Nuclear excitation by electronic transition of 235U

Background: Nuclear excitation by electronic transition (NEET) is a rare nuclear excitation that can occur in isotopes containing a low-lying nuclear excited state. Over the past 40 yr, several experiments have attempted to measure NEET of U-235 and those experiments have yielded conflicting results. Purpose: An experiment was performed to determine whether NEET of U-235 occurs and to determine its excitation rate. Method: A pulsed Nd:YAG laser operating at 1064 nm with a pulse energy of 790 mJ and a pulse width of 9 ns was used to generate a uranium plasma. The plasma was collected on a catcher plate and electrons from the catcher plate were accelerated and focused onto a microchannel plate detector. An observation of a decay with a 26-min half-life would suggest the creation of U-235m and the possibility that NEET of U-235 occurred. Results: A 26-min decay consistent with the decay of U-235m was not observed and there was no evidence that NEET occurred. An upper limit for the NEET rate of U-235 was determined to be lambda(NEET) < 1.8 x 10(-4) s(-1), with a confidence level of 68.3%. Conclusions: The upper limit determined from this experiment is consistent with most of the past measurements. Discrepancies between this experiment and past measurements can be explained by assuming that past experiments misinterpreted the data.