Plasma Focus Neutron Energy Measurements Using Zirconium and Beryllium Activation Detectors

Zirconium and beryllium fast-neutron activation detectors are used to investigate the DD fusion neutron emission from the NX3 Plasma Focus (PF) device operated in deuterium gas. The differing energy dependence of the Zr and Be activation cross-sections enables an effective neutron energy
${E_{\mathrm{n}}}^{\text{eff}}$
to be inferred from the Zr/Be count ratio for individual PF shots. For the fast-neutron activation detector geometry, the relationship between the Zr/Be count ratio and
${E_{\mathrm{n}}}^{\text{eff}}$
is established by MCNP5 simulation. Zirconium and beryllium detector pairs are positioned at 0° and 90° to the PF axis, so that the anisotropy of the neutron energy is measured from shot to shot. The NX3 PF is operated at 12 kV, corresponding to 7.2 kJ capacitor bank energy. Shots are performed for D2 gas pressures ranging from 1.5 to 10 mbar. The highest neutron yields of ∼10
⁹
neutron/shot are observed for 5 mbar D2 gas pressure. Typical effective neutron energies
${E_{\mathrm{n}}}^{\text{eff}}$
for the 0° and 90° directions are ∼2.8 MeV and ∼2.5 MeV, respectively, and the neutron energy-anisotropy remains close to
$\Delta E_{\mathrm{n}}\cong 0.3\ \text{MeV}$
over the range of D2 gas pressures investigated. The effect of blocking the forward fast-deuteron beam with an obstacle plate positioned in front of the PF anode is also studied. For this case, shots are performed for D2 gas pressures ranging from 1 to 6 mbar. Insertion of the beam-obstacle plate results in a large reduction in neutron yield, while the effective neutron energy
${E_{\mathrm{n}}}^{\text{eff}}$
increases for both the 0° and 90° directions. The results indicate that DD fusion contributions from thermonuclear or gyrating-particle processes are negligible. The collected results are entirely consistent with beam-target fusion being the dominant mechanism within the plasma pinch and throughout an extended cone on the forward (0°) axis of the NX3 PF.