GNO

Home page of the Experiment: http://www.lngs.infn.it/site/exppro/gno/Gno_home.htm

The detection reaction is 71Ga(n e,e)71Ge, which has a threshold of 233 keV, well below the maximum energy of the pp neutrinos (420 keV). The 71Ge is produced by solar neutrinos inside a 30 tons Ga target in the form of 100 tons of a GaCl3 solution, at a rate of about 0.6 atoms per day. The Ge is chemically extracted by flushing 2000 m3 of N2 during 10 hours every 4 weeks; it is extracted by the N2 outside the solution and concentrated into a few litres of water; it is then converted into GeH4 gas, and introduced in small proportional counters (active volume ~ 1cm3), where the 71Ge e-capture (t=16.5 days) is detected. The solar neutrino interaction rate is finally deduced from the number of 71Ge decays observed. Due to the very low statistics (on the average ~ 5 71Ge atoms are counted in each extraction), the Ge extractions have to be repeated many times in order to reach a low statistical error on the measured rate.

This procedure, apparently simple, requires a lot of care to suppress fake signals in such a way to recognize the few 71Ge events: for instance during the synthesis 'ancient' tritium-free water is used; the synthesis line has to be carefully evacuated to avoid the presence of Radon; the proportional counters have to be shielded against environmental radioactive background and against e.m. noise; the signals from proportional counters have to be selected according to their amplitude and shape in order to suppress background. In general every operation requires extreme cleanliness and continuous monitoring.

GNO is the successor project of GALLEX, which continuously took data with 30 tons of gallium between 1991 and 1997 at LNGS and was checked in performance tests with two man-made MCi 51Cr neutrino sources. After 5 years of data taking (1992-1997) GALLEX measured the neutrino interaction rate with a global uncertainty of 10%; then the collaboration proposed a new program of measurement of the solar neutrino interaction rate on Ga over a long period (at least one solar cycle) with improved systematic (GNO), in order to:

- reach a sensitivity on the neutrino interaction rate of ~5% (half of GALLEX). This requires:

- (i) continuation of solar neutrino observations for several years possibly with increased target mass;
- (ii) decreasing of the systematic error.

- Investigate possible time variations of the signal (seasonal or longer)

Since April 1998 GNO is monitoring the solar neutrino flux with regular 4 week extractions.

We are presently considering a third exposure of the gallium solution to an intense (>2.5 MCi~ 90 PBq) neutrino source. At present we are performing the final design for the source in contact with the Research Institute of Atomic Reactors of Dimitrovgrad (Ulyanovsk region, Russia).