Publisher：Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics  

The calcium isotopes are an ideal system to investigate the evolution of shell structure and magic numbers. Although the properties of surface nucleons in calcium have been well studied, probing the structure of deeply bound nucleons remains a challenge. Here, we report on the first measurement of unbound states in 53Ca and 55Ca, populated from 54,56Ca(p,pn) reactions at a beam energy of around 216 MeV/nucleon at the RIKEN Radioactive Isotopes Beam Factory. The resonance properties, partial cross sections, and momentum distributions of these unbound states were analyzed. Orbital angular momentum l assignments were extracted from momentum distributions based on calculations using the distorted wave impulse approximation (DWIA) reaction model. The resonances at excitation energies of 5516(41) keV in 53Ca and 6000(250) keV in 55Ca indicate a significant l = 3 component, providing the first experimental evidence for the ν0f7/2 single-particle strength of unbound hole states in the neutron-rich Ca isotopes. The observed excitation energies and cross-sections point towards extremely localized and well separated strength distributions, with some fragmentation for the ν0f7/2 orbital in 55Ca. These results are in good agreement with predictions from shell-model calculations using the effective GXPF1Bs interaction and ab initio calculations and diverge markedly from the experimental distributions in the nickel isotones at Z=28.

DOI： 10.1016/j.physletb.2024.138828

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Publisher：Physical Review C  

Background: The proton-induced deuteron knockout reaction, (p,pd), is one of the interests in studies probing the deuteron-like p-n correlation in nuclei. According to a recent study of the inclusive deuteron-induced reaction, (d,d′x), the refraction effect of the deuteron has a significant effect on the elementary process, nucleon-deuteron (N-d) binary scattering inside a nucleus, of the reaction. In the paper, it is shown that proper treatment of the local N-d relative momentum in the elementary process is crucial in (d,d′x) reactions at 100 MeV and below. Purpose: In the present work, we investigate the deuteron refraction effect in the exclusive (p,pd) reactions. We also discuss the incident energy dependence of the refraction effect. Method: The refraction effect on the p-d elementary process is taken into account by the local semiclassical approximation to the distorted waves. The results are compared with those obtained with the asymptotic momentum approximation, which is standardly applied to the distorted wave impulse approximation framework. Result: It is shown that the refraction effect drastically changes the energy sharing distribution of the O16(p,pd)N14 reaction at 101.3 MeV and gives a better agreement with experimental data. In contrast, it is confirmed that the effect is negligibly small at 250 MeV. Conclusion: We have clarified that the deuteron refraction effect is significant in the O16(p,pd)N14 reaction at 101.3 MeV and the experimental data are well reproduced. The refraction effect plays a significant role in both the shape and magnitude of the (p,pd) cross section, while the effect is negligible at 250 MeV.

DOI： 10.1103/PhysRevC.110.014617

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Publisher：Computer Physics Communications  

PIKOE, Proton-Induced KnockOut reaction calculation for Exclusive processes, is a Fortran 90 program that calculates triple- and quadruple-differential cross sections, vector analyzing powers, and momentum distributions of reaction residues, for proton-induced nucleon knockout reactions in normal and inverse kinematics. The distorted-wave impulse approximation with the factorization approximation to the nucleon-nucleon transition matrix is adopted, and the distorted waves of the incoming proton and the outgoing two nucleons are calculated quantum mechanically. Kinematics of the reaction particles are treated in a relativistic manner, which gives the proper asymptotics of the three-body scattering wave in the plane-wave limit. Program summary: Program Title: pikoe CPC Library link to program files: https://doi.org/10.17632/m594h58kck.1 Licensing provisions: MIT Programming language: Fortran 90 Supplementary material: See https://www.rcnp.osaka-u.ac.jp/~kazuyuki/pikoe/index.php Nature of problem: Calculation of triple- and quadruple-differential cross sections, vector analyzing powers, and momentum distributions of reaction residues, for nucleon knockout reactions with proton beams/targets in normal/inverse kinematics. Solution method: Numerov method for solving single-channel second-order differential equations; multi dimensional integration with the Gauss-Legendre quadrature rule. Additional comments including restrictions and unusual features: The global optical potential by Koning and Deraloche [1] for nucleons at energies from 1 keV to 200 MeV, for (near-)spherical nuclei with mass number between 24 and 209, is implemented in the program. Tables of nucleon-nucleon (NN) elastic differential cross sections and NN transition amplitudes calculated with the NN effective interaction by Franey and Love [2] are provided as external files for PIKOE (included in the package). Users are encouraged to prepare tables for the optical potentials and the cross sections of the elementary process according to the reaction system of interest by themselves. With external files thus prepared, PIKOE can be applied to various kinds of knockout processes such as α or deuteron knockout by proton, α-induced knockout reactions, etc. Calculation of spin observables, the vector analyzing power in fact, however, is restricted to nucleon-induced nucleon knockout processes with the coplanar kinematics.

DOI： 10.1016/j.cpc.2023.109058

Scopus

Publisher：Physical Review C  

The effective polarization of the residual nucleus in the proton-induced α knockout reaction is investigated within the distorted wave impulse approximation framework. The strong absorption of the emitted α particle results in strong selectivity on the reaction position depending on the third component of the single-particle orbital angular momentum of the α particle inside a nucleus, hence on the spin direction of the reaction residue. This is caused by a mechanism that is similar to the Maris effect, the effective polarization in the proton-induced proton knockout reactions. However, as a distinct feature of the effective polarization in the α knockout process, the spin degrees of freedom of the reacting particles play no role. The α knockout process with complete kinematics can be a useful polarization technique for the residual nucleus, without actively controlling the spin of the proton.

DOI： 10.1103/PhysRevC.107.054603

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Scopus

Publisher：Physical Review C  

Background: An isoscalar pn pair is expected to emerge in nuclei that have similar proton and neutron numbers and it may be a candidate for a deuteron "cluster."There is, however, no experimental evidence for it. Purpose: The purpose of this paper is to construct a new reaction model for the (p,pd) reaction including the deuteron breakup in the elementary process and the deuteron reformation by the final-state interactions. How these processes contribute to the observables of the reaction is investigated. Methods: The distorted wave impulse approximation is extended twofold. The elementary processes of the (p,pd), i.e., the p-d elastic scattering and the d(p,p)pn reaction, are described with an impulse picture employing a nucleon-nucleon effective interaction. The three-body scattering waves in the final state of the (p,pd) reaction are calculated with the continuum-discretized coupled-channels method. The triple-differential cross section (TDX) of the (p,pd) reaction is calculated with the new model. Results: The elementary processes are described reasonably well with the present model. As for the (p,pd) reaction, the deuteron reformation can either increase or decrease the TDX height depending on the interference between the elastic and breakup channels of deuteron, while the back-coupling effect always decreases it. Conclusions: It is shown that the deuteron reformation significantly changes the TDX of the (p,pd) reaction through the interference. It is important to include this process to quantitatively discuss the (p,pd) cross sections in view of the deuteron formation in nuclei. For more quantitative discussion regarding the experimental data, further improvement will be necessary.

DOI： 10.1103/PhysRevC.106.064613

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