WO2003034052A1

WO2003034052A1 - Gamma ray imaging apparatus

Info

Publication number: WO2003034052A1
Application number: PCT/GB2002/004362
Authority: WO
Inventors: Ian David Jupp; Stephen James Hunter; Tony Carter; David Ramsden
Original assignee: The Secretary Of State For Defence
Priority date: 2001-10-11
Filing date: 2002-09-30
Publication date: 2003-04-24
Also published as: GB0124424D0

Abstract

An imaging system is described which utilises pairs of gamma ray photons, the members of each pair travelling in mutually opposite directions. Position sensitive detection of the first of a photon pair allows determination of the direction of the second. This information is used in conjunction with information derived from back scattering of the second from the target volume permits generation of a two dimensional image of the density distribution within the target volume.

Description

Gamma Ray Imaging Apparatus

This invention relates to portable imaging apparatus and particularly to apparatus for imaging objects on the remote side of a barrier.

It is well known to carry out screening of articles using x-ray apparatus. For example, at airports, passenger hand luggage is screened for explosive articles. Such screening tends to be carried out using shadowing techniques. By means of a moveable x-ray beam from a continuously driven x-ray source, the luggage may be line scanned from one side. To the other side is a large array of x-ray detectors. Any x-ray impermeable objects within the luggage cast a shadow on the array, and the shadows are converted to screen images which are scrutinised by an operator. X-ray transmission imaging is also termed two-sided imaging.

Customs, security and law enforcement personnel have a requirement for a non- invasive system of detecting objects behind an obscuring barrier. For example, the detection of drugs or explosives hidden within the body panels of a vehicle where access to both sides of the panel is often not possible. Similarly, unclaimed luggage left on the platform of an underground train station may require explosive threat assessment, but moving the luggage in order to determine its contents is of itself risky. In both cases, there is a need to be able to determine the contents in situ.

X-ray or γ-ray photons impinging on an object may undergo Compton scattering. This is the result of interaction between the photons and the electrons of the object. The photons may be scattered at various angles and flux intensities, according to the composition of the object. Some will be back-scattered to the same side of the object as the source. Detection of Compton back-scattered photons, produced from incident photons thus provides a single-sided imaging capability. Known back-scatter x-ray screening apparatus which derives an image from x- rays back-scattered from an object is not easily portable, not least because it typically tends to use a mechanically scanned x-ray beam, necessitating equipment of a size and weight which does not lend itself to portability. Conventional thinking is that back-scattered x-rays produced by a portable pulsed source offer insufficient penetration for imaging applications.

Radioisotopes that decay via emission of a positron and which, on annihilation of the positron with an electron, produce a pair of γ-ray photons are well known to those skilled in the art; for example ²²Na. The radioisotope ²²Na has been used previously in single-sided anomaly detection instrumentation, see "Compton gamma flaw detection" by V E Rad'ko, Pribory I Teknika Eksperimenta 4, ppl74-193, (1993). The difficulty of imaging the back-scattered photons with an adequate degree of spatial resolution has prevented the use of such radioisotopes in imaging applications.

It is an object of this invention to provide a method and apparatus to allow imaging of objects behind barriers using γ-ray radiation.

According to a first aspect of this invention, apparatus for imaging an object on the remote side of a barrier comprises;

a radioisotope source that, via positron emission and subsequent positron- electron annihilation, produces pairs of gamma ray photons, each pair of gamma ray photons comprising a first photon and a second photon where the first photon travels in the opposite direction to the second photon,

a primary detector having two dimensional spatial resolution,

a secondary detector for detecting scattered radiation, wherein the radio-isotope is located between the primary detector and the object, and

the secondary detector is located to receive photons scattered from the object.

In a preferred embodiment, the secondary detector is located substantially in the plane of the radio-isotope and is oriented so as to receive photons back-scattered from the object.

Conveniently, the secondary detector is shielded from non-scattered radiation emitted from the radio-isotope source. This ensures only the radiation scattered from the object is detected by the secondary source.

Advantageously, the apparatus further comprising image generation means for compiling information obtained from the primary detector and the secondary detector to produce a two dimensional image of the object. The image generation means for compiling information may, for example, comprise a personal computer.

In a preferred embodiment, the secondary detector has two dimensional spatial resolution. In such an embodiment, the apparatus may further comprising image generation means for compiling information obtained from the primary detector and the secondary detector to produce a three dimensional image of the object.

Conveniently, the radioisotope source is Na,

In a preferred embodiment, the primary detector is an LSO scintillation array coupled to a multi-pixel hybrid photodiode and/or the secondary detector is a fast scintillation detector element coupled to a photo-multiplier tube. Advantageously, the Compton cross-section of the object is greater than that of the barrier.

Conveniently, the apparatus is portable.

Advantageously, the apparatus is incorporated in a kit for detecting objects on the remote side of a barrier, such packets of drugs on the remote side of a vehicle body panel.

According to a second aspect of this invention, a method of imaging an object on the remote side of a barrier, comprises the steps of,

using a radioisotope source, that decays via positron emission, to produce pairs of gamma ray photons which are emitted in opposite directions,

using a two dimensional primary detector to detect one photon of an emitted photon pair to establish the direction of incidence of the associated photon on the barrier and object,

using a secondary detector to detect photons scattered from the object and barrier,

whereby the directional information obtained from the primary detector is correlated with information gathered from the secondary detector allowing an image of the object and barrier to be constructed.

The invention will now be described by reference to the accompanying drawing in which figure 1 is an illustration of the single-side imaging apparatus according to the invention. With reference to figure 1, an object 2 under investigation is situated to one side of a barrier 4. The object 2 is represented as a three dimensional body but could be a packet of drugs, a gun, a knife or explosives etc. The barrier 4 could be the skin of a brief case or the metal body panel of a vehicle. Ideally, the object should consist of a material with a higher Compton cross-section than the barrier. Herein, the barrier 4 and the object 2 are collectively termed the target volume 5.

A radio-isotope source 6, that decays via the emission of a positron, is situated on the other side of the barrier 4. A suitable radioisotope is ²²Na which decays to

99 Ne via the emission of a positron and, on subsequent annihilation of the positron with an electron, gives rise to two 51 IKeN gamma rays that are emitted in opposite directions.

A primary detector 10 is located on the opposite side of the radio-isotope source 6 to the target volume 5. The primary detector 10 has two dimensional spatial resolution; in other words it is capable of detecting the position of impact of gamma rays incident on it from the radio-isotope source 6.

The simultaneous production by the radio-isotope source 6, via positron annihilation, of two gamma rays photons (8 and 9) which travel in opposite directions, allows one photon from each pair to be used to tag the direction of the other photon. In other words, the measurement of the position of incidence of the gamma ray photon 8 on the primary detector 10 permits the direction of travel of the associated photon 9 to be determined and hence its position of incidence on the target volume 5 can be ascertained.

A secondary detector 12 is positioned in substantially the same plane as the radio-isotope 6, and is suitable for detecting any Compton back-scattered photons incident upon it. If a Compton back-scattered photon 14 is produced by the photon 9 when it interacts with the target volume 5, this is detected by the secondary detector 12 in coincidence with its associated photon 8 being detected by the primary detector 10. The back-scattered flux measured by the secondary detector 12, . in combination with the positional information obtained from the primary detector 10, permits a two dimensional image of the density distribution within the target volume 5 to be produced by the image generation means 16. The intensity distribution in this image will be proportional to the Compton cross-section integrated along the path of each photon incident on the target volume 5.

To shield the secondary detector 12 from the gamma ray emissions from the radio-isotope source 6, appropriate radiation shielding (not shown) may also be provided.

The spatial resolution of the device is determined primarily by the spatial resolution of the two-dimensional primary detector, and also on the relative distances between the primary detector 10, secondary detector 12, radio-isotope source 6 and target volume 5.

A person skilled in the art would recognise that a variety of detector types could be used as primary and secondary detectors.

The primary detector 10 should have a high stopping power, good energy resolution and good two-dimensional spatial resolution. These requirements are met by, for example, an LSO scintillation array coupled to a Multi-pixel Hybrid Photodiode (MHD). Typical LSO MHD detectors comprise an array of 40x40 pixels, and are of approximately 80mm x 80mm in size.

The secondary detector 12 should have a fast response, a large area and high stopping power for the back scattered photons. These requirements can be met with a fast plastic scintillation detectors readout using photo-multiplier tubes. The depth of this detector ideally provides sufficient stopping power for the back-scattered photons and also shields the primary detector from the back- scattered flux.

A secondary detector 12 may also be employed which has, in addition to energy resolution, a degree of spatial resolution. The energy of the back-scattered photons and the position at which they interact with the secondary detector are thus known. This allows the angle through which incident photons have been scattered to be calculated using the kinematics of the Compton scattering process. The details of the Compton scattering process are well known to those skilled in the art.

Having both a primary and a secondary detectors with spatial resolution makes it is possible to calculate the position (in 3-D space) of the scattering volume, therefore making it possible to reconstruct a 3-D density distribution of the target volume. The spatial resolution of the 3-D density distribution map is determined by the spatial resolution of the primary and secondary detectors as would be known to a person skilled in the art.

By utilising the energy resolution of the primary detector and secondary detector it is possible to select energy windows that allow the optimisation of the imaging performance through the reduction of unwanted background noise and random coincidences.

Apparatus produced in accordance with this invention may be readily made portable, that is to say, it may be of a size and weight which lends itself to portability. Portability enables the apparatus to be taken to the location of an object requiring assessment.

Claims

1 Apparatus for imaging an object on the remote side of a barrier comprising;

a radioisotope source that, via positron emission and subsequent positron-electron annihilation, produces pairs of gamma ray photons, each pair of gamma ray photons comprising a first photon and a second photon where the first photon travels in the opposite direction to the second photon,

a primary detector having two dimensional spatial resolution,

a secondary detector for detecting scattered radiation,

wherein the radio-isotope is located between the primary detector and the object, and

the secondary detector is located to receive photons scattered from the object.

2 Apparatus for imaging an object on the remote side of a barrier as claimed in claim 1 wherein the secondary detector is located substantially in the plane of the radioisotope and is oriented so as to receive photons back-scattered from the object.

3 Apparatus for imaging an object on the remote side of a barrier as claimed in any preceding claim wherein the secondary detector is shielded from non-scattered radiation emitted by the radioisotope source.

4 Apparatus for imaging an object on the remote side of a barrier as claimed in any preceding claim and further comprising image generation means for compiling information obtained from the primary detector and the secondary detector to produce a two dimensional image of the object. 5 Apparatus for imaging an object on the remote side of a barrier as claimed in any of claims 1 to 3 wherein the secondary detector has two dimensional spatial resolution.

6 Apparatus for imaging an object on the remote side of a barrier as claimed n claim 5 and further comprising image generation means for compiling information obtained from the primary detector and the secondary detector to produce a three dimensional image of the object

7 Apparatus for imaging an object on the remote side of a barrier as claimed in any preceding claim wherein the radioisotope source is ²²Na.

8 Apparatus for imaging an object on the remote side of a barrier as claimed in any preceding claim wherein the primary detector is an LSO scintillation array coupled to a multi-pixel hybrid photodiode.

9 Apparatus for imaging an object on the remote side of a barrier as claimed in any preceding claim wherein the secondary detector is a fast scintillation detector element coupled to a photo-multiplier tube.

10 Apparatus for imaging an object on the remote side of a barrier as claimed in any preceding claim wherein the Compton cross-section of the object is greater than that of the barrier.

11 Apparatus for imaging an object on the remote side of a barrier as claimed in any preceding claim wherein the apparatus is portable.

12 Apparatus for imaging an object on the remote side of a barrier as hereinbefore described with reference to figure 1.

13 A kit for detecting objects on the remote side of a barrier incorporating the apparatus of any of claims 1 to 12. 14 A kit for detecting packets of drugs on the remote side of a vehicle body panel incorporating the apparatus of any of claims 1 to 12.

15 A method of imaging an object on the remote side of a barrier, comprising the steps of,

whereby the directional information obtained from the primary detector is correlated with information scattered from the secondary detector allowing an image of the object and barrier to be constructed.

16 A method of imaging an object on the remote side of a barrier as hereinbefore described with reference to figure 1.