The Gallium solar Neutrino Observatory (GNO) [1] at Laboratori Nazionali del Gran Sasso detects solar neutrinos via the reaction ⁷¹Ga(n_e,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%) [2].

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 capture).
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 [3]. 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 [4] 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 [5] 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 resolution.
       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  [6]

(1 SNU Solar Neutrino Unit = 10-36 captures per second and per absorber nucleus.)
This result has important physical implications both for astrophysics and for particle physics.

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  [7]

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  [7]
 

Future activities

  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 error;
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.
 

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References

1. 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
   hep-ex/0006034.
2. 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
3. J.N. Bahcall, Phys.Rev.C56:3391-3409,1997; hep-ph/9710491
4. E. Henrich et al., Proc. IV Int'l  Solar Neutrino Conf., ed. W. Hampel. MPI Kernphysik, Heidelberg (1997)  151-162
5. R. Wink et al., Nucl. Inst. and Meth. A329 (1993) 541
6. GALLEX collaboration, Phys. Lett. B447 (1999) 127
7. 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.)

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