Stars as Laboratories for Fundamental Physics - MPP Theory Group

394 Chapter 10
yielding about 50 times more light at the relevant energies below about
1 MeV. Naturally, as a target one needs to use an appropriate scintillator
rather than water. The advantage of a lowered threshold is bought
at the price of losing all directional information. However, because of
the good energy resolution the monochromatic beryllium neutrinos at
862 keV should be clearly detectable as a distinct shoulder in the energy
spectrum of the recoil electrons. Optimistically, a scintillation detector
could have a threshold as low as E ν = 250 keV.
The main challenge at implementing this method is to lower the
radioactive contamination of the scintillator, its vessel, and the surrounding
water bath to an unprecedented degree of purity. For example,
the allowed mass fraction of 238 U of the scintillator is less than
about 10 −16 g/g. The feasibility of this method is currently being studied
at the CTF (Counting Test Facility) experiment, located in the
Gran Sasso underground laboratory. Assuming a positive outcome,
BOREXINO would be built, consisting of 300 tons of scintillator, surrounded
by 3000 tons of water. Optimistically, data taking with this
facility could commence in 1997.
10.9.4 Homestake Iodine Detector
Currently, a modular 100-ton iodine detector is under construction in
the Homestake mine. It is similar to the Homestake chlorine detector,
except that it uses the 127 I → 127 Xe transition to measure the ν e flux. It
has an effective threshold of 0.789 MeV, similar to the chlorine detector.
However, for a standard solar neutrino flux the detection rate should
be about four times higher if the cross section calculations are correct.
The currently built detector should take up operation in mid-1995. It
may be expanded at a later time after running experience has been
obtained, and after the neutrino cross sections have been measured
(Bahcall et al. 1995; Engel, Krastev, and Lande 1995).
10.9.5 Summary
The hypothesis of neutrino flavor oscillations is strongly supported by
the results of all existing solar neutrino experiments. With the new
generation of detectors which will begin to take up operation in 1996
it looks plausible that nonstandard neutrino properties can be firmly
established on the basis of the solar neutrino flux before the millenium
ends.