An Attempt to Detect Artificial Radioactivity

A. C. Shenstone. An Attempt to detect Induced Radioactivity resulting from x-Ray Bombardement.
Phil. Mag. 43, p. 938 (1922).

As Rutherford’s experiments have shown, as a result of bombardment by $\alpha$-particles the nuclei of a whole series of elements are split, ejecting a hydrogen nucleus.

It is obvious that in the residual nucleus the equilibrium is thereby disturbed. It is possible that the degree of this instability will be such that the entire nucleus is in general destroyed instantaneously. But the opposite case is also possible: a nucleus from which one component (an $H$-particle) has been knocked out, after a corresponding rearrangement, rapidly comes to equilibrium, forming a certain new atom. The existence of isotopes in many elements definitely speaks in favor of the feasibility of the latter possibility. Finally, between these two extreme cases intermediate gradations are also conceivable—a gradual decay of the residual nucleus, resembling radioactive decay. The author made an attempt to detect such a decay1. A strip of the substance under investigation was glued with shellac to a disk of nickel steel. At a distance of 1.5 from the disk a lead bar with an aperture was fixed, in which a radioactive source of $\alpha$-particles was mounted. Close to the lead bar (at a distance of 1.2 cm from the source) a ZnS screen was placed, firmly attached to a microscope. The disk was set into rapid rotation, so that the number of revolutions could be brought up to 250 per second. Under these conditions the time elapsed between the moments when the investigated material passed before the source and before the screen amounted to $\frac{1}{12500}$ second, while those parts of the investigated strip that were bombarded by obliquely directed $\alpha$-particles passed before the screen only after $\frac{1}{30000}$ second.

The following were investigated: carbon (paper), aluminum, and the material of the disk itself (nickel steel). In all cases the results obtained were negative, although the possibility is not excluded that the expected artificial radioactive decay is accompanied by the emission of $\beta$- and $\gamma$-rays, which cannot be detected with the arrangement described.

E. Shpolsky.

1. The work was carried out in Cambridge, in Rutherford’s laboratory. ↩