Speaker
Description
Physics aim
A recent measurement at the Spiral1 facility at GANIL has revealed that the $^{46}$Ar nucleus is a bubble nucleus with a filled $\pi d_{3/2}$ shell and an empty $\pi s_{1/2}$ [1]. This finding also explain the small B(E2) value for $^{46}$Ar 2$^+$ state.
This important discovery is in agreement with ab-initio calculations with the NNLOsat parametrization of the chiral effective Hamiltonian, but in disagreement with shell-model calculations with the well-established SDPF-U interaction. This latter Hamiltonian predicts the $^{46}$Ar 0$^+$ ground state to be an even mixture of the $\pi d_{3/2}$ and $\pi s_{1/2}$ shells. This also somehow what would be suggested by an heuristic reasoning, considering that in $^{47}$K the $\pi d_{3/2}$ and $\pi s_{1/2}$ shells appear to be separated by only 360 keV.
A key point then arises: if the $^{46}$Ar 0$^+$ ground state has the $\pi d_{3/2}^{4} s_{1/2}^0$ proton configuration, at which energy is the second $0^+$ with the $\pi d_{3/2}^{2} s_{1/2}^2$ wave function ? And why it does not mix with the ground state ? A measurement of its excitation energy would provide crucial information to understand if the lack of mixing is coming from a large energy difference between the two $0^+$ levels or from the small diagonal pairing between them.
Proposed measurement
We propose to populate the $\pi d_{3/2}^{2} s_{1/2}^2$ excited $0^+$ state $^{46}$Ar using a $^{48}$Ca($^6$Li,$^8$B)$^{46}$Ar reaction at about 10 MeV/u. Gamma rays will be detected by the AGATA array, while the $^8$B ejectile by a DSSD detector like TRACE or OSCAR. The reaction can be performed in direct kinematics, using a $^6$Li 56 MeV beam from the Tandem and a CaF target or in inverse kinematics with a Tandem-ALPI or PIAVE-ALPI $^{48}$Ca beam at around 500 MeV.
Considering that a cross section of about 1 $\mu$b can be intercepted by the DSSD detector, we predict that about seven days of beam time will be sufficient to detect the desired $\pi d_{3/2}^{2} s_{1/2}^2$ excited $0^+$ state. The use of PRISMA can be considered for ancillary detection of heavy recoils, at least in A/Q.
[1] D. Brugnara et al., to be submitted
