Absolute measurements of neutron yields from DD and DT implosions at the OMEGA laser facility using CR-39 track detectors

The response of CR-39 track detectors to neutrons has been characterized and used to measure neutron yields from implosions of DD- and DT-filled targets at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], and the scaling of neutron fluence with R (the target-to-detector distance) has been used to characterize the fluence of backscattered neutrons in the target chamber. A Monte-Carlo code was developed to predict the CR-39 efficiency for detecting DD neutrons, and it agrees well with the measurements. Neutron detection efficiencies of ${\text{(1.1±0.2)×10}}^{\text{−4}}$ and ${\text{(6.0±0.7)×10}}^{\text{−5}}$ for the DD and DT cases, respectively, were determined for standard CR-39 etch conditions. In OMEGA experiments with both DD and DT targets, the neutron fluence was observed to decrease as $R^{\text{−2}}$ up to about 45 cm; at larger distances, a significant backscattered neutron component was seen. The measured backscattered component appears to be spatially uniform, and agrees with predictions of a neutron-transport code. As an additional application of the calibration results, it is shown that the neutron-induced signal in CR-39 used in charged-particle spectrometers on OMEGA can be used to determine DD and DT yields ranging from about ${10}^{10}$ up to ${10}^{14}.$ With further improvements in the processing and analysis of CR-39, this upper limit can be increased by at least two orders of magnitude.