UC Berkeley

The β decays of Br87, Br88, and Kr87 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 β-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 Br87 and Br88 suffer from the so-called pandemonium effect. Unique MTAS properties enable direct neutron measurements. We present MTAS-derived β-delayed neutron spectra, β-delayed neutron emission probabilities of Pn(Br87) =2.36(24)%, and Pn(Br88) =6.4(6)%, and the β-delayed neutron transitions intensity of 4(2)% to the first excited Kr87 state, populated in the β-neutron decay of Br88. Incorporating new data into calculations of the electromagnetic decay heat component emitted during thermal neutron fission of U235 and Pu239 improves agreement with experimental data up to approximately 80 s after fission. The estimation of the nuclear reactor ν¯e flux results in changes of up to 1% in the expected ν¯e interactions with the detector material for U235, U238, Pu239, and Pu241.