Decay studies of the β-delayed neutron emitters Br87 and Br88 measured by means of the Modular Total Absorption Spectrometer at ORNL HRIBF

Abstract

The $\ensuremath{\beta}$ decays of $^{87}\mathrm{Br}$, $^{88}\mathrm{Br}$, and $^{87}\mathrm{Kr}$ were measured with the Modular Total Absorption Spectrometer (MTAS) at Oak Ridge National Laboratory's Holifield Radioactive Ion Beam Facility (HRIBF). Both bromine isotopes are $\ensuremath{\beta}$-delayed neutron emitters that have large cumulative fission yields and were identified as top-priority cases for total absorption study by the Nuclear Energy Agency in 2007. Our investigations corroborate that the decay schemes of $^{87}\mathrm{Br}$ and $^{88}\mathrm{Br}$ suffer from the so-called pandemonium effect. Unique MTAS properties enable direct neutron measurements. We present MTAS-derived $\ensuremath{\beta}$-delayed neutron spectra, $\ensuremath{\beta}$-delayed neutron emission probabilities of ${P}_{n}(^{87}\mathrm{Br})$ $=2.36(24)%$, and ${P}_{n}(^{88}\mathrm{Br})$ $=6.4(6)%$, and the $\ensuremath{\beta}$-delayed neutron transitions intensity of 4(2)% to the first excited $^{87}\mathrm{Kr}$ state, populated in the $\ensuremath{\beta}$-neutron decay of $^{88}\mathrm{Br}$. Incorporating new data into calculations of the electromagnetic decay heat component emitted during thermal neutron fission of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ improves agreement with experimental data up to approximately 80 s after fission. The estimation of the nuclear reactor ${\overline{\ensuremath{\nu}}}_{e}$ flux results in changes of up to 1% in the expected ${\overline{\ensuremath{\nu}}}_{e}$ interactions with the detector material for $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, $^{239}\mathrm{Pu}$, and $^{241}\mathrm{Pu}$.