Experimental review of double beta decay

Nuclear Physics B (Proc. Suppl.) 19 (1991) 158-176 North-Holland

158

E X P E R I M E N T A L R E V I E W OF DOUBLE BETA DECAY M.K. MOE Physics Department, University of California, Irvine Irvine, California 92717, USA* The unique sensitivity of nuclear double beta decay to the existence of Majorana neutrinos of nonzero mass, and to right-handed weak leptonic currents, has inspired the more than 30 experiments now underway or being planned to search for this rare disintegration. These experiments are reviewed, with some discussion of recent positive results for the two-neutrino mode, and the projected nentrino-mass sensitivity of the zero-neutrino

experiments. 1. INTRODUCTION Although it is the rarest of Natuze's known processes, double beta decay is also one of her most fascinating. The implications of a yet-unseen neutrinoless decay mode have intrigued theorists and experimentalists alike since the po~'bKity was first suggested by

observation of neutrinoless double beta decay would answer the first two, and under some clrcnm~tances all three of these questions in the affirmative, opening the door to new physics beyond the Standard Model. A number of good review articles can be found in the literature. 2-17 Double beta decay is possible for some three dozen even-even nuclei for which pairing forces or angular

T

e

momentum differences prevent ordinary beta decay to an adjacent isobar, but which are nevertheless unstable with respect to a next-nearest isobar (Fig. 1). In

o*

t

2"

Q

o

A,Z

A,Z + I

the standard, allowed mode, two neutrons simultaneously convert to protons and emit two electrons and two antinentrinos (Fig. 2a). The s, lm of the kinetic

A,Z+2

{o) ~.Szv

{b) ~,8ov

P FIGUi~.E 1 Energy level scheme for double beta decay. In this example, single beta decay to the intermediate isobar is energetically forbidden.

i

\ Wendell Furry I in 1939. As we push the search for neutrinoless decay to ever greater sensitivity, we are asking whether the neutrino has mass, whether it is its own antiparticle, and whether there exists a righthanded component in the weak leptonic current. The

/ n

P

P

n

n

1/

FIGURE 2 (a) Two-neutrino double beta decay. (b) The zeroneutrino mode

*Supported by the U.S. Department of Energy, Contract No. DE AT03-76SF00010. 0920--5632/91/$3.50 © Elsevier Science Publishers B.V. (North-IIolland)

M.K. Moe / Double beta decay

energies of the two electrons is a broad distribution (Fig. 3) This two-neutrino mode has been observed in the laboratory for S~Se with a half life of about 10a° years, m Recently there have been reports of laboratory

t~

by the emis~_~n of a m a : ~ a ~. ~ cently ruled out by ~ . The ~ nance e x ~ neutrino-~~ ~

~ ~ c~" the ~ ~ a

re-

measurements of two-neutrino decay of 7eGe and m°Mo as well.:9-ax We also have loag-staoding geochemical measurements of the presumably two-neutrino half ~ves

of e~Se and Z~Te.~ In Fm'ry's alternative, a virtual antineutdno emitted by the first neutron is absorbed as a neutrino by the second, and two electrons but no a n t i n e u ~ n ~ appear in the final state (Fig. 2b) This can happea oaly if the neutrino and antineutrino ate identical, a possibility first suggested by Majoraua24. Reabso~tion of the neutrino also requires an helicity reversal of parity nonconservation in the weak interaction. reversal would be a m~ifestation of neutrino m a s ~ .

L

~~tT. The ma~']x ~ , have been a c h a l k n ~ ~ r ~

dN dE

st~cmre c a l ~

~

0.5

ESUM/Q FIGURE 3 The sum of the kinetic energies of the two emitted electrons for two-neutrino and zero-neutrino decay. For the neutrinoless mode the electron sum spectrum would be a distinctive spike at the full transition energy. If neutrinoless decay to the first excited (2 +) level is also observed it would signal the presence of right handed currents. Neither form of neutrinoless decay has been observed, the best lower limit on the half life being 2.4x 1024 y e a r s 26 for the ground-state transition in ¢°Ge. A third possible mode that received much attention in recent years is neutrinoless decay accompanied

d ~ =

I.o The first serious ~

c~.~r T~~

of t ~ ~

dent of , m l r a ~ ~ 0.0

ca ~ ~ .

~ .

iM ~

(z} ~ c a s

s m

early SO's~ some ~ - ~ d ~ ~ ~ ~ sured h-lf lives which were ~ ~ ; ~ m s for a2Se and la°Te. ~ disczepancy k d to ~ tionin~ of the geochemicaI ~ ~ w~da t e n d ~ to be longer than p ~ c t e d by theory, the p~blem especially severe~9 for ~ T e . The confirmation of ~he g ~ c a l S2Se ~ by direct experiment led to further theoretical ~ . Calculations in the quasipartide random phase approximation (QRPA) ~°-~ have shown that the twoneutrino rates are strongly suppressed by ground-state particle-hole correlations. It is now possible to create realisticaUy small two-nentrino matrix dements with the QRPA, but extreme sensitivity to the particle-

160

M.E. Moe/Double beta decay

particle interaction strength limits the predictive power of the theory. This sensitivity is particularly acute in the case of l°°Mo.

to-noise ratios to the point where the murky business of dummy-source subtraction is unuecessary.

The majority of theorists argue that zero-neutrino matrix elements do not suffer this sensitivity, and are

4. RANGE OF < rn, > ACCESSIBLE TO DOUBLE BETA DECAY EXPERIMENTS

more readily calculable. This is fortunate, but since there is currently no way to test the zero-neutrino calculations the situation would be more satisfying if theory were more than loosely accountable for the two-

A criticism sometimes heard by those of us who do

neutrino matrix elements that do face experimental constraints. It is worth while, therefore, to continue measuring two-neutrino half lives for additional isotopes in the hope that future improvements to theory will be testable. Limits, or ultimately a value for neutrino mass from neutrinoless double beta decay will be only as reliable as 15t °~ 12. 3. THE RECENT 76Ge AND l°°Mo TWO-NEUTRINO MEASUREMENTS The two-neutrino measurements reported to date for ~ZGe and l~lHo have been derived from low signalto-noise ratios by the subtraction of a natural source spectrum from that of a source enriched in the double beta isotope. 19-21 We should be cognizant of the history of false positive measurements from the twosource technique, sam and be cautions in interpreting

double beta decay experiments is that we are "searchLug under the lamppost" for neutrino mass, i.e., we are looldng in a small patch of light in a vast region of darkness. Furthermore, a filter on this lamp lets us see only Majorana neutrinos. To be sure, the predictive power of grand unified theories for < rn~ > is very poor, with values ranging from several eV all the way down to 10-11 eV. The current upper limit on < my > from double beta decay searches is about 1 eV. How far down in < rn, > can we expect to probe with double beta decay before the lamp light becomes too dim? Searches for the 0u spike usually involve looking for a bump among the statistical fluctuations in the background at the expected energy. The fluctuations go as the square-root of the number of background counts, and the half-life limits so determined have this square-root dependence. Since the neutrino mass ]iraits, in turn, go as the square-root of the half life, the mass limits depend on the fourth root of the number of counts. The resulting equation is W

) 112

these results. Indeed, the failure of the 90~ confidence limits of the two recent l°°Mo two-neutrino measure-

<:mv:>m,n - 2.50x 10-s .~zeO°-~lM°vl~

ments to overlap (see Table I, collaborations jj and

with < m~ >,~i~ in electron volts. W = molecular weight (g), ~ = isotopic abundance, z - no. /~/~ iso-

kk) shows that at least one of these measurements is wrong. The difBculty is that the mix of radioactive impu-

~('SAE~'I4Mt / (3)

tope atoms/molecule, e = efficiency, b = background (keV.kg-y)-I, A E = FWHM resolution (keV), M =

rities in and around the enriched and natural sources

source mass (kg), t = run time (y).

may not be identical. Double beta decay experiments are constantly fighting with backgrounds that can easily overwhelm the signal. A slightly higher background in the enriched source is all is needed ~o give the ap-

factor may be somewhat different for different statistical methods of analysis, but the equation gives the

pearance of double beta decay in the difference spectrum. The problem persists because most independent methods of measuring impurity levels have been less sensitive than the double beta decay experiment itself. One goal experimentalists share is to improve signal-

The constant

essential dependence of the neutrino mass limit on the parameters involved. (A claim of discovery, however, would require more stringent conditions.) The mass < my >,nin is proportional to the product of two factors, one to the I/2 power, and one to the I/4 power. For germanium detectors, which are responsible for the best existing limits on < rnv ) , the

M.K. Moe / Double beta decay

I~]

only exploitable parameter in the 1/2 power term is

Some b]m~l~values can be est~ma~t~l~ gor ~

the isotopic fraction ~ . Since natural germanium is

FWHM resolution for Oe ~

only 7.8% ~6Ge, a si~ficant improvement in sensitivity can result from the use of isotopically enriched m a t e r i a l (See Fig. 4).

table since they are ~

The other controllable parameters for ~ennanium are all to the 1/4-power. This is a very tough game!

me+hod, and a b r i d ~

To achieve an order of mag3aitude improvement in sensitivity to < m~ >, one must increase the detector mass

me=musty s=pplied by ~ simkmme= ~ ~ collabora+Jous, and is ~ ~ The accept+ respomibiti~v ~ rmr ~ ~

by four orders of magnitude, or reduce the backs~und

the

is ~

ab~

3 ~V. Completed exlm~iments ~ ~ + e d ~ by ea,-lisr ~ 0 ~ The tab~ is ~ ~ to e z ~ a m ~

;,,the t e x t M u ~ o t ~

of~

~

is~

in~¢mation ~ ~

~

~

by four orders, or some combination of both. One certainly c~-not run I0 ~ times as long. Enriched germanium experiments now being as-

sembled are expected to reach a few tenths of an eV. It may be possible with great effort, e~ecially in terms of back~ound and resolution, to eventually reach 0.01 eV with large detectors of some less expensive isotope

tO

having a favorable G°~ I.M °" I~ product, as -If < m~ >is much less than 0.01 eV it will have to be found some other way - - perhaps as a mass dill]~erencein neutrino oscillations. It should be pointed out that in the absence of background we can escape the 1/4 power dependence, and trade it for the 1/2 power. By ass-m~ng 1.14 counts, the 68% confidence ]im~t for zero events during the run time, we fiud a m+n+mum detectable mass W ~ V2 < m+, > m , . = 2.67

x

10 - s

~ z e G o , ' I~10,, l= M=/

(4)

for zero background. One then follows the steeper slope in Fig. 4 until the first background count ap-

++ UCS8-

++

LeL

A

> ~efo~

q

|0-+

=6 z

+o

~oz

,o3

Mt ( k g - y )

pears, after which the 1/4 power again prevails. 5. CURRENTLY ACTIVE OR PLANNED EXPERIMENTS More than 30 double beta decay experiments are underway or in various stages of planning and construction. A summary of the direct counting experiments, including the available parameters relevant to < rn~ >mi, sensitivity, is included in Table I. In cases where all entries but "source thickness" are complete the reader can combine the parameters as in equation 3 (or equation 4 if the background is zero) to arrive at the m~, accessible to a particular experiment.

FIGURE 4 The mlnlm,m detectable neutrino ~ vs. the product of detector m,_~ and run time for germanium +
16"2

M.K. M o e / Double beta decay

selenium that decay to the noble gases xenon and krypton. Because of their volatility and chemical inertness, noble gases are strongly depleted in minerals at the time of crystalization. The great sensitivity of noble gas msss spectrometry makes it p~__~ble to see the tiny excess of daughter atoms built up over ~eologic time.~ The method does not reveal the electron spectrmn and so does not differentiate directly between the twoneutrino and zero-nentrino modes. A famous argumeat by Pontecorvo, 37 however, suggests that the ratio of half lives of 12~Te and IS°Te should be sensitive to the existence of zero-neutrino decay. Assureing similar matrix elements, the half lives of the two isotopes should be quite ~ t because of their unequal transition energies. But how different, the reasoning goes, depends on the decay mode since the energy dependence of haK life much is stronger for twoneuL~ino than fur zero-neutrino decay. (The assumption of equal matrix elements is probably not valid,29 and one would have to use calculated values whose present uncertainty compromises the sensitivity of the tellurium ratio test. It is apparent that the cm~.~ut 1 eV mass l~mlts from Other experiments are already approarMng the tellurium ratio sensitivity.) The 13~I~ half life is f ~ l y wen established, but the lon~..rand more difficult lZ~re value remains controversial.~ s ~ There is work underway at the MaxPlanck Institute Fur Kernphysik in Heidelberg aimed at improving the sensitivity to 12STe decay through resonance ionization mass spectrometry using time-offlight. Measurements could begin very soon. 4° Other geochemical measurements on selenium and telluri-m are being carried out at the University of Missouri at Ptolla. s9 A n experiment to detect l°°Ru from the double beta decay of l°°Mo in 2-hiUion-yearold molybdenum sulfide mineral is underway at Los Alamos National Laboratory and the University of New Mexico. This work is described briefly under "Other Geochemical Experiments" in reference 43. Attempts to contact the spokesman were unsuccessful. 5.2 Radiochemical (milking) experiments Similar to the geochemical experiments in that they

look for accumulations of daughter nuclei, radiochemical experiments are carried out on a much shorter time scale. The ter~hnlque exploits daughters that are radioactive, and therefore detectable in smaller numbers than atoms of stable noble gases, so geologic integration times are mmecessary. And .nlilre the small, preeious samples typical of geochemical measurements, large quantities of the donble-beta parent isotope can be isolated in a controlled environment for a period of one to several tens of years. The sample then is ~ i l k e d z to collect the radioactive daughter for count-

ing. These experiments are free of uncertainties of ore age, primordial concentration of daughter isotope, and long-term e~ects of gawous dflSzsion. Two good doublebeta candidates for the radiochemical technique are ~U

~Th

~ ~Pu

~~ U

The daughters are alpha unstable with hag lives of 88 and 72 years respectively--longer than ideal, but still quite convenient. One experimental hurdle is the development of alpha particle detectors of sumciently low background. In 1983, Haxton, Cowan, and Goldhaber,41 in the spirit of Pontecorvo's argument for tellurium, pointed out that the ratio of double-beta-decay rates for 238Uand ~2Th should differ by about a factor of six for pure zero-neutrino and pure two-neutrino decay. An experiment being carried out by the University of Chicago, Los Alamos National Laboratory, and the Santa Fe Institute is counting the 238pu alpha patticles from 4 kg of 2asU aged 32 years since the initial purging of plutonium. They have set a half life limit of 6×1020 years in smaller samples, and are aiming for a sensitivity of 1022 years.4~ Another experiment on 2ssU is being done at the Centre de Recherches Nucl~aires, Strasbourg, where 300 kg of uranium has been purged of plutonium and stored one year in a 200 meter deep mine. Extraction chromatography is expected to begin shortly, and the alpha spectroscopy will be done in the Laboratoire Souterrain de Modane. Approximately 200 al-

M.K. Moe / Double beta decay

pha counts per year are expected if the ~asU half ilfe is 10~ years.4a

1~

two-neutrh~ decay ~

butioa of this ~

the broad ~

is not ~

~ -

by ~

5.3 ~eGe experiments (source = detector)

The most sensitiveneutrinolessdouble beta decay measurements to date have been done with large germ a n h m =ystah that contain 7.8% natural abundance of the double beta isotope, ~eGe (Fig. 5). The highpurity germanium is at once the source and detector, a situation that permits the mcmitoring of large numbers of ~eOe atoms with high eltldeney. The typically excellent 3 keY FWHM energy resolution is beautifully suited to searching for the zero-neutrino spike at

is actually~

~

for aa

~y.

I n ~ d ~ ~

L m ium

6).

>

¢D

xSO0 0

_oe0o m l-. 4 o o z :~2oo

0 o

O~

20

LO

IO

A ~ c e

~

FIG~ ~

by 0

|.5

ENERGY ( ~ V )

'

2000

2020 2040 2060 ENERGY ( k e V )

2080

FIGURE 5 A portion of the energy spectrum from the UCSB-LBL germanium experiment near the 2041 keY Q-vahe. In 21 kg.ycars there is no evidence of a peak corresponding to neutrinoless double beta decay. Instead there is a dip at the expected energy (arrow). A 68% contldence limit for T~1~2was set at 2.4x 1024years.~

6 7aGe ~

n EP-YEPI

2.5 kg.y. The ¢o~ts are a ~ ~ to t ~ - ~ double beta ~ y dCSGe ~ a h a h ~ d (9+1)x 1 ~

years (eS% c.L) As ~

~

to d,-op, and ~

tithed detectors come on ~ p for two-~mtdno ~ in ~

~

20.5% of natural germanium. Fortunately 7°Ge (and of course 6SGe) will be strongly depleted in the enriched 7~Ge detectors.

Germanium is less well suited to the detection of

=smmeu~ ~

quite strong. And enriched germanium ~

be hard to beat for sensitivity to ~

decay.

5.4 Semiconductor-wa~r One can get away from the l i m i t ~

the Q value of 2.041 MeV. A rather persistent background in these experiments has been the 8 + decay of eSGa following:electron capture in 288-day cosmogenic eSGe. This activity comes from T°Ge(n,3n)eSGe, and 7°Ge makes up

~-

on

selection by desi~'n;n~ detectors that are ~ from the source. A price h ~ to be paid in ~ of energy resolution, efficiency, and loss of ~ o n on what goes on within the source itse]L But one is free to choose an isotope of high Q-walue to relieve pressure from low-energy backgrounds and to enjoy a faster double beta transition rate. An example is silicon semiconductors interleaved with 91m~ of x°°Mo (Q = 3.00 MeV).

76Ge

76Ge

76Ge

76Ge

7eGe

76Ge

76Ge

Caltech Neuchatel Paul Scherrer Institute ==

IGEXbb

ITEP YEPI cc

MPIH-KIAEdd

PNL-USC ~c

PNL-USC ITEP//

UCSB-LBLg0

0.078

0.085

0.76"

0.76"

0.078

0.86

0.85

0.85

0.078

0.76"

0.76"

0.76"

0.76"

0.76"

76Ge E x p e r l m e n t s (Source = Detector)

1.2 =

0.3h

0.6_+0.3/

2.2

0.3b

2.9=

3.3

3

3.11

4.3 d

3.2

6.9

0.25

2.1

12e

1.2

2.1c

5.8

Operating in Oroville dam. Mt=21 kg-y T~2 >2.4(1.2)×1024y, 68% (90%) c.1. Detectors being diverted to dark matter search.

Operating in Homestake gold mine in PNL-USC low background cryostat.

Operating in Homestake gold mine. Deep-mined Ge with minimum cosmic ray exposure, 450 year-old Pb. T~/"2 ---(1.1_o.3)×10 +0.e 21 y, 95% c.l.e~

16.9 kg of 86% ~6Ge raw material in hand. First 1.0 kg enriched detector has been made. Space allocated at Gran Sasso. Experiment fully funded.

T°/"2 >1.3x1024y, 68% c.1. T2/"2 -(9_+1)x102°y, 68% c.1.

M t = 2.5 kg-y

Operating in Avan salt mine.

5 kg of 85% T6Ge raw material to be delivered Oct. 90. Trying to obtain additional 5 kg.

Running in St. Gotthard Tunnel. Mt = 7.0 kg-y TI°/%>2.7(1.7)x10~y, 68% (90%) c.l. Cosmogenic background dropping.

TABLE I. Direct counting double beta decay experiments that are currently in operation~ under construction~ or being proposed. Efficiency Isotopic 0z, Resolution Total Source for 0v fraction background at endpoint source mass thickness Collaboration Isotope e .(4 b(keV.kg.y) -1 AE (keV) M(kg) (mg/cm 2) Current status and recent results

O

t~

j.d ¢b

7eGe 0.34

Z°°Mo

0.15

S2Se

UC Irvine LANL"

Bordeaux Caen Orsay Strasbourg m~

X°°Mo

Osaka kk

Z00Mo

Z°°Mo

X°°Mo

INRjj

0.97 0.97

0.94

0.36

0.21 0.21

0.90

0.94

0.19

Gas Detectors (Solid Sources)

LBL MHC UNM INEL"

0.078

Efficiency Isotopic for Ov fraction

Semiconductor-Wafer Stacks

Zaragozahh

ZSGe 0 + --,2 + Search

Collaboration IsotOpe

TABLE I. (Continued)

1.3 m

0.5 ~

background b(keV.kg.y) -~

0//

300 300

250P

500

390

2.4J 56 k

Resolution at endpoint A E (keV)

0.014 0.008

0.104

0.046

0.022

1.1

Total source mass M(kg)

7.5 6.4

40.5

67

34

Source thickness (mg/cm 2)

Building I m 2 prototype detector to study backgrounds in Modane Underground Laboratory. Aiming for a factor of 100 improvement on present limits for T~1~2.

TPC operating Hoover Dam. Se is elemental. Mo is as MoOs. See Fig. 9.

Operating in Kamiot--~ mine. Tz°/~ >1.2(0.72)×I02Zy, 68% (90%) c.l. T2/~2---~. - r I 1 R'!'0"34(0"71) ~,v . . . . . 0.22(0.33)J^lOZmy, 68% (90%) c.l.

T21~ -- -t~qv . Uq+2.0~xlnlS. 90% c.1. _l.0/ u #~

Operating in Baksan Neutrino Observatory. Tz°/~2 >7.1× 102°y, 68% c.l.

Operating in Consolidated silver mine. Mr=0.0204 kg-y Tz°/"2>4xl02Zy, 68% c.l. T°~2(0+--*2+)>4x102°y, 68% c.l. T~/"2 =5.5xl0ZSy was derived from a two-parameter fit,but a limit was reportedy. Increasing from 40 detectors to 148. Purifying 150 g Z°°Mo to
Operating in Canfranc t,m-el. Mr=0.188 kg-y T°~z(0+ --'2+)>1.4× 1022y, 68% e.1.

Current status and recent, results

¢.n

¢b

~=

¢b

O

t~

¢b

L_(Continucd)

l~°Xe

136Xe

136Xe

136Xe

136Xe

Milan~

Caltech Neuchatel Paul Scherrer Iustitutd'P

ITEP~q

I N R - I T E P ~"

Yale~s

0.02 r

0.37

0V

3.3

0.46

94

0.93

50

I0

280"

4.4

M(kg)

Total source mass

0.96 t

124

(keY)

Resolution at endpoint

5.9

13 ~

background ~keV-kg.y) -1

176q

0.64

Efficiency Isotopic for 0u fraction

Gas Detectors (Source m Detector)

Co]]aboration Isotope

TABLE ~ource (mg/cm9

thickness

3 liter, 58-atmosphere ionization chamber. Working on improved resolution and larger detector.

3.14 liter, 25-atmosphere ionization chamber operating in Baksan Laboratory. T°l~ >3.3×I021y, 68% c.l. T~/~2 >8.4×1019y, 68% c.l. Recently reduced background by a factor of 20.

13 m 3 atmospheric pressure TPC built and ready for testing. 0.32 m 3 prototype tested: T~/u2 >2×1019y, 68% c.L from 130 hours in prototype.

5-atmosphere TPC, 208 1 fiducial volume. Operating in St. Gotthard Laboratory with depleted Xe for background studies.

T2/~ >1 x 102°y,Analysis continuing.

1O y, 90% c.L

9.5-atmosphere multi-ceil proportional chamber operating in Gran Sasso Laboratory. 'I~1~2>1.5×102~y, 90~ c.L

Current status and recent results

¢D t~ ¢b

O

¢B

Efficiency Isotopic for 0v fraction

136Xe

l°°Mo n6Cd 0.83

0v background b(keV.kg-y)-'

120

.53

Resolution Total at endpoint source mass A E (keV) M(kg)

l°°Mo neCd

l°°Mo

INR-ITEP ~"

ITEP PNL-USC UM~

0.007

0.013

0.98 0.83 174

566

2.1" 2.6

1.0

0.31 0.50

Search for G a m m a Rays Following Double B e t a Decay to Excited States

ITEP uu

Liquid Detectors (Source ~ Detector)

Columbia Waseda"

Liquid Detectors (Source -- Detector)

Collaboration Isotope

T A B L E I. (Continued)

50

Opeiating in Sudan mine with. germanium detector. T°~+2~(0+ -*2 +)> 2 × 102°y. Expect 2×1021y by December 1990.

Window used is 2 × FWHM.

+ --, 2+)U Cd> 1.7 x 10 °y, 68% c.L

T°~'+2v(0+-.2+)10°Mo>1.4 ×102°y, 68% c.1.

Isotope sample on top of a shielded germani-m detector within NaI and plastic veto at sea level.

Liquid argon multi-section ionization chamber to be located in Baksan Laboratory. Starts 1991. Anticipated sensitivity: T~1~2>3× 10~y, T2/~ - ( 3 to 9)×102Sy, for isotopes of Qpp >2.6 MeV. Will also use S2Se, 13°Te, l~Xe, and 1raNd.

3.5 liter liquid Xe prototype TPC proposal to Japanese Ministry of Science & Education. Progressing with investigations of purification, energy resolution, ionization and scintillation yields in liquid Xe.

Source thickness (mg/cm') Current status and recent results

¢b

¢b

O e-

¢b

Isotope

for Or.

~dency.

0.87

116Cd

4sCa

0.87

50

l~Te

l°°Mo

13 •

214 ~

177

218

A E (keV)

~Ge

10.1

~(keV-kg.y) -l

Resolution at endpoint

0.0021

0.011

0.043 ~

0.341

0.116

M(kg)

Total source mass

1990.

Working on a superconducting detector and a dilution refrigerator for Oroville Dam.

Ge thermistor glued to a 2 cm 3 Ge crystal at 35 inK. Ge thermistor glued to a 0.35 cm 3 Te crystal at 44 mK. Dilution refrigerator specially constructed of low-radioactivity materials operating in (]ran Sasso.

CaF2 scintillation crystal with 30 g of enriched 4SCa in preparation. CaF2 to be purified to 10-gg/g. Measurements begin

T~/~ >4.5×10my, 99% c.L

>(3- 5)×10 y, 6s% c.l.

cm 3 CdW04 scintillation crystal operating in Solotvina underground laboratory. Mt -- 0.034 kg.y 56 cm 3 CdWO4 crystal M ~ = 0.09 kg.y 19

(mg/cm 2) Current status and recent results

Source thickness

a) Average of declining background over the entire run. b) Achieved by the collaboration in natural Ge. The r~Ga x-ray intensity implies that this is mostly r~Ge, and that only 0.1(kev.kg-y) -z will remain in enriched 76Ge crystals which are depleted in 7°Ge and aSGe. c) Anticipated detector-grade crystal yield from 5 kg of enriched ~6Ge. Will require some alteration of normal crystal-growing procedure. d) Estimated by the author, based on published value of 4.0 keV at 1.764 MeV. e) Approximate detector-grade crystal yield anticipated from 16.9 kg of enriched T6Ge. f) Measured for first enriched detector.

Oxfor& ~

UCSB-LBL

Milan =

f~

Isotopic Or. fraction background

u6Cd

T h e r m a l Detectors

(Moscow)

IA~MIFI~

INR (Kiev)~

Scintillators (Source = Detector)

Collaboration

TABLE_ I.

e~

O

O0

g) Measured in pilot experiment at Gran Sasso. h) The eSGa x-ray intensity indicates that eSGe accounts for all but 0.1(keV-kg.y) -1 i) At 1480 keV with g~.mma ray in coincidence. j) At 1480 keV 0+ --+2+ endpoint. k) In Nal at 560 keV gamma-ray energy. 1) Calculated by the author based on 69.9 mole-days in 8317 hours and fA = 0.94, ref. 49 (PRL). m) Calculated by the author from 8 events in 300 keV in 8317 hours, ref 49 (PRL). n) Assllmlng 100% detection efficiency, and 0.76% of signal in FWHM window. p) Plastic scintillator only. Ref. 62. q) Calculated by the author based on 11% resolution for I MeV electrons. Ref. 63. r) 0.02 in prototype for two-neutrino double beta decay, ITEP preprint of ref. 54. s) Calculated by the author from 180 keV FWHM for 1 MeV electrons in prototype, ITEP preprint of ref. 54. t) In prototype. u) For 1332 keV g~mma line of e°Co. v) Anticipated. w) Estimated by the author, ass-ming a high enrichment factor. x) At 1.33 MeV. y) Ref. 64. z) Calculated by the author from numbers in ref. 52. aa) ref 44. bb) International Gallium EXperiment: Pacific Northwest Laboratories, South Carolina, Zaragoza, Minnesota, Institute of Nuclear Research (Moscow), ref. 45. cc) Institute of Theoretical and Experimental Physics (Moscow), Yerevan Physical Institute, ref 19. dd) Max-Planck Institute fiir Kemphysik (Heidelberg), Kurchatov institute of Atomic Energy (Moscow), ref. 46. ee) Pacific Northwest Laboratories, University of South Carolina, Post-conference two-neutrino result added in proof. Ref. 47. if) Pacific Northwest Laboratories, University of South Carolina, Institute of Theoretical and Experimantal Physics (Moscow), ref. 48. gg) University of California Santa Barbara, Lawrence Berkeley Laboratory, ref. 26. hh) Ref. 16. ii) Lawrence Berkeley Laboratory, Mr. Holyoke College, University of New Mexico, Idaho National Engineering Laboratory, ref. 49. jj) Institute for Nuclear Research (Moscow), ref. 20. kk) Ref. 21. U) University of California Irvine, Los Alamos National Laboratory, refs. 18 and 50. ram) Ref. 51. nn) Ref. 52. pp) Ref. 53. qq) Institute of Theoretical and Experimental Physics (Moscow), ref. 54. rr) Institute for Nuclear Research (Moscow), Institute of Theoretical and Experimental Physics (Moscow), ref. 55. ss) Ref. 56.

TABLE I. (Continued)

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M,K, Moe/Double beta d~=y 5.5 Gas detectors (solid sources) These devices share some similarities with the semiconductor-wafer stacks, but additionally incorporate tracking of the two beta particles in gas. T~acking, supplemented by a magnetic field or time-of-flight, is a powerful background suppressant, and can give the opening angles of the electrons and their energy distributions singly as well as in sum. The signature of twoneutrino double beta decay under these circumstances is well defined. On the negative side, the tracking requirement leads ~o reduced efficiency, the need for thin sources, and b-|kier, more complex detectors ~or monitoring a given mass of isotope. Data from three track'

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ENERGY (MeV)

FIGURE 7 (a) The two-electron sum-energy spectra of enriched l°°Mo (black stars) and natural molybdenum (open boxes) measured by the INR group. 2° (b) The difference of the two spectra above (open stars) and a Monte Carlo simulation of two-neutrino double beta decay (black stars). A 250 keV threshold has been imposed on the single electrons of the two-electron events. These and later measurements yield T ~ 2 -----(3.3+~:°)x10's years, (90% c.1.).

0

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SUM ENERGY(MeV) FIGURE 8 The two-electron sum-energy spectrum of enriched l°°Mo (top), natural molybden-m (middle), and the difference of the two spectra (bottom) as measured by the Osaka group. 2: (The difference spectrum represents somewhat more data than is represented in the individual spectra.) The bottom section includes a histogram of the two-neutrino Monte Carlo spectrmn. A 200 keV threshold has been imposed on the single electrons of the two-electron events. The calculated half life is Tt=~2 -(1.16_o.33)×10 +o,71 z. years, (90% c.1.).

172

M.K. Moe/Double bets decay

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easily and inexpensively enriched by ultracentrifugation, and large-mass experiments are possible.

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FIGURE 9 The two-electron sum-energy spectrum, enriched 16°Mo measured by the UC Irvine - LANL group (histogram). Nothing has been subtracted. The detector is the same TPC used earlier 5° to determine the half life of S2Se, more than an order of magnitude longer than implied here for l°°Mo. The TPC has been moved underground, and background is lower than for the selenium measurement. A small residue of 214Pb activity from radon contributes below 800 keV. The solid curves and shading are Monte Carlo simulations of the twoneutrino spectrum normalized to the Osal~ and INR half lives and their 90% confidence limits for l°°Mo. A 200 keV threshold has been imposed on the single electrons of the two-electron events. 5.6 Gas detectors (source - detector) 136Xe as a noble gas can, like germanium, serve as both source and detector. It can operate as a scintillator or an ionization chamber, or in combination. Alternatively it can work in the proportional mode. The energy resolution in xenon is more than an order of magnitude worse than in germanium, but the Qvalue is higher (2.48 MeV), and spatial resolution can give additional leverage against background. Electron trajectories in xenon suffer badly from scattering, but at least one experiment is having some success distinguishing pairs from single tracks by the increased ionization density near the end of the electron range. Perhaps the greatest ad~,~ntage of xenon i~ that it is

5.8 Liquid detectors (source ~ detector) Liquid argon ionization chambers can be used as a detector for double beta decay sources placed nearby. This scheme has the advantages and disadvantages of separate source and detector discussed in section 5.4. 5.9 Search for gamma rays following double beta decay to excited states The daughter nuclei produced by double beta decay of isotopes with favorable Q-values typically have a first excited level with jr __ 2+, on the order of 500 keV above the ground state. Transition probabilities to these and other excited states are expected to be significantly lower than to the ground state for the two-neutrino mode, and 2+ transitions for the z~roneutrino mode require the existence of right-handed currents. But the gamma rays emitted as the excited levels relax to the ground state present an opportunity for detection of double beta decay of any so~rce placed ~n proximity to a high-resolu~,ion gamma detector such as germanium. (This technique should not be confused with germanium source-equals-detector experiments that try to detect the beta electrons in germanium in coincidence with de-excitation gamma rays in a surrounding medium such as NaI.) 5.10 Scintillators (source ffi detector) Some double bets isotopes can be grown into scintillating crystals which operate as calorimeters, much as germanium diodes do. The energy resolution is nowhere near as good as in germanium, and the necessary photomultiplier tubes are a notorious source of radioactive background. The phototubes, however, can be bacl~d away with light pipes, and the imtopu can have h | ~ e r Qovalues thaa germanium for f u t ~

M.K. Moe / lJouble beta decay

rates and operation in a lower background region of the spectrum. Examples are 4Sea (Q = 4.3 MeV) in CaF2 crystals and **6Cd (Q = 2.8 MeV) in crystals of CdWO4. 5.11 Thermal detectors Sometimes called cryogenic detectors because they operate at low temperatures, these devices respond to the ~o~al energy/deposited m non.ionizing as well as ionizing events are detectable. This effect leads to calorimeters that, in principle, are capable of very high energy resolution. Examples are tun,!eling junctions, superheated superconducting granules that flip to "normal" when struck by a particle, and bolometers. Bolometers generally take the form of a pure single crystal with a sensor in thermal contact. For a dielectric crystal the Debye relation C(T) c~ T 8

between specific heat C(T) and temperature T leads to very small specific heats at milliKelvin temperatures. Since the temperature rise AT of the crystal is AT

=

E

C(T)

the e~ect can be large enough to detect for energy deposits E in the MeV range. (The quantity e(T) is the heat conversion efficiency for the particle, which to first approximation can be taken as unity.) Small bolometers have already been successfully built and tested. If problems of operating the sensors (thermistors) at a few milliKelvin can be overcome, these devices will be excellent high-resolution detectors for use with suitable high Q-value isotopes. 6. CONCLUSIONS Many experiments using a wide variety of techniques are in operation, or being Msernbled to search for double beta decay in laboratories world wide. Half' live8 for the twomentrtno mode from direct counting

173

of 76Ge, S2Se, and *°°Mo have been reported, although the dubious practice of subtracting dummy sources makes some of these results questionable. In the case of *°°Mo, dummy-so~ce subtraction has yielded inconsistent half lives from two separate experiments. Limits from searches in 78Ge have ruled out zero-neutrino half lives in this isotope of less than 2.4×1024 years, and effective Majorana neutrino masses larger than 1 eV. Measurements of the two-neutrino mode are still in the realm of small experiments, as are prototype developments for new techniques applicable to zeroneutrino detection. For serious assaults on the zeroneutrino mode with established technology such as the germanium calorimeter, however, we are seeing the formation of international conaboratious to attract the necessary resources to mount increasingly expensive experiments. Faced with a fourth-root dependence of neutrino mass sensitivity on background and detector mass we have a choice between brute force (bigger mass) and finesse (lower background). Limited resources win phsh us in the direction of finesse, keeping background close to zero as we struggle for funding to push up the mass. Herein lies the challenge, and the hope of one day seeing that elusive spike. ACKNOWLEDGEMENTS I express my warm thanks to the spokesmen for the experiments discussed, for sending me up-to-date information on their progress and results. I also gratefully acknowledge the contribution of my colleagues, Steve Elliott, Matt Nelson, and Mike Vient in producing the *°°Mo spectrum from the Irvine TPC. REFERENCES 1. W.H. Furry, Phys. Rev. 56, 1184 (1939). 2. H. PrimakoiY and S.P. Rosen, Rep. Prog. Phys. 22, 121 (1959). 8. V.R. Lazarenko, Usp. Fiz. Nauk. 90, 601 (1966). 4. E. Fiorini, Revista del Nuovo Cimento 2,1 (1972).

174

M.K. Moe / Double beta decay

5. D. Bryman and C. Picciotto, Rev. Mod Phys.

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Rev. Nud.

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so. 1'. v , ~ ~

w~, ~d ~ ~ ~ e ~

Rev. C.~¥, 731 (19~),, P. Yogi and MR. Zimbauer, P]~n~ ~mv. ~ sT, m4s Oos~).

T. ~

and A ~emler, ~ Phys. ~

41. W . C . ~ C~ ~ a = ~ Phys. Z~,. C ~ , ~ ( ~ ) .

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,

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31. T. ~

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(zuo).

44. P. I ~ e r . ~ ~ ~ ~ D. R ~ , d. ~ , ~

~ ~ C ~ ~ ~ ~

Left. B19~,

B194,11 (1987).

~v. ~ , z. P ~ ~ ss4, ~ 0989~, K. M ~ and H.V. Klapd~, P k ~ L ~ . B~01, 420 (1988).

FT. X v ~ = ~ ~

~

( ~

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(1990). See

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(~).

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p

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Lett. BBBS, 273

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