The [formula omitted] reaction through isobaric analogue resonances in 141Pr

Abstract

The 140Ce(p, γ 0) 141Pr reaction proceeding through the 2 f 7 2 isobaric analogue state |IA〉 of 141Ce was measured with a 20 cm 3 Ge(Li) crystal. A high-energy line of 14.95 MeV was identified as the E1 gamma ray from the 7 2 − IA resonance to the 5 2 + ground state in 141Pr on the basis of the correct gamma energy and resonance behavior. The excitation function for the gamma rays observed at θ 0 = 125°, where P 2(cos θ 0) ≈ 0, displayed a slightly asymmetric resonance cross section superimposed on a smooth background cross section. The excitation function was analysed in terms of two interfering processes through the isobaric analogue and the semidirect giant resonances. The matrix element of the E1 gamma transition 〈 m γ〉 = 〈 r 〉 from the | IA〉 = (2T 0) − 1 2 T −| i〉 of the 2 f 7 2 ground state |i〉 in 141Ce to the 2 d 5 2 ground state |f〉 in 141Pr was obtained to be |〈f| e m γ|IA〉| = (0.063±0.015) e fm. This is smaller than a simple shell model estimates by a factor F 1 2 ≈ 5 . The corresponding first-forbidden beta matrix element |〈 f|ɡ v mβ| i〉| , where ɡ v mβ = (ɡ v /e)[e mγ, T −] , was deduced from the observed gamma matrix element. By use of the 〈 mβ〉 = ɡ v 〈 r〉 and the log ft value, relations between the beta matrix elements 〈 σ× r 〉, 〈 α〉 and 〈 r 〉 where obtained. Discussions concerning the gamma and beta matrix elements and the (p, γ) reaction mechanism are given in detail.