Compound-Nucleus and Doorway-State Decays of β -Delayed Neutron Emitters K51,52,53

We investigated decays of 51,52,53K at the ISOLDE Decay Station at CERN in order to understand the mechanism of the β-delayed neutron-emission (β⁢n) process. The experiment quantified neutron and $\gamma$-ray emission paths for each precursor. We used this information to test the hypothesis, first formulated by Bohr in 1939, that neutrons in the β⁢n process originate from the structureless “compound nucleus.” The data are consistent with this postulate for most of the observed decay paths. The agreement, however, is surprising because the compound-nucleus stage should not be achieved in the studied β decay due to insufficient excitation energy and level densities in the neutron emitter. In the 53K β⁢n decay, we found a preferential population of the first excited state in 52Ca that contradicted Bohr’s hypothesis. The latter was interpreted as evidence for direct neutron emission sensitive to the structure of the neutron-unbound state. We propose that the observed nonstatistical neutron emission proceeds through the coupling with nearby doorway states that have large neutron-emission probabilities. The appearance of “compound-nucleus” decay is caused by the aggregated small contributions of multiple doorway states at higher excitation energy.