THE EMERGENCE OF POSITRONS UNDER THE ACTION OF β-RAYS ON MATTER

Working with a Wilson chamber, inside which a weak radium preparation had been placed, Skobeltzyn and Stepanowa¹ established that the preparation emits not only electrons but also positrons. In addition, they showed that the number of positrons appearing depends to a considerable degree on the substance by which the radioactive source is surrounded. Thus, for example, replacing Pb by C in the shell surrounding the source, they obtained an increase in the number of positrons by a factor of 2–3. This fact indicates that at least some of the positrons are born outside the source, in the shell surrounding it. The cause of their appearance is the interaction of β-rays with the material of the shell.

From the increase in the number of positrons on going from Pb to C one may conclude that positron emission is not caused by the γ-radiation of the source, since in the latter case the dependence of the positron yield on the atomic number would have the opposite course. Calculations of the positron yield for C lead to the same conclusion. Assuming that positrons appear under the action on matter only of fast electrons (by virtue of their kinetic energy), whose energy is greater than 1 MeV (the energy necessary for the creation of a pair), the authors find for the positron yield a value not less than 2% and for the radius of the effective cross section, \(0.5 \cdot 10^{-12}\) cm per atom. This value exceeds by a large factor the corresponding data for the appearance of positrons from the γ-rays of ThC″.

The large positron yield also rules out the possibility of explaining the observed effect by induced radioactivity caused in the shell by the α-particles of the source, since the experiments of Curie and Joliot give a considerably smaller probability for the formation of positrons in radioactivity induced by α-particles.

The same question of the emission of positrons under the action of β-rays on matter has been investigated in the recent work of Alichanow and Kossodaew². The authors applied the method of magnetic spectroscopy. The presence of positrons and their number were established by observing simultaneous discharges in two Geiger counters placed in the path of flight of the positrons. They established that a radioactive source \([\mathrm{Ra}(\mathrm{B}+\mathrm{C})]\), covered with a layer of aluminum, gives a considerable number of positrons, although the thickness of the aluminum is sufficient to absorb most of the positrons emitted by the source itself. These positrons should be ascribed to the action of β-rays on aluminum. Estimating the positron yield, they obtain for 1 mm of aluminum a value of 0.4%, if only fast electrons with energy greater than 1 MeV are taken into account.

The experiments described establish the fact of the birth of positrons from β-rays. (Let us note that in one of his earlier works Skobeltzyn³ succeeded in observing the birth of a pair from a β-electron directly in the gas filling the Wilson chamber.) However, under the conditions of the experiment two possibly independent effects are superposed upon one another—the emission of positrons by the radioactive source itself and their birth from β-rays. All this makes it difficult to elucidate the details of the processes occurring here.

It should not be omitted that analogous experiments with Pt, carried out by Chadwick, Blackett, and Occhialini⁴, led to a negative result.

Literature

1. Skobeltzyn a. Stepanowa, Nature 133, 565, 646, 1934.
2. Allichanow u. Kossodaew, Z. Physik 90, 249, 1934.
3. Skobeltzyn, Nature 133, 23, 1934.
4. Chadwick, Blackett a. Occhialini, Proc. Roy. Soc. 144, 235, 1934.

L. Groshev