High energy x-y neutron detector and radiographic/tomographic device

• US 5,410,156 A
• Filed: 08/13/1993
• Issued: 04/25/1995
• Est. Priority Date: 10/21/1992
• Status: Expired

• Current Assignee: Thomas G. Miller
• Original Assignee: Thomas G. Miller
• Inventors: Miller, Thomas G.
• Primary Examiner(s): Dzierzynski, Paul M.
• Assistant Examiner(s): HANIG, RICHARD E
• Application Number:
  US08/106,437
• Time in Patent Office: 620 Days
• Field of Search: 250/390.04, 250/390.11, 250/390.12, 250/390.02, 250/390.03
• US Class Current: 250/390.04
• CPC Class Codes: G01N 23/09 the radiation being neutrons
• CPC Class Codes: G01N 23/18 Investigating the presence of flaws defects or foreign matter
• CPC Class Codes: G01T 3/06 with scintillation detectors
• CPC Class Codes: G01V 5/22 Active interrogation, i.e. by irradiating objects or goods using external radiation sources, e.g. using gamma rays or cosmic rays
• CPC Class Codes: G01V 5/226 using tomography

Abstract

An improved fast neutron x-y detector and radiographic/tomographic device utilizing a white neutron probe (4). The invention includes a multiple scattering filter (44), radiographic and tomographic imaging of the number densities of atoms in small volume increments through a sample 32 and the atomic, chemical and physical structure of a sample, (32), and neural net analysis techniques, where the neural net is trained through use of simulated volume increments. The invention detects fast neutrons over a two dimensional plane, measures the energy of the neutrons, and discriminates against gamma rays. In a preferred embodiment, the detector face is constructed by stacking separate bundles (6) of scintillating fiber optic strands (20) one on top of the other. The first x-y coordinate is determined by which bundle (6) the neutron strikes. The other x-y coordinate is calculated by measuring the difference in time of flight for the scintillation photon to travel to the opposite ends of the fiber optic strand 20. In another embodiment, the detector is constructed of discrete scintillator sections (48) connected to fiber optic strands (52) by couplers (50) functioning as lens. The fiber optic strands (52) are connected to a multi-anode photomultiplier (100) tube (56). The x-y coordinate of a neutron interaction is determined by the row and column of the affected scintillation section (48). Neutron energy for both detectors is calculated by measuring the flight time of a neutron from a point source (2) to the detector face.

Claims

1. A method for analyzing neutrons of multiple energies which have passed through a sample to determine the presence or absence of certain atoms in specified number densities and ratios, for purposes of explosives detection or analysis of a sample, comprising the steps of:

producing a white neutron beam ( a beam of neutrons of multiple energies);

determining the neutron attenuation of the white neutron beam without a sample in the path of the beam;

directing the white neutron beam through the sample;

reducing the multiple scattering of said neutrons;

measuring the attenuation of the neutrons which travel through the sample without scattering;

comparing the baseline white neutron beam directed onto the sample with the unscattered neutrons passing through the sample, and determining neutron attenuation as a function of neutron energy;

comparing the resulting attenuation with known neutron cross-sections;

creating a radiographic or tomographic image showing the number densities and ratios of atoms throughout volume increments of the sample through such comparison; and

determining whether an explosive or other specific substance is present in any such volume increment by comparing the resulting number densities and ratios of atoms in said volume increments of the sample to known number densities and ratios of atoms in explosives or other substance sought to be identified.

2. Apparatus for producing a radiographic/tomographic view of a sample showing the number densities of atoms in volume increments through a sample, consisting of a first means for producing a beam of white neutrons and directing said beam;

a second means for conveying samples into place for exposure to the beam;

a third means for directing the beam through the sample;

a fourth means for reducing the multiple scattering of neutrons by the sample;

a fifth means of detecting neutrons;

a sixth means of determining the location in the sample through which such neutrons pass;

a seventh means of measuring the intensity of neutrons both before and after the sample is placed in the neutron beam;

an eighth means of comparing the neutrons intensities which reach the detector means without a sample in the neutron beam path with the neutrons passing through the sample;

a ninth means of determining the number densities or ratios of atoms, atomic, chemical or physical structure of the sample through such comparison;

a tenth means of creating a tomographic image of the number densities or ratios of atoms, atomic, chemical or physical structure of the sample; and

an eleventh means of comparing said samples with a database of samples with known features for features sought to be identified in the unknown samples.

3. Apparatus and means of claim 2 whereby

(A) the first means comprises a neutron accelerator producing a pulsed beam of neutrons of multiple energies from 0.5-15 MeV;

(B) the second means comprises a conveyor track system in which samples are conveyed one at a time onto a turntable for purposes of exposure to said neutron beam;

(C) the third means comprises a sample turntable in front of a neutron x-y detector so that all portions of said sample are exposed to said beam of neutrons emanating from a neutron point source produced by said accelerator;

(D) the fourth means comprises the frustum of a cone placed between said sample and detector, which frustum consists of a neutron attenuating material and is constructed in a dartboard configuration, in which sections of the dartboard alternate between solid segments and hollow passages through such frustum, such that the wall of each hollow passage consists of the outside edge of a solid segment, and the hollow passages are constructed along straight lines from said point source, which lines are perpendicular to said detector, and means for rotating such frustum on its axis;

(E) the fifth means comprises;

(i) one or more scintillating fiber optic strands of a predetermined geometric shape and length,

(ii) which fiber optic strands are formed into a predetermined number of discrete bundles (consisting of one or more of said strands) stacked linearly one on top of the other;

(iii) with one or more scintillation sensors attached to the end of each bundle, so that all fiber optic strands are coupled to a scintillation sensor at each end of the bundle in which the fiber optic strand is located;

(F) the sixth means comprises means of determining the specific bundle containing a fiber optic strand in which a neutron interaction occurs, thereby providing the first two dimensional coordinate of the neutron interaction, and means for calculating the other two dimensional coordinate of said neutron interaction by measuring the difference in time which it takes a photon to travel to opposite ends of such strand, a time delay being place in one end to facilitate calibration;

(G) the seventh means comprises means for calculating the energy of a neutron by calculating the time of flight of the neutron from the neutron point source to the interaction on said fiber optic strand;

(H) the eighth means comprises a means to determine the neutron attenuation by first measuring and recording in a computer the neutron spectrum with the sample out of the neutron beam and then comparing such data with a measurement of the neutron spectrum with the sample in the neutron beam;

(I) the ninth means comprises a means to reconstruct the number densities or ratios of atoms, atomic, physical or chemical structure of the sample by using known total neutron cross sections to determine which elements in the sample and their number densities caused the measured neutron attenuation;

(J) the tenth means comprises a means for creating a tomographic image of the number densities of atoms, atomic, physical or chemical structure through volume increments in the sample by determining the neutron attenuation through the sample for several angles through the sample;

(K) the eleventh means comprises a database to train a neural network to identify features of volume increments provided in a tomographic image of an unknown sample, using said database containing actual or simulated results of radiographic scans of volume increments equal to the volume increments provided by said tomographic scan, where the volume increments in the database contain known features sought to be identified, so that the neutral network can identify features of a volume increment of said unknown sample when the same features appear in the volume increments of such database.

4. Apparatus and means of claim 3, whereby there is a plurality of said fiber optic strands described in the fifth means which are routed to two or more scintillation sensors at each end of a bundle in an alternating pattern, so that for any given scintillating strand attached to a given scintillation sensor, all contiguous fiber optic strands are routed to a different scintillation sensor on the same end of the bundle, thereby allowing discrimination of gamma rays from neutron scattering events.

5. Apparatus and means of claim 3, in which the scintillation sensor described in the fifth means is a photomultiplier tube and the bundles are approximately 4 centimeters by 4 centimeters thick, comprised of approximately 64 fiber optic strands per bundle, and are greater than one meter in length.

6. Apparatus and means of claim 3, in which the scintillation sensor described in the fifth means is a photomultiplier tube and the bundles are comprised of a single bar of scintillating fiber optic material approximately 4 centimeters by 1 centimeter thick, and are equal to or less than one meter in length.

7. Apparatus and means of claim 3 in which the scintillation sensors described in the fifth means of claim 2 have discrete channels, such as multichannel photomultiplier tubes, microchannel plates, or CCD type detectors, allowing each strand in a bundle to be attached to a discrete anode or channel, so that the scintillation sensor detects which of said strands registers a scintillation.

8. Apparatus of claim 3 in which the fifth means is a neutron detector constructed as follows:

a neutron detector consisting of discrete sections of a certain size constructed of a material which scintillates upon interaction with a neutron;

each scintillator section is connected by a coupler to a non-scintillating fiber optic cable, which coupler is constructed to concentrate the light, and which fiber optic cable is connected to one anode of a multi-anode photomultiplier tube;

the photomultiplier tube is connected to means for voltage and signaling which in turn is connected to a specific memory bank in a computer, whereby neutrons contacting a scintillator section cause the creation of a photon, which travels down the fiber optic cable to said anode of the photomultiplier tube and is recorded in the memory bank of a computer; and

the sixth means comprises a means for identifying the specific scintillator section in which a neutron interaction occurs; and

the seventh means comprises a means for measuring the time of flight of a neutron from the point source to a scintillator section, and producing an output signal containing such information.

9. Apparatus of claim 3 whereby the fifth means comprises a high energy neutron detector and radiographic/tomographic device, comprising:

(a) fiber optic scintillating strands of a predetermined geometric shape and length, comprising a material which scintillates when a neutron interaction occurs emitting light,

(b) one or more of said scintillating strands are fastened into a discrete bundle of a predetermined width and depth,

(c) a predetermined number of said bundles are attached linearly one on top of the other, whereby a detector face with two dimensional coordinates is formed, with one set of coordinates being the separate rows formed by the discrete bundles stacked one on top of the other, and the other set of coordinates being the points along the length of the scintillating strands constituting the bundles,

(d) means for attaching the respective ends of the scintillating strands in each bundle to one or more scintillation sensors attached to each end of each of said bundles,

(e) a means for determining the bundle in which a neutron interaction occurs by means of registering on one or more of said scintillation sensors attached to each end of such bundle, thereby determining one coordinate of the two dimensional location of the neutron on the detector face, and an apparatus combining or/sum and sum circuits, time to amplitude converters and other common electronic equipment such as discriminators, cabling and power supplies and a specific location in the memory bank of a computer for storing output information, for measuring the difference in time that it takes a scintillation photon in a said fiber optic strand, caused by a neutron incident on said fiber optic strand, to travel to the opposite ends of such fiber optic strand, and producing an output signal containing such information, thereby allowing calculation of the other coordinate of the two dimensional location of the neutron on the detector face,

(f) the detector is constructed so that the neutrons from said point source strike the detector face approximately perpendicular to the lengths of one or more fiber optic strands constituting each of such bundles; and

a combination of or/sum and sum circuits, time to amplitude converters and other common nuclear electronic equipment such as discriminators, cabling and power supplies and a specific location in the memory bank of a computer for storing output information, in order to measure the time of flight of a neutron from the point source to the detector face, and producing an output signal containing such information, thereby allowing calculation of the energy of the neutron.

10. Apparatus of claim 9, whereby said fiber optic strands are routed to scintillation sensors at each end of a bundle in an alternating pattern, so that for any given scintillating strand attached to a given scintillation sensor, all contiguous scintillating strands are routed to a different scintillation sensor on the same end of the bundle, thereby allowing discrimination of gamma rays from neutron interactions.

11. Apparatus of claim 9 in which the scintillation sensors have discrete channels, such as multichannel photomultiplier tubes, microchannel plates, or CCD type detectors, allowing a plurality of strands to be attached to each anode or channel of the sensor, or allowing the scintillation sensor to discriminate which specific scintillating strand in a bundle registers a scintillation.

12. Apparatus of claim 9 having a sample turntable or similar device between the point white neutron source and the detector array in order that several neutron radiographic/spectroscopic views may be taken through the sample from 0°

to 180°

or from 0°

to 360°

.

13. In a device for analyzing neutrons of multiple energizer means to reduce or eliminate the multiple scattering of radiation emanating from an object towards a detector, consisting of radiation attenuating material divided into sections of a specified geometric shape consisting of alternating solid segments and hollow passages, which device is placed between a sample and a detector, and configured so that (1) the said hollow passages are rotated or oscillated so as to expose the entire detector surface, at different moments in time, to radiation proceeding through the sample, and (2) the dimensions of said device, including its width and diameter of its said hollow passages and said segments, are constructed so that radiation which is scattered in said sample will not proceed through a hollow passage to the detector face.

14. Means of claim 13 for reducing or eliminating the multiple scattering of radiation, consisting of the frustum of a cone divided into alternating segments in a "dart board" configuration around its axis, with sections consisting of alternating hollow segments and solid passages, consisting of a radiation attenuating material, and includes a means for rotating said frustum through its axis, and is constructed so that said hollow passages lie along a straight line proceeding from a point neutron source through a sample onto and perpendicular to the face of a radiation detector.

15. Means of claim 13 for reducing the scattering of radiation from an object onto a detector, consisting of a means of exposing the entire face of said detector at different moments in time through the rotation or oscillation of solid portions of radiation attenuating material and hollow passages in such material.