Levels in ${}^{99}\mathrm{Zr}$ observed in the decay of ${}^{99}\mathrm{Y}$

A new investigation of the decay scheme of ${}^{99}\mathrm{Y}$ to ${}^{99}\mathrm{Zr}$ removes minor discrepancies between decay and prompt-fission data. The existence of a strongly coupled band on the 725-keV level postulated in an earlier β-decay work is not confirmed. Calculations in the interacting boson fermion framework allow an alternative interpretation of the even-parity levels above 500 keV. Still, the nature of most of these levels above remains unclear.

Figure 1

Projection gated by the 614-keV transition obtained with the nine-detector array. A constant visible at the very left has been added to avoid display of negative counts. The labels r and u refer to random coincidences and an unassigned transition, respectively. Other lines are placed in the scheme.

Figure 2

Projection gated by the 725-keV transition obtained with the nine detector array. See caption of Fig. 1. The sharp line at the foot of the 127.6 keV peak is a residual of the random coincidence with the 130.2-keV transition (not marked).

Figure 3

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Figure 4

Even-parity levels in the N = 59 isotones ${}^{97}\mathrm{Sr}$ and ${}^{99}\mathrm{Zr}$ discussed in the text. Not all transitions are shown. Intensities are those observed in β decay. The dotted lines indicate the states forming decoupled 3/2${}^{+}$ and 5/2${}^{+}$ bands according to Ref. 8, but also interpreted as a strongly coupled K = 3/2${}^{+}$ band in ${}^{97}\mathrm{Sr}$ [2, 19]. The dashed lines connect possible spherical 2${}^{+}\otimes$ $s_{1/2}$ levels, see Ref. 19 for ${}^{97}\mathrm{Sr}$.