α-Radioactivity of Cerium-142

Abstract

JOHNSON AND NIER¹ have measured the atomic masses of some of the rare-earth isotopes and have shown that the mass difference cerium-142—(barium-138 + helium-4) is equivalent to 1.68 ± 0.10 MeV. Similar results for the naturally occurring samarium and neodymium isotopes show that the α-active isotope of each element is the one having the largest possible decay energy. Rasmussen and others² suggest that the two or three neutrons just beyond the closed shell of 82 neutrons have decreased binding energies and hence the α-energy has a maximum about 84 neutrons. Johnson and Nier suggest that the α-decay of cerium-142 may take place with enough energy to be experimentally observable. Porschen and Riezler³ examined a sample of un-enriched cerium ammonium citrate using nuclear track plates sensitive to α-particles. No α-activity was observed after a 30-day exposure of 1.2 mgm. of the cerium salt. In 1957 Riezler and Kauw⁴ reported an alpha activity for an enriched sample of cerium-142. From their results they calculated a half-life of 5.1 × 10¹⁵ years with an uncertainty factor of 2.