Gamma decay of the 238U shape isomer

Abstract A search for a competing γ-decay branch of the spontaneously fissioning 238m U isomer is reported. The 200 nsec 238 U shape isomer was produced by the (d, pn) reaction with a pulsed beam of 18 MeV deuterons incident on natural uranium targets. A γ-ray line at 2.514 MeV has been observed which is attributed to the decay of the shape isomer to the 0.045 MeV 2 + rotational state of 238 U. It has a half-life in agreement with that observed for the fission branch and is produced with a yield consistent with predictions of a more penetrable inner barrier. A second weaker line has been observed at 1.879 MeV corresponding in energy to the transition from the shape isomer to the first excited 1 − state at 0.680 MeV in 238 U. The decay of this line is also consistent with the measured half-life of 238m U.