Photoneutron Reactions: C12, N14, O16, and F19 near Threshold

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Abstract
Photoneutron reactions in carbon, nitrogen, oxygen, and fluorine have been studied in the region of threshold using improved efficiency for the detection of the residual activity. The betatron energy calibration used is based on thresholds of deuterium, bismuth, copper, and for scattering from the 15.12-Mev level in carbon. Results show that the thresholds for nitrogen and fluorine correspond well with the expected values for the respective neutron separation energies. For oxygen, the position of threshold is also in good agreement. Assuming a linear extrapolation of the betatron calibration above 15 Mev, it is found that the carbon threshold is 52 kev above the accepted value of the separation energy. The successful correlation between the assignment of known resonance energies with the positions of many of the breaks in the yield curves corroborates the assumed linearity of the betatron energy scale above 15 Mev. It follows that previous betatron calibrations using the carbon threshold must be in error by approximately 100 kev at 18.7 Mev.