Some features of odd Au nuclei revealed by half-life measurements

2014 The half-life (1.2 ± 0.1 ns) of the 789.9 keV 9/2level in 193Au is measured. The retardation of the deexciting M 1 and E 2 transitions suggests that this 9/2state, as well as those found in 187, 189, 191 gold nuclei, issues from the h9/2 orbit. The average shape of the Au nuclei in these states and of their even Pt cores is interpreted with the particle + asymmetric rotor model. LE JOURNAL DE PHYSIQUE TOME 36, JUILLET-AOÛT 1975, Classification Physics Abstracts 4.1b2 The great interest focused on transitional nuclei in the Os to Pb region is connected with observations made precedently : Signs of a prolate to oblate shaped transition revealed by the study of level positions in even Pt nuclei [1, 2]. Establishment of 9/2states issued from the h9/2 orbital in Tl nuclei [3]. The discovery and interpretation of negative parity states in Ir nuclei [4]. The existence of unexpectedly large retardations of transitions in Ir and Pt nuclei [5]. The large isotopic shift measured between 185Hg and 18’Hg [6]. Most of the above information was obtained at ISOLDE, CERN and studies of odd Au and Pt nuclei are actually continued at ISOCELE, Orsay. Half-life measurements of excited states [7] and heavy ion reaction experiments [8, 9] have led to the proposal of two decoupled bands in 189 Au. They are based on 9/2and 11/2levels, joined as in 1871r [4] by a very retarded M 1 transition (78 keV, Fw 15 000). The 11/2band is supposed to result from the coupling of an h11/2 proton hole to the « rotation » of a Hg core and has, in fact, « rotation » energies similar to those of even Hg isotopes. In these states the deformation of the nucleus should be oblate as in Hg nuclei. The band appears in all odd Au isotopes (*) And University Paris VII. and is observed to have a very stable structure [7-14]. As seen in figure 2 the band head lowers slowly, when the neutron number decreases. The 9/2state is proposed to arise from the coupling of a proton in the h9/2 shell to the « rotation » of a Pt core and has, according to this interpretation, a prolate shape. States issued from the high lying h9/2 orbital, have already been observed in Tl nuclei and their appearance at low excitation energy is explained by the gain in pairing energy [3]. In Tl, and, as shown below, in Au these states lower rapidly with decreasing mass number. In 187Au the 9/2state is placed below the 11/2isomer (Fig. 2) and the decoupled character of the band is well developed [ 13]. The half-life of the 11 /2 state has been measured, but the retardation factors for the 11/2to 9/2transition cannot be given, as the M 1 to E 2 mixing ratio has not yet been determined. (The experimental rësults on 185 Au are now being analysed.) In 195 Au the 9/2level is proposed as lying at 1 068 keV [15]. More information is available in 191Au : Hôglund et al. [14] have found the 9/2band at about 200 keV higher excitation energy than in 189Au (Fig. 2), and measured a similar hindrance as in the latter nucleus for the 9/2to 11/2M 1 transition (Table I). For the identification of the 9/2state in 193Au, the extensive studies by Vieu and Dionisio [ 11 give aid. Their table of analogue states in 193 Au and 195 Au, i.e. states with the same spin and parity, nearly same excitation energy and identical depopulation mode, shows that the 789.9 keV 9/2level in 113 Au has no equivalence in the 195 isoArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01975003607-8061300