Stars as Laboratories for Fundamental Physics - MPP Theory Group

362 Chapter 10
The recognized systematic errors are 1.5% for the extraction efficiency,
3% for the proportional counter efficiency, 3% for the cosmic-ray
background, 5% for the neutron background, and 2% for the
proportional counter background, which amounts to a total of 7%
or 0.18 SNU.
The measured counting rate for the individual runs at Homestake
is shown in Fig. 10.8. The current global best-fit average for the solar
neutrino flux measurement, derived from runs 18−124 is (Lande 1995)
(2.55 ± 0.17 stat ± 0.18 syst ) SNU = (2.55 ± 0.25) SNU (10.12)
where the errors were combined in quadrature. The average argon
production rate per day in the detector is obtained by dividing the
SNUs by 5.31 so that it is found to be 0.48 37 Ar/day. Eq. (10.12) is
to be compared with the predictions from different solar models shown
in Tab. 10.5.
10.3.3 Gallium Detectors (SAGE and GALLEX)
The gallium experiments involve far more complicated chemical extraction
procedures for the neutrino-produced 71 Ge which is not a noble
gas. The Soviet-American (now Russian-American) Gallium Experiment
(SAGE) used at first 27 tons, later 55 tons of metallic gallium
while the European GALLEX collaboration uses 100 tons of an aqueous
gallium chloride solution, corresponding to 30.3 tons of gallium.
The SAGE experiment is located in the Baksan Neutrino Observatory
in Mount Andyrchi, Caucasus Mountains (Russia) while GALLEX is
located in the Gran Sasso tunnel near Rome (Italy). SAGE has been
taking data since January 1990, GALLEX since May 1991. The current
results of SAGE were published by Abdurashitov et al. (1994)
and Gavrin (1995), those of GALLEX by the GALLEX Collaboration
(1994, 1995b) and Kirsten (1995).
The half-life of 71 Ge is 11.43 days so that one needs only about
a three-week exposure, allowing for frequent extractions. SAGE has
already accumulated a total of 21 analyzed runs, GALLEX a total
of 39. In Fig. 10.9 the individual counting rates are shown as well
as the global best-fit averages and the distribution of counting rates
in 25 SNU bins. For the SAGE data, formally negative rates after
background subtraction are forced to zero. In both cases, recognized
background signals of 7−8 SNU were subtracted. The average counting