Optimizing recoil-isomer tagging with the Argonne fragment mass analyzer

A new focal plane detector arrangement for the Fragment Mass Analyzer (FMA) has been built and tested at Argonne National Laboratory. This set-up is particularly sensitive for performing Recoil-Isomer Tagging on nuclei with isomeric states with lifetimes in the microsecond range. Recoiling nuclei from fusion–evaporation reactions at the target position are dispersed by their ratio of mass to charge (A/qA/q) by the FMA and stopped in low pressure gas (air) at the focal plane. Subsequent gamma decays from isomeric states in the reaction products are observed using Ge detectors. A constant gas flow through the focal plane chamber efficiently removes longer-lived beta-decaying species from sight of the detectors. This set-up has been commissioned successfully with the microsecond isomer in 80Rb, populated via the 52Cr(32S, 3pn) reaction at 135 MeV.