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GALLEX solar neutrino observations: complete results for GALLEX II
31 August 1995 PHYSICS
ELSEVIER
LETTERS
B
Physics Letters B 357 (1995) 237-247
GALLEX solar neutrino observations: complete results for GALLEX II 1y2y3~4 GALLEX Collaboration P. Anselmann a, W. Hampel ‘, G. Heusser ‘, J. Kiko a, T. Kirstena, M. Laubenstein a, E. Pernickaa, S. Pezzoni a, U. Rijnn a, M. Sann a, C. Schlosser a, R. Wink a, M. Wojcik a,51 R. v.Ammon b, K.H. Ebert b, T. Fritschb, D. Heidt b, E. Henrich b, L. Stieglitz b, F. Weirich b, M. BalataC, H. LallaC, E. Bellottid, C. Cattadori d, 0. Cremonesid, N. Ferrarid, E. Fiorinid, L. Zanotti d, M. Altmann”, F. v. Feilitzsche, R. Mii13bauere, U. Schandae,6, G. Berthomieu f, E. Schatzman fY7,I. Carmig, I. Dostrovsky g, C. Bacci h*8,P. Belli h, R. Bernabei h, S. d’Angelo h, L. Paoluzi h, A. Bevilacquaiv9, S. Charbit i, M. Cribier’, L. Gosset i, J. Rich i, M. Spiro’, T. Stolarczyk’, C. Tao iylo, D. Vignaud’, J. Bogerj, R.L. Hahnj, F.X. Hartmann jJ1 , J.K. Rowleyj, R.W. Stoennerj, J. Weneserj a Max-Planck-lnstitut fur Kernphysik (MPIK), Postfach 103980, D-69029 Heidelberg, Germany’ b lnstitut fiir Technische Chemie, Kernforschungszentrum Karlsruhe (KFK), Postfoch 3640, D-76021 Karlsruhe. Germany ’ INFN - Laboratori Nazionali de1 Gran Sass0 (LNGS). S.S. I7/bis Km I8+910, I-67010 L’Aquila, Italy 2 ’ Dipartimento di Fisica, Universita di M&no e INFN - Sezione di Milano, VTaCeloria 16. I-20133 Milano, Italy2 e Physik Department ElS, Technische Universitat Mtinchen (TUM), James-Franck Straje, D-85748 Garching b. Miinchen, Germany f Observatoire de la C&e d’Azur; Departement Cassini, BP 229, 06004 Nice Ceder 4, France s Department of Environmental and Energy Research, The Weizmann Institute of Science (WI), PO. Box 26, 76100 Rehovot, Israel h Dipartimento di Fisica, II Universita di Roma “Tor Vergata” e INFN - Sezione di Roma2, Via della Ricerca Scientihca. i-00133 Roma. Italy2 i CEA, DAPNIA, CE Saclay, F-911 91 Gtf-w--Yvette Cedex, France3 i Brookhaven National Laboratory (BNL), Upton, NY 11973, USA4
Received 5 July 1995 Editor: K. Winter
Abstract
We report the solar neutrino results from the complete set of runs in the exposure period, GALLEX II, from 19 August 1992 - 23 June 1994. Counting for these runs was completed on 10 December 1994. The GALLEX II result (24 runs) is [ 75.2 f 9.7 (stat) ?‘& (syst) ] SNU ( 1 a). After three years of recording the solar neutrino flux with the GALLEX detector, the combined result from the 39 completed solar runs (GALLEX I+II) is [77.1 f 8.5 (stat) ‘-“;f,(syst)] SNU ( 1 a) or 77.1 ?$, SNU with errors combined in quadrature. The combined error (f 13%) has now approached a level where the limits on the derived contribution of 7Be neutrinos to the GALLEX signal confront the predictions of solar models.
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@ 1995 Elsevier Science B.V. All rights reserved
SSDIO370-2693(95)00897-7
238
GALLEX Collaboration/Physics
1. Introduction
The GALLEX detector at the Gran Sasso Underground Laboratories (LNGS) monitors solar neutrinos with energies above the 233 keV threshold for the inverse beta-decay reaction 71Ga (Y, , e- ) Ge. In order to use the 16.5 day mean-life of 71Ge to maximum advantage, a typical solar exposure lasts about one month, followed by a 6 month period of counting of the 7’Ge after its extraction from the gallium target at the end of the exposure. We publish our results at regular intervals, including the data of the most recent runs, which we consider to be preliminary if the samples have not been counted to completion. Final data on these runs routinely appear in the followup publication. In this fourth release of solar data we present the completed data set for GALLEX II covering the 24 runs which were performed in the A tank of the GALLEX installation between 19 August 1992 and 22 June 1994 (GALLEX II was preceded by the GALLEX I series of runs performed in the B tank between 14 May 1991 and 29 April 1992). In addition, we give the results of an analysis of the combined data from GALLEX I and GALLEX II. The development of the GALLEX data base, as reported in previous publications [l-3] and in the present paper, is shown in Table 1 for solar runs (SR) and for short exposure
’ This work has been supported by the German Federal Minister for Research and Technology (BMFT). This work has been generously supported by the ALFRIED KRIJPP von BOHLEN und HALBACH-Foundation, Germany. * This work has been supported by Istituto Nazionale di Fisica Nucleare (INFN), Italy. 3 This work has been supported by the Commissariat a l’bnergie atomique (CEA) , France. 4 This work has been supported by the Office of High Energy and Nuclear Physics of the U.S. Department of Energy, United States. 5 Permanent address: Instytut Fizyki, Uniwersytet Jagiellonski, ul. Reymonta 4, PL-30059 Krakow, Poland. 6 Present address: Max-Planck-Institut ftlr Physik, Fijhringer Ring 6, Postfach 401212, D-80805 Munchen, Germany. 7 Permanent address: DASGAL, Bfrtiment Copemic, Observatoire de Paris, 5 place Jules Janssen, F-92195 Meudon Principal, France. s Permanent address: Dipartimento di Fisica, III Universim di Roma, via C. Segre 2, 1-00100, Roma, Italy. 9 Permanent address: SRS/SAPR, CEN Grenoble, F-38041 Grenoble Cedex, France. to Permanent address: LPC du College de France, 11, Place Marcelin Berthelot, F-75231 Paris Cedex 05, France. ” Present address: Laboratori Nazionali dei Gran Sasso (LNGS)
Letters B 357 f 1995) 237-247
blank runs (BL). The GALLEX II series was ended by the start of the scheduled GALLEX Cr source experiment in which the gallium target was exposed to a strong calibrated man-made neutrino source. The results of this new experiment, which were published recently [41, unequivocally demonstrated the reliability of the GALLEX detector. The previous GALLEX results a) provided the first observation of pp neutrinos from the Sun, with a signal that is large enough to be consistent with a “minimal” solar model which requires only that the present Sun produce all its energy through fusion of hydrogen via the pp and pep (PPI) cycle (about 79 SNU or about 60% of the SSM expectation) ; and b) indicated a deficit of the expected fluxes of the other neutrino branches, in particular of 7Be neutrinos, the second largest expected contributor to the GALLEX detector signal. The significance of this 7Be neutrino problem has evolved only gradually, as the experimental errors on the GALLEX result have decreased. Further elucidation of this important result will require continued error reduction (as discussed in Section 3B). We report the new results in this paper without repeating those details of the experimental and data-evaluation methods that were described before [ l-3,5-7]. However, in addition to including the completed counting data for all GALLEX II runs, we report here on a) additional Ge chemical-yield corrections that were applied to these data, compared to what was done in [ 31, and b) on a new experimental test for side reactions.
2. GALLEX-II runs and results. GALLEX II spans runs A59 to A105, comprising consecutive solar runs SR16-SR39 and Blank runs BL6-BL27. The run characteristics are given in Table 2. The only change in the data sets that were previously given in Table 1 of [ 31 for runs A59-A79 is the addition of the Ge-yield corrections that are discussed next. 2.1.
Germanium
yield determination
Isotopically distinct germanium carriers (70Ge, 72Ge 74Ge, 76Ge) are used alternatingly to monitor the yields of the various steps in desorbing and pro-
GALLEX Collaboration/Physics
239
L.etters B 357 f 1995) 237-247
Table 1 Summary of pubfished GALLEX data. Numbers in parentheses indicate the number of runs in whichthe standard counting period of 6 months had not been completed at the time of the respective data release Date of data release
May 1992 June 1993 February 1994 June 1995
Reference
111 I21 131 this work
Blank runs
Solar runs GALLEX I
GALLEX II
total
result (in SNU)
14 I5 15 15
0 6 15 24
14 21 30 39
83 f 21 87f 16 79 f 12 71 f 10
cessing the germanium [ 1,2] _ The ‘integral yield’ is defined as the ratio of the measured quantity of germanium that is filled into the counter to the amount of germanium that was initially added to the target tank. Supported by independent cross-checks, this yield is primarily determined by (P,V,T) determination of the gaseous germane, GeH4, inserted into the counter. It is these yields that we have routinely reported [ l-31 and used, but we do have the additional possibility to cross-check them after the completion of the 6 month counting period by performing a mass spectrometric analysis of the counter filling. For this, the GeH4 is thermally decomposed to a Ge-metal mirror, dissolved in hydrogen peroxide and analyzed on the Re-filament in a Finnigan MAT/THQ thermion mass spectrometer. The Ge-recovery in this method is quantitative and confirms that the counter filling is preserved intact for > 180 days. Analysis of the mass spectra revealed that the analyzed germanium is typically composed of: a) The carrier isotope with its known satellite abundances (at most 4% per isotope), b) 0.3 f 0.2% carry-over of the carrier isotope used in the preceding run. This value is consistent with the values observed in the GALLEX I runs after scaling for the larger volume of nitrogen gas swept through the target tank in the GALLEX II runs. The correction for this carry-over is not included in Table 2, since it was discussed previously in [ I ] _ c) Small carry-overs from runs before the previous one (“memory”). We have calculated an average correction factor based upon the isotopic analyses of the counting samples from the majority of the runs. The measured integral yields for these runs have thus been multiplied by this correction factor, 0.996. d) Natural germanium impurities in the HCl used in
(9) (2) (4) (0)
5 (1) 11 (2) 19 (4) 27 (0)
the desorption and concentration steps. These impurities are introduced in those runs where either it was necessary to replenish a large quantity of HCI (about 100 kg) in the target tank before the respective run, or where the HCI gas routinely used for column acidification during the run came from almost empty HCl bottles, in which Ge-impurities seem to concentrate. The appropriate correction has been applied to each of these runs individually. We apply the required corrections to the integral yields and list them in Table 2 together with the uncorrected yields for comparison. In the majority of runs (37 of 47)) we have multiplied the integral yield by 0.996 according to item c) above. The other cases are marked * and corrected on the basis of their individual mass spectrum. The mean corrected yield of normal runs is 96.8 f 0.3%. The average loss of 3.2% agrees well with the sum of the 1% loss expected from the desorption and concentration steps and the 2% loss expected from the CC14 extraction, Ge& synthesis and counter filling. All of the runs have either been corrected for carry-over or for the addition of small quantities of natural Ge, excepting run A92 (a blank run, where the value listed in Table 2 was used). Since the overall effect of the corrections on these yields is small, the systematic error introduced in the GALLEX result is also small. 2.2. Experimental test for side reactions We have previously reported on the exclusion of all potential side reactions from radioimpurities in the target solution except for a minor loophole concerning hypothetical transuranium-element contamination which might produce 71Ge through neutrons from spontaneous fission. We have now excluded this pos-
240
GALLEX Collaborabon / Physics Letters B 357 (1995) 237-247
Table 2 Characteristics Type”
of GALLEX
Run #
II runs A 59
Time period
- A 105
Duration [days1
Carrier b
Ge yield c
[%I
Ge yieldd corrected by MS
Position e
label HD2 s
[%I SR 16 BL 6 SR 17 BL 7 SRI8 BL 8 SR 19 BL 9 SR 20 BL 10 SR21 BL II SR22 BL12 SR 23 BL13 SR 24 _h SR 25 BL 14 SR 26 BLl5 SR27 BL16 SR 28 BLIJ SR 29 BL 18 SR 30 BL 19 SR31 BL 20
A 59 A 60 A 61 A 62 A63 A 64 A 65 A 66 A 67 A 68 A69 A70 A71 A72 A 73 A74 A 75 A 76 A 77 A 78 A 19 A80 A81 A82 A 83 A84 A 85 A 86 A 87 A 88 A89 A 90
SR32 BL21 SR 33 BL 22 SR 34 BL 23 SR 35 BL 24 SR 36 BL25 SR 37 BL 26 SR38 BL27 SR 39
A91 A92 A 93 A 94 A 95 A 96 A 97 A 98 A 99 Al00 A 101 A 102 A103 Al04 A 105
For footnotes
19.08.92 - 16.09.92 16.09.92 - 17.09.92 17.09.92 - 14.10.92 14.10.92 - 15.10.92 15.10.92 - 11.11.92 ll.ll.9212.11.92
28.0 1.0 27.0 1.0 27.0 I.0
12.11.92 09.12.92 10.12.92 06.01.93 07.01.93 03.02.93 04.02.93 03.03.93 04.03.93 02.04.93 03.04.93 28.04.93 29.04.93 26.05.93 27.0593 23.06.93 24.06.93 21.07.93 22.07.93 18.08.93 19.08.93 15.09.93 16.09.93 13.10.93
27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 29.0 1.0 25.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0
14.10.93 10.11.93 11.11.93 08.12.93 09.12.93 05.01.94 06.01.94 02.02.94 03.02.94 02.03.94 03.03.94 30.03.94 3 I .03.94 27.04.94 2804.94 25.05.94 2605.94
-
-
-
09.12.92 10.12.92 06.01.93 07.01.93 03.02.93 04.02.93 03.03.93 04.03.93 02.04.93 03.04.93 28.04.93 29.04.93 26.05.93 27.05.93 23.06.93 24.06.93 21.07.93 22.07.93 18.08.93 19.08.93 15.09.93 16.09.93 13.10.93 14.10.93
- 10.11.93 - 11.11.93 -
-
-
-
see next page.
08.12.93 09.12.93 05.01.94 06.01.94 02.02.94 03.02.94 02.03.94 03.03.94 30.03.94 3 1.03.94 27.04.94 28.04.94 25.05.94 26.05.94 22.06.94
27.0 1.0 27.0 1.o 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 21.0
74 72 76 74 72 76 74 72 76 74 72 70 76 74 72 76 70 74 72 70 76 72 74 70 72 74 70 72 74 70 72 74 70 72 74 70 72 74 70 72 74 70 72 74 70 76 72
95.6 97.0 93.5 91.1 95.9 97.1 96.1 97.4 97.0 94.9
95.3 96.7 93.1 88.2 * 95.6 96.8 95.8 97.0 96.6 94.6
96.3 98.8 97.6 95.2 98.8 99.0 99.2 97.3 99.1 96.0 95.3 94.1 103.3 96.6 96.8 93.8 101.7 97.9 95.3 99.3 99.2 101.2 98.9 104.6 83.4’ 100.4 98.4 96.9 99.7 98.8 95.9 98.6 109.2 95.7 100.0 97.1 92.2
96.0 98.4 97.2 94.9 98.4 98.6 98.8 91.8” 97.8 * 95.7 95.0 93.8 93.9* 96.3 96.5 93.5 98.9 97.5 95.0 98.9 97.0’ 95.5’ 98.5 104.2 83.1 100.0 98.0 93.7” 99.3 98.4 95.6 98.2 98.3 * 95.4 99.6 95.4” 91.9
Counter
a a a a a a P P P P P P P P a a a _ a a P P P P P P P P a a a a P P P P P P P P P P P P P P P
(Si) I14 (Fe)103 (Fe)115 (Fe) 107 (Si) 102 (a)119 (Fe)43 (Si)106 (Fe)39 (Fe)118 (Fe)112 (Si)ll3 (Si)108 (Fe)99 (Fe) 103 (Si)ll4 (Fe) 107 (Fe)115 (%)I19 (Si)lO2 (Si) 106 (Fe)43 (Fe)118 (Fe)39 (Si)ll3 (Fe)112 (Fe)115 (Si)108 (SC) 140 (SC) 139 (SC) 137 (SC)135 (Si) 102 (Si)119 (Fe)43 (Si) 106 (Fe)39 (Fe)118 (Fe)112 (Si)l13 (Si) 108 (Fe)103 (SC)139 (SC) 138 (Sc)136 (SC) 130 (Si)l19
End of counting etiiciencyr
[ %]
L
K
29.2 29.0 28.7 28.6 29.4 28.9 28.5 29.9 28.9 28.0 29.1 30.3 30.2 26.7 29.0 29.2 28.6 27.7 28.9 29.4 29.9 28.5 28.0 28.9 30.3 29.1 28.7 30.2 33.5 32.5 32.2 33.1 29.4 28.9 28.5 29.9 28.9 28.0 29.1 30.3 30.2 29.0 32.5 32.9 32.7 32.6 28.9
31.3 34.4 34.0 33.9 31.4 30.9 33.7 32.0 34.3 33.2 34.4 32.4 32.3 31.5 34.4 31.3 33.9 34.0 30.9 31.4 32.0 33.7 33.2 34.3 32.4 34.4 34.0 32.3 38.8 37.6 37.4 38.4 31.4 30.9 33.7 32.0 34.3 33.2 34.3 32.4 32.3 34.4 37.7 38.2 38.0 37.8 30.9
Counting live time [days]
23.03.93 23.03.93 23.04.93 23.04.93 2 I .05.93 21.05.93 18.06.93 18.06.93 19.07.93 16.07.93 13.08.93 13.08.93 13.09.93 13.09.93 07.10.93 07.10.93 08.1 1.93 _
168 167 127 174 177 I75 171 177 177 177 179 178 182 176 173 176 179
03.12.93 03.12.93 03.01.94 03.01.94 24.01.94 24.0 I .94 25.02.94 25.02.92 25.03.94 25.03.94 22.04.94 22.04.94 21.05.94 21.05.94 18.06.94 18.06.94 01.07.94 22.06.94
178 177 I81 175 179 179 I81 170 181 179 180 178 180 179 181 181 162 158 145 151 161 137 167 167 156 158 170 170 162
06.07.94 13.07.94 18.08.94 25.07.94 23.09.94 23.09.94 09.10.94 09.10.94 19.11.94 19.11.94 10.12.94
GALLEX Collaboration/Physics Letters B 357 (1995) 237-247
241
Table 2 -continued il SR = solar neutrino run, BL = short exposure, blank run. h 70, 72, 74, 76 indicate the use of carrier solutions enriched in 70Ge, 72Ge, 74Ge, 76Ge, respectively. ’ Tank-to-counter yield of Ge-carriers (see text), errors are 3~1.7%. d See text (section 2) for an explanation of Ge yieldscorrectedby massspectrometry. Runs marked * are individually corrected. The effective 7’Ge yields are about 1% smaller for the SR runs, before considering the decay between end of desorption and start of counting. c a = active (NaI) counting position; p = passive counting position [ 21. f Values include rise time cut where applicable. s Counters have either iron or silicon cathode. SC = silicon counter with shaped cathode. h Malfunction during counting (no useful data can be extracted). i Sample partly lost during CC4 extraction.
sibility by establishing an experimental upper limit on the maximal possible level of spontaneous fission. For this purpose we have measured the fission product 133Xe within the entire 101 tonnes of GALLEX target solution. We selected 5.25 day ‘33Xe because it is produced in good yield in the asymmetric fission of heavy-element nuclei (since it lies near the heavy peak of the fission-product mass-distribution). The xenon was desorbed in 20 hour ‘presweeps’ with 80 m3 of nitrogen two days before the regular desorptions of runs A99 and Al01 respectively. (We note that during this procedure, about 20% of the Ge-carrier was also unavoidably desorbed. However, this early removal of Ge did not adversely affect the normal operation of the ensuing solar run, since this Ge, which was separated from the Xe, could subsequently be recovered from the first absorber column and simply added to the remainder of the germanium that was later desorbed from the gallium target.) The desorption of the xenon was monitored with M 0.7 cc of 124Xeor ‘26Xe which was added to the target 6 days before desorption. Xenon was collected in a charcoal cold trap at 123 K after having undergone gas purification (for CO2 removal with dilute KOH) and drying (concentrated H2SO4 scrubbers downstream from the second absorber column) . The xenon fraction was further purified by applying standard rare-gas handling techniques, and its yield was finally determined both volumetrically and mass spectrometrically. The Xe-yields were routinely 85%, so we made no additional attempts to maximize the recovery. The Xe sample then became part of the regular GALLEX Xe-Ge counting gas mixture for proportional counting. It was counted for > 2 months in an active position, just as in a regular solar run. The observation of P--y coincidences in the NaI detector was the sought-after signal for ‘33Xe (P-decay to 133Cs).
No positive ‘33Xe-signal was seen. From this result and the respective deduced detection efficiencies, we conclude that spontaneously fissioning nuclides produce less than 10m4of the measured solar signal [ 81. This test has special relevance in that it involves the whole GALLEX target. Consequently, it also covers contrived scenarios that involve ‘particulate spots’ or ‘wall contaminants’ which could have escaped sampling in regular radiopurity checks. 2.3. Results The total GALLEX II exposure time (excluding the 23 days for blank exposures) amounts to 649 days, twice the 324 days for solar exposures in GALLEX I. Note that only 1 day was lost out of 672 exposure days. The results from the individual runs in GALLEX II for the net solar production rates of 71Ge (based on the counts in the K and L energy and rise-time windows [ 1,2] ) are given in Table 3 after the usual subtraction [ 1,2] for side reactions and background effects (7.4 SNU for GALLEX II). In addition, they are plotted in Fig. 1 together with the earlier data from GALLEX I. For GALLEX II, the total number of observed (L+K) decays of 71Ge due to solar neutrinos is M 115 in 24 runs, or M 4.8 per run. This small number illustrates why the results of individual runs necessarily have little statistical significance, while the combined result of the maximum-likelihood analysis for all 24 GALLEX II runs is statistically significant. The result of GALLEX II is 75.2 f 9.7 (stat.) ?i;k (syst.) SNLJ, in full agreement with the preliminary result of 78 f 13 i 5 SNU given in [ 31, with the statistical error reduced from 17% to 13%. A global maximum-likelihood fit to the data from the full 39 runs gives the result for GALLEX I + II of 77.1 f 8.5 ‘_j SNU. The mean life of the decaying
242
GALLEX Collaboration/Physics
Table 3 Results for individual solar neutrino runs in GALLEX II. All SNU-values shown are net solar production rates of ‘lGe after subtraction of 7.4 SNU for side reactions (see text). The quoted errors are statistical only Run number
K + L result (SNU)
SRI6
AS9
SRI7
A61
SRI8
A63
SRl9
A65
SR20
A67
SR21
A69
SR22
A71
SR23
A73
115 :z
SR24
A75
138 :$
SR25
A77
36 +4x -35
SR26
A79
62 +54 -42
32
f3Y -2s
100
T’,:
58 +YJ -W 70 +;; -42
SR27
A81
$9 +53 -42
SR28
A83
56 +@ -29
SR29
A85
SR30
A87
SR31
A89
SR32
A91
64 +‘% -36 4‘j
+49 -39
71 +‘I’ -37 -9
+39 -26
SR33
A93
39 +4y -35
SR34
A95
52 +‘w -36
SR35
A97
8.5 +52 -41
SR36
A99
SR37
Al01
107 :z
SR38
A103
112:;
SR39
A105
90
51
f5* -43
+63 -50
component deduced from all L + K data is r = 15.1 122: d, in agreement with the known value for 7’Ge of 16.5 days. The summary of the results for GALLEX I and GALLEX II is given in Table 4, and includes not only the combined L + K data but also separate analyses of the L- and K-regions. Note that the ratio of the SNU-values of the L- and K-regions has converged towards the expected value of unity as more data have been obtained. We note here that the latest value from the SAGE gallium experiment in the Baksan Laboratory is 69 f 13 SNU [ 91, in agreement with our result.
Letters B 357 (I 9951237-247
We have investigated the stability of the GALLEX results against changes in our standard cuts for the energy and pulse-shape acceptance windows: a) In one analysis, we have removed the sharp energy cut (a step function) and instead used the known shape of the “Ge K and L spectra to assign weights to the individua1 counts. The result obtained in this way agrees well (within 2%) with our mean result. b) In another analysis, we have also investigated the stability of the GALLEX result with respect to the pulse rise-time criterion, by performing an independent alternative pulse shape analysis, which is based on a model of the energy deposition of ionizing radiation in a gas-filled counter. The parameters of the model, which characterize the microscopic charge deposition structure inside the counter’s active volume in terms of, e.g. the number of primary charge clouds, the energy deposited in each of these clouds and their radial extensions, are least-squares-fitted directly to the recorded transient digitizer pulse. For a counter pulse to be accepted, all of these parameters must fulfill the criteria that characterize Ge-decays. Details of this method are described in [ 101. Utilizing this approach for the full data set yields an overall result in excellent agreement with that derived in our standard rise-time analysis: the difference between the results from the two different methods corresponds to the 2 percent uncertainty we assign to the rise-time cut [ 21. c) Variations in the energy and rise-time/pulseshape cuts were also studied in [ 111 and shown to have little effect on the stability and reliability of the final SNU values. The fact that different pulse shape analyses show such close agreement is indicative of the robustness of the GALLEX result. From the very beginning of the GALLEX solar exposures, we have performed blank runs in order to check the whole experimental procedure. These blanks are in every respect like production runs except that the exposure time is only 1 day, the minimum time needed to perform a run. The blank-run results have consistently been reassuring in that the measured contribution from unknown time-independent sources has always been consistent with zero. Our most recent published figure [ 3 I of - l&7 SNU was based on 19 blanks, with four of them not completely counted (see Table 1). The present updated value is based on 27 completed blanks and yields a result of -5.0 f 5.4 SNU.
GALLEX
Collaboration/Physics
243
Letters B 3S7 (1995) 237-247
- 320
3.0 -
GALLEX II
CALLEX I
h! !,
d L,! A,! 1.2 I,,!‘J “0
1991
rtrcllt - 280
71t IO SNU
8lfl7SNU
,$ In,:
cornbintd
J L
I”~““’ ?.a “II
II
XI
3””
6,
1993
1991
lIIII(I
y*r 1994
“6%
PJL
Fig. I. Results for the 24 solar neutrino runs of GALLEX II (labels 16-39) together with the IS GALLEX I solar runs [ 21 (labels I-15). The left-hand scale is the measured “Ge production rate; the right-hand scale, the net solar neutrino production rate (SNU) after subtraction of side reaction contributions. Error bars am f 1CT,statistical only. The point labelled “combined” applies to the mean global value for the total of all 39 runs. Horizontal bars represent run duration; their asymmetry reflects the “mean age” of the “Ge produced Table 4 Results from GALLEX II (complete) and from GALLEX lfll
combined (in SNU) GALLEX II
GALLEX I + GALLEX II
number of runs
19.08.92 - 22.06.94 649 24
14.05.91 - 22.06.94 973 39
Result, L only (stat. error only) Results, K only (stat. error only) Results, L + K (stat. and syst. error)
67.6 f 14.4 8l.Of 13.1 75.2 f 9.7+‘.’ -4.6
77.6 f 13.1 76.7f 11.1 77.1 f 8.5+_4jf’,
time period exposure days
3. Discussion 3.1. Does the GALLEX result vary over time? We have investigated the question of how well the GALLEX data are described by a production rate that is constant in time, by performing a maximumlikelihood ratio test (see for instance [ 12,131) . This test employs the ratio A = QH)IQIfu): C(H) is the maximized value of the likelihood function for the hypothesis being tested, namely that there is a single true value of the production rate; C( Ho) is its maximum value for the hypothesis that the production rate varies from run to run, evaluated by setting the expectation value for each run in the combined analysis
equal to the value actually measured in that run. For a sufficiently large number of runs (this is certainly the case for 30 or more runs, as has been tested in a Monte Carlo simulation), the quantity -2 In A is distributed like x2 for n degrees of freedom, where n is the difference of free parameters for HO and H, equal to N - 1, where N is the number of runs. In this way we obtain a goodness-of-fit confidence level of 88%, i.e. only in 12% of all cases is a fit better than the actual one to be expected. This result implies that the distribution of results and errors of individual runs in GALLEX agrees with the expectation that they are quasi-normally distributed about a constant production rate. To quantify the statement further that the run data
244
GALLEX Collaboration/Physics
are consistent with a production rate constant in time, we have done two additional maximum-likelihood analyses of all data from GALLEX I and GALLEX II (39 runs). In the first of these, we assumed a periodic variation in time of the production rate about a constant value, Q, and solved the equation: P(t) -a+bsin($(t-to))
(1)
without any restrictions on the amplitude, period and phase. The best fit to the data gave the values, a = 77.0 f 9.0 SNU, b = 10.8 f 12.0 SNU, T = 312 f 79 days, r. = 2.2 f 2.6 days (zero time is May 1, 1991) . The goodness-of-fit confidence level for this model using the likelihood ratio test is 82%. In the second analysis, as an example of a different type of temporal variation, we checked for a possible correlation of the production rate P(t) with the variation of the sunspot number Ns ( t) relative to the mean < Ns> for the GALLEX data-taking period: P(t) -a+b(
Ns(t)--
< Ns >
< Ns >
1
This model has previously been applied to the data of the Womestake detector in Ref. [ 131. Our result for the GALLEX data is a = 78.5 f 9.1 SNIJ for the constant part and b = 6.5 & 17.6 SNU for the variable amplitude, with a goodness-of-fit confidence level of 86%. The results from both of these calculations are consistent with zero variation of the GALLEX production rate with time: ( 1) the calculated time-independent production rate, a, agrees with the present measured GALLEX mean value, (2) the time-dependent amplitude, 6, is consistent, within the errors, with a value of zero, and (3) the goodness-of-fit does not improve by invoking time dependence. 3.2. Relevance of the GALLEX data to the “‘Be neutrino problem”
The standard solar model (SSM) predictions for the gallium detector span a relatively narrow range varying from 113 SNU [ 141 to 122.5 f 14SNU (2~) [15],tol31.5f14SNU (2~) [16].Inthefollowing discussion, in order to explore the range of validity of the conclusions we will draw, we compare our experimental results with values from the two most widely
Letters B 357 (1995) 237-247
quoted SSM’s: Turck-Chieze and Lopes (TCL) [ 151 and Bahcall and Pinsonneault (BP) [ 161. Our present experimental result is 77.1’_\‘, SNU (quoted as 2a errors). Compared to the SSM values, with all of the theoretical values also quoted at the 2a-level (95% confidence statement), this result corresponds to 63 f 18 % and 59 f 16 %, respectively, of the predictions of TCL [ 151 and BP [ 161. These ratios agree almost exactly with what is expected from the minimal solar model. As we will show below, the decrease in the errors (statistical and systematic) of the GALLEX determination of the solar neutrino flux is of marked importance in focusing on the nature of the solar neutrino problem. We should emphasize here that the validation of the GALLEX result by the recently performed “Cr neutrino source experiment [ 41 demonstrates the absence of any unexpected systematic errors and therefore adds confidence in the conclusions that can be drawn from our solar neutrino results. Many recent phenomenological analyses [ 171 have pointed out that, relative to solar model predictions, the flux of ‘Be neutrinos derived from the different solar neutrino detectors appears to be strikingly low, especially when compared to the measured Kamiokande flux of *B neutrinos. This conclusion appears to argue against an “astrophysical solution” to the solar neutrino problem [ 181: even those (non-standard) solar models that are able to reduce the neutrino fluxes from both ‘Be and 8B still cannot come close to reproducing the experimentally derived ‘Be and 8B fluxes. The simple calculations shown below demonstrate the sharp constraints on 7Be production that the present GALLEX results force upon the SSM, and indicate how much tighter these constraints will become as the GALLEX experimental uncertainty is further reduced by the accumulation of additional data. In these calculations, done each time within a particular SSM, we calculate the effect on the signal in gallium (in SNU) due to the reduction of the ‘Be flux by the factor, fh. In doing so, we keep the solar luminosity constant by calculating the fractional increase in the (pp + pep) cycle that is required to offset the reduction in the ‘Be cycle. For the contribution of the 8B neutrino flux we take the value directly measured by Kamiokande [ 191. (We note that we have done similar calculations that use the result from the radiochemical chlorine detector instead of from
GALLEX
Collaboration/Physics
Letters B 357 (1995) 237-247
245
Table 5 Upper limits on the ‘Be neutrino flux suppression values, fee, deduced from GALLEX and Kamiokande. fa is the ratio to the solar model prediction of the *B flux measured by Kamiokande [ 191. (a) is for the present GALLEX 1-w error ( IO SNU) and (b) for the anticipated I -TVerror at the termination of GALLJZX (8 SNU). Solar model
TCL [IS] TCL [IS] (no CNO) BP [16] BP [I61 (no CNO)
fBe
h
0.62 0.62 0.48 0.48
f f f f
0.09 0.09 0.07 0.07
(a)
fBe
for 2u - 20 SNU
for 3~ - 30 SNU
for 2~ - 16 SNU
for 3a - 24 SNU
5 2 5 5
5 5 5 2
<_ 0.05 < 0.25 6 -0.04 5 0.19
5 5 5 5
0.20 0.40 0.09 0.32
Kamiokande. However, the resulting limits are not as stringent as those that we list below in Table 5.) In our first trial, we include the total predicted flux of CNO neutrinos. Comparing the calculated production rate with the GALLEX measurement, including its overall error, then gives a range of allowed values of f&, and more importantly, its maximum allowed value. Examples of these calculations follow. The SNU-value calculated in this way, in the framework of the SSM, is given by the sum of
+ [6.01
(3)
for the TCL model [ 151 and [73.9+5.8(1-f&] + [7.91
+135.8&J
(b)
+[6.7fl.O] (4)
for the BP model [ 161, where the terms in the brackets are respectively: the (pp + pep) value, as the sum of the SSM value plus the increase due to the constant luminosity requirement; the SSM value for 7Be modified by the reduction factor; the number of SNU corresponding to the ‘B flux measured by Kamiokande; and the full SSM value for CNO. By setting this sum equal to the measured GALLEX result, 77.1 SNU, and taking the total 2a error to be f 20 SNU, we find that fk I +0.20 for TCL and fae 5 +0.09 for BP. These resultant values for the ratio of the 7Be signal to the SSM predictions are much smaller than the Kamiokande result for ‘B neutrinos (fa-O.62 f 0.09 relative to the TCL prediction and 0.48 f 0.07 relative to the BP prediction [ 191) . These values are reported in Table 5, as are the corresponding values when the errors are taken to be 3~.
0.59 0.79 0.43 0.65
0.36 0.56 0.23 0.45
A striking result is obtained if we reduce the error on the GALLEX result from the current 2u error of f 20 SNU to the f 16 SNU expected after 4 years of data taking (last columns of Table 5). We find that this reduction in the error translates into a value of fh <+O.OS for TCL and I -0.04 for BP, several times smaller than the present limits. The 20% improvement in statistics makes a drastic difference in setting a stringent upper limit on the 7Be flux and defining the scope of the “7Be neutrino problem”. The potential benefits of even further shrinkage in the GALLEX errors are easily estimated by linear extrapolation of the above results. Reasonable concerns might be based on our taking the CNO neutrino flux at the full SSM value. In a less model dependent approach, we set it instead to zero in our calculations, and raise the pp + pep flux further to maintain the luminosity value. The corresponding results are also reported in Table 5. At 2a, fk is < +0.40 for TCL [ 151 and I f0.32 for BP [ 161. These limits on the 7Be flux are not as severe as the values above, but they involve the extreme assumption of the complete suppression of the CNO flux. We note that the evidence for a 7Be problem has strengthened over time as GALLEX data have accumulated. Our earlier results (see Table I), with their larger error ranges, hinted at but could not precisely define, the scope of the deficit of 7Be neutrinos. The evolution of our understanding of this question is illustrated in Fig. 2. Part (a) compares the SSM predictions of TCL [ 151 and BP [ 161 for gallium, divided into the contributions from the different solar neutrino branches, with the published results of GALLEX during the past four years. Also shown is the expected error from 60 GALLEX runs (assuming
246
GALL.EX Collaboration/
Physics Letters B 357 (1995) 237-247
a) CALLEX 7.C uppcrlimil ..__.__.__._..._._.__________.__~_.~-..~________----..__------_-_ 1
)Pr
1992 1993 1994 1991 ‘1997”
BP
b)
Fig. 2. (a)
TCL
lw)
The GALLEX
39 (this work), corresponding f
results after 14 [I],
21 [2],
30 (31,
and 60 (expected in 1997) solar runs, with their 2u error ranges. The two left columns indicate
the predicted contribution from the pp + pep neutrino branches plus a reduced *B contribution as measured by Kamiokande. The resulting 86-SNU value agrees with the present GALLEX result, within the quoted 2a errors. Little room remains for the contributions from 7Be and CNO neutrinos (see Table 5). In part (b), the calculated SNU values from Eqs. (3) and (4) are plotted vs. the upper limits on the values of fk. We see that as the GALLEX errors become smaller, the maximum allowable values of fBe are drastically reduced. In calculations similar to those above, we can use the GALLEX result to derive an upper limit on the CNO cycle contribution to the energy generation in the Sun. In order to get the most conservative upper limit, it is assumed that the Sun operates only via the PPI and the CNO cycles; the 7Be and ‘B branches are completely suppressed. The result (at the 95% confidence level) is that less than 2.4% of the solar energy is produced by the CNO cycle. This experimental limit is only about a factor of 2 larger than the 1.3% value predicted by standard solar models. In summary, the GALLEX result, with its current 13% error, puts strict limits on the 7Be flux, and also gives us some approximate idea about the CNO flux. It is very clear that, even at this stage of GALLEX data taking, we have an interesting confrontation with the astrophysical model. Collecting additional solar data will tighten these constraints even further.
standard solar model expectations [ 16,151 for gallium, with their respective partitioning due to the different solar neutrino branches. The column on the right (“‘heuristic model”)
contains only the
3.3.
Future
plans
contribution from the pp + pep neutrino branches and a reduced *B contribution as measured by Kamiokande. It is seen to account for the present GALLEX
result, leaving little room within the 2a
error bars for any contributions from ‘Be and CNO neutrinos. (b) The SNU values from Eqs. (3) f&;
and (4).
plotted as functions of
the full SSM contributions of the pp + pep and CNO cycles,
and a reduced contribution from aB (as listed in Table 5 from the Kamiokande GALLEX
result),
experimental
ate assumed. Also shown are the 2u upper limits from (a).
It is obvious that
the evidence for a small value of fne has only gradually evolved as the errors have shrunk in the continuing GALLEX
measurements.
that the mean value does not change appreciably). We note that, while the mean GALLEX SNU values have changed only slightly, their combined 2~ errors have decreased substantially. The general result of our calculations is indicated by the right-hand column (called “heuristic model” in the figure), which contains only
At present, we are taking solar data in the run sequence GALLEX III. This series will last until October 1995, when the second “Cr source will be inserted into the GALLEX tank for a period of 4 months. Afterwards, further solar runs will resume until about the end of 1996, when we will have reached our goal of having done 60 solar runs. The projected total 1 (T error (statistical and systematic) at that time will be 10% (about 8 SNU) .
Acknowledgements
We acknowledge the technical staffs of the participating institutions for their invaluable contributions to the development and subsequent consistently suc-
GALLEX Collaboration /Physics
cessful operation of the GALLEX neutrino detector facility. References [I ] GALLEX Collaboration, I? Anselmann et al., Phys. L&t. B 285 ( 1992) 376. [2] GALLEX Collaboration, P Anselmann et al., Phys. Lett. B 314 (1993) 445. [3] GALLEX Collaboration, P Anselmann et al., Phys. Lett. B 32-l ( 1994) 371. [4] GALLEX Collaboration, P Anselmann et al., Phys. Lett. B 342 (1995) 440. [S] E. Hemich and K.H. Ebert, Angew. Chem., Int. Ed. (En&) 31 (1992) 1283. [6] P. Anselmann and EX. Hartmann, Prog. Part. Nucl. Phys. 32 (1994) 35. [7] T. Kirsten, F.X. Hartmann, R. Wink and l? Anselmann, Nucl. Phys. B (Proc. Suppl.) 35 ( 1994) 418. [8] S. Pezzoni, Ph.D. Thesis, Univ. of Heidelberg, 1995. [9] J.N. Abdurashitov et al., Phys. Lett. B 328 ( 1994) 234; SAGE Collaboration, Contribution to the Glasgow ICHEP Conference, August 1994. [ 1O] M. Altmann, “Pulse. shape analysis by fitting the TDF-pulse”, GALLEX internal note GX-69, March 95. [ I I ] S. Charbit, Ph.D. Thesis, University of Paris VII, July 1994, DAPNIA/SPP 94-25;
Letters B 357 f 1995) 237-247
241
SCharbit, M. Cribier and D. Vignaud, GALLEX internal note GX-57, September 1994. [ 121 W&die et al., in “Statistical methods in experimental physics”, North-Holland, Amsterdam, 1971, Section 10.5. [ 131 B.W. Filippone and P Vogel, Phys. Lett. B 246 ( 1990) 546. [ 141 A. Dar and G. Shaviv, Preprint Technion Ph-94-5; G. Shaviv, Nuc1.Phys.B (Proc. Suppl.) 38 ( 1995) 81. [ 151 S. Turck-Chieze and 1. Lopes, Ap. J. 408 ( 1993) 347. [ 161 J.N. Bahcall and M.H. Pinsonneault, Rev. Mod. Phys. 64 ( 1992) 885. [ 171 See for example J.N. Bahcall, Phys. Lett. B 338 ( 1994) 276; W. Kwong and S.P. Rosen, Phys. Rev. L&t. 73 ( 1994) 369; V. Berezinsky, G. Fiorentini and M. Lissia, Phys. Lett. B 341 (1994) 38. [ 181 See for example N. Hata, S. Bludman and PG. Langacker, Phys. Rev. D 49 ( 1994) 3622; V. Berezinsky, Comments on Nucl. and Part. Phys. ( 1994); J.N. Bahcall and H.A. Bethe, Phys. Rev. D 47 ( 1993) 1298; S. Turck-Chitze, H. Dzitko and I. Lopes, in “Neutral Currents, 20 years after”, Paris, July 1993, ed. by U. NguyenKhac and A.M. Lutz, p. 423; V. Castellani et al., Phys. Rev. D 50 ( 1994) 4749; X. Shi, D.N. Schramm and D.S.P. Dearborn, Phys. Rev. D 50 (1994) 2414. [ 191 K.S. Hirata et al., Phys. Rev. D 44 ( 1991) 2241; Y. Suzuki, Nucl. Phys. B (Proc. Suppl.) 35 ( 1994) 407.
ELSEVIER
LETTERS
B
Physics Letters B 357 (1995) 237-247
GALLEX solar neutrino observations: complete results for GALLEX II 1y2y3~4 GALLEX Collaboration P. Anselmann a, W. Hampel ‘, G. Heusser ‘, J. Kiko a, T. Kirstena, M. Laubenstein a, E. Pernickaa, S. Pezzoni a, U. Rijnn a, M. Sann a, C. Schlosser a, R. Wink a, M. Wojcik a,51 R. v.Ammon b, K.H. Ebert b, T. Fritschb, D. Heidt b, E. Henrich b, L. Stieglitz b, F. Weirich b, M. BalataC, H. LallaC, E. Bellottid, C. Cattadori d, 0. Cremonesid, N. Ferrarid, E. Fiorinid, L. Zanotti d, M. Altmann”, F. v. Feilitzsche, R. Mii13bauere, U. Schandae,6, G. Berthomieu f, E. Schatzman fY7,I. Carmig, I. Dostrovsky g, C. Bacci h*8,P. Belli h, R. Bernabei h, S. d’Angelo h, L. Paoluzi h, A. Bevilacquaiv9, S. Charbit i, M. Cribier’, L. Gosset i, J. Rich i, M. Spiro’, T. Stolarczyk’, C. Tao iylo, D. Vignaud’, J. Bogerj, R.L. Hahnj, F.X. Hartmann jJ1 , J.K. Rowleyj, R.W. Stoennerj, J. Weneserj a Max-Planck-lnstitut fur Kernphysik (MPIK), Postfach 103980, D-69029 Heidelberg, Germany’ b lnstitut fiir Technische Chemie, Kernforschungszentrum Karlsruhe (KFK), Postfoch 3640, D-76021 Karlsruhe. Germany ’ INFN - Laboratori Nazionali de1 Gran Sass0 (LNGS). S.S. I7/bis Km I8+910, I-67010 L’Aquila, Italy 2 ’ Dipartimento di Fisica, Universita di M&no e INFN - Sezione di Milano, VTaCeloria 16. I-20133 Milano, Italy2 e Physik Department ElS, Technische Universitat Mtinchen (TUM), James-Franck Straje, D-85748 Garching b. Miinchen, Germany f Observatoire de la C&e d’Azur; Departement Cassini, BP 229, 06004 Nice Ceder 4, France s Department of Environmental and Energy Research, The Weizmann Institute of Science (WI), PO. Box 26, 76100 Rehovot, Israel h Dipartimento di Fisica, II Universita di Roma “Tor Vergata” e INFN - Sezione di Roma2, Via della Ricerca Scientihca. i-00133 Roma. Italy2 i CEA, DAPNIA, CE Saclay, F-911 91 Gtf-w--Yvette Cedex, France3 i Brookhaven National Laboratory (BNL), Upton, NY 11973, USA4
Received 5 July 1995 Editor: K. Winter
Abstract
We report the solar neutrino results from the complete set of runs in the exposure period, GALLEX II, from 19 August 1992 - 23 June 1994. Counting for these runs was completed on 10 December 1994. The GALLEX II result (24 runs) is [ 75.2 f 9.7 (stat) ?‘& (syst) ] SNU ( 1 a). After three years of recording the solar neutrino flux with the GALLEX detector, the combined result from the 39 completed solar runs (GALLEX I+II) is [77.1 f 8.5 (stat) ‘-“;f,(syst)] SNU ( 1 a) or 77.1 ?$, SNU with errors combined in quadrature. The combined error (f 13%) has now approached a level where the limits on the derived contribution of 7Be neutrinos to the GALLEX signal confront the predictions of solar models.
0370-2693/95/$9.50
@ 1995 Elsevier Science B.V. All rights reserved
SSDIO370-2693(95)00897-7
238
GALLEX Collaboration/Physics
1. Introduction
The GALLEX detector at the Gran Sasso Underground Laboratories (LNGS) monitors solar neutrinos with energies above the 233 keV threshold for the inverse beta-decay reaction 71Ga (Y, , e- ) Ge. In order to use the 16.5 day mean-life of 71Ge to maximum advantage, a typical solar exposure lasts about one month, followed by a 6 month period of counting of the 7’Ge after its extraction from the gallium target at the end of the exposure. We publish our results at regular intervals, including the data of the most recent runs, which we consider to be preliminary if the samples have not been counted to completion. Final data on these runs routinely appear in the followup publication. In this fourth release of solar data we present the completed data set for GALLEX II covering the 24 runs which were performed in the A tank of the GALLEX installation between 19 August 1992 and 22 June 1994 (GALLEX II was preceded by the GALLEX I series of runs performed in the B tank between 14 May 1991 and 29 April 1992). In addition, we give the results of an analysis of the combined data from GALLEX I and GALLEX II. The development of the GALLEX data base, as reported in previous publications [l-3] and in the present paper, is shown in Table 1 for solar runs (SR) and for short exposure
’ This work has been supported by the German Federal Minister for Research and Technology (BMFT). This work has been generously supported by the ALFRIED KRIJPP von BOHLEN und HALBACH-Foundation, Germany. * This work has been supported by Istituto Nazionale di Fisica Nucleare (INFN), Italy. 3 This work has been supported by the Commissariat a l’bnergie atomique (CEA) , France. 4 This work has been supported by the Office of High Energy and Nuclear Physics of the U.S. Department of Energy, United States. 5 Permanent address: Instytut Fizyki, Uniwersytet Jagiellonski, ul. Reymonta 4, PL-30059 Krakow, Poland. 6 Present address: Max-Planck-Institut ftlr Physik, Fijhringer Ring 6, Postfach 401212, D-80805 Munchen, Germany. 7 Permanent address: DASGAL, Bfrtiment Copemic, Observatoire de Paris, 5 place Jules Janssen, F-92195 Meudon Principal, France. s Permanent address: Dipartimento di Fisica, III Universim di Roma, via C. Segre 2, 1-00100, Roma, Italy. 9 Permanent address: SRS/SAPR, CEN Grenoble, F-38041 Grenoble Cedex, France. to Permanent address: LPC du College de France, 11, Place Marcelin Berthelot, F-75231 Paris Cedex 05, France. ” Present address: Laboratori Nazionali dei Gran Sasso (LNGS)
Letters B 357 f 1995) 237-247
blank runs (BL). The GALLEX II series was ended by the start of the scheduled GALLEX Cr source experiment in which the gallium target was exposed to a strong calibrated man-made neutrino source. The results of this new experiment, which were published recently [41, unequivocally demonstrated the reliability of the GALLEX detector. The previous GALLEX results a) provided the first observation of pp neutrinos from the Sun, with a signal that is large enough to be consistent with a “minimal” solar model which requires only that the present Sun produce all its energy through fusion of hydrogen via the pp and pep (PPI) cycle (about 79 SNU or about 60% of the SSM expectation) ; and b) indicated a deficit of the expected fluxes of the other neutrino branches, in particular of 7Be neutrinos, the second largest expected contributor to the GALLEX detector signal. The significance of this 7Be neutrino problem has evolved only gradually, as the experimental errors on the GALLEX result have decreased. Further elucidation of this important result will require continued error reduction (as discussed in Section 3B). We report the new results in this paper without repeating those details of the experimental and data-evaluation methods that were described before [ l-3,5-7]. However, in addition to including the completed counting data for all GALLEX II runs, we report here on a) additional Ge chemical-yield corrections that were applied to these data, compared to what was done in [ 31, and b) on a new experimental test for side reactions.
2. GALLEX-II runs and results. GALLEX II spans runs A59 to A105, comprising consecutive solar runs SR16-SR39 and Blank runs BL6-BL27. The run characteristics are given in Table 2. The only change in the data sets that were previously given in Table 1 of [ 31 for runs A59-A79 is the addition of the Ge-yield corrections that are discussed next. 2.1.
Germanium
yield determination
Isotopically distinct germanium carriers (70Ge, 72Ge 74Ge, 76Ge) are used alternatingly to monitor the yields of the various steps in desorbing and pro-
GALLEX Collaboration/Physics
239
L.etters B 357 f 1995) 237-247
Table 1 Summary of pubfished GALLEX data. Numbers in parentheses indicate the number of runs in whichthe standard counting period of 6 months had not been completed at the time of the respective data release Date of data release
May 1992 June 1993 February 1994 June 1995
Reference
111 I21 131 this work
Blank runs
Solar runs GALLEX I
GALLEX II
total
result (in SNU)
14 I5 15 15
0 6 15 24
14 21 30 39
83 f 21 87f 16 79 f 12 71 f 10
cessing the germanium [ 1,2] _ The ‘integral yield’ is defined as the ratio of the measured quantity of germanium that is filled into the counter to the amount of germanium that was initially added to the target tank. Supported by independent cross-checks, this yield is primarily determined by (P,V,T) determination of the gaseous germane, GeH4, inserted into the counter. It is these yields that we have routinely reported [ l-31 and used, but we do have the additional possibility to cross-check them after the completion of the 6 month counting period by performing a mass spectrometric analysis of the counter filling. For this, the GeH4 is thermally decomposed to a Ge-metal mirror, dissolved in hydrogen peroxide and analyzed on the Re-filament in a Finnigan MAT/THQ thermion mass spectrometer. The Ge-recovery in this method is quantitative and confirms that the counter filling is preserved intact for > 180 days. Analysis of the mass spectra revealed that the analyzed germanium is typically composed of: a) The carrier isotope with its known satellite abundances (at most 4% per isotope), b) 0.3 f 0.2% carry-over of the carrier isotope used in the preceding run. This value is consistent with the values observed in the GALLEX I runs after scaling for the larger volume of nitrogen gas swept through the target tank in the GALLEX II runs. The correction for this carry-over is not included in Table 2, since it was discussed previously in [ I ] _ c) Small carry-overs from runs before the previous one (“memory”). We have calculated an average correction factor based upon the isotopic analyses of the counting samples from the majority of the runs. The measured integral yields for these runs have thus been multiplied by this correction factor, 0.996. d) Natural germanium impurities in the HCl used in
(9) (2) (4) (0)
5 (1) 11 (2) 19 (4) 27 (0)
the desorption and concentration steps. These impurities are introduced in those runs where either it was necessary to replenish a large quantity of HCI (about 100 kg) in the target tank before the respective run, or where the HCI gas routinely used for column acidification during the run came from almost empty HCl bottles, in which Ge-impurities seem to concentrate. The appropriate correction has been applied to each of these runs individually. We apply the required corrections to the integral yields and list them in Table 2 together with the uncorrected yields for comparison. In the majority of runs (37 of 47)) we have multiplied the integral yield by 0.996 according to item c) above. The other cases are marked * and corrected on the basis of their individual mass spectrum. The mean corrected yield of normal runs is 96.8 f 0.3%. The average loss of 3.2% agrees well with the sum of the 1% loss expected from the desorption and concentration steps and the 2% loss expected from the CC14 extraction, Ge& synthesis and counter filling. All of the runs have either been corrected for carry-over or for the addition of small quantities of natural Ge, excepting run A92 (a blank run, where the value listed in Table 2 was used). Since the overall effect of the corrections on these yields is small, the systematic error introduced in the GALLEX result is also small. 2.2. Experimental test for side reactions We have previously reported on the exclusion of all potential side reactions from radioimpurities in the target solution except for a minor loophole concerning hypothetical transuranium-element contamination which might produce 71Ge through neutrons from spontaneous fission. We have now excluded this pos-
240
GALLEX Collaborabon / Physics Letters B 357 (1995) 237-247
Table 2 Characteristics Type”
of GALLEX
Run #
II runs A 59
Time period
- A 105
Duration [days1
Carrier b
Ge yield c
[%I
Ge yieldd corrected by MS
Position e
label HD2 s
[%I SR 16 BL 6 SR 17 BL 7 SRI8 BL 8 SR 19 BL 9 SR 20 BL 10 SR21 BL II SR22 BL12 SR 23 BL13 SR 24 _h SR 25 BL 14 SR 26 BLl5 SR27 BL16 SR 28 BLIJ SR 29 BL 18 SR 30 BL 19 SR31 BL 20
A 59 A 60 A 61 A 62 A63 A 64 A 65 A 66 A 67 A 68 A69 A70 A71 A72 A 73 A74 A 75 A 76 A 77 A 78 A 19 A80 A81 A82 A 83 A84 A 85 A 86 A 87 A 88 A89 A 90
SR32 BL21 SR 33 BL 22 SR 34 BL 23 SR 35 BL 24 SR 36 BL25 SR 37 BL 26 SR38 BL27 SR 39
A91 A92 A 93 A 94 A 95 A 96 A 97 A 98 A 99 Al00 A 101 A 102 A103 Al04 A 105
For footnotes
19.08.92 - 16.09.92 16.09.92 - 17.09.92 17.09.92 - 14.10.92 14.10.92 - 15.10.92 15.10.92 - 11.11.92 ll.ll.9212.11.92
28.0 1.0 27.0 1.0 27.0 I.0
12.11.92 09.12.92 10.12.92 06.01.93 07.01.93 03.02.93 04.02.93 03.03.93 04.03.93 02.04.93 03.04.93 28.04.93 29.04.93 26.05.93 27.0593 23.06.93 24.06.93 21.07.93 22.07.93 18.08.93 19.08.93 15.09.93 16.09.93 13.10.93
27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 29.0 1.0 25.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0
14.10.93 10.11.93 11.11.93 08.12.93 09.12.93 05.01.94 06.01.94 02.02.94 03.02.94 02.03.94 03.03.94 30.03.94 3 I .03.94 27.04.94 2804.94 25.05.94 2605.94
-
-
-
09.12.92 10.12.92 06.01.93 07.01.93 03.02.93 04.02.93 03.03.93 04.03.93 02.04.93 03.04.93 28.04.93 29.04.93 26.05.93 27.05.93 23.06.93 24.06.93 21.07.93 22.07.93 18.08.93 19.08.93 15.09.93 16.09.93 13.10.93 14.10.93
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-
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08.12.93 09.12.93 05.01.94 06.01.94 02.02.94 03.02.94 02.03.94 03.03.94 30.03.94 3 1.03.94 27.04.94 28.04.94 25.05.94 26.05.94 22.06.94
27.0 1.0 27.0 1.o 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 27.0 1.0 21.0
74 72 76 74 72 76 74 72 76 74 72 70 76 74 72 76 70 74 72 70 76 72 74 70 72 74 70 72 74 70 72 74 70 72 74 70 72 74 70 72 74 70 72 74 70 76 72
95.6 97.0 93.5 91.1 95.9 97.1 96.1 97.4 97.0 94.9
95.3 96.7 93.1 88.2 * 95.6 96.8 95.8 97.0 96.6 94.6
96.3 98.8 97.6 95.2 98.8 99.0 99.2 97.3 99.1 96.0 95.3 94.1 103.3 96.6 96.8 93.8 101.7 97.9 95.3 99.3 99.2 101.2 98.9 104.6 83.4’ 100.4 98.4 96.9 99.7 98.8 95.9 98.6 109.2 95.7 100.0 97.1 92.2
96.0 98.4 97.2 94.9 98.4 98.6 98.8 91.8” 97.8 * 95.7 95.0 93.8 93.9* 96.3 96.5 93.5 98.9 97.5 95.0 98.9 97.0’ 95.5’ 98.5 104.2 83.1 100.0 98.0 93.7” 99.3 98.4 95.6 98.2 98.3 * 95.4 99.6 95.4” 91.9
Counter
a a a a a a P P P P P P P P a a a _ a a P P P P P P P P a a a a P P P P P P P P P P P P P P P
(Si) I14 (Fe)103 (Fe)115 (Fe) 107 (Si) 102 (a)119 (Fe)43 (Si)106 (Fe)39 (Fe)118 (Fe)112 (Si)ll3 (Si)108 (Fe)99 (Fe) 103 (Si)ll4 (Fe) 107 (Fe)115 (%)I19 (Si)lO2 (Si) 106 (Fe)43 (Fe)118 (Fe)39 (Si)ll3 (Fe)112 (Fe)115 (Si)108 (SC) 140 (SC) 139 (SC) 137 (SC)135 (Si) 102 (Si)119 (Fe)43 (Si) 106 (Fe)39 (Fe)118 (Fe)112 (Si)l13 (Si) 108 (Fe)103 (SC)139 (SC) 138 (Sc)136 (SC) 130 (Si)l19
End of counting etiiciencyr
[ %]
L
K
29.2 29.0 28.7 28.6 29.4 28.9 28.5 29.9 28.9 28.0 29.1 30.3 30.2 26.7 29.0 29.2 28.6 27.7 28.9 29.4 29.9 28.5 28.0 28.9 30.3 29.1 28.7 30.2 33.5 32.5 32.2 33.1 29.4 28.9 28.5 29.9 28.9 28.0 29.1 30.3 30.2 29.0 32.5 32.9 32.7 32.6 28.9
31.3 34.4 34.0 33.9 31.4 30.9 33.7 32.0 34.3 33.2 34.4 32.4 32.3 31.5 34.4 31.3 33.9 34.0 30.9 31.4 32.0 33.7 33.2 34.3 32.4 34.4 34.0 32.3 38.8 37.6 37.4 38.4 31.4 30.9 33.7 32.0 34.3 33.2 34.3 32.4 32.3 34.4 37.7 38.2 38.0 37.8 30.9
Counting live time [days]
23.03.93 23.03.93 23.04.93 23.04.93 2 I .05.93 21.05.93 18.06.93 18.06.93 19.07.93 16.07.93 13.08.93 13.08.93 13.09.93 13.09.93 07.10.93 07.10.93 08.1 1.93 _
168 167 127 174 177 I75 171 177 177 177 179 178 182 176 173 176 179
03.12.93 03.12.93 03.01.94 03.01.94 24.01.94 24.0 I .94 25.02.94 25.02.92 25.03.94 25.03.94 22.04.94 22.04.94 21.05.94 21.05.94 18.06.94 18.06.94 01.07.94 22.06.94
178 177 I81 175 179 179 I81 170 181 179 180 178 180 179 181 181 162 158 145 151 161 137 167 167 156 158 170 170 162
06.07.94 13.07.94 18.08.94 25.07.94 23.09.94 23.09.94 09.10.94 09.10.94 19.11.94 19.11.94 10.12.94
GALLEX Collaboration/Physics Letters B 357 (1995) 237-247
241
Table 2 -continued il SR = solar neutrino run, BL = short exposure, blank run. h 70, 72, 74, 76 indicate the use of carrier solutions enriched in 70Ge, 72Ge, 74Ge, 76Ge, respectively. ’ Tank-to-counter yield of Ge-carriers (see text), errors are 3~1.7%. d See text (section 2) for an explanation of Ge yieldscorrectedby massspectrometry. Runs marked * are individually corrected. The effective 7’Ge yields are about 1% smaller for the SR runs, before considering the decay between end of desorption and start of counting. c a = active (NaI) counting position; p = passive counting position [ 21. f Values include rise time cut where applicable. s Counters have either iron or silicon cathode. SC = silicon counter with shaped cathode. h Malfunction during counting (no useful data can be extracted). i Sample partly lost during CC4 extraction.
sibility by establishing an experimental upper limit on the maximal possible level of spontaneous fission. For this purpose we have measured the fission product 133Xe within the entire 101 tonnes of GALLEX target solution. We selected 5.25 day ‘33Xe because it is produced in good yield in the asymmetric fission of heavy-element nuclei (since it lies near the heavy peak of the fission-product mass-distribution). The xenon was desorbed in 20 hour ‘presweeps’ with 80 m3 of nitrogen two days before the regular desorptions of runs A99 and Al01 respectively. (We note that during this procedure, about 20% of the Ge-carrier was also unavoidably desorbed. However, this early removal of Ge did not adversely affect the normal operation of the ensuing solar run, since this Ge, which was separated from the Xe, could subsequently be recovered from the first absorber column and simply added to the remainder of the germanium that was later desorbed from the gallium target.) The desorption of the xenon was monitored with M 0.7 cc of 124Xeor ‘26Xe which was added to the target 6 days before desorption. Xenon was collected in a charcoal cold trap at 123 K after having undergone gas purification (for CO2 removal with dilute KOH) and drying (concentrated H2SO4 scrubbers downstream from the second absorber column) . The xenon fraction was further purified by applying standard rare-gas handling techniques, and its yield was finally determined both volumetrically and mass spectrometrically. The Xe-yields were routinely 85%, so we made no additional attempts to maximize the recovery. The Xe sample then became part of the regular GALLEX Xe-Ge counting gas mixture for proportional counting. It was counted for > 2 months in an active position, just as in a regular solar run. The observation of P--y coincidences in the NaI detector was the sought-after signal for ‘33Xe (P-decay to 133Cs).
No positive ‘33Xe-signal was seen. From this result and the respective deduced detection efficiencies, we conclude that spontaneously fissioning nuclides produce less than 10m4of the measured solar signal [ 81. This test has special relevance in that it involves the whole GALLEX target. Consequently, it also covers contrived scenarios that involve ‘particulate spots’ or ‘wall contaminants’ which could have escaped sampling in regular radiopurity checks. 2.3. Results The total GALLEX II exposure time (excluding the 23 days for blank exposures) amounts to 649 days, twice the 324 days for solar exposures in GALLEX I. Note that only 1 day was lost out of 672 exposure days. The results from the individual runs in GALLEX II for the net solar production rates of 71Ge (based on the counts in the K and L energy and rise-time windows [ 1,2] ) are given in Table 3 after the usual subtraction [ 1,2] for side reactions and background effects (7.4 SNU for GALLEX II). In addition, they are plotted in Fig. 1 together with the earlier data from GALLEX I. For GALLEX II, the total number of observed (L+K) decays of 71Ge due to solar neutrinos is M 115 in 24 runs, or M 4.8 per run. This small number illustrates why the results of individual runs necessarily have little statistical significance, while the combined result of the maximum-likelihood analysis for all 24 GALLEX II runs is statistically significant. The result of GALLEX II is 75.2 f 9.7 (stat.) ?i;k (syst.) SNLJ, in full agreement with the preliminary result of 78 f 13 i 5 SNU given in [ 31, with the statistical error reduced from 17% to 13%. A global maximum-likelihood fit to the data from the full 39 runs gives the result for GALLEX I + II of 77.1 f 8.5 ‘_j SNU. The mean life of the decaying
242
GALLEX Collaboration/Physics
Table 3 Results for individual solar neutrino runs in GALLEX II. All SNU-values shown are net solar production rates of ‘lGe after subtraction of 7.4 SNU for side reactions (see text). The quoted errors are statistical only Run number
K + L result (SNU)
SRI6
AS9
SRI7
A61
SRI8
A63
SRl9
A65
SR20
A67
SR21
A69
SR22
A71
SR23
A73
115 :z
SR24
A75
138 :$
SR25
A77
36 +4x -35
SR26
A79
62 +54 -42
32
f3Y -2s
100
T’,:
58 +YJ -W 70 +;; -42
SR27
A81
$9 +53 -42
SR28
A83
56 +@ -29
SR29
A85
SR30
A87
SR31
A89
SR32
A91
64 +‘% -36 4‘j
+49 -39
71 +‘I’ -37 -9
+39 -26
SR33
A93
39 +4y -35
SR34
A95
52 +‘w -36
SR35
A97
8.5 +52 -41
SR36
A99
SR37
Al01
107 :z
SR38
A103
112:;
SR39
A105
90
51
f5* -43
+63 -50
component deduced from all L + K data is r = 15.1 122: d, in agreement with the known value for 7’Ge of 16.5 days. The summary of the results for GALLEX I and GALLEX II is given in Table 4, and includes not only the combined L + K data but also separate analyses of the L- and K-regions. Note that the ratio of the SNU-values of the L- and K-regions has converged towards the expected value of unity as more data have been obtained. We note here that the latest value from the SAGE gallium experiment in the Baksan Laboratory is 69 f 13 SNU [ 91, in agreement with our result.
Letters B 357 (I 9951237-247
We have investigated the stability of the GALLEX results against changes in our standard cuts for the energy and pulse-shape acceptance windows: a) In one analysis, we have removed the sharp energy cut (a step function) and instead used the known shape of the “Ge K and L spectra to assign weights to the individua1 counts. The result obtained in this way agrees well (within 2%) with our mean result. b) In another analysis, we have also investigated the stability of the GALLEX result with respect to the pulse rise-time criterion, by performing an independent alternative pulse shape analysis, which is based on a model of the energy deposition of ionizing radiation in a gas-filled counter. The parameters of the model, which characterize the microscopic charge deposition structure inside the counter’s active volume in terms of, e.g. the number of primary charge clouds, the energy deposited in each of these clouds and their radial extensions, are least-squares-fitted directly to the recorded transient digitizer pulse. For a counter pulse to be accepted, all of these parameters must fulfill the criteria that characterize Ge-decays. Details of this method are described in [ 101. Utilizing this approach for the full data set yields an overall result in excellent agreement with that derived in our standard rise-time analysis: the difference between the results from the two different methods corresponds to the 2 percent uncertainty we assign to the rise-time cut [ 21. c) Variations in the energy and rise-time/pulseshape cuts were also studied in [ 111 and shown to have little effect on the stability and reliability of the final SNU values. The fact that different pulse shape analyses show such close agreement is indicative of the robustness of the GALLEX result. From the very beginning of the GALLEX solar exposures, we have performed blank runs in order to check the whole experimental procedure. These blanks are in every respect like production runs except that the exposure time is only 1 day, the minimum time needed to perform a run. The blank-run results have consistently been reassuring in that the measured contribution from unknown time-independent sources has always been consistent with zero. Our most recent published figure [ 3 I of - l&7 SNU was based on 19 blanks, with four of them not completely counted (see Table 1). The present updated value is based on 27 completed blanks and yields a result of -5.0 f 5.4 SNU.
GALLEX
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243
Letters B 3S7 (1995) 237-247
- 320
3.0 -
GALLEX II
CALLEX I
h! !,
d L,! A,! 1.2 I,,!‘J “0
1991
rtrcllt - 280
71t IO SNU
8lfl7SNU
,$ In,:
cornbintd
J L
I”~““’ ?.a “II
II
XI
3””
6,
1993
1991
lIIII(I
y*r 1994
“6%
PJL
Fig. I. Results for the 24 solar neutrino runs of GALLEX II (labels 16-39) together with the IS GALLEX I solar runs [ 21 (labels I-15). The left-hand scale is the measured “Ge production rate; the right-hand scale, the net solar neutrino production rate (SNU) after subtraction of side reaction contributions. Error bars am f 1CT,statistical only. The point labelled “combined” applies to the mean global value for the total of all 39 runs. Horizontal bars represent run duration; their asymmetry reflects the “mean age” of the “Ge produced Table 4 Results from GALLEX II (complete) and from GALLEX lfll
combined (in SNU) GALLEX II
GALLEX I + GALLEX II
number of runs
19.08.92 - 22.06.94 649 24
14.05.91 - 22.06.94 973 39
Result, L only (stat. error only) Results, K only (stat. error only) Results, L + K (stat. and syst. error)
67.6 f 14.4 8l.Of 13.1 75.2 f 9.7+‘.’ -4.6
77.6 f 13.1 76.7f 11.1 77.1 f 8.5+_4jf’,
time period exposure days
3. Discussion 3.1. Does the GALLEX result vary over time? We have investigated the question of how well the GALLEX data are described by a production rate that is constant in time, by performing a maximumlikelihood ratio test (see for instance [ 12,131) . This test employs the ratio A = QH)IQIfu): C(H) is the maximized value of the likelihood function for the hypothesis being tested, namely that there is a single true value of the production rate; C( Ho) is its maximum value for the hypothesis that the production rate varies from run to run, evaluated by setting the expectation value for each run in the combined analysis
equal to the value actually measured in that run. For a sufficiently large number of runs (this is certainly the case for 30 or more runs, as has been tested in a Monte Carlo simulation), the quantity -2 In A is distributed like x2 for n degrees of freedom, where n is the difference of free parameters for HO and H, equal to N - 1, where N is the number of runs. In this way we obtain a goodness-of-fit confidence level of 88%, i.e. only in 12% of all cases is a fit better than the actual one to be expected. This result implies that the distribution of results and errors of individual runs in GALLEX agrees with the expectation that they are quasi-normally distributed about a constant production rate. To quantify the statement further that the run data
244
GALLEX Collaboration/Physics
are consistent with a production rate constant in time, we have done two additional maximum-likelihood analyses of all data from GALLEX I and GALLEX II (39 runs). In the first of these, we assumed a periodic variation in time of the production rate about a constant value, Q, and solved the equation: P(t) -a+bsin($(t-to))
(1)
without any restrictions on the amplitude, period and phase. The best fit to the data gave the values, a = 77.0 f 9.0 SNU, b = 10.8 f 12.0 SNU, T = 312 f 79 days, r. = 2.2 f 2.6 days (zero time is May 1, 1991) . The goodness-of-fit confidence level for this model using the likelihood ratio test is 82%. In the second analysis, as an example of a different type of temporal variation, we checked for a possible correlation of the production rate P(t) with the variation of the sunspot number Ns ( t) relative to the mean < Ns> for the GALLEX data-taking period: P(t) -a+b(
Ns(t)--
< Ns >
< Ns >
1
This model has previously been applied to the data of the Womestake detector in Ref. [ 131. Our result for the GALLEX data is a = 78.5 f 9.1 SNIJ for the constant part and b = 6.5 & 17.6 SNU for the variable amplitude, with a goodness-of-fit confidence level of 86%. The results from both of these calculations are consistent with zero variation of the GALLEX production rate with time: ( 1) the calculated time-independent production rate, a, agrees with the present measured GALLEX mean value, (2) the time-dependent amplitude, 6, is consistent, within the errors, with a value of zero, and (3) the goodness-of-fit does not improve by invoking time dependence. 3.2. Relevance of the GALLEX data to the “‘Be neutrino problem”
The standard solar model (SSM) predictions for the gallium detector span a relatively narrow range varying from 113 SNU [ 141 to 122.5 f 14SNU (2~) [15],tol31.5f14SNU (2~) [16].Inthefollowing discussion, in order to explore the range of validity of the conclusions we will draw, we compare our experimental results with values from the two most widely
Letters B 357 (1995) 237-247
quoted SSM’s: Turck-Chieze and Lopes (TCL) [ 151 and Bahcall and Pinsonneault (BP) [ 161. Our present experimental result is 77.1’_\‘, SNU (quoted as 2a errors). Compared to the SSM values, with all of the theoretical values also quoted at the 2a-level (95% confidence statement), this result corresponds to 63 f 18 % and 59 f 16 %, respectively, of the predictions of TCL [ 151 and BP [ 161. These ratios agree almost exactly with what is expected from the minimal solar model. As we will show below, the decrease in the errors (statistical and systematic) of the GALLEX determination of the solar neutrino flux is of marked importance in focusing on the nature of the solar neutrino problem. We should emphasize here that the validation of the GALLEX result by the recently performed “Cr neutrino source experiment [ 41 demonstrates the absence of any unexpected systematic errors and therefore adds confidence in the conclusions that can be drawn from our solar neutrino results. Many recent phenomenological analyses [ 171 have pointed out that, relative to solar model predictions, the flux of ‘Be neutrinos derived from the different solar neutrino detectors appears to be strikingly low, especially when compared to the measured Kamiokande flux of *B neutrinos. This conclusion appears to argue against an “astrophysical solution” to the solar neutrino problem [ 181: even those (non-standard) solar models that are able to reduce the neutrino fluxes from both ‘Be and 8B still cannot come close to reproducing the experimentally derived ‘Be and 8B fluxes. The simple calculations shown below demonstrate the sharp constraints on 7Be production that the present GALLEX results force upon the SSM, and indicate how much tighter these constraints will become as the GALLEX experimental uncertainty is further reduced by the accumulation of additional data. In these calculations, done each time within a particular SSM, we calculate the effect on the signal in gallium (in SNU) due to the reduction of the ‘Be flux by the factor, fh. In doing so, we keep the solar luminosity constant by calculating the fractional increase in the (pp + pep) cycle that is required to offset the reduction in the ‘Be cycle. For the contribution of the 8B neutrino flux we take the value directly measured by Kamiokande [ 191. (We note that we have done similar calculations that use the result from the radiochemical chlorine detector instead of from
GALLEX
Collaboration/Physics
Letters B 357 (1995) 237-247
245
Table 5 Upper limits on the ‘Be neutrino flux suppression values, fee, deduced from GALLEX and Kamiokande. fa is the ratio to the solar model prediction of the *B flux measured by Kamiokande [ 191. (a) is for the present GALLEX 1-w error ( IO SNU) and (b) for the anticipated I -TVerror at the termination of GALLJZX (8 SNU). Solar model
TCL [IS] TCL [IS] (no CNO) BP [16] BP [I61 (no CNO)
fBe
h
0.62 0.62 0.48 0.48
f f f f
0.09 0.09 0.07 0.07
(a)
fBe
for 2u - 20 SNU
for 3~ - 30 SNU
for 2~ - 16 SNU
for 3a - 24 SNU
5 2 5 5
5 5 5 2
<_ 0.05 < 0.25 6 -0.04 5 0.19
5 5 5 5
0.20 0.40 0.09 0.32
Kamiokande. However, the resulting limits are not as stringent as those that we list below in Table 5.) In our first trial, we include the total predicted flux of CNO neutrinos. Comparing the calculated production rate with the GALLEX measurement, including its overall error, then gives a range of allowed values of f&, and more importantly, its maximum allowed value. Examples of these calculations follow. The SNU-value calculated in this way, in the framework of the SSM, is given by the sum of
+ [6.01
(3)
for the TCL model [ 151 and [73.9+5.8(1-f&] + [7.91
+135.8&J
(b)
+[6.7fl.O] (4)
for the BP model [ 161, where the terms in the brackets are respectively: the (pp + pep) value, as the sum of the SSM value plus the increase due to the constant luminosity requirement; the SSM value for 7Be modified by the reduction factor; the number of SNU corresponding to the ‘B flux measured by Kamiokande; and the full SSM value for CNO. By setting this sum equal to the measured GALLEX result, 77.1 SNU, and taking the total 2a error to be f 20 SNU, we find that fk I +0.20 for TCL and fae 5 +0.09 for BP. These resultant values for the ratio of the 7Be signal to the SSM predictions are much smaller than the Kamiokande result for ‘B neutrinos (fa-O.62 f 0.09 relative to the TCL prediction and 0.48 f 0.07 relative to the BP prediction [ 191) . These values are reported in Table 5, as are the corresponding values when the errors are taken to be 3~.
0.59 0.79 0.43 0.65
0.36 0.56 0.23 0.45
A striking result is obtained if we reduce the error on the GALLEX result from the current 2u error of f 20 SNU to the f 16 SNU expected after 4 years of data taking (last columns of Table 5). We find that this reduction in the error translates into a value of fh <+O.OS for TCL and I -0.04 for BP, several times smaller than the present limits. The 20% improvement in statistics makes a drastic difference in setting a stringent upper limit on the 7Be flux and defining the scope of the “7Be neutrino problem”. The potential benefits of even further shrinkage in the GALLEX errors are easily estimated by linear extrapolation of the above results. Reasonable concerns might be based on our taking the CNO neutrino flux at the full SSM value. In a less model dependent approach, we set it instead to zero in our calculations, and raise the pp + pep flux further to maintain the luminosity value. The corresponding results are also reported in Table 5. At 2a, fk is < +0.40 for TCL [ 151 and I f0.32 for BP [ 161. These limits on the 7Be flux are not as severe as the values above, but they involve the extreme assumption of the complete suppression of the CNO flux. We note that the evidence for a 7Be problem has strengthened over time as GALLEX data have accumulated. Our earlier results (see Table I), with their larger error ranges, hinted at but could not precisely define, the scope of the deficit of 7Be neutrinos. The evolution of our understanding of this question is illustrated in Fig. 2. Part (a) compares the SSM predictions of TCL [ 151 and BP [ 161 for gallium, divided into the contributions from the different solar neutrino branches, with the published results of GALLEX during the past four years. Also shown is the expected error from 60 GALLEX runs (assuming
246
GALL.EX Collaboration/
Physics Letters B 357 (1995) 237-247
a) CALLEX 7.C uppcrlimil ..__.__.__._..._._.__________.__~_.~-..~________----..__------_-_ 1
)Pr
1992 1993 1994 1991 ‘1997”
BP
b)
Fig. 2. (a)
TCL
lw)
The GALLEX
39 (this work), corresponding f
results after 14 [I],
21 [2],
30 (31,
and 60 (expected in 1997) solar runs, with their 2u error ranges. The two left columns indicate
the predicted contribution from the pp + pep neutrino branches plus a reduced *B contribution as measured by Kamiokande. The resulting 86-SNU value agrees with the present GALLEX result, within the quoted 2a errors. Little room remains for the contributions from 7Be and CNO neutrinos (see Table 5). In part (b), the calculated SNU values from Eqs. (3) and (4) are plotted vs. the upper limits on the values of fk. We see that as the GALLEX errors become smaller, the maximum allowable values of fBe are drastically reduced. In calculations similar to those above, we can use the GALLEX result to derive an upper limit on the CNO cycle contribution to the energy generation in the Sun. In order to get the most conservative upper limit, it is assumed that the Sun operates only via the PPI and the CNO cycles; the 7Be and ‘B branches are completely suppressed. The result (at the 95% confidence level) is that less than 2.4% of the solar energy is produced by the CNO cycle. This experimental limit is only about a factor of 2 larger than the 1.3% value predicted by standard solar models. In summary, the GALLEX result, with its current 13% error, puts strict limits on the 7Be flux, and also gives us some approximate idea about the CNO flux. It is very clear that, even at this stage of GALLEX data taking, we have an interesting confrontation with the astrophysical model. Collecting additional solar data will tighten these constraints even further.
standard solar model expectations [ 16,151 for gallium, with their respective partitioning due to the different solar neutrino branches. The column on the right (“‘heuristic model”)
contains only the
3.3.
Future
plans
contribution from the pp + pep neutrino branches and a reduced *B contribution as measured by Kamiokande. It is seen to account for the present GALLEX
result, leaving little room within the 2a
error bars for any contributions from ‘Be and CNO neutrinos. (b) The SNU values from Eqs. (3) f&;
and (4).
plotted as functions of
the full SSM contributions of the pp + pep and CNO cycles,
and a reduced contribution from aB (as listed in Table 5 from the Kamiokande GALLEX
result),
experimental
ate assumed. Also shown are the 2u upper limits from (a).
It is obvious that
the evidence for a small value of fne has only gradually evolved as the errors have shrunk in the continuing GALLEX
measurements.
that the mean value does not change appreciably). We note that, while the mean GALLEX SNU values have changed only slightly, their combined 2~ errors have decreased substantially. The general result of our calculations is indicated by the right-hand column (called “heuristic model” in the figure), which contains only
At present, we are taking solar data in the run sequence GALLEX III. This series will last until October 1995, when the second “Cr source will be inserted into the GALLEX tank for a period of 4 months. Afterwards, further solar runs will resume until about the end of 1996, when we will have reached our goal of having done 60 solar runs. The projected total 1 (T error (statistical and systematic) at that time will be 10% (about 8 SNU) .
Acknowledgements
We acknowledge the technical staffs of the participating institutions for their invaluable contributions to the development and subsequent consistently suc-
GALLEX Collaboration /Physics
cessful operation of the GALLEX neutrino detector facility. References [I ] GALLEX Collaboration, I? Anselmann et al., Phys. L&t. B 285 ( 1992) 376. [2] GALLEX Collaboration, P Anselmann et al., Phys. Lett. B 314 (1993) 445. [3] GALLEX Collaboration, P Anselmann et al., Phys. Lett. B 32-l ( 1994) 371. [4] GALLEX Collaboration, P Anselmann et al., Phys. Lett. B 342 (1995) 440. [S] E. Hemich and K.H. Ebert, Angew. Chem., Int. Ed. (En&) 31 (1992) 1283. [6] P. Anselmann and EX. Hartmann, Prog. Part. Nucl. Phys. 32 (1994) 35. [7] T. Kirsten, F.X. Hartmann, R. Wink and l? Anselmann, Nucl. Phys. B (Proc. Suppl.) 35 ( 1994) 418. [8] S. Pezzoni, Ph.D. Thesis, Univ. of Heidelberg, 1995. [9] J.N. Abdurashitov et al., Phys. Lett. B 328 ( 1994) 234; SAGE Collaboration, Contribution to the Glasgow ICHEP Conference, August 1994. [ 1O] M. Altmann, “Pulse. shape analysis by fitting the TDF-pulse”, GALLEX internal note GX-69, March 95. [ I I ] S. Charbit, Ph.D. Thesis, University of Paris VII, July 1994, DAPNIA/SPP 94-25;
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