Last Update: October 2000

GNO (Gallium Neutrino Observatory)

Collaboration:

LNGS, L'Aquila, Milano-Bicocca, Rome Tor Vergata, M. Planck Heidelberg

T.U. Munich

Spokesperson - E. Bellotti (Milano-Bicocca).

General description of the experiment

The activity on solar neutrinos at Gran Sasso laboratory started more than ten years ago with GALLEX, a radiochemical experiment using gallium as target.

After Gallex has completed its program, the above mentioned collaboration proposed a new program of measurement of the solar neutrino interaction over a long period (at least one solar cycle).

The proposed GNO program forecasts three steps with 30, 60 and 100 tons of natural gallium as target. The first step has been approved by the funding authorities of Italy and Germany.

The interesting reaction is:

71Ga (ne,e)71Ge

Natural gallium contains 40% of Ga.

The energy threshold is 233 keV only, well below the maximum energy of the so-called p-p neutrinos (420keV), produced in the basic reaction - fusion of two protons - of the hydrogen cycle. For that reason, the expected rate is modestly dependent on details of solar models.

A single large tank contains about 30 tons of natGa in hydrochloric solution.

Every 4-5 weeks the produced 71Ge atoms (typically less than 10 atoms) and about a milligram of a Ge stable isotope purposely introduced in the solution are extracted by a nitrogen purge.

The extracted Ge is transformed in GeH4 (synthesis) which is inserted , together with old Xenon, in purposely designed proportional counters.

Pulses from counters are processed by an electronic chain consisting in preamplifier, amplifiers (300MHz band width) and fast transient digitizer (5 GHz sampling).

Data are acquired on Alphaserver.

Counting lasts six months, to evaluate the 71Ge genuine signal as well the background.

The measured number of detected 71Ge atoms allows to estimate the interaction rate on Ga, which can be compared with the model predictions.

Major aims of the experiment are:

- the measurement of the interaction rate with an global accuracy of 5 SNU ;
- to investigate possible time variations ( seasonal or longer).

Share of responsibility

* Extraction: LNGS and L'Aquila

* Synthesis: LNGS, L'Aquila and Heidelberg

* Counters: Heidelberg

* Electronics: Milano Bicocca -Heidelberg

* DAQ: Rome - Tor Vergata

* Data analysis: mainly in LNGS, Milano and Heidelberg

* New counters: Munich (also Heidelberg and Milano)

Activity

1997 - Activity was mainly concentrated on the installation of new electronic chain and DAQ system ( the GALLEX electronics and DAQ were designed more than ten years ago) and on a general maintenance of the set-up.

1998 - A new series of " solar runs" started. Installation of the electronics continued, as well minor improvement in the set-up.

1999 - The measurement on solar neutrino as well the other activities continued as in 1998. An X-ray tube was installed to routinely calibrate the counters .

2000 - First data collected in more than 500 days have been published (GNO I results)

The obtained result is :

66± 11 SNU

(1 SNU = 1 interaction/ 10**36 target atoms and second)

to be compared with the model prediction of ~130SNU's.

This result is in good agreement with the previous GALLEX data and the Sage result.

The central value is below the minimum rate of 80 SNU, computed imposing quite extreme values (sometimes also non physical) of some input parameters.

If one conservatively assumes that the observed value is due only to pp neutrinos, no room is left to other components ( the so called 7Be, 8B and CNO neutrinos) of the solar neutrino flux, in contradiction with the observation of a fraction (~45%) of 8B neutrinos by Superkamiokande.

However, owing the 10% error, contributes from other neutrinos cannot be excluded, although not at the level required by the standard solar models.

2001 Program

For the 2001 year, we plan :

- more solar runs and a few "blank" runs
- a new analysis of data based on a complete pulse shape analysis
- to re-evaluate radon background in counters
- to calibrate most of our proportional counters with radioactive Ge isotopes.
- to study the feasibility of a irradiation of the target using a 51Cr source
- to begin a quantitative study of the planned CERN-GS neutrino beam effects
- to continue the development of cryogenic detectors and investigate the feasibility of a new type of hybrid detectors

Publications (2000)

Phys. Rev.Lett..

Invited talk at NU 2000

Communication at ICHEP

Many other presentations specifically devoted to GNO or, more in general, to the problem of solar neutrino physics at International or National Conferences.

Students ( Italian Component of the Collaboration)

Five students ( two in chemistry and three in physics) have done or are working for their Diploma

One student in physics is working for his Ph.D. degree in Italy and second ( Italian ) in Germany.