ABSTRACT
The first neutron-rich radioactivity to be discovered was 6He with a half-life of 0.8 sec, produced and studied in the (n,α) reaction on 9Be by Bjerge and Broström 1 in 1936. A wider field was opened up when medium-weight neutron-rich isotopes formed in neutron bombardments of uranium were identified by Hahn and Strassmann 2 in 1939, an observation that signalled the discovery of fission 3 and the birth of modern nuclear physics. Already the same year, Roberts et al. 4 reported that uranium continued to emit neutrons for one to two minutes after the end of an irradiation with neutrons; they suggested that the neutrons were caused by, or accompanied, beta and gamma radiation with the same period, about 13 sec — an interpretation that turned out to be correct. It was soon realized that the beta-delayed neutrons are observed with the beta half-life of the mother isotope (the “precursor”), and that the neutron emission occurs from excited states in the daughter nucleus (the “emitter”). This process is illustrated in Figure 1. Schematic level scheme illustrating the beta-delayed emission of one and two neutrons. The beta decay intensity is, apart from well-known phase space and Coulomb corrections, determined by the reduced beta transition probability S<sub>β</sub> per unit interval of excitation energy. This quantity, called the strength function, see <xref ref-type="sec" rid="sec6_3_1">Subsection III. A</xref>, is strongly influenced by the two giant resonances: the isobaric analogue state (IAS) and the Gamow-Teller giant resonance (GTGR). For neutron-rich nuclei, both are situated outside the window of observation, and the tail of the latter leads to an enhancement of the intensities of the beta branches to high-lying levels. The arrows indicate the possible decay routes of excited states that exceed the binding energy of one or two neutrons. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781351075381/487c276b-e14e-400f-9b2d-536d191f255a/content/fig6_1.tif"/>
