Two-neutron knockout as a probe of the composition of states in Mg22,Al23 , and Si<

Abstract

Simpson and Tostevin proposed that the width and shape of exclusive parallel momentum distributions of the A − 2 residue in direct two-nucleon knockout reactions carry a measurable sensitivity to the nucleon single-particle configurations and their couplings within the wave functions of exotic nuclei. We report here on the first benchmarks and use of this new spectroscopic tool. Exclusive parallel momentum distributions for states in the neutron-deficient nuclei 22 Mg, 23 Al, and 24 Si populated in such direct two-neutron removal reactions were extracted and compared to predictions combining eikonal reaction theory and shell-model calculations. For the well-known 22 Mg and 23 Al nuclei, measurements and calculations were found to agree, supporting the dependence of the parallel momentum distribution width on the angular momentum composition of the shell-model two-neutron amplitudes. In 24 Si, a level at 3439(9) keV, of relevance for the important 23 Al(p,γ) 24 Si astrophysical reaction rate, was confirmed to be the 2 + 2 state, while the 4 + 1 state, expected to be strongly populated in two-neutron knockout, was not observed. This puzzle is resolved by theoretical considerations of the Thomas-Ehrman shift, which also suggest that a previously reported 3471-keV state in 24 Si is in fact the (0 + 2) level with one of the largest experimental mirror-energy shifts ever observed.

In-beam γ -ray spectroscopy of Cr62,64

“…The only shell-model effective interaction available so far that captures well both the onset of collectivity in the neutron-rich Cr and Fe isotopes around N = 40 and the presence of coexisting structures in 68 Ni is the LNPS one [9]. This is in part attributed to its extensive model space which encompasses the full f p shell for protons and the 0 f 5/2 , 1p 3/2 , 1p 1/2 , 0g 9/2 , and 1d 5/2 orbitals for neutrons, relative to a 48 Ca core [9]. Neutron particle-hole excitations across the N = 40 harmonic oscillator shell gap into the g 9/2 and d 5/2 orbitals emerge as critical factors in the description of the collective properties.…”

“…[44], complex 48 Ca + 26 Mg reactions at energies roughly 200% above the Coulomb barrier resulted in the production of a large number of isotopes with Z = 25-28 comprising both proton-and neutron-rich products. It is possible that reactions of the same type such as 48 Ca + 22 Ne might reach the neutron-rich Cr isotopes. Alternatively, fusion or few-neutron transfer reactions with a radioactive projectile 12.…”

“…As already demonstrated in Refs. [25,45,46], the shape of the knockout residue's parallel momentum distribution depends strongly on the total angular momentum of the two removed nucleons, and, at a more detailed level, on the components of the total orbital angular momentum [47,48]. By combining an eikonal model of the reaction dynamics [49] and the LNPS shell-model calculations of the two-proton amplitudes connecting the 64 Fe ground state and the 62 Cr final states, the cross sections for two-proton knockout from the 64 Fe ground state to the 0 + 2 and 6 + 1 states of 62 Cr and associated parallel momentum distributions were calculated.…”

In-beam $γ$-ray spectroscopy of $^{32}$Mg via direct reactions

“…In general, odd-spin cross sections are very small as compared to even-spin states, as was the case for 22 Mg in Ref. [51]. For SDPF-M and EEdf1, the proton sdpf model space allows the population of negative-parity and J = 5 + states in the two-proton knockout reaction.…”

“…[44][45][46][47]. In the reactions of this kind, the parallel momentum distribution of the knockout residue is indicative of the total angular momentum of the removed nucleons [48][49][50][51], and thus the experimental momentum distribution associated with each final state can be used to infer the spin and parity.…”