M. Balata

GALLEX solar neutrino observations: Results for GALLEX IV

Abstract We report the GALLEX solar neutrino results for the measuring period GALLEX III, the period from 12 October 1994-4 October 1995. Counting for these runs was completed on 29 March 1996. The GALLEX III result (14 runs) is [53.9 ± 10.6(stat.) ± 3.1 (syst.)] SNU (1σ). This is 15.8 SNU below but statistically compatible with the new combined result for GALLEX (I+II+III) (53 runs), which is [69.7 ± 6.7(stat.) −4.5 +3.9 (syst.)] SNU (1σ) or (69.7 −8.1 +7.8 ) SNU with errors quadratically added. We also give the preliminary result from our second 51 Cr-source experiment: the measured detector response is 83±10% of expectation. The combined result from both GALLEX 51 Cr-source experiments is 92±8% of expectation.

GNO solar neutrino observations: Results for GNO I

Abstract We report the first GNO solar neutrino results for the measuring period GNO I, solar exposure time May 20, 1998 till January 12, 2000. In the present analysis, counting results for solar runs SR1–SR19 were used till April 4, 2000. With counting completed for all but the last 3 runs (SR17–SR19), the GNO I result is [65.8 ± 10.2 9.6 (stat.) ± 3.4 3.6 (syst.)] SNU (1 σ ) or [65.8 ± 10.7 10.2 (incl. syst.)] SNU (1 σ ) with errors combined. This may be compared to the result for Gallex (I–IV), which is [77.5 ± 7.6 7.8 (incl. syst.)] SNU (1 σ ). A combined result from both GNO I and Gallex (I–IV) together is [74.1 ± 6.7 6.8 (incl. syst.)] SNU (1 σ ).

Solar neutrinos observed by GALLEX at Gran Sasso.

We have measured the rate of production of 71Ge from 71Ga by solar neutrinos. The target consists of 30.3 t of gallium in the form of 8.13 M aqueous gallium chloride solution (101 t), shielded by ≈ 3300 m water equivalent of standard rock in the Gran Sasso Underground Laboratory (Italy). In nearly one year of operation, 14 measurements of the production rate of 71Ge were carried out to give, after corrections for side reactions and other backgrounds, an average value of 83 + 19 (stat.) ± 8 (syst.) SNU (1σ) due to solar neutrinos. This conclusion constitutes the first observation of solar pp neutrinos. Our result is consistent with the presence of the full pp neutrino flux expected according to the “standard solar model” together with a reduced flux of 8B + 7Be neutrinos as observed in the Homestake and Kamiokande experiments. Astrophysical reasons remain as a possible explanation of the solar neutrino problem. On the other hand, if the result is to be interpreted in terms of the MSW effect, it would fix neutrino masses and mixing angles within a very restricted range.

Complete results for five years of GNO solar neutrino observations

Abstract We report the complete GNO solar neutrino results for the measuring periods GNOu2009III, GNOu2009II, and GNOu2009I. The result for GNOu2009III (last 15 solar runs) is [ 54.3 −9.3 +9.9 ( stat ) ± 2.3 ( syst ) ] SNU ( 1 σ ) or [ 54.3 −9.6 +10.2 ( incl. syst ) ] SNU (1σ) with errors combined. The GNO experiment is now terminated after altogether 58 solar exposure runs that were performed between 20 May 1998 and 9 April 2003. The combined result for GNO (Ixa0+xa0IIxa0+xa0III) is [ 62.9 −5.3 +5.5 ( stat ) ± 2.5 ( syst ) ] SNU (1σ) or [ 62.9 −5.9 +6.0 ] SNU (1σ) with errors combined in quadrature. Overall, gallium based solar observations at LNGS (first in GALLEX, later in GNO) lasted from 14 May 1991 through 9 April 2003. The joint result from 123 runs in GNO and GALLEX is [ 69.3 ± 5.5 ( incl. syst ) ] SNU (1σ). The distribution of the individual run results is consistent with the hypothesis of a neutrino flux that is constant in time. Implications from the data in particle- and astrophysics are reiterated.

First results from the 51Cr neutrino source experiment with the GALLEX detector

Abstract 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.

GALLEX results from the first 30 solar neutrino runs

Abstract We report new GALLEX solar neutrino results from 15 runs covering 406 days (live time) within the exposure period 19 August 1992–13 October 1993 (“GALLEX II”). With counting data considered until 4 January 1994, the new result is [78±13 (stat.) ±5 (stat.)] SNU (1σ). It confirms our previous result for the 15 initial runs (“GALLEX I”) of [81±17( stat .)±9( syst .)] SNU. After two years of recording the solar neutrino flux with the GALLEX detector the combined result from 30 solar runs (GALLEX I + GALLEX II) is [79±10( stat .)±6( syst .)] SNU (1 σ ). In addition, 19 “blank” runs gave the expected null result. GALLEX neutrino experiments are continuing.

Final results of the Cr-51 neutrino source experiments in GALLEX

Abstract The radiochemical GALLEX experiment, which has been measuring the solar neutrino flux since May 1991, has performed an investigation with two intense 51 Crneutrino 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 71 Geproduction, divided by the directly determined source strength is R=1.01xa0 +0.12 −0.11 for the first source and R=0.84xa0 +0.12 −0.11 for the second one. The combined value of R for the two source experiments is R=0.93xa0±xa00.08. It shows that the >xa040xa0% 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 10xa0% level.

Implications of the GALLEX determination of the solar neutrino flux

Abstract The GALLEX result 83 ± 19 (stat.) ± 8 (syst.) SNU is two standard deviations below the predictions of stellar model calculations (124–132 SNU). To fit this result together with those of the chlorine and Kamiokande experiments requires severe stretching of solar models but does not rule out such a procedure, leaving the possibility of massless neutrinos. It clearly implies that the pp neutrinos have been detected. The Mikheyev-Smirnov-Wolfenstein (MSW) mechanism provides a good fit, and the GALLEX result fixes the Δm 2 and sin 2 2 θ parameters in two very confined ranges (around Δm 2 = 6 × 10 −6 eV 2 and sin 2 2 θ = 7 × 10 −3 and around Δm 2 = 8 × 10 −6 eV 2 and sin 2 2 θ = 0.6). Explanations of the solar neutrino problems based on the decay or magnetic interactions of neutrinos are disfavoured.

GALLEX solar neutrino observations: Complete results for GALLEX II

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±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.

GALLEX solar neutrino observations. The results from GALLEX I and early results from GALLEX II

Abstract 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.