Search for Two-Neutrino Double Electron Capture on $^{124} $ Xe

Home , Double electron capture

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Y. Z νβ a and aik bevtr,IsiuefrCsi a eerh th Research, Ray Cosmic for Institute Observatory, Kamioka − 124 a,d b obeeeto atr,Nurn,Lqi xenon Liquid Neutrino, capture, electron Double β etro negon hsc,IsiuefrBscScience Basic for Institute Physics, Underground of Center . d A 3 − al nttt o h hsc n ahmtc fteUnive the of Mathematics and Physics the for Institute Kavli f,d .Hiraide K. , ei efre sn 6. aso aacletdwt h XM the with collected data of days 165.9 using performed is Xe ) × r h tmcnme n tmcms number mass atomic and number atomic the are a,d .H Kim H. Y. , b k [email protected] . [email protected] 10 .Tasaka S. , .Norita T. , eateto hsc,Fclyo niern,Ykhm Na Yokohama Engineering, of Faculty Physics, of Department ( ( Z Z ν 21 , , CC,i loalpo ubrviolating number lepton a also is ECEC), A A er t9%cndnelvla well. as level confidence 90% at years ) ) + → a,d .Takiya H. 2 i g,b ( oa ersra niomn aoaoy aoaUnivers Nagoya Laboratory, Environment Terrestrial Solar e .Ichimura K. , a c,1 Z .Ogawa H. , − h .S Lee S. J. , eateto hsc,Myg nvriyo dcto,Sen Education, of University Miyagi Physics, of Department .Liu J. , + → g j oe eerhIsiueo tnad n cec,Daejeon Science, and Standards of Institute Research Korea 2 eateto hsc,TkiUiest,Hrtua Kanag Hiratsuka, University, Tokai Physics, of Department ( , Z A c nomto n utmdacne,Gf nvriy iu5 Gifu University, Gifu center, multimedia and Information i f ) .Uchida H. , eateto hsc,Kb nvriy oe yg 657-85 Hyogo Kobe, University, Kobe Physics, of Department − d,2 + 2 g .Martens K. , a,d 2 , .B Lee B. K. , a,d e A .Sekiya H. , − ) .Kishimoto Y. , , , i .Nishijima K. , 10,Japan. -1205, g d .K Lee K. M. , .Suzuki Y. , Vermillion, , h nvre aoaUiest,Fr-h,Ciuak,N Chikusa-ku, Furo-cho, University, Nagoya Universe, the d MS Collaboration XMASS a,d h h Uni- the ch, double s .Takachio O. , eoti oe ii nthe on limit lower a obtain We a,d nvriyo oy,HgsiMzm,Kmoa ia Gifu Hida, Kamioka, Higashi-Mozumi, Tokyo, of University e 0Ysogdeo18-i,Ysogg,Deen 305-811 Daejeon, Yuseong-gu, 1689-gil, Yuseong-daero 70 , aelpo ubrvoain erhfrtonurn dou two-neutrino for search A violation. number lepton rate .Kobayashi K. , (2) (1) ta n e s WI,teUiest fTko ahw,Cia 277-858 Chiba, Kashiwa, Tokyo, of University the (WPI), rse - j c w ria lcrn r atrdsmlaeul nth in simultaneously captured are electrons orbital two ich .Fujii K. , g d e eeec o h aclto fncermti eleme matrix nuclear of calculation the for reference new a e .Fukuda Y. , .Fujita R. , l-iea 4 as alf-life inlUiest,Ykhm,Kngw 4-51 Japan 240-8501, Kanagawa Yokohama, University, tional enosre nmr hntniooe,teeeitol fe a 2 only for exist there results isotopes, experimental ten positive than more in observed been er 1]adadrc esrmn for measurement direct a and [14] years a esrmn for measurement cal n w-etiodul lcrncpue(2 capture electron double two-neutrino and r loe ihntesadr oe.Atog 2 Although model. standard the within allowed are obebt lsdcy(0 decay plus beta double osbeehneeto h atr aeb atra large as factor a a However, by rate energy. 10 capture decay as the the of away enhancement carries possible that accompani photon and life-time a longer by a T have to simultaneously. expected captured is are process electrons orbital two where osbergthne ekcret[2 13]. [12, current weak right-handed possible e he the could modes determine decay nuclear to these of Detection nuclei. di final mass the on depending cleus n xeietly[,8 ,1,1] oevr neutrinoles Moreover, (0 11]. capture 10, 4, electron 9, [3, positron-emitting 8, theoretically [7, both pro experimentally attention decay and attracting nuclear this also hence is and cess [2], degenerate are nucleus a .Takeda A. , nteohrhn,tonurn obebt ea (2 decay beta double two-neutrino hand, other the On k 6 .Murayama I. , a cu ftemse fteiiiladfia (excited) final and initial the of masses the if occur can f a,d t,Ngy,Aci4480,Japan 464-8602, Aichi Nagoya, ity, .Hosokawa K. , ∗ h . .Kobayashi M. , .Itow Y. , 7 124 a,Myg 8-85 Japan 980-0845, Miyagi dai, × S- iudxnndtco.N infiatexcess significant No detector. xenon liquid ASS-I w 5-22 Japan 259-1292, awa 0-4,SuhKorea South 305-340, a,d 10 ewt h MS- detector XMASS-I the with Xe 119,Japan 01-1193, .Yamashita M. , 21 i,e er t9%cndnelvl h obtained The level. confidence 90% at years 1 Japan 01, ff 126 .Kegasa R. , k cienurn asadprmtr fa of parameters and mass neutrino ective 130 .Nakamura S. , etonurn obeeeto capture electron double two-neutrino Xe f .Miuchi K. , awt aflf f(2 of half-life a with Ba a .Moriyama S. , νβ + β a,d ff i + .Kobayashi K. , rnebtenteiiiland initial the between erence a cu ntesm nu- same the in occur may ) .S Yang S. B. , gy,Aci 6-62 Japan 464-8602, Aichi, agoya, ν f k CCs a:ageochemi- a far: so ECEC .Oka N. , νβ 0-25 Japan 506-1205, , + a,d ot Korea South , C n neutrinoless and EC) 78 .Nakagawa K. , ,Japan 2, rwt half-life a with Kr f ν .Onishi Y. , a,d CC processes ECEC) i . .Y Kim Y. N. , .Masuda K. , 2 ± l electron ble a 7 2016 27, May 0 νβ . 5) − f same e νβ β , × a − ,6] 5, , − 10 b β has i , nts his , ed − lp 21 w s ) - along the central vertical axis of the detector. Measuring with 124 Table 1: Calculated half-lives for 2νECEC on Xe. The lower and upper the 57Co source from the center of the detector volume the values are calculated for the axial-vector coupling constant gA = 1.26 and 1.0, respectively. photoelectron yield is determined to be 13.9 photoelectrons (PEs)/keV [28]. This large photoelectron yield is realized by 2νECEC 21 a large inner surface photocathode coverage of >62% and the Model T1/2 (×10 yr) Reference QRPA 0.4-8.8 [19] large PMT quantum efficiency of approximately 30%. The non- QRPA 2.9-7.3 [13] linear response in scintillation light yield for electron-mediated events in the detector was calibrated with 55Fe, 57Co, 109Cd, and SU(4)στ 7.0-18 [20] 241 PHFB 7.1-18 [21] Am sources. When a PMT signal exceeds the discriminator PHFB 61-160 [22] threshold equivalent to 0.2 PE, a “hit” is registered on the chan- MCM 390-980 [23] nel. Data acquisition is triggered if ten or more hits are asserted within 200ns. Each PMT signal is digitized with charge and timing resolution of 0.05PE and 0.4ns, respectively [31]. The liquid xenondetector is located at the center of a cylindrical wa- +5.5 21 of (9.2−2.6(stat) ± 1.3(sys)) × 10 years [11]. In the case that ter Cherenkov veto counter and shield, which is 11m high with after the 2νECEC process the nucleus is in the ground state, a 10m diameter. The veto counter is equipped with 72 20-inch the observable energy comes from atomic de-excitation and nu- PMTs. Data acquisition for the veto counter is triggered if eight clear recoil; depending on the nucleus, the energy deposited or more of its PMTs register a signal within 200ns. XMASS- by nuclear recoil may become negligible, leading to a well de- I is the first direct detection dark matter experiment equipped fined energy deposit dominated by the atomic de-excitation - a with such an active water Cherenkov shield. line spectrum. Nevertheless, little attention has been paid to direct detection of this process because of difficulties due to small natural abundance and the energy threshold of large volume de- 3. Expected Signal and Detector Simulation tectors. Any measurement of 2νECEC will provide a new ref- The process of 2νECEC on 124Xe is erence for the calculation of nuclear matrix elements from the 124 − 124 proton-rich side of the mass parabola of even-even isobars [15]. Xe + 2e → Te + 2νe (3) Although the matrix element for the two-neutrino mode is different from that for the neutrinoless mode, it gives constraints with a Q-value of 2864 keV. In the case that two K-shell elec- on the relevant parameters within a chosen model [16]. trons in the 124Xe atom are captured simultaneously, a daughter The XMASS detector uses liquid xenon in its natural iso- atom of 124Te is formed with two vacancies in the K-shell and topic abundance as its active target material. Among others it de-excites by emitting atomic X-rays and/or Auger electrons. 124 contains the double electron capture nuclei Xe (0.095%) and The total energy deposition in the detector is 2Kb = 63.63 keV, 126 136 Xe (0.089%), as well as the double beta decay nuclei Xe where Kb is the binding energy of a K-shell electron in a tel- (8.9%) and 134Xe (10.4%). It has been pointed out that large lurium atom. The energy deposition from the recoil of the volume dark matter detectors with natural xenon as targets have daughter nucleus is ∼30 eV at most, which is negligible. Al- the potential to measure the 2νECEC on 124Xe [17, 18]. Among though 126Xe can also undergo 2νECEC, this reaction is ex- the different models for calculating the corresponding nuclear pected to be much slower than that on 124Xe since its Q-value of matrix element, there exists a wide spread of calculated half- 920 keV is smaller. The Q-values are taken from the AME2012 lives for this process: between 1020 and 1024 years as summa- atomic mass evaluation [32]. rized in Table 1. The Monte Carlo (MC) generation of the atomic de- A previous experiment used enriched xenon. A gas propor- excitation signal is based on the atomic relaxation package in tional counter containing 58.6 g of 124Xe (enriched to 23%) was Geant4 [33]. While the X-ray and Auger electron tables refer to looking for the simultaneous capture of two K-shell electrons emission from singly charged ions, 2νECEC produces a doubly on that isotope, and published the latest lower bound on the charged ion. The energy of the double-electron holes in the K- 2ν2K 21 124 half-life T1/2 as 2.0 × 10 years [24, 25]. shell of Te is calculated to be 64.46 keV [34], which is only In this paper, we present the result from a search for 2νECEC 0.8 keV different from the sum of the K-shell binding energy of on 124Xe using the XMASS-I liquid xenon detector. the singly charged ion. Therefore, this difference is negligible in this analysis. Simulated de-excitation events are generated 2. The XMASS-I Detector uniformly throughout the detector volume. The MC simulation includes the nonlinearity of the scintillation response [26] XMASS-I is a large single phase liquid xenon detector [26] as well as corrections derived from detector calibrations. The located underground (2700m water equivalent) at the Kamioka absolute energyscale of the MC is adjusted at 122keV.The sys- Observatoryin Japan. An active targetof 835kg of liquid xenon tematic difference of the energyscale between data and MC due is held inside of a pentakis-dodecahedral copper structure that to imperfect modeling of the nonlinearity in MC is estimated as holds 642 inward-looking photomultiplier tubes (PMTs) on its 3.5% by comparing 241Am data to MC. The decay constants of approximately spherical inner surface. The detector is cali- scintillation light and the timing response of the PMTs are mod- brated regularly with 57Co and 241Am sources [27] inserted eled to reproduce the time distribution observed with the 57Co 2 (122keV) and 241Am (60keV) gamma ray sources [35]. The PEs 500 1000 1500 2000 group velocity of the scintillation light in liquid xenon is calcu- 103 lated from its refractive index (∼11cm/ns for 175nm) [36].

Entries/keV 2 4. Data Sample and Event Selection 10

The data used in the present analysis were collected between December 24, 2010 and May 10, 2012. Since we took exten- 10 sive calibration data and various special runs by changing the detector conditions to understand the general detector response and the background, we select periods of operation under what 1 we call normal data taking conditions with a stable temperature (174 ± 1.2 K) and pressure (0.160-0.164 MPa absolute). Af- 20 40 60 80 100 120 140 Energy (keV) ter furthermore removing periods of operation with excessive PMT noise, unstable pedestal levels, or abnormal trigger rates, the total livetime becomes 165.9 days. Figure 1: Energy spectra of the simulated events after each reduction step. From Event selection proceeds in four stages: pre-selection, fidu- top to bottom, the simulated energy spectrum after pre-selection and radius cial volume cut, timing balance cut, and band-like pattern cut. cut (black solid), timing balance cut (red dashed), and band-like pattern cut (blue filled) are shown. The vertical dashed lines indicate the 56-72 keV signal The pre-selection requires that no outer detector trigger is asso- window. ciated with the event, that the event is separated in time from the nearest event by at least 10 ms, and that the RMS spread of the inner detector hit timings of the event is less than 100 ns. The fiducial volume, timing balance, and band pattern cut This pre-selection reduces the total effective lifetime to 132.0 values are optimized to maximize sensitivity to a monoener- days in the final sample. getic peak in the 60 keV region. For the fiducial volume cut, the In order to select events occurring in the fiducial volume, an range of the cut value was restricted in the optimization process event vertex is reconstructed based on a maximum likelihood to be larger than 15 cm in order to avoid too small of an ac- evaluation of the observed light distribution in the detector [26]. ceptance, and this restriction turns out to determine the optimal We select events satisfying that the radial distance of their re- value [37]. constructed vertex from the center of the detector is smaller than In the present analysis, the total energy deposition of events 15 cm. The fiducial mass of natural xenon in that volume is is reconstructed from the observed number of photoelectrons 41 kg, containing 39 g of 124Xe. correcting for the non-linear response of scintillation light During the data-taking period, a major background in the yield. The correction is performed assuming the light origi- relevant energy range comes from radioactive contaminants in nates from two X-rays with equal energy. Finally, the signal the aluminum seal of the PMTs. These background events of- window is defined such that it contains 90% of the simulated ten occur at a blind corner of the nearest PMT and are mis- signal with equal 5% tails to either side after all the above were reconstructed in the inner volume of the detector. The remain- applied, which results in a 56−72 keV window. Fig. 1 shows ing two cuts deal with these mis-reconstructed events. The tim- energy spectra of the simulated events after each reduction step. ffi ing balance cut uses the time difference between the first hit in From the simulation, signal detection e ciency is estimated to an event and the mean of the timings of the second half of all be 59.7%. the time-ordered hits in the event. Events with smaller time difference are less likely to be events from the detector’s inner sur- 5. Results and Discussion face that were wrongly reconstructed and are kept. The timing balance cut reduces the data by a factor of 5.9 in the signal en- Fig. 2 shows energy distributions of data events remaining ergy window defined later, while it keeps 80% of signal events after each reduction step. After all cuts, 5 events are left in the remaining after the fiducial volume cut. The band-like pattern signal region but no significant peak is seen. The main con- cut eliminates events that reflect their origin within grooves or tribution to the remaining background in this energy regime crevices in the inner detector surface through a particular illu- is the 222Rn daughter 214Pb in the detector. The amount of mination pattern: The rims of the groove or crevice act as an 222Rn was estimated to be 8.2 ± 0.5 mBq from the observed aperture that is projected as a “band” of higher photon counts rate of 214Bi-214Po consecutive decays. Given the measured de- onto the inner detector surface. This band is characterized by cay rate the expected number of background events in the sig- the ratio of the maximumPEs in the band of width 15 cm to the nal region from this decay alone is estimated to be 5.3 ± 0.5 total PEs in the event [35]. Events with smaller ratio are less events. The concentration of krypton in the xenon was mea- likely to originate from crevices and are selected. The band sured to be <2.7 ppt [26], and thus background from 85Kr is pattern cut reduces the data by a factor of 24.6 while it keeps negligible in this analysis. The background from 2νβ−β− of 136 21 214 70% of signal events remaining after the fiducial volume and Xe (T1/2 = 2 × 10 years [38]) is smaller than the Pb timing balance cuts. background by a factor of 7 and is negligible for this analysis. 3 PEs 3 500 1000 1500 2000 10 35

30 102 Entries/keV 25 Entries/16keV 20 10 15

10 1 5

0 20 40 60 80 100 120 140 20 40 60 80 100 120 140 Energy (keV) Energy (keV)

Figure 2: Energy spectra of the observed events after each reduction step for Figure 3: Energy distribution of the observed events (black) overlaid with the the 165.9 days of data. From top to bottom, the observed energy spectrum after 214Pb background simulation (green) after all cuts except for the energy window pre-selection and radius cut (black solid), timing balance cut (red dashed), and cut. The vertical dashed lines indicate the 56-72 keV signal window. band-like pattern cut (blue filled) are shown. The vertical dashed lines indicate the 56-72 keV signal window. The expected 214Pb background (green hatched) together with the signal expectation for the 90% confidence level upper limit (magenta hatched) are also shown. versus the negative side of the distribution center as described below. Table 2 summarizes the systematic uncertainties in exposure, Fig. 3 shows the energy distribution of the observed events detection efficiency, and event selection. The systematic uncer- overlaid with the 214Pb background simulation after all cuts ex- tainty in the detector exposure is dominated by the uncertainty cept for the energy window cut. The energy spectrum after cuts 124 214 in the abundance of Xe in the xenon. A sample was taken in data is consistent with the expected Pb background spec- from the detector and its isotope composition was measured at trum. Although the total number of observed events is 26% 214 the Geochemical Research Center, the University of Tokyo us- larger than that of the expected Pb background in the energy ing a modified VG5400/MS-III mass spectrometer [39]. The range between 24 keV and 136 keV but outside the signal win- result is consistent with that of natural xenon in air, and we σ dow, the tension is still at a 1.4 level with this small statistics. treat the uncertainty in that measurement as a systematic er- Note that an excess in the highest energybin is due to a gamma- ror. The systematic uncertainty in the detection efficiency is ray from 131mXe in liquid xenon, and thus this energy bin is not estimated from comparisons between data and MC simulation included in the calculation. We derive a conservative limit un- 241 214 for Am (60 keV γ-ray) calibration data at various positions der the assumption of the Pb background constrained by the within the fiducial volume. The systematic uncertainty in the 214Bi-214Po measurement. energy scale is evaluated to be ±5%, summing up in quadra- A lower limit on the 2νECEC half-life is derived using the ture the uncertainties from the nonlinearity of the scintillation following Bayesian method that also accounts for systematic yield (±3.5%), position dependence (±2%), and time variation uncertainties to calculate the conditional probability distribu- (±3%). Changing the number of photons generated per unit tion for the decay rate as follows: energy deposited in the simulation by this amount, the signal 0 −(Γλǫ+b)(1+δ) n ffi ± e ((Γλǫ + b)(1 + δ)) obs e ciency changes by 8.6%. Since we apply the energy cut on P(Γ|nobs) = lower and upper sides, both increasing and decreasing number ZZZZ n ! obs of photons in MC makes signal efficiency smaller. The energy ×P(Γ)P(λ)P(ǫ)P(b)P(δ)dλdǫdbdδ (4) resolution in the calibration data is found to be 12% worse than ff where Γ is the decay rate, nobs is the observed number of events, that in the simulation. The uncertainty due to this di erence λ is the detector exposure including the abundance of 124Xe, ǫ is evaluated by worsening energy resolution in the simulation, is the detection efficiency, b is the expected number of back- which leads to a 5.3% reduction in signal efficiency. ground events, and δ is a parameter representing the systematic The uncertainty in modeling the scintillation decay constant 1.5 uncertainty in the event selection which affects both signal and as a function of energy is evaluated to be ±0 ns, resulting in 0 background. The decay rate prior probability P(Γ)is1for Γ ≥ 0 an uncertainty in the signal efficiency of ±7.1%. The radial po- and otherwise 0. The prior probability distributions incorporat- sition of the reconstructed vertex for the calibration data differs ing systematic uncertainties in the detector exposure P(λ), de- from the true source position by 5 mm, which causes a 6.7% tection efficiency P(ǫ), background P(b), and event selection reduction in efficiency. For the timing balance and band-like P(δ) are assumed to be the split normal distribution centered at pattern cuts, we evaluate the impact on the signal efficiency by the nominal value with two standard deviations since some er- again taking the difference of their acceptance for calibration ror sources are found to have a different impact on the positive data and the respective simulation. The resulting change in sig- 4 2νECEC half-life of 4.3 × 1021 years at 90% CL. A future de- Table 2: Summary of systematic uncertainties in exposure, detection efficiency, and event selection. tector with XMASS-II characteristics establishes a path toward covering the whole range of half-lives obtained in the model calculations cited in introduction. Item Errorsource Fractional uncertainty (%) Acknowledgments Exposure Abundanceof 124Xe ±8.5 Liquid xenon density ±0.5 We thank Prof. Nagao and his group for the measurement ffi ±0 E ciency Energyscale 8.6 of the isotope composition of the xenon used in the XMASS- ±0 Energy resolution 5.3 I detector. We gratefully acknowledge the cooperation of the ±0 Scintillation decay time 7.1 Kamioka Mining and Smelting Company. This work was sup- 0 Event selection Fiducial volume cut ±6.7 ported by the Japanese Ministry of Education, Culture, Sports, 3.0 Timing balance cut ±0 Science and Technology, Grant-in-Aid for Scientific Research, Band-like pattern cut ±5.0 JSPS KAKENHI Grant No. 19GS0204 and 26104004, and partially by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2011-220-C00006).

3.0 nal efficiency is ±0 % for the timing balance cut, and ±5.0% for the band-like pattern cut. References Finally, we calculate the 90% confidence level (CL) limit us- [1] W. Buchmuller, R. D. Peccei, T. Yanagida, Ann. Rev. Nucl. Part. Sci. 55 ing the relation (2005) 311. Γ [2] J. Bernabeu, A. De Rujula, C. Jarlskog, Nucl. Phys. B 223 (1983) 15. limit Γ| Γ [3] Z. Sujkowski, S. Wycech, Phys. Rev. C 70 (2004) 052501. 0 P( nobs)d R ∞ = 0.9 (5) [4] D. Frekers, hep-ex/0506002. Γ| Γ 0 P( nobs)d [5] M. I. Krivoruchenko et al., Nucl. Phys. A 859 (2011) 140. R [6] J. Kotila, J. Barea, F. Iachello, Phys. Rev. C 89 (2014) 064319. to obtain [7] A. S. Barabash et al., Nucl. Phys. A 785 (2007) 371. [8] A. S. Barabash et al., Phys. Rev. C 80 (2009) 035501. ln 2 [9] P. Belli et al., Phys. Rev. C 87 (2013) 034607. T 2ν2K 124Xe = > 4.7 × 1021 years. (6) 1/2 Γ [10] P. Belli et al., Nucl. Phys. A 930 (2014) 195. limit [11] Y. M. Gavrilyuk et al., Phys. Rev. C 87 (2013) 035501. [12] W. C. Haxton, G. J. Stephenson, Prog. Part. Nucl. Phys. 12 (1984) 409. Note that the total systematic uncertainty worsens the obtained [13] M. Hirsch et al., Z. Phys. A 347 (1994) 151. limit by 20%. [14] A. P. Meshik et al., Phys. Rev. C 64 (2001) 035205. In addition, the fact that we do not observe significant excess [15] J. Suhonen, O. Civitarese, Phys. Rept. 300 (1998) 123. above background allows us to give a constraint on 2νECEC on [16] S. M. Bilenky, C. Giunti, Int. J. Mod. Phys. A 30 (2015) 1530001. 126 [17] D.-M. Mei et al., Phys. Rev. C 89 (2014) 014608. Xe in the same manner. The fiducial volume contains 36 g of [18] N. Barros, J. Thurn, K. Zuber, J. Phys. G 41 (2014) 115105. 126Xe and the uncertaintyin the abundanceof 126Xe is estimated [19] J. Suhonen, J. Phys. G 40 (2013) 075102. 2ν2K 126 21 [20] O. A. Rumyantsev, M. H. Urin, Phys. Lett. B 443 (1998) 51. to be 12.1%, and we obtain T1/2 Xe > 4.3 × 10 years at [21] S. Singh et al., Eur. Phys. J. A 33 (2007) 375. 90% CL. [22] A. Shukla, P. K. Raina, P. K. Rath, J. Phys. G 33 (2007) 549. The XMASS project uses a single phase liquid xenon de- [23] M. Aunola, J. Suhonen, Nucl. Phys. A 602 (1996) 133. tector with a natural abundance target. This straightforward [24] Y. M. Gavrilyuk et al., Phys. Part. Nucl. 46 (2015) 147. technology offers easy scalability to larger detectors. The fu- [25] Y. M. Gavrilyuk et al., arXiv:1507.04520 [nucl-ex]. [26] K. Abe et al., XMASS Collaboration, Nucl. Instrum. Meth. A 716 (2013) ture XMASS-II detector will contain 10 tons of liquid xenon 78. in its fiducial volume as target, and the expected sensitivity of [27] N. Y. Kim et al., XMASS Collaboration, Nucl. Instrum. Meth. A 784 XMASS-II will improve by more than two orders of magnitude (2015) 499. over the current limit after 5 years, assuming a backgroundlevel [28] This photoelectron yield is smaller than the value reported in Ref. −5 −1 −1 −1 − − [26, 29, 30] since we changed a correction on the charge observed in of 3 × 10 day kg keV . This backgroundis due to 2νβ β the electronics. This correction is within the uncertainty reported earlier, of 136Xe and pp+7Be solar neutrinos. ±1.2PE/keV. [29] K. Abe et al., XMASS Collaboration, Phys. Lett. B 719 (2013) 78. [30] K. Abe et al., XMASS Collaboration, Phys. Lett. B 724 (2013) 46. 6. Conclusions [31] Y. Fukuda et al., Super-Kamiokande Collaboration, Nucl. Instrum. Meth. A 501 (2003) 418. In conclusion, we have searched for 2νECEC on 124Xe us- [32] M. Wang et al., Chin. Phys. C 36 (2012) 1603. [33] S. Guatelli et al., IEEE Trans. Nucl. Sci. 54 (2007) 585; ing an effective live time of 132.0 days of XMASS-I data in a S. Guatelli et al., IEEE Trans. Nucl. Sci. 54 (2007) 594. fiducial volume containing 39 g of 124Xe. No significant excess [34] D. A. Nesterenko et al., Phys. Rev. C 86 (2012) 044313. over the expected background is found in the signal region, and [35] H. Uchida et al., XMASS Collaboration, Prog. Theor. Exp. Phys. (2014) we set a lower limit on its half-life of 4.7 × 1021 years at 90% 063C01. [36] S. Nakamura et al., in Proceedings of the Workshop on Ionization and CL. The obtained limit has ruled out parts of some theoretical Scintillation Counters and Their Uses, vol. 27, 2007. expectations. In addition, we obtain a lower limit on the 126Xe [37] K. Abe et al., XMASS Collaboration, Phys. Rev. Lett. 113 (2014) 121301. 5 [38] K. A. Olive et al., Particle Data Group, Chin. Phys. C 38 (2014) 090001. [39] K.-I. Bajo et al., Earth Planets Space 63 (2011) 1097.