Double beta decays of 116Cd

Nuclear Physms A577 (1994) 405c~10c North-Holland, Amsterdam

NUCLEAR PHYSICS A

Double beta decays of 116Cd K Kume ~, H Ejlrl a'b, K Fushxml~,R I-Iazama~,K Kawasakl ~ *, V Koutz ~, N Kudoml~,K Nagata~,V Mahshko ¢, H Ohsuml ~, K Okada d, H Sano~,T Senoo ~, T Shlbata ~, T Shlma f, J Tanaka ~ and Yu Zdesenko ¢ ~Department of Physms, Osaka Umverslty, Toyonaka, 560, Japan bResearch Center for Nuclear Physics, Osaka Umverslty, Smta 567, Japan ¢Instltute for Nuclear Research, Kmv, Prospect Naukl, 47 Kmv 252028,Ukraine dFacluty of Science, Kyoto Sangyo Umverslty, Kyoto 603,Japan ~Instxtute for Nuclear Study, Umverslty of Tokyo, Tanashl 188, Japan fDepartment of Applied Physms,Tokyo Institute for Technology, Tokyo 152, Japan A fimte half-hfe of T~y2 = 2 "-0~+°95 1019 yr(68%CL) for the two neutrmo double beta decay of n ° C d to the ground state of n6Sn was found for the first txme by means of the E L E G A N T S V detector Nuclear matrix elements for the two neutrino double beta decay was deduced as M 2~ = (0 073 4- 0 010)m21 from the measured half hfe A stnngent lower hmxt on the half hfe of neutrmoless double beta decay of 116Cd to the ground state m n6Sn was obtained as T°~2 > 5 4 x 1021yr (68%CL) Double beta decays (/3/3) are current interesting subjects for studying the fundamental propemes of neutrino and for checking the vall&ty of electroweak standard model[l] The two neutrino double beta ( 2uflt3 ) decay, (A, Z - 2) --+ (A, Z) + 2e- + 2P, is the process within the framework of the electroweak standard model On the other hand, the neutrmoless double beta ( 0u~3/3 ) decay, (A, Z - 2) ~ (A, Z) + 2e-, is relevant to theories beyond the standard model Experimentally, ~/3 decays have been extensively studied for 76Ge by using sohd state Ge detectors [2][3] The lower hmlt on the half hfe T°y2 for the 0u¢]/3 mode has been obtained to be T~ly2 > 2 5 x 1024yr (68% CL) , whmh leads to the upper hmlt of 1 5 eV on the Majorana neutrino mass ( < m~ > )[2] Here one has to know the nuclear matrix element M °" for the 0 v ~ mode m order to deduce the neutrino mass So far, there is no experimental way to get M °~ Thus one has to rely on theoretmal calculations for M °~ Experimental observation of the 2uj3~ mode gives the nuclear matrix element M 2~ for the 2u/~j3 mode The comparison of the obtained value for M 2" and calculated ones provides one with good test for the theoretmal ealculatmns of M °~ as well as M 2~ "Present address Nomura Research Instltute,I~amakura 247, Japan 0375-9474/94/$07 00 © 1994 - Elsevier Science B V All nghts reserved 0375-9474(94)00404-8

SSDI

406c

K Kume et al I Double beta decays ofll6cd

41~6In 1+

Half lives of 2u/3z3 decays have been observed m darect counting methods for the isotopes r6Ge[3J,S2Se[4], and

41~6Cd 0 +

1°°Mo[5][61

Qn~ =28

In the present work ll6Cd with the large Q value of Q~a = 2 80MeV (figure 1) was chosen because of the large phase spaces of 2u/3~ and 0u/3~3 Measurements of 2u9S and 0u/3!3 decays for 116Cd wele camed out by means of ELEGANTS V[?](EL V) at the Kamaoka underground laboratory(2,700m ~ e ) Details of EL V have been presented elsewhere [5][7] EL V consists of a couple of drift chainbers (DC), plastic scintillator arrays (PL), and sodium iodide detector arrays (NaI), The whole detector system was shielded by 10era thick oxygen flee high conductive coppei (OFHC)

li6~_ 50 ~n 0 +

Figure 1 Decay scheme of il6Cd

L

r ~l!--II

Pb

°,'-," Xe,0, e_l~r%.

J P

DC

NaiCu' i- L

SH

0

0 5

IJIIIIIJIIr

1

~

Figure 2 Schematic view of ELEGANTS V detector (EL V), where the left-side part of the movable shield (SH), whwh consists of lead and copper bricks, is opened to show the detectors msade the shield box Pb lead bricks, Cu OFHC bricks, NaI NaI detector array, LG light guide, PM photomultlpher, PL-A and PL-B plastic scintillators, DC-A and DC-B drift chambers, S source plane, CD cable duct, and AT alrtaght eontmner Typacal ,39 tracks (~1,92) and a Compton scattered electron ( e ) and "7 ray are illustrated

K Kume et al / Double beta decays o f l l 6 c d

407c

brinks and 15cm think lead bricks It was put in an air - tight contaaner to keep the system free from radioactive Rn gas m the air Dry N2 gas evaporated from hquld N 2 was introduced reside the container, to purge radmactlve Rn gas Enriched 116Cd (90 7% enrmhment) source with 91 lg m weight and natural Cd (nCd) one with 89 3g m weight were used for comparison Both enriched 116Cd metal and natural nCd one were rolled into foils with 33mg/cm 2 m thmkness Two runs were carned out, the first run (A) for 920 hours and the second one (B) for 955 hours To cancel out instrumental systematm errors due to detector efficmncms and non-umform backgrounds, the position of ll6Cd and "Cd was exchanged m the second run The sum energy (E~ + E~,) spectrum for each run was obtained by summing signals from two PL's Here low energy signals below 0 3MeV were cut off (E-cut) to avoid low energy background events Rejection of fl~tracks with I 0 - 180° I< 30 ° (0-cut), where was the angle between the two/3 tracks, was made to avoid spurmus ~3events caused by single electron events It m important to notice a definite excess in the energy regmn of 0 7 - 2 0 MeV m the sum energy spectrum, where the 2~j3~3 decay of 116Cd m expected, Here most of major background events m the 116Cd and nCd spectra are considered to be due to radmactlvltms around and/or m the detector elements Internal conversmn electrons (ICE) following the ~ decays give background events of two fl rays Among them, 2°ST1 in 232Th series and 214B1 m 238U series are important The contents of 238U and 232Th isotopes m both 116Cd and nCd foils were checked to be less than 0 5 ppb by using ICPMS (mductlvely coupled plasma separation method) Assuming radmactlve equlhbrmm, the expected background rates due to these isotopes gives the upper hmlt for 2vj3~ decay hfe time of 1 3 x 1021yr and 6 0 x 1021yr for U and Th serms Radmactlve Rn gas around the source fods is another origin of 214B1 The background rate from the possible 214B1 was checked by 3-ICE-7 comcxdence method Here "r ray is emitted from excited states of 214po and xs observed by the NaI detector m EL V Contributions from rad~oactlve Rn around source foils are smaller by almost one order of magnitude than the 116Cd 2r~3fl event rate Thus background events are common for both H6Cd and "Cd , and are corrected for by subtracting the "Cd spectrum from the ll6Cd spectrum The sum energy spectrum of 116Cd for the first run (A), being corrected for the spectrum of "Cd , is obtained as shown in figure 3 The fimte contributmn from H6Cd is clearly observed The sohd line shows a Monte Carlo calculation (MC) for ~3/3decay of H6Cd with a half-life T2/2 = 2 6 1019yr Together with data for the second run (B) the half-hfe is derived as T2/~2 = 2 6+°-059 1019yr with 68% CL There has been observed no defimte excess of counts beyond stastmal errors at the 0L,~3~3peak regmn of E = 2 3 - 2 9MeV The lower hmlts on half hves hfe times of the 0~3~3 mode are 2~2 > 5 4(2 8) 1021yr for < m~ > mode and T°y2 > 3 6(2 0) 10~lyr for < A > mode, with 68(90)% CL m 2794 hour measurement (see table 1), respectively The nuclear matrix element M ~ is derived from the measured T2~ The obtained value of M 2~" = (0 073 4- 0 010)m~-1, where m~ stands for the electron mass, is smaller by one order of magmtude than the Q R P A + O E M calculation[9], while it is of the same order of magmtude with the QRPA calculatmn[8] The values M 2~ deduced from the T~/2 measured by the direct counting method are plotted m figure 4 They are uniformly

408c

K Kume et al / Double beta decays ofll6cd

small, and are scattered around 0 0 7 m [ 1 The present experimental limits for two 0u/33 modes are the most stringent hmIts ever pubhshed Using the nuclear matrix element M °" calculated by QRPA[8], two upper hmltsof<9eVand<14 10 .5 a r e o b t a m e d w l t h 6 8 % C L

Table 1 Summaries of 2u/3/3 and 0v/3/3 half-h~es of 116Cd mode

RUN-ID

2v##

A B A+B A B A+B A B A+B

o.##

0~'f13

I

i

I

i

hve time ( hour ) 920 955 1875 1445 1349 2749 1445 1349 2749 [

70--I-27 5 102 9 + 4 1 2

T1/2 × 1019(yr). v-06 2 7+1 ~-0

~ 7

2 6 +0 ~ -0 5

<22 <32

226

<22 <32

143 158

>49×102 > 3 2 × 10e > 5 4 × 10 e > 3 1 × 102 > 2 1 × 10 ~ > 3 6 × 10e

22 2

I ....

I'

_natcd

lteCd

detection efficmncy ( % ) 73 90

ymld

''1'''

I ....

.,

20 Qaa=2 80MeV "'

> ~9

10

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(

--'~

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U]

0

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0

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-10 ,,r

,I,,,,I 05

....

1

15

f .... 2

[ l l l l

25

3

ENERGY

(MeV) Figure 3

The difference between the ll6Cd and nCd spectra (plot) together with Monte Carlo (MC) calculation for 2u/?/? mode (sohd hne) for the 920 hour run

K Kume et al

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Double beta decays ofll6cd

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1021

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(Y)

Figure 4 M 2~ derived by the dxrect measurements of (a) n6Cd (this work),(b) 1°°Mo[5][6],(c) S2Se[4], (d) 76Ge[3] REFERENCES

1 2 3

4 5 6 7 8 9

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