D. VIGNAUD DAPNIA/SPP. CEA/Saclay Gif-sur-Yvette, France

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1 FINAL RESULTS OF THE GALLEX SOLAR NEUTRINO EXPERIMENT D. VIGNAUD DAPNIA/SPP CEA/Saclay Gif-sur-Yvette, France For the GALLEX Collaboration The radiochemical GALLEX experiment is based on the reaction e + Ga! Ge + e,. The detector has been exposed to solar neutrinos for. years, from May 1991 to January The nal results are presented here. From the 6 3- or 4-week solar runs, the solar neutrino production rate has been measured to be 77.6:2 (stat.) +4:3,4:7 (syst.)snu (one SNU is one solar neutrino unit and corresponds to 1,36 capture/atom/second). This value represents a signicant decit compared to the predictions of the standard solar models, which range presently from 12 to 13 SNU. The consequences of this decit being crucial for particle physics and/or astrophysics, it was very important to test the trustworthiness of the experimental techniques. Two experiment using a very intense (more than 6 PBq) articial neutrino source (neutrinos from 1 Cr decay) have been performed. The ratio of the neutrino source strength derived from the measured rate of Ge production to the directly determined source strength is.93.8, showing that the decit observed cannot be attributed to experimental artifacts. Moreover, after the end of the experiment, experiments using about 1 As radioactive atoms ( As decays into Ge) have checked at the 1 % level the recovery of Ge, ruling out hot-atom chemical eects. 1 INTRODUCTION The main objective of the radiochemical gallium experiments is the detection of pp-neutrinos which are produced in the primary pp fusion reaction in the core of the Sun. The GALLEX collaboration 1 has been recording the solar neutrino ux for more than years, using a 3.3 tons gallium target. The nal results are presented here. Results for 6 completed solar runs (3-4 weeks each) are given (section 2), showing a signicant decit (about 4 %) compared to the predictions of standard solar models (SSM) 2;3. The consequences of such a decit are potentially important for stellar models or for neutrino physics beyond the standard model of particle physics. To check the reliability of the detector, two high intensity neutrino sources ( 6 PBq) made of 1 Cr have been built 4 and irradiated the detector for 3-4 months. The number of Ge atoms produced by the 7 kev neutrinos from the source has been compared to the number expected from the measurement of the activity of the source and the agreement isgood (section 3). After the end of the experiment, experiments using about 1 As radioactive atoms ( As decays into Ge) have been performed to check at the 1 % level the recovery of Ge, in order to rule out hot-atom chemical eects (section 4). The time schedule of the GALLEX operations is given in table 1 Table 1: Time schedule of the GALLEX operations. Time period Operations May 91-May92 solar observations (GALLEX I) May 92 - Aug 92 change of the target tank Aug 92 - Jun 94 solar observations (GALLEX II) Jun 94 - Oct 94 1 st source experiment Oct 94 - Sep 9 solar observations (GALLEX III) Sep 9 - Feb 96 2 nd source experiment Feb 96 - Jan 97 solar observations (GALLEX IV) Feb 97 - Apr 97 As tests 2 GALLEX RESULTS FOR SOLAR NEUTRI- NOS The radiochemical GALLEX detector monitors solar neutrinos with energies above 233 kev using the reaction Ga ( e ; e, ) Ge. It is sensitive to the pp which are produced in the primary pp fusion reaction in the core of the Sun, and for which the energy spectrum extends to a maximum value of 42 kev ; but it is also sensitive to 7 Be- and to 8 B-neutrinos ( Be and B ), coming from other nuclear reactions in the pp chain. The radioactive Ge produced decay (with a half-life of days) by electron capture (87.7 %, 1.3 % and 2 % respectively for K, L and M peaks), emitting Auger electrons and X-rays. The principle of the experiment is as follows : expose the gallium to solar neutrinos for 3-4 weeks in a low background environment, extract by a chemical method the Ge atoms which are produced, transform into counting gas GeH 4, ll a proportional counter and count them. 6 runs have been performed. GALLEX is installed in the Gran Sasso Underground 1

2 Laboratory (Italy), 12 km east of Rome. The detector (see gure 1) is a cylindrical tank (8 m high and 3.8 m diameter) containing 3.3 tons of gallium in the form of a solution of GaCl 3 (8.2 M/l of GaCl 3 and 1.9 M/l of HCl). The germanium (either the few Ge atoms produced by neutrinos or the one mg of stable germanium ( 7 Ge, 72 Ge, 74 Ge or 76 Ge) used as carrier and introduced in the tank at the beginning of each exposure) is extracted by circulating a large ow of nitrogen (3 m 3 =h) through the tank for 1 hours. The germanium, in the form of GeCl 4,isvery volatile in presence of HCl ; it is ushed out with the nitrogen and then absorbed into pure water 6. It is then transformed into GeH 4 and put into a small proportional counter (. cm 3 ) made with ultrapure material. In the counter, Ge decay is signed by a count observed in the L (1.2 kev) or K (1.4 kev) peak. The discrimination from the counter background itself (mainly Compton electrons) is done by a pulse shape analysis. The germanium extraction eciency is about 9 % and the counting eciency about 6 %. N 2 N 2 + GeCl 4 1 À,, AC ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáãac ƒáã,@acdfg ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,@ACDFG ƒ ÀÁÃÄÆÇ,QRTUWX, GaCl 3 Figure 1: Scheme of the GALLEX cylindrical tank and of the germanium desorption system. The cylindrical thimble in the middle is used to install the 1 Cr neutrino source. The results from the individual runs for the net solar production rates of Ge are based on the counts in the L and K energy and rise-time windows (see 7;8 for details). They are shown in gure 2 and the corresponding numbers can be found in ref. 7;8;9;1. The main sources of background, i.e. non solar neutrinos sources for producing Ge atoms, are given in table 2 and are subtracted from the result.,@ À,@ À N 2 H O 2 Table 2: Background sources (in SNU) Muon induced background Fast neutrons Ge produced by muons and B and falsely attributed to Ge Rn outside the counters.3.3 Rn-cut ineciency Total to subtract Table 3: Systematic errors in GALLEX Counting eciency including energy and and risetime cuts 4. % Target size and chemical yield 2.2 % 68 +:9 Ge correction error,2:6 % Side reaction subtraction error 2.2 % Total systematic error +:6,6:1 % The combined result of the maximum-likelihood analysis for GALLEX I-IV (6 solar runs) is [ (stat.) 4:7 4:3 (syst.) ] SNU (1). More detailed results for each GALLEX period and for L- and K- peak only are given in table 4. The scatter of single run results (see gure 2) is compatible with a Monte-Carlo generated distribution of single run results for a constant production rate of 77. SNU under the conditions of normal single runs, using the actual conditions of the dierent runs in appropriate proportion (see 1 for the gure). We note however that the results for the four measuring periods show appreciable scatter. GALLEX III is 2.2 below, and GALLEX IV 2.2 above the mean value. The 2 - test, for the result of the four data taking periods to be compatible with the mean, yields only a 1.% probability ( 2 =1.4 with 3 D.O.F.), which is quite low, but nevertheless compatible with a normal distribution. However there is nothing peculiar in this way of grouping the runs. For example, if we divide all 6 solar runs into four equal quarters (subsequent run numbers 1-17, 18-33, 34-49, - 6), the respective probability is 26.7% ( 2 =3.9 with 3 D.O.F.). Other groupings tend also to give probabilities > 1%; in this sense it is just the GALLEX grouping of measuring periods (I-IV) which behaves extreme. The main factors contributing to the systematic error are given in table 3. Details can be found in ref. 1. The result of the 36 blank runs performed during the GALLEX operation (1-day exposure following a solar run) is (stat.+syst.) (1) SNU, consistent with anull result, as expected. We note here that the latest value from the SAGE gallium experiment is :8 SNU 11,8:1, in good agreement with our result. The total number of observed decays of Ge due to solar neutrinos is less than per run (about 3 2

3 3 77. ± 8 SNU 2 number of SNU 1 <I> <II> <III> <IV> -1 May-91 Nov-91 May-92 Nov-92 May-93 Nov-93 May-94 Nov-94 May-9 Nov-9 May-96 Nov-96 Date of the exposure Figure 2: Results of the 6 GALLEX solar runs, together with the combined value. Errors bars are 1, statistical only. The upper band around SNU represents the predictions of the standard solar models. Table 4: Results from solar exposure periods. a : statistical error only; b : statistical and systematic error ; c : statistical and systematic error combined in quadrature. GALLEX I GALLEX II GALLEX III GALLEX IV GALLEX I-IV Nb of days Nb of runs L only (SNU) a K only (SNU) a L+K (SNU) b :2 +6:8,9: 7.9 9:7 +4:2,4: :6 3: :8 6: :2 +4:3,4:7 L+K (SNU) c :,19: :,1: :6,7:8 Ge counts have been observed). It corresponds to a production rate by solar neutrinos of.7 Ge per day. Our value, about 6 % of the predictions of SSM which presently range from 12 to 13 SNU 2;3, is signicantly smaller than the model predictions. This puts strict limits on the 7 Be-neutrino ux, as has been already discussed (see for example ref. 8 ). The consequences of such a decit are important for stellar models and/or for neutrino physics beyond the standard model of particle physics (it could imply neutrino masses through the MSW mechanism and for a recent review, see 12 ). To check the reliability of the detector, it has been exposed to a high intensity articial neutrino source. 3 THE Cr-SOURCE EXPERIMENTS 1 Cr has been chosen as the most convenient neutrino source. It is produced by neutron capture on Cr and decays by electron capture with an half-life of days. 9 % of the decays correspond to the emission of a 7 kev neutrino, and 1 % to the emission of a 43 kev neutrino plus a 32 kev gamma. 36 kg of chromium enriched to about 4 % in Cr were irradiated for more than three weeks in the core of the Siloé reactor, in Grenoble. The source itself was formed by the irradiated chromium surrounded by gamma shielding made of tungsten (8. cm thick). Two sources have been made, one in June 94 and the other in September 9. Dierent methods have been used to measure their activity, all based on the measurement of the 32 kev gamma. The resulting mean activity of the 1 Cr sources at the end of bombardment by neutrons in Siloé, more than 6 PBq (1 PBq corresponds to 1 1 /s), is given in table. These are the strongest neutrino sources ever produced by man. See ref. 4 for details on the production of the sources. The sources were installed for 3-4 months in the reentrant tube in the center of the GALLEX detector (see gure 1). The experimental conditions for the runs performed were kept as close as possible as for the solar runs. The individual results can be found in reference and 3

4 are plotted in gure 3. Production rate Ge/d 2 1 Prediction for 63.4 PBq 1 Sun + Side reactions Time since EOB (days) 2 Table : Characteristics of the two source experiments. The combined value for the ratio of the activity deduced from the Ge measurement and of the activity directly measured, R, is :93 :8. First source Second source Irradiation period June 94 Sept. 9 Duration 23.8 d 26. d Start of exposure June 23, 94 Oct. 1, 9 End of exposure Oct., 94 Feb. 14, 96 Nb of extractions 11 7 End of counting May 2, 9 Sept. 17, 96 Activity directly 63:4 +1:1,1:6 69:1 +3:3,2:1 measured (PBq) Activity deduced 64: +7:3,6:9 7:9 +7:6,7:2 from Ge (PBq) Ratio R 1:1 +:12,:11 :84 +:12,:11 Production rate Ge/d 1 1 Sun + Side reactions Prediction for 69.1 PBq Time since EOB (days) Figure 3: Number of Ge atoms produced per day a) during the rst source experiment, b) during the second source experiment. The points for each run are plotted at the beginning of each exposure (the horizontal lines show the duration of the exposures). The predicted curve (dotted line) decreases with the known halflife of 1 Cr. Vertical bars are statistical errors. The horizontal line (small dots) corresponds to the constant production rate due to solar neutrinos and side reactions. The main characteristics and the results of the two sources are summarized in table. The agreement between the predicted and the measured signal is less satisfactory for the second source experiment than for the rst, being 1.7 from the rst Cr result. Possible systematic eects have been considered, but no indications have been found of any experimental eects that might lower the results. A combined analysis of the two source experiments has been performed and gives R = This shows that the decit of solar neutrino ux observed by GALLEX cannot be attributed to experimental artifacts and demonstrates the absence of any unexpected systematic errors. More details and discussion can be found in ref.. The validation of the GALLEX result by the 1 Cr neutrino source experiment adds condence in the conclusions that can be drawn from the solar neut- rino results. 4 VERIFICATION TESTS WITH Ge PRO- DUCED IN-SITU FROM -DECAY OF As A complementary approach to the articial neutrino source is to add a known amount of As activity to the target solution where it decays with 2.72-day half-life into Ge. The test consists of comparing the amountof Ge that is recovered from the solution to the known amount of Ge that was formed in situ by As decay, under variable conditions of extraction, standing time, stable germanium carrier addition and mixing. Two samples of As (A and B) were prepared at the Heidelberg Tandem Accelerator and transported at the Gran Sasso and 4 experiments have been performed. The arsenic solutions were split in three parts, one to measure the activity by-ray spectroscopy, the second one to put in the tank (about 1 atoms) and the third one to measure the Ge activity after Ge-synthesis and counting, but by-passing the target tank. In the rst experiment, the carrier and the arsenic were mixed together with much stronger conditions than for solar runs ; the second experiment was carrier-free and with normal mixing ; the third and the fourth experiments had normal carrier and normal mixing ; they used the same As spike, but there was a rst Ge extraction after 3 days (short exposure), and a second one 22 days later (long exposure) The results are summarized in table 6. They are all consistent with full recovery (1 %) and the mean value of the four experiments is (99.8.8) %. The sensitivity of the As radioactive labeling used to trace potential losses in the GALLEX procedures is then better than one percent. No indications exist for 4

5 Table 6: Results of the As experiments. As preparation A (January, 17, 1997) B (March, 3, 1997) As experiment tank residence time long long short long description of addition of mixture carrier free short exp. long exp. the experiment of As and Ge-carrier (3 d) (22 d) recovery (in %) any trapping mechanisms, either of the hot or cold chemistry variety, even under the most critical of conditions. All Ge atoms formed in the GALLEX acidic target solution end up as volatile extractable GeCl 4. CONCLUSION The nal GALLEX result is 77: 8 SNU, only about 6 % of the predictions of the standard solar models. The decit has been validated by the two Cr-source experiments which nd a ratio of the production rate of chromium-produced Ge to the rate expected from the known source activity equal to.93.8 and by the tests with Arsenic 13 which demonstrate the absence of any unexpected systematic error at the 1 % level. The result by itself shows that GALLEX has observed a ux of solar neutrinos in sucient quantity to account for the solar luminosity as reected in the ux of low-energy pp-neutrinos from the primary hydrogen fusion reaction in the solar core. But the present result, with its current 1 % error, puts strict limits on the 7 Be ux, and it is very clear that we have already an interesting confrontation with the astrophysical model. An appealing solution to the observed decit is neutrino oscillation via the MSW mechanism. GALLEX counting stopped in June After a major overhaul and modernisation of the experimental set-up, the GNO (Gallium Neutrino Observatory) experiment, the successor project to GALLEX, started in April 1998, with 3 tons of gallium ; the extension to 1 tons is still under discussion. Its main purpose is the monitoring of the solar neutrino ux (mainly the pp one) during a 11-year solar cycle. It is a pleasure to thank M. Cribier, J. Rich and M. Spiro for comments and discussions. 3. S. Turck-Chièze and I. Lopes, Ap. J. 48 (1993) 347 ; H. Dzitko et al., Ap. J. 447 (199) 428 ; S. Brun et al., Ap. J., October M. Cribier et al., Nucl. Instr. Meth. A378 (1996) GALLEX Collaboration, P. Anselmann et al., Phys. Lett. B342 (199) 44 ; W. Hampel et al., Phys. Lett. B42 (1998) E.Henrich and K.H.Ebert, Angew.Chem., Int. Ed. (engl.) 31 (1992) GALLEX Collaboration, P. Anselmann et al., Phys. Lett. B28 (1992) 376 ; Phys. Lett. B314 (1993) 44 ; Phys. Lett. B327 (1994) GALLEX Collaboration, P. Anselmann et al., Phys. Lett. B37 (199) GALLEX Collaboration, W. Hampel et al., Phys. Lett. B388 (1996) GALLEX Collaboration, W. Hampel et al., submitted to Phys. Lett. B (October 1998). 11. J. N. Abdurashitov et al., Phys. Lett. B328 (1994) 234; V. N. Gavrin, SAGE Collaboration, talk given at '98, Takayama, June J. N. Bahcall, P. I. Krastev and A. Yu. Smirnov, Where do we stand with solar neutrino oscillations, preprint hep-ph/987216, 2 July GALLEX Collaboration, W. Hampel et al., Veri- cation tests of the GALLEX solar neutrino detector with Ge produced in-situ from the -decay of As, Phys. Lett. B (in press). 14. E. Bellotti et al., GNO proposal, LNGS Report INFN/AE-96/27, also available at 1. GALLEX collaboration : Heidelberg, Karlsruhe, Munich, Milan, Rome, Nice, Saclay, Rehovot, Brookhaven. 2. J. N. Bahcall and M. H. Pinsonneault, Rev. Mod. Phys. 67 (199) 781 ; J. N. Bahcall, S. Basu and M. H. Pinsonneault, Phys. Lett. B433 (1998) 1.