Radioactivities by light fragment (C, Ne, Mg) emission

Abstract

Recently, heavy nuclei known as α-emitters like Ra and U isotopes have been found to present parallel emissions of 14C, 24Ne and 28Mg with very low branching ratios (< 10-9). Today, the spontaneous emission by heavy nuclei of fragments intermediate between 4He and fission fragments has become a well established field of research. The theory has been developed since 1980 and the first experimental discovery was published in 1984. Now, there are four known 14C emitters (222,223,224, 226Ra) and four 24Ne emitters (230Th, 231Pa, 232, 233U). Probably 25Ne is present in the results of 233U. Two kinds of radioactivity, 28Mg and Ne (probably 24Ne and 26Ne), have been measured from 234U, previously known for its cold fission. Some measurements gave upper limits in branching ratios ; in particular, an upper limit for the Si emission from 241 Am of ~ 10-15 relative to α emission has been set. After a brief review of the theoretical models which have oriented the experimental discoveries, the experiments are described. They are grouped according to the technique used to identify the emitted fragment : ΔE x E telescope in direct view of the source, magnetic spectrometers and solid state track detectors. The experimental results are compared with the theoretical predictions. We present the systematics studies which have given evidence of odd-even effects in the parent nucleus, have analysed the nuclear deformation effects and have proposed a unified approach of α-decay, fragment emissions and cold fission. Finally, it is shown how these new radioactivities are being used to test and refine very elaborate models of α-decay and fission.