One-proton and one-neutron knockout reactions from N=Z=28Ni56 to the A=55

Abstract

Article:Spieker, M., Gade, A., Weisshaar, D. et al. (15 more authors) (2019) One-proton and one-neutron knockout reactions from N=Z=28 Ni 56 to the A=55 mirror pair Co 55 and Ni 55. Physical Review C. 051304.We present a high-resolution in-beam γ -ray spectroscopy study of excited states in the mirror nuclei 55 Co and 55 Ni following one-nucleon knockout from a projectile beam of 56 Ni. The newly determined partial cross sections and the γ -decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. The new experimental data are compared to large-scale shell-model calculations in the full pf space which include charge-dependent contributions. A mirror asymmetry for the partial cross sections leading to the two lowest 3/2 − states in the A = 55 mirror pair was identified as well as a significant difference in the E 1 decays from the 1/2 + 1 state to the same two 3/2 − states. The mirror asymmetry in the partial cross sections cannot be reconciled with the present shell-model picture or small mixing introduced in a two-state model. The observed mirror asymmetry in the E 1 decay pattern, however, points at stronger mixing between the two lowest 3/2 − states in 55 Co than in its mirror 55 Ni.

Structure of Si33 and the magicity of the N=20 gap at Z=14Jongile1, Lemasson2, Sorlin3 et al. 2020Phys. Rev. CSelf Cite10030View full textAdd to dashboardCite

“…In concluding this discussion, we note also that in recent one-proton and one-neutron removal studies on 56 Ni, core-coupled final states in 55 Ni and 55 Co with complex structures, of 2 + 1 ( 56 Ni) ⊗f −1 7/2 character, were observed to be weakly populated, having shifted momentum distributions [41]. This suggests mechanisms involved in the high-spin states population may include nucleon knockout from an excited state of the projectile [29,37,41].…”

“…In concluding this discussion, we note also that in recent one-proton and one-neutron removal studies on 56 Ni, core-coupled final states in 55 Ni and 55 Co with complex structures, of 2 + 1 ( 56 Ni) ⊗f −1 7/2 character, were observed to be weakly populated, having shifted momentum distributions [41]. This suggests mechanisms involved in the high-spin states population may include nucleon knockout from an excited state of the projectile [29,37,41]. Continued experimental advances from earlier, more inclusive data limitations to higher-statistics, final-statesexclusive cross sections and, importantly, their momentum distributions thus begin to allow such contributions to be identified and quantified.…”

“…4(b), suugesting that this state may be partially or exclusively fed by a mechanism that is beyond the single-step direct removal of a neutron. In recent one-proton and one-neutron removal studies on 56 Ni, complex-structure states of core-coupled 2 + 1 ( 56 Ni) ⊗f −1 7/2 character were observed to be weakly populated in 55 Ni and 55 Co with shifted momentum distributions [41], lending credence to earlier speculations that the formation process of the unexpected high-spin states includes contributions from nucleon knockout from an excited state of the projectile [37,40,41]. As the momentum distribution of the 7/2 − isomer could not be disentangled from that of the ground state, we could not determine how much of the 7/2 − isomer pop-ulation was due to direct or indirect population, via a weaker higher-order mechanism.…”

Experimental identification of the $T = 1$, $J^π = 6^+$ state of $^{54}$Co and isospin symmetry in $A = 54$ studied via one-nucleon knockout reactions

“…Nevertheless, even though missed feeding contributions and contributions from more complex excitation mechanisms cannot be excluded, yrare states are more strongly populated than predicted by the present LSSM calculations in both 54 Co and 54 Fe. The population of the 3 − 1 state of 54 Fe in one-proton knockout from 55 Co might indicate that considering (1f 7/2 ) −1 (1s 1/2 ) 1 configurations at higher energies is important, as was also recently discussed in [35]. However, as shown by the comparison of the c coefficients, the influence of cross-shell mixing with the sd-shell configurations on the yrast states is negligible.…”

Quenching of single-particle strength from direct reactions with stable and rare-isotope beams

“…The population of higher-spin core-coupled complex-structure states in removal residues was reported for 35 Si, 35 P and the A = 55 Co and Ni mirrors in Refs. [272,51,305]. While concrete experimental evidence as to the origin of the population of these states is lacking, all of the observed states could, in principle, be reconciled with a picture where a nucleon is removed from the projectile inelastically excited to its first 2 + state, for example.…”