Accelerating fissile material detection with a neutron source

Patent metadata

Field	Value
Publication number	US-10557950-B2
Application number	US-201815865099-A
Country	US
Kind code	B2
Filing date	Jan 8, 2018
Priority date	Sep 24, 2004
Publication date	Feb 11, 2020
Grant date	Feb 11, 2020

Abstract

A method of discriminating fissile material from non-fissile material with a digital data acquisition system that collects data at high rate, and processes large volumes of data directly to count neutrons from the unknown source and detect excess grouped neutrons to identify fission. The system includes a Poisson neutron generator for in-beam interrogation of a possible fissile neutron source and inducing neutron emission therefrom, and a DC power supply that exhibits electrical ripple of less than one part per million. A neutron count histogram and Poisson count distribution are overlaid to provide a visual indication of the difference in correlation of natural and induced emitted neutrons from the radiation source to characterize the neutron source as fissile material or non-fissile material.

First claim

What is claimed is:

1. A method of determining whether a material as fissile or non-fissile, comprising:
generating an interrogation beam of neutrons from a pulsed electric source of neutrons;
irradiating the material by bombarding the material using the interrogation beam of neutrons;
detecting, through a detector, neutrons emitted from the material while the interrogation beam is irradiating the material, wherein the detector is configured to distinguish between electric source neutrons in the interrogation beam and induced fission neutrons from the material;
powering, with a DC power supply, a system to analyze the material;
performing an analysis of the material using the system; and
providing a Poisson neutron source comprising the electric source to generate the interrogation beam, wherein the Poisson neutron source is configured to impose no electrical ripple to distort correlation of generated neutrons and to eliminate any problematic effect of electrical ripple on the DC power supply.

2. The method of claim 1 wherein the DC power supply comprises:
two separate power supply stages configured such that their outputs are summed with a sinusoidal ripple added 180 degrees out of phase; and
a Cockroft-Walton voltage multiplier configured to operate at a frequency that enhances an effectiveness of series resistor-indictor circuit components to reduce ripple on an output DC signal.

3. The method of claim 2 further comprising:
detecting the neutrons emitted from the material in a first detector of the detector calibrated to a fast time scale on the order of nanoseconds to generate a first set of detected neutrons;
detecting the neutrons emitted from the material in a second detector of the detector calibrated to a slow time scale on the order of milliseconds to generate a second set of detected neutrons; and
analyzing the first and second sets of detected neutrons to determine the number of times that a group of n simultaneously emitted neutrons is observed from the material after a defined measurement period is repeated a defined number of times to derive a neutron count measurement based on at least one of the fast time scale and slow time scale to determine whether the material is fissile versus non-fissile.

4. The method of claim 3 further comprising comparing a neutron correlation distribution shape of the neutrons emitted from the material against a generalized Poisson distribution, a second count distribution associated with cosmic sourced neutrons correlated with the fast time scale, and a third count distribution associated with fission neutrons correlated with the slow time scale to determine, as a result of the comparing, whether or not the neutrons emitted from the material are fission neutrons emitted from the material.

5. The method of claim 4 wherein the defined measurement time period is ½ millisecond, and wherein a fissile source creates real correlations between the neutrons emitted from the material, and anon-fissile source creates no correlation or only accidental correlations between the neutrons emitted from the material.

6. The method of claim 5 further comprising:
producing a histogram representing a number of times different group sizes occur from a number of measurement time periods;
deriving a Poisson count distribution for a hypothetical Poisson neutron source; and
overlaying the histogram and the Poisson count distribution to provide a visual indication of the difference in correlation of the neutrons emitted from the material to determine whether the material as fissile or non-fissile.

7. The method of claim 6 further comprising determining the value of a coefficient, Cn, which represents a number of times that a group of neutrons is observed after repeating the ½ millisecond measurement time period a defined number of times, and wherein a mean count is calculated by the formula:

C
_

=

∑

n
=
0

∞

⁢
nCn

∑

n
=
0

∞

⁢
Cn

.

8. The method of claim 7 wherein the shape of the histogram is given by the formula:

Cn
⁢

-

⁢
poisson

=

c
_

n

n
!

⁢

⁢

e

-

c
_

.

9. The method of claim 3 wherein the detector comprises a scintillator and energy selector system that is configured to detect fast and direct neutrons emitted from the material.

10. The method of claim 9 wherein the second detector comprises at least one of a moderated neutron capture detector or a scintillator-based detector configured to detect neutrons emitted from the material in correspondence with the slow time scale.

Assignees

• L Livermore Nat Security Llc

Inventors

• Rowland Mark S
• Snyderman Neal J

Classifications

• G01N23/005

  by using neutrons (G01N23/02 - G01N23/227 take precedence) · CPC title

• G01T3/00 Primary

  Measuring neutron radiation (G01T5/00 takes precedence) · CPC title

• G01V5/281

  detecting special nuclear material [SNM], e.g. Uranium-235, Uranium-233 or Plutonium-239 · CPC title