Search

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.

This paper describes the main features of the proposed low energy solar neutrino detector Borexino, planned to be installed at the Gran Sasso Laboratory. This real time detector is based on a massive, calorimetric, liquid scintillation spectroscopy technique, whose high luminosity is the base for the attempt to achieve a low signal detection threshold. After a description of the main structural components of the detector, of its performances in term of spatial and energy resolution, and of the neutrino reactions occurring in the liquid scintillator, a description of the crucial background topic is given. Finally the main implications of the physics program of the experiment are briefly illustrated.

Silvia BONETTI University and INFN Milan, Via Celoria 16, 20133 Milan, Italy Borexino * , planned to operate in the Laboratori Nazionali del Gran is a future real time detector that, providing high luminosity, radiopurity and low energy threshold, will give unique information solar neutrinos specifically from the 7 Be source . + Borexino Collaboration : C . Ar esella',A . Donati l ,D . Franciotti l ,R . Tartagli l ,M . Deutsch2 ,F . Von Feilitz q ch3 ,S . Schoe ert 3 ,G . Manuzio 4 ,S . Pakvasa P . Trincherini 6 ,G . Bellini S . Bon~tti M . C~mpanella 7 ,M . Giammarchi 7 ,D . Giugni 7 ,P . Inzani 7 ,I . Manno ,E . Meroni F . Ragusa 7 ,G . Ranucci 7 ,p . Ullucci 7 ,T . hovacs 8 ,3 . Mitchell s ,P . Raghavana ,R .S . RaghavanB ,G . Cecchet 9 ,A . De Bari 9 ,A . Perotti 9 ,R . Steinbergl° 1 Laboratori Nazionali del Gran Sasso, 2 Massachusetts Inst . of Technology, 3 Technical Univ . Munich, ~Genova Univ . and INFN, Univ . of Hawaii at Minoa, 6 Euratom ISPRA, 7 M~lano Univ . and INFN, 8 AT&T Bell Lab ., Murray Hill, Pavia Univ . and INFN, ° Drexel Univ . Philadelphia 1 . INTRODUCTION The two experiments running from longtime' with the goal to measure the flux of neutrinos born inside the Sun have emphasized a puzzling situation : the detected flux is lower than the expected one, evaluated on the basis of the SSM, and the indication of a time-variation found by one of them, is not confirmed by the other . A decisive answer is expected from the two "Gallium" experiments 2 in the neutrino energy region . The neutrino high energy region, will be deeply investigated by the SuperKamiokande and SNO experiments, that are now in project . BOREXINO 3 is an experiment that has been conceived to perform a neutrino spectroscopy with the unique feature of a real-time sensitivity to the low energy 7 Be monochromatic solar source (E,=0.860 MeV) . Significative results are expected also in the 8 B region . The detector will provide high luminosity, low detection threshold and very " p-p" " 8 B"