A Suggested Explanation of β-ray Activity

M. N. Saha and D. S. Kothari

Editor’s Note

Radioactive beta decay had proven most baffling. Unlike alpha particles, beta particles were emitted not with a well-defined energy, but with a broad range of energies, and physicists had established that these energies were created in the nuclear process itself, not as the electron interacted with shell electrons. Here Indian physicists Meghnad Saha and Daulat Singh Kothari suggest that the mystery might be settled by the recently discovered process in which a gamma photon can create an electron and its antimatter partner, a positron. The electron so created might have any kinetic energy, depending on the photon’s energy. This attempted explanation seemed promising, though physicists still knew nothing of the weak nuclear force or of neutrinos, which a proper explanation would require.　　中文

THE β-ray activity of radioactive bodies has until now proved to be a very baffling problem. The points at issue are summarised in Gamow’s “Constitution of Atomic Nuclei”, etc. (pp. 52–54), and in “Radiations from Radioactive Bodies” by Rutherford, Chadwick and Ellis (p. 385). They are also discussed at some length by Bohr in his Faraday lecture (1930).　　中文

Briefly speaking, the chief points under discussion are the following: the disintegration electrons (β-rays) from a radioactive body are not emitted with a single velocity as in the case of α-rays, but show a distribution of velocities over wide ranges, though the breaking-up of the atom is a unitary process, as is proved by the fact that the life-period is definite and there is one electron for each disintegrating atom. It has further been proved that the continuous distribution of velocities is a nuclear process, and not due to action of the surrounding shell of electrons.　　中文

It appears that the β-ray disintegration admits of a very simple explanation on the basis of the recent experiments by Anderson and Neddermeyer, and Curie and Joliot on the production of positrons by the impact of hard γ-rays with the nuclei of elements. These experiments have been interpreted by Blackett and Occhialini as indicating the conversion of a γ-ray quantum into an electron and a positron near the nucleus. Curie and Joliot have brought further evidence in favour of this view by showing that γ-rays of thorium C" (energy 2.6×106 electron volts) are converted inside all matter into an electron (mass 9×10－28 gm., energy m0c2 = 0.51×106 eV) and a positron (having the same mass and energy as the electron), the excess energy being distributed as the kinetic energy of the two particles, and the energy of the residual quantum. They have denoted this phenomenon by the term “materialisation of light quanta”. They have further shown that a proton is a complex structure, being a compound of the neutron and a positron. As pointed out by Blackett and Occhialini, this explains the anomalous absorption of γ-ray quanta observed by Gray and Tarrant, which Gentner has found to commence with the γ-ray possessing the limiting energy 1.1 million electron volts.　　中文

The discovery, which is confirmed by so many workers, promises to be of great importance, as it establishes for the first time, on experimental grounds, the splitting up of a quantum into two charged particles of opposite sign. Many astrophysicists have postulated the probability of the annihilation of the proton and the electron with their mass energies converted into quanta, but the actual process, as revealed by these experiments, seems to be very different. For the quantum breaks up into charged particles possessing opposite charges, but having equal mass, and the positron being absorbed by the neutron forms the proton which is thus seen to be complex. The phenomenon is therefore not a “materialisation of the quantum” as Curie and Joliot suggest, for the neutron appears to be the fundamental mass-particle, but it consists in a splitting of the quantum into two fundamental opposite charges. We may call it “electro-division of the quantum”.　　中文

Let us see how we can explain β-ray activity. If the “electro-division of a quantum” can be brought about by a nucleus when the quantum hits it from the outside, it is much more probable that the γ-rays produced within the nucleus itself should be completely split up into an electron and a positron. The electron will come out as a β-ray, but a positron will not be able to come out if the conversion takes place within the potential barrier. It will attach itself mainly to some one of the numerous neutrons which are present in the nucleus, and thus form a proton. The positive charge of the nucleus will therefore be increased by unity.　　中文

It is not difficult to explain the continuous distribution of β-ray energy. The γ-ray may suffer this “internal electro-division” anywhere within the nucleus, and hence the velocities imparted to the resulting electrons may vary within wide limits. The exact mathematical calculation can be carried out only when more data are forthcoming. The positron combining with the neutron will give rise to the softer γ-rays which are always present in a β-ray disintegration.　　中文

According to the above view, the β-ray emission is only a secondary process, the primary phenomenon which starts this chain of events being the generation of a primary γ-ray. We can now ask ourselves: How is this γ-ray generated? It must be due to the passage of an α-particle or proton from one barrier to another. Gamow, and also Condon and Gurney have postulated the existence of only one barrier in a radioactive nucleus for explaining the emission of α-rays, with definite velocity, but several lines of argument indicate that there may be more than one barrier present in the nucleus. When an α-particle crosses from one barrier to the other, the γ-ray responsible for the whole chain of events leading to the β-ray disintegration is emitted. The life-period is therefore determined by the time of leakage of an α-particle or proton from one barrier to another, and this explains why the life-periods of β-ray bodies are of the same order as those of α-ray bodies, and have a definite value.　　中文

(132, 747; 1933)

M. N. Saha and D. S. Kothari: Department of Physics, Allahabad University, Oct. 20.