SAGE the Argon Neutrino Source Development

All four solar neutrino experiments (Chlorine, SUPERKAMIOKANDE, SAGE, and GALLEX) show that the measured solar neutrino flux at the orbit of the Earth is considerably less than predicted by the Standard Solar Model. Because the reduction of the solar neutrino spectrum is most pronounced at intermediate energies (~1 MeV), new detectors that can measure the neutrino radiation from the Sun in this energy regime are especially needed. Several such detectors are in various stages of development and deployment, such as BOREXINO at Gran Sasso, KAMLAND in Japan, and the iodine detector at Homestake. To be certain of their response to low-energy neutrinos, these detectors must be calibrated by exposure to an intense artificially-produced source of neutrinos of about 1 MeV. Because of the extremely small cross section for neutrino capture, source intensities of about 1 MCi are necessary.

Two possible sources for this purpose are ⁵¹Cr and ³⁷Ar. Sources of ⁵¹Cr of ~1 MCi activity have been used to measure the low-energy neutrino cross section of Ga. An even more useful source is ³⁷Ar, which decays by electron capture and emits a monoenergetic neutrino. ³⁷Ar has several advantages compared to ⁵¹Cr. The neutrino is higher energy (814 keV vs. 747 keV). It has a longer half-life (35 d vs. 28 d). Because the Ar is easy to separate, there are almost no accompanying gamma rays from contamination thus reducing the need for bulky shielding and simplifying source transportation. Finally an Ar source can be very precisely calibrated by counting a small aliquot. Both KAMLAND and the iodine project have expressed an interest to calibrate their detectors with an intense ³⁷Ar source. These experiments are sensitive to Compton scattered electrons and thus require a calibration source, like ³⁷Ar, that has a reduced high-energy photon emission.

The use of ³⁷Ar to calibrate solar neutrino detectors was originally proposed by Haxton in 1988(W. Haxton, Phys. Rev. Lett. 60, 768 (1988).). ³⁷Ar will be produced by fast neutrons using the reaction ⁴⁰40Ca(n,a)³⁷Ar. Previous work(V.N. Gavrin, et al., Preprint INR 777/92 (1992); and J.N. Abdurashitov, et al., to be published in Instruments and Experimental Techniques.) showed that calcium oxide, CaO (2.4 g Ca/cm³) was the most suitable calcium-containing target. A 1-MCi ³⁷Ar source could be produced by placing about 200 kg of Ca in a fast flux reactor. Argon is easily removed from this target by dissolving the CaO in water or acid and flowing a carrier gas (such as He) through the solution. Argon is easily separated from helium with a trap at low temperature and reactive gases, such as hydrogen, can be removed from argon by flowing the gas through a getter at high temperature. After purification, the ³⁷Ar will be encapsulated in a minimal volume as the prototype of a source. The activity of this prototype will be determined by various methods, such as by measuring the rate of decay of a small aliquot of Ar in a proportional counter and by measuring the inner bremsstrahlung activity.

The project is funded by the International Scientific and Technology Center (ISTC) award number 1431.
 
 

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