Double-beta decay of $^{130}$Te to the first 0$^{+}$ excited state of $^{130}$Xe with CUORICINO

0.72 MB·English

by  E. Andreotti

Double-beta decay of 130Te to the first 0+ excited state of 130Xe with CUORICINO
E. Andreotti,1,2,a C. Arnaboldi,3 F. T. Avignone III,4 M. Balata,5 I. Bandac,4 M. Barucci,6,7
J. W. Beeman,8 F. Bellini,9,10 C. Brofferio,2,3 A. Bryant,11,12 C. Bucci,5 L. Canonica,13,14 S. Capelli,2,3
L. Carbone,2 M. Carrettoni,2,3 M. Clemenza,2,3 O. Cremonesi,2 R. J. Creswick,4 S. Di Domizio,13,14
M. J. Dolinski,12,15 L. Ejzak,16 R. Faccini,9,10 H. A. Farach,4 E. Ferri,2,3 E. Fiorini,2,3,b L. Foggetta,1,2,c
A. Giachero,2 L. Gironi,2,3 A. Giuliani,1,2,d P. Gorla,5,e E. Guardincerri,5,11,14 T. D. Gutierrez,17
E. E. Haller,8,18 K. Kazkaz,15 L. Kogler,11,12 S. Kraft,2,3 C. Maiano,2,3 C. Martinez,4,f M. Martinez,2,19,g
R. H. Maruyama,16 S. Newman,4,5 S. Nisi,5 C. Nones,1,2,h E. B. Norman,15,20 A. Nucciotti,2,3
F. Orio,9,10 M. Pallavicini,13,14 V. Palmieri,21 L. Pattavina,2,3 M. Pavan,2,3 M. Pedretti,15 G. Pessina,2
S. Pirro,2 E. Previtali,2 L. Risegari,6,7 C. Rosenfeld,4 C. Rusconi,1,2 C. Salvioni,1,2 S. Sangiorgio,16,i
D. Schaeffer,2,3 N. D. Scielzo,15 M. Sisti,2,3 A. R. Smith,22 C. Tomei,10 G. Ventura,6,7 and M. Vignati9,10
2
1Dipartimento di Fisica e Matematica, Universita` dell’Insubria, Como I-22100 - Italy
1
2INFN - Sezione di Milano Bicocca, Milano I-20126 - Italy
0
2 3Dipartimento di Fisica, Universita` di Milano-Bicocca, Milano I-20126 - Italy
4Department of Physics and Astronomy, University of South Carolina, Columbia, SC 29208 - USA
n 5INFN - Laboratori Nazionali del Gran Sasso, Assergi (L’Aquila) I-67010 - Italy
a 6Dipartimento di Fisica, Universita` di Firenze, Firenze I-50125 - Italy
J
7INFN - Sezione di Firenze, Firenze I-50125 - Italy
0 8Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA
3 9Dipartimento di Fisica, Sapienza Universita` di Roma, Roma I-00185 - Italy
10INFN - Sezione di Roma, Roma I-00185 - Italy
x] 11Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA
e 12Department of Physics, University of California, Berkeley, CA 94720 - USA
- 13Dipartimento di Fisica, Universita` di Genova, Genova I-16146 - Italy
cl 14INFN - Sezione di Genova, Genova I-16146 - Italy
u 15Lawrence Livermore National Laboratory, Livermore, CA 94550 - USA
n 16Department of Physics, University of Wisconsin, Madison, WI 53706 - USA
[ 17Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407 - USA
18Department of Materials Science and Engineering,
2
University of California, Berkeley, CA 94720 - USA
v
19Laboratorio de Fisica Nuclear y Astroparticulas,
3
Universidad de Zaragoza, Zaragoza 50009 - Spain
1
20Department of Nuclear Engineering, University of California, Berkeley, CA 94720 - USA
3
21INFN - Laboratori Nazionali di Legnaro, Legnaro (Padova) I-35020 - Italy
4
22EH&S Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA
.
8 (Dated: January 31, 2012)
0
1 TheCUORICINOexperimentwasanarrayof62TeO2single-crystalbolometerswithatotal130Te
1 mass of 11.3kg. The experiment finished in 2008 after more than 3 years of active operating time.
: Searchesforboth0ν and2ν double-betadecaytothefirstexcited0+ statein130Xewereperformed
v
by studying different coincidence scenarios. The analysis was based on data representing a total
i
X exposureof N(130Te)·t=9.5×1025y. Noevidenceforasignal was found. Theresulting lowerlimits
r on the half lives are T12ν(cid:0)130Te→130 Xe∗(cid:1)>1.3×1023y (90% C.L.), and T10ν(cid:0)130Te→130 Xe∗(cid:1)>
2 2
a 9.4×1023y (90% C.L.).
PACSnumbers: 23.40.Hc,23.40.Bw,21.10.Bw,27.60.+j
I. INTRODUCTION
Two-neutrinodouble-beta(2νββ)decayandneutrino-
a Presentlyat: JointResearchCenter,InstituteforReferenceMa- less double-beta (0νββ) decay have been known for over
terialsandMeasurement,2440Geel-Belgium
b Correspondingauthor;e-mail: [email protected]
c Presentlyat: Laboratoiredel’Acc´el´erateurLin´eaire,CentreSci-
entifiqued’Orsay,91898Orsay-France Canada
d Presently at: Centre de Spectrom´etrie Nucl´eaire et de Spec- g Presently at: Institut d’Astrophysique Spatial, 91045 Orsay -
trom´etriedeMasse,91405OrsayCampus-France France
e Presently at: INFN - Sezione di Roma Tor Vergata, Roma I- h Presentlyat: CEA/Saclay, 91191Gif-sur-Yvette-France
00133-Italy i Presently at: Lawrence Livermore National Laboratory, Liver-
f Presently at: Queen’s University, Kingston, ON K7L 3N6 - more,CA94550-USA
2
70 years now [1, 2] (a recent review can be found in [3]).
While experimental evidence for 2νββ-decay has been Decay Transition Theoretical (y) Experimental (y)
found there is still no observation for the 0νββ-decay, 0+ →0+ 7.5×1025 [17, 18] >3.1×1022 [19]
0ν 1
however several limits for the half-life have been set in 0+ →0+ (1.6÷15)×1023 [20] >2.8×1024 [21]
thepastwithvaluesgreaterthan1021y. Inbothofthese 0+ →0+ (5.1÷14)×1022 [15, 16]a >2.3×1021 [16]
2ν 1
processesthelifetime isproportionaltothesquareofthe 0+ →0+ (1.7÷70)×1019 [20] 7.0×1020 [22]
Nuclear Matrix Elements (NME). Two neutrino double
a Correctedvaluesfor[15](discussionintext)
betadecayhasbeendetectedintennucleiontheground
state of the daughter nucleus and in two nuclei on the TABLEI.Theoreticalevaluations(formββ=1eV)andexper-
excited state of it, and the corresponding extracted val- imental best limits (90% CL) for the half-life of 130Te 0νββ
ues for the NME are in reasonable agreement with the and 2νββ decay.
theoretical expectation. In the case of 0νββ-decay their
value is very important since it plays the same role in
the prediction of the decay time as m , the effective
ββ
neutrino mass [3–6]. indicated in reference [15] since it was based on a wrong
evaluation of the phase-space. The reported value is the
TheCUORICINOexperimentwasanarrayof62TeO
2
one re-elaborated by A.S. Barabash [16] on the basis of
bolometers operated at a temperature of about 10mK.
the correct phase space factor.
A bolometer [7, 8] detects an energy release as a tem-
perature rise in the absorber crystal. Thermal pulses
are converted into electric signals by means of neutron
transmutation doped (NTD) thermistors [9], which are II. SEARCH STRATEGY AND EVENT
SELECTION
coupled to each absorber. CUORICINO was organized
in 13 planes. All of these planes were composed of four
crystals with dimensions of 5×5×5cm3 and a mass of In this analysis, we consider only configurations in
790geach,exceptforthe11th and12th (fromtoptobot- which the electrons are contained in the crystal where
tom). Each of these two particular planes had 9 crystals the decay takes place, and each de-excitation photon is
withdimensionsof3×3×6cm3andamassof330g. Two completely absorbed in one crystal. With these require-
of these smaller crystals were enriched to 82.3% of 128Te ments, three different scenarios are possible (see Fig-
and two others to 75% of 130Te. All the other crystals ure 3). Scenario 1 takes place when both gammas es-
had the natural isotopic abundance of 130Te (33.8%). A cape from the original crystal. In scenario 2, the low-
monthlycalibrationwasperformedusinga232Thsource. energy gamma (536.09keV) is trapped in the original
The energy spectrum of the events collected by CUORI- crystal with the betas, while the high-energy gamma
CINO can be seen in Figure 1. A more detailed descrip- (1257.41keV) escapes. Scenario 3 is the opposite of sce-
tion of the experiment can be found in [10]. nario2: thehigh-energygamma(1257.41keV)istrapped
CUORICINO’s geometry provides a unique opportu- in the original crystal with the betas, while the low-
nity to search for 0νββ and 2νββ decay to the first 0+ energyone(536.09keV)escapes. Thesignaturesandthe
excited state in 130Xe in an essentially background-free correspondingefficienciesarereportedinTableII.Afur-
environment. This is due to the fact that these pro- ther explanation of the calculation of the efficiencies can
cesses can be studied using a coincidence-based analysis be found in Section III.
by searching for two γ lines of well defined energy. As The first-level analysis of the CUORICINO data is
canbe seenin Figure 2, the decayto the first0+ excited common to all physics processes to be studied and is de-
state in 130Xe differs from the one to the ground state scribed in detail in [21]. It starts from raw events and
inthat itproduces agammacascade. Giventhe Q-value ends with a set of energy-calibratedhits associated with
of the decay, Q =2527.5keV[11–13], the two electrons atime,acrystal,andotherancillaryinformation,suchas
ββ
are left with a totalenergy of734.0keV.The mostprob- pulse shape parameters. In this phase of the analysis, a
able de-excitation pattern, with a 86% branching ratio, channel-andtime-dependentenergythresholdis applied
proceeds through the emission of a 1257.41keV and a tothedata,basedontheperformanceofeachbolometer.
536.09keVgamma. Though 2νββ and 0νββ decay both For the processes studied in this paper, the analysis
result in the emission of two electrons, the spectra of consistsofdefiningsignaturesaccordingtothethreesce-
the sum energy of the two electrons differ drastically. In narios reported in Table II, using them to select events
the first case, the two resulting betas have a continuous from the CUORICINO data and evaluating the corre-
spectrum in the range (0÷734.0)keV, while in the sec- sponding efficiencies from GEANT4-based Monte Carlo
ond case, the result is just a monochromatic beta peak simulations [23].
centered at 734.0keV. Theoretical evaluations and ex- Event selection criteria can be grouped into three
perimental limits for these two processes can be found categories: global, event-based and coincidence-based.
in Table I. It is important to note that the theoretical Globalandevent-basedcutsarenotspecifictothisanal-
calculationfor the half life of 2νββ-decay to the first ex- ysis, and here we only outline them briefly (refer to [21]
citedstate0+ reportedinTableIisnottheoneoriginally for details). Defined a priori,globalcuts are used to dis-
3
V 104
e
k
nts / 103
u
o
c
102
10
1
600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600
energy (keV)
FIG. 1. Single-hit (black line) and double-hit (red dashed line) energy spectra collected by CUORICINO in the range
(500÷2700)keV.
Decay mode Scenario Signature (energies in keV) Efficiency
MC Instrumental Total
1 734 (β) + 536 (γ) + 1257 (γ) (0.60±0.02)% (86±2)% (0.44 ± 0.02)%
0ν 2 1257 (γ) + 1270 (β+γ) (2.29±0.04)% (90±1)% (1.77 ± 0.04)%
3 536 (γ) + 1991 (β+γ) (1.41±0.03)% (90±1)% (1.09 ± 0.03)%
1 (0 ÷ 734) (β) + 536 (γ) + 1257 (γ) (0.53±0.02)% (86±2)% (0.39 ± 0.02)%
2ν 2 (536 ÷ 1270) (β+γ) + 1257 (γ) (3.04±0.04)% (90±1)% (2.35 ± 0.04)%
3 (1257 ÷ 1991) (β+γ) + 536 (γ) (1.28±0.03)% (90±1)% (0.99 ± 0.03)%
TABLEII.Signaturesandefficienciesforthethreescenariosfor0ν and2ν decay. Wedenotewiththe+signthecoincidenceof
energiesreleasedindifferentcrystals. EfficiencieslabeledasMCwerecomputedbasedonMonteCarlosimulations. Instrumental
efficiencies were computed based on CUORICINO data. Total efficiencies are given by the product of MC and instrumental
efficiencies, times a factor of 0.86 to account for thebranching ratio of theconsidered decay scheme(see Figure 2).
5+ sionofnon-physicalpulses(electronicspikesorcryogenic-
130I induced pulses) andphysicalpulses for whichthe energy
53 Β
0+ is not estimated correctly (pile-up or excessive noise su-
130Te perimposed on the pulse).
52 ΒΒ
Coincidence-based cuts rely on the properties of a
01+ 86 14 1793.5keV groupofeventsthatoccurredwithinafixedtimewindow.
Events can be selected based on the number of involved
22+ 13 87 1122.15keV crystals, the spatial distance among them, the sum en-
ergy or the energy of the single hits. In this paper, a
21+ 100 536.09keV 100mstimewindowwasusedtodefinecoincidentevents.
Physicalcoincidencesinduced by130Xe de-excitationoc-
QΒΒ=2527.5keV
0+ cur on much shorter time scales, but such a large time
130Xe window must be chosen to accountfor the slow response
54
of the bolometers.
FIG. 2. Decay scheme for 130Te, showing the energy levels Thecoincidence-basedeventselectioncriteriawerede-
(keV)and thebranchingratios for theγ-rays[14].
cidedbasedonthescenariosdescribedatthebeginningof
this section. Becausethe twoelectronsemittedinthe 2ν
decayhaveacontinuousspectrum,awideenergywindow
mustbechosenforoneofthecrystals. Thishastheeffect
card time windows in which one or more detectors per- ofintroducinga muchbiggerbackgroundthanis present
formed poorly. This could happen because of external in the analysis of the 0ν decay mode. As a consequence,
noise or cryogenic instabilities, which in turn result in a besides the criteria reported in Table II, additional re-
badenergyresolution. Event-basedcuts allowthe exclu- strictionswereappliedtotheeventstobeincludedinthe
4
one crystal after undergoing a Compton interaction in
a different one. The computed values reported in the
last column of Table II also include inefficiencies due to
event-based cuts, channel- and time-dependent energy
thresholds, and discarded time windows in which one
or more detectors were not performing properly (global
cuts). Inefficiencies induced by channel based cuts were
evaluatedontheCUORICINOdatainthesamewaydis-
cussedin[21]. Theeffectofglobalcutswastakenintoac-
(a)Scenario 1 (b)Scenario 2 countbyremovingthesimulatedeventslyinginthetime
windowsthatwerediscardedfromtherealCUORICINO
data, after said time windows were rescaled by the ra-
tio between the total duration of the simulation and the
real CUORICINO live time. The same procedure was
used to associate energy thresholds to the simulation.
Because the effect of global cuts was taken into consid-
eration when determining the efficiencies, the exposure
used in this work corresponds to the complete CUORI-
CINO statistics without any subtractions: N(130Te)·t =
9.5×1025y.
(c)Scenario 3
Figure 4 shows the energy spectra obtained from the
CUORICINOdataafterapplyingtheeventselectioncuts
described in Section II. For each scenario, the spectrum
FIG. 3. Possible capture scenarios. The blue lines represent
wasbuiltasfollows. Coincidencecutswereappliedbased
the1257.41keVγ,whiletheredlinesrepresentthe536.09keV
one. For each scenario, the available energy for the emitted on Table II, requiring that the accepted events be in co-
βs is 734.0keV. incidence with events satisfying each component hit of
the signature except for the hit corresponding to the
highest-energy γ. The signal search could then consist
2ν analysis. To reduce random coincidences, a cut was of a search in the resultant spectrum for evidence of the
imposed on the distance between the crystals involved highest-energyγofthesignature,whichisthecomponent
in the events, as it was seen from the simulation that with the lowest background. Moreover, the acceptance
there is a low chance for the investigated processes to widthforeachcutwasenlargedby±10keVwithrespect
involve crystals that are far apart from each other. The to the energies and energy ranges listed in Table II, to
most relevant backgroundfrom physicalprocesses is due account for the finite energy resolution of the detectors
to gamma rays that undergo a Compton interaction in (σ ≃ 2keV; see discussion below). The energy windows
one crystal and are then absorbed in another crystal. used for the spectra were chosen to be much larger than
While the sumenergyofthese eventsis fixed,the energy the detector resolution, but small enough that at most
released in each crystal has a continuous distribution. one radioactive background peak was included, and the
Toreduce this background,eventswhosesumenergyfell continuum could be assumed to be flat or linear.
intoawindowof±8keVaroundthemostintensegamma
lines(1729.60keV,1764.49keV,1847.42keV,2118.5keV,
Noevidenceforasignalwasfoundinanyoftheenergy
2204.21keV and 2447.86keV from 214Bi, 2505keV from
spectra. For the zero-neutrino decay mode, the back-
60Co and 2615keVfrom 208Tl) were removed.
ground is negligible, and no fit was performed. In this
case,a condition of zero signaland zero backgroundwas
assumed. Incontrast,thebackgroundisnotnegligiblefor
III. ANALYSIS
the 2ν decay mode, and therefore a Bayesian maximum
likelihood fit was performed for the 2ν analyses. The
As stated in Section II, Monte Carlo simulations were best-fit curves are represented by the blue lines in Fig-
usedtocalculatetheefficienciesfortheprocessesstudied ure 4. Depending on the scenario, different background
in this paper. This was achievedby comparing the num- models were adopted for the 2ν spectra. The continuum
ber of events passing the coincidence cuts to the total was fitted with a constant (scenarios 1 and 2) or linear
number of simulated events. The relatively low efficien- shape (scenario 3), while the possible additional peaks
cies reported in Table II arise from the fact that most of (1238keVfrom 214Bi for scenario 2, 511keVfor scenario
the gammas escape the crystals undetected and are ab- 3) were fitted with a Gaussian shape. The free parame-
sorbed by inert materials surrounding them. Moreover, tersinthefitwereasfollows: thenumberofsignalcounts,
the signatures sought only consider the case of photons the number of events from the flat background and the
that are completely absorbed in one crystal, thus reject- numberofcountsundertheadditionalbackgroundpeaks
ing events in which at least one photon is absorbed in (scenarios 2 and 3). The energy resolution was fixed to
5
A 444...555 D
3
444
2.5 333...555
ents/(1 keV) 1.25 nts / ( 1 keV )nts / ( 1 keV )nts / ( 1 keV ) 222...222333555
v eee
E vvv
1 EEE 111...555
111
0.5
000...555
0 000
1100 1150 1200 1250 1300 1350 1400 111222000000 111222222000 111222444000 111222666000 111222888000 111333000000
energy (keV) eeennneeerrrgggyyy (((kkkeeeVVV)))
222444
B E
222222
3
222000
2.5 111888
V) V )V )V ) 111666
ents/(1 ke 1.25 nts / ( 1 kents / ( 1 kents / ( 1 ke 111111111024024024
v eee
E EvEvEv 888
1
666
0.5 444
222
0 000
1200 1220 1240 1260 1280 1300 1320 1340 111222333000 111222333555 111222444000 111222444555 111222555000 111222555555 111222666000 111222666555 111222777000 111222777555 111222888000
energy (keV) eeennneeerrrgggyyy (((kkkeeeVVV)))
C F
3 777000
666000
2.5
V) V )V )V ) 555000
vents/(1 ke 1.25 ents / ( 1 keents / ( 1 keents / ( 1 ke 343434000000
E vvv
EEE
1
222000
0.5 111000
0 000
1920 1940 1960 1980 2000 2020 2040 2060 555000000 555111000 555222000 555333000 555444000 555555000 555666000 555777000
energy (keV) eeennneeerrrgggyyy (((kkkeeeVVV)))
FIG. 4. CUORICINO energy spectra after the event selection cuts applied for the 0ν (left) and 2ν (right) analyses. For the
0ν decay, the signal was expected at 1257.41keV (plot A), 1270keV (plot B) and 1991keV (plot C) for scenarios 1, 2 and 3
respectively. Forthe2ν decay,thesignalwasexpectedat1257.41keVforscenarios1(plotD)and2(plotE)andat536.09keV
for scenario 3 (plot F).
σ=1.8keV.It was evaluatedon the 511keVpeak and on ties, they were found to be negligible.
the two 60Co peaks at 1173keV and 1332keV that are
visible in the CUORICINO energy spectrum (see Fig-
ure 1), and it was found to be comparable for all three
IV. RESULTS
peaks. A summary of the best-fit values for the 2νββ
searches is reported in Table III. Systematic uncertain-
ties were evaluated by repeating the fitting procedure For each decay mode and for each of the three sce-
with different backgroundmodels, fitting ranges and en- narios,the posterior probability density function (p.d.f.)
ergy resolutions, and, compared to statistical uncertain- for the number of signal counts, P(N ), was extracted
S
using a Bayesian approach and assuming flat priors in
6
2 ordersof magnitude, for both the 0ν and 2ν processes,
Scenario NS NB with respect to the results of past experiments. It is
[counts] [counts/keV] worth noting that the new lower limit on the half life of
1 1.1±1.4±0.29 0.12±0.03 the 2ν decay mode is close to the upper bound of the
2 -0.4±6.6±2.6 6.31±0.41 theoreticalcalculationpresentedinTableI.Amoreclear
3 -3.0±6.8±2.8 6.73±0.33
picture will be available once CUORE, CUORICINO’s
successor [24], comes online. This is due to the increase
in target mass and improved background reduction that
TABLEIII.2νanalysisbest-fitvaluesforthenumberofsignal
will be achieved in CUORE.
(NS) and background (NB) counts. For NS, both statistical
and systematic uncertainties are reported.
the physical region (N > 0). For the 0ν decay mode,
s
because there was no evidence of a signal and the back- ACKNOWLEDGMENTS
ground was negligible, a Poisson p.d.f. for zero observed
events was assumed for all three scenarios. For the 2ν
TheCUORICINOCollaborationowesmanythanksto
decay mode, the p.d.f.s were obtained as a result of the
the Directors and Staff of the Laboratori Nazionali del
maximum likelihood fits on the spectra shown in Fig-
Gran Sasso over the years of the development, construc-
ure 4. For each decay mode, a global p.d.f. for the
tionandoperationofCUORICINO,andtothe technical
decay rate was obtained as the product of the three in-
staffs of our Laboratories. In particular we would like to
dividual p.d.f.s, P (Γ) = Q P (Γ). In this formula
P (Γ)=P (N )·εT·ONT(130Te)·ti, wihere the index i runs thank R. Gaigher, R. Mazza, P. Nuvolone, M. Perego,
i i S i B.Romualdi,L.TatananniandA.Rotilioforcontinuous
over the three scenarios and ε is the corresponding de-
i and constructive help in various stages of this experi-
tectionefficiency fromTable II.Systematic uncertainties
ment. We are grateful to our colleagues Y.G. Kolomen-
were included in the P (Γ) according to the procedure
i ski and L. Zanotti for help and fruitful discussions. The
described in [21]. This resulted in the following half life
CUORICINO experiment was supported by the Istituto
lower limits:
NazionalediFisicaNucleare(INFN), the Commissionof
the European Community under Contract No. HPRN-
T (2νββ∗)>1.3·1023y, 90%C.L. CT-2002-00322,bytheU.S.DepartmentofEnergyunder
1/2
T (0νββ∗)>9.4·1023y, 90%C.L. Contract No. DE-AC03-76-SF00098,and DOE W-7405-
1/2
Eng-48, and by the National Science Foundation Grant
These new limits represent an improvement of almost Nos. PHY-0139294and PHY-0500337.
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