M. Spiro

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.

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.

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.

Study of a high intensity 746 keV neutrino source for the calibration of solar neutrino detectors

Abstract We describe the test production of a 0.94 PBq (25 400 Ci) source of 746 keV neutrinos coming from the electron capture decay of 51 Cr. This is the first time that such a source has been produced. It was obtained by neutron activation of 12.5 kg of natural granular chromium at the Siloe reactor at Grenoble. If this nuclear reactor were devoted exclusively to the production of 51 Cr from natural chromium, we calculate that the highest activity that could be produced would be around 29 PBq (0.8 MCi) in 45 days of irradiation, for 125 kg of target. With chromium enriched in 50 Cr, we could get around 40 PBq (1.08 MCi) in an irradiation of 21 days, for 45 kg of target containing 17% 50 Cr. Such sources are planned for use in calibrating forthcoming solar neutrino detectors, especially the gallium detector to be installed in the Gran Sasso Underground Laboratory. To obtain a precision of 10% in such calibrations would require either three sources made from natural chromium (i.e. three reactor irradiations of 45 days each) or two sources made from enriched chromium (two irradiations of 21 days each).

PRODUCTION OF A 62 PBQ 51CR LOW ENERGY NEUTRINO SOURCE FOR GALLEX

Abstract We describe the production of a 62 PBq (62 × 1015 Bq) source of ∼ 750 keV neutrinos coming from the electron capture decay of 51Cr. This is the first time that such a high-intensity, low-energy neutrino source has been produced. The rationale for having such a source is to check the overall procedures of the radiochemical solar neutrino experiment, GALLEX. The source was obtained by neutron activation of 36 kg of enriched chromium at the Siloe reactor at Grenoble. The enriched chromium (containing 38.6% of 50Cr compared to 4.35% for natural chromium) was produced by the Kurchatov Institute in Moscow, in the form of CrO3. It was then electrolyzed in Saclay to obtain chromium metal tubes, which were subsequently broken into coarse chips of typically 1 mm3 volume. The chromium chips were put inside 12 special zircalloy irradiation cells that were then placed around the Siloe reactor core, which had been specially reconfigured for this irradiation. The irradiation lasted for 23.8 days. Then the activated chromium was transferred in a stainless-steel container into a sealed tungsten shield having a wall thickness of 8.5 cm. The 51Cr activity at the end of the irradiation (EOB) has been measured to have a mean value of (62.5 ± 0.4) PBq, using several techniques: by neutronics and gamma scanning in the reactor shortly after EOB, with an ionization chamber, by calorimetry, and, after a considerable decay period, by gamma-ray spectroscopy and measurement of the non-radioactive 51V daughter. The source resided in the center of the GALLEX detector at the Gran Sasso Underground Laboratory between June 23, 1994 and October 10, 1994, irradiating the gallium target with a neutrino intensity well above the solar neutrino background. The results of this full-scale test are in good agreement with the expected efficiency of the entire GALLEX experiment calculated as a product of the known efficiencies of the various parts of the experimental procedure.

New experimental limits on radiative neutrino decay

Abstract We have searched for radiative decays of electron-antineutrinos produced by a nuclear reactor giving photons with wavelenghts in the sensitivity range of a photomultiplier. The absence of signal puts stringent limits on such a decay, and excludes it as an explanation for the solar neutrino problem, even for nearly degenerate masseigenstates, provided that Δm / m > 3 × 10 −7 . No previous limits, including those coming from SN 1987A, could exclude such as explanation for small values of Δm / m .

The central electromagnetic calorimeter of UA1

Abstract We describe the construction, calibration and performance of the central electromagnetic calorimeter of the UA1 experiment at the CERN proton-antiproton collider. The calorimeter is of the lead-scintillator sandwich type. It is 26.4 radiation lengths thick and covers a surface of about 50 m 2 . We estimate the resolution of the calorimeter for electrons of energy greater than 1 GeV to be the sum in quadrature of 15% √E ( E in GeV) and a constant 3%. The first term comes from the inherent resolution of the calorimeter due to sampling fluctuations and photostatistics. The second term comes from uncertainties in the calibration procedure and dominates the resolution for electrons from W and Z 0 decay. The uncertainty in the overall energy scale also reflects the uncertainties in the calibration procedure and is estimated to be 3%.

Limits on neutrino oscillation parameters from the chlorine solar-neutrino experiment

Abstract MSW regeneration of solar ν e in the earth can lead to a seasonal variation in the capture rate in the chlorine solar-neutrino experiment. The absence of such an effect in the data allows us to set a limit on the neutrino oscillation parameters for Δm 2 near 3 × 10 −6 eV 2 . The limit thus obtained is only weakly dependent on solar-model inputs.

The performance of a UV sensitive multiwire proportional chamber filled with TMAE

Abstract We tested a low pressure multiwire proportional chamber filled with TMAE under various conditions. The asymptotic quantum efficiency (infinite thickness) for gaseous TMAE was measured to be 8% at 210 nm. No signal could be extracted from an adsorbed or condensed layer of TMAE. However, we expect that such a device with gaseous TMAE could be very efficient (> 50%) in the 100–180 nm range for detecting UV light, especially for astrophysical applications.