Nuclear reactions with radioactive, isomer beams: Coulomb excitation of 18Fg.s. and its Jπ=5+ isomer 18Fm using a large position-sensitive NaI array

Intermediate-energy projectile Coulomb excitation of the unstable nucleus 18Fg.s. and its short-lived Ex=1.1 MeV (Jπ=5+) isomeric state (t1/2=162 ns) 18Fm have been studied at 30 MeV per nucleon. Radioactive beams of 18Fg.s. and 18Fm nuclei were produced by nucleon transfer on a carbon production target and delivered to a secondary 197Au target for Coulomb excitation. Photons emitted in the vicinity of the 197Au target were detected in a large-solid-angle position-sensitive NaI(Tl) array, which allowed for the identification of the beam and target excited states populated by intermediate-energy Coulomb excitation. Two possible 18F ground-state excitations were identified, and B(Eλ↑) values were deduced. Upper limits were established for reduced transition probabilities B(E1↑) and B(E2↑) of several predicted transitions from the 18Fm isomeric state, 5+→4+, 4−, 3+, and compared with values inferred from γ-decay lifetimes. An indication of an enhanced in-flight isomer decay, possibly due to the Coulomb interaction with the target is observed and discussed. The special detector configurations and analytical techniques needed for these types of measurements are described together with suggested improvements.