Speaker
Description
Observation of high 2$^{+}$ excitation energy in $^{68}$Ni (Z = 28, N = 40) had drawn a clear signature of double magic character in this nucleus [1]. And while $^{68}$Ni can be described as spherical isotope, 2$^{+}$ excitation energy of $^{66}$Fe drops significantly [2], indicating deformed shape of $^{66}$Fe. $^{67}$Co isotope is in between $^{68}$Ni and $^{66}$Fe nuclei and found to share coexistence of both spherical and deformed structures in low-lying excited states [3]. This effect can be described as superposition of a proton f$_{7/2}$ hole coupled to neighbouring spherical even-even nickel isotope and a prolate proton-intruder state coupled to the $^{66}$Fe isotope [4]. Discovery of shape coexistence in $^{67}$Co rose an interesting question about further shape evolution in Co nuclei, namely $^{69,71,73}$Co and shell transformation from N = 40 to N = 50.
In-beam gamma experiment was performed at Radioactive Isotope Beam Factory, RIKEN Nishina centre, Japan. Secondary beam of $^{70,72,74}$Ni and $^{72}$Co isotopes at energy of 260 MeV/µ bombarded liquid hydrogen target (MINOS) to produce $^{69,71,73}$Co nuclei via (p, 2p) and (p, pn) reactions. DALI2 NaI(Tl) detector array was used to measure $\gamma$-rays. Energy levels were studied using $\gamma$-$\gamma$ coincidence technique. Systematics of excited states of cobalt isotopes was compared with Lenzi-Nowacki-Poves-Sieja (LNPS) model [5] of nuclear interaction using fpgd model space. Experimental results of $^{69,71}$Co spectrums show that isotopes share shape coexistence, as spherical structure coexists with deformed band. In case of $^{73}$Co nucleus, due to the lack of statistics only spherical band is confirmed.
In this talk the evolution of shell structure in $^{69,71,73}$Co isotopes will be discussed together with physics behind the shape coexistence in neutron-rich Co nuclei.
References.
1. R. Broda, B. Fornal, W. Krolas, and T. Pawkat, Phys. Rev. Lett. 74, 868 (1995);
2. S. Lunardi, S. M. Lenzi, F. Della Vedova, Phys. Rev. C, 76, 034303 (2007);
3. F. Recchia et al., Phys. Rev. C 85, 064305 (2012);
4. D. Pauwels et al., Phys. Rev. C 78, 041307(R) 2008;
5. S. M. Lenzi, et. al., Phys. Rev. C 82, 054301 (2010).
