Z. Phys. A 350, 9-10 (1994) Short note ZEITSCHRIFT FORPHYSIKA 9 Springer-Verlag 1994 Spin and parity of isomeric and ground states of the doubly-odd nucleus 184Au F. Ibrahim 1, P. Kilcher 1, B. Roussi6re 1, j. Sauvage 1, j. Genevey 2, A. Gizon 2, A. Knipper 3, G. Marguier 4, D. Barn60ud 2, R. B6raud 4, G. Cata-Danil 2, J. Blachot 2, I. Deloncle s, R. Duffait 4, A. Emsallem 4, D. Hojman 6, A.J. Kreiner 6, F. Le Blanc 1, J. Libert 7, J. Oms 1 1Institut de Physique Nucl~aire, F-91406 Orsay, France 2 Institut des Sciences Nucl6aires, F-38026 Grenoble Cedex, France 3 Centre de Recherches Nucl6aires, F-67037 Strasbourg, France 4 Institut de Physique Nucl6aire, F-69622 Villeurbanne Cedex, France 5 Centre de Spectrom6trie Nucl6aire et de Spectrom6trie de Masse, F-91405 Orsay, France 6 Departamento de Fisica, CNEA 1429 Buenos Aires, Argentina 7 Centre d'Etudes Nucl6aires de Bordeaux Gradignan, Le Haut Vigneau, F-33170 Gradignan, France Received: 11 July 1994 Abstract : Low-spin states in doubly-odd 184Au have been populated in the [~+/EC decay of 184Hg produced by bombarding 148Sm targets with an 185 MeV 40Ar beam. Radioactive Hg nuclei were transported by a He jet system. A new level scheme has been established from T-y-t and X-y-t coincidence measurements. Spin and parity values Ix = 5+, IX = 2+ and In = 3- were assigned to the ground state and the two isomeric states of 184Au, respectively. The structure of these states is discussed using B(E1) values. PACS : 23.20.Lv, 23.40.-s, 27.70.+q, 21.10. Hw, 21.10.Tg The NICOLE group discovered that the alleged ground state of the doubly-odd nucleus 184Au is, actually, an isomer with spin value I = 2 or 3 linked to the ground state with spin value I = 5 or 6 by an M3 isomeric transition [1]. An unusual fact is that the ground-state decay half-life is shorter than the isomer one, which explains why it was unknown until now[2,3]. Moreover, the ground-state spin value and the existence of an isomer are difficult to understand with the v | x prolate configurations that are expected in 184Au from the neutron and proton states lying at low energy in the neighbouring odd-A nuclei [4]. This led us to search for answers to the following questions : is the ground-state nuclear shape oblate or prolate, and what are the spin and parity values of the isomeric and ground states ? We answered the first point by studying the properties of the rotational bands observed in in-beam experiments. They indicate clearly that, at low excitation energy, the t84Au nucleus has a prolate shape [4,5]. The purpose of the present work is to answer the second question by performing a study of the [3+/EC decay of the 184Hg ground state (TI/2 = 30.6 s) to the excited states of the 184Au nucleus. The radioactive Fig isotopes were produced by bombarding 2 mg/cm2 14SSm targets with an 185 MeV energy 40Ar beam delivered by the SARA accelerator in Grenoble. A He-jet system was used to carry the produced radioactive atoms to a low-background counting room. Transportation of Hg was obtained by introducing iodine aerosols in the He gas as previously recommended [6,7]. The radioactive Hg isotopes were collected on a mylar- aluminum tape and moved to the counting set-up including two v-ray, 60% and 34% efficiency, Ge (HP) coaxial detectors and one X-ray Ge (HP) detector. Singles v-ray and X-ray spectra as well as y-y-t and X-y-t coincidence events were recorded. A new level scheme has been established at variance with the previous one [8]. In particular a 25.7 keV M1 transition recently observed [4,9] has been located in cascade E keV &O0 300 1 + 236.7 v 2- 69 + ~ 25.7 6ns 3- 142.0 4 + ,oo , 5+ 1 Fig. 1 . Partial level scheme of doubly-odd 184Au. The isomer and ground-state half-lives have been estimated to be either Tl/2,m = 48 s and TI/2 ~ = 19 s [2] or Tl/2m = 28 s and TI/2,~ = 12 s [3]. Arro~,s marked by a stal: have a width divid&! by 5