DREB Conference 2024

Unlike standard like-particle pairing (neutron-neutron, proton-proton) that only exists in the T=1 channel, proton-neutron pairing can exist in both T=1 and T=0 channels. The consequences of this coexistence are not yet fully understood, but could explain phenomena such as the overbinding of self-conjugate nuclei.

Proton-neutron pairing can be studied by spectroscopy as in ref. [1], or by transfer reactions, as in ref. [2] , since the two-nucleon transfer reaction cross-section is expected to be enhanced by pairing. The relative proton-neutron pairing strengths between T=1 and T=0 channels can be accessed by measuring transfer cross-sections to the low-lying (J=0${}^{+}$, T=1) and (J=1${}^{+}$, T=0) states in odd-odd N=Z nuclei. We chose to use (p,${}^3$He) reaction as its selection rules allow to populate both states at once.

As pairing is a collective effect, it is expected to be stronger in the middle of high j orbitals. The f${}_{7/2}$ shell is the highest j shell currently accessible with sufficient beam intensity for two-nucleon transfer reactions in N=Z nuclei. We are thus investigating ${}^{48}$Cr, which lies in the middle of this shell, and comparing it with previous experiments in the same region [2]. Moreover, ${}^{48}$Cr is a good candidate to study the interplay between pairing and deformation since it is known to be a good rotor up to spin 10${}^{+}$ [3].

The experiment to measure the two-nucleon transfer reaction ${}^{48}$Cr(p,${}^3$He)${}^{46}$V was performed at GANIL. A radioactive ${}^{48}$Cr beam was produced by fragmentation of a primary ${}^{50}$Cr beam and selected by the LISE spectrometer, before impinging on a CH${}_2$ target. A forward array of DSSD-CsI telescopes (MUGAST) was used to detect and identify light charged particles, and was coupled to 12 EXOGAM Germanium clovers around the target, a zero-degree detection (ZDD) and MWPC detectors to reconstruct event by event the beam position on the target.

I will present preliminary cross-sections and angular distributions for the low-lying states of ${}^{46}$V. They will be compared with second-order Distorted Wave Born Approximation (DWBA) calculations for two-nucleon transfer performed with both realistic and single particle two-nucleon amplitudes (TNA). The results will be put in perspective with the systematics in the f-shell : ${}^{56}$Ni(p,${}^3$He), ${}^{52}$Fe(p,${}^3$He) and ${}^{40}$Ca(p,${}^3$He).

[1] Cederwall, B., Moradi, F., Bäck, T. et al. Evidence for a spin-aligned neutron-proton paired phase from the level structure of Pd. Nature 469, 68-71 (2011). https://doi.org/10.103/nature09644

[2] Le Crom, B., Assié, M. et al. Neutron-proton pairing in the N=Z radioactive fp-shell nuclei Ni and Fe probed by pair transfer, Physics Letters B 829 (2022), 137057. https://doi.org/10.1016/j.physletb.2022.137057

[3] Robinson, S. J. Q. and Hoang, T. and Zamick, L. and Escuderos, A. and Sharon, Y. Y. Shell model calculations of B(E2) values, static quadrupole moments, and g factors for a number of N = Z nuclei. Phys. Rev. C 89 (2014), 014316. https://link.aps.org/doi/10.1103/PhysRevC.89.014316