Structure of 40Mg

H.L. Crawford et al., PRC 89, 041303 (2014)


A measurement of the direct two-proton removal from 42Si has provided the first structural information on the N = 28 isotone 40Mg. The value for the inclusive cross section for two-proton removal from 42Si of 40+27−17 μb is significantly lower than that predicted by structure calculations using the recent SDPF-MU shell-model effective interaction combined with eikonal reaction theory. This observed discrepancy is consistent with the interpretation that only one of the predicted low-lying 0+ states in 40Mg is bound. A two-state mixing analysis describing two-proton knockout cross sections along N = 28 provides support for the interpretation of a prolate-deformed 40Mg ground state.

Why do we study 40Mg?

Why not?

What did we do? And where?

Layout of the RIBF at RIKEN, located just outside of Tokyo, Japan.

To study 40Mg, we first have to produce it. We chose to do this by removing two protons from 42Si, which we were able to obtain as a radioactive beam at the RI Beam Factory (RIBF) at RIKEN Nishina Center. At this facility, 48Ca, a stable nuclide is accelerated through three coupled cyclotrons to an energy of 345 MeV/nucleon, or about 50% the speed of light. This high energy ion beam is then fragmented on a C target, and produces, with some probability, every nucleus lighter than the original beam of 48Ca. All of these fragments then fly forward and 42Si is separated from most of the rest through a series of bends in the magnets of the BigRIPS separator. We can identify each beam particle based on the time it takes to fly through the separator, and the energy it loses in various detectors, and so we are able to ensure we look only at reactions occurring on 42Si.

What did we learn?

It turns out, we could put together a fairly nice physics picture with our 5 events of 40Mg. Those 5 events gave us access to the inclusive, or the total, cross-section for producing 40Mg following the removal of 2 protons from 42Si. By itself, this doesn't tell us so much, other than the fact that this isn't a very likely process! But we also have knowledge about the cross-section for removing 2 protons from 44S to form 42Si. And by comparing these two cross-sections within a theoretical framework, we can deduce information regarding the structure of 40Mg, relative to 42Si.