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We describe the results of the GALLEX solar neutrino experiment after completion of the project. In particular, we summarize the results for GALLEX IV (12 solar runs) and for our 71As - spiking experiments. The integral GALLEX result for all 65 solar runs (GALLEX I–IV) is (78 ± 8) SNU (1σ). This is only slightly more than half of the expected rate. The significance of this deficit is assured. • directly by the results from our two 5Cr neutrino source experiments (at the 10% - level), and • indirectly by means of experiments in which 71Ge is generated within the target solution from beta-decaying 71As (at the percent level). GALLEX at Gran Sasso is now succeeded by Gallium Neutrino Observatory). The GNO status as of October 1998 is reported.

Since the pioneering experiment of R. Davis et al., which started neutrino astronomy by measuring the solar neutrinos via the inverse beta decay reaction on 37Cl, all solar neutrino experiments find a considerably lower flux than expected by standard solar models. This finding is generally called the solar neutrino problem. Many attempts have been made to explain this result by altering the solar models, or assuming different nuclear cross sections for fusion processes assumed to be the energy sources in the sun. There have been performed numerous experiments recently to investigate the different possibilities to explain the solar neutrino problem. These experiments covered solar physics with helioseismology, nuclear cross section measurements, and solar neutrino experiments. Up to now no convincing explanation based on “standard” physics was suggested. However, assuming nonstandard neutrino properties, i.e. neutrino masses and mixing as expected in most extensions of the standard theory of elementary particle physics, natural solutions for the solar neutrino problem can be found. It appears that with this newly invented neutrino astronomy fundamental information on astrophysics as well as elementary particle physics are tested uniquely. In this contribution an attempt is made to review the situation of the neutrino astronomy for solar neutrino spectroscopy and discuss the future prospects in this field.

After 5.5 years of data taking, GALLEX ended its experimental phase. The solar neutrino production rate, 76.4 ± 8 SNU, represents about 55% of the predicted rate. The As tests prove, at the 1% level, the reliability of the technique and the detection of 51Cr neutrino, with the nominal efficiency (93.0±8%), control the response of the detector to neutrinos in the solar energy range.

We searched for the ν j → ν i + e + + e − decay mode of heavy neutrinos in an experiment performed nearby a nuclear power reactor in Bugey (France) using a detector consisting of multiwire proportional chambers. New limits on the neutrino mixing parameter ∥ U e 3 ∥ 2 in the mass range 1 MeV m ν h

The beta decay of Lutetium 177 was investigated for a possible heavy neutrino mixing with a mass of m νh = 17 keV. The beta spectrum was measured with the magnetic spectrometer BILL at the Institute Laue Langevin Grenoble. No evidence for ν-mixing was observed.

The radiochemical GALLEX experiment, which has been measuring the solar neutrino flux since May 1991, has performed an investigation with an intense man-made 51 Cr neutrino source (61.9 ± 1.2 PBq). The source, produced via neutron irradiation of ≈ 36 kg of chromium enriched in 50 Cr, primarily emits 746 keV neutrinos. It was placed for a period of 3.5 months in the reentrant tube in the GALLEX tank, to expose the gallium chloride target to a known neutrino flux. This experiment provides the ratio, R , of the production rate of Cr-produced 71 Ge measured in these source exposures to the rate expected from the known source activity: R = 1.04 ± 0.12. This result not only constitutes the first observation of low-energy neutrinos from a terrestrial source, but also (a) provides an overall check of GALLEX, indicating that there are no significant experimental artifacts or unknown errors at the 10% level that are comparable to the 40% deficit in observed solar neutrino signal, and (b) directly demonstrates for the first time, using a man-made neutrino source, the validity of the basic principles of radiochemical methods used to detect rare events (at the level of 10 atoms or less). Because of the close similarity in neutrino energy spectra from 51 Cr and from the solar 7 Be branch, this source experiment also shows that the gallium detector is sensitive to 7 Be neutrinos with full efficiency.

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±9.7 (stat) −4.6+4.1 (syst)] SNU (1 σ). 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±8.5 (stat) −5.4+4.4 (syst) SNU (1 σ) or 77.1 −10.1+9.6 SNU with errors combined in quadrature. The combined error (± 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.

The first period (GALLEX I) of data taking in the GALLEX solar neutrino experiment has been completed. From 14 May 1991 to 29 April 1992, the experiment observed the solar neutrino flux using neutrino capture by 71 Ga to form 71 Ge. Counting ended on 2 November 1992. The final result from the 15 runs of this periodpis (81±17 [ stat .]±9 [ syst .]) SNU (1 σ ). The first 6 runs of GALLEX II recorded the neutrino signal from 19 August 1992 to 3 February 1993. The preliminary result for this period is (97±23 [ stat .]±7 [ syst .]) SNU (1 σ ). With counting data considered until 29 April 1993, the joint result for all 21 runs is (87±14 [ stat .]±7 [ syst .]) SNU (1 σ ). The present neutrino recording period GALLEX II is continuing with one solar exposure every four weeks.

The radiochemical GALLEX experiment, which has been measuring the solar neutrino flux since May 1991, has performed an investigation with two intense 51Crneutrino sources (> 60 PBq) that were produced in the Siloe nuclear reactor and used at the Gran Sasso National Laboratory, one between June and October 1994, and the second between October 95 and February 96. The ratio, R, of the the neutrino source strength derived from the measured rate of 71Geproduction, divided by the directly determined source strength is R=1.01 +0.12−0.11 for the first source and R=0.84 +0.12−0.11 for the second one. The combined value of R for the two source experiments is R=0.93 ± 0.08. It shows that the > 40 % deficit of solar neutrino flux observed by GALLEX cannot be attributed to experimental artifacts and demonstrates the absence of any significant unexpected systematic errors at the 10 % level.

Previously, it was demonstrated that the GALLEX solar neutrino detector responds properly to low energy neutrinos, by exposing it to two intense 51 Cr-neutrino sources; the recovery yield of the product 71 Ge was reported to be 93%±8%. New experiments, in which known amounts of radioactive 71 As have decayed to 71 Ge in the full-scale gallium detector, strongly support this evidence. In several experiments, the gallium detector has been spiked with ∼10 5 71 As atoms, under varying conditions of how the 71 As was added (either carrier free, or with Ge carrier), how the gallium solution was mixed, and how long the 71 Ge remained in the gallium. 71 As decays by electron capture and positron emission to 71 Ge, with a half life of 2.72 d. Hot atoms are produced by these decay modes with kinematics that mimic solar neutrino capture, although the 51 Cr neutrino source provided a more perfect match. This relative disadvantage is offset by the much better statistics obtainable with the 71 As. In all 71 As experiments, the recovery of 71 Ge from the gallium was 100%, with uncertainties of only ±1%. The combined results from the 51 Cr sources and the 71 As spikes rule out any loss mechanisms for 71 Ge, including hot-atom chemical effects. Chemical processes in the aqueous gallium trichloride - hydrochloric acid solution guarantee that the 71 Ge atoms formed in the GALLEX target will be quickly converted to extractable, volatile GeCl 4 .