GNO
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page of the Experiment: http://www.lngs.infn.it/site/exppro/gno/Gno_home.htm
The detection reaction is 71Ga(ne,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:
- 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.
The project for a third exposure of the gallium solution to an intense neutrino source has been presented to the scientific and funding authorities.