In I=1 bands of doubly odd nuclei, the two signatures, i.e. the odd-spin and even-spin members, are usually not equivalent energetically. Due to the Coriolis force acting on the valence particles, one signature, called favoured is lower in energy that the other signature. For a single particle in a j shell, the favoured signature is given by the simple rule: _f=j mod 2. Hence for a doubly odd nucleus: _f = [ j_p + j_n ] mod 2, which can be either 0 (even-spin levels) or 1 (odd-spin levels), depending on the configuration.

Doubly odd nuclei of the mass A~125 light rare-earth region exhibit the phenomenon of signature inversion whereby the unfavoured signature component _u of certain bands actually lies lower in energy than the corresponding favoured _f component at low spin. Specifically, for the h_11/2 h_11/2 configuration the favoured =1 signature, defining odd-spin levels, only becomes yrast (i.e. energetically favoured) at spin I~18 relative to the =0 signature, defining even-spin levels. The systematics of the critical inversion spin have been the subject of several recent studies and this phenomenon is still not fully understood theoretically. Triaxiality, strong neutron-proton interactions, and quadrupole pairing have been invoked to explain signature inversion. In ¹²⁴La [Ref] twin h_11/2 h_11/2 bands have been identified in a GAMMASPHERE plus MICROBALL plus NEUTRON SHELL experiment (GSFMA79). The yrast band shows signature inversion in its level energies below I=18.5, while the excited band shows signature inversion above I=18.5, as shown in the following diagram. The expected favoured signatures are denoted by the solid symbols.

A signature dependence of the B(M1)/B(E2) ratios is also observed in the yrast h_11/2 h_11/2 configuration of ¹²⁴La, as shown in the following figure. In this case, the ratios are larger for transitions from the `favoured' signature to the `unfavoured' signature, but unlike the level energies, show no inversion (change of phase). The staggering, however, reaches a maximum around I=18.5 - the inversion spin of the level energies! The staggering arises solely from Coriolis effects of the B(M1) reduced transition probabilities. B(E2) values are not expected to show any signature dependence and behave smoothly with spin.

Signature inversion has systematically been observed in doubly odd nuclei of other mass regions. These include g_9/2 g_9/2 bands of mass 80 nuclei, h_11/2 i_13/2 bands of mass 160 nuclei, and recently g_9/2 h_11/2 bands of mass 100 nuclei [Ref].

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Maintained by Eddie Paul <esp@ns.ph.liv.ac.uk>