WO2003075037A1 - Détecteur de rayons x et de neutrons - Google Patents

Détecteur de rayons x et de neutrons Download PDF

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Publication number
WO2003075037A1
WO2003075037A1 PCT/US2003/005958 US0305958W WO03075037A1 WO 2003075037 A1 WO2003075037 A1 WO 2003075037A1 US 0305958 W US0305958 W US 0305958W WO 03075037 A1 WO03075037 A1 WO 03075037A1
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WO
WIPO (PCT)
Prior art keywords
scintillator
detector
capture
photons
neutrons
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Application number
PCT/US2003/005958
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English (en)
Inventor
Lee Grodzins
Peter Rothschild
Original Assignee
American Science And Engineering, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US10/156,989 external-priority patent/US20030165211A1/en
Priority claimed from US10/330,000 external-priority patent/US20040256565A1/en
Application filed by American Science And Engineering, Inc. filed Critical American Science And Engineering, Inc.
Publication of WO2003075037A1 publication Critical patent/WO2003075037A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T3/00—Measuring neutron radiation
    • G01T3/06—Measuring neutron radiation with scintillation detectors

Definitions

  • the present invention relates to devices and methods for detecting neutrons and high-energy photons such as x-rays and gamma rays.
  • a detector for detecting neutrons has a first scintillator containing high neutron-capture-cross-section atoms for capturing neutrons, the high neutron- capture-cross-section atoms being such as to emit electrons in a decay process upon neutron capture, and the first scintillator being such as to emit electromagnetic radiation in response to electrons emitted by the high neutron-capture-cross-section atoms; and an optical detector for detecting the emitted electromagnetic radiation and for generating an electrical signal.
  • the high neutron-capture-cross-section atoms may be chosen from the group including gadolinium, boron, and cadmium, and the scintillator, in particular, may be Gd 2 O 2 S or doped Gd 2 O 2 S.
  • the detector may additionally have a moderator for converting fast neutrons into thermal neutrons to be captured by the high neutron-capture- cross-section atoms.
  • the optical detector may include a photodiode or a photomultiplier tube.
  • a detector for detecting high-energy photons with enhanced efficiency.
  • the detector includes a heavy element backing in a path of photons that have traversed the first scintillator for generating Auger electrons and concomitant secondary photons.
  • Another embodiment of the detector further includes a second scintillator in a path of photons that have traversed the first scintillator for increasing x-ray detection efficiency.
  • the second scintillator may also be a neutron-detecting scintillator, a particular embodiment utilizing gadox.
  • a detector for detecting neutrons includes a first scintillator containing high neutron-capture-cross- section atoms that emit massive particles in a decay process upon neutron capture, the first scintillator emitting electromagnetic radiation in response to the emission of massive particles.
  • the detector also includes a heavy element backing for photons that have traversed the first scintillator that generates Auger electrons and concomitant secondary photons, and an optical detector that detects the emitted electromagnetic radiation and the concomitant secondary photons and generates an electrical signal.
  • the detector may utilize atoms from a group including gadolinium, boron, cadmium, and lithium for the high neutron-capture-cross-section atoms.
  • a method for detecting neutrons.
  • a scintillator is provided that contains high neutron-capture-cross-section atoms for capturing the neutrons and emitting electromagnetic radiation, with at least one dimension of the scintillator exceeding the mean free path of an optical scintillation photon of a specified wavelength range in the scintillator.
  • Photons at the specified wavelength range are detected with a photodetector characterized by a position with respect to the scintillator.
  • a direction of a detected neutron may be inferred based on the position of the photodetector detecting scintillation photons.
  • Another embodiment includes the step of moderating incident fast neutrons for capture by the high neutron-capture-cross-section atoms.
  • a method for detecting concealed fissile material.
  • a first scintillator for absorbing massive fission products and generating visible light
  • a second scintillator in a path of photons that have traversed the first scintillator
  • Photons arising in the scintillators are detected.
  • An alternative to this embodiment may be practiced by additionally providing a heavy element backing in the path of photons that have traversed the second scintillator for generating Auger electrons and concomitant secondary photons, and detecting the concomitant secondary photons in the first and second scintillators.
  • FIG. 1 is a schematic view of essential elements of a thermal neutron detector in accordance with preferred embodiments of the present invention
  • FIG. 2 is a schematic view of a 4 ⁇ thermal neutron detector in accordance with embodiments of the present invention
  • FIG. 3 is a schematic view showing the use of backscatter detectors of a cargo inspection system for detection of thermal neutrons in accordance with preferred embodiments of the present invention
  • FIG. 4 is schematic view of essential elements of an enhanced x-ray or gamma ray detector in accordance with preferred embodiments of the present invention
  • FIG. 5 is a schematic view of a combined enhanced photon detector and thermal neutron detector in accordance with further embodiments of the present invention.
  • a high-efficiency detector for thermal neutrons is described.
  • a new type of detector is described herein that has high efficiency for detecting thermal and epi-thermal (intermediate energy, typically 1-10 4 eV) neutrons.
  • the detector uses the scintillator Gd 2 O S, commonly known, and referred to herein, as "gadox," to stop both neutrons and the photons.
  • Gadox is well-known as a scintillating material for x-rays and is used in x-ray detection applications, such as in sheets for lining the inner surfaces of x-ray detector boxes in x-ray inspection products.
  • X-ray-induced scintillations from the gadox in the visible portion of the spectrum are then detected, typically by photomultipliers or photodiodes.
  • Gadox has good efficiency for stopping photons of energies below about 100 keV and converting the ionization energy into optical light that can be detected by a photomultiplier tube (PMT) or photodiode.
  • PMT photomultiplier tube
  • Gadox may be doped with various elements, typically rare earths, with the dopants determining the optical spectrum and the lifetime of the optical transitions.
  • Gd 2 O 2 S:Pr is a preferred dopant in that the lifetime of the light output is short, ⁇ 5 ⁇ s (depending upon the amount of Pr), and the light is emitted primarily at a single wavelength, 511 nanometers, so there is very little "afterglow” that occurs from multiple wave length emission some of which are long lived. It is to be understood that for purposes of neutron detection, the use of any dopant falls within the scope of the present invention.
  • the nucleus of the gadolinium isotope 157 Gd is also a highly efficient absorber of thermal neutrons, with a thermal neutron cross section of 255,000 barns.
  • the mean free path ⁇ n of the Gd isotope for capturing thermal neutrons is only 1 mg/ cm , whereas the mean free path of scintillator gadox, made from naturally occurring gadolinium, is 8.2 mg/cm".
  • the absorption of a neutron by 157 Gd leads to 158 Gd, with an excess energy of 7.9 MeV.
  • This energy is dissipated in a cascade of gamma rays as the nucleus passes from upper to lower quantum states.
  • the 79.5-keV first excited state of Gd takes part in most of the cascades.
  • the 79.5 keV state with a half -life of 2.52 nsecs, decays by either of two modes, either direct emission of a gamma ray photon, or ejection of an electron.
  • a 79.5 keV gamma ray is emitted in about 15% of the decays, The majority of the 79.5 keV states decay, however, by internal conversion, a process by which the energy is transferred to atomic electrons.
  • strongly neutron absorbing materials may also be utilized, such as materials incorporating lithium, for example LiF. Interaction of any of these high neutron capture cross-section materials with a captured neutron may result in the emission of one or more massive particles, non-limiting examples including alpha particles, neutrons, and electrons, which interact with the scintillator material to generate photons.
  • the present invention is distinguished from the incidental use of gadolinium (or boron) in plastic scintillators where the element is not used for scintillation but merely to produce radiations that are subsequently detected in plastic scintillators.
  • Gd in the plastic is an inert element; it takes no part in the scintillation process and is not required for scintillation; indeed it deteriorates the scintillators effectiveness by shortening the mean free path of the optical emission, which is why, typically, only 1% by weight can be used.
  • gadox contains 79.5% Gd, has an efficiency exceeding that of plastic scintillators by about two orders of magnitude.
  • the present invention may also advantageously provide smaller, lighter, and lower-cost detectors, and versatility for special geometries.
  • GSO Cerium-doped Gd 2 SiO 5
  • phosphors containing elements of high neutron-capture cross- section may be employed, in accordance with other embodiments of the present invention, to yield a screen providing a direct image of thermal neutrons that may be optically imaged by a camera such as a pinhole or multiple pin-hole camera.
  • phosphors include, for example, Y x Gd ⁇ - x BO 3 :Eu.
  • gadox screens are typically made of polycrystalline Gd O 2 S, which has a short mean free path for the optical light generated when ionizing radiation passes through.
  • Gadox self-absorbs its own emitted light with a mean free path of from 75 to 150 mg/cm 2 depending on the amount of dopant in the scintillator. Thicknesses much greater than the mean free path of the optical light are of no use since much of the light is absorbed before reaching the PMTs.
  • the gadox is viewed by a PMT that efficiently detects the emitted optical light, and the signals are analyzed in an energy-dispersive mode, then x-rays and neutrons can be effectively sorted since the neutrons result in a spectrum of lines around 30 keV and 70 keV, while the signals from x-rays and gamma rays typically produce quite different pulse heights.
  • the 79.5 keV gamma ray emitted in the decay of Gd when captured by the gadox, produces the same signals as does the internal conversion electrons, and thereby increases the efficiency of detecting thermal neutrons. Even without energy discrimination, the x-rays and neutrons can be sorted by virtue of details of the detection mechanism, as described below.
  • a thickness of 20 mg/cm 2 of gadox is sufficient to stop the neutrons (>2 mean free paths) as well as the electrons produced in the (n, ⁇ ) reaction.
  • a 20 mg/cm 2 gadox thickness produces a readily detectable signal of from 500 to 1,000 or so optical photons from the stopped electrons.
  • the short stopping thickness of gadox for thermal neutrons is in sharp contrast to the long mean free paths for x-rays: the mean free paths of x-rays of 40 keV, 100 keV, and 185 keV are 200 mg/cm 2 , 400 mg cm 2 , and 1800 mg/cm 2 , respectively.
  • the thickest gadox screens that one can use are about 150 mg/cm 2 , as discussed above.
  • a 20 mg/cm" gadox screen has a detection efficiency of only about 5% for detecting a 100 keV x-ray, indeed, it is almost transparent to photons in the range of 100 keV and above.
  • the different mean free paths can be used to make a "4 ⁇ " neutron only detector, or a directional neutron detector, or a "4 ⁇ " neutron plus x-ray detector, or a directional detector for both x-rays and neutrons, as discussed in greater detail below.
  • FIG 1 shows the essential elements of a neutron detector 6.
  • a gadox scintillator screen 10 covered by an optical shield 8 that reflects the internally generated light, is viewed by a PMT 20.
  • Thermal neutrons entering the gadox 10 along path 12 are absorbed by the 157 Gd and Auger (internal conversion) electrons 14 are produced.
  • the optical photons 16 are captured by the photocathode 18 of the PMT 20, producing a signal at the anode that is processed by pulse electronics 21.
  • the neutrons 12 are stopped in the first 20 mg/cm 2 of the gadox 10.
  • the optical photons 16 have a maximum travel of about 200 mg/cm 2 in gadox.
  • a region 4 of a moderator material, such as paraffin, for example, may be provided in order to slow any fast neutrons 2 and enable their detection in the manner herein described with respect to thermal neutrons. If the gadox is thicker than the maximum optical photon travel distance, then neutrons stopping in the outer layer are not detected. Neutrons entering the gadox from the side facing the PMT photocathode are readily detected.
  • a "4 ⁇ " neutron-only detector is made by placing a 20 mg/cm thick gadox screen 10, with its accompanying optical shield and reflector 8 and photodetector(s) 20, in a box 22 shielded by a material 24 opaque to x-rays.
  • Shield 24 may be 5mm thickness of bismuth or 1cm of lead, to cite two examples.
  • Bismuth with a mean free path of 17cm for thermal neutrons, is essentially transparent to the neutrons but effectively shields the gadox from x-rays and gamma rays that have any appreciable interaction with the gadox.
  • the gadox need only be much thicker than the mean free path of the optical light, say 300 mg/cm".
  • the light detected by a PMT must have come from neutrons that entered from the side of the gadox facing the photocathode of the PMT.
  • This embodiment when unshielded with respect to x-rays, is also a directional detector for x-rays that have a mean free path much less than 150 mg/cm 2 in gadox. And when this detector is placed in a bismuth-lined box, it may be rendered sensitive to neuttons only.
  • a moderator such as paraffin
  • fast neutrons such as those emitted by Plutonium
  • a hand-held thermal neutron detector employs one of a variety of different configurations.
  • One example is a hand-held, thermal neutron detector that signals the general direction of the origin of the neutron.
  • a gadox screen typically on the order of 300 mg cm" thick, is sandwiched between two, rectangular, side-window PMTs that efficiently detect the fluorescent light emitted by the gadox.
  • Each photomultiplier only detects neutrons that enter the gadox from the face facing the PMT.
  • the signals from each PMT are processed and analyzed by techniques well known in the art; commercial signal processors of PMT signals are available that consume little space and power and easily fit in a hand-held .instrument.
  • the pulse heights from each PMT are sorted into those produced by neutrons and x-rays.
  • the detector thus described, without a collimator gives only a two- value directionality of the neutrons. More precise directionality can be attained with the use of collimators made from material such as gadolinium, boron or cadmium that 5. strongly absorb the neutrons.
  • gadox detectors are in x-ray inspection systems to find neutron-emitting material in baggage at airports or in freight cargo.
  • baggage or cargo 300 is irradiated by x-ray beam 310, typically swept as a pencil beam in a scanning pattern across object 300.
  • x-ray beam 310 typically swept as a pencil beam in a scanning pattern across object 300.
  • Other beam shapes are 0 within the scope of the present invention.
  • Detectors 312 of backscattered x-rays 314 are hollow rectangular boxes whose inner surfaces are lined with gadox 316, approximately 150 mg/cm thick, or, alternatively, gadox and another scintillator that does not absorb neutrons.
  • Photomultipliers 20 intrude into the boxes to detect the fluorescent light.
  • the gadox 318 on the surfaces facing the inspected container 300 are thin enough that fluorescent light from the neutrons stopped in the outer 10 mg/cm" layer can efficiently escape out of the scintillator and be detected by the PMTs.
  • a more effective solution is to add a separate section to the backscatter detectors with additional PMTs.
  • thick gadox can be placed on the surface furthest away from the 0 inspected container, and a scintillator that is essentially transparent to neutrons can be placed on the surface facing the inspected container.
  • the neutrons will be absorbed on the inner surface of the gadox, allowing the scintillation light to be detected by the PMTs.
  • Gamma rays and x-rays will be absorbed and detected in both the gadox and the other scintillator. In this way, the efficiency for absorbing and 5 detecting high energy x-rays or gamma rays may be maximized.
  • Gadox scintillation screens can be placed, in accordance with other embodiments of the invention, on traditional gamma ray detectors that have excellent efficiency for detection of both high and low energy x-rays or gamma rays.
  • the gamma ray detectors act as light conduits for the fluorescent light produced by the gadox 0 screens.
  • the gadox can be optically coupled to plastic scintillators viewed by PMTs, the latter may be used to efficiently detect high energy gamma rays.
  • the gadox can be optically coupled to high-Z gamma ray detectors such as Nal(Tl), BGO, CsI(Tl), etc.
  • the signals from the two distinct scintillators can generally viewed by a single PMT with the signals from the two scintillators distinguished by their different pulse shapes, a technique well known in the art.
  • the signals from the two scintillators can be separated by placing a notch filter for the 511 nanometer line on one of the PMTs so that it only counts light from the gadox.
  • the detection efficiency of scintillation screen detectors is enhanced for x-rays above about 70 keV, with particular utility for x-ray energies in the 100 keV to 200 keV range.
  • Advantage is taken of the fact that heavy materials such as tungsten, lead and uranium are excellent converters of higher energy photons to lower energy photons, which, in turn, are more efficiently detected by the gadox. The invention is now described with reference to the schematic shown in Fig. 4.
  • the detector 40 which consists of a scintillator 30, such as gadox, lining the inside of the front face, a scintillator 32, such as gadox, lining the inside of the back face of the detector, PMTs 36 viewing the interior of the detector to measure the intensity of the light emitted from the gadox, and a sheet 34 of a heavy element, such as lead, backing the scintillator 32.
  • the detector may not utilize a scintillator on the back face of the detector, the back face including a sheet of the heavy element.
  • the operation of the detector is illustrated by imagining that 100, 100 keV x-rays (such as the K x-ray of uranium) and 100, 185 keV gamma rays (from the decay of fissionable 235 U) impinge on the detector.
  • the screens 30, 32 are assumed to be 150 mg/cm" gadox, their maximum effective thickness.
  • the backing 34 is assumed to be 5 mm of lead, which is thick enough to stop the 100 keV and 185 keV photons.
  • the front gadox layer 30 stops and detects about 30 of the 100 keV x-rays, letting 70 x-rays through.
  • the back gadox layer 32 stops 21 of the 70 x-rays so that if the lead sheet 34 were not present, 49 x-rays would pass out the back end of the detector; the efficiency of the detector for 100 keV x-rays would be -50%.
  • the remaining 100 keV x-rays 48 stop in the lead sheet, 34.
  • the stopping is primarily the result of the 100 keV x-rays ejecting K electrons 50 from the lead atoms (the photoelectric effect).
  • K x-rays are emitted with energies of 72 and 75 keV; for illustration we use the dominant 75 keV x-ray.
  • the fraction of incident x-rays that result in K x-rays emitted backwards into the detector is given by the ratio,
  • Figure 5 shows a simple detector 80 that has excellent efficiency for thermal neutron capture, and good efficiency for both the K x-rays of uranium or plutonium and the 185 keV gamma rays emitted by fissionable uranium. Detection of fast neutrons by employing an intervening moderator is discussed elsewhere herein.
  • the configuration is similar to that of Figure 4, the only substantive difference is that the first gadox layer 50 is only 20 mg/cm 2 thick. That front layer stops the thermal neutrons 52 producing strong signals in the PMTs corresponding to the deposition of 25 keV and 70 keV electrons. A fraction of the 100 keV and 185 keV photons 54 stop in a back scintillator layer 60, the remainder 56 stop in the lead backing 62. The back
  • scintillator layer 60 in accordance with one embodiment, may itself be gadox or another high-neutron-capture material, and, more specifically, may have a thickness of 150 mg/cm 2 .
  • the stopping of the remainder photons 56 in the lead backing 62 can result in the ejection of K electrons 64 which may be subsequently detected by the back scintillator layer 60, as described above.
  • the overall detection efficiency for the 100 keV and 185 keV radiations is about 60% and 25% respectively.
  • the detection efficiency for thermal neutrons is -50%.
  • the second detector can be any scintillator with good stopping power, including gadox.
  • the first detector may be a scintillator other than gadox while the second detector is gadox.
  • the first detector can be thin gadox or a scintillator other than gadox.

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  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
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  • High Energy & Nuclear Physics (AREA)
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  • Spectroscopy & Molecular Physics (AREA)
  • Measurement Of Radiation (AREA)

Abstract

L'invention concerne un appareil et un procédé permettant de détecter des neutrons, notamment, à l'aide d'une sensibilité directionnelle. Cet appareil est un détecteur muni d'un scintillateur (10) contenant des atomes (20) présentant une importante section transversale servant à capturer des neutrons et à émettre un rayonnement électromagnétique et un détecteur optique permettant de détecter le rayonnement électromagnétique émis et de générer un signal électrique. Les atomes (20) présentant une importante section transversale et destinés à la capture des neutrons peuvent être, notamment, des atomes de gadolinium, et le détecteur peut également comprendre un modérateur servant à convertir les neutrons rapides en neutrons thermiques qui sont ensuite capturés par lesdits atomes. L'appareil et le procédé selon l'invention permettent également de détecter des photons à haute énergie. La capture de ces derniers peut être améliorée en incluant un scintillateur supplémentaire (32, 60) ou un support (34, 64) constitué d'un élément lourd qui produit des électrons d'Auger et des photons secondaires concomitants qui sont ultérieurement détectés par un ou plusieurs scintillateurs.
PCT/US2003/005958 2002-03-01 2003-02-27 Détecteur de rayons x et de neutrons WO2003075037A1 (fr)

Applications Claiming Priority (6)

Application Number Priority Date Filing Date Title
US36085402P 2002-03-01 2002-03-01
US60/360,854 2002-03-01
US10/156,989 2002-05-29
US10/156,989 US20030165211A1 (en) 2002-03-01 2002-05-29 Detectors for x-rays and neutrons
US10/330,000 2002-12-26
US10/330,000 US20040256565A1 (en) 2002-11-06 2002-12-26 X-ray backscatter mobile inspection van

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WO2003075037A1 true WO2003075037A1 (fr) 2003-09-12

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10901113B2 (en) 2015-03-20 2021-01-26 Rapiscan Systems, Inc. Hand-held portable backscatter inspection system
US11143783B2 (en) 2002-07-23 2021-10-12 Rapiscan Systems, Inc. Four-sided imaging system and method for detection of contraband
US11175245B1 (en) 2020-06-15 2021-11-16 American Science And Engineering, Inc. Scatter X-ray imaging with adaptive scanning beam intensity
US11340361B1 (en) 2020-11-23 2022-05-24 American Science And Engineering, Inc. Wireless transmission detector panel for an X-ray scanner
US11525930B2 (en) 2018-06-20 2022-12-13 American Science And Engineering, Inc. Wavelength-shifting sheet-coupled scintillation detectors
US11579327B2 (en) 2012-02-14 2023-02-14 American Science And Engineering, Inc. Handheld backscatter imaging systems with primary and secondary detector arrays

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US5734166A (en) * 1996-09-20 1998-03-31 Mission Support Incorporated Low-energy neutron detector based upon lithium lanthanide borate scintillators

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NOVIKOV V M: "A METHOD FOR MONITORING OF GD CONCENTRATION IN GD-LOADED SCINTILLATORS", NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH, SECTION - A: ACCELERATORS, SPECTROMETERS, DETECTORS AND ASSOCIATED EQUIPMENT, NORTH-HOLLAND PUBLISHING COMPANY. AMSTERDAM, NL, vol. 366, no. 2/3, 1 December 1995 (1995-12-01), pages 413 - 414, XP000559674, ISSN: 0168-9002 *

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US11143783B2 (en) 2002-07-23 2021-10-12 Rapiscan Systems, Inc. Four-sided imaging system and method for detection of contraband
US11579327B2 (en) 2012-02-14 2023-02-14 American Science And Engineering, Inc. Handheld backscatter imaging systems with primary and secondary detector arrays
US10901113B2 (en) 2015-03-20 2021-01-26 Rapiscan Systems, Inc. Hand-held portable backscatter inspection system
US11300703B2 (en) 2015-03-20 2022-04-12 Rapiscan Systems, Inc. Hand-held portable backscatter inspection system
US11561320B2 (en) 2015-03-20 2023-01-24 Rapiscan Systems, Inc. Hand-held portable backscatter inspection system
US11525930B2 (en) 2018-06-20 2022-12-13 American Science And Engineering, Inc. Wavelength-shifting sheet-coupled scintillation detectors
US11175245B1 (en) 2020-06-15 2021-11-16 American Science And Engineering, Inc. Scatter X-ray imaging with adaptive scanning beam intensity
US11340361B1 (en) 2020-11-23 2022-05-24 American Science And Engineering, Inc. Wireless transmission detector panel for an X-ray scanner
US11726218B2 (en) 2020-11-23 2023-08-15 American Science arid Engineering, Inc. Methods and systems for synchronizing backscatter signals and wireless transmission signals in x-ray scanning

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