Discriminating Uranium Isotopes Using the Time-Emission Profiles of Long-Lived Delayed Neutrons

Abstract

In nuclear nonproliferation and safeguards, detecting and accurately characterizing special nuclear material remains one of the greatest challenges. Uranium enrichment determination is typically achieved by measuring the ratio of characteristic γ-ray emissions from 235 U and 238 U. Fission also produces β-delayed neutrons, which have been used in the past to determine uranium enrichment from the time dependence of the long-lived delayed neutron emission rate. Such measurements typically use moderated 3 He tube detectors. We demonstrate a new measurement technique that employs a fast neutron active interrogation probe and a scintillation detector to measure the enrichment of uranium using both the buildup and decay of β-delayed neutron emission. Instead of 3 He tubes, a capture-based heterogeneous composite detector consisting of scintillating Li-glass and polyvinyl toluene has been constructed and used, offering a prospect to scale delayed neutron measurements to larger detector sizes. Since the technique relies on the existing tabulated nuclear data, no calibration standards are required. It is shown that the buildup of delayed neutron emission can be used to distinguish between uranium samples and infer the uranium enrichment level, with accuracy that rivals the method that employs the time-dependent decay of delayed neutron emission.

Discriminating Uranium Isotopes Based on Fission Signatures Induced by Delayed Neutrons

“…The coincidence-based approach presented here could supplement the methods described in Ref. [23], providing an additional point of distinction between isotopes.…”

“…The experimental data are fitted with a parameterized model based on a six-group superposition of delayed neutrons, whose decay constants are obtained from tabular nuclear data; the procedure is described in depth in Ref. [23]. The experimental results show close agreement with the model (χ 2 = 200.2/198), which confirms that the coincidence events are caused by delayed neutron-induced fission and suggests that discrimination based on the delayed neutron time emission profile can also be performed using fast neutron measurements, provided that the material is fissionable by lower-energy delayed neutrons.…”

“…Previous studies have applied this principle to differentiate SNM samples by measuring the decay of the delayed neutron rate for short-lived [19] and long-lived groups [20][21][22]. In our own prior work, we have utilized both the buildup and decay time profiles of long-lived delayed neutron groups to perform isotopic discrimination and infer the enrichment level of uranium [23]. While the energy and timing characteristics of delayed neutrons have been determined by dedicated precision measurements, the observed delayed neutron signature for bulk materials can be complicated by additional interactions before the delayed neutrons escape the object.…”

A New Era of Nuclear Criticality Experiments: The First 10 Years of Radiation Test Object Operations at NCERC

“…The University of Michigan studied uranium isotopics of the RF hemishells using gamma spectroscopy with CZT detectors 93,94 and delayed neutron information from AI with lithium glass and PVT detectors. 95 The University of Michigan used OSs (Ref. 96), ISs (Refs.…”

Detecting and characterizing special nuclear material for nuclear nonproliferation applications

“…For completeness, we present results demonstrating shielded uranium characterization. The technique uses delayed neutron emissions during active interrogation of uranium using a neutron generator 15 – 17 .…”