$\beta$-delayed neutrons from oriented $^{137,139}$I and $^{87,89}$Br nuclei

We propose a world-­‐first measurement of the angular distribution of $\beta$-­‐delayed n and $\gamma$- radiation from oriented $^{137, 139}$I and $^{87,89}$Br nuclei, polarised at low temperature at the NICOLE facility. $\beta$-­‐delayed neutron emission is an increasingly important decay mechanism as the drip line is approached and its detailed understanding is essential to phenomena as fundamental as the r‐process and practical as the safe operation of nuclear power reactors. The experiments offer sensitive tests of theoretical input concerning the allowed and first-­‐forbidden $\beta$‐decay strength, the spin-­‐density of neutron emitting states and the partial wave barrier penetration as a function of nuclear deformation. In $^{137}$I and $^{87}$Br the decay feeds predominantly the ground state of the daughters $^{136}$Xe and $^{86}$Kr whereas in $^{139}$I and $^{89}$Br we will explore the use of n-$\gamma$- coincidence to study neutron transitions to the first and second excited states in the daughters $^{138}$Xe and $^{88}$Kr. These measurements will use the new “Versatile Array of Neutron Detectors at Low Energy” (VANDLE) and high purity Ge detectors for neutron-­‐$\gamma$- coincidence and monitoring of the nuclear orientation experiment. This instrument had its first use in on-­‐line experiments using fission fragments at HRIBF and in direct reaction studies at NSCL and Notre Dame University. The experiments described in this proposal have direct relevance to a recently initiated Coordinated Research Project (CRP) at the International Atomic Energy Agency (IAEA) in Vienna on the development of a “Reference Database for $\beta$-­delayed Neutron Emission Evaluation”.