GALLEX SOLAR NEUTRINO OBSERVATIONS:

RESULTS FROM THE TOTAL DATA SET

Te chnische Universitiit Mii.nchen, MichaelPhysik Department Altmann E15, D-85747 Garching, Germany and Sonderforschungsbereich 975 "Research in Astra Particle Physics " on behalf of the GALLEX Collaboration*

Abstract: With the successful completion of GALLEX after more than six years of operation and the smooth transition to GNo a milestone in radiochemical solar neutrino recording has been reached. The GALLEX solar neutrino result, 76 ± 8 SNU, being significantly below all solar model predictions, confirms the long standing solar neutrino puzzle and constitutes an ., indication for non-standard neutrino properties. This conclusion is validated by the 51Cr neu- trino source experiments and 71 As doping tests which proved the full efficiency of the GALLEX detector to neutrinos in the solar energy range.

"GALLEX Collaboration: MPIK Heidelberg, FZK Karlsruhe, LNGS Assergi, Univ. e INFN Milano, TU Miinchen, Observatoire de la Cote d'Azur Nice, WI Rehovot, Univ. e INFN Roma II, CE Saclay, BNL Upton. (M. Altmann, R. v. Ammon, S. d'Angelo, C. Bacci, M. Balata, P. Belli, E. Bellotti, R. Bernabei, G. Berthomieu, Boger, I. Carmi, C. Cattadori, 0. Cremonesi, M. Cribier, I. Dostrovsky, K. Ebert, F. v. Feilitssch, N. Ferrari, E. Fiorini, T. Fritsch, L. Gosset, R. Hahn, W. Hampel, J. Handt, F. Hartmann, E. Henrich, G. Reusser, J. Kiko, J. T. Kirsten, M. Laubenstein, R. MoBbauer, L. Pao!U1i, E. Pernicka, J. Rich, J. Rowley, E. Schatsman, M. Spiro, L. Stieglits, R. Stoenner, C. Tao, D. Vignaud, S. Winninger, F. Weirich, J. Weneser, M. Wojcik, L. Zanotti) 345 The G ALLEX Experiment 1 The radiochemical gallium detector GALLEX has been measuring the integral solar neutrino flux from 5/1991 - 1/1997, exploiting the capture reaction 71 Ga + 71 Ge + e-. The energy threshold being only 233 keV, this reaction allows to detect the pp-neutrinos from the initial solar fusion reaction which contribute about 90 3 to the total solarVe neutrino--+ flux. The GALLEX target consists of 30 tons of natural gallium in the form of 101 tons of aqueous GaCla solution, acidified in HCl. It is stored in one of two 70 m3 glass fibre reinforced vinylester resin tanks, lined with PVDF on the inside. In a typical solar neutrino run the target is, after the addition of "" mg1 of a stable germanium carrier1, exposed to the solar neutrino flux for three to four weeks. Then both, the neutrino produced 71Ge and the stable carrier isotope, present in the form of GeCl.1 which is highly volatile in acidic environment, are extracted by purging the tank with nitrogen. The GeCl.1 then is dissolved in water and after some concentration steps, organic extraction and aqueous back-extraction, germane GeH.1 is synthesized, which is a suitable counting gas [12, 15]. Together with old xenon it is filled into a miniaturized low-background proportional counter [17] {counting gas Xe/GeH.1=70/30), inserted in an air-tight, low background shielding tank [13], calibrated [16], and counted for six months in order to allow for a reliable characterization of the counting background. The shielding tank and the data acquisition electronics are installed in a Faraday cage. With an optical fibre cable the electronics is linked to an outside µVAX, which allows for continuous on­ line monitoring of the counting system. Up to 23 counters can be operated simultaneously. In the proportional counter the decay 71 Ge+ e- --+ 71 Ga + is observed. 71Ge decays via electron capture with a half-life of 11.4 days. Thereby Auger electrons and/or X-rays are emitted leading to energy deposits mainly in two regions around 10 keVVe {K peak) and 1 keV (L peak). From a typical solar run we expect to end up with order of 10 neutrino produced 71 Ge atoms, thus, offline data analysis aims at separating these few events from the bulk of recorded back­ ground events. This is achieved by performing cuts on energy and a pulse shape. Then, in a maximum likelihood analysis a time constant background plus a component exponentially decreasing with the known life-time of 71Ge is fitted to the time sequence of the candidate events, yielding the initial 71Ge activity (and therefrom the solar neutrino flux). In GALLEX several different and largely independent analysis methods are employed [5, 1, 14]. Their results are in good agreement which underlines the quality of the GALLEX data set and the trustworthiness of the result.

GALLEX Solar Neutrino Measurements 2 As can be seen in table 1 GALLEX has been continuously operating for solar neutrino recording and large scale detector tests from may 1991 to march 1997, with counting extending till mid 1997. Altogether 65 solar runs and, in addition, 36 minimal exposure blank runs2 have been per­ formed, grouped into four series (GALLEX-1 - GALLEX-IV). The 65 GALLEX solar neutrino

1The carrier allows to determine the extraction and gas synthesis yields and, in addition, serves as ahold back carrier to saturate potential trace impurities in the target solution which otherwise might capture the neutrino­ produced 71 Ge. 'Blank runs are performed to test for unknown spurious contributions to the observed signal which are independent of the exposure time. The blank run result being compatible with zero allows to severely constrain such contributions. 346 Table 1: GALLEX experimental program. It comprises 4 periods of solar neutrino observations (Gallex 1 - Gallex 4), two chromium neutrino source experiments (Source I and Source and the arsenic test. II) date exposure period number of runs result reference 5/91-4/92 Gallex I 15 solar + 5 blank 81 ±17 ± 9SNU [6] 8/92-6/94 Gallex II 24 solar + 22 blank 75 ± 10!� SNU [8] 6/94-10/94 Source I 11 source runs R 1.01 !3::/, = [7] 10/94-10/95 Gallex(51C III r) 14 solar + 4 blank 54 ±11±3SNU [9] Source source runs R [10] 10/95-9/96 II 7 = 0.84 !g:]i 9/96-1/97 Gallex(51 IVCr) 12 solar + 5 blank 117 ± 19 ± 8SNU 1/97-3/97 As-test 4 arsenic runs 1.00 ± 0.03 71 y =

runs correspond to 1593 net days of target exposure.

The combined result of 65 GALLEX solar runs is 6 ± 6.3 SNU. We note, however, all 7 .4 !t� that this number though being derived from the entire data set is not yet to be considered the 'ultimate' GALLEX result. In contrast, it will be subject to a slight revision in the percent range. This is due to two reasons: Firstly, some input parameters for the off-line analysis could only be determined after the solar neutrino program was completed, mainly due to contamination risks. These investigations are just being completed. Hence, during the ongoing GALLEX experimental program not the all (then still unknown) correct numbers but instead some 'best estimates' had to be applied. The most important parameters of this kind are the exact efficiencies of every individual counter, a number which enters linearly in the calculation of the SNU-result. Secondly, during GALLEX-IV problems with the front-end electronics affected the pulse shape recording of low energy L-pulses. Therefore, the SNU-number reported in this article was determined by applying no pulse shape cuts at all for L-pulses in GALLEX-IV, contrary to the previous data taking periods. K-pulses, however, are treated as usual with full pulse shape analysis. Obviously, applying no pulse shape cuts to GALLEX-IV L-pulses worsens the signal­ to-background ratio, resulting in an increase of the statistical error, as can be seen from table 1. The comparably large systematic uncertainty quoted for GALLEX-IV in this table is due to the same reason. Its dominating contribution is caused by the fact that the omission of pulse shape cuts for GALLEX-IV L-events influences the efficiency of the time cut which is applied to the data to suppress events originating from radon decay chains. At present3, however, we are finalizing an improved analysis procedure which allows to utilize the available pulse shape information also for GALLEX-IV L-pulses without introducing a major systematic uncertainty.

Though the results of individual runs due to their extremely weak event statistics have only negligible physical meaning, the graphical representation shown in figure 1 gives a good feeling of what large scatter is typical for an experiment of the GALLEX size, i.e. a thirty ton target. Bearing in mind that the distribution of the 65 single run results is fully compatible with expectation from Monte Carlo simulations, this figure may also help to judge the scatter of the four results from the data taking periods GALLEX-1, -II, -III, and -IV. It is not the results

3May 1998

�47 themselves, but, if anything, the time sequence of high and low results that shows a rather large statistical deviation from the mean.

3.0 320 2.5 I II Ill IV 280 � 5' GALLEX GALLEX GALLEX ..GALLEX 240 Ci!E 2.0 "C 200 � 1.5 160 z 0 ·!!l;:: §.iii 1.0 120 ::::l a:CD �_::ult__ 80 0.5 40 0 c: ---f :5c: 0 0 ·;:: t5 0.0 ... �11i1 CD e ,�-� ::J j � i eo m "Ca.. -0.5 t -40 0z .. �·� ��*�- ' -80 ....CD -1.0 -120 £' j j 1�i en 1991 1992 1993 1994 1995 1996 1997

Figure 1: Individual results of the 65 GALLEX solar exposures. Errors are statistical only. The combined results of the four data taking periods and the overall GALLEX result from 65 runs are run indicated by open and closed circles, respectively.

Without going deeply into the interpretation of the GALLEX solar neutrino result , we just mention that it corresponds to only about 60% of the predictions from solar model calculations. In this context it is worthwhile to stress, however, that even the most 'extremely tuned' low­ flux solar models cannot even marginally accommodate the GALLEX result. Obviously, this substantial discrepancy by itself constitutes an indication for non-standard neutrino properties. This evidence is even more conclusive when considering in addition the results from other solar neutrino experiments. Homestake, Ka.mioka.nde, Superkamioka.nde a.ndSage all did also observe a substantial solar neutrino deficit.

51Cr Neutrino Source Experiments 3 Beyond doubt, however, a far-reaching statement like this must rely on the fa.ct that the ex­ periments work correcly, i.e. that the neutrino deficit is not faked by unidentified major mal­ functions of the detectors. In order to validate this prerequisite by checking the reliability and efficiency of the detector the GALLEX collaboration hasprepared a.n intense ( � 2 MCi) neutrino source by neutron irradiating isotopically enriched Cr [4] . By exposing the gallium target to neutrinos from this source one can compare the rate at which 71 Ge is produced by capture of 51 Cr-neutrinos on 71Ga with the rate expected from the known activity of the source. This allows to test the the efficiency of the entire experimental procedure a.nd, relying on the result from the 71 As-tests detailed below, also the cross section for neutrino capture on 71Ga. As a side remark we note that with 51 Cr-neutrinos not only the transition to the 71 Ge nuclear ground state but also to the first two excited levels at 175 ke V and 500 ke V is being tested. ·" Cr decays by EC a.nd emits neutrinos of energy 750 ke V (decay to V ground state, 90%) a.nd 430 keV (decay to 320 keV excited level, 103). 51

348 The source has been inserted in the central tube of the target tank. Altogether 18 extractions have been performed with the source in place, divided into two series of measurements with the source being re-activated in between. The nominal source activity has been determined by various independent methods like calorimetry, gamma-spectroscopy of the 320 kc V line, and measurement of the 51 V-content, the decay daughter of 51 Cr. For the first and second activation of the chromium nominal source strengths at end of neutron irradiation of Ao 63.4 PBq = :::� and Ao 69.1 PBq, respectively, have been determined. = :�:� These numbers are to be compared to the activities determined fromthe neutrino measurement. Figure 2 shows the individual run results of the 18 source neutrino experiments.

1.0 R OSo ource 1: R 1.01±0.11 < Source 2: 0.84±0.12 0 R =

Combined: = 0.93±0.08

0 0.4

� 0.2

0.0

c 20 1, 0 6:) 80 100 12:1 140 [d) TIME SINCE END OF BOMBARDMENT Figure 2: Individual run results of the GALLEX chromium neutrino source experiments, normalized to the known source activity, a function of time after the end of neutron irradiation of the source. The horizontal bars indicate the durations of the exposures. The exponential curve is not a fit to the as data points but depicts the expectation from the decay of the source. Source I and Source runs are II represented by circles and diamonds, respectively.

An analysis of Source I and Source II yields R = 1.01 and R 0.84 respectively, :8::� = :g:n, where R is the source activity as deduced from the neutrino measurement normalized to the true source activity A0• As the results from both series agree within their statistical uncertainty it is straightforward to perform a combined maximum-likelihood analysis of the entire data set. Such a combined analysis of both series results in R 0.93 0.08, clearly demonstrating the = ± absence of large unknown systematic errors which could account for the observed 40% solar neutrino deficit. In particular, the 51Cr neutrino energies nicely accommodating those from the solar 7Be-branch, the fullefficiency of the radiochemical gallium experiment to 7Be-neutrinos is demonstrated. We just mention, that the these 7Be-neutrinos play a crucial role for explaining the solar neutrino puzzle as it appears in the light of the new results from GALLEX, Sage, and Superkamiokande.

349 71 As experiments 4 The 51 Cr neutrino source experiments, testing the experimental procedure including neutrino all capture, clearly are that kind of verificationexperiment that resembles most closely the steps all taking place for solar neutrino measurements. It must be stressed, however, that though the 51Cr source in GALLEX outperformed the sun by more than a factor 15 after insertion into the target tank, these experiments still were low statistics, involving only several dozens of neutrino produced 71 Ge atoms. For verification of the chemical behaviour of 71 Ge produced at trace levels by a nuclear reaction inside the target solution, however, there is an elegant possibility fora< 33-accuracy test. This large-scale test has been performed by GALLEX the collaboration the very end of the experi­ ment, after the solar neutrino measurements were completed. The goal was to investigate all potential effects of hot chemistry, which might lead to a different chemical behaviour of 71 Ge produced in a nuclear reaction compared to the stable Ge carrier isotope. If such hot chemistry effects existed, they might lead to the capture of the neutrino produced 71Ge into non-volatile compounds, whereas the added stable germanium carrier forms volatile GeCLi, as expected. Thus the determined extraction yield would not apply properly to the neutrino produced 71Ge. A suitable reaction to test on a large scale for such effects is the in-situ production of 71 Ge by ,13-decay of 71 As. A known quantity of 71 As (0(105) atoms) which had been produced at the Heidelberg tandem accelerator, has been added to the tank (t-sample), where it decayed with T 1;2 2.9 d to 71Ge. Four runs have been made under different operating conditions (mixing, carrier= addition, standing time), cf. table 2. For every spike a reference sample (e-sample) was kept aside, making possible to calculate the ratio of t- and e-sample. This ratio does not suffer from most of the systematic uncertainties associated with 71Ge-counting.

Table 2: Experimental conditions and preliminaryresults (ratio t-sample/e-sample) of the 71 As runs. run mixing conditions Ge-carrier standing prel. result [h m3/h] addition time (tank / external) 22 5.5 + 0.17 60 with 19.9d 1.01 ± 0.02 Al As 2 6x 19.9d 1.00 ± 0.01 A 5.5 no Ge-carrier B3-1 x 24 5.5 x after 2.0d 1.00 ± 0.02 x As B3-2 - after 22.0 d 0.99 ± 0.02 x As

In cases a quantitative recovery of 1003 was achieved, demonstrating on < 33-level the all absence of withholding effects even under unfavourable conditions like carrier-free operation·1 [11]. a

Gallium Neutrino Observatory 5 With the 71 As-tests GALLEX has completed its large-scale experimental program. Solar neu­ trino measurements with a gallium target at Gran Sasso, however, have been re-commenced in

· As mentioned before the stable germanium carrier is not only used for determination of the extraction t 1 yield, but also plays the role of an 'insurance' to saturate potential trace impurities which might capture 7 Ge in non-volatile complexes. 350 April 1998 in the frame of the Gallium Neutrino Observatory (GNO) [3]. GNO is designed for long-term operation covering at least one solar cycle. The future of the Sage experiment being unclear to a certain extent, GNO might one day well be the only running experiment which is sensitive to the low energy pp-neutrinos from the initial solar fusion reaction. We should keep in mind that all the currently prepared modern real-time experiments have an energy threshold too high to detect pp-neutrinos which contribute about 903 of the integral solar neutrino flux. The flux of pp-neutrinos which is closely linked to the well known solar luminosity and thus can be predicted quite accurately will be monitored by GNO concurrently to the 7Be- and 8B-neutrino measurements of Borexino, SNO and Superkamiokande. Obviously, to have this number in addition to the individual fluxes in the medium and high energy part of the solar neutrino spectrum is an important issue for judging proposed neutrino mass/mixing scenar­ ios. Toghther with the aim to look for possible time variations and the goal to increase the accuracy of the integral fluxmeasurement to about 53 this important pp-neutrino monitoring constitutes the major motivation for performing a long-term radiochemical gallium experiment.

The time between the end of the GALLEX 71 As-experiments in March 1997 and the first GNO solar run in April 1998 during which the residual 71 Ge from 71 As decay decreased to less than one atom was used to modernize and improve the DAQ-electronics and some other installations at Gran Sasso. In its first phase GNO uses the 30 ton target of GALLEX. However, for a second phase it is planned to increase the target mass to 60 tons and later to 100 tons. Recalling figure 1 it is obvious that an increase of the statistical significance of single runs and small groups of runs is an inevitable prerequisite for any serious analysis of short-period time variations of the integral solar neutrino flux. Singleruns being dominated from statistics rather than systematics, lowering their errors means primarily to increase target mass and counting efficiency. On the other hand, however, increasing the accuracy of the combined result requires a decrease of the systematic uncertainty. In this respect effort is made to improve 1 Ge counting, both by improving the presently used proportional counters, and by investigating7 novel techniques like semiconductor devices and cryogenic detectors [2].

Acknowledgements It is a pleasure to thank the organizers of the workshop fortheir kind invitation. I did appreciate very much the lively,stimulating atmosphere all during the conference. Special thanks to Stefan Schonert for his continuous heroic endeavour to improve my skiing technique. Our contribution to GALLEX and GNo is supported by grants from the german BMBF, the DFG (SFB-375), and the Beschleunigerlaboratorium Garching.

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