The Neutrino

H. Bethe and R. Peierls

Editor’s Note

Bethe and Peierls again write on the perplexing matter of the neutrino. It seems very unlikely, they note, that this particle would create any detectable ionisation after emerging from a nucleus. Nevertheless, its existence might still be demonstrated experimentally. One way, they suggest, would be to measure the recoil of a nucleus during beta decay. Though this energy in natural beta decays would be too small, it could be much bigger in artificial decays, for example of 13N. If the neutrino hypothesis were correct, its existence would be evident from the missing energy or momentum in individual decays. Although this was not how the neutrino’s existence would ultimately be demonstrated, the suggestion showed that doing so was not considered hopeless.ft　　中文

ALTHOUGH it seems very unlikely that neutrinos, after having been emitted in a nuclear process, give rise to any detectable ionisation1, we would like to point out that it is not impossible in principle to decide experimentally whether they exist.ft　　中文

One possible experiment would be to check the energy balance for the artificial β-decay. Take, for example, the process

B10 + α → N13 + neutron

N13 → C13 + e+ + neutrino.

One can safely assume that if the positive electron is emitted with the greatest possible energy, the kinetic energy of the neutrino will just be zero. The balance of energy in this case will therefore determine the mass of the neutrino. For this purpose one would have to know the mass defects of B10, C13 and the neutron*, the kinetic energy of the α-particles and the neutrons and the upper limit of the spectrum of the emitted positive electrons.ft　　中文

A second way of deciding the question would be to observe the recoil of the nucleus in β-decay. With natural β-rays this is in practice impossible because the recoil energy is too small, but the nuclei involved in artificial β-decay are much lighter. The kinetic energy of recoil of a disintegrating N13 nucleus would be of the order of some hundreds of volts if there were no neutrinos. If the neutrino hypothesis is correct, there would be a defect of momentum which would be uniquely connected with the lack of observable energy in each individual process.ft　　中文

In addition to the nuclear processes mentioned in our previous communication, it may also be expected that a nucleus catches one of its orbital electrons, decreases by one in atomic number, and emits a neutrino. (A corresponding process with increase in atomic number is not possible because of the absence of positive electrons.) This process further limits the possible mass differences between stable neighbouring isobares, and particularly between neutron and proton. If the hydrogen atom is to be stable, we must have (for the masses):

Protom + electron < neutron + neutrino.

ft　　中文

The probability of such a process is less than that of a process involving emission only, the energy of the neutrino being the same. The reason is that the momentum of the electron, which enters in the third power, is about a hundred times smaller. But even for a surplus energy of 105 volts, the life-period of hydrogen would be only 1010 years, which seems incompatible with experimental facts. If therefore the neutrino is not heavier than the electron, the neutron must be at least as heavy as the proton.ft　　中文

(133, 689-690; 1934)

H. Bethe and R. Peierls: Physical Laboratory, University, Manchester, April 1.

Reference:

  1. H. Bethe and R. Peierls, Nature, 133, 532, April 7, 1934.

* The accuracy with which the mass of the neutron can be determined at present is, however, far from being sufficient for this purpose.