GNO : Gallium Neutrino Observatory

Collaboration

- Italy: Dip. di Fisica dell' Università di Milano 'La Bicocca', and INFN sez. di Milano, Milano; INFN, Laboratori Nazionali del Gran Sasso (LNGS), Assergi (AQ); Dip. di Fisica, Università di Roma 'Tor Vergata' and INFN sez. di Roma-II, Roma; Dip. di Ingegneria Chimica e Materiali, Università dell'Aquila , L'Aquila
- Germany: Max Planck Institut fuer Kernphysik (MPIK), Heidelberg; Institut fuer Technische Chemie, Forschungszentrum Karlsruhe (Fzk), Karlsruhe; Physik Department E15, Technische Universitaet Muenchen (TUM), Muenchen

Location of the experiment: LNGS for solar neutrino measurements, MPIK Heidelberg for proportional counter construction and absolute calibrations.

National Responsible: E.Bellotti (Mi)

1. Goal of the experiment

The goal of GNO is to monitor the low energy solar neutrino flux with a radiochemical gallium detector underground at Laboratori Nazionali del Gran Sasso. The measurement of the solar neutrino flux and their energy spectrum is crucial both for astrophysics and for particle physics (neutrino flavor oscillations). Gallium-based radiochemical detectors are the only working experiments able to detect the low-energy pp neutrinos, produced in the sun in the basic reaction of the hydrogen chain.

More on the experiment…

2.

Activities during 2001

The following physics items were achieved during 2001:

- 13 solar runs have been successfully performed; the total number of solar runs of GNO is now 43. These runs can be added to the 65 solar runs performed in GALLEX.
- 4 short-exposure runs (blanks) have been successfully performed during 2001; the total number of blanks in GNO is 8. The analysis of blanks is an important tool to check the absence of any signal faking 71Ge from solar neutrinos.
- Upgrade of the electronics from 12 to 16 independent counting lines (analog, digital, and DAQ)
- Development of an advanced pulse shape analysis. Data analysis was carried on up to now with a two parameter pulse shape cut (energy-rise-time). A more sophisticated analysis based on fitting the whole pulse and using a neural network to select 71Ge events was developed; efficiency and systematic was deeply investigated, and the new method will be adopted starting from the next data release.
- Counter calibrations. The most relevant systematic error of the experiment is represented by the uncertainty in the determination of the counting efficiency of the proportional counters. A substantial reduction of this component (from 4% to about 1.5%) was achieved by direct absolute calibration of several proportional counters doped with 69Ge. It is foreseen to calibrate all other used counters in the future.
- A Radon test started in 1999 was successfully completed. The aim of the test was to improve the characterization and understanding of Rn events, and decrease the systematic uncertainty due to the so-called 'Rn cut' applied to the data.
- Data analysis. Data from the first 35 GNO solar runs were analyzed and results were presented at the TAUP 2001 conference. The neutrino interaction rate as measured from the first 35 GNO runs is 67.7 +/- 7.2 (stat) +/- 3.2 (sys) SNU; the global result from the 65 GALLEX + 35 GNO solar runs is 73.3 +/- 4.7 (stat) +/- 4.0 (sys) SNU.
- The systematic error in GNO is significantly reduced with respect to the GALLEX one (+/- 4.5 SNU); therefore a relevant reduction of the global error is expected to come from continuation of the experiment.
- Data were also analyzed in terms of possible seasonal variations; no significant effect was found.

The following achievements have been obtained:

Routine solar and blank runs execution. | 31-12-2001 | 100%
Development of new pulse and data analysis methods. | 30-09-2001 | 100%

Milestones 2002:

Uninterrupted acquisition of solar runs | 31-10-2002
Preliminary work to obtain a 51Cr neutrino source | 31-10-2002
51Cr neutrino source production and Gallium solution irradiation (if possible, within 30/04/2003). | 31-12-2002

3. INFN contribution to the experiment in terms of manpower and financial support

Manpower: 13 researchers (7.5 FTE), and support of the LNGS workshop and chemical labs

Budget for the year 2002:1216,5 kEuro

4.Publications in refereed journals (in 2001):

5. Number of conference talks (in 2001): 6

6. Undergraduate and PhD (italian groups):

Undergraduate students: 3 students completed their 'tesi di laurea' during 2001 (1 at Università degli Studi di Milano, 1 at Phys. Dep. of Università dell'Aquila, 1 at Chem. Dep. of Università dell'Aquila)

7. Leadership roles and primary responsibilities of the experiment

- E. Bellotti - principal investigator
- INFN LNGS and L'Aquila University : Ge Extraction, maintenance and upgrade of the chemical plants;
- INFN Milano and LNGS: hardware and electronics for 71Ge counting;
- INFN Milano: DAQ;
- INFN Roma and LNGS: Data storage and backup, web page;
- INFN Milano and LNGS: Data monitoring and analysis;
- INFN Milano and MPI Heidelberg: Counter absolute calibrations.

8. Innovative instruments

- Development of extremely low radioactivity techniques
- Development of extremely-low-background cryogenic-detectors able to detect a few 71Ge decays. These instruments, under study at TUM Muenchen, could eventually be employed besides the gas proportional counters presently used in GNO, with improved counting efficiency and independent systematic.

9. Competing experiments

Radiochemical gallium detectors are the only one presently able to detect pp solar neutrinos. Next generation real-time experiments able to lower the threshold to cover the pp energy region are still in a R&D phase and are not expected to start observations at least in the next 5 years.

The SAGE experiment, situated in the Baksan mine in Caucasus is the only other radiochemical gallium solar neutrino detector in the world. The gallium in SAGE is in metal form with a mass of about 50 tons.

10. International committee which has reviewed the experiment

The experiment is reviewed by the funding authorities of INFN and periodically by the Scientific Committee of the LNGS.