THE L-FORBIDDEN M1 DECAY OF THE 2522 KEV LEVEL IN K-39

The mean lifetime of the first excited 1/2+ level in K-39 has been measured to be tau = 93 +/- 13 fs by high recoil velocity DSAM. The level was excited by bombarding a He-4-implanted Ta target with a 100 MeV K-39 beam. The lifetime was determined by fitting the Doppler-broadened lineshape of the 2522 keV gamma-ray transition to the 3/2+ ground state. Together with the measured B(E2), the B(Mi;d3/2 --> S1/2) for this l-forbidden M1 transition was determined to be 0.0100 +/- 0.0027 mu-N2. A theoretical calculation, taking into account core polarization, delta excitation and mesonexchange current contributions, underestimates the B(M1) 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.