We report results from the NEMO-3 experiment based on an exposure of 1275 days with 661 g of Te-130 in the form of enriched and natural tellurium foils. The beta beta decay rate of Te-130 is found to be greater than zero with a significance of 7.7 standard deviations and the half-life is measured to be T-1/2(2v)=[7.0 +/- 0.9(stat) +/- 1: 1(syst)] x 10(20) yr. This represents the most precise measurement of this half- life yet published and the first real-time observation of this decay

Arnold, R

Augier, C

Baker, J

Barabash, AS

Basharina-Freshville, A

Blondel, S

Bongrand, M

Broudin-Bay, G

Brudanin, V

Caffrey, AJ

Chapon, A

Chauveau, E

Durand, D

Egorov, V

Flack, R

Garrido, X

Grozier, J

Guillon, B

Hubert, P

Hugon, C

Jackson, CM

Jullian, S

Kauer, M

Klimenko, A

Kochetov, O

Konovalov, SI

Kovalenko, V

Lalanne, D

Lamhamdi, T

Lang, K

Liptak, Z

Lutter, G

Mamedov, F

Marquet, C

Martin-Albo, J

Mauger, F

Mott, J

Nachab, A

Nemchenok, I

NEMO-3 Collaboration,

Nguyen, CH

Nova, F

Novella, P

Ohsumi, H

Pahlka, RB

Perrot, F

Piquemal, F

Reyss, JL

Richards, B

Ricol, JS

Saakyan, R

Sarazin, X

Shitov, Y

Simard, L

Simkovic, F

Smolnikov, A

Soldner-Rembold, S

Stekl, I

Suhonen, J

Sutton, CS

Szklarz, G

Thomas, J

Timkin, V

Torre, S

Tretyak, VI

Umatov, V

Vala, L

Vanyushin, I

Vasiliev, V

Vorobel, V

Vylov, T

Zukauskas, A

NEMO-3 Collaboration

UCL Discovery

Measurement of the  Decay Half-Life of 130Te with the NEMO-3 DetectorR. Arnold,1 C. Augier,2 J. Baker,3,* A. S. Barabash,4 A. Basharina-Freshville,5 S. Blondel,2 M. Bongrand,2G. Broudin-Bay,6,7 V. Brudanin,8 A. J. Caffrey,3 A. Chapon,9 E. Chauveau,10 D. Durand,9 V. Egorov,8 R. Flack,5X. Garrido,2 J. Grozier,5 B. Guillon,9 Ph. Hubert,6,7 C. Hugon,6,7 C.M. Jackson,10 S. Jullian,2 M. Kauer,5 A. Klimenko,8O. Kochetov,8 S. I. Konovalov,4 V. Kovalenko,6,7,8 D. Lalanne,2 T. Lamhamdi,11 K. Lang,12 Z. Liptak,12 G. Lutter,6,7F. Mamedov,13 Ch. Marquet,6,7 J. Martin-Albo,14 F. Mauger,9 J. Mott,5 A. Nachab,6,7 I. Nemchenok,8 C.H. Nguyen,6,7,15F. Nova,12 P. Novella,14 H. Ohsumi,16 R. B. Pahlka,12 F. Perrot,6,7 F. Piquemal,6,7 J. L. Reyss,17 B. Richards,5 J. S. Ricol,6,7R. Saakyan,5 X. Sarazin,2 L. Simard,2 F. Sˇimkovic,19 Yu. Shitov,8,18 A. Smolnikov,8 S. So¨ldner-Rembold,10 I. Sˇtekl,13J. Suhonen,20 C. S. Sutton,21 G. Szklarz,2 J. Thomas,5 V. Timkin,8 S. Torre,5 V. I. Tretyak,1,8 V. Umatov,4 L. Va´la,13I. Vanyushin,4 V. Vasiliev,5 V. Vorobel,22 Ts. Vylov,8,* and A. Zukauskas22(NEMO-3 Collaboration)1IPHC-DRS, Universite´ Louis Pasteur, CNRS, F-67037 Strasbourg, France2LAL, Universite´ Paris-Sud 11, CNRS/IN2P3, Orsay, France3INL, Idaho National Laboratory, Idaho Falls, Idaho 83415, USA4ITEP, Institute of Theoretical and Experimental Physics, 117259 Moscow, Russia5University College London, London WC1E 6BT, United Kingdom6Universite´ de Bordeaux, CENBG, UMR 5797, F-33175 Gradignan, France7CNRS/IN2P3, CENBG, UMR 5797, F-33175 Gradignan, France8JINR, Joint Institute for Nuclear Research, 141980 Dubna, Russia9LPC, ENSICAEN, Universite´ de Caen, Caen, France10University of Manchester, Manchester M13 9PL, United Kingdom11USMBA, Universite´ Sidi Mohamed Ben Abdellah, 30000 Fes, Morocco12University of Texas at Austin, Austin, Texas 78712-0264, USA13IEAP, Czech Technical University in Prague, CZ-12800 Prague, Czech Republic14IFIC, CSIC-Universidad de Valencia, Valencia, Spain15Hanoi University of Science, Hanoi, Vietnam16Saga University, Saga 840-8502, Japan17LSCE, CNRS, F-91190 Gif-sur-Yvette, France18Imperial College London, London SW7 2AZ, United Kingdom19FMFI, Comenius University, SK-842 48 Bratislava, Slovakia20Jyva¨skyla¨ University, 40351 Jyva¨skyla¨, Finland21MHC, Mount Holyoke College, South Hadley, Massachusetts 01075, USA22Charles University in Prague, Faculty of Mathematics and Physics, CZ-12116 Prague, Czech Republic(Received 11 April 2011; published 4 August 2011)We report results from the NEMO-3 experiment based on an exposure of 1275 days with 661 g of 130Tein the form of enriched and natural tellurium foils. The  decay rate of 130Te is found to be greater thanzero with a significance of 7.7 standard deviations and the half-life is measured to be T21=2 ¼ ½7:00:9ðstatÞ  1:1ðsystÞ  1020 yr. This represents the most precise measurement of this half-life yetpublished and the first real-time observation of this decay.DOI: 10.1103/PhysRevLett.107.062504 PACS numbers: 23.40.s, 21.10.Tg, 14.60.PqThe first evidence of  decay (2) appeared in1950 through the observation of 130Xe from the decay of130Te in rock samples [1]. This result was met with scep-ticism for the ensuing 15 years until the results of a numberof other geochemical experiments began to confirm theobservation. There was, however, significant disagreementbetween two distinct sets of these measurements that wasnot immediately resolved. Several groups measured a longhalf-life of  2:7 1021 yr [2,3] while others obtained asignificantly shorter half-life of  0:8 1021 yr [4–7].One hypothesis to explain the difference is based on theobservation that shorter half-lives were measured in rockof relatively young age ( 107–108 yr), while the longerhalf-lives were measured in relatively old rock ( 109 yr)[2,4]. It has even been suggested that there is a timedependence in the value of the weak interaction couplingconstant [8]. Recent papers [9] attempt to explain the long-held discrepancy between these measurements as beingcaused by catastrophic xenon loss in the older samples.To date, the only direct evidence for the 130Te 2PRL 107, 062504 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending5 AUGUST 20110031-9007=11=107(6)=062504(4) 062504-1  2011 American Physical Societyprocess comes from the MIBETA experiment, which re-ported a half-life of ½6:1 1:4ðstatÞþ2:93:5ðsystÞ  1020 yr[10] by comparing different crystals isotopically enrichedin 130Te and 128Te, assuming that any difference in ratewas due to 2 events (128Te has a much longer 2half-life). However, a systematic uncertainty of about 50%rendered this result somewhat inconclusive. In this Letter,we present the first direct, high-precision measurement of2 decay of 130Te with the NEMO-3 detector. In addi-tion, a search for neutrinoless  decay (0) and forthe decay with Majoron emission (00) is reported.The NEMO-3 detector is located in the ModaneUnderground Laboratory. The detector [11] contains al-most 9 kg of seven different  isotopes in the form ofthin foils. It provides direct detection of electrons from the decay by the use of a tracking device based on openGeiger drift cells and a calorimeter made of plastic scin-tillator blocks coupled to low-radioactive photomultipliers(PMTs). For 1 MeV electrons the timing resolution is250 ps and the energy resolution (full width at half maxi-mum) is about 15%. A magnetic field surrounding thedetector provides identification of electrons by the curva-ture of their tracks. In addition to the electron and photonidentification through tracking and calorimetry, the calo-rimeter measures the energy and the arrival time of theseparticles while the tracking chamber can measure the timeof delayed tracks associated with the initial event for up to700 s. The calorimeter energy scale is calibrated ap-proximately once per month using a 207Bi source providingconversion electrons of 482 and 976 keV (K-lines).Stability of the calorimeter response is surveyed twice aday by a laser system. The data presented in this Lettercorrespond to 1275 days of data taking between October2004 and December 2009. Two different foils are used inthe analysis: a Te foil, enriched at a level of 89:4 0:5%corresponding to 454 g of 130Te, and a natural Te foil whichcontains 33.8% 130Te, corresponding to 207 g of 130Te.When searching for rare processes, the background esti-mation is paramount as it will limit the final sensitivity ofthe experiment. An exhaustive program of work has beencarried out to measure the very large number of sources ofbackground present in the NEMO-3 detector. The methodof the background measurement and its validation with ahighly radiopure Cu foil is described in [12]. There are threecategories of background: the external background, origi-nating from radioactivity outside the tracking chamber; thetracking volume background, which includes radon inthe tracking gas and the drift cell wire contamination;and the internal background due to radioactive impuritiesinside the source foils whose dominant isotopes are 40K,234mPa, 210Bi, 214Bi, and 208Tl. The different backgroundcontributions are estimated by measuring independentevent topologies, both for enriched and natural Te.The external background originates from componentsoutside the tracking volume. PMTs are the main contrib-utors since they have glass and electronic components that,though at a very low level, contain traces of 214Bi, 208Tl and40K. The external background is measured with -rayCompton scattering in the scintillators, either producingan electron that crosses the tracking chamber or depositingenergy in one scintillator, followed by an electron emittedfrom the source to another scintillator. The reliabilityof the external background model is illustrated by theenergy distributions of the one-electron crossing eventsin Figs. 1(a) and 1(b).The source foil activities in 234mPa, 40K, and 210Bi aredetermined with single-electron events coming from thefoil. The energy distribution of the observed events and theresult of the fit of the different components of the back-ground are presented in Figs. 1(c) and 1(d). The goodagreement between the data and the fit demonstrates thereliability of the internal background model. The foil ac-tivity in 214Bi is measured using events with a singleelectron accompanied by a delayed -particle track. Thistopology is a signature of the  decay of 214Bi to 214Pofollowed by  decay of 214Po to 210Pb. The foil activity in208Tl is measured with events that contain one electron andeither two or three photons emitted from the foil. Theresults of measurements of the internal contamination by214Bi and 208Tl are reported in [12]. The measured foilactivities are summarized in Table I.The background from single  decay in the trackingvolume is of importance if the decay occurs near the foil.The main source of this background is due to daughters ofradon: 214Pb, 214Bi, and 210Bi. The radon activity in thetracking chamber is measured using e events as for the0100020003000400050006000700080000 0.5 1 1.5 2 2.5 3ETOT (MeV)Number of events / 0.05 MeV     DataTlAcBiKCoBiTotal MC(a)01000200030004000500060000 0.5 1 1.5 2 2.5 3ETOT (MeV)Number of events / 0.05 MeV     DataTlAcBiKCoBiTotal MC(b)1101021031041050 0.5 1 1.5 2 2.5 3 3.5Ee (MeV)Number of events / 0.05 MeV    DataPaKBi foilBi wiresRadonExt bkgTotal MC(c)1101021031040 0.5 1 1.5 2 2.5 3 3.5Ee (MeV)Number of events / 0.05 MeV    DataPaKBi foilBi wiresRadonExt bkgTotal MC(d)FIG. 1 (color online). Energy sum distribution for crossing-electron events for (a) 130Te and (b) natTe (b); Energy distribu-tion of electron events coming from the source foil for (c) 130Teand (d) natTe. Dots correspond to the data and histograms to thefit of the background model.PRL 107, 062504 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending5 AUGUST 2011062504-2measurement of internal 214Bi background. The distribu-tion of these events is measured as a function of thelocation in the tracking volume. For the data presentedhere, the mean 222Rn activity in the whole gas volume is209 2 mBq. Unlike the preceding radon daughters,210Pb has a long half-life of about 22 years. It is thereforenot in equilibrium with 222Rn in the tracking volume andmost 210Pb was deposited during the construction of thedetector. 210Bi produced in  decay of 210Pb contributes tothe low-energy background below 1 MeV. The 210Pb dep-osition on drift cell wires, measured by detecting electronsfrom 210Bi  decay, was found to be vary significantly indifferent sectors [12]. In contrast to 222Rn the concentrationof 220Rn in NEMO-3 is very small and its contribution tothe total background in the 130Te sectors is less than 1%.The measured activities are used to estimate the back-ground contribution in the two-electron channel with aMonte Carlo (MC) simulation. Signal and backgroundMC events are generated using a GEANT-based simulation[13] of the detector with the initial kinematics given by theevent generator DECAY0 [14].The two-electron events are selected with the followingrequirements. Two tracks of a length greater that 50 cmwith curvature corresponding to a negative charge arereconstructed. Both tracks originate from a common vertexin the foil and terminate in isolated scintillators with asingle energy deposit greater than 0.2 MeV. The time-of-flight information is consistent with the hypothesis that twoelectrons were emitted from the same point on the sourcefoil. No photon or delayed  track is detected in the event.These selection criteria lead to a 2 detection efficiencyof 3.5% for enriched 130Te and 2.8% for natTe, where thedifference is mainly due to the source foil thickness.Several factors contribute to this low efficiency, but themost important are the geometrical acceptance of thedetector, the effect of the energy threshold, and the trackingalgorithm inefficiencies.We measure the 130Te half-life with data from the en-riched Te source foil by performing a likelihood fit to thebinned energy sum distribution in the interval [0.9–2] MeV.This interval is chosen using the MC simulation to max-imize the signal significance. It reduces the 2 effi-ciency by a factor of 0.7. The result of the fit is presented inFigs. 2(a), 2(c), and 2(e). Using the MC simulation thenumber of background events in the interval [0.9–2] MeVis estimated to be 363 25 events, of which 141 events areassociated with the external background, 179 with theinternal background, and 43 with radon induced back-ground. The number of events in excess of the backgroundin the interval [0.9–2] MeV is determined to benð2Þ ¼ 178 23; (1)with a signal significance of 7.7 standard deviations and asignal-to-background ratio of S=B ¼ 0:5. This correspondsto a 130Te half-life ofT21=2 ¼ ð7:0 0:9Þ  1020 yr: (2)The main systematic uncertainty on the measured 130Tehalf-life is associated with the background estimation andis due to the small signal-to-background ratio. The uncer-tainty on the number of expected background events hasbeen obtained by applying the largest variations of thecomponent activities in the background model. The corre-sponding uncertainty on the 130Te half-life is 14%. Anothersystematic uncertainty is associated with the two-electrondetection efficiency in NEMO-3 which is found to beTABLE I. Background contaminations measured in the Tesource foils (in mBq).Impurity 130Te natTe208Tl 0:17 0:04 0:24 0:04214Bi 0:29 0:05 0:25 0:12234mPa 2:49 0:05 0:73 0:04210Bi 19:9 0:4 18:4 0:340K 14:7 0:2 12:3 0:2 02550751001251501752000.5 1 1.5 2 2.5 3E1+E2(MeV)Number of events / 0.125 MeV   Dataββ TeRadonExt bkgInt bkgBiNEMO-3(a)0204060801001200.5 1 1.5 2 2.5 3E1+E2(MeV)Number of events / 0.125 MeV   Dataββ TeRadonExt bkgInt bkgBiNEMO-3(b)01002003004005006000.5 1 1.5 2Ee (MeV)Number of tracks / 0.075 MeV   Dataββ TeRadonExt bkgInt bkgBiNEMO-3(c)0501001502002503003504000.5 1 1.5 2Ee (MeV)Number of tracks / 0.075 MeV   Dataββ TeRadonExt bkgInt bkgBiNEMO-3(d)020406080100120140160-1 -0.5 0 0.5 1cosΘNumber of events   Dataββ TeRadonExt bkgInt bkgBiNEMO-3(e)020406080100120-1 -0.5 0 0.5 1cosΘNumber of events   Dataββ TeRadonExt bkgInt bkgBiNEMO-3(f)FIG. 2 (color online). (a),(b) Distribution of the sum of theelectron energies; (c),(d) individual electron energy; and (e),(f) cosine of the angle between the electron tracks for two-electron events selected from the two Te foils: (a),(c),(e)enriched in 130Te and (b),(d),(f) natural Te.PRL 107, 062504 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending5 AUGUST 2011062504-3correct within an accuracy of 5%. This uncertainty isdetermined with a calibrated 207Bi source and a dedicated90Sr source which decays to 90Y, a pure  emitter ofQ ¼ 2:28 MeV. Finally, the source foil thickness andthe GEANT model of electron energy losses in dense thinmedia contribute a systematic uncertainty of 4% whichis estimated by comparing signals from metallic andcomposite 100Mo source foils [15]. The total systematicuncertainty of 15% is obtained by adding the individualcontributions in quadrature.The measurement of the 2 half-life is verified usingthe natural Te foil. The energy and angular distributions ofthe two-electron events of the natural Te data are presented inFigs. 2(b), 2(d), and 2(f) and are comparedwith the expectedMC distribution using the measured half-life T21=2 ¼7:0 1020 yr and the background model of the natural Tefoil. There are 65 8  events and 316 28 backgroundevents expected in the electron energy sum interval[0.9–2]MeV. The total number of 381 29 expected eventsis in good agreement with the 377 observed events.The 130Te data [Fig. 2(a)] are also used to set a limit onthe 0 and 00 processes with the modified fre-quentist analysis [16]. Themethod uses the full informationof the binned energy sum distribution for signal and back-ground, as well as the statistical and systematic uncertain-ties and their correlations as described in [17].The total efficiency to detect 0 decay of 130Te isestimated to be ð13:9 0:7Þ% yielding a limit ofT01=2 > 1:3 1023 yr ð90% C:L:Þ; (3)which is an order of magnitude less stringent than the limitobtained by the CUORICINO experiment [18] based on11 kg of 130Te.The detection efficiency for the decay with ordinary(spectral index n ¼ 1) Majoron emission (see discussionin [19] and references therein) is ð9:6 0:5Þ% and the limitis determined to beT001=2 > 1:6 1022 yr ð90% C:L:Þ; (4)which is a factor of 7 more stringent than the previous bestlimit from MIBETA [10]. The corresponding limit on thecoupling constant of the Majoron to the neutrino is gee <ð0:6–1:6Þ  104 (using nuclear matrix elements from[20–24]) and is comparable with the best present limits.In summary, the 2 decay 130Te half-life measuredwith the NEMO-3 detector isT21=2 ¼ ½7:0 0:9ðstatÞ  1:1ðsystÞ  1020 yr: (5)With this result, the corresponding nuclear matrix elementcan be extracted according toðT21=2Þ1 ¼ G2jM2j2; (6)where G2 ¼ 4:8 1018 yr1 (for gA ¼ 1:254) is theknown phase space factor [25], which yields the resultM2 ¼ 0:017 0:002 (scaled by the electron mass). Thisvalue for M2 may be used to fix the gpp parameter of thequasiparticle random-phase approximation model, whichcorresponds to the strength of the nucleon-nucleon inter-action inside the nucleus. It has been suggested that thiswill improve the M0 calculations [20,21,26].The NEMO-3 result for the 130Te half-life is consistentwith the geological measurements made in younger rocksamples and is the most precise measurement of this iso-tope half-life to date.We thank the staff at the Modane UndergroundLaboratory for its technical assistance in running theexperiment and Vladimir Tretyak for providing theMonte Carlo event generator [14]. We acknowledge sup-port by the Grants Agencies of the Czech Republic, RFBR(Russia), STFC (U.K.) and NSF (U.S.).*Deceased.[1] M.G. Inghram and J. H. Reynolds, Phys. Rev. 78, 822(1950).[2] T. Kirsten et al., in: Nuclear Beta Decays and the Neutrinoedited by T. Kotani, H. Ejiri, and E. Takasugi (WorldScientific, Singapore, 1986), p. 81.[3] T. Bernatowicz et al., Phys. Rev. C 47, 806 (1993).[4] O. K. Manuel, in Nuclear Beta Decays and the Neutrino,edited by T. Kotani, H. Ejiri, and E. Takasugi (WorldScientic, Singapore, 1986), p. 71.[5] W. J. Lin et al., Nucl. Phys. A481, 477 (1988).[6] N. Takaoka and K. Ogata, Z. Naturforsch. A 21, 84(1966).[7] N. Takaoka, Y. Motomura, and K. Nagano, Phys. Rev. C53, 1557 (1996).[8] A. S. Barabash, JETP Lett. 68, 1 (1998); Eur. Phys. J. A 8,137 (2000); Astrophys. Space Sci. 283, 607 (2003).[9] A. P. Meshik et al., Nucl. Phys. A809, 275 (2008); H. V.Thomas et al., Phys. Rev. C 78, 054606 (2008).[10] C. Arnaboldi et al., Phys. Lett. B 557, 167 (2003).[11] R. Arnold et al., Nucl. Instrum. Methods Phys. Res., Sect.A 536, 79 (2005).[12] R. Arnold et al., Nucl. Instrum. Methods Phys. Res., Sect.A 606, 449 (2009).[13] R. Brun et al., CERN Program Library W 5013, 1984.[14] O. A. Ponkratenko et al., Phys. At. Nucl. 63, 1282 (2000).[15] R. Arnold et al., Nucl. Phys. A781, 209 (2007).[16] T. Junk, Nucl. Instrum. Methods Phys. Res., Sect. A 434,435 (1999).[17] R. Argyriades et al., Phys. Rev. C 80, 032501(R)(2009).[18] C. Arnaboldi et al., Phys. Rev. C 78, 035502 (2008).[19] R. Arnold et al., Nucl. 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Measurement of the beta beta Decay Half-Life of Te-130 with the NEMO-3 Detector

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Measurement of the beta beta Decay Half-Life of Te-130 with the NEMO-3 Detector

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UCL Discovery