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Limits on low-energy neutrino fluxes with the Mont Blanc liquid scintillator detector
Astroparticle North-Holland
Astroparticle Physics
Physics 1 (1992) l-9
Limits on low-energy neutrino fluxes with the Mont Blanc liquid scintillator detector M. Aglietta a, P. Antonioli b, G. Badino b, G. Bologna b, C. Castagnoli a,b, A. Castellina ‘, V.L. Dadykin ‘, W. Fulgione a, P. Galeotti a,b, F.F. Khalchukov ‘, E.V. Korolkova ‘, P.V. Kortchaguin ‘, V.B. Kortchaguin ‘, V.A. Kudryavtsev ‘, A.S. Malguin ‘, L. Periale a, V.G. Ryassny ‘, O.G. Ryazhskaya ‘, 0. Saavedra a*b,G. Trinchero a, S. Vernetto a, V.F. Yakushev ’ and G.T. Zatsepin ’ oIstituto di Cosrnogeofuica de1 C.N.R., Corso Fiume 4, 10133 Toritw, Italy ’ Istituto di Fisica Generale dell’Unicersita’ di Torino, cia P. Giuria 1, 10125 Torino, Italy ’ Institute for Nuclear Research of the Russian Academy of Sciences, 60th Annicersary of October Recolution, Moscow VI-11 7312. Russian Federation Received
20 May 1992; in revised
form 25 June
Prospect
7a,
1992
The LSD liquid scintillation detector has been operating since 1985 as an underground neutrino observatory in the Mont Blanc Laboratory with the main objective of detecting antineutrino bursts from collapsing stars. In August 1988 the construction of an additional lead and borex paraffin shield considerably reduced the radioactive background and increased the sensitivity of the apparatus. In this way the search for steady fluxes of low-energy neutrinos of different flavours through their interactions with free protons and carbon nuclei of the scintillator was made possible. No evidence for a galactic collapse was observed during the whole period of measurement. The corresponding 90% cl. upper limit on the galactic collapses rate is 0.45 y-’ for a burst duration of AT 5 10 s. After analysing the last 3 years data, the following 90% c.l. upper limits on the steady neutrino and antineutrino fluxes were obtained: <9.0X
lo4 V, s-‘cm-2,
9
@(Ge) <8.2X
@(cc)
lo3 G, s-‘cm-*,
20 I E, I 50 MeV,
@(IJ,) <6.8X
lo3 v, s~tcm-*,
25 I E, I 50 MeV,
) < 3.0X 10’ vpfi
s1’cm12,
20 I E, I 100 MeV,
@( Yp+7) < 3.3X 10’ 5,+,
s-‘cm-*,
20 I E, 5 100 MeV.
@(u,+,
In particular comparing the obtained upper limit on the i7, flux for 9 I E, I 20 MeV to the solar ve flux predicted by the standard solar model in the same range of energy, we can exclude the possibility that more than 6.3% of the solar U~‘Sflux can change to 5,. Finally the first limits on the flux of relic supernova neutrinos of all flavours as a function of the neutrino sea temperature are presented.
1. The Mont Blanc neutrino telescope
The Mont Blanc, liquid scintillation detector (LSD) has been running since January 1985 in the Correspondence too: Prof. 0. Saavedra, Istituto di Cosmogeofisica de1 C.N.R., Corso Fiume 4, 10133 Torino, Italy. 0927-6505/92/$05.00
0 1992 - Elsevier
Science
Publishers
Mont Blanc Tunnel at a vertical depth of 5200 hg cm-*. The experimental characteristics of the apparatus are described elsewhere [ 1,2]. Briefly, the detector consists of 90 tons of liquid scintillator (Cr0Hz2) [3], contained in 72 stainless-steel tanks (1.0 X 1.0 X 1.5 m3) placed on three layers; each counter is watched by 3 photomultipliers
B.V. All rights reserved
2
M. Aglietta et al. / Limits on low-energy neutrino fluxes
(diameter 15 cm) operating in threefold coincidence. Since its conception [4], the LSD experiment has been dedicated to the detection of antineutrino bursts from the gravitational collapse of stars in our Galaxy, by the antineutrino capture on free protons (ije energy threshold E,, = 1.8 MeV):
counter); furthermore, a large fraction of them give signals in time coincidence with different counters. The absolute time of each event is recorded with an accuracy of 2 ms.
&),+p -
In the present work we define an $e candidate as an event recorded in a single scintillation counter with an energy release E < 60 MeV. Most of these events are due to local radioactivity; a small fraction of them is due to unidentified muons hitting the corner-edges of the detector and/or to gamma rays or to neutrons produced by muons in the surrounding material. The present rate of Ge candidates is 8.8 h-‘. We define an 5, burst candidate as a cluster of N such events recorded during a time interval At, which can be simulated by statistical fluctuations of the background less than once every 100 years (for example, with the present background rate a burst with N = 5 and At = 10 s is produced once every 890 years). Our search includes bursts with time durations At ranging from 1 ms to 600 s. A monitoring task running during the data acquisition time performs an on-line preliminary search for 5, bursts on the raw data, followed by a more accurate off-line analysis. The maximum distance of a detectable collapse is a function of the burst duration At; as the distance to the collapsing stars increased the number of interactions decreases, and only by lowering At can the signal to noise ratio remain unchanged. Figure 1 shows the efficiency of LSD to detect a “standard” collapse (i.e. one producing on average about 50 interactions in our detector from a distance of 10 kpc) as a function of distance, for different burst durations. Integrating over the distance distribution of the stars, we calculated the efficiency E to detect a galactic collapse. If At I 0.1 s, E = 0.999; if At = 10 s, E = 0.995; even in the extreme case of At = 600 s, the efficiency is still 0.95. The Galaxy was monitored from August 1988 to November 1991 with a total exposure of 244.8 ton year. During this period 217792 events were
n + e+ Ln+p
-
d+r
This interaction gives 2 signals in time coincidence: the prompt positron pulse with energy E, = E, - 0.8 MeV followed by a gamma pulse of energy E, = 2.2 MeV with an average delay of 190 us. The energy threshold of the detector is 5 MeV for the 16 inner counters and 7 MeV for the external ones. After each trigger the threshold is lowered to 0.8 MeV for a time of 500 ps to detect the gamma ray signal. The efficiency in detecting the gamma pulse is 0.5 and the probability to have such a delayed coincidence in a single counter by chance is less than 10-i. In August 1988 an additional shield, made of borex paraffin and lead, was added to the detector to decrease the local radioactivity background from the surrounding rock. The results of this improvement in background reduction at different energies and the corresponding increase in sensitivity have been discussed in a previous paper [5]. A careful and systematic study of the low-energy radioactivity background spectrum 161 was performed at the same time with two additional detectors: a NaI 5” x 5” crystal with energy threshold Et,, = 220 KeV and a radon-meter (alpha spectrometer). The results showed a strong correlation of low-energy pulses ( > 0.8 MeV) with Rn content, and indicate that a high sensitivity to low-energy particles of LSD can be achieved 161. In this new background condition of the experiment the total trigger rate is 14.8 events h-‘. The rate of cosmic muons crossing the apparatus is 6.0 h-l. They can be easily identified and rejected because of their high-energy release (160 MeV for a vertical throughgoing muon in one
2. Galactic supernova
monitoring
M. Aglietta et al. / Limits on low-energy neutrino fluxes
0
I
I
87
95
10
I 98
I 100
20
If the data recorded from August 1985 to 1988 are also taken into consideration (847.3 days of measurement), then, apart the burst of 5 pulses detected in real time on 23th February 1987 [7], and taking into account the slightly lower sensitivity of the detector, the upper limits on the rate of galactic collapses are 0.45 year- ’ (At I 10 s) and 0.47 year-’ (At I 600 s), at 90% confidence level. This supernova survey, based on the on-line and off-line burst analysis we have developed many years ago, has been recently used also by MACRO at Gran Sasso with a total of 45 tons of liquid scintillator.
% of stars
30
50
40 Distance
3
(kpc)
Fig. 1. Efficiency to detect an antineutrino burst from a collapsing star versus distance. On the upper scale the percentage of stars in our Galaxy with distances < 15, 20, 25, 30 kpc is reported.
recorded and no significant burst candidate was found. Figure 2 shows the distribution of clusters with a number of events N > 3, 4, 5,. . . ,12 versus At, compared with the expected number calculated, assuming a random behaviour of the background.
”
N I3 N 24
3. Steady neutrino fluxes In the study of steady neutrino fluxes of energy E < 50 MeV, the problem of background rejection is fundamental. Therefore, even neutrino interaction channels with lower cross-sections, but with a stronger signature, become interesting. In particular, the interactions with carbon nuclei of the liquid scintillator present a very good signature and furthermore, the neutral current gives one the possibility to study low energy u& and v~. In this section we present the results on the search for steady neutrino fluxes using different target nuclei; their interpretation in terms of past supernovae background will be discussed in section 4.
N 35
O3
N 16
3.1. Ve- proton interactions
N 27 N 2.3
02
N Z9
N 210
0
N 212
1 0
100
200
300
400
Burst
500
durotion
600 (SK)
Fig. 2. Distribution of clusters with a number of events N > 3, 4, s,..., 12 as a function of time duration, recorded during the period August 1988 - November 1991, and the expected curves from a Poissonian background.
The measurement of a steady +, flux is performed, as for Ge bursts, using the free proton inverse P-decay. This interaction presents three advantages: (a) higher cross-section [8] in comparison with other interactions in the same energy range, i.e. charged and neutral current on carbon nuclei and elastic scattering on electrons (see fig. 3.); (b) low-energy threshold: E,, = 1.8 MeV; (c) good signature due to the detection of the gamma emitted after neutron capture. In this section we present our results on the search for 5, in two ranges of energy: (a) 9 I E,
4
M. Aglietta et al. / Limits on low-energy neutrino fluxes
lid
0
’
’
’
1’
20
’
’
’
’
’
40
’
’
’
’
’ 60
’
’
j
u Em&
”
’
1
100
(Mev)
Fig. 3. Cross-sections of neutrino and antineutrino interactions with protons and carbon nuclei. Cross-sections (d) and (e) refer to the interactions Y~+~ +‘“C + r’C* + Y~,+,~ and v+T + 1% + %* + Ge,a,T respectively, where C* is the “C 15.11 MeV excited state.
I 50 MeV, where the energy lower limit is set by the detector energy threshold, (b) 20 I E, I 50 MeV, where the radioactivity background component is absent and the sensitivity of the detector is higher. An internal core of the apparatus (made by the 28 counters presenting at least 5 faces shielded by contiguous counters and placed on the second and third layers) was used in this analysis, corresponding to a total exposure of 93.9 ton year. The choice of this subset reduces the radioactivity background and eliminates most of the events due to muons hitting the edge-comers of the detector (these kind of events increase the highenergy tail of the background spectrum). The search for antineutrinos of energy 9 I E, I 50 MeV is done in two steps: (1) the events with deposited energy 7 I E, 5 55 MeV contained in one single counter are selected, (2) the number, N, of low-energy pulses occurring in the same counter within 500 l.~s after the trigger is compared with the number, Nb, of pulses expected from a uniform background. In the whole period 11128 events and 483 low-energy pulses were recorded. To be independent of possible electronic noise, we considered
only the delayed pulses occurring between 20 and 450 us after the trigger; this reduces the efficiency to detect the gamma pulse by a factor of 0.81. Figure 4 shows the integral energy spectrum of all events considered in this analysis (curve a) and the fraction of them which are followed by one or more delayed low-energy pulses (curve b); for comparison the corresponding distributions for the events recorded in the whole apparatus (72 tanks) are also given (curve c and d, respectively). Using the counting rates for each counter for pulses over the energy threshold (0.8 MeV), we obtained the value N, = 470.2 for the expected number of low-energy pulses (the counting rates for the 72 tanks are measured and recorded every 7 minutes to check the stability of the apparatus). Taking into account the total efficiency of detection the delayed gamma pulse (E = 0.50 x 0.81 = 0.411, the resulting number of antineutrino interaction candidates is 31.6 + 56.0. The same analysis was performed on events with energy deposited in the range 17
10
20
30
40 Energy
50 (MeV)
Fig. 4. Integral energy spectrum of the events with energy 7 < E < 55 MeV recorded during the whole period of measurement considered in this work: (a) events in the 28 tanks core, (b) events in the 28 tanks core followed by one or more low energy pulses, (c) events in the whole apparatus (72 tanks), (d) events in the whole apparatus followed by one or more low-energy pulses.
M. Aglietta et al. / Limits on low-energy neutrino fluxes
MeV, in order to search for antineutrinos with 20 I E, I 50 MeV. In this case we recorded 554 events: the number of delayed pulses, N, was 37 and the expected number, Nb, was 22.7. The resulting number of c, interaction candidates is 35.3 + 16.8. In both cases no significant excess of low-energy pulses was observed and the corresponding 90% cl. upper limits on the Ge flux with energy in the range 9 I E, I 50 MeV and with 20 I E, I 50 MeV are 9.0 X lo4 $e s-1cm-2 and 8.2 X lo3 Ge s-icm-*, respectively. These limits have been conservatively calculated using the values of the cross-section at 9 and 20 MeV, respectively, and taking into account the energy resolution of the detector [9]. 3.2. Neutrino interactions with carbon nuclei
-
‘*N+e-
L
‘*C+e+
Et,, = 17.3 MeV Ge +‘*c
-
(2)
‘*B + e+
Et,, = 14.4 MeV
L
background, we used all of the 72 counters, corresponding to a total exposure of 244.8 ton year. Taking into account the different trigger rate of each counter (depending on its position inside the apparatus), the expected number of pairs due to chance was calculated and compared with the number of pairs experimentally found. No significant excess was observed (1 and 4 candidates have been selected for (2) and (3), respectively while 1.6 and 3.0 were expected). The corresponding upper limits to the v, and ce fluxes in the energy range 25 I E, I 50 MeV (where the lower limit on the energy is set by the energy threshold of the detector) were calculated taking into account the efficiency to detect both pulses [13] and by using the values of the crosssections at 25 MeV: @(vJ < 6.8 x lo3 V~s-‘cm-*,
Neutrinos of different flavours are detectable through their interactions with ‘*C nuclei. The energy thresholds of these reactions are higher and the cross-sections (see fig. 3) are about one order of magnitude lower than the inverse beta decay (the calculated cross-sections used in this analysis [lo] have been recently confirmed by two experimental studies [11,121X The charge current interactions ve +12c
5
12C+ e(3)
give 2 signals: 1) the prompt electron (positron) pulse, 2) the positron (electron) pulse from the 12N (‘*B) decay, delayed by 15.9 (29.4) ms. We define a candidate for interactions (2) and (3) as a pair of events with E I 60 MeV occurring in the same counter within 48 and 90 ms, respectively. In this search, because of the very low
@( cek) < 1.2 X lo4 5, s-‘cm-*, at 90% confidence level. The neutral current interactions
Eth = 15.1 MeV
(4)
also allow us to study v~+~ and Gp+? fluxes. The deexcitation of the ‘*C* nucleus (occurring with a 90% branching ratio) gives a 15.1 MeV photon. A Monte Carlo simulation based on the EGS4 code was performed in order to evaluate the LSD detection efficiency of the emitted gamma ray in the energy range between 12 and 18 MeV, giving E = 35% [9]. The result of this search for an exposure of 93.9 ton year (we used a core of 28 counters) shows no evidence of a peak among the 335 events in the region between 12 and 18 MeV (fig. 5). Given the results previously obtained on the flux of G, and ver their contribution can be neglected, and the corresponding 90% c.1. upper limits to the up+7 and Gkf7 fluxes with energy > 20 MeV, conservatively calculated using the cross-section value at 20 MeV, are: @(VP,, ) < 3.0 X 10’ up+7 s-‘cmP2, @($+J
< 3.3 X 10’ GP+, s-‘cm-2.
6
M. Aglietta ei al. / Limits on low-energy neutrino fluxes
4. Neutrinos from past supernovae
Relic neutrinos from past supernovae (i.e. resulting from the sum of the single contributions of al1 gravitational collapses occurred at different times and distances during the cosmological evolution) are expected to have a spectrum given by a superposition of many Fermi-Dirac spectra, with an average temperature of a few MeV. This spectrum would be red-shifted by a factor of about 0.5 [143 compared to the spectrum of a recent supernova. Disagreement exists among current theoretical models about the order of magnitude of the neutrino flux. With a conservative approach, Woosley et al. 11.51predicted a cosmic Ge signal peaked at 6-10 MeV with a flux @= 1.0 CCs-‘cm -*. More optimistic estimations give larger values by several orders of magnitude [ 16-181. Following the method used by the Kamiokande group [19], we searched for a relic Ce flux in a definite energy window, characterized by a lower background rate and reasonable overlapping with the expected Se spectrum. We chose events in the energy range 12
Energy (MeV) Fig. 5. Energy distribution of the candidates for neutrinos neutral current interaction with carbon nuclei. An excess is expected in the shaded region.
0
4
8
12
16
20
24
Energy
28 (MeVV)
Fig. 6. Integral energy spectrum of the trigger events with E < 30 MeV recorded in the 8 tanks core during the whole period of measurement considered in this analysis: (a) all events, (b) events with the double signature of the CCC+p) interaction; only one event was detected in the range 12 < E < 30 MeV.
MeV. We limited this analysis to the data recorded in the internal core of the apparatus, i.e. in the 8 most shielded counters, corresponding to a total exposure of 22.5 ton year. We found only 1 event with the double signature of the Q inverse decay in the whole set of data. As in section 3.1, to avoid electronic noise we considered only pulses delayed between 20 and 450 trs after the trigger. Figure 6 (curve a) shows the integral energy spectrum of the trigger pulses with energy E < 30 MeV recorded in the 8 tanks core; the spectrum of the events followed by one or more low-energy pulses is shown in curve b. We can thus write
where @($,I is the antineutrino flux, a, E* is the cross-section for inverse beta decay, L is the total exposure, NP is the number of free protons in
M. Aglietta et al. / Limits on low-energy neutrino fluxes
tures for both these flavours. The temperature of the Fermi-Dirac spectrum of muon and tau neutrinos is expected to be higher than the electron neutrino one. Therefore, assuming that the electron neutrinos and antineutrino fluxes are negligible at higher energies, we can write
7 Lo
YE10 .!3 8
7
7
1
10
10
xdE,_
10
F
1c I3
0
2
4
6
8
10
T,,, (MeV) Fig. 7. Upper limit to the relic SN antineutrinos as a function of the temperature of the neutrino sea. The upper limit obtained by the Kamiokande experiment is shown by the dashed curve.
one ton of scintillator, 4Ed) is the efficiency to detect a signal of energy E,, lP = 0.50is the efficiency to detect the 2.2 MeV delayed gamma ray. F(E, , T’.,) is a normalized Fermi-Dirac spectrum with an effective equilibrium temperature Ten, G(Eve- 0.8-Ed) is a normalized Gaussian distribution with r.m.s = 0.25 JEd to take into account the energy resolution of the detector and N,, = 3.89 is the upper limit at 90% c.1. on the number of observed events Nabs = 1. The corresponding upper limit at a 90% c.1. on the flux @CC,) is shown in fig. 7 as a function of the temperature T,,of the antineutrino sea. The same figure also shows the upper limits obtained by the Kamiokande group (dashed line) with an exposure of 0.58 kton year and a higher energy window, 19 < E, < 35 MeV [193. It is interesting to remark how our detector, even with a total exposure 25 times lower than Kamiokande, can give significant results for low temperatures of the neutrino sea due to its high sensitivity at low energies. The results obtained in section 3.2 will now be used in order to estimate the flux of relic u&T and t,7, . In this case, according to the standard model of collapse, we assume equal fluxes and tempera-
where @ = @(v,) = @(v,) = @(V,) = @CC,), NC is the number of carbon nuclei in one ton of scintillator, L = 93.9 ton year is the exposure for the 28 counters core used in this analysis, E = 35% is the efficiency to detect the 15.11 MeV gamma ray in the energy range between 12 and 18 MeV, and N,, = 358 is the upper limit on the number Nabs of neutral current candidates obtained in section 3.2. The corresponding 90% c.1. upper limit on the flux, @, is shown in fig. 7 as a function of the temperature T,,.This limit, although still far from the predicted value, is, however, the only experimental measurement of relic muon and tau neutrino fluxes.
5. Summary
Figure 8 shows the obtained upper limits on the different neutrino and antineutrino flavour fluxes obtained by LSD during the last 3 years of measurement, after the construction of additional shielding which considerably reduced the radioactivity background. Due to the high sensitivity of the detector at low energies and to the low-energy threshold of the inverse beta decay interaction, we could evaluate the upper limit on the G, flux at energies as low as 9 MeV. A better limit has been obtained in the range 20 I E, s 50 MeV, where the background is due only to unidentified muons hitting the edge-corners of the counters. We would like to stress that the upper limits on the Ce flux obtained in this work can provide strong constraints on the fraction of electron neutrinos produced in the Sun that could transform into electron antineutrinos during their trip to-
8
M. Aglietta et al. / Limits on low-energy neutrino fluxes
-1091 I!
k +
P
I 102r,““““‘““““.‘..’ 0 10
k +
P
;I 20
i. + “C (c.c) Y. + ‘t (c.c)
30
40 v energy
Fig. 8. Upper
limits
to the integral fluxes different flavours.
50
(MeV)
of neutrinos
of
wards the Earth. It is well known that neutrino oscillations are among the best candidates able to solve the neutrino puzzle; i.e., the disagreement between the solar r~, flux measured over 20 years by the Homestake and recently by the Kamiokande neutrino detectors [20,21] and that predicted by the standard solar model (SSM). For example, in the so-called hybrid models [22-241, the spinflavour transitions v, -+ Z,, and the mass oscillations 5, -+ Ge can account for the observed depletion in neutrino flux. In this scenario the antineutrinos are assumed to have approximately the same spectrum as the solar electron neutrinos. Comparing the obtained upper limit on the c= flux in the energy range 9 I E, I 20 MeV (@(Ze,,) < 8.2 x lo4 Ge s-‘cm-2) to the solar v= flux predicted by the SSM in the same range of energy (@(v,> < 1.3 X lo6 v, s-1cm-2 [251), we can conclude that less than 6.3% of the v, flux goes to the above transmutation. This value is comparable with the upper limit (6%) derived from Kamiokande data [26]. These results could provide interesting bounds to the mixing parameters and to the magnetic moment of the neutrino. In this measurement, as in the search for antineutrinos from past supernovae, (in the range of temperatures of the neutrino sea around l-2
MeV) our detector proved to be comparable to much more massive detectors thanks to the clear double signature of the 5,-p interaction and the consequent large background rejection. The high-energy threshold of the interactions on carbon nuclei does not allow us to study neutrinos with energy as low as in the inverse beta decay case. However in the search for charge current interaction candidates, the clear signature of this process allows an almost complete rejection of the background, thus leading to a limit on the v, and V, fluxes with energy 25 I E, I 50 MeV comparable with the one obtained through the inverse beta decay, despite the fact that the cross-section is about one order of magnitude lower. The neutral current interaction on carbon has a slightly lower cross-section than the charged one and a much larger background. It can, however, be used to search for muon and tau neutrinos and antineutrinos. The first upper limits on their flux in the energy range 20 I E, I 100 MeV and the first upper limit on their flux from past supernovae were reported.
Acknowledgements
We wish to acknowledge the courtesy of the Mt. Blanc Tunnel Society for the facilities and collaboration and assistence in the relation of our logistic and technical problems and facilities always given to our Institute. Special thanks to our technical staff in particular to C. Barattia, R. Bertoni, M. Canonico, G. Pirali and A. Romero for their continues assistence during the experiment.
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G. Badino et al., Nuovo Cimento C 7 (1984) 573. M. Aglietta et al., Nuovo Cimento C 9 (1986) 185, 270. A.V. Voevodsky et al., Prib. Tekh. Eksp. 1 (1970) 85. G. Badino et al., Proc. Int. Conf. NEUTRINO ‘84, Dortmund, W. Germany, eds. K. KIeinknecht and E.A. Paschos (World Scientific, Singapore, 1984) p. 566. [5] M. Aglietta et al., Proc. 21th Int. Cosmic Ray Conf,
M. Aglietta et al. / Limits on low-energy neutrino fluxes
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Adelaide, ed. R.J. Protheroe (1990), vol. 10, p.44. M. Aglietta et al., Nucl. Phys. Suppl. B 14 (19901 205. M. Aglietta et al., Nucl. Phys. Suppl. B, in press. M. Aglietta et al., Europhys. Lett. 3 (1987) 1315. P. Vogel, Phys. Rev. D 28 (1984) 1918. P. Antonioli et al., Nucl. Instrum. Methods A 309 (1991) 569. M. Fukugita et al., Phys. Lett. B 212 (19881 139. B. Bodmann et al., Phys. Lett. B 267 (1991) 321. B. Bodmann et al., Phys. Lett. B 280 (1992) 198. D.A. Krakauer et al., Phys. Rev. C 45 (1992) 2450. M. Aglietta et al., Proc. 21th Int. Cosmic Ray Conf., Adelaide, ed. R.J. Protheroe (1990), vol. 10, p. 48. G.S. Bisnovatyi-Kogan and Z.F. Seidov, Ann. NY Acad. Sci. 422 (1984) 319. S.E. Woosley, J.R Wilson and R. Mayle, Astrophys. J. 302 (1986) 19.
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[16] G.S. Bisnovatyi-Kogan and Z.F. Seidov, Astron. Zh. 59 (1982) 213. [17] J.N. Bahcall et al., Astrophys. J. 267 (1983) L77. [18] L.M. Krauss et al., Nature 310 (1984) 191. [19] W. Zhang et al., Phys. Rev. Lett. 61 (1988) 385. [20] R. Davis Jr., Proc. 13th Int. Conf. Neutrino Physics and Astrophysics, Boston, NEUTRINO’88 (World Scientific, Singapore, 1988), p. 502. [21] K.S. Hirata et al., Phys. Rev. Lett. 65 (1990) 1297. [22] C.S. Lim and W.J. hlarciano, Phys. Rev. D 37 (1988) 1368. 1231 E.Kh. Akhmedov, Phys. Lett. B 213 (1988) 64. [24] C.S. Lim et al., CERN-TH.5680/90, KEK preprint 90-9 t 1990). [25] J.N. Bahcall and R.K. Ulrich, Rev. Mod. Phys. 60 (1988) 297. 1261 Barbieri et al., Phys. Lett. B 259 (1991) 119.
Astroparticle Physics
Physics 1 (1992) l-9
Limits on low-energy neutrino fluxes with the Mont Blanc liquid scintillator detector M. Aglietta a, P. Antonioli b, G. Badino b, G. Bologna b, C. Castagnoli a,b, A. Castellina ‘, V.L. Dadykin ‘, W. Fulgione a, P. Galeotti a,b, F.F. Khalchukov ‘, E.V. Korolkova ‘, P.V. Kortchaguin ‘, V.B. Kortchaguin ‘, V.A. Kudryavtsev ‘, A.S. Malguin ‘, L. Periale a, V.G. Ryassny ‘, O.G. Ryazhskaya ‘, 0. Saavedra a*b,G. Trinchero a, S. Vernetto a, V.F. Yakushev ’ and G.T. Zatsepin ’ oIstituto di Cosrnogeofuica de1 C.N.R., Corso Fiume 4, 10133 Toritw, Italy ’ Istituto di Fisica Generale dell’Unicersita’ di Torino, cia P. Giuria 1, 10125 Torino, Italy ’ Institute for Nuclear Research of the Russian Academy of Sciences, 60th Annicersary of October Recolution, Moscow VI-11 7312. Russian Federation Received
20 May 1992; in revised
form 25 June
Prospect
7a,
1992
The LSD liquid scintillation detector has been operating since 1985 as an underground neutrino observatory in the Mont Blanc Laboratory with the main objective of detecting antineutrino bursts from collapsing stars. In August 1988 the construction of an additional lead and borex paraffin shield considerably reduced the radioactive background and increased the sensitivity of the apparatus. In this way the search for steady fluxes of low-energy neutrinos of different flavours through their interactions with free protons and carbon nuclei of the scintillator was made possible. No evidence for a galactic collapse was observed during the whole period of measurement. The corresponding 90% cl. upper limit on the galactic collapses rate is 0.45 y-’ for a burst duration of AT 5 10 s. After analysing the last 3 years data, the following 90% c.l. upper limits on the steady neutrino and antineutrino fluxes were obtained: <9.0X
lo4 V, s-‘cm-2,
9
@(Ge) <8.2X
@(cc)
lo3 G, s-‘cm-*,
20 I E, I 50 MeV,
@(IJ,) <6.8X
lo3 v, s~tcm-*,
25 I E, I 50 MeV,
) < 3.0X 10’ vpfi
s1’cm12,
20 I E, I 100 MeV,
@( Yp+7) < 3.3X 10’ 5,+,
s-‘cm-*,
20 I E, 5 100 MeV.
@(u,+,
In particular comparing the obtained upper limit on the i7, flux for 9 I E, I 20 MeV to the solar ve flux predicted by the standard solar model in the same range of energy, we can exclude the possibility that more than 6.3% of the solar U~‘Sflux can change to 5,. Finally the first limits on the flux of relic supernova neutrinos of all flavours as a function of the neutrino sea temperature are presented.
1. The Mont Blanc neutrino telescope
The Mont Blanc, liquid scintillation detector (LSD) has been running since January 1985 in the Correspondence too: Prof. 0. Saavedra, Istituto di Cosmogeofisica de1 C.N.R., Corso Fiume 4, 10133 Torino, Italy. 0927-6505/92/$05.00
0 1992 - Elsevier
Science
Publishers
Mont Blanc Tunnel at a vertical depth of 5200 hg cm-*. The experimental characteristics of the apparatus are described elsewhere [ 1,2]. Briefly, the detector consists of 90 tons of liquid scintillator (Cr0Hz2) [3], contained in 72 stainless-steel tanks (1.0 X 1.0 X 1.5 m3) placed on three layers; each counter is watched by 3 photomultipliers
B.V. All rights reserved
2
M. Aglietta et al. / Limits on low-energy neutrino fluxes
(diameter 15 cm) operating in threefold coincidence. Since its conception [4], the LSD experiment has been dedicated to the detection of antineutrino bursts from the gravitational collapse of stars in our Galaxy, by the antineutrino capture on free protons (ije energy threshold E,, = 1.8 MeV):
counter); furthermore, a large fraction of them give signals in time coincidence with different counters. The absolute time of each event is recorded with an accuracy of 2 ms.
&),+p -
In the present work we define an $e candidate as an event recorded in a single scintillation counter with an energy release E < 60 MeV. Most of these events are due to local radioactivity; a small fraction of them is due to unidentified muons hitting the corner-edges of the detector and/or to gamma rays or to neutrons produced by muons in the surrounding material. The present rate of Ge candidates is 8.8 h-‘. We define an 5, burst candidate as a cluster of N such events recorded during a time interval At, which can be simulated by statistical fluctuations of the background less than once every 100 years (for example, with the present background rate a burst with N = 5 and At = 10 s is produced once every 890 years). Our search includes bursts with time durations At ranging from 1 ms to 600 s. A monitoring task running during the data acquisition time performs an on-line preliminary search for 5, bursts on the raw data, followed by a more accurate off-line analysis. The maximum distance of a detectable collapse is a function of the burst duration At; as the distance to the collapsing stars increased the number of interactions decreases, and only by lowering At can the signal to noise ratio remain unchanged. Figure 1 shows the efficiency of LSD to detect a “standard” collapse (i.e. one producing on average about 50 interactions in our detector from a distance of 10 kpc) as a function of distance, for different burst durations. Integrating over the distance distribution of the stars, we calculated the efficiency E to detect a galactic collapse. If At I 0.1 s, E = 0.999; if At = 10 s, E = 0.995; even in the extreme case of At = 600 s, the efficiency is still 0.95. The Galaxy was monitored from August 1988 to November 1991 with a total exposure of 244.8 ton year. During this period 217792 events were
n + e+ Ln+p
-
d+r
This interaction gives 2 signals in time coincidence: the prompt positron pulse with energy E, = E, - 0.8 MeV followed by a gamma pulse of energy E, = 2.2 MeV with an average delay of 190 us. The energy threshold of the detector is 5 MeV for the 16 inner counters and 7 MeV for the external ones. After each trigger the threshold is lowered to 0.8 MeV for a time of 500 ps to detect the gamma ray signal. The efficiency in detecting the gamma pulse is 0.5 and the probability to have such a delayed coincidence in a single counter by chance is less than 10-i. In August 1988 an additional shield, made of borex paraffin and lead, was added to the detector to decrease the local radioactivity background from the surrounding rock. The results of this improvement in background reduction at different energies and the corresponding increase in sensitivity have been discussed in a previous paper [5]. A careful and systematic study of the low-energy radioactivity background spectrum 161 was performed at the same time with two additional detectors: a NaI 5” x 5” crystal with energy threshold Et,, = 220 KeV and a radon-meter (alpha spectrometer). The results showed a strong correlation of low-energy pulses ( > 0.8 MeV) with Rn content, and indicate that a high sensitivity to low-energy particles of LSD can be achieved 161. In this new background condition of the experiment the total trigger rate is 14.8 events h-‘. The rate of cosmic muons crossing the apparatus is 6.0 h-l. They can be easily identified and rejected because of their high-energy release (160 MeV for a vertical throughgoing muon in one
2. Galactic supernova
monitoring
M. Aglietta et al. / Limits on low-energy neutrino fluxes
0
I
I
87
95
10
I 98
I 100
20
If the data recorded from August 1985 to 1988 are also taken into consideration (847.3 days of measurement), then, apart the burst of 5 pulses detected in real time on 23th February 1987 [7], and taking into account the slightly lower sensitivity of the detector, the upper limits on the rate of galactic collapses are 0.45 year- ’ (At I 10 s) and 0.47 year-’ (At I 600 s), at 90% confidence level. This supernova survey, based on the on-line and off-line burst analysis we have developed many years ago, has been recently used also by MACRO at Gran Sasso with a total of 45 tons of liquid scintillator.
% of stars
30
50
40 Distance
3
(kpc)
Fig. 1. Efficiency to detect an antineutrino burst from a collapsing star versus distance. On the upper scale the percentage of stars in our Galaxy with distances < 15, 20, 25, 30 kpc is reported.
recorded and no significant burst candidate was found. Figure 2 shows the distribution of clusters with a number of events N > 3, 4, 5,. . . ,12 versus At, compared with the expected number calculated, assuming a random behaviour of the background.
”
N I3 N 24
3. Steady neutrino fluxes In the study of steady neutrino fluxes of energy E < 50 MeV, the problem of background rejection is fundamental. Therefore, even neutrino interaction channels with lower cross-sections, but with a stronger signature, become interesting. In particular, the interactions with carbon nuclei of the liquid scintillator present a very good signature and furthermore, the neutral current gives one the possibility to study low energy u& and v~. In this section we present the results on the search for steady neutrino fluxes using different target nuclei; their interpretation in terms of past supernovae background will be discussed in section 4.
N 35
O3
N 16
3.1. Ve- proton interactions
N 27 N 2.3
02
N Z9
N 210
0
N 212
1 0
100
200
300
400
Burst
500
durotion
600 (SK)
Fig. 2. Distribution of clusters with a number of events N > 3, 4, s,..., 12 as a function of time duration, recorded during the period August 1988 - November 1991, and the expected curves from a Poissonian background.
The measurement of a steady +, flux is performed, as for Ge bursts, using the free proton inverse P-decay. This interaction presents three advantages: (a) higher cross-section [8] in comparison with other interactions in the same energy range, i.e. charged and neutral current on carbon nuclei and elastic scattering on electrons (see fig. 3.); (b) low-energy threshold: E,, = 1.8 MeV; (c) good signature due to the detection of the gamma emitted after neutron capture. In this section we present our results on the search for 5, in two ranges of energy: (a) 9 I E,
4
M. Aglietta et al. / Limits on low-energy neutrino fluxes
lid
0
’
’
’
1’
20
’
’
’
’
’
40
’
’
’
’
’ 60
’
’
j
u Em&
”
’
1
100
(Mev)
Fig. 3. Cross-sections of neutrino and antineutrino interactions with protons and carbon nuclei. Cross-sections (d) and (e) refer to the interactions Y~+~ +‘“C + r’C* + Y~,+,~ and v+T + 1% + %* + Ge,a,T respectively, where C* is the “C 15.11 MeV excited state.
I 50 MeV, where the energy lower limit is set by the detector energy threshold, (b) 20 I E, I 50 MeV, where the radioactivity background component is absent and the sensitivity of the detector is higher. An internal core of the apparatus (made by the 28 counters presenting at least 5 faces shielded by contiguous counters and placed on the second and third layers) was used in this analysis, corresponding to a total exposure of 93.9 ton year. The choice of this subset reduces the radioactivity background and eliminates most of the events due to muons hitting the edge-comers of the detector (these kind of events increase the highenergy tail of the background spectrum). The search for antineutrinos of energy 9 I E, I 50 MeV is done in two steps: (1) the events with deposited energy 7 I E, 5 55 MeV contained in one single counter are selected, (2) the number, N, of low-energy pulses occurring in the same counter within 500 l.~s after the trigger is compared with the number, Nb, of pulses expected from a uniform background. In the whole period 11128 events and 483 low-energy pulses were recorded. To be independent of possible electronic noise, we considered
only the delayed pulses occurring between 20 and 450 us after the trigger; this reduces the efficiency to detect the gamma pulse by a factor of 0.81. Figure 4 shows the integral energy spectrum of all events considered in this analysis (curve a) and the fraction of them which are followed by one or more delayed low-energy pulses (curve b); for comparison the corresponding distributions for the events recorded in the whole apparatus (72 tanks) are also given (curve c and d, respectively). Using the counting rates for each counter for pulses over the energy threshold (0.8 MeV), we obtained the value N, = 470.2 for the expected number of low-energy pulses (the counting rates for the 72 tanks are measured and recorded every 7 minutes to check the stability of the apparatus). Taking into account the total efficiency of detection the delayed gamma pulse (E = 0.50 x 0.81 = 0.411, the resulting number of antineutrino interaction candidates is 31.6 + 56.0. The same analysis was performed on events with energy deposited in the range 17
10
20
30
40 Energy
50 (MeV)
Fig. 4. Integral energy spectrum of the events with energy 7 < E < 55 MeV recorded during the whole period of measurement considered in this work: (a) events in the 28 tanks core, (b) events in the 28 tanks core followed by one or more low energy pulses, (c) events in the whole apparatus (72 tanks), (d) events in the whole apparatus followed by one or more low-energy pulses.
M. Aglietta et al. / Limits on low-energy neutrino fluxes
MeV, in order to search for antineutrinos with 20 I E, I 50 MeV. In this case we recorded 554 events: the number of delayed pulses, N, was 37 and the expected number, Nb, was 22.7. The resulting number of c, interaction candidates is 35.3 + 16.8. In both cases no significant excess of low-energy pulses was observed and the corresponding 90% cl. upper limits on the Ge flux with energy in the range 9 I E, I 50 MeV and with 20 I E, I 50 MeV are 9.0 X lo4 $e s-1cm-2 and 8.2 X lo3 Ge s-icm-*, respectively. These limits have been conservatively calculated using the values of the cross-section at 9 and 20 MeV, respectively, and taking into account the energy resolution of the detector [9]. 3.2. Neutrino interactions with carbon nuclei
-
‘*N+e-
L
‘*C+e+
Et,, = 17.3 MeV Ge +‘*c
-
(2)
‘*B + e+
Et,, = 14.4 MeV
L
background, we used all of the 72 counters, corresponding to a total exposure of 244.8 ton year. Taking into account the different trigger rate of each counter (depending on its position inside the apparatus), the expected number of pairs due to chance was calculated and compared with the number of pairs experimentally found. No significant excess was observed (1 and 4 candidates have been selected for (2) and (3), respectively while 1.6 and 3.0 were expected). The corresponding upper limits to the v, and ce fluxes in the energy range 25 I E, I 50 MeV (where the lower limit on the energy is set by the energy threshold of the detector) were calculated taking into account the efficiency to detect both pulses [13] and by using the values of the crosssections at 25 MeV: @(vJ < 6.8 x lo3 V~s-‘cm-*,
Neutrinos of different flavours are detectable through their interactions with ‘*C nuclei. The energy thresholds of these reactions are higher and the cross-sections (see fig. 3) are about one order of magnitude lower than the inverse beta decay (the calculated cross-sections used in this analysis [lo] have been recently confirmed by two experimental studies [11,121X The charge current interactions ve +12c
5
12C+ e(3)
give 2 signals: 1) the prompt electron (positron) pulse, 2) the positron (electron) pulse from the 12N (‘*B) decay, delayed by 15.9 (29.4) ms. We define a candidate for interactions (2) and (3) as a pair of events with E I 60 MeV occurring in the same counter within 48 and 90 ms, respectively. In this search, because of the very low
@( cek) < 1.2 X lo4 5, s-‘cm-*, at 90% confidence level. The neutral current interactions
Eth = 15.1 MeV
(4)
also allow us to study v~+~ and Gp+? fluxes. The deexcitation of the ‘*C* nucleus (occurring with a 90% branching ratio) gives a 15.1 MeV photon. A Monte Carlo simulation based on the EGS4 code was performed in order to evaluate the LSD detection efficiency of the emitted gamma ray in the energy range between 12 and 18 MeV, giving E = 35% [9]. The result of this search for an exposure of 93.9 ton year (we used a core of 28 counters) shows no evidence of a peak among the 335 events in the region between 12 and 18 MeV (fig. 5). Given the results previously obtained on the flux of G, and ver their contribution can be neglected, and the corresponding 90% c.1. upper limits to the up+7 and Gkf7 fluxes with energy > 20 MeV, conservatively calculated using the cross-section value at 20 MeV, are: @(VP,, ) < 3.0 X 10’ up+7 s-‘cmP2, @($+J
< 3.3 X 10’ GP+, s-‘cm-2.
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M. Aglietta ei al. / Limits on low-energy neutrino fluxes
4. Neutrinos from past supernovae
Relic neutrinos from past supernovae (i.e. resulting from the sum of the single contributions of al1 gravitational collapses occurred at different times and distances during the cosmological evolution) are expected to have a spectrum given by a superposition of many Fermi-Dirac spectra, with an average temperature of a few MeV. This spectrum would be red-shifted by a factor of about 0.5 [143 compared to the spectrum of a recent supernova. Disagreement exists among current theoretical models about the order of magnitude of the neutrino flux. With a conservative approach, Woosley et al. 11.51predicted a cosmic Ge signal peaked at 6-10 MeV with a flux @= 1.0 CCs-‘cm -*. More optimistic estimations give larger values by several orders of magnitude [ 16-181. Following the method used by the Kamiokande group [19], we searched for a relic Ce flux in a definite energy window, characterized by a lower background rate and reasonable overlapping with the expected Se spectrum. We chose events in the energy range 12
Energy (MeV) Fig. 5. Energy distribution of the candidates for neutrinos neutral current interaction with carbon nuclei. An excess is expected in the shaded region.
0
4
8
12
16
20
24
Energy
28 (MeVV)
Fig. 6. Integral energy spectrum of the trigger events with E < 30 MeV recorded in the 8 tanks core during the whole period of measurement considered in this analysis: (a) all events, (b) events with the double signature of the CCC+p) interaction; only one event was detected in the range 12 < E < 30 MeV.
MeV. We limited this analysis to the data recorded in the internal core of the apparatus, i.e. in the 8 most shielded counters, corresponding to a total exposure of 22.5 ton year. We found only 1 event with the double signature of the Q inverse decay in the whole set of data. As in section 3.1, to avoid electronic noise we considered only pulses delayed between 20 and 450 trs after the trigger. Figure 6 (curve a) shows the integral energy spectrum of the trigger pulses with energy E < 30 MeV recorded in the 8 tanks core; the spectrum of the events followed by one or more low-energy pulses is shown in curve b. We can thus write
where @($,I is the antineutrino flux, a, E* is the cross-section for inverse beta decay, L is the total exposure, NP is the number of free protons in
M. Aglietta et al. / Limits on low-energy neutrino fluxes
tures for both these flavours. The temperature of the Fermi-Dirac spectrum of muon and tau neutrinos is expected to be higher than the electron neutrino one. Therefore, assuming that the electron neutrinos and antineutrino fluxes are negligible at higher energies, we can write
7 Lo
YE10 .!3 8
7
7
1
10
10
xdE,_
10
F
1c I3
0
2
4
6
8
10
T,,, (MeV) Fig. 7. Upper limit to the relic SN antineutrinos as a function of the temperature of the neutrino sea. The upper limit obtained by the Kamiokande experiment is shown by the dashed curve.
one ton of scintillator, 4Ed) is the efficiency to detect a signal of energy E,, lP = 0.50is the efficiency to detect the 2.2 MeV delayed gamma ray. F(E, , T’.,) is a normalized Fermi-Dirac spectrum with an effective equilibrium temperature Ten, G(Eve- 0.8-Ed) is a normalized Gaussian distribution with r.m.s = 0.25 JEd to take into account the energy resolution of the detector and N,, = 3.89 is the upper limit at 90% c.1. on the number of observed events Nabs = 1. The corresponding upper limit at a 90% c.1. on the flux @CC,) is shown in fig. 7 as a function of the temperature T,,of the antineutrino sea. The same figure also shows the upper limits obtained by the Kamiokande group (dashed line) with an exposure of 0.58 kton year and a higher energy window, 19 < E, < 35 MeV [193. It is interesting to remark how our detector, even with a total exposure 25 times lower than Kamiokande, can give significant results for low temperatures of the neutrino sea due to its high sensitivity at low energies. The results obtained in section 3.2 will now be used in order to estimate the flux of relic u&T and t,7, . In this case, according to the standard model of collapse, we assume equal fluxes and tempera-
where @ = @(v,) = @(v,) = @(V,) = @CC,), NC is the number of carbon nuclei in one ton of scintillator, L = 93.9 ton year is the exposure for the 28 counters core used in this analysis, E = 35% is the efficiency to detect the 15.11 MeV gamma ray in the energy range between 12 and 18 MeV, and N,, = 358 is the upper limit on the number Nabs of neutral current candidates obtained in section 3.2. The corresponding 90% c.1. upper limit on the flux, @, is shown in fig. 7 as a function of the temperature T,,.This limit, although still far from the predicted value, is, however, the only experimental measurement of relic muon and tau neutrino fluxes.
5. Summary
Figure 8 shows the obtained upper limits on the different neutrino and antineutrino flavour fluxes obtained by LSD during the last 3 years of measurement, after the construction of additional shielding which considerably reduced the radioactivity background. Due to the high sensitivity of the detector at low energies and to the low-energy threshold of the inverse beta decay interaction, we could evaluate the upper limit on the G, flux at energies as low as 9 MeV. A better limit has been obtained in the range 20 I E, s 50 MeV, where the background is due only to unidentified muons hitting the edge-corners of the counters. We would like to stress that the upper limits on the Ce flux obtained in this work can provide strong constraints on the fraction of electron neutrinos produced in the Sun that could transform into electron antineutrinos during their trip to-
8
M. Aglietta et al. / Limits on low-energy neutrino fluxes
-1091 I!
k +
P
I 102r,““““‘““““.‘..’ 0 10
k +
P
;I 20
i. + “C (c.c) Y. + ‘t (c.c)
30
40 v energy
Fig. 8. Upper
limits
to the integral fluxes different flavours.
50
(MeV)
of neutrinos
of
wards the Earth. It is well known that neutrino oscillations are among the best candidates able to solve the neutrino puzzle; i.e., the disagreement between the solar r~, flux measured over 20 years by the Homestake and recently by the Kamiokande neutrino detectors [20,21] and that predicted by the standard solar model (SSM). For example, in the so-called hybrid models [22-241, the spinflavour transitions v, -+ Z,, and the mass oscillations 5, -+ Ge can account for the observed depletion in neutrino flux. In this scenario the antineutrinos are assumed to have approximately the same spectrum as the solar electron neutrinos. Comparing the obtained upper limit on the c= flux in the energy range 9 I E, I 20 MeV (@(Ze,,) < 8.2 x lo4 Ge s-‘cm-2) to the solar v= flux predicted by the SSM in the same range of energy (@(v,> < 1.3 X lo6 v, s-1cm-2 [251), we can conclude that less than 6.3% of the v, flux goes to the above transmutation. This value is comparable with the upper limit (6%) derived from Kamiokande data [26]. These results could provide interesting bounds to the mixing parameters and to the magnetic moment of the neutrino. In this measurement, as in the search for antineutrinos from past supernovae, (in the range of temperatures of the neutrino sea around l-2
MeV) our detector proved to be comparable to much more massive detectors thanks to the clear double signature of the 5,-p interaction and the consequent large background rejection. The high-energy threshold of the interactions on carbon nuclei does not allow us to study neutrinos with energy as low as in the inverse beta decay case. However in the search for charge current interaction candidates, the clear signature of this process allows an almost complete rejection of the background, thus leading to a limit on the v, and V, fluxes with energy 25 I E, I 50 MeV comparable with the one obtained through the inverse beta decay, despite the fact that the cross-section is about one order of magnitude lower. The neutral current interaction on carbon has a slightly lower cross-section than the charged one and a much larger background. It can, however, be used to search for muon and tau neutrinos and antineutrinos. The first upper limits on their flux in the energy range 20 I E, I 100 MeV and the first upper limit on their flux from past supernovae were reported.
Acknowledgements
We wish to acknowledge the courtesy of the Mt. Blanc Tunnel Society for the facilities and collaboration and assistence in the relation of our logistic and technical problems and facilities always given to our Institute. Special thanks to our technical staff in particular to C. Barattia, R. Bertoni, M. Canonico, G. Pirali and A. Romero for their continues assistence during the experiment.
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[16] G.S. Bisnovatyi-Kogan and Z.F. Seidov, Astron. Zh. 59 (1982) 213. [17] J.N. Bahcall et al., Astrophys. J. 267 (1983) L77. [18] L.M. Krauss et al., Nature 310 (1984) 191. [19] W. Zhang et al., Phys. Rev. Lett. 61 (1988) 385. [20] R. Davis Jr., Proc. 13th Int. Conf. Neutrino Physics and Astrophysics, Boston, NEUTRINO’88 (World Scientific, Singapore, 1988), p. 502. [21] K.S. Hirata et al., Phys. Rev. Lett. 65 (1990) 1297. [22] C.S. Lim and W.J. hlarciano, Phys. Rev. D 37 (1988) 1368. 1231 E.Kh. Akhmedov, Phys. Lett. B 213 (1988) 64. [24] C.S. Lim et al., CERN-TH.5680/90, KEK preprint 90-9 t 1990). [25] J.N. Bahcall and R.K. Ulrich, Rev. Mod. Phys. 60 (1988) 297. 1261 Barbieri et al., Phys. Lett. B 259 (1991) 119.