Absence of Species Other Than ⁸⁷Br Contributing to the 55.4-sec Delayed Neutron Activity in Fission

Two experiments have shown that ⁸⁷Br is the major contributor to the 55-sec delayed-neutron activity in ²³⁵U thermal-neutron fission products. An upper limit of 0.5% has been set on the possible contribution to the total 55-sec delayed-neutron activity by unknown precursors with half-life of the order of 55-sec or longer. The half-life of⁸⁷Br has been accurately measured by observing the neutron decay in chemically pure bromine samples and was found to be 55.4 ± 0.7 sec.