Springham, Stuart Victor

The effect of varied concentrations of deuterium-krypton (D₂ - Kr) admixture on the neutron emission of a fast miniature plasma focus device was investigated. It was found that a judicious concentration of Kr in D₂ can significantly enhance the neutron yield. The maximum average neutron yield of (1 ± 0.27) x 10⁴ n/shot for pure D₂ filling at 3 mbars was enhanced to (3.14 ± 0.4) x 10⁵ n/shot with D₂ + 2% Kr admixture operation, which represents a ˃ 30-fold increase. More than an order of magnitude enhancement in the average neutron yield was observed over the broader operating range of 1–4 mbars for D₂ + 2 % Kr and D₂ + 5% Kr admixtures.

The anisotropies of neutron and high energy deuteron emissions from the NX2 plasma focus device [M. V. Roshan et al., Phys. Lett. A 373, 851 (2009)] are studied. The nuclear activation of graphite targets is used to measure the fluences of high energy deuterons in the axial and radial directions. Two bismuth germanate scintillation detectors connected to multichannel analyzer systems are used for the detection of 511 keV gamma rays resulting from positron annihilation in the two targets. In addition, fast neutron activation detectors are employed to measure the axial and radial fluences of fusion neutrons. These detection systems are calibrated using the simulation code _MCNPX [L. S. Waters et al., AIP Conf. Proc. 896, 81 (2007)]. Two distinct regimes of neutron and deuteron anisotropies are observed for the NX2 device. For deuterium gas pressures below 10 mbar, the neutronanisotropy increases with increasing pressure, while the overall neutron yield remains low. For gas pressures of 10–14 mbar, the neutronanisotropy is essentially constant, while, with increasing pressure, the neutron yield rises rapidly and the deuteron anisotropy falls.

This work aims to test the concept of an activation detector for pulsed DD fusion neutron sources, based on the production of metastable ^79m within a LaBr₃(Ce) scintillator crystal via (n, n') inelastic scattering. The pulsed neutron source employed is the NX3 Plasma Focus (PF) device operated in deuterium gas, which yields about 10⁹ neutrons per shot. A range of D₂ gas pressures, from 1 to 13 mbar are used to vary the test conditions. For the sake of comparison, a beryllium fast-neutron activation detector is used simultaneously with the LaBr₃(Ce), and for each NX3 PF shot we derive neutron yield values from both Be and LaBr₃(Ce) detectors, denoted Y_n^Be and Y_n^LaBr. The two detectors are positioned in the equatorial plane (θ=90˚) of the NX3 to expose them to bursts of neutrons with energies close to 2.5 MeV, to simulate a thermonuclear DD fusion source. Overall, the shot-to-shot values of Y_n^Be and Y_n^LaBr obtained compare reasonably well. At each D₂ gas pressures the 10-shot averaged values〈Y_n^Be〉 and 〈Y_n^LaBr〉are mostly within 10% of one another; for the worst case (10 mbar)〈Y_n^LaBr〉is 25% higher than〈Y_n^Be〉. Overall, it is concluded that LaBr₃(Ce) scintillation detectors can function as a capable and readily obtainable fast-neutron activation detector for measuring neutron yields from pulsed DD fusion sources.