Measurement of prompt neutron polarization asymmetries in photofission of 235^,238U, 239Pu, and 232Th

A photofission experiment was performed on targets of 235U, 238U, 239Pu, and 232Th using nearly 100% linearly polarized, high-intensity (∼107 γ/s), and nearly monoenergetic γ-ray beams having energies between 5.6 and 7.3 MeV at the High Intensity γ-ray Source (HIγS). An array of 18 liquid scintillator detectors was used to measure prompt fission neutron polarization asymmetries. An asymmetry close to zero was found for 235U and 239Pu while a significant asymmetry (∼0.5) was found for 238U and 232Th. A simplified model of near-threshold photofission has been developed in an attempt to explain the systematic difference between the even-even and even-odd targets. The results of a simulation, based on this model and using previous measurements of fission fragment angular distributions, are shown to accurately reproduce the essential features of the data. © 2012 American Physical Society.

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Author Diane M Markoff Physics