Stars as Laboratories for Fundamental Physics - MPP Theory Group

360 Chapter 10
10.3.2 Chlorine Detector (Homestake)
Solar neutrino observations were pioneered by the chlorine detector of
Davis (1964) which is located in the Homestake Gold Mine at Lead,
South Dakota (U.S.A.). The target consists of about 615 tons of perchloroethylene
(C 2 Cl 4 ), a cleaning fluid, from which argon is extracted
every few months by an intricate chemical procedure. Because 37 Ar
has a half-life of 35.0 days the amount of neutrino-produced 37 Ar begins
to saturate after a couple of months whence a longer exposure
time is not warranted. The extracted argon—only a few atoms in a
small amount of carrier gas—is then viewed by a proportional counter
which registers the Auger electrons which are ejected when 37 Ar decays
by electron capture. The introduction in 1970 of an electronic
system which analyzes the pulse rise time greatly enhanced the sensitivity.
Therefore, usually only the results after 1970 are quoted, beginning
with run 18. Except for a period from May 1985 to October
1986 where the experiment was down due to successive electrical
failures of the circulation pumps, data have been taken continuously
since 1967.
The known backgrounds for the experiment are cosmic-ray produced
37 Ar atoms which correspond to (0.29±0.08) SNU, and an average neutron
background corresponding to (0.13 ± 0.13) SNU. Because these
average backgrounds as well as the backgrounds of the proportional
Fig. 10.7. Distribution of the counting rates of 99 runs (18−117). The
black bars represent the data shown in Fig. 10.7, the shaded histogram the
expected distribution (adapted from Lande 1995). The absorption rate in
SNU is obtained by multiplying the argon production rate by 5.31.