Collective contributions to E1, ?K=1 matrix elements in odd-odd deformed nuclei

Abstract

The half-life of the 184.3keV state in 165Dy has been remeasured as "F1/2 = 1.0+_0.1 ns in order to deduce a reliable value for the transition matrix element of the depopulating E 1, AK=I transition. Experimental El, AK=I transition matrix elements in odd-odd deformed nuclei are found to be systematically larger than similar values in adjacent odd-A isotones. This effect is supposed to be associated with higher-order vibrational admixtures coupled via RPC and p-n interaction mixings.Earlier theoretical studies [1] suggested that the quasiparticle-phonon interaction in odd-odd deformed nuclei is expected to play a more important role than in odd-A nucleides. Model-dependent investigations of transition probabilities performed earlier on the basis of Nilsson hindrance factors [2] did not reveal any appreciable differences between these quantities in both kinds of nucleides. The present work deals with a new measurement in t65Dy-relevant for the present considerations-and a more detailed analysis of E1 transition matrix elements including also recent experimental results [3,4]. A preliminary communication of some of the present results was already given in [3] ponents in the time distribution cannot unambiguously be separated in this case. In the present work, lifetime measurements were performed in the (n, ?) reaction by means of the method of delayed ;,-y coincidences using a plastic scintillator and a planar Ge(Li) detector (cf. Fig. 1). The target consisted of t64Dy20 3 enriched to 97.0~. The experimental methods and techniques used at the Rossendorf research reactor as well as the analysing procedure have been described earlier (cf. e.g. Refs. 3, 7). According to this procedure, time distributions of ;,-ray transitions were obtained after subtracting the ,.'-ray background contributions. Then, information on the lifetimes was deduced applying the centroid shift and the slope methods. For the 184.3 keV state in 165Dy, which is of special interest here, a half-life T1:2(184.3 keV)= 1.0 +0.1 ns has been determined ( Fig. 1) in accordance with the previous result TI~ 2 = 1.07 _+0.36ns (Ref. 5). Additionally, the half-life of the 180.9 keV level in 165Dy was observed to be T1:2~2.5 (_+l.0)ns in contradiction to the value of T1 ~=10.7ns given ill Reference 5. The relatively large experimental un-0340-2193/78/0289/0107/$01.00