Probing the rapid onset of deformation below ⁶⁸Ni through the beta decay of ⁶⁷Mn

One of the best-known divergences from the independent-particle shell model description is the existence of islands of inversion [1]. The IoI of the region N=40 draws particular attention since the neutron number 40 was postulated as a non-traditional “magic” number.In stable nuclei, the neutron g_9/2 orbital is close enough to the pf shell to reduce this shell gap resulting in a more stable subshell closure at N = 50. Measurements of B(E2) values and E(2⁺) in the neutron-rich region show increased collectivity through the N = 40 shell gap, with the clear exception of ⁶⁸Ni [2,3]. Deformation and shape coexistence have been identified in the area, LNPS calculations predict triple shape coexistence for ⁶⁷Co (N=40), with three rotational bands [4]. And, recent experiments on ⁶⁷Fe (N=41) propose a spin-parity of 5/2⁺ or 1/2⁻ for its ground state [5] which indicates a significant deformation. In addition, shape coexistence is also expected for ⁶⁷Fe. Despite the high interest in the region, very limited information is available, to this end, an experiment was performed at the TRIUMF-ISAC facility utilizing the GRIFFIN spectrometer [6], where the β and βn decay of 67Mn populated the ^67,66Fe isotopes.

This data set contains orders of magnitude more statistics than previous studies allowing us to build for the first time a complete level scheme of ⁶⁷Fe, as well as by improving the precision of branching ratios and ground-state half-life measurement. For the ⁶⁷Fe isotope, a good level of statistics will make it possible to measure the energy of the identified isomeric state and improve the lifetime measurement. These results can provide further insight into the structure of the states by comparison to simple models and large-scale shell model calculations to confirm or refute the shape coexistence picture predicted by LNPS calculations and the shrinking of the N=40 gap just one proton below ⁶⁸Ni. Preliminary results from the analysis will be presented and discussed.

[1] B. A. Brown. Physics, 3:104 (2010).

[2] S. Naimi et al.,Phys. Rev. C 86 (2012), p. 014325

[3] M. Hannawald et al., Phys. Rev. Lett. 82 (1999), pp. 1391–1394.

[4] F. Recchia et al.,Phys. Rev. C, 85:064305 (2012)

[5] M. Sawicka et al., The European Physical Journal A - Hadrons and Nuclei, 16(1):51–54, 2003

[6] Garnsworthy et al., Nucl. Inst. Meths. A 918, 9 (2019)