The $E2$ transition matrix elements of isobaric multiplets are expected to follow a linear trend as a function of isospin projection. However, measurements of the 2⁺→0⁺ transitions in the $A=38$ triplet of Ca, K, and Ar show a deviation from this trend with an enhanced transition strength in ³⁸Ca with respect to its mirror ³⁸Ar. We have studied analogue 11/2^-→7/2^- E2 transitions in ³⁹Ca and its mirror partner ³⁹K to determine if this enhancement persists in neighboring Ca isotopes. Recoil-distance lifetime measurements of ³⁹Ca and ³⁹K were performed utilizing a ⁴²Sc secondary beam, the TRIPLEX plunger, the GRETINA array, and the S800 spectrograph. Our data provide a lifetime measurement of the (11/2^-) state in ³⁹Ca as well as an improved lifetime result for the (9/2^-) state, while the ³⁹K data are used to validate the present analysis. A comparison of the present data to shell-model calculations suggests an enhanced transition strength in ³⁹Ca, pointing to both proton and neutron contributions to core excitations across the $Z=N=20$ shell gaps in close proximity to ⁴⁰Ca.
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Sanchez et al. (2024) studied this question.
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