Lightweek (vol.1 Issue 3- Sept.15-21, 2009)

Vol. I

September 15th – 21st, 2009

Issue 3

THE GALLIUM NUTRINO OBSERVATORY – FROM GALLEX TO GNO [PART-II] Looking back at the latest results of GALLEX

performed in the time period 1991-1997.

GALLEX was operating between 1991 and 1997 with a 30 tons gallium detector at LNGS. In 1997, GALLEX completed its scientific program: the achieved results can be summarized in the following manner:

* The target solution was exposed two times to a very intense (>60 PBq) 51Cr source of monochromatic neutrinos (750 keV, 90%; 430 keV, 10%). The ratio measured signal / expected signal was found to be R=0.93 ± 0.08.

* The measured solar neutrino interaction rate on 71Ga is 77.5 ± 6.2 (stat.) +4.3 −4.7(sys.) SNU 1 (1σ) (1SNU = 10−36 captures per second)

* A test of the systematics on the extraction efficiency was performed at the end of the experiment by introducing in the solution several thousands atoms of radioactive 71As (decaying into 71Ge with a kinematic which closely mimics solar neutrinos); the

This result was obtained after 5 years of measurements (65 ’solar run’ extractions) ©The Online Physics Club

http://theonlinephysicsclub.clubdiscussion.net

result confirms that the 71Ge signal is as expected within 1%. The measured value of the rate is substantially below the predictions of the various standard solar models (about 130 SNU). The results of the two Cr source experiments and of the As experiments validate the GALLEX solar neutrino experiment, and limit any systematic biases to less than a few percent. Given that the solar luminosity is provided by the pp-fusion reaction in the core of the Sun, and the 8B solar neutrino flux is as measured by Superkamiokande, one can conclude from the GALLEX result alone that the 7Be solar neutrino flux is much below what is expected from the standard solar model. The GNO project takes over GALLEX completed its experimental program in March 1997. There was then the question, if to dismantle the detector or to go on with ©The Online Physics Club

the observations, possibly with some improvements in the experimental apparatus, and an enlargement of the gallium mass. The scientific community pushed for a continuation of the observations with the gallium detector at LNGS for a time period of the order of 10 years. The GNO (Gallium Neutrino Observatory) project was then proposed. For an extensive discussion of the over all GNO project see the proposal. The main motivation for GNO is that the only experiments sensitive to the main component of the solar neutrino flux (the p-p neutrinos) which can operate in the next decade are those based on the use of gallium, namely GNO and SAGE. Unfortunately, SAGE suffers of many difficulties related to the present economical and political situation in Russia: thus the gallium detector at Gran Sasso is going to be probably the only low energy solar neutrino detector http://theonlinephysicsclub.clubdiscussion.net

continuously operating in the next years. The GNO targets are the following: 1. Measurement of the interaction rate of low energy solar neutrinos on gallium, with an accuracy of 5 SNU if the central value will be 77 SNU 2. Study of possible time variation of the rate, with a sensitivity of about 15%. Three steps are planned: 1. To start a new series of solar runs with 30 tons of gallium using the same but

renovated experimental set-up of GALLEX for the extraction and synthesis, and a new electronics and DAQ for counting (GNO30); 2. to increase the target mass to about 66 tons, the maximum amount of gallium that can be accommodated in the two available tanks (GNO66); 3. to further increase the mass up to about 100 tons, with the last 35 tons possibly in metallic form (GNO100).

This work is licensed under the Creative Commons Attribution-No Derivative Works 3.0 United States License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nd/3.0/us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

You can download a copy of this and other issues of LIGHTWEEK at http://theonlinephysicsclub.clubdiscussion.net

©The Online Physics Club

http://theonlinephysicsclub.clubdiscussion.net