Spontaneous Emission of Neutrons by Artificially Produced Radioactive Bodies

I. CURIE, F. Joliot and P. Preiswerk¹ observed that after bombarding silicon or phosphorus by neutrons, the artificially produced radioactive nuclei emitted: (1) negative electrons as previously observed by Fermi²; (2) γ-rays of high energy (∑ 5 × 10⁶ e.v.); (3) positrons (which they tentatively suggest to be due to the creation of `pairs' by the γ-rays, an explanation which does not seem very probable as they state that the positrons have an upper energy limit of ~ 1 × 10⁶ e.v. only); and (4) neutrons. They assume that some of the radioactive nuclei are ₁₃Al²⁸ and ₁₄Si³¹, and that the spontaneously emitted neutrons are due to the transformation processes: