C. Tao

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.

Pulse shape discrimination and dark matter search with NaI(Tl) scintillator

Abstract An extensive study of NaI(Tl) as a Dark Matter particle detector is presented. Emphasis is put on the response of the detector, both in energy and pulse shape, to all particles interacting in the detector, namely high energy (MeV) photons, low energy photons (X-rays), betas from external radioactivity, and neutrons, which induce nuclear recoils. The initial hope that the shorter decay times of nuclear recoils induced by WIMPs could be statistically separated from Compton interactions is weakened by the fact that low energy X-rays and betas exhibit pulse shapes similar to recoils. As a consequence, any indication of shorter decay time pulses leads to an ambiguous interpretation. Underground data for the WIMP search were obtained in a low activity environment at the Laboratoire Souterrain de Modane (LSM), with a 10 kg crystal having high photoelectron yield and 2 keV energy threshold. The data contain pulse shapes with decay times shorter than for Compton interactions and are not compatible with calibration reference shapes or a mixture of these. The effect is shown to come from a population with decay times even shorter than nuclear recoils but its origin was not identified. The Compton rejection efficiency is limited to factors ranging from 3 at 5 keV to 8 at 20 keV (electron equivalent energy). It is shown that the sensitivity of NaI(Tl) detectors to the cross section of Spin Independent coupling WIMPs is only slightly improved by the pulse shape analysis, while it is mostly determined by the differential energy rate at threshold. The sensitivity to the cross section of Spin Dependent coupling WIMPs is improved by about an order of magnitude by the pulse shape analysis.

Calibration of a Ge crystal with nuclear recoils for the development of a dark matter detector

Abstract The ionization deposited in a Ge crystal by the scattering of ≈ 1 MeV neutrons on Ge nuclei is measured and its lowenergy behavior is investigated down to recoil energies of 3 keV. This calibration study is fundamental for the discrimination of Weakly Interacting Massive Particles (WIMPs) from the radioactive background. Experimental results are compared with theoretical predictions.

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.

On the possibility to measure neutrino magnetic moment down to 10−11μB

Abstract We discuss a possibility to search for a neutrino magnetic moment in the range 10 −10 -10 −11 μ B using a 147 Pm antineutrino source and a large mass high radiopurity NaI(TI) detector deep underground

Implications of the GALLEX results after the Chromium source experiment

Since May 1991, the GALLEX experiment — installed in the Gran Sasso Underground Laboratory — is continuously monitoring the solar neutrino signal by measuring the 71 Ge production rate in a target of 30.3 tons of nat Ga. The measured signal 77.1 ± 8.5 (stat) +4.4 – 5.4 SNU (1 σ ) [1] is well below the Standard Solar Model (SSM) prediction: ⋍ 120–130 SNU. To exclude unknown systematic effects as responsible for the observed deficit, an experiment with a (62.5 ± 0.4) PBq 51 Cr neutrino source has been performed 2. , 3. . The ratio R=0.97 ± 0.11 between measured and expected Cr-produced 71 Ge demonstrates that the deficit cannot be ascribed to unknown experimental systematics. Although astrophysical reasons could still remain as a possible explanation of the “solar neutrino puzzle”, we recall that the GALLEX result, when considered together with the results of the other solar neutrino experiments, would allow to strongly constrain the neutrino masses and mixing angles in the framework of the MSW effect.

Particle Dark Matter search with low activity scintillators

A review updating on particle Dark Matter direct detection by low activity solid scintillators (BPRS collaboration) and by liquid xenon target-detector (DAMA group) is presented.