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There is a growing interest in studying harmonic oscillator nuclear shell closures, particularly far from the valley of stability. This is where the concept of the island of inversion arises [1], i.e., a region of the chart of nuclides where strong quadrupole correlations overcome the spherical mean-field shell gaps, leading to the emergence of energetically favoured deformed intruder states. In even-even nuclei, this phenomenon typically manifests as spherical and deformed 0$^{+}$ states lying close in energy.
The boundary between the negative-parity pf shell and the positive-parity sdg shell in the N=40 region has been extensively investigated. A key reference is $^{68}$Ni, which lies at the intersection of the N=40 neutron and Z=28 proton shell closures, the latter a well-established magic number. This nucleus exhibits an abrupt change in the B(E2) and E(2$_{1}$$^{+}$) systematics, resembling a doubly-magic behaviour, while also showing evidence of shape coexistence [2]. However, in the vicinity of $^{68}$Ni, the N=40 island of inversion emerges, and the region is characterized by enhanced collectivity and shape coexistence, as indicated by the B(E2) and E(2$_{1}$$^{+}$) systematics [3, 4, 5]. This interplay between competing configurations motivates experimental studies to extend our understanding of nuclear structure in this area.
To further explore these phenomena, an experiment was carried out with GRIFFIN at TRIUMF to study $^{62, 64, 66}$Fe isotopes populated via $\beta$-decay of Mn beams. The primary goal was to firmly assign the spins and parities of low-energy excited states in these $^{62, 64, 66}$Fe isotopes through high-precision $\gamma$-$\gamma$ angular correlation measurements [6]. Preliminary results include the firm identification of the 0$_{2}$$^{+}$ states in Fe, in some cases discarding previous tentative assignments. Additionally, the determination of several excited states in the chain places particular emphasis on the 2$_{2}$$^{+}$ states. In combination with theoretical calculations, these will provide crucial insight into shape coexistence along the neutron-rich Fe isotopes in the N=40 island of inversion.
[1] S. M. Lenzi, F. Nowacki, A. Poves, and K. Sieja, Phys. Rev. C. 82, 054301
(2010).
[2] T. Otsuka and Y Tsunoda J. Phys. G: Nucl. Part. Phys. 43 024009 (2016).
[3] S. Naimi, G. Audi, D. Beck, K. Blaum et al. Phys. Rev. C. 86, 014325 (2012).
[4] H. L. Crawford, R. M. Clark, P. Fallon, A. O. Macchiavelli et al. Phys. Rev. Lett. 110, 242701 (2013).
[5] M. Rocchini, P. E. Garret, M. Zielinska, S. M. Lenzi, D. D. Dao, F. Nowacki, V. Bildstein, A. D. MacLean, B. Olaizola et al. Phys. Rev. Lett. 130, 122502 (2023).
[6] J. K. Smith, A.D. MacLean, W. Ashfield, A. Chester, A. B. Garnsworthy, C. E.
Svensson Nuc. Instrum. Methods Phys. Res., Sect. A, 922, 0168-9002 (2019).