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, TUM Muenchen: R&D on cryogenic detectors; L’Aquila University: R&D on desorption plant.

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 2002

The following physics items were achieved during 2002:

- 12 solar runs have been successfully performed; the total number of solar runs of GNO is now 54. These runs can be added to the 65 solar runs performed in GALLEX.
- 2 short-exposure runs (blanks) have been successfully performed during 2002; the total number of blanks in GNO is 10. The analysis of blanks is an important tool to check the absence of any signal faking 71Ge from solar neutrinos.
- Application of a new advanced pulse shape analysis studied in detail during 2001. In the past data analysis was carried out 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. Application of this method allowed to reduce the background level by about 10%, and a better better recognition of double pulses.
- 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. In total 15 counters have been calibrated. The remaining few counters will be calibrated in 2003.
- Data analysis. Data from the first 43 GNO solar runs were analyzed and results were presented at the Neutrino 2002 conference. The neutrino interaction rate as measured from the first 43 GNO runs is 65.3 +/- 6.4 (stat) +/- 3.0 (sys) SNU; the global result from the 65 GALLEX + 43 GNO solar runs is 70.8 +/- 4.5 (stat) +/- 3.8 (sys) SNU.
- The systematic error in GNO (+/- 3.0) is significantly reduced with respect to the GALLEX one (+/- 4.5 SNU); therefore a relevant reduction of the global error has been achieved by GNO.
- Data were also analyzed in terms of possible seasonal variations; no significant effect was found.
- We are presently considering a third exposure of the gallium solution to an intense (>2.5 MCi~ 90 PBq) neutrino source. At present we have a design of the source very close to the final one provided by the Research Institute of Atomic Reactors of Dimitrovgrad (Ulyanovsk region, Russia) after strict contacts with the GNO collaboration. Many technical aspects (e.g. absolute calibration of the source, transportation) have been investigated. The source project if finally approoved and funded will allow to check the stability of the experiment and improve our knowledge on the neutrino cross section on gallium.

The following achievements have been obtained:

- Routine solar and blank runs execution.(data from one solar run was lost due high voltage instability) (93%)
- Preliminary work to obtain a 51Cr neutrino source (100%)

Milestones 2003:

Completion of 12 solar runs and of blank runs | 31-12-2003
Absolute calibration of 4 proportional counters | 30-06-2003
Re-calibration of the carrier concentration | 30-09-2003
Definition of the final program for the 51Cr source project | 30-09-2003
Release of updated results on solar neutrinos | 30-09-2003

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

Manpower: 14 researchers (7.4 FTE), and support of the LNGS workshop and chemical labs

Budget for the year 2003: 1.0 % of the CSNII budget

4.Publications in refereed journals (in 2002): 1

5. Number of conference talks (in 2002): 5

6. Undergraduate and PhD (italian groups):

1 students (Milano Bicocca Univ.) is doing his 'tesi di laurea'.

1 PhD student (L’Aquila University) is doing his PhD studies (1st year).

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.
- INFN Milano, LNGS, MPI Heidelberg, TUM Muenchen: 51Cr Source project

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.
- High intensity neutrino source and related technologies

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.

We plan to have a more strict than in the past exchange of information among the two experiments.

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.