A Measurement of Neutron Polarization Asymmetries in Photofission of 235,238U Using Polarized γ Rays at HIγS


Jonathan M. Mueller

A photofission experiment was performed on 235U and 238U using nearly 100% linearly polarized, high intensity (~107 γs per second), and nearly-monoenergetic γ-ray beams of energies between 5.8 and 7.0 MeV at the High Intensity γ-ray Source (HIγS). An array of 18 liquid scintillating detectors, located at 55, 72, 90, 107, 125, and 142 degrees in theta and 0, 90, 180, and 270 degrees in phi, was used to measure prompt fission neutron angular distributions. The ratio of prompt fission neutron yields parallel to the plane of beam polarization to the yields perpendicular to this plane was measured as a function of beam and neutron energy. A ratio of unity was found for 235U while a significant ratio (~3 at 90 degrees in theta) was found for 238U. This significant difference could potentially be used to distinguish 235U and 238U. In addition to our experimental work, we are in the process of developing a phenomenological model to attempt to explain this large effect. A simulation, based on our model and using previous measurements of fission fragment angular distributions, is being used to interpret our experimental findings.

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