Resonance Neutron Capture in 138Ba and 140Ce and the Prompt Neutron Correction to ?-ray Detectors

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Abstract
The neutron capture cross sections of 138Ba and 140Ce have been measured with high energy resolution between 3 and 100 keY using the capture cross section facility at the 40 m station of the Oak Ridge Electron Linear Accelerator. The average s-wave level spacings are <D)s = 6�3 � 1 �7 and 3�2 � 0�8 keY for 138Ba and 140Ce respectively. The deduced s-wave neutron strength functions are 104So = 1�0�0�4 and 1�54�0�53 respectively, and the average s-wave radiative widths are <r,)s = 55�20 and 35�9meY. The p-wave neutron strength functions are 104S1 ~ 0�03 and 0�32�0�12 respectively. The average 30 keY Maxwellian capture cross sections are 3�9�0�8mb for 138Ba and 7�7 � 0�9 mb for 140Ce. A more rigorous treatment of the prompt neutron correction for l'-ray detectors is described, and it is shown that previously published results for 138Ba have underestimated this correction. The 140Ce data provide excellent corroboration for the calculated magnitude and time dependence of the prompt neutron correction.

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