A high-resolution measurement of the photofission spectrum of 232Th near threshold

The Chalk River bremsstrahlung monochromator at the University of Illinois Microtron Laboratory has been used with a multiwire fission chamber to measure the photofission cross section of ²³²Th between 4.95 and 6.76 MeV with a resolution of 12–14 keV. This cross section, which on the average increases exponentially with photon energy, shows three plateaus 5.20 to 5.80, 5.90 to 6.15, and above 6.30 MeV respectively. Structure is observed on eached plateau. In particular there appear to be well-resolved peaks, separated by ≈ 100 keV, at 5.50 and 5.60 MeV on the lowest plateau and other prominent peaks are observed at 5.92, 6.03 and 6.11 MeV on the middle plateau. The closely spaced structure in each region might be interpreted as evidence for the excitation of vibrational states in shallow outer wells of a multiwell potential.