The l-forbidden m1 decay of the 2522 keV level in 39K

The mean lifetime of the first excited 12+ level in 39K has been measured to be τ = 93 ± 13 fs by high recoil velocity DSAM. The level was excited by bombarding a 4He-implanted Ta target with a 100 MeV 39K beam. The lifetime was determined by fitting the Doppler-broadened lineshape of the 2522 keV gamma-ray transition to the 32+ ground state. Together with the measured B(E2), the B(M1; d32 → S12) for this l-forbidden M1 transition was determined to be 0.0100 ±0.0027 μ2N. A theoretical calculation, taking into account core polarization, delta excitation and mesonexchange current contributions, underestimates the B(M)1) by more than a factor of six. The experiment also measured the lifetimes of the 3019 keV and 3883 keV levels to be 20 ±4 fs and 11 ± 6 fs, respectively.