Revised spin-parity assignment and a new interpretation of the high-spin isomer in ¹⁵¹Er

The high-spin isomer in Er-151 (E-x = 10.3MeV, T-1/2 = 420 ns) has been studied by the Sn-116(Ar-40, 5n) Er-151 reaction at 197MeV. From the gamma gamma coincidence relations, a new transition with an energy of 1514 keV was found. This finding requires the revision of the spin-parity assignment from previous 67/2(-) to 61/2(+) or 65/2(-). The 61/2+ assignment is the one which was predicted by the deformed independent particle model as an isomer with a large oblate deformation (beta = -0.17) of the [v(f(7/2)h(9/2)i(13/2))pi(h(11/2)(4))](61/2+) configuration. The isomerism may be attributed to the sudden shape change of the isomer from the nearly spherical shape of the lower-spin yrast states to the oblate shape.