Radioactivity Induced by a Local Mass Injection Initiated Dense Plasma Focus

Abstract

Radioactivity induced in material irradiated by accelerated deuterons has been investigated in experiments conducted using a unique configuration of dense plasma focus (DPF) driven by the Hawk pulsed-power generator at the Naval Research Laboratory. The high inductance (607 nH) and associated high voltage (640 kV) and fast rise time (1.2 µs) of Hawk are unusual for a DPF driver, as is the initialization of the DPF using local mass injection rather than a conventional neutral gas fill. The local mass injection involves injecting transient neutral gas and plasma into a vacuum chamber at prescribed locations just prior to application of the main current pulse that drives the DPF. Evidence of ions accelerated to energies over an order of magnitude higher than the applied generator voltage has been observed in ion multi-pinhole images obtained with absorbers of varying thickness and in neutron time-of-flight detector signals. In this work an aluminum plate was positioned for irradiation by accelerated deuterons escaping the plasma during the DPF pulse. Following irradiation, a NaI(Tl) detector was used to record the energy spectrum and decay rate of the radioactivity induced in the plate. At the same time, an x-ray/γ-ray sensitive image plate placed in contact with the irradiated plate was used to record the spatial distribution of that radioactivity. The measurements showed γ-ray spectra and half-lives consistent with induced 27 Al(d,p) 28 Al and 12 C(d,n) 13 N reactions in the aluminum and from carbon impurities in the chamber. The induced radioactivity was concentrated at the center of the plate, which was located on the axis of the machine during irradiation.