Plasma focus neutron energy and anisotropy measurements using zirconium–beryllium​ pair activation detectors

Zirconium and beryllium fast-neutron activation detectors are used to investigate the fusion neutron emission from the NX3 Plasma Focus (PF) device operated in deuterium gas, with 7.2 kJ bank energy. Both Zr and Be activation cross-sections increase with energy, but with different trends, enabling an effective neutron energy Eneff to be inferred from the Zr/Be count ratio. The relationship between Eneff and Zr/Be count ratio is established by MCNP5 simulation. Compact Zr/Be detector pairs were positioned at 0° and 90° to the PF axis, permitting measurements of neutron yield, energy- and fluence-anisotropy to be made for each shot. Series of NX3 shots were performed for D2 gas pressures ranging from 1.5 to 10 mbar. Typical effective neutron energies Eneff for the 0° and 90° directions are found to be ∼2.8 MeV and ∼2.5 MeV, respectively. The highest neutron yields of ∼109 neutron/shot were observed for 5 mbar D2 gas pressure. Neutron fluence-anisotropy AnBe for individual shots ranged from ∼2.5 to ∼4.5. The mean value AnBe exhibits a steady decline with increasing D2 gas pressure. By contrast, neutron energy-anisotropy ΔEn remains almost constant as the D2 gas pressure is varied. The effect of blocking the forward D+ ion beam with an obstacle plate positioned 6 cm in front of the anode tip is also studied. Marked reductions in both neutron yield and fluence-anisotropy are observed, whilst the effective neutron energy Eneff increases slightly at both 0° and 90° directions. Fusion contributions from thermonuclear or gyrating-particle processes are found to be negligible. All results are completely consistent with a straightforward beam–target model of PF fusion.