Nuclear reactions with radioactive, isomer beams: Coulomb excitation of ¹⁸F_g.s. and its J^π=5⁺ isomer ¹⁸F^m using a large position-sensitive NaI array

Intermediate-energy projectile Coulomb excitation of the unstable nucleus ¹⁸F_g.s. and its short-lived E_x=1.1 MeV (J^π=5⁺) isomeric state (t_1/2=162 ns) ¹⁸F^m have been studied at 30 MeV per nucleon. Radioactive beams of ¹⁸F_g.s. and ¹⁸F^m 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 ¹⁸F^m 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.