Homestake gold mine experiment

The pioneering experiment in this direction was performed deep in the Homestake Gold Mine in South Dakota starting in the early 1970's. It was organized by a chemist, Ray Davis. The experiment is deep underground to protect it from high energy particles from outer space called cosmic rays. The detection method was based on the reaction

37Cl + neutrino --> 37Ar + electron.

(Chlorine, Cl has 17 protons while argon, Ar has 18 protons. Thus one neutron got converted into a proton.)

After a few days, the argon decays back to chlorine:

37Ar --> 37Cl + neutrino + antielectron .

The idea is to tell that the reaction happened by seeing the antielectron.

It's not so easy. Neutrios interact so infrequently that you need to have a lot of chlorine, and then expect to make one argon atom per day. Here is a picture of how it works.

This experiment ran for many years. The first results were announced in the early 1970s. The result was that about 1/3 of the expected number of reactions occurred. This suggested three possibilities

  1. The experiment was wrong.
  2. The standard solar model was wrong.
  3. The standard picture of neutrinos was wrong. Electron neutrinos could oscillate to become muon neutrinos, which don't interact with chlorine.
At the time, it seemed that explanations 1 or 2 were most likely.

ASTR 122 course home page

Updated 22 Octobber 2007

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA