Evolution of shell gaps in the neutron-poor calcium region from invariant-mass spectroscopy of Sc 37,38, Ca 35, and K 34

A fast secondary beam of Ca37 impinged on a Be9 target resulting in a set of reactions populating proton-rich nuclei including Ca35 and the first observations of Sc37,38 and K34. Invariant-mass spectroscopy, used to reconstruct proton decays for these nuclei, yielded three new ground-state masses and information on their low-lying structures. The newly measured mass excesses are: ΔM(Sc37)=3500(410)keV, ΔM(Sc38)=-4656(14)keV, and ΔM(K34)=-1487(17)keV. These nuclei straddle the well-known Z=20 shell closure as well as the N=16 subshell closure. Trends in separation energies help elucidate how nuclear structure evolves showing a fading of the Z=20 shell gap for N≤18 and indications of a N=16 subshell gap.