Low-lying collective excitations in (18)F(g.s.) and (18)F(m) studied via intermediate-energy Coulomb excitation.

Abstract

The unstable nuclei <super>18</super>F_g.s. and <super> 18</super>F<super>m</super> have been studied via intermediate-energy Coulomb excitation in an attempt to populate collective states above the short-lived 18Fm 1.1 MeV (J<super>pi</super> = 5<super>+</super>) isomeric (t_1/2 = 162 ns) state. A radioactive beam of <super>18</super>F and <super> 18</super>F<super>m</super> nuclei was produced by nucleon transfer to energies per nucleon (E/A) of 40 MeV/u, delivered to a <super>197</super>Au target for Coulomb-excitation. Photons emitted in the vicinity of the <super>197 </super>Au target were detected in a large-angle NaI(Tl) array, which allowed for the identification of the excited states populated by intermediate-energy Coulomb excitation. Upper limits were established for reduced transition probabilities B(E2; 5<super>+</super> &rarr; 4<super>+</super>,3<super>+</super>) of several predicted transitions from the <super>18</super>F<super>m</super> isomeric state. The reduced transition strengths, B(Elambda &uarr; ), of two <super>18</super>F ground state excitations were also directly measured via intermediate-energy Coulomb excitation for the first time. The energies and B(Elambda &uarr; ; 1<super>+</super>_g.s. &rarr; 2<super>+</super>₂₅₂₃,2<super>--</super>₂₁₀₁) values for these transitions are reported. An indication of enhanced isomer decay, possibly due to the Coulomb interaction with the target is seen and discussed.