Production of a 62 PBq 51Cr low energy neutrino source for GALLEX

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 Siloé 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 Siloé 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.