From Current Literature

Positron Radiation Emitted by Radioactive Substances

A number of works have established that certain radioactive substances, in addition to negative electrons, also emit positrons. The velocity distribution of these positrons was measured by Alikhanov, Alikhanyan, and Kozodaev[^1]. To register the positrons they used two Geiger–Müller counters placed in the path of a positron beam bent by a preliminary magnetic field along an arc of definite radius (the coincidence method). In their works it was established that part of the positrons emitted by radioactive sources is a consequence of internal conversion of $\gamma$-radiation at negative-energy levels. In such a process a $\gamma$-quantum is transformed into an electron pair, the positive component of which is a constituent part of the positron spectrum investigated by the authors. The theory of internal conversion of $\gamma$-rays through pair creation was developed by Hulme and Jaeger[^2]. The coefficient of internal conversion calculated on the basis of their theory is in good agreement with the values obtained for it by Alikhanov.

The experimental results show, however, that the entire positron spectrum cannot be explained by pair production in internal conversion alone. There are also positrons arising in some other way. Alikhanov, Alikhanyan, and Kozodaev ascribe their appearance to the action of $\beta$-radiation. For such positrons a continuous spectrum is obtained, extending in the case of Th up to energies of $1.2 \cdot 10^6$ eV and for RaC up to $1.7 \cdot 10^6$ eV (they are found as the difference between the measured positron spectrum and the spectrum of positrons arising in the internal conversion of $\gamma$-rays and calculated according to the theory of Hulme and Jaeger). The number of positrons produced by the action of $\beta$-rays is approximately $1/10000$ of the number of negative electrons emitted by the given radioactive source, and is of the same order of magnitude as the number of positrons created in the internal conversion of $\gamma$-rays.

In a recent paper Møller[^3] showed that the positron radiation which Alikhanov, Alikhanyan, and Kozodaev ascribe to the action of $\beta$-rays can be explained as follows. An electron, possessing an electric charge, can interact with electrons occupying negative-energy levels. As a result of this interaction it may happen that an electron from one of these levels is transferred to a positive-energy level, i.e. the simultaneous appearance of an electron and a positron will occur under the action of another electron emerging from the nucleus. For the ratio of the number of positrons appearing to the number of negative electrons emitted by the nuclei, Møller gives, in the first approximation, a value of order $10^{-4}$, which is in good agreement with the experimental data of Alikhanov, Alikhanyan, and Kozodaev. In addition, it follows from the theory that the upper limit of the positron spectrum must lie by $2mc^2$ ($\sim 10^6$ eV) below the upper limit of the $\beta$-ray spectrum of the same element. This fact also agrees with the experimental data, since for the upper limit of the $\beta$-spectrum for ThC$'$ we have the value $2.2 \cdot 10^6$ eV, and for RaC respectively $2.9 \cdot 10^6$ eV, while for the upper limit of the positron spectrum from the data of Alikhanov, Alikhanyan

and Kozodaev give values of \(1.2 \cdot 10^6\) and \(1.7 \cdot 10^6\) eV, respectively; the less satisfactory agreement between the theoretical and experimental data for RaC is explained, in all probability, by the fact that here the curve of the positron spectrum falls off at the boundary not very sharply, which makes it difficult to establish the exact value of the spectral limit.

L. Groshev, Moscow

Literature

1. Allichanow, Allichanjan, Kosodaew, Nature, 136, 475, 712, 1935.
2. Hulme and Jaeger, Proc. Roy. Soc., 148, 708, 1935.
3. Moller, Nature, 137, 314, 1936.