Applications of Fast Neutrons in the Development of Novel Sources and Detection Signatures

Abstract

Fast neutrons are closely linked to nuclear fission, making them important in many nonproliferation applications, both as a measurable signal and as a source of probing radiation. This work examines a number of novel applications of fast neutrons in both the signal and source domains.

Fast neutrons are particularly attractive as an active interrogation probe due to their high penetrability and high cross-section for induced fission in special nuclear material (SNM). Because of this propensity to induce fission, radiographic imaging techniques based on fast neutron transmission can be used to combine geometric verification with detection of fissionable material. A simple experimental method is demonstrated for realizing crude imaging of the geometric configuration of special nuclear material and confirming its fissionable content based on spectroscopic fast neutron transmission measurements, which may be attractive in treaty verification scenarios where more complex or high-precision measurements are undesirable.

In addition to source transmission and the prompt fission signal, delayed neutron emission is a significant aspect of the overall fast neutron signature induced by neutron active interrogation of SNM. Delayed neutrons have unique isotope-specific spectral and temporal characteristics, which can provide the basis for isotope identification. Previous work has shown that measurement of the buildup and decay time profiles of long-lived delayed neutron groups can be used to perform isotopic discrimination in uranium and infer enrichment. However, in bulk materials with an appreciable fissile content (e.g., 235U), delayed neutrons have a high probability of inducing additional fission events, leading to the emission of prompt fission radiation during the delayed neutron time window. Two methods are presented for exploiting the composite nature of this signal to perform isotopic discrimination in uranium based on measurement of the delayed neutron energy spectrum and detection of coincident radiation from delayed-neutron-induced fission. Furthermore, because the long-lived delayed neutron precursors exhibit decay times on the order of tens of seconds, their time profiles are insensitive to the delay associated with scattering or diffusion in shielding, which occur on much shorter time scales. A study of the effects of neutron-moderating shielding on measurements of the delayed neutron time-emission profile is presented.

Fast neutron sources are also useful for inducing activation or transmutation reactions that cannot be produced with lower-energy neutrons or other source particles. One application of this transmutation capability is in the production of certain radionuclides that may be useful for the calibration of nuclear instrumentation. In particular, 16N and 17N are radionuclides that can be produced via (n,p) reactions in oxygen-containing targets, and are of interest for the calibration of large water-based Cherenkov detectors used for antineutrino detection. A demonstration of the production of 16N and 17N using a DT generator neutron source is presented, as well as the design, construction, and testing of a specialized beta-tagging detector. Beta-correlated measurements are performed to examine the feasibility of 17N as a time-tagged neutron source for antineutrino detector calibration.