We describe an experimental campaign to confirm the production of deuterium-deuterium (DD) fusion neutrons by an inertial confinement fusion target that is driven by a one-sided impact. Experiments were performed on First Light Fusion’s 38 mm bore, two-stage light gas gun. This facility can launch a 100 g solid projectile at 6.5 km s−1. Neutron diagnostics included scintillators and moderated helium-3 proportional counters.
We have demonstrated repeatable, coincident detection of particles on multiple scintillator detectors, within the expected time window. Background contribution to the dataset was found to be negligible. In null shots, where the DD fuel was replaced with hydrogen, no particles were observed. These results indicate that dynamics within a DD target result in the generation of energetic particles. The number of scintillator hits recorded dropped off according to the solid angle subtended by detector groups at different distances, consistent with a particle source at the target location.
The nature of the energetic particles was determined using He-3 detectors and scintillator-based time-of-flight measurements. The shot-aggregated He-3 detector dataset demonstrates that neutron levels within the anticipated detection time window are well above background. The particle arrival time at scintillators positioned at different distances from the source was used to determine the particle velocity. Additionally, a fusion event timing fiducial inserted into each target provided an additional point (at x = 0) for assessing particle time-of-flight. On multiple shots, time-of-flight data was recorded that is consistent with the propagation of 2.45 MeV DD fusion neutrons. Assuming a neutron mass, an average particle energy of 2.46±0.3 MeV was determined, which, within error, is in agreement with the energy of an unscattered DD fusion neutron. No data was recorded that is consistent with the propagation of target generated photons.
Estimates of neutron yield from the detected number of hits on both detector types are in agreement. The scintillators and He-3 proportional counters were independently, absolutely calibrated for their 2.5 MeV counting efficiency. Consistency between the two detector types therefore provides further evidence that observed counts were induced by a common source of DD fusion neutrons.
The collective evidence presented leads to the conclusion that DD fusion neutrons have been observed and that they were produced within targets driven by FLF’s large two-stage gas gun. The UK Atomic Energy Authority (UKAEA) have verified the experimental setup, modelled the detector responses and witnessed two experiments. They have also reviewed the full campaign dataset, analysis and interpretation of the results.