUP 2007. E-mail: [email protected] 1 Department of Physics, Princeton University, Princeton, NJ 08544, USA 2 INFN and Dipartimento di Fisica, University of L’Aquila, L’Aquila 67100, Italy 3 INFN and Dipartimento di Fisica, University of Padua, Padua 35131, Italy 4 School of Earth, Atmospheric, and Environmental Sciences, University of Machester, M13 9PL, United Kingdom 5 AirSep Corporation, Buffalo, NY 14228, USA 6 Kansas Refined Helium, Otis, KS 67565, USA 7 Schlumberger Princeton Technology Center, Princeton, NJ 08550, USA 8 INFN and Dipartimento di Fisica Nucleare e Teorica, University of Pavia, Pavia 27100, Italy 9 Department of Engineering, Princeton University, Princeton, NJ 08544, USA 10 INFN and Dipartimento di Scienze Fisiche, University of Naples “Federico II”, Naples 80216, Italy 11 Department of Geosciences, University of Houston, Houston, TX 77204, USA 12 Department of Physics, Notre Dame University, Notre Dame, IN 46556, USA 13 Linde Engineering, Murray Hill, NJ 07974, USA 14 INFN, Laboratori Nazionali del Gran Sasso, Assergi (AQ) 67100, Italy 15 Felician College, Lodi, NJ 07644, USA 16 Argonne National Laboratories, Argonne, IL 60439, USA 17 Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland 18 Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA 19 Department of Physics, Stanford University, Stanford, CA 94305, USA

c 2008 IOP Publishing Ltd 1 10th Int. Conf. on Topics in Astroparticle and Underground Physics (TAUP2007) IOP Publishing Journal of Physics: Conference Series 120 (2008) 042015 doi:10.1088/1742-6596/120/4/042015

20 INFN and Dipartimento di Fisica, University of Milano Bicocca, Milano 20126, Italy 21 Linde Gas, Murray Hill, NJ 07974, USA 22 Instytut Fizyki Jadrowej PAN, 31-342 Krakow, Poland

Abstract. We report on the ﬁrst measurement of 39Ar in argon from underground natural gas reservoirs. The gas stored in the US National Helium Reserve was found to contain a low level of 39Ar. The ratio of 39Ar to stable argon was found to be ≤4×10−17 (84% C.L.), less than 5% the value in atmospheric argon (39Ar/Ar=8×10−16). The total quantity of argon currently stored in the National Helium Reserve is estimated at 1000 tons. 39Ar represents one of the most important backgrounds in argon detectors for WIMP dark matter searches. The ﬁndings reported demonstrate the possibility of constructing large multi-ton argon detectors with low radioactivity suitable for WIMP dark matter searches.

Argon is an ideal target for WIMPs searches as it allows detection of rare WIMP-induced nuclear recoils down to a few keV by scintillation and/or ionization. Liquid argon, in particular, is an excellent material for use as a detector of ionizing particles. It produces copious scintillation light, allows the drift of the ionization charge over long distances, and it has been used for large detectors [1]. Moreover, the difference in the stopping power between nuclear recoils and β/γ events leads to significant differences in the ratio of ionization charge to scintillation light detected and in the scintillation light pulse shape, providing powerful tools to separate WIMP- induced events from natural radioactivity [2, 3]. Studies with the WARP 3.2-kg liquid argon detector demonstrated that pulse shape alone permits a separation of 1 recoil in 108 β’s, and the separation by the ratio of scintillation to ionization is 1 in 102 [2]. For a liquid argon WIMP detector, the separation of recoil events from β events is particularly important because of the intrinsic background from β decays of 39Ar, present in atmospheric 39 argon. The specific activity of Ar (Q = 565 keV, t1/2 = 269 y) is ∼1 Bq/kg of atmospheric argon [4, 5]. 39Ar is produced by cosmic ray interactions in the atmosphere, principally via the 40Ar(n,2n)39Ar reaction [6, 7]. The WARP 3.2-kg detector [2] published results from a first search for WIMPs obtaining a sensitivity comparable to the best current limits [8]. The high selectivity for argon recoils should permit a sensitive WIMP search with a 140 kg liquid argon detector employing atmospheric argon, currently under construction at Laboratori Nazionali del Gran Sasso [9]. However, in spite of its favorable β/recoil discriminating power, it is highly desirable to use argon with a much lower 39Ar contamination for future, larger detectors. Based on the proven β/recoil discrimination, a 10-fold or more reduction of 39Ar with respect to the atmospheric level would enhance the prospects of future multi-ton argon WIMP detectors. Argon from natural gas wells is a promising source of 39Ar-depleted argon because 39Ar production induced by cosmic rays is strongly suppressed underground. In the subsurface, 39Ar can be produced by a number of reactions, mainly neutron reactions on potassium, such as 39K(n,p)39Ar [10]. Argon samples collected in groundwater from U and Th rich crystalline rocks revealed an enhanced 39Ar activity, with 39Ar/Ar ratios exceeding atmospheric values by up to 16 times [11]. The enhanced activity is likely due to an abundant local neutron flux, originating from fission or (α,n) reactions induced by decays in the U and Th chains. The 39Ar/Ar ratio in subsurface gases thus depends on the local U, Th, and K concentration, on the porosity of the surrounding rocks, and may vary significantly among geological formations. We report on the first measurement of the 39Ar/Ar ratio in natural gas wells [12]. We measured argon samples originating from the US National Helium Reserve, located in the Cliffside Storage Facility outside Amarillo, TX [13]. The facility stores crude helium separated by chromatography and/or cryogenic distillation from the nearby helium-rich natural gas fields. The separation processes employed also transfer a limited fraction of the argon contained in

2 10th Int. Conf. on Topics in Astroparticle and Underground Physics (TAUP2007) IOP Publishing Journal of Physics: Conference Series 120 (2008) 042015 doi:10.1088/1742-6596/120/4/042015 the natural gas along with the crude helium stream. All plants producing crude helium are connected to the US National Helium Reserve by the Helium Conservation Pipeline, run by the Bureau of Land Management. Crude helium samples from the National Helium Reserve were collected from the Helium Conservation Pipeline at the Kansas Refined Helium plant in Otis, Kansas. The composition of the crude helium, as measured by mass spectrometry, can be found in Ref. [12]. The crude helium was processed with a novel Pressure Swing Adsorption plant developed at Princeton to remove non-argon components [12]. Measurements of 39Ar activity were performed at the Low Level Counting Underground Laboratory at the Physics Institute at the University of Bern with a low background proportional counter [4]. The detector background is comparable to the rate due to 39Ar in atmospheric argon and must be determined accurately. Three measurements are performed to measure the 39Ar content in the sample of interest: one with a reference 39Ar-depleted sample to measure intrinsic background, one with standard atmospheric argon for reference, and one with the underground sample itself. The 39Ar activity of the sample under investigation is evaluated by subtracting the detector background from the total measured activity and is then compared to the 39Ar activity in atmospheric argon (background subtracted). More details can be found in Ref. [12]. The best estimate for the 39Ar activity indicates a 39Ar/Ar ratio ≤4×10−17, which is less than 5% of the 39Ar/Ar ratio in atmospheric argon. The depletion factor from the atmospheric activity of the sample is therefore ≥20 at 84.1% C.L. (≥10 at the 97.7% C.L.). This result is, to the best of our knowledge, the first measurement of the concentration of 39Ar in subsurface gas. It represents an upper limit which is based on the background of the proportional counting system. It therefore motivates the development of new and potentially more sensitive techniques, such as Accelerator Mass Spectrometry (AMS) [14] and 39Ar-decay counting in liquid argon detectors [5], for a more accurate determination of the 39Ar/Ar ratio. The discovery of low 39Ar/Ar ratio in the US National Helium Reserve is part of a larger program of exploration of several possible underground argon sources. During this investigation, the authors developed the technology required to separate and collect large quantities of argon from natural gas wells. The quantity of argon processed by the Kansas Refined Helium plant is 25 tons per year. Production of a 10 ton target of liquid argon for a dark matter search experiment would take a 1-yr production campaign after construction and commissioning of dedicated separation plants. These findings lead the way for future multi-ton, low background, argon detectors for WIMP dark matter, able to reach sensitivities for the WIMP-nucleon cross section of 10−46 cm2 or smaller.

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