Observation of isomeric states in neutron deficient A È 80 nuclei following the projectile fragmentation of 92 Mo C. Chandler, 1 P. H. Regan, 1 B. Blank, 2 C. J. Pearson, 1 A. M. Bruce, 3 W. N. Catford, 1 N. Curtis, 1, * S. Czajkowski, 2 Ph. Dessagne, 4 A. Fleury, 2 W. Gelletly, 1 J. Giovinazzo, 2,4 R. Grzywacz, 5,6 Z. Janas, 2,5 M. Lewitowicz, 6 C. Marchand, 2 Ch. Miehe ´ , 4 N. A. Orr, 7 R. D. Page, 8 M. S. Pravikoff, 2 A. T. Reed, 8 M. G. Saint-Laurent, 6 S. M. Vincent, 1,† R. Wadsworth, 9 D. D. Warner, 10 J. S. Winfield, 1,6 and F. Xu 1 1 School of Physical Sciences, University of Surrey, Guildford, GU2 5XH, United Kingdom 2 CEN Bordeaux-Gradignan, Le Haut-Vigneau, F-33175 Gradignan Cedex, France 3 School of Engineering, University of Brighton, Brighton, BN2 4GJ, United Kingdom 4 IReS, BP28, F-67037 Strasbourg Cedex, France 5 Institute of Experimental Physics, Warsaw University, Pl-00681 Warsaw, Poland 6 GANIL, BP 5027, F-14000 Caen Cedex, France 7 LPC, ISMRA et Universite ´ de Caen, Bld. du Marechal Juin, 14050 Caen Cedex, France 8 Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool, L69 7ZE, United Kingdom 9 Department of Physics, University of York, Heslington, York, Y01 4DD, United Kingdom 10 CLRC Daresbury Laboratory, Warrington, WA4 4AD, United Kingdom Received 13 August 1999; published 3 March 2000 -ray decays depopulating isomeric states have been observed in a number of very neutron deficient nuclei around A 80 following the projectile fragmentation of a 92 Mo primary beam. Previously unobserved decays have been identified in the N =Z +2 nuclei 39 80 Y and 41 84 Nb and the N =Z nucleus, 43 86 Tc, making the latter the heaviest N =Z nucleus to date in which a discrete -ray transition has been assigned. The lifetime of the previously reported I  = 9 2 + isomeric state in 73 Kr has also been measured and a clearer picture of its decay properties has been deduced. Isomeric ratios have been measured and have been interpreted in terms of the yrast or nonyrast nature of the isomeric state. PACS numbers: 21.10.Tg, 25.70.Mn, 27.50.+e I. INTRODUCTION The richness observed in the structure of the neutron de- ficient nuclei with A 80 is the result of the low level density in the nuclear potential for 30Z 40. This leads to shell gaps in the nuclear mean field at nucleon numbers 34, 36 oblate, 34, 38 prolate, and 40 spherical1. The reduc- tion in the excitation energy of the first excited state between the N =Z =36 system 36 72 Kr and the N =Z =38 system 38 76 Sr has been interpreted 1–4 as being due to a sudden alter- ation in the nuclear shape, from deformed oblate in 72 Kr to deformed prolate in 76 Sr. Lister et al. 2 have used the Grodzins estimate to establish that the most deformed nucleus in the region is 38 76 Sr 38 with a prolate deformation of  2 0.4. The coexistence of neighboring oblate and prolate shell gaps also causes the nuclear deformation to change dra- matically with the addition or subtraction of only a few nucleons. This effect is enhanced in nuclei with near equal numbers of protons and neutrons as the single particle spec- tra are similar for the two types of nucleon. The nuclear shape can also vary with excitation energy and spin as well as nucleon number. Competition between prolate, oblate, and spherical shapes has been investigated in this region and con- vincing evidence for shape coexistence between prolate and spherical shapes has been found in 76,78 Kr 5,6. The influ- ence of the positive parity g 9/2 single particle intruder orbital on the structure of these mass 80 nuclei also becomes appar- ent when investigating states with oblate deformation in nu- clei in this region. Isomeric states arising from the g 9/2 single particle orbital have been observed in 69,71 Se 7 and have been associated with oblate deformed configurations. The observation of isomeric states allows the investiga- tion of nuclear phenomena such as shape coexistence since they provide information regarding the excitation energies of intrinsic states. This provides a crucial test of mean field models far from the valley of  stability where theoretical descriptions are often extrapolations of data pertaining to near-stable nuclei. The investigation of isomeric states pro- vides information about the competition between single par- ticle and collective structures in nuclei at, or close to, the proton drip line. Information regarding isomeric states present in the neutron deficient mass 80 nuclei is also essen- tial for our understanding of the path of the rp process 8. The projectile fragmentation of heavy ion beams has been shown to be an excellent mechanism for the production of exotic nuclei due to the high degree of selectivity provided by modern projectile fragment separators such as the LISE3 spectrometer at GANIL 9, the A1200 at MSU 10 and the FRS at GSI 11. Because of the short flight time from pro- duction to detection typically less than 1 s, the technique is particularly suited to the study of isomeric states in exotic nuclei. *Present address: Department of Physics, Florida State Univer- sity, Tallahassee, FL 32306. † Present address: Department of Physics, University of Notre Dame, IN 46556. PHYSICAL REVIEW C, VOLUME 61, 044309 0556-2813/2000/614/04430914/$15.00 ©2000 The American Physical Society 61 044309-1