Crane & Halpern detect neutrinos in 1937

There were many efforts to find the neutrino after Pauli proposed it in late 1930s. Though it was hardly a quantitative experiment, what is generally regarded as the first observation of neutrino momentum was made in 1936 by Alexander Leipunski. He measured the distribution of momentum in nuclei that were recoiling from the beta decay of carbon-11 and found more recoil momentum than could be attributed to electrons whose beta-ray spectrum had been previously measured.   Discovery of the neutrino is commonly attributed to Cowan and Reines for their 1956 experiment in which neutrinos incident on protons produced neutrons that underwent detectable capture as well as positrons that annihilated with emission of characteristic gamma rays:   ν + p+  –> n0 + e+ .   This was certainly convincing evidence.

But almost two decades earlier, in 1937, Crane and his postdoctoral associate Jules Halpern had used a cloud chamber to observe, in individual events, not only the recoiling nucleus but also the associated beta particle. They put a chlorine-38 beta source in the chamber and applied a magnetic field so that the beta particle momentum could be determined from the curvature of its track. Although the recoiling nucleus did not leave a track long enough to have a discernable curvature, its motion did generate ionization that they assumed to be proportional to the kinetic energy of recoil motion. To measure that energy, Crane and Halpern shut off the clearing field in the cloud chamber for a fraction of a second before expanding the cloud chamber. This gave the ions time to diffuse several millimeters outward before droplets condensed around them. The well-separated droplets could be counted, thus providing a measure of the kinetic energy of the recoiling nucleus. With this experiment Crane and Halpern became the first to measure the recoil momentum of both charged particles in a given beta decay and to show thereby that the neutrino must carry momentum if energy and momentum are conserved in the decay.

Going further, Crane recognized that it would be good to do an experiment in which a neutrino passing through a target material would produce an element not present in the target.  Since sulfur-35 undergoes beta decay to chlorine-35 with a half-life of 80 days,

                                               35S –> 35Cl + e + ν,

he set out to detect the inverse process by putting a source of neutrinos (1 millicurie of radium) into a 3-pound bag of table salt, waiting 3 months, then testing for the presence of radioactive sulfur.  This established an upper limit of 10-30 cm2 for neutrino capture by chlorine-35.  Crane then described how a modest extension of his experiment could rule out the possibility that capture processes prevent neutrinos from escaping from the sun. That work was submitted for publication in January of 1939.

Almost a decade later, Crane was asked to contribute an article to the upcoming 1948 Reviews of Modern Physics issue that was to be a festschrift for Millikan’s 80th birthday.  Crane chose to write on energy and momentum relations in beta decay and on the search for the neutrino. Comprehensive, broad ranging, and admiringly cited by many, this review article was Crane’s way of closing his involvement with the neutrino problem.
——————————————-
Crane and Halpern: New experimental evidence for the existence of a neutrino. Phys. Rev. 53:789-794, 1938.   And later: Further experiments on the recoil of the nucleus in beta-decay. Phys. Rev. 56:232-237, 1939.
Crane: Energy and momentum relations in beta decay and the search for the neutrino.
Rev. Mod. Phys. 20:278-295, 1948.

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