$\beta$-Particles of High Energies

Using a Wilson chamber with a magnetic field (1500 gauss) to study the energy distribution of electrons emitted by artificially radioactive elements, Lauritsen, Fowler, and others established that, for some of these elements, the electrons emitted by them possess energies several times greater than the energies of $\beta$-particles emitted by naturally radioactive substances. Such fast $\beta$-particles usually appear in artificially radioactive elements with very short lifetimes.

In the experiments of Lauritsen and others, the active elements were obtained by irradiating a substance with deuterons having energies from 600 to 1000 keV at a current of about one microampere; in this case the deuterons struck the substance being activated in a special thin-walled tube placed inside the Wilson chamber, so that investigation of the radiation emitted by the active element was carried out either during irradiation of the substance by deuterons or immediately after its cessation, which is especially important in the case of short-lived radioactive elements. Let us consider some of the results obtained by these authors.

${}^{12}\mathrm{B}$. When boron is irradiated with deuterons, a radioactive element with a very short lifetime arises, $^{1,2}$ decaying with the emission of negative electrons. To determine the lifetime of this element, the authors counted the number of electron tracks in the Wilson chamber, expanding it after various intervals of time following the cessation of irradiation of the boron by deuterons. From these data, the half-life of the active element was found to be approximately 0.02 second. Subsequently, because of the smallness of the half-life, in studying the electrons emitted by the radioactive substance arising when boron is irradiated with deuterons, the boron was irradiated during the expansion of the Wilson chamber. It is true that here an additional difficulty arises in that, at the moment of irradiation, $\gamma$-rays are produced, which then produce electrons in the chamber that are superposed on the electrons emitted by the radioactive atoms. However, by special additional experiments with an absorbing screen placed between the irradiated boron and the gas of the Wilson chamber, it was shown that the electrons produced by the $\gamma$-rays do not play a noticeable role, i.e., the electrons observed in it arise mainly in the decay of radioactive atoms. The authors’ measurement of the energy of these electrons (1773 tracks) from their deflection in the magnetic field showed that their energy spectrum is continuous, just as in the case of $\beta$-particles of naturally radioactive elements. This continuous spectrum has an upper limit of about 11 MeV and gives, for the mean electron energy, a value of 4.75 MeV. Thus in this case we have a source of very fast electrons.

The proposed nuclear reaction for the formation of the active element under consideration may be written in the following form:

\[ {}^{11}_{5}\mathrm{B} + {}^{2}_{1}\mathrm{H} \to {}^{12}_{5}\mathrm{B} + {}^{1}_{1}\mathrm{H}, \]

\[ {}^{12}_{5}\mathrm{B} \to {}^{12}_{6}\mathrm{C} + \bar e, \]

i.e., the active element is ${}^{12}_{5}\mathrm{B}$.

${}^{8}\mathrm{Li}$. When lithium is irradiated with deuterons, $^{1,3}$ a radioactive element with a short lifetime likewise arises. It decays with the emission of negative electrons. For its half-life the authors obtained, by the same method as in the case of boron, a value of about 0.5 sec. In studying the electrons emitted by this active element, the Wilson chamber was expanded after the cessation of irradiation of the lithium by deuterons. Measurement of the energy of the $\beta$-particles showed that in this case particles with energies up to 10 MeV are observed. Thus here, too, we have very fast electrons.

The proposed reaction is the following:

\[ {}^{7}_{3}\mathrm{Li}+{}^{1}_{2}\mathrm{H}\to{}^{8}_{3}\mathrm{Li}+{}^{1}_{1}\mathrm{H}, \]

\[ {}^{8}_{3}\mathrm{Li}\to{}^{8}_{4}\mathrm{Be}+\bar e, \quad \text{or} \quad {}^{8}_{3}\mathrm{Li}\to 2{}^{4}_{2}\mathrm{He}+\bar e . \]

The active element is \({}^{8}_{3}\mathrm{Li}\).

Fast \(\beta\)-particles are also observed for a number of other elements, for example for \({}^{20}\mathrm{F}\), produced when calcium fluoride is irradiated with deuterons. For it \(\beta\)-particles with energies up to 6 MeV were found. Similar particles with energies up to 5 MeV were found by Kurie, Richardson, and Paxton\(^4\), who irradiated substances (C, O, F, Si, P, Cl, A, K) with deuterons of energy up to 5.3 MeV. When substances are irradiated with neutrons, electrons of high energies also sometimes arise\(^5\).

In conclusion we note that in the work of Lauritsen et al., and also in the work of Kurie, Richardson, and Paxton, it was shown that for artificially radioactive elements decaying with the emission of positrons, the upper limit of the continuous electron spectrum did not exceed 2 MeV.

L. Groshev, Moscow

LITERATURE

1. Fowler, Delsasso a. Lauritsen, Phys. Rev. 49, 561, 1936.
2. Crane, Delsasso, Fowler a. Lauritsen, Phys. Rev. 47, 887, 1935.
3. Crane, Delsasso, Fowler a. Lauritsen, Phys. Rev. 47, 971, 1935.
4. Kurie, Richardson a. Paxton, Phys. Rev., 49, 368, 1936.
5. Naiduy, Siday, Proc. Phys. Soc., 48, 332, 1936.