Table Of ContentResults of NEMO 3 and Status of SuperNEMO
9
0 L. Va´laa∗ on behalf of the NEMO and SuperNEMO Collaborations
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2 aCzech Technical University in Prague, IEAP, Horsk´a 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)intheFr´ejustun- 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 parame-
ters 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
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