Isomeric decay of 67Fe -: Evidence for deformation

An extension of nuclear spectroscopy studies to nuclei with a large neutron excess is both interesting and challenging. It is interesting since there are theoretical predictions for drastic changes of nuclear structure with increasing neutron number N -see recent review articles [1,2] and earlier references therein. The experimental verification of these predictions is challenging, as the yield of any nuclear reaction, like fission or high-energy fragmentation employed for production of neutron-rich nuclei, drops down very fast with increasing distance from the line of beta-stability. One has to develop experimental methods of very high sensitivity. In this context, the in-flight identification of reaction products in a fragment separator, combined with the decay study of isomers, has proven to be especially successful [3]. During the past five years a number of new experimental results on the isomeric properties like half-life and level schemes has been obtained in neutron-rich nuclei between Ca-48 and Ni-78. Isomers within a half-life range from about 20 ns to over 800 mus have been observed. The experiment we presently report is a continuation of the studies performed with the 60 MeV/u Kr-86 beam on a Ni-nat target in which several new isomers around the Z = 28 and N = 40 shell closure were observed [4,5]. Among these an isomer in Fe-67 was reported to decay via a 367 keV gamma-transition with a half-life of 43(30) mus. This half-life suggested the M2 character of the transition.