Masses of the Tz = −3/2 nuclei 27P and 29S

Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the T-z = -3/2 nuclei P-27 and S-29. The new mass excess value ME(S-29) = -3094(13) keV is 66(52) keV larger than the result of the previous S-32(He-3, He-6)S-29 reaction measurement in 1973 and a factor of 3.8 more precise. The new result for S-29 together with those of the T = 3/2 isobaric analog states in P-29, Si-29, and Al-29 fit well into the quadratic form of the isobaric multiplet mass equation (IMME). The mass excess of P-27 has been remeasured to be ME(P-27) = -685(42) keV. By analyzing the linear and quadratic coefficients of the IMME in the T-z = -3/2 sd-shell nuclei, it was found that the ratio of the Coulomb radius parameters is R approximate to 0.96 and is nearly the same for all T-z = 3/2 isospin multiplets. Such a nearly constant R value, apparently valid for the entire light mass region with A > 9, can be used to set stringent constraints on the isovector and isotensor components of the isospin nonconserving forces in theoretical calculations.