Data from the DBA liquid argon ionization chamber experiment have been used to obtain an estimate on the concentration of 42Ar in the Earth's atmosphere, View the MathML source6.8−3.2+1.7·10−21 atoms of 42Ar per atom of 40Ar corresponding to the 42Ar activity of View the MathML source1.2−0.5+0.3μBq per cubic meter of air

Barabash, AS

Saakyan, RR

Umatov, VI

UCL Discovery

arXiv:1609.08890v1  [nucl-ex]  28 Sep 2016On concentration of 42Ar in the Earth’s atmosphereA.S. Barabasha,∗, R.R. Saakyanb, V.I. UmatovaaNational Research Center “Kurchatov Institute”, Institute of Theoreticaland Experimental Physics, B. Cheremushkinskaya 25, 117218 Moscow, RussiabUniversity College London, London WC1E 6BT, United KingdomAbstractData from the DBA liquid argon ionization chamber experiment have beenused to obtain an estimate on the concentration of 42Ar in the Earth’s atmo-sphere, 6.8+1.7−3.2 ·10−21 atoms of 42Ar per atom of 40Ar corresponding to the 42Aractivity of 1.2+0.3−0.5 µBq per cubic meter of air.Keywords:42Ar, low background experiments2010 MSC: 00-01, 99-001. IntroductionThe long-lived isotope of argon, 42Ar, is a potential background source inargon-based low background detectors. The isotope undergoes a beta decaywith a half-life of 32.9 years and the beta decay of its daughter isotope, 42K,has the maximum electron energy of 3.52 MeV. The decay scheme is shown inFigure 1. The potential background problem from 42Ar was first identified bythe ICARUS experiment [1, 2]. Ray Davis pointed out that a significant amountof 42Ar could be formed in the Earth’s atmosphere due to nuclear weapon testsin the upper atmosphere carried out at the end of the 1950s−beginning of the1960s [3].Estimates of the 42Ar concentration in atmospheric argon were carried out∗Corresponding authorEmail address: barabash@itep.ru (A.S. Barabash)Preprint submitted to Journal of LATEX Templates September 29, 20160+ 32.9 y4218Ar✲β−100%Qβ− = 599 keV2− 12.36 h4219KQβ− = 3525.45 keVβ−✲17.64% 2+1524.6❄✲81.90% 0+4220CarrrrFigure 1: A simplified decay scheme of 42Ar.in 1995 using the available information on nuclear tests [4, 5]. During such tests42Ar can be produced via a two-step neutron capture reaction:40Ar(n,γ)41Ar (1)41Ar(n,γ)42Ar (2)The above mechanism of 42Ar production requires a very high neutron flux,the condition fulfilled during nuclear tests in the upper atmosphere. In addition,the half-life of 41Ar is sufficiently long (T1/2 = 1.83 h) in comparison with atypical time of a nuclear explosion (of the order of milliseconds) to allow anefficient production of 42Ar. It was shown that the maximum concentration of42Ar caused by nuclear tests is of the order of ∼ 10−22−10−23 atoms 42Ar/atom40Ar. An alternative mechanism of 42Ar production was suggested in 1997 [6]leading to a substantially higher level of the isotope content in the atmosphere.In this mechanism 42Ar is produced by cosmic ray interactions in the upperatmosphere via the reaction 40Ar (α,2p) 42Ar. The corresponding 42Ar/40Arratio was estimated to be ∼ 10−20 atoms 42Ar/atom 40Ar.The first experimental limit on the content of 42Ar in atmospheric argonwas obtained in 1992, < 10−18 atoms 42Ar/atom 40Ar [7]. Later on, a moresensitive estimate came from the DBA experiment that ran in the Gran SassoUnderground Laboratory during 1995-2000 [8, 9]. The primary goal of the2experiment was the search for the double beta decay of 100Mo using a liquidargon ionization chamber. The first limit on the 42Ar content was obtainedfrom a partial data set, < 6 · 10−21 atoms 42Ar/atom 40Ar [10], followed by anupdated limit from the full data set, < 4.3 · 10−21 atoms 42Ar/atom 40Ar at90% C.L. [9]. It should be noted that a surplus of events above the expectedbackground was observed in the DBA experiment in a single electron energyspectrum. However, due to multiple other sources that can contribute to thisevent topology it was difficult to attribute these events to 42Ar. A conservativeapproach was therefore adopted and only an upper limit established.More recently, the GERDA-I experiment carried out an independent mea-surement of the 42Ar concentration [11]. The aim of the experiment is to searchfor the neutrinoless double beta decay (0νββ) of 76Ge. The experimental sig-nature of 0νββ is a peak at the Q-value of the decay (2039 keV). The isotopeof 42K is therefore a potential background source for this experiment.GERDA-I started data taking in 2011. High-purity germanium (HPGe) de-tectors were immersed directly in a vessel with ∼ 90 tons of liquid argon. The1525 keV gamma line from 42K was used to estimate the 42Ar content. Theinterpretation of the observed count rate in terms of the 42Ar concentration de-pends crucially on the assumption of the spatial distribution of 42K ions. It wasshown previously (see for example [13]) that positive 42K ions are preferentiallyformed as a result of the 42Ar β− decay. Their spatial distribution in liquidargon is determined by the electric field configuration in the detector. A highvoltage of ∼ 4 kV is typically applied to HPGe detectors. As a result, 42K ionsdrift in the electric field and are accumulated in the vicinity of the detectorsleading to a higher counting rate around the 1525 keV gamma line than thatexpected from a uniform distribution of 42K in liquid argon.The GERDA collaboration carried out a thorough study of the influenceof the electric field on the level of background and methods of its reduction.Thin-walled copper containers, mini-shrouds, were constructed around HPGedetectors to isolate them from the main liquid argon volume. It resulted ina significant reduction of the background in the 0νββ region and allowed the342Ar concentration in liquid argon to be estimated, giving a range of (7 − 12) ·10−21 atoms 42Ar/atom 40Ar [11] 1. Although this value is higher than thelimit obtained in [9], it is worth noting that, within errors, the observed effectcorresponds to the excess seen in the DBA detector. The new analysis of theDBA single electron events presented here has been triggered by the GERDA-Iresult.2. Updated DBA result on 42Ar concentrationThe data obtained in the DBA experiment [9] have been reanalyzed to esti-mate the 42Ar content in the Earth’s atmosphere. The detector was located inthe Gran Sasso Underground Laboratory at a depth of 3500 m of water equiva-lent. The experimental setup consisted of a liquid Ar ionization chamber (Fig.2) placed in a 15 cm lead passive shielding, an Ar purification and delivery sys-tem, readout electronics and a data acquisition system. The fiducial part of thedetector is composed of alternating circular planes of anodes and cathodes withFrisch screening grids placed between them. The cathodes are made of a molyb-denum foil approximately 50 mg/cm2 thick. The chamber contains 14 cathodes,15 anodes and 28 screening grids. The grid-anode distance is 5.5 mm and thegrid-cathode is 14.5 mm. The fiducial volume diameter is 30 cm and its heightis 56 cm. Each anode is connected to a charge-sensitive preamplifier, followedby an amplifier and a shaper. Shaped and unshaped signals are digitized by two8-bit flash ADC with a 50 ns sampling time. Unshaped pulse analysis providesspatial information of the events. The trigger for data collection requires thatat least one anode signal exceeds the threshold (∼ 600 keV). The trigger causes1The GERDA low-background test facility LArGe was also used to study the 42Ar back-ground. Its concentration was found to be (2.2 ± 1.0) · 10−21 atoms 42Ar/atom 40Ar. Theassumption of a uniform distribution of 42K in liquid argon was used [12]. The effect of theelectric field from PMTs immersed in the liquid argon volume was not taken into account inthe estimation. Consequently, the 42Ar concentration could have been underestimated, whichwas mentioned in [12]).4digitized signals from all anodes to be recorded. A more detailed description ofthe experiment can be found in [9].During the data taking period the electric field was 1.9 kV/cm in the cathode-grid gap and 4 kV/cm in the anode-grid gap. The detector energy scale wascalibrated using 22Na (Eγ = 1275 keV) and88Y ((Eγ = 1836 keV) radioactivesources (see description in [9]). The energy resolution of the chamber was ∼ 6%(FWHM) at the 0νββ transition energy of 100Mo, 3 MeV.A single electron signature is used to select candidate events from 42K decays.This signature requires a single energy deposition detected on one of the anodeswith no signals on all other anodes. The 3.0-3.5 MeV energy interval of thesingle electron spectrum accumulated over 2706 hours is used to determine the42Ar concentration in liquid argon.The new analysis of the experimental data has shown a small shift in theenergy scale of the calibration spectra that was unaccounted for in the originalanalysis. A 5th-degree polynomial is used to fit the high energy edge of theCompton spectrum. The inflection point of the fitting function corresponds tothe maximum energy of Compton electrons. However, multiple interactions of γrays in a single cathode-grid volume shift the inflection point to higher energies.This effect is illustrated in Figure 3.Decays of 208Tl from materials surrounding the detector are used as an ad-ditional point of the energy calibration that takes into account multiple gamma-ray interactions in a single cathode-grid gap. Figure 4 shows the fit of simulated208Tl events in materials surrounding the fiducial volume of the detector to theexperimental single energy spectrum around 2381.76 keV. This energy corre-sponds to the maximum energy of Compton electrons from the 2614 keV line of208Tl.The energy scale correction has been applied to the single electron energyspectrum, which is shown in Figure 5.MC simulations have been carried out to evaluate the detection efficiency inthe 3.0-3.5 MeV energy interval for two possible distributions of 42K in liquidargon. In the first case, positive ions of 42K created in the β decay of 42Ar are5Figure 2: Diagram of the liquid-argon ionization detector used in the DBA experiment: ( 1) chamber filled with liquid argon; ( 2 ) thermostat volume filled with liquid nitrogen; ( 3 )vacuum volume of the thermostat; ( 4 ) space filled with nitrogen vapor; and ( 5 ) system ofheaters.60100020003000400050006000700080009000800 900 1000 1100 1200 1300 1400a)010002000300040005000600070001400 1500 1600 1700 1800 1900 2000b)Energy (keV)Counts/10 keVFigure 3: Simulated energy spectra of single electrons in the DBA liquid argon detector fromthe sources of 22Na (a) and 88Y (b). Arrows show inflection points at 1099 keV (a) and at1644 keV (b). Vertical lines correspond to the maximum energy of Compton electrons, 1061.70keV and 1611.78 keV, respectively.705000100001500020000250003000035000400002000 2200 2400 2600 2800 3000 3200 3400ENERGY (keV)COUNTS/100 keVFigure 4: High energy part of the single electron spectrum observed in data (solid line). Thedashed line shows the simulated 208Tl events in detector materials fitted to the experimentaldata. The inflection point of the experimental spectrum is shown by the arrow at 2475 keV.8010002000300040005000600070008000x 10 20 1000 2000 3000 4000 5000a)0204060801001201403000 3250 3500 3750 4000 4250 4500 4750 5000 5250 5500b)Energy (keV)Counts/100 keVFigure 5: Single electron energy spectrum in the region of 0-5.5 MeV (a) and 3.0-5.5 MeV(b). The measurement time is 2706 hours.9assumed to maintain their charge for a sufficiently long time to be transportedto negatively charged cathodes of the detector and undergo their β decay fromthere, as was shown in [13]. In the second case, a uniform distribution of 42K inthe liquid argon volume of the detector is assumed. The efficiency is calculatedto be 0.87% and 1.07% in the first and second case respectively. The calcula-tions below assume the accumulation of positive 42K ions on negatively chargedcathodes of the chamber. However, a uniform distribution of neutral 42K atomsin the detector volume will not alter the final result by more than 15%. Re-cently, EXO-200 reported the fraction of 214Bi+ ions in the 214Pb β decay (inliquid Xe) to be (76.4± 5.7)%, while the remainder of 214Bi atoms are neutral[14]. Assuming the same ratio of positive to neutral 42K atoms from the 42Ardecay (in liquid Ar) the above efficiency is estimated to be 0.92%. Ultimately,the uncertainty on the efficiency due to the distribution of the 42K ions in thevolume of the liquid argon detector is included in the systematic uncertainty onthe 42Ar concentration calculated below.Table 1 shows possible other contributions to the single electron events inthe 3.0-3.5 MeV energy interval which are treated as background for the 42Armeasurement. The contribution from nγ reactions are evaluated by extrapo-lating the flat part of the spectrum at high energies down to lower energies.The external background from 214Bi is estimated using the available informa-tion about the radiopurity of titanium (< 30 mBq/kg), which was the primarymaterial of the detector vessel. The 208Tl external background is estimated byfitting the Compton edge of the single electron spectrum around 2400 keV (seeFigure 4), and evaluating its contribution to the single electron events in the3.0-3.5 MeV energy region.Limits obtained from screening measurements with low-background HPGedetectors are used to constrain the background from the internal contaminationinside the molybdenum cathode foils (the specific activities of 208Tl and 214Biare < 1.6 mBq/kg and < 10 mBq/kg, respectively). Taking into account theabove background sources the contribution of 42Ar to the single electron eventsobserved in the 3.0-3.5 MeV interval over the period of data taking is estimated10Table 1: Possible background contributions (in addition to 42Ar) to single electron events inthe energy region of 3.0-3.5 MeV. The limits are given at 95% C.L.Background Number of eventsγ-rays from neutrons 66208Tl (external) 87214Bi (external) < 0.4208Tl (Mo cathodes) < 89214Bi (Mo cathodes) < 10to be 240± 23(stat)+55−110(syst) events. The main source of the systematic erroris a relatively weak constrain on 208Tl in the Mo cathode foils (see Table 1).Other sources of the systematic error include the energy scale calibration, the42K ion distribution in liquid argon and its effect on the detection efficiency.The argon used in the detector had been produced ∼ 10 years before themeasurements were carried out. Thus, given the half-life of 42Ar (32.9 years)the concentration of the isotope in the detector’s volume is 20% lower than thatin the Earth’s atmosphere.Using the above parameters and accounting for the total fiducial Ar massof 52 kg, the concentration of 42Ar in the Earth’s atmosphere can be estimatedas 6.8+1.7−3.2 · 10−21 atoms of 42Ar/atom of 40Ar, which corresponds to the 42Aractivity of 1.2+0.3−0.5 µBq/m3 of air. This leads to a specific activity of freshlyproduced liquid argon of 68+17−32 µBq/kg, in agreement with the earlier GERDA-I result [11]. We also note that the presented result is consistent with themechanism of 42Ar production suggested in [6].AcknowlegementsThis work was supported by RFBR under the research project No. 15-02-02919.11ReferencesReferences[1] J.N. Bahcall, M. Baldo-Ceolin, D.B. Cline and C. Rubia, Phys. Lett. B 178(1986) 324.[2] P. Cennini et al., Proposal by the ICARUS Collaboration, IV, 1992, p. 80.[3] R. Davis , 1979 (private communication)[4] A.S. Barabash, V.N. Kornoukhov and V.E. Jants, Nucl. Instr. Meth. A 385(1997) 530 ; Preprint ITEP 18/95 (1995).[5] P. Cennini et al., Nucl. Instr. Meth. A 356 (1995) 526.[6] A.J. Peurung, T.W. Bowyer, R.A. Craig and P.L. Reeder, Nucl. Inst. Meth.A 396 (1997) 425.[7] C. Arpesella et al., Preprint LNGS 92/27 (1992).[8] V.D. Ashitkov et al., JETP Lett. 74 (2001) 529.[9] V.D. Ashitkov et al., Inst. Exp. Tech. 46 (2003) 153.[10] A.S. Barabash et al., Nucl. Instr. Meth. A 416 (1998) 179.[11] M. Agostini et al., Eur. Phys. J. C 74 (2014) 2764.[12] M. Heisel, thesis, Heidelberg, 2011.[13] A.S. Barabash et al., Sov. J. Nucl. Phys. 30 (1979) 559.[14] J.B. Albert et al., Phys. Rev. C 92 (2015) 045504.12

On concentration of Ar-42 in the Earth's atmosphere

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On concentration of Ar-42 in the Earth's atmosphere

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