β-delayed Neutron Emission

Principal Investigator: Krzysztof Rykaczewski, HRIBF

β-delayed Neutron Emission

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The process of beta-delayed neutron emission (βn) from 238U fission products contributes to the total number of neutrons inducing fission in power reactors. The same process also fuels nucleosynthesis during SuperNova events providing more neutrons to be captured and changing the isobaric distribution of the ashes created in the rapid-neutron capture process (r-process). The rates of βn-emission increase for nuclei with larger β-decay energies and smaller neutron separation energies, i.e., for neutron rich nuclei far from β-stability.

The process of β-delayed neutron emission may occur after population of the neutron-unbound excited state by a β transition from a neutron-rich precursor nucleus, see Fig. 1.


Fig. 1 - Radiation modes expected to follow beta decay of the very neutron-rich isotope 94Br. The beta decay energy of 13.34 MeV, and the 1n and 2n separation energies were taken from the extrapolations of measured masses [G.Audi et al., Nucl. Phys. A729 (2003) 3].

The decay of the unbound level may result in emission of either a neutron, a gamma-ray or even a conversion electron. For neutron-rich nuclei, the energy window for β-delayed neutron emission is

Qβ - Sn

where Qβ is the precursor β-decay energy and Sn is the neutron separation energy in the daughter nucleus. As we move away from the stable nuclei, Qβ typically gets larger and on the neutron-rich side of stability, Sn typically gets smaller. Thus as this energy window increases, so does the probability of neutron emission. For nuclei very far from beta stability, other decay channels like β-delayed-2n emission may become energetically possible and this is also illustrated in Fig. 1.

The properties of βn-emission are affected by the structure of the nuclei, in particular by the Qβ and Sn energies and by the energies of the levels primarily populated in beta decay. Since neutron emission follows promptly after the beta transition, the measured time distribution of the observed neutrons can define the half-life of the parent nucleus. Therefore, the studies of β-delayed neutron emission are usually the first experiments which identify the basic decay properties of new neutron-rich isotopes.

Presently, we measure beta-delayed neutron emission by detecting the excited states of the nucleus through subsequent γ-ray emission. However, if the neutron emission goes directly to the ground state, we can only infer such transistions by following subsequent daughter β decay. In order to improve these measurements, we are constructing a neutron detector based on 3He-filled ionization chambers imbedded in a matrix of high-density polyethylene. This detector is called 3Hen

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This file last modified Friday March 28, 2008