Lightweek (vol.1 Issue 2- Sept.7-14, 2009)

Vol. I

September 7th – 14th, 2009

Issue 2

THE GALLIUM NUTRINO OBSERVATORY – WHAT ARE THEY DOING? [PART-I] A gallium solar neutrino detector is installed at Aboratori Nazionali del Gran Sasso (Abruzzo, Italy) since 1990. It looks for solar neutrinos via the reaction 71Ga (νe, e ) 71Ge, which has a threshold of 233 keV. The detector is sensitive mainly to pp-neutrinos. In the present configuration the target consists of 101 tons solution of GaCl3 in water and HCl, containing 30.3 tons of natural gallium; this amount corresponds to ~ 1029 71Ga nuclei. The solar neutrino interaction rate on 71Ga is deduced from the number of 71 Ge atoms observed. The ©The Online Physics Club

latter are identified through their decay after chemical separation from the target. In short the experimental procedure is the following: 1. The solution is exposed to solar neutrinos for about 4 weeks; at the end of this exposure about 16 71Ge nuclei should be present in the solution if the standard solar model (SSM) is correct and all neutrinos do reach the Earth. 2. The 71Ge , present in the solution as volatile GeCl4, is chemically extracted into water by pumping ~ 3,000 m3 of Nitrogen through the solution. http://theonlinephysicsclub.clubdiscussion.net

3. The extracted 71Ge is converted into GeH4, Germane gas, and introduced into miniaturized proportional counters 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. 71 4. The Ge e-capture 71 (meanlife 16.5 days) Ge (e−, νe ) 71Ga is observed for a period of 6 months, allowing the complete decay of 71Ge and a good determination of the counter background.

5. Data are analyzed to obtain the most probable number of 71 Ge introduced in the counter. Counter backgrounds are minimized by rigorous application of low level technology in counter design and construction. The residual background is mostly rejected through application of amplitude and shape analysis on the recorded ©The Online Physics Club

pulses. 71Ge decays produce pulses corresponding to an energy around 10.4 keV (Kcaptures) or 1.1 keV (Lcaptures); the ionization produced by the decay is point-like, so that the pulses are fast compared with most of the natural radioactivity background. 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. In case of instabilities, the calibrations are performed every month. 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, http://theonlinephysicsclub.clubdiscussion.net

extraction and filling efficiencies) gives the number of 71Ge produced in the solution during the exposure and, therefore, the 71Ge production rate. 6. A correction is made to account for contributions to the observed signal from sources other than solar neutrinos, mainly due to interactions in the solution

generated by high energy muons from cosmic rays and by products due to natural radioactivity. 7. Another correction is made to account for background signals in the counter that can be confused with 71Ge decays. [The total subtraction for points 6 and 7 is of the order of a few percent of the signal.]

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