The Gallium solar Neutrino Observatory (GNO)  at
Laboratori Nazionali del Gran Sasso detects solar neutrinos via the reaction 71Ga(ne,e-)71Ge,
which has a threshold of 233 keV. The detector is sensitive mainly to pp-neutrinos
(53 % of the interaction rate according
to the standard solar model), with smaller contributions from 7Be n (27%), 8B n (12
%), and CNO n (8%) .
The target consists of 101 tons of a GaCl3 solution in water and HCl, containing
30.3 tons of natural gallium; this amount corresponds to ~1029, 71Ga
nuclei. The solution is contained in a large tank.
71Ge produced by neutrinos is radioactive, and decays back by
electron capture into 71Ga (the reverse process of the solar neutrino
The meanlife of a 71Ge nucleus is about 16 days: thus the 71Ge
accumulates in the solution, reaching equilibrium when the number of 71Ge
atoms produced by neutrino interactions is just the same as the number of the
decaying ones. When this equilibrium condition is reached, about a dozen 71Ge
atoms are present inside the 103 tons gallium chloride solution (containing
~1029 Ga nuclei) .
The solar neutrino flux above threshold is deduced from the number of 71Ge
atoms produced. using the theoretically calculated cross sections .
The 71Ge are identified through their decay after chemical separation
from the target.
In short the experimental procedure for the measurement of the solar neutrino
flux is the following:
The solution is exposed to solar neutrinos
for about 4 weeks; at the end of this time ~10 71Ge nuclei are
present in the solution, due to solar neutrino interactions on 71Ga.
71Ge, present in the
solution as volatile GeCl4, is chemically extracted  into
water by pumping ~ 3,000 m3 of Nitrogen through the solution.
The extracted 71Ge is converted
into GeH4 (Germane gas), and introduced into miniaturized proportional
counters  mixed with Xenon as counting gas. At the
end ~ 95-98% of the 71Ge present in the solution at the time of
the extraction is in the counter; extraction and conversion efficiencies
are under constant control using non radioactive germanium isotopes as carriers.
Decays and interactions in the counter,
including 71Ge e-capture (meanlife 16.5 days) 71Ge(e-,nu)71Ga,
are observed for a period of 6 months, allowing the complete decay of 71Ge
and a good determination of the counter background. The charge pulses produced
in the counters by decays are recorded by means of fast transient digitizer
operating at 0.2 ns/chan for a depth of 400 ns.
Data are analyzed to obtain the most
probable number of 71Ge introduced in the counter. Counter backgrounds
are minimized by rigorous application of low-level-radioactivity technology
in counter design and construction.
The residual background is mostly rejected through application of amplitude
and shape analysis on the recorded pulses. 71Ge decays produce
pulses corresponding to an energy around 10.4 keV ('K peak)' or 1.2
keV ('L peak'); the ionization produced by the decay is point-like, so that
the pulses are fast compared with most of the natural radioactivity background
(producing diffuse ionization and 'slow' pulses).
Counters are calibrated by an external Gd/Ce X-ray source, in order to carefully
define amplitude and pulse shape cuts with known efficiency for each measurement.
The event amplitude and shape selection reduces the mean background rate
to less than 0.1 counts per day.
Normally, each counter is calibrated 5 times
during the 6 month counting time, to check the stability of the gain and of the
The selected data are analyzed with a maximum
likelihood method to obtain the most probable number of 71Ge nuclei
at the beginning of counting, which (after correcting for counting, extraction
and filling efficiencies) gives the number of 71Ge produced in the
solution during the exposure and, therefore, the 71Ge production rate.
A correction is applied to account
for contributions to the observed signal from processes other than solar
neutrino capture, producing 71Ge as well ( the so called "side
reactions"), mainly due to interactions in the solution generated by high
energy muons from cosmic rays and by natural radioactivity. Another correction
is made to account for background signals in the counter that can be misidentified
as 71Ge decays. The total subtraction is tipically of the order
of a few percent of the signal.
Data taking and results
The gallium detector was operated between 1991 and 1997 by the GALLEX collaboration:
65 `solar runs' were performed. The solar neutrino capture rate on 71Ga
was measured with a global uncertainty of 10% as:
GALLEX : 77.5
+/- 6.2 (stat.) +4.3 -4.7 (sys.) SNU 
(1 SNU Solar Neutrino Unit = 10-36 captures per second and per absorber
This result has important physical implications both for astrophysics and for
After maintenance of the chemical plants and renovation of the DAQ and electronics,
a new series of measurements was started in April 1998, within the GNO (Gallium
Neutrino Observatory) project. The experiment is presently running with 30
tons of gallium (GNO30).
Data from the first 43 GNO solar runs (extractions performed from May 1998
until Jan 2002) have been evaluated and results have been presented at the
Neutrino 2002 conference. A total of ~ 200 decaying 71Ge atoms
were identified from the 1240 days of exposure in solar runs
SR1-SR43. The corresponding nu interaction rate is:
GNO : 65.2 +/-
6.4 (stat.) +/- 3.0 (sys.) SNU 
The combined result for GALLEX and GNO (65+35=100 solar runs, corresponding
to 1594+1240=2834 days of live time) is
GNO+GALLEX : 70.8
+/- 4.5 (stat.) +/- 3.8 (sys.) SNU 
1. Monitoring the low energy neutrino flux
The gallium detector at LNGS is, together with SAGE, the only observatory
continously monitoring the low energy neutrino flux.
In the next future only gallium radiochemical experiments will be able to detect
pp neutrinos: also they will be a reference for the low energy solar neutrino
detectors of the 'second generation', which hopefully should start the operations
in the next decade.
GNO is presently monitoring the low energy solar neutrino flux with 30 tons
of gallium. Data are taken with regular 4-week exposure solar runs.
The aim of the experiment is to reach a global uncertainty (statistic+systematic) < 5%
on the measured solar neutrino interaction rate, and to monitor the solar neutrino
flux over a time scale of the order of one solar cycle. This target requires
long and stable measurements, possibly an incresing of the gallium mass, and
a reducton of the systematic error.
2. Decreasing the systematic error
One of the most important goals for GNO is to substantially reduce the systematic
Very important results have been otained; the systematic error of GALLEX (4.5
SNU) was reduced down to 3.0 SNU in GNO through a combination of R&D activities.
- GNO collaboration, "Proposal for a permanent
Gallium Neutrino Observatory (GNO) at LNGS", LNGS annual report 1995; GNO
collaboration, "GNO progress report for 2001', LNGS annual report 2001
; GNO collaboration, Phys. Lett. B490(2000) 15-25
- J.N. Bahcall et al., Astroph. J. 555(2001)
990; S. Turck-Chieze et al. Nucl. Phys. (Proc. Suppl.) 87 (2000) 162-171;
V. Castellani et al.,
Nucl. Phys. (Proc. Suppl.) 70 (1999) 301-304
- J.N. Bahcall, Phys.Rev.C56:3391-3409,1997;
- E. Henrich et al., Proc. IV Int'l Solar
Neutrino Conf., ed. W. Hampel. MPI Kernphysik, Heidelberg (1997) 151-162
- R. Wink et al., Nucl. Inst. and Meth.
A329 (1993) 541
- GALLEX collaboration, Phys. Lett. B447
- T. Kirsten, for the GNO collaboration,
talk at the Neutrino 2002 conference, Muenchen (Germany) May 2002, to be
published on Nucl.
Phys. B (Proc. Suppl.)