Magnetic Spectrum of β-Rays Excited by γ-Rays

C. D. Ellis. The Magnetic Spektrum of the β-rays excited by γ-rays. Proceedings of the Royal Society. 99, p. 261, 1921.

We still know too little about γ-rays. It is known that γ-rays are similar to X-rays, but have an even shorter wavelength, of the order of \(10^{-10}\)–\(10^{-11}\) cm. But we know neither the exact wavelength, nor the mechanism of excitation, nor the connection of γ-rays with the β-rays emitted by various radioactive substances. There is Rutherford’s theory, according to which the primary event is the ejection of an electron from the nucleus; then this electron excites a γ-ray; the latter, acting on the electronic atmosphere of the atom, tears out one of the atom’s electrons, and these electrons ejected by the atom are what we perceive as β-rays. The very mechanism of the generation of γ-rays, as will be seen below, is not at all clear to us. By analogy with X-rays one might suppose that γ-rays are produced by the braking of electrons ejected from the nucleus by the substance of the radioactive element. Then one would have to expect a spectrum of γ-rays and, in addition, characteristic radiation, which in fact is not the case. Ellis studies a kind of photoelectric effect in γ-rays and, by his investigation, resolves the problems mentioned above.

The γ-rays of Ra B pass through thin layers of metals (W, Pt, Pb, Ur, and Ba). The β-rays produced are deflected by a strong magnetic field and are recorded on a photographic plate in the form of separate lines corresponding to one or another

energy of $\beta$-rays. Knowing the value $Hs$, where $H$ is the intensity of the magnetic field and $s$ is the radius of the circular trajectory of the $\beta$-ray, one can determine for each line the corresponding energy of the $\beta$-ray. In the following table are given data expressing the energy of the $\beta$-rays in volts, corresponding to the three most intense lines in the $\beta$-spectra for a number of elements:

The difference in energy for W as against the electron energy for Pt is $0{,}08 \cdot 10^5$ volts for all three lines.

According to Rutherford’s theory it follows that the energy of the $\beta$-ray equals the energy of the $\gamma$-ray minus the energy required to transfer an electron from one of the inner orbits to infinity.

\[ W = W_n - W_a, \tag{1} \]

where $W_a$ is an energy characteristic of one or another element.

The energy required to transfer an electron from the $K$ ring to infinity can be found from the edges of the absorption bands of X-rays in one or another element. The difference of these energies for W and Pt is precisely 8000 volts. The data for these energies are indicated in the following table:

From equation (1) it is evident that the energy of the $\gamma$-rays $W_n$ will be found if to the energy of the $\beta$-ray one adds the energy $W_a$; then we obtain Table III, expressing the energy of the $\beta$-ray corresponding to a definite line in Ellis’s $\beta$-spectra.

These numbers show that Ra B emits three groups of $\gamma$-rays, and that the $\beta$-rays observed from the elements studied are electrons ejected by the $\beta$-rays from the $K$ ring.

A more detailed study of the magnetic spectrum of $\beta$-rays from Ra B showed that Ra B emits groups of $\gamma$-rays whose energy in volts and wavelengths will be:

Ellis’s experiments only confirm the above-mentioned theory of Rutherford. The existence of definite groups of $\gamma$-rays makes unclear the mechanism of excitation of $\gamma$-rays, apparently very different from the mechanism of excitation of X-rays.

N. Selyakov.