Measurement of the response of a Ga solar neutrino experiment to neutrinos from a ${}^{37}\mathrm{Ar}$ source

An intense source of ${}^{37}\mathrm{Ar}$ was produced by the $(n,\alpha )$ reaction on ${}^{40}\mathrm{Ca}$ by irradiating 330 kg of calcium oxide in the fast neutron breeder reactor at Zarechny, Russia. The ${}^{37}\mathrm{Ar}$ was released from the solid target by dissolution in acid, collected from this solution, purified, sealed into a small source, and brought to the Baksan Neutrino Observatory where it was used to irradiate 13 tonnes of gallium metal in the Russian-American gallium solar neutrino experiment SAGE. Ten exposures of the gallium to the source, whose initial strength was $409\pm 2$kCi, were carried out during the period April to September 2004. The ${}^{71}\mathrm{Ge}$ produced by the reaction ${}^{71}\mathrm{Ga}({\nu }_e,e^{-}){}^{71}\mathrm{Ge}$ was extracted, purified, and counted. The measured production rate was $11.0_{-0.9}^{+1.0}\text{(stat)}\pm 0.6\text{(syst)}$ atoms of ${}^{71}\mathrm{Ge}$/d, which is $0.{79}_{-0.10}^{+0.09}$ of the theoretically calculated production rate. When all neutrino source experiments with gallium are considered together, there is an indication the theoretical cross section has been overestimated.

Figure 1

Levels in ${}^{71}\mathrm{Ge}$ that can be reached by capture in ${}^{71}\mathrm{Ga}$ of a neutrino from ${}^{37}\mathrm{Ar}$ or ${}^{51}\mathrm{Cr}$ sources.

Figure 2

${}^{37}\mathrm{Ar}$ decay scheme showing the neutrino energies.

Figure 3

Plan view of the laboratory showing the 10 chemical reactors, irradiation reactor 6 with the adjacent calorimeter, and the pump for transferring Ga among reactors.

Figure 4

Source power measurements at Baksan. The solid curve is a weighted fit of the data to a decaying exponential with the half-life of ${}^{37}\mathrm{Ar}$. In the lower panel the power is normalized to the time shown and the 68% confidence band for the fit is indicated by the dashed lines.

Figure 5

(Upper panel) Energy vs. rise-time histogram of all events after time cuts observed in all 10 exposures during the first 30 d after extraction. The live time is 289.0 d and 472 events are shown. The expected location of the ${}^{71}\mathrm{Ge}$ L and K peaks based on the ${}^{55}\mathrm{Fe}$ calibrations is shown darkened. (Lower panel) The same histogram for the 217 events that occurred during an equal live-time interval beginning at day 100 after extraction. The ${}^{71}\mathrm{Ge}$ has decayed away and is absent. The number of events outside the peaks is about the same in both panels as these are mainly due to background.

Figure 6

(Upper panel) Comparison of measured total production rate for each extraction with predicted rate. (Lower panel) Measured rates from the ${}^{37}\mathrm{Ar}$ source extrapolated back to the start of the first extraction. The combined results for events in the L and K peaks and for all events are shown separately at the right and compared to the predicted rate.

Figure 7

Results of all neutrino source experiments with Ga. The hashed region is the weighted average of all four experiments.