Speaker
Description
We have studied the $^{29}$Mg(d,p)$^{30}$Mg reaction in inverse kinematics, with the first use of a reaccelerated rare-isotope beam from FRIB delivered to the SOLARIS solenoidal spectrometer. The N=20 Island of Inversion (IoI) at $^{32}$Mg is well known, arising from a diminished gap between the $sd$ and $fp$ neutron shells. Evidence for this modification comes from a variety of studies including mass measurements [1], neutron knockout [2], Coulomb excitation [3] and proton scattering [4]. A pioneering study of the two-neutron transfer reaction $^{30}$Mg(t,p)$^{32}$Mg [5] and theoretical analyses [6] of those data suggested that the $^{32}$Mg ground state is strongly deformed with significant contributions of 2p-2h and 4p-4h neutron excitations out of the $sd$ shell. Differing interpretations of the approach to N=20 in the Mg isotopes exist, however. Coulomb-excitation work from the MINIBALL experiment suggest that $^{30}$Mg is a spherical nucleus residing fully outside of the IoI [7], while other Coulex measurements [8] indicate that $^{30}$Mg is deformed, and knockout data [9] identify non-negligible $fp$-shell strength in $^{30}$Mg that is approximately 30% of that seen in $^{32}$Mg. The question of whether the transition to N=20 IoI is sudden, or occurs more gradually, with particle-hole excitations already playing a role in the structure of low-lying states in $^{30}$Mg, can also be addressed by studying neutron transfer to the low-lying 0$^+$ states in $^{30}$Mg with the $^{29}$Mg(d,p)$^{30}$Mg reaction. Furthermore, little information exists about the negative-parity states in $^{30}$Mg, which inform us about the $fp$-shell single-particle energies. Shell-model calculations yield different predictions about the energies of negative-parity excitations, and single-neutron transfer strongly populates such states. We studied the $^{29}$Mg(d,p)$^{30}$Mg reaction in inverse kinematics using a reaccelerated $^{29}$Mg beam produced by the ReA6 facility at FRIB. The $^{29}$Mg beam, with an intensity of approximately 40000 particles per second and energy of 8.46 AMeV, bombarded a 200 μg/cm$^2$ CD$_2$ target. Protons and recoiling $^{30}$Mg nuclei were detected with SOLARIS, providing high-resolution measurements for final states in $^{30}$Mg. The data suggest significant mixing between the ground and first-excited 0$^+$ states, clear evidence for several negative-parity excitations, and one new previously unobserved state. We will present these results, and discuss comparisons of the experimental observations with predictions from shell-model calculations.
This material is based upon work supported by the U. S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract Numbers DE-SC0014552 (UCONN) and DE-AC02-6CH11357 (ANL) and used resources of the Facility for Rare Isotope Beams (FRIB) Operations, which is a DOE Office of Science User Facility under Award Number DE-SC0023633. SOLARIS is funded by the DOE Office of Science under the FRIB Cooperative Agreement DE-SC0000661.
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[6] A.O. Macchiavelli et al., Phys. Rev. C 94, 051303(R) (2016).
[7] O. Niedermaier et al., Phys. Rev. Lett. 94, 172501 (2005).
[8] V. Chisté et al., Phys. Lett. B 514, 233 (2001).
[9] J. R. Terry et al., Phys. Rev. C 77, 014316 (2008).