Table Of Content

Results of NEMO 3 and Status of SuperNEMO 9 0 L. Va´laa∗ on behalf of the NEMO and SuperNEMO Collaborations 0 2 aCzech Technical University in Prague, IEAP, Horská 3a/22, CZ – 128 00 Prague, Czech Republic n a The NEMO 3 experiment is devoted to the search for neutrinoless double beta decay, as well as for accurate J measurement of two-neutrino double beta decay. The detector has been taking data in the LSM laboratory 5 since 2003 and the latest NEMO 3 results for seven ββ isotopes are presented here for both decay modes. The SuperNEMO project aims to extend the NEMO techniqueto a 100 – 200 kg isotope experiment with the target ] x half-life sensitivity of 1−2×1026 y. Thecurrent status of theSuperNEMO R&D programme is described. e - p e 1. Introduction and providing a three-dimensional measurement h of the charged particle tracks by recording the [ ThecurrentlyrunningNEMO3experiment[1] drifttimeandthetwoplasmapropagationtimes. (NEMO = Neutrino Ettore Majorana Observa- 1 The calorimeter,whichsurroundsthewirecham- tory) is devoted to the search for neutrinoless v ber, consists of 1940 plastic scintillators coupled 3 double beta decay (0νββ) and to the accurate to very low-radioactivity PMTs and gives an en- 7 measurement of two-neutrino double beta decay ergyresolutionof14 17%/√E FWHMat1MeV 4 (2νββ) by means of the direct detection of the − and the time resolution of 250 ps. 0 two electrons. 0νββ decay is a process beyond . SeventeensectorsofNEMO3accommodateal- 1 Standard Model because of the violation of lep- most 10 kg of enriched ββ isotopes (see Table 1) 0 tonnumberconservationbytwounits. Detection 9 in the form of thin foils. Three sectors are also of this process is the best experimental test for 0 used for external background measurement and : Majorana character of neutrinos. This kind of are equipped with pure Cu and natural Te. v decay constrains also the type of mass spectrum i The detector is surroundedby a solenoidalcoil X hierarchyandthe absolutemassofthe neutrinos. generating magnetic field of 25 Gauss for the r e−/e+ recognition, and is covered by two types a 2. NEMO 3 detector ofshieldingagainstexternalγ-raysandneutrons. TheNEMO3detectorislocatedintheModane undergroundlaboratory(LSM)intheFréjustun- 3. Event selection and background nel in France at the depth of 4800 m w.e. The A candidate for a ββ decay is a two-electron set-up is cylindrical in design, it is divided into event which is selected by requiring two recon- twenty equal sectors and combines two detection structed tracks with a curvature corresponding techniques – particle identification provided by a tothe negativechargeandcomingfromthe same wiretrackingchamberandenergyandtime mea- vertex in the source foils. Each track has to be surements of particles with a calorimeter. Thus, NEMO 3 is able to identify e−, e+, photons, and associatedwithafiredscintillator,energyofeach − e measured in the calorimeter should be higher α-particles which allows recognition of different than 200 keV and the time-of-flight has to corre- ββ decay modes, measurement of internal and spond to the case of two electrons emitted at the external backgrounds, as well as good discrimi- same time from the common vertex in the foils. nation between signal and background. A complete study of background in the 0νββ The tracking detector is made of 6180 open window has been performed. The level of each octagonal drift cells operated in Geiger mode background component has been directly mea- ∗E-mail: [email protected] sured from data using different analysis chan- 1 2 nels. The background can be classified in three perNEMO will consist of about twenty identical groups: 1) internal radioactive contamination of modules, each housing around 5 – 7 kg of iso- thesource,2)externalbackgroundfromincoming tope in the form of thin foil ( 40 mg/cm2). ∼ γ-rays and 3) radon inside the tracking volume. The tracking volume contains more than 2000 Thedominantbackgroundduringthefirstrun- wire drift cells operated in Geiger mode (Geiger ning period from February 2003 to September cells),whicharearrangedinninelayersparallelto 2004(PhaseI)wasduetoradondiffusionintothe the foil. The calorimeter is divided into 1000 ∼ wire chamber through tiny air leaks. The radon blocks, which cover most of the detector outer level inside NEMO 3 during the second running area and are read out by low background PMTs. periodafterinstallationofthe radontrappingfa- cility in November 2004 (Phase II) has been re- 6. SuperNEMO R&D duced by a factor of ten. Remaining low radon activity inside NEMO 3 is due to detector com- The R&Dprogrammefocuses onfourmainar- ponent degassing. eas of study: (i) calorimeter, (ii) isotope enrich- ment, (iii) tracking detector, and (iv) ultra-low background materials and measurements. 4. NEMO 3 results SuperNEMO aims to improve the calorimeter Measurements of the 2νββ decay half-lives energy resolution to 7%/√E FWHM at 1 MeV. havebeenperformedforallthesevenββ isotopes Toreachthis goal,severalongoingstudies arein- in NEMO 3. The obtained half-lives, combining vestigating the choice of calorimeter parameters new preliminary results for 150Nd, 130Te, 96Zr, suchasscintillatormaterial(plasticorliquid)and and 48Ca with higher statistics (Phase I and II the shape, size and coating of calorimeter blocks data) and previous results for 100Mo and 82Se [9]. These are combined with dedicated develop- (Phase I data), are given in Table 1. ment of PMTs with very low radioactivity and No evidence was found for the 0νββ decay of high quantum efficiency. The collaboration ex- 100Mo, 82Se, 150Nd, 96Zr, and 48Ca. The 0νββ pectstomakethefinaldecisiononthecalorimeter decay half-life limits and limits on the effective design in mid-2009. Majorana neutrino mass m have been derived The tracking detector design study has for its ν h i and are summarised in Table 2. goal optimisation of the wire chamber parameters to obtain high efficiency and resolution in measuring the trajectories of electrons from ββ 5. SuperNEMO detector design decay, as well as of α-particles for the purpose of SuperNEMO aims to extend and improve background rejection. The first 9-cell prototype the experimental techniques use by the current was successfully operated demonstrating propa- NEMO 3 experiment in order to search for 0νββ gation efficiency close to 100% over a wide range decay with a targethalf-life sensitivity of 1 2 ofvoltages[10]. Recently,a90-cellprototypehas 1026 year,whichcorrespondstotheeffective−neu×- been set in operation. trino mass sensitivity m of 40 100 meV, de- The main candidate ββ isotopes for Su- ν pendingonnuclearmahtrixielemen−ts(NME)used. perNEMO are 82Se and 150Nd. A sample of 4 kg The SuperNEMO project is a 100 200 kg of 82Se has been enriched and is currently un- ∼ − sourceexperimentandiscurrentlyinathreeyear dergoing purification. The SuperNEMO collab- design study and R&D phase. oration is investigating the possibility of enrich- Like NEMO 3, SuperNEMO will combine inglargeamountsof150Ndviatheatomicvapour calorimetry and tracking. This technological laser isotope separation (AVLIS) method. choiceallowsthe measurementofindividualelec- In order to reach required sensitivity, Su- tron tracks, event vertices, energies and time- perNEMO has to maintain ultra-low levels of of-flight, and thus provides the full reconstruc- background (one order lower than for NEMO 3). tion of kinematics and topology of an event. Su- Contamination of sources has to be less than 3 Table 1 ββ isotopes installed in NEMO 3 and results for 2νββ decay half-life measurement. Isotope Mass (g) Q (keV) T2ν (y) S/B ββ 1/2 100Mo 6914 3034 [7.11 0.02(stat) 0.54(syst)] 1018 [2] 40 82Se 932 2995 [9.6 ±0.3(stat) ±1.0(syst)] 1×019 [2] 4.0 150Nd 37.0 3367 [9.11±+0.25(stat)± 0.63(syst)×] 1018 [3] 2.8 130Te 454 2529 [7.6 −01..252(stat) ±0.8(syst)] ×1020 0.25 116Cd 405 2805 [2.8±0.1(stat)±0.3(syst)]×1019 7.5 96Zr 9.4 3350 [2.3±0.2(stat)±0.3(syst)]×1019 1.0 48Ca 7.0 4272 [4.4+±0.5(stat) ±0.4(syst)] ×1019 6.8 −0.4 ± × Table 2 NEMO 3 results: 0νββ decay half-life limits at 90% C.L. Isotope 0νββ mode T0ν limit (90% C.L.) NME 1/2 100Mo (V – A) >5.8 1023 y [2] m <0.8 1.3 eV [4,5] ν (V + A) >3.2×1023 y [2] h i − 82Se (V – A) >2.1×1023 y [2] m <1.4 2.2 eV [4,5] ν (V + A) >1.2×1023 y [2] h i − 150Nd (V – A) >1.80× 1022 y [3] m <3.7 5.1 eV [6] ν (V + A) >1.07×1022 y [3] h i − 96Zr (V – A) >8.6 ×1022 y m <7.4 20.1 eV [4,7] ν 48Ca (V – A) >1.3×1022 y hm i<29.6−eV [8] ν × h i 2 µBq/kg for 208Tl and less than 10 µBq/kg for REFERENCES 214Bi. Inordertoevaluatetheseactivities,adedi- 1. R. Arnold et al. (NEMO Collaboration), catedBiPodetectoris being developed. Thefirst Nucl. Instr. and Meth. in Phys. Res. A536 prototype, BiPo1, installed in the LSM labora- (2005) 79. tory in February 2008, reached the background level of < 7.5 µBq/kg for 208Tl (90% C.L.) [11]. 2. R. Arnold et al. (NEMO Collaboration), Phys. Rev. Lett. 95 (2005) 182302. Another prototype, BiPo2, was installed in the 3. J. Argyriades et al. (NEMO Collabora- LSM in July 2008. tion), submitted to Phys. Rev. Lett., arXiv:0810.0248. 7. Conclusion 4. M. Kortelainen and J. Suhonen, Phys. Rev. C75 (2007) 051303(R); M. Kortelainen and The NEMO 3 detector has been routinely tak- J. Suhonen, Phys. Rev. C76 (2007) 024315. ing data since 2003. The 2νββ decay half-lives 5. V.A. Rodin et al., Nucl. Phys. A793 (2007) of seven isotopes have been measured with high 213. statisticsandwith better precisionthaninprevi- 6. V.A. Rodin et al., Nucl. Phys. A766 (2006) ous measurements. No evidence for the 0νββ de- 107. cayhasbeenfoundindataandtheT and m 1/2 h νi 7. F. Sˇimkovic et al., Phys. Rev. C77 (2008) limits have been set. The next generation exper- 045503. iment SuperNEMO will extrapolate the NEMO 8. E.Caurrieretal.,Phys.Rev.Lett.100(2008) technique of calorimetry plus tracking to 100 – 052503. 200kgofββ isotopescaleexperiment. Due toits 9. M. Kauer, arXiv:0807.2188. modulardesign,SuperNEMOcanstartoperation 10. I. Nasteva, arXiv:0710.4279. in stages, with the first module installed in 2011 11. M. Bongrand, J. Inst. 3 (2008) P06006. and all the modules running by 2014.