New observation of 22 decay of 100Mo to the 0+

1 level of 100Ru in the ARMONIA experiment

F. CappellaF. CappellaINFN-RomaINFN-Roma

SILAFAESILAFAE

Valparaiso, CHILEValparaiso, CHILE

6-12 December 20106-12 December 2010

SILAFAESILAFAE

Valparaiso, CHILEValparaiso, CHILE

6-12 December 20106-12 December 2010

DAMA/R&DDAMA/LXe low bckg DAMA/Ge

for sampling meas.

DAMA/NaI

DAMA/LIBRA

http://people.roma2.infn.it/dama

Roma2,Roma1,LNGS,IHEP/Beijing

measurement with 100Mo

+ by-products and small scale experiments.: INR-Kiev+ neutron meas.: ENEA-Frascati+ in some studies on decays (DST-MAE project): IIT Kharagpur, India

DAMA/LXe: results on rare processes DAMA/LXe: results on rare processes Dark Matter Investigation• Limits on recoils investigating the DMp-129Xe

elastic scattering by means of PSD • Limits on DMp-129Xe inelastic scattering• Neutron calibration• 129Xe vs 136Xe by using PSD SD vs SI signals to

increase the sensitivity on the SD component

PLB436(1998)379PLB387(1996)222, NJP2(2000)15.1PLB436(1998)379, EPJdirectC11(2001)1

foreseen/in progress

Other rare processes:• Electron decay into invisible channels• Nuclear level excitation of 129Xe during CNC processes• N, NN decay into invisible channels in 129Xe

• Electron decay: e- e• 2 decay in 136Xe • 2 decay in 134Xe

• Improved results on in 134Xe,136Xe • CNC decay 136Xe 136Cs• N, NN, NNN decay into invisible channels in 136Xe

Astrop.P.5(1996)217

PLB465(1999)315

PLB493(2000)12

PRD61(2000)117301

Xenon01

PLB527(2002)182

PLB546(2002)23

Beyond the Desert (2003) 365

EPJA27 s01 (2006) 35

NIMA482(2002)728

• 2 decay in 136Ce and in 142Ce• 2EC2 40Ca decay• 2 decay in 46Ca and in 40Ca• 2+ decay in 106Cd• 2 and decay in 48Ca• 2EC2 in 136Ce, in 138Ce and decay in 142Ce• 2+ 0, EC + 0 decay in 130Ba• Cluster decay in LaCl3(Ce)• CNC decay 139La 139Ce

• Particle Dark Matter search with CaF2(Eu)

DAMA/R&D set-up: results on rare DAMA/R&D set-up: results on rare processesprocesses NPB563(1999)97,

Astrop.Phys.7(1997)73Il N. Cim.A110(1997)189Astrop. Phys. 7(1997)73 NPB563(1999)97Astrop.Phys.10(1999)115 NPA705(2002)29NIMA498(2003)352

NIMA525(2004)535NIMA555(2005)270UJP51(2006)1037

• RDs on highly radiopure NaI(Tl) set-up;• several RDs on low background PMTs;• qualification of many materials • measurements with a Li6Eu(BO3)3 crystal

(NIMA572(2007)734)• measurements with 100Mo sample investigating decay in the 4π low-bckg HP Ge facility of LNGS (NPA846(2010)143 )

• search for 7Li solar axions (NPA806(2008)388) decay of 96Ru and 104Ru (EPJA42(2009)171)• measurements with a Li2MoO4 (NIMA607(2009) 573) decay of 136Ce and 138Ce (NPA824(2009)101)

+Many other meas. already scheduled for near future

DAMA/Ge & LNGS Ge DAMA/Ge & LNGS Ge facilityfacility

decay of natural Eu decay of 113Cd decay of 64Zn, 70Zn, 180W, 186W decay of 108Cd and 114Cd

NPA789(2007)15PRC76(2007)064603PLB658(2008)193, NPA826(2009)256EPJA36(2008)167

DAMA results on DAMA results on decay decay

Experimental limits on T1/2 obtained by DAMA (red) and by

previous experiments (blue)

all the limits are at 90% C.L. (except for 2+0 in 136Ce and 2-

0 in 142Ce - 68% C.L.)

and in 2010:

new observation of 2 decay of 100Mo to the first excited 0+ level of 100Ru

Double beta decay of 100Mo

100Mo: one of the most interesting 2 candidates

1) Natural abundance: = 9.824%;

2) Inexpensive enrichment feasible;

3) High Q2 = 3034 keV

22 decay is allowed in the Standard Model, it is the most rare nuclear decay ever observed in nature, with T1/2 in the range of 1018–1024 yr

20is forbidden in the SM; however, it is predicted by many SM extensions where neutrinos are naturally expected to be Majorana particles with small but non-zero mass

Observation of 22 decays is also an important tool to test the theoretical models used for the calculations of the nuclear matrix elements for the 2processes.

Allowed 22 decay to the g.s. of 100Ru observed in several direct experiments, with T1/2 in the range (3.3–11.5) × 1018 yr

The most accurate value comes from NEMO-3 (7 kg of 100Mo):

T1/2 (2ν; g.s. – g.s.) = (7.1± 0.5) × 1018 yr

Double beta decay of 100Mo

22 decay100Mo100Ru(0+

1

)

Armonia(meAsuReMent of twO-NeutrIno ββ decAy of 100Mo to the first excited 0+ level of 100Ru )

In addition to the transition to the g.s., the 22 decay of 100Mo was registered also for the transition to the first excited 0+

1 level of 100Ru

T1/2 measured in several experiments:

The aim of the experiment was a remeasurement of 1 kg ofMo enriched in 100Mo to 99.5% used before in the Frejus exp

The experimental set-up

Set-up with 4 low-background HPGe detectors (~225 cm3 each one) mounted in one cryostat with a well in the center

Set-up enclosed in a lead and copper passive shielding and with a nitrogen ventilation system in order to avoid radon

First data taking (1927 h): sample of metallic 100Mo powder with mass = 1009 g and (99.50.3)% enrichment in 100Mo, but counting rate ~3 times higher than the background rate of the set-up without the sample

Further purification of the sample from radioactive pollutions with procedure based on chemical transformation of metal molybdenum to molybdenum oxide 1199 g of purified 100MoO3

Effectively removed the pollution of 40K (14 times lower), 137Cs (6 times lower), and U/Th concentration (2 and 4 times lower)

If the 0+1 excited level of 100Ru (E=1130 keV)

is populated, two quanta with energies of 591 keV and 540 keV will be emitted in cascade in the following deexcitation process

Measurements

The background of the set-up was collected before and after the measurements with the sample (total time 7711 h) with consistent results;

Sample of 100MoO3 measured for 18120 h

1-dimensional sum spectrum of all 4 HP Ge detectors

background

DAQ accumulates both the energy spectra of the individual HP Ge detectors and their coincidences

Energy calibration performed before and after the measurements. Internal peaks of known origin (U/Th chains, 40K, 60Co, 137Cs) used to control the energy scale during data taking.The final energy resolution over 3 years of running time is 2.5 keV on the 583 keV line and 4.0 keV on the 1461 keV line

Analysis of the 1-dimensional energy spectrum

Background (normalized)

100MoO3

data

Both peaks at 540 keV and 591 keV expected for 100Mo→100Ru(01

+) 22 decay are observed in the data collected with 100MoO3

In the background spectrum they are absent

N = 4.85 1024 nuclei of 100Mo

t = 18120 h

= peaks efficiencies (e540=3.0%; 591=2.9%)

= conversion coefficient (=4.2810-3; 591=3.3210-3)

S = number of event in the peaks

Peak @ 539.5 keV

Fit in [480,560] keV with sum of exponential distribution and two Gaussians:

Peak position = 539.40.2 keV

S540 = 31956 events

2/dof = 0.76

Peak @ 590.8 keV

Fit in [560,625] keV with sum of exponential distribution and four Gaussians:

Peak position = 590.90.2 keV

S540 = 27853 events

2/dof = 1.4

Background (normalized)

100MoO3

data

Analysis of the 1-dimensional energy spectrum

similar results within uncertainties obtained changing exponential to straight line or energy intervals in the fit

Systematic uncertainties related with:- the mass of the 100MoO3 sample (0.01%);- the enrichment in 100Mo (0.3%);- the calculation of the live time (0.5%);- the calculation of the efficiencies (10%), estimated comparing calculated and measured

efficiencies for a voluminous water source containing several radioactive isotopes in the centre of the 4 HP Ge set-up

Analysis of the 2-dimensional energy spectrum

Spectrum of the events with multiplicity 2 accumulated in coincidence mode

Fixing the energy of one of the detectors to (6095) keV ( line of 214Bi)

Fixing the energy of one of the detectors to (26155) keV ( line of 208Tl)

Example

Analysis of the 2-dimensional energy spectrum

Taking into account the efficiency calculated for the coincidence (8.0 × 10−4) we obtain:

T = 17807 hEnergy of one detector fixed at (5402) keV

Energy of one detector fixed at (5912) keV

Energy of one detector fixed at (5452) keV (background)

Fixing the energy of one of the detectors to the quanta (591 or 540 keV) emitted in the 22 decay 100Mo100Ru(01

+), we observe the coincidence peak at the corresponding supplemental energy

Eight events detected (red)

in agreement with the half life derived in 1-d analysis

Possible processes mimicking the 100Mo100Ru(01+)

decayA. 100Mo + n 101Mo (=0.199 b; thermal n @ LNGS= 5.4-11 × 10−7 n cm−2 s−1)

101Mo (T1/2=14.61 m) decay 101Tc ’s from 101Mo decay: 540.1 keV (0.094%)

590.1 keV (5.6%) 590.9 keV (16.4%) + many other ’sExcluded:

(a) The ratio of the 540 and 591 keV peaks (1:234) is fully different from observed (1:1)

(b) The other lines from 101Mo decay are absent (c) Expected only 35–70 captures during 18120 h 10−3 counts in the 540 keV

peak

B. 100Mo + p → 100Tc + n (protons produced by fast neutrons or cosmic rays muons)

100Tc (T1/2=15.46 s) decay 100Ru the same levels @ 539.5 keV (0.75%) and 1130.3 keV (5.36%)

are populatedExcluded: The contribution from the (p,n) reaction can be ruled out because of low fluxes of fast neutrons (510−8 n cm−2s−1) and cosmic muons (310−8 cm−2s−1) at LNGS and because of the absence of H containing materials close to the 100MoO3 sample

Possible processes mimicking the 100Mo100Ru(01+)

decay

C. 100Mo + e → 100Tc + e- (capture rate of solar e by 100Mo: 93910−36

atom−1s−1)

100Tc (T1/2=15.46 s) decay 100Ru as before, levels @ 539.5 keV and 1130.3 keV are populatedExcluded:

Capture of solar neutrinos would give only 0.3 νe captures in our 100MoO3 sample during 18120 h.

D. 100Mo 20 decay to 100Ru(01+) (20g.s.: T1/2 > 1.1 × 1024 yr at 90%

C.L.)

This decay cannot be distinguished observing only emitted deexcitation

Excluded: Supposing equal the nuclear matrix elements for g.s. and 01

+ transition and accounting for the different Q2 values (T1/2 Q2

) we can expect a number of events 4 orders of magnitude lower than that observed in our measurements.

Search first proposed in [Proc. Nat. Ac. Sci. 45 (1959)

1301] to test the law of conservation of the electric charge.

If we suppose that in the decay:

(A,Z)→(A,Z + 1) +e-+e e- is replaced by some massless particle (e.g. a e or a quantum) the energy available in the (A,Z) decay would be increased by 511 keV (the e- rest mass).

This would allow transitions to the daughter (A,Z+1) nucleus which are energetically forbidden for the normal decay.

by

product Limit on charge non-conserving decay of

100Mo

Limit on charge non-conserving decay of 100Mo

In the used experimental configuration it is impossible to distinguish the CNC decay of 100Mo from the 2 process 100Mo→100Ru(01

+ ) - tracks of electrons are not registered.

If we consider the observed quanta as a result of the 2 decay of the 100Mo, we can calculate for Slim:

: yield of quantum in 100Tc decay (540= 6.60%; 591= 5.39%)

Taking into account only the more conservative value as the final result, we obtain:

From the obtained τCNC limit we can derive constraints on the CNC admixture in the weak interactions:

by

product

Conclusions

Data collected at the LNGS with 4 low-background HP Ge detectors for 18120 h with a 1199 g sample of 100MoO3, enriched in 100Mo to 99.5%, allow to observe the 22 decay 100Mo→100Ru(01

+).

The derived half life value is:

This value measured is in agreement with the results of previous experiments and does not confirm a previous negative result where a limit of T1/2 > 12× 1020 yr (90% C.L.) was set.

Moreover, the measured half life is in reasonable agreement with recent theoretical calculations: 1.8 × 1020 yr (inside the MAVA approach) , (2.6–4.4) × 1020 yr (SSDH), 4.2 × 1020 yr (SSDH), 1.9 × 1020 yr (MCM), 4.5 × 1020 yr (SSDH).

The result has been published on Nucl. Phys. A 846 (2010) 143