Some Upper Limits of Possible Alpha-Activity

Abstract

WITH the single exception of samarium-152 ¹, no α-ray emitter of nuclear charge smaller than 83 is known. On the other hand, it is concluded from the general trend of the mass-defect curve that many heavy nuclei must be unstable against a-decay. Circumstantial evidence for the existence of α-ray emitters hitherto unknown is provided by certain pleochroic haloes² and, possibly, by ‘excess helium’ in minerals, notably beryl³. The difficulty in identifying additional α-active nuclear species must be due to the slowness of their decay, or to their low abundance in the natural elements.