High-power, photofission-inducing bremsstrahlung source for intense pulsed active detection of fissile material

Abstract

Intense pulsed active detection (IPAD) is a promising technique for detecting fissile material to prevent the proliferation of special nuclear materials. With IPAD, fissions are induced in a brief, intense radiation burst and the resulting gamma ray or neutron signals are acquired during a short period of elevated signalto-noise ratio. The 8 MV, 200 kA Mercury pulsed-power generator at the Naval Research Laboratory coupled to a high-power vacuum diode produces an intense 30 ns bremsstrahlung beam to study this approach. The work presented here reports on Mercury experiments designed to maximize the photofission yield in a depleted-uranium (DU) object in the bremsstrahlung far field by varying the anode-cathode (AK) diode gap spacing and by adding an inner-diameter-reducing insert in the outer conductor wall. An extensive suite of diagnostics was fielded to measure the bremsstrahlung beam and DU fission yield as functions of diode geometry. Delayed fission neutrons from the DU proved to be a valuable diagnostic for measuring bremsstrahlung photons above 5 MeV. The measurements are in broad agreement with particlein-cell and Monte Carlo simulations of electron dynamics and radiation transport. These show that with increasing AK gap, electron losses to the insert and outer conductor wall increase and that the electron angles impacting the bremsstrahlung converter approach normal incidence. The diode conditions for maximum fission yield occur when the gap is large enough to produce electron angles close to normal, yet small enough to limit electron losses.

Simulations of the generation and transport of a 5 MV end-point x-ray beam on a pulsed power generator

“…The power pulse is driven on the left boundary of the simulation. In the simulation, the center conductor (cathode) has a step-down in radius (from 10.46 cm to 8.42 cm) just before the axial location where the electron emission from the cathode starts, similar to the step used in previous simulation work [2,12]. A spacecharge limited (SCL) electron emission model was applied from the location of this step-down along the rest of the axial length of the cathode.…”

“…During this campaign, an appropriate target was fielded in this x-ray beam, and the pulse of photoneutrons produced in this target was measured. This method of producing a pulse of x-rays and neutrons has various applications, such as detection of special nuclear material [2].…”

Beryllium and Indium Activation Measurements of Total Neutron Yield from a Fast-Pulsed Photoneutron Source

“…Washington, D.C. [1]. The objective of this experiment was to verify the feasibility of using a pulsed power source to create a large number of photoneutrons in a short period of time, leveraging the low energy threshold for photoneutron production in 9 Be to maximize neutron dose.…”

Active Interrogation of Depleted Uranium Using a Single Pulse, High-Intensity Photon and Mixed Photon-Neutron Source

“…A large area x-ray diode was used to generate the x-ray source [15]. Electrons were accelerated into a m thick, 35.56 cm-diameter tantalum convertor layer covered by a m-thick aluminium foil and backed by a 1.91 cm-thick aluminium beam stop.…”