Speaker
Description
The shell structure of nuclei has served as a backbone of nuclear theory. A large energy gap with the completely filled spherical orbitals defines shell closure and magic number. One of the intriguing experimental findings is disappearance of the “normal” filling at certain N or Z, which is not predicted from the classical shell model [1]. This Island of Inversion (IOI) has been successfully explained through the shell model with variants of dynamical SU(3) symmetry [2]. The present work focused on the N = Z nucleus $^{84}$Mo in which we probed unexpected large deformation.
The experiment was conducted at the NSCL (now FRIB), Michigan State University. A 140-MeV/u $^{92}$Mo beam impinged to a 235-mg/cm$^{2}$ $^{9}$Be target to produce an $^{86}$Mo secondary beam. The HPGe tracking array GRETINA and the TRIPLEX plunger [3] were used to measure the lifetime of the first $2^+$ state. The extracted B(E2; $2_1^+ \rightarrow 0_1^+$) shows a significant difference between $^{84}$Mo and $^{86}$Mo. Interestingly, it departs from a similar B(E2; $2_1^+ \rightarrow 0_1^+$) trend of other N = Z and N = Z + 2 nuclides. We employed the DNO-SM [4] and other theoretical approaches to explain the difference of the B(E2; $2_1^+ \rightarrow 0_1^+$) values. The study revealed that an abrupt change of B(E2; $2_1^+ \rightarrow 0_1^+$) between $^{84}$Mo and $^{86}$Mo is attributed to an increase of the energy gap between the $g_{9/2}$ and $d_{5/2}$ orbitals, leading to different particle-hole configurations. The experimental finding and the interpretation demarcate an Isospin-symmetric IOI lying on the N = Z line for the first time.
References
[1] F. Nowacki, A. Obertelli and A. Poves, Prog. Part. Nucl. Phys. 120, 103866 (2021).
[2] S. M. Lenzi, F. Nowacki, A. Poves and K. Sieja, Phys. Rev. C 82, 054301 (2010).
[3] H. Iwasaki et al., Nucl. Instrum. Methods Phys. Res. A 806, 123 (2016).
[4] D. D. Dao and F. Nowacki, Phys. Rev. C 105, 054314 (2022).