VOLUME 37, NUMBER 5 P H Y S I C A L R E V I E W L E T T E R S 2 AUGUST 1976

observa t ion that would be expected for a s ta te pos ses s ing the proposed new quantum number charm.9*1 0

fWork supported by the U. S. Energy Research and Development Administration.

* Miller Institute for Basic Research in Science, Berkeley, Calif. (1975-1976).

^Permanent address% Centre d'Etudes Nucleaires de Saclay, Saclay, Francec

§ Fellow of Deutsche Forschungsgemeinschaft. || Permanent address° Varian Associates, Palo Alto,

California. **Permanent address: Laboratoire de Physique Nu-

cleaire et Haute Energie, Universite Paris VI, Par i s , France.

t tPermanent address: Laboratori Nazionali, Fra-scatti, Rome, Italy.

JlPermanent address; histitut de Physique Nucleaire, Orsay, France.

i j . -E , Augustin et al0, Phys. Rev. Lett. 34, 233 (1975).

2 J . -E. Augustin^ al., Phys. Rev. Lett. 34, 764 (1975).

The p e r s i s t i n g d i spar i ty between theore t ica l expectat ions of the so l a r -neu t r ino flux and the observat ional r e s u l t s of the Davis, H a r m e r , and Hoffman exper iment 1 based on the inve r se j3 d e ­cay of 37Cl-~ 37Ar has intensified a r e - e x a m i n a ­tion of quest ions on the cu r ren t models of the s o ­l a r in t e r io r as well as those concerning bas ic physical theory . 2 The predic t ions of low-energy ( -0 .4 MeV) so l a r -neu t r i no flux f rom the p ro ton-proton reac t ion is cons idered to be independent of a s t ronomica l uncer ta in t i es and r e q u i r e s only the pr inciple that nuc lear fusion is the bas ic en­e rgy-produc ing mechan i sm in the sun and that neut r inos a r e s tab le . 3 Since the 37C1 exper iment i s dominantly sens i t ive only to h igh-energy (5 -10 MeV) neut r inos , p red ic t ions of which a r e m o r e cr i t ica l ly dependent on models of the so la r i n t e r -

3The only other feature in the Kir system that we ob­serve in this data sample is the If * (890).

4A. M. Boyarski et al., Phys. Rev. Lett. 351, 196 (1975).

5This cut rejects most nucleons (p andf) as well as tracks accompanied by extra particles in the TOF count­er.

6G. S. Abrams et al., Phys. Rev. Le t t 34, 1181 (1975). 7A. M. Boyarski et al., Phys. Rev. Lett. 34, 1357

(1975). 8J. Siegrist et al., Phys, Rev. Lett. 3<6, 700 (1976). 9S„ L0 Glashow, J„ Iliopoulos, and L. Maiani, Phys.

Rev. D 2, 1285 (1970); S0 L„ Glashow, in Experimental Meson Spectroscopy—1974, AIP Conference Proceed­ings No. 21, edited by D. A. Garelick (American Insti­tute of Physics, New York, 1974), p. 387.

10Other indications of possible charmed-particle pro­duction have come from experiments involving neutri­no interactions. See, for example, A. Benvenuti et al0, Phys. Rev. Lett. 34, 419 (1975); E. G. Cazzoli et al., Phys. Rev. Lett, 34, 1125 (1975); J. Bleitschau et al., Phys. Lett. 60B, 207 (1976); J0 von Krogh et al., Phys. Rev. Lett. 36, 710 (1976); B0 C. Barish et al., Phys. Rev. Lett. 369 939 (1976).

ior , the necess i ty of an exper iment which c lea r ly es tab l i shes the validity of the phys ica l b a s i s , namely, one which is sens i t ive most ly to the low-energy pp neut r inos , i s becoming increas ingly u rgen t . Although many inverse-jS-decay candi ­da tes have been considered from this point of view over the y e a r s , at p r e s e n t only r a d i o c h e m i ­cal exper iments on the c a s e s of 71Ga-~ 71Ge or 7Li— 7Be a r e judged to be even hopeful.2 The p u r ­pose of th is Le t t e r i s to p ropose a new candidate: the inverse /3 decay of 115In to the excited s ta te at 614 keV in 115Sn (15Sn*) having r 1 / 2 = 3 . 2 6 jixsec and de-exci t ing by the emiss ion of two y r a y s of energ ies 116 and 498 keV. The p r e s e n t ca lcu la ­t ions show that th is case i s sens i t ive to the low-energy pp neut r inos and has a very l a rge capture r a t e , <£>cx~750 so la r neutr ino uni ts (SNU's) [ l SNU

Inverse $ Decay of 115In -> 115Sn*: A New Possibility for Detecting Solar Neutrinos from the Proton-Proton Reaction

R. S. Raghavan Bell Laboratories, Murray Hill, New Jersey 07974

(Received 8 April 1976)

The basis for a low-threshold, high-efficiency, direct-counting detector for solar neu­trinos from the p-p fusion reaction is proposed. The inverse (3 decay of 115In to the 614-keV excited state of 115Sn (Ti/2 = S.26 /usee) provides a unique delayed-coincidence signa­ture and is estimated to yield a solar-neutrino capture rate of ~ 750 solar neutrino units, ~85 of which is due topp +pep neutrinos.

259

VOLUME 37, NUMBER 5 P H Y S I C A L R E V I E W L E T T E R S 2 AUGUST 1976

Q=128(10)-keV-

7/2 t|/2=3.26/i.s<

t1 / 2=5.1x10 l 4y 9 / 2 * ) ? 3/2* _Ji e/y = 0.8

49 x "66

(95.7%)

, 1/2+

e/y = 6 x 1 0

AM=486(9)keV H 5 Q n

5 0 i r i 6 5

FIG. 1. Nuclear data for 115In and 115Sn (Ref. 4).

= 10"36 captures/(target nucleus) • sec]. It also has a delayed-coincidence signature for a neutrino event due to the microseconds lifetime of 115Sn*. Taking into account the fact that In is almost mo-nisotopic in mass 115, the amount of material needed for an experiment with an event rate of 1/day is only about 3 tons. This is the smallest weight of any material with natural isotopic abun­dance which has yet been proposed to yield this given solar-neutrino capture rate.

The relevant nuclear spectroscopic data4 avail­able to date on 115In and 115Sn are shown in Fig. 1. The isomeric state at 614 keV in 115Sn was dis­covered by Ivanov, Iordachescu, and Pascovici.5

It has been observed in the decay6 of 115Sb and in many charged-particle reaction studies7"9 which have confirmed the spin of \ for this level. It is clearly part of the systematic appearance of a low-lying •7

r+ state in odd Sn isotopes [see Fig.

2(b)] and could be identified as the g7/2(v) shell-model state. Allowed Gamow-Teller /3 transitions have also been systematically observed between these Sn |-+ levels and the corresponding ground states of odd In nuclides [all with P = |-+; £9/2" H77) shell-model state]. The only unobserved case (because of competition from y decay) is 115Sn •* 115In. The ft value of the inverse of this t ransi­tion, needed to compute neutrino-capture rates, is estimated from systematics and theory as fol­lows.

The very extensive experimental and theoreti­cal data on Sn isotopes,10 which form a simple system because of the closed proton shell at Z = 50, provide a well-understood basis for pre­dicting relative ft values of these g9/2 proton •~g7/2 neutron j8 transitions. The systematics of ft values [see Fig. 2(a)] have been theoretically examined by several authors11 who obtained ex­cellent agreement for the dependence of these ft values on mass number with the predictions of the pairing-correlations theory.12,13 According to theory,11"13 the relative variation is caused by

1— •

— — - " " • C k > > , /

1 1 1 1 1 113 115 117 119 121

A - 9 2 5 ^ -8 4 0 ^ '

7 3 0 /

/ _ / / / /

(a)

, DERIVED

' F R O M dp.dt

REACTIONS

EXPT. - THEORY I

THEORY n

(b)

FIG. 2. (a) Systematics of log ft values of /3 transi­tions of odd In(jr+) and Sn(|-+) levels compared with rel­ative log ft values (normalized to A =117 based on Vn

2(N) factors derived from stripping reactions (Ref. 7) and theory (I, Ref. 12; II, Ref. 13). (b) Systematics of the energy of the jf-+ states in odd Sn nuclei.

the change in the g7/2 neutron-pair occupation probability12 Vn

iN) with the neutron number. Thus ftcc (21 f + l)/Vn

2(N), where I{ is the spin of the ini­tial state of the /3 transition. Values of Vn(N) ob­tained from theory12'13 or from experiments on stripping reactions in Sn isotopes7 show general­ly that for A^ 115, Vn{N) changes only a few per­cent, although for A< 115 it drops more sharply. This tendency, presumably governed by the ap­proximate filling of the g7/2 shell at A = 114, is also shown nicely by the sharp break between A = 113 and 115 in the systematics of the energy of the 1 + state [Fig. 2(b)]. Thus the I + levels of odd Sn isotopes with mass 115 and above should have very similar properties; indeed, the ex­perimental logft values for A = 117, 119, 121 are all reported to be the same: log/2 = 4.4.14 Figure 2(a) shows the /3-decay data compared with theo­retical predictions as well as estimates (relative to mass 117) based on Vn(N) values derived from stripping reactions.7 These data show that, to a good approximation (-10%), one can write15

(^)invB<115M# + ) lnVerSe6; i l5Sn(-7f + ))

= ( /O 0 - ( 1 1 7 In( | + ) l" 1 1 7 Sna + ) ) . (1)

The most recent value for (ff)6-(117In£"117Sn) is 2.5(2) xlO4 sec.14 We shall thus adopt this value for ft to be used below.

The cross section for neutrino capture is given

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V O L U M E 37, N U M B E R 5 P H Y S I C A L R E V I E W L E T T E R S 2 A U G U S T 1976

by1

CT = -2ir

• ( — ) '

\m0cj

ln2

(/OiE • W e

2 G ( Z , W e , £ e (2)

where We is the total (including rest-mass) ener­gy of the electron following neutrino capture: We

=EV-Q + 1, where Ev is the energy of the solar neutrino and Q the threshold energy, all in units of m0c

2. G(Z,We,pe) is the modified Fermi func­tion17 which depends on the charge number of the final nucleus [Z(Sn) = 50] as well as the energy and momentum pe (in units of m0c) of the emitted electron. Figure 1 shows that Q = 128(10) keV. For the monoenergetic solar neutrinos [E v(pep) = 1.44 MeV; Ev(

7Be) = 0.861 MeV], the cross sec­tions of Table I were evaluated using (2) and (/-Oinve obtained above. For pp neutrinos with continuous energies [Ev

m*x(pp) = 0A2 MeV], Eq. (2) was averaged over the neutrino spectrum. The cross sections thus obtained were used to compute the capture rates cpo using the neutrino fluxes tabulated by Bahcall et al.18 for different models of solar opacities. These results are summarized in Table I. For comparison the ta­ble also shows the rates for 7 1Ga- 71Ge tabulated most recently.19 The total event rates for 115In, being some 3.5 times larger than for 71Ga, are in fact significantly superior to all the other neu­trino targets19 proposed in the last thirty years.

The nuclear data of Fig. 1 dictate that the ex­perimental method applicable is one of direct counting with the attendant advantages20 of dis­crimination of solar-neutrino types, possibility of observing slow time variations, etc. To as ­sist in suppressing the background, the present

approach utilizes a characteristic neutrino sig­nature defined by a pair of pulses (due to the electron emitted after neutrino capture and the radiations de-exciting the 614-keV level) with a delay time of up to about 10 jusec. It is thus sim­ilar in principle to the classic reactor antineu-trino experiment of Reines et al.21 The back­ground might nevertheless be the most serious problem to overcome since low-energy radia­tions must be detected. A possible solution is in­dicated by the fact that in addition to constraints on energies, pulse shapes, and delay time, it could also be demanded that the two pulses origi­nate in the same location of the neutrino detec­tor. One might thus consider an aggregate of relatively small and separate volumes of In-con­taining detectors, e.g., a liquid scintillator, each with its own photomultiplier recording the pulses in that volume. The totality of pulses observed by all the phototubes could be analyzed to sepa­rate neutrino events producing a pair of pulses with the proper characteristics of (a) type of ra ­diation, (b) energy, (c) time delay, and (d) loca­tion in the "pile." The pile idea has the further important advantage that it is inherently an elabo­rate anticoincidence arrangement leading to con­siderable background rejection since events sat­isfying (a) to (d) by Compton scattering of a ran­dom y ray could be vetoed by demanding, in ad­dition, anticoincidence with the nearest neighbors in the close-packed pile. Thus the geometry of the pile and the size of the subunit are important. Theoretically, for a given total amount of In, the random coincidence rate in the pile as a whole, in a double-coincidence scheme, decreases in-

TABLE I. E s t i m a t e s of neutr ino cap ture r a t e s for 115In—- 115Sn*, The contr ibut ions for 8 B , 13N, and 1 50 neu t r i nos , not l i s ted h e r e , const i tu te a negligible f ract ion of the tota l r a t e s l i s ted above0

Solar neut r ino

PP, j 5 y

m a x = 0 . 4 2 MeV

pep, £ v = 1.44 MeV

7 Be, Ev =0 .861 MeV

Total

cr (10~4 5cm2)

11

60

28

115m Capture r a t e (SNU)

Using <pt a

693

10

43 746

Using <p2

671

9

105 785

(10' ac

"45 cm2)

3

35

15

71Ga Capture r a t e (SNU)

Using cp j a

189

6

21 216

Using cp 2

183

5

51 239

a Neut r ino flux (in uni ts of cm" 2 s ec" 1 ) , new s tandard ( l o w Z ) ; Ref. 18: (pi(PP) =6.3X 1010; <p1(pep) = lm6x 108; (Pi(7Be) = 1 . 4 x i o 9 .

^Neut r ino flux (in uni ts of cm" 2 s ec" 1 ) , new s tandard ; Ref. 18: <p2(PP) = 6 . 1 * 1010; <p2(pep)=1.5* 108; cp^Be) =3„4 x i O 9 .

cRef. 19.

261

V O L U M E 37, N U M B E R 5 PHYSICAL REVIEW LETTERS 2 A U G U S T 1976

versely as the number of subunits in the pile. A signal rate of 1/day indicates the need for a large number of subunits. It may be possible to achieve considerably better background rejection with ev-r!-y2 triple-coincidence arrangements (see Fig. 1). Consider a liquid scintillator loaded with 1 kg of In and surrounded by an independent detector. Let the inner detector record ev (100-300 keV) and y1 (116 keV) and be thick enough to stop elec­trons and y rays < ~ 120 keV from reaching the outer mantle. y2 (500 keV) escapes and is r e ­corded in the outer detector. The random coin­cidence rate in a pile made of 3000 such units is R « Z0Q0N1N2N371r2 x 105/day. With T1 ~ 10"5 sec, T 2 ~ 1 0 " 8 sec, A^-iN^-lOO/sec due mostly to the radioactivity of 1 kG of In in the inner detector, we find N3 ^ 3/sec for R = l. This upper limit on the background rate N3 in the outer detector is far from severe. The triple-coincidence signa­ture thus appears to offer reasonable hope for the feasibility of the experiment.

In conclusion, a new high-efficiency detector scheme based on the inverse j8 decay of 115In - 115Sn* is proposed for the observation of low-energy p-p neutrinos from the sun. A prelimi­nary analysis of its properties indicate that it could possibly achieve a breakthrough in the so­lar-neutrino problem if it could be shown that correlated and uncorrelated background effects in a practical detector subunit could in fact be suppressed to tolerable levels. Experiments are being initiated in this laboratory towards the de­velopment of a suitable In scintillator.

I wish to thank A. P. Mills, J r . , L. N. Pfeiffer, and L. J. Lanzerotti for stimulating my interest in solar neutrinos in general.

*See R. Davis , J r . , D. S0 H a r m e r , and K0 C. Hoffman, Phys„ Rev. Let t . £ 0 , 1205 (1968), for a descr ip t ion of th is exper iment .

2 J . N. Bahcall and R.Davis , J r . , Science 191, 264 (1976),

3 B. Pon tecorvo , Zh. Eksp . T e o r . F i z . 53 , 1717 (1967)

[Sov. P h y s . J E T P 26, 984 (1968)]; J . N. Bahcal l , N. Ca -bibbo, and A, Yahil , P h y s . Rev. Let t . 28 , 316 (1972).

4S. Raman and H. J . Kim, Nucl. Data Sheets JL6, 195 (1975).

5 E. A. Ivanov, A. Iordachescu , and G. P a s c o v i c i , Rev. Roum0 P h y s . 13, 879 (1968).

e O. Rahmouni , Nuovo Cimento B 57, 389 (1968). 7 E, J . Schneid, A. P r a k a s h , and B . L. Cohen, P h y s .

Rev. 156^, 1316 (1967). 8 P 0 E . Cavanagh, C. F0 Coleman, A. G. H a r d a c r e ,

G. A. G a r d s , and J . F . T u r n e r , Nucl. P h y s . A141, 97 (1970).

9 D. G. F leming , M. Blann, and H. W. Fulbr ight , Nucl. P h y s . A163, 401 (1971).

10See E, U. B a r a n g e r , Adv. P h y s . 5, 261 (1971), for a rev iew of the nuc lea r s t r u c t u r e of In, Sn, and Sb nucle i .

U L . S i lverberg and A. Winther , P h y s . Le t t . 3 , 158 (1963); J . I. Fuj i ta , Y. Fu t ami , and K. Ikeda, P r o g . Theo r . P h y s . 3 8 , 107 (1967).

1 2L. S, Kiss l inger and R. A. Sorensen , K. Dan. Vi -densk. Selsk. , M a t . - F y s . Medd. 32 , No. 9 (1960).

1 3L. S, Kiss l inger and R. A. Sorensen , Rev. Mod. P h y s . 35 , 853 (1963).

1 4For.A = 113, M. Schmorak, G. T . E m e r y , and G. Scharff-Goldhaber, P h y s . Rev. 124, 1186 (1961); for A = 117, M. J . Mar t in , p r iva te communicat ion, and to be publ ished; for A^ 119, B. Foge lberg , L. E . de Gee r , K. F r a n s s o n , and M. af Ugglas , Z. P h y s . A276, 381 (1976).

15Note that in the approximat ion of Eq. (1),

^ ) E c ( 1 1 5 S n ( r ) E X I n 1 1 5 ( | + ) )

^O^^^-C^ ln^)^ 1 1 ^^) )

a s a r e s u l t of the change in the in i t i a l - s t a t e spin. 1 6E. J . Konopinski, Ann. Rev. Nucl. Sci. 9, 99 (1959). 17Values of G(Z,We9pe) w e r e taken f rom the tab les of

M. E . Rose , in Beta and Gamma Spectroscopy, edited by K. Siegbahn (North-Holland, A m s t e r d a m , 1955), p . 875.

1 8J. N. Bahcal l , W. F . Huebner , N. H. Magee, J r . , A. L. M e r t s , and R. K. Ulr ich , Ap. J . 184, 1 (1973).

19See J . Evans , in P roceed ings of the Conference on Solar Neut r inos , I rv ine , Cal ifornia , 1972, edited by F . Reines and V. T r imb le (unpublished), pp . B 6 - B 1 3 , for extensive tabulat ions on poss ib le and feasible neu ­t r ino t a r g e t s with complete data on each c a s e .

2 0F o Re ines , P r o c . Roy. Soc. London, Ser . A 301 , 159 (1967).

2 1 F . Re ines , C. L. Cowan, J r . , F . B. H a r r i s o n , H. W. K r u s e , and A. D. McGuire , P h y s . Rev. .117, 159 (1960).

262