Orateur
Description
Proton-neutron pairing is an exotic type of pairing that appears in nuclei with $N\sim Z$, as pairing requires an important overlap of the wave functions at the Fermi surface. Unlike standard like-particle pairing (neutron-neutron, proton-proton) that exists only in the isovector ($T = 1$) channel, proton-neutron pairing can exist in both the isovector and isosacalar ($T = 0$) channels. The possibility to observe pairing condensation of two different fermionic fluids is a unique phenomenon, and its understanding is crucial to infer the structure of $N=Z$ nuclei.
Pairing can be probed using transfer reactions since the two-nucleon transfer cross section is enhanced by the collectivity of the nucleon pairs condensate. For proton-neutron pairing, the relative contribution of the isovector and isoscalar 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 the (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. 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$^+$ [1].
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 twelve 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 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 shell model and single particle two-nucleon amplitudes (TNA). The results will be put into perspective with the systematics in the $f_{7/2}$ shell, and will also be discussed in light of Anderson's limit, which predicts a disappearance of superfluidity if the level spacing at the Fermi surface becomes greater than the BCS gap [2].
[1] Robinson, S. J. Q. and Hoang, T. and Zamick, L. and Escuderos, A. and Sharon, Y. Y., Phys. Rev. C, 89, (2014), 014316. https://link.aps.org/doi/10.1103/PhysRevC.89.014316.
[2] P.W. Anderson, Journal of Physics and Chemistry of Solids, 11, (1959), https://doi.org/10.1016/0022-3697(59)90036-8.