GB2456302A - Screening system and method for scanning a target for radioactive isotopes - Google Patents

Abstract

A screening system (102) for scanning a target for radioactive isotopes comprises a gamma-ray spectrometer comprising at least one scintillation body (201) for generating photons in response to incident gamma-rays, at least one photodetector (203) coupled to the respective scintillation body for detecting the photons and generating output signals therefrom, and a processing unit (106) for receiving the output signals from the photodetector(s). The screening system further comprises a drive mechanism (103) coupled to the gamma-ray spectrometer which is arranged to move the spectrometer relative to the target to perform a scan. The drive mechanism may be an electrically powered vehicle (e.g. fork-lift truck). The processing unit calculates energy loss spectra for incident gamma-rays at intervals along the target to identify radioactive isotopes present in the target from individual or combined spectra. Thus, a full spectroscopic analysis can be obtained for the target.

Description

TITLE OF THE INVENTION

SCREENING SYSTEM

BACKGROUND ART

The present invention relates to a radiation screening system. In particular the invention relates to a radiation screening system of the type used to scan cargo for radioactive materials.

It is common for radioactive materials to be present in cargo passing through a port, border crossing, or other control point. Often these radioactive materials will be innocent in nature, e.g. they may be radioactive materials for legitimate use in medical or industrial processes, or they may be naturally occurring radioactive isotopes. For example, sanitary ware, roofing tiles, cat litter and scouring pads can all contain detectable amounts of naturally occurring radioactive materials. However, there is also a risk of illegitimate radioactive materials being smuggled.

Accordingly it is common practice to screen cargo passing through a control point (e.g. at entry/exit borders or other control points, such as at military installations or nuclear power stations) to monitor for nuclear materials. The ultimate aim of such screening is to identify the presence of radioactive materials in a cargo or other screening target, and also to identify the nature of the radioactive isotopes present, to the extent the screening system allows this to be done. in general radioactive isotope screening systems are based on gamma-ray detection. The presence of radioactive materials in a cargo is determined from a measured increase in detected gamma-ray intensity, and the nature of the radioactive isotopes present is identified, so far as possible, from the spectral characteristics of the detected radiation.

Policing illegitimate trafficking in nuclear materials is one of the most challenging areas for radioactive material screening because it generally requires that large numbers of people / cargos are scanned relatively quickly, but with a high degree of reliably. Reliability is important as not only is there a clear desire to be able to correctly identify undesirable radioactive isotopes, e.g. plutonium, passing through a control point, it is also important that legitimate sources of radiation are not wrongly identified as undesirable sources. This is because false-alarms arising from this kind of ) misidentification can be very costly, both financially in terms of lost operation time while the alarm is investigated, and in terms of the degree of inconvenience to people passing through the facility.

At present, two-step screening processes are generally employed in scanning targets at control points.

In a first step, targets having elevated levels of gamma-ray emission are identified ("flagged") as requiring further investigation. This step does not include any discrimination as to the source of radiation, but is based solely on overall gamma-ray intensity. Targets that do not show elevated levels of gamma-ray emission are deemed clear to continue through the control point. This step is often performed by large-volume plastic scintillation detectors arranged so as to provide a scanning portal through which targets pass. These types of detectors may be referred to as primary scanners. Generally they are relatively cheap and can be highly sensitive to gamma-rays, but provide little or no reliable spectral information. Thus large-volume plastic scintillation detectors are useful for quickly identifying "hot" targets (i.e. those emitting gamma rays at an intensity deemed to be significant for the application at hand), but less useful in identifying the nature of the source of the radiation.

Accordingly, in a second step, flagged targets are re-scanned using a detector better able to identify the radioactive isotopes present from their gamma-ray emission spectra, and so identify the material that gave rise to the alarm. Scanners used for this second step may be referred to as secondary screening systems.

The situation can be complicated by the possibility that an illegitimate source can be masked by a stronger innocent source of radiation. In general, the overall intensities of gamma-ray emission expected from illegitimate nuclear materials will be relatively low compared to typical emissions from legitimate sources (e.g. from targets containing naturally occurring radiation). Because of this, a primary scanning system used to initially identify "hot" targets requiring further investigation needs to be relatively sensitive. This means targets containing legitimate sources of radiation are very often identified as being hot and passed through the secondary screening system (because the primary screening system is unable to discriminate between different sources of radiation, it flags all sources of gamma-ray emission above its operating threshold whether innocent or illegitimate in nature). Thus the secondary screening ) systems are pressed into service frequently, and not only on the rare occasions that a truly illegitimate nuclear source is present.

Figure 1 shows a simplified schematic representation of typical primary scanning system 501, in which two upright pillars 502, each housing a gamma ray detector (not shown), are arranged so that a car or lorry 504 may drive between them.

The pillars typically contain large-volume plastic scintillation detectors for detecting gamma-ray radiation. The pillars 502 extend in height above the height of the vehicle 504, and will generally have a lesser extent in the horizontal direction. In operation the vehicle 504 is driven between the two pillars 502 during the scanning process. A processor 503 is arranged to receive and process signals from the respective scintillation detectors, and provide an appropriate readout indicative of whether a predetermined detected gamma-ray emission intensity threshold has been exceeded. If the threshold is not exceeded, the lorry 504 is considered safe and allowed to proceed.

If the threshold is exceeded, a flag is raised so that the lorry can be diverted for secondary screening.

As noted above, once a target is flagged for secondary screening based on the intensity of its gamma-ray emission as determined by the primary scanner, a second scan is made using a secondary scanner to identify the nature of the source of the radiation (i.e. the radioactive isotopes present). There are two techniques generally used for this secondary screening. These techniques are based on using the so-called new generation of Advanced Spectroscopic Portals (ASPs), or based on manual scanning of a target by an operator using a hand-held radioactive isotope identifier detector (RIID).

The new generation of Advanced Spectroscopic Portals (ASP) are generally based on crystal scintillator detectors, e.g. sodium-iodide doped with thallium (Nal(Tl)) detectors, or high purity germanium (HPGe) detectors. ASPs are operated in a manner generally similar to primary portal detectors, as shown in Figure 1, in that a target is scanned as it is driven relatively slowly (e.g. around 8 kmh') through the ASP. In this regard, ASPs are designed to scan a vehicle and provide an indication of the nature of any radioactive materials present in as little as 6-10 seconds. With an appropriate processing system, this can allow enough time for a full spectroscopic analysis of the martial, contained in the scanned vehicle, to be carried out. Thus the ) resulting spectroscopic scan is ideally able to identify and accurately locate all, if any, of the radioactive materials that may be contained in the scanned vehicle.

These proposed ASP systems are expensive and their routine use in secondary screening may be difficult to justify.

The current most widely used secondary screening method is a manual one based on the use of a hand-held isotope identifier. The main drawbacks of this approach are: a) it is a slow process because of the limited sensitivity of the hand-held isotope identifier. This limitation is imposed by a maximum weight of the hand-held isotope identifier that an operator can realistically wield.

b) it provides a non-systematic coverage of the cargo c) the thoroughness of the search is limited by the reach of the inspector compared with the height of the truck.

Thus, this type of manual screening process is time consuming and provides an uncontrolled map of the radioactivity present in the target. For example, when using a hand-held isotope identifier, the operator will often naturally concentrate on those regions having the highest count rates, whilst missing some regions which might conceal a material that imposes a much greater threat. The subjective nature of hand-held scans is unavoidable. Thus a manual search of a lorry, for example, might take up to 30 minutes to finish, but even then it may not provide a complete scan of cargo contained in the lorry.

In a large container port, e.g. handling perhaps 10 million containers per year, the number of targets identified by primary scanning systems as requiring secondary scanning can be around 600 per day. In these circumstances, the use of manual scanning techniques using hand- held isotope identifiers is impractical. Accordingly, Advanced Spectroscopic Portals with their faster screening times will generally be preferred.

However, for less-busy control points, for example at control points where traffic flow might be less than 1 million targets per year, the number of flagged' vehicles that require a secondary scan, will be correspondingly less, e.g. perhaps around 50 per day. This level of traffic may not justify the expense of a full ) spectroscopic portal (ASP). But, for the reasons given above, it might not be acceptable to use the less effective manual searching method.

Thus there is a need for a radioactive screening system that does not suffer from the above mentioned drawbacks of known screening systems. 1)

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a screening system for scanning a target for radioactive isotopes, the screening system comprising a gamma-ray spectrometer comprising a scintillation body for generating photons in response to incident gamma-rays and a photo-detector coupled to the scintillation body for detecting the photons and generating output signals therefrom, and a processor for receiving the output signals from the photo-detector and calculating energy loss spectra for incident gamma-rays, and for identifying radioactive isotopes present in the target based on the energy loss spectra. The screening system further comprises a drive mechanism coupled to the gamma-ray spectrometer and arranged to move the gamma-ray spectrometer relative to the target to perform a scan. The drive mechanism may be operable to move the gamma-ray spectrometer horizontally and/or vertically. Thus a full spectroscopic analysis can be made at different locations along the target.

The drive mechanism may be operable to move the gamma-ray spectrometer at a fixed speed relative to the target while obtaining data, calculating energy loss spectra and identifying radioactive isotopes at intervals during the scan. This allows for a spectroscopic analysis of the target having a consistent spatial resolution along the target. The speed may be relatively slow, e.g. typically less than 1, 0.5 or 0.25 ms' and may be altered according to the desired spatial resolution. A faster speed takes less time to complete a scan of the target, whereas a slower speed provides for increased sensitivity and spatial resolution (e.g. because the spectrometer does not move so far in the time taken to acquire enough data for a statistically significant energy loss spectrum to be calculated).

The drive mechanism may be electrically powered. This can provide for a readily controllable motion of the spectrometer relative to the target. For example, the drive mechanism might comprise an electrically powered vehicle (device) arranged to carry the gamma-ray spectrometer. E.g. the electrically powered vehicle may be either a simple hand held steered unit or a conventional fork-lift truck. The speed of an electrically powered vehicle or fork-lift truck can generally be controlled easily at the required speeds. If a fork-lift truck is used, the gamma-ray spectrometer can be deployed at any location where there is a fork-lift truck available and with a great deal ) of flexibility in the locations at which scanning may be performed. For example, targets that might be too large for a conventional ASP portal device can be readily scanned simply by a suitable positioning vehicle, e.g. forklift truck, carrying the screening system.

The screening system may further comprise rails for guiding the gamma-ray spectrometer as it moves relative to the target. By mounting the screening system on rails the scanning process could be readily automated.

The drive mechanism may further comprise a controller configured to automatically move the gamma-ray spectrometer relative to the target in a pre-determined manner. Thus the target could be automatically scanned using an unmanned screening system.

The processor may be configured to calculate an energy loss spectrum and identify radioactive isotopes present in the target based on the spectrum at intervals as the gamma-ray spectrometer moves relative to the target. Thus the radioactive isotope analysis will provide a detailed map of the location of the radioactive materials that may be contained within the target.

The processor may be operable to calculate the energy loss spectra and identify radioactive isotopes at intervals corresponding to a movement of 1, 0.5, 0.25 m of the gamma-ray spectrometer relative to the target. Therefore, by varying the intervals of the radioactive isotope analysis, the spatial resolution and sensitivity of the radioactive isotope analysis can be changed.

The gamma-ray spectrometer may comprise at least one further scintillation body coupled to at least one further photo-detector. For example, multiple scintillation bodies could be arrayed in a direction orthogonal to a direction in which the spectrometer is arranged to move. For example a vertical stack of multiple scintillation bodies, e.g. four, could be used where the motion is horizontal. The spectrometers may be distributed in a vertical column such as this so as to provide full coverage of the height of the vehicle and its cargo. Thus full coverage of the vehicle and its cargo across several separate horizontal bands, e.g. bands at different heights, which are scanned as the spectrometers are moved relative to the target by the, or their respective, drive mechanism(s) may be provided. Thus the gamma-ray spectrometer may be used for more quickly scanning targets of extended height, for example. )

Where there are multiple scintillation bodies with associated multiple photo-detectors, the processor (or separate processors) may be operable to generate respective energy loss spectra from output signals from the respective photo-detectors.

Alternatively, a single processor may be operable to generate energy loss spectra from combined output signals from the respective photo-detectors. This can help improve counting statistics when signal intensities are relatively low, for example.

According to a second aspect of the invention there is provided a method for scanning a target for radioactive isotopes, the method comprising providing a gamma-ray spectrometer comprising, a scintillation body for generating photons in response to incident gamma-rays, a photo-detector coupled to the scintillation body for detecting the photons and generating output signals therefrom, a processor for receiving the output signals from the photo-detector and calculating energy loss spectra for incident gamma-rays and for identifying radioactive isotopes present in the target based on the energy loss spectra, and moving the gamma-ray spectrometer relative to the target using a drive mechanism coupled to the gamma-ray spectrometer. )

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show how the same may be carried into effect, reference is now made by way of example to the accompanying drawings, in which: Figure 1 shows a schematic view of a typical vehicle portal comprising two upright pillars, each pillar housing a gamma ray spectrometer, arranged so that a car or lorry can pass between them; Figure 2 shows a schematic view of a screening system according to an embodiment of the invention arranged to scan a lorry; Figure 3 shows a further schematic view of the screening system of Figure 2; Figure 4 shows a schematic view of a screening system according to another embodiment of the invention arranged to scan two freight containers; and Figure 5 shows a schematic view of a screening system according to an embodiment of the invention arranged to scan a lorry. )

DETAILED DESCRIPTION

Figure 2 shows a screening system 102 and a stationary vehicle 101 according to a first embodiment of the present invention. The screening system 102 comprises a drive mechanism 103, mounted on which is a battery 104, a processing unit 106 and a plurality of detection bodies (in this case four) 105. The drive mechanism 103 comprises a motor (not shown) and a plurality of wheels 107 (in this case four). Each of the detector bodies contains a scintillation counter coupled to a photomultiplier tube (PMT) via an appropriate light guide (not shown). Although not shown, it will be appreciated that the drive mechanism 103 allows the operator of the screening system 102 to move the screening system 102 forward and backward and steer it using front, rear or all-wheel steering.

The drive mechanism 103 includes an operator speed-control (not shown). This could be a go / stop button' that when pressed a first time results in the drive mechanism 103 driving the screening system 102 at a predetermined speed and direction. Accordingly, when the go / stop button' is pressed for a second time the drive mechanism 103, will no longer drive the screening system 102. The operator control could also be of the form of a foot pedal or a hand dial, such that the operator could select the speed at which the screening system 102 will be driven. If the operator has a speed-control mechanism to control the speed of the drive mechanism 103, there may also be provided an accurate means for displaying the speed at which the drive mechanism is moving, for example a digital speedometer.

All of the elements of the screening system 102 are powered using the battery 104, which is coupled to the processing unit 106 and the drive mechanism 103.

The height of the screening system (i.e. the height / vertical extent of the plurality of detection bodies) may be at least the same height / extent as the largest vehicle that will be expected at the location that the screening system 102 is deployed.

The drive mechanism 103 is used to drive the screening system 102 during the scanning process. The screening system 102 may also be driven to a location adjacent the stationary vehicle 101 using the drive mechanism 103 prior to the scanning process.

At the beginning of the scanning process, the vehicle 101 is directed to a designated scanning bay for the scanning procedure to be carried out. A parking brake, -*1 1-like those typically found on motor vehicles, may be applied. The designated scanning bay may be at a control point, such as a port, and arranged such that the screening system 102 can readily access both sides of the vehicle. Thus the screening system 102 may be operable to scan both sides of the vehicle in carrying out a complete scan.

The screening system 102 is driven to a position adjacent the designated scanning bay. This is because during the scanning process, the screening system 102 should typically be driven along a path that is parallel to the vehicle being scanned.

When the vehicle is in the designated scanning bay, the screening system 102 is driven along the length of the stationary vehicle 101, using the drive mechanism 103, e.g., at a constant velocity. The velocity of the vehicle will typically be in the range from 0.25 to I ms', but may be lower or higher. The distance between the stationary vehicle 101 and the screening system 102 is typically kept constant (i.e. the screening system 102 remains on a path that is parallel to a side of the stationary vehicle 101).

As the screening system 102 is driven along the length of the vehicle 101, data is collected from the detection bodies 105, and stored on the processing unit 106.

In addition to the data from the detection bodies 105, the revolutions of the wheels 107 are counted. This may be achieved by using a revolution-counter (not shown) coupled to the wheels 107 of the drive mechanism 103. Data from the revolution-counter are transmitted to the processing unit 106 and are recorded.

The position of the detector system with respect to the start of the scan may be computed by the processing unit 106, using the data from the revolution-counter. This allows the screening system 102 to map detailed isotope analyses based on subsets of the data to different positions along the length of the scanned vehicle.

To obtain the position of the detector system with respect to the start of the scan, the time since the start of the scan and the speed of the screening system 102 may be used to produce a position-in-time map of the screening system. The distance moved by the screening system may be obtained from the number of revolutions of the wheels and the circumference of the wheels 107, for example. In other examples any type of conventional position sensor may be used.

Once the detection system 102 has completed the first scan along the length of the stationary vehicle 101, the detection system 102 may be relocated to the opposite side of the vehicle 101, whereby another scan may be carried out, in the manner ) described above. The scanning process of the opposite side of the vehicle is typically started at the same position with respect to the stationary vehicle 101 (i.e. the front or the rear of vehicle). Alternatively, only one side of the vehicle may be scanned, or each side may be scanned, possibly simultaneously, using two screening systems.

The operator of the screening system 102 will typically complete a scan of the entire vehicle (i.e. both sides of the vehicle) before requesting an isotope analysis report. However, during the scanning procedure the operator may also be provided with an audible signal that conveys the radiation intensity detected at every point during the scan. When the scanning process of the vehicle is complete the data from the detector bodies 105 and the drive mechanism 103 may be processed by the processing unit 106, as described below.

Figure 3 shows the screening system 102 shown in Figure 2. In Figure 3, the panels from one of the plurality of detection bodies 105 (the topmost in the figure) are shown transparent such that the interior can be viewed. The plurality of detection bodies 105 each contain a scintillation body 201, e.g. a 4 x 2 x 16 inch (approximately x 50 x 400 mm) Thallium-doped Sodium iodide (NaI(Tl)) crystal scintillation body, which is coupled to respective photomultiplier tubes (PMTs) 203, via respective light guides 202. In the example shown in Figure 3, each scintillation body 201 is coupled to a photomultiplier at each end. However, it will be appreciated that in some examples, only one photomultiplier tube may be used, or indeed more than two could be used. Other photo-detectors, e.g. photodiodes, could be used.

The photomultipliers contained in each of the detector bodies are coupled to the processing unit 106, via connection lines 204.

The processing unit 106 may be specific to the detector, or may be provided by a suitably programmed general purpose computer, for example, a personal data assistant (PDA) type device or a general purpose computer coupled to the detector. It will be appreciated that if each detection body contains its own specific processing hardware, control signals and processed data may be transmitted to and received from a single central processing unit, for example a general purpose computer. The processing unit 106 may also be provided with a display screen (not shown), for example a liquid crystal display (LCD) screen, for displaying information to a user. In embodiments where the processing unit 106 is provided by a general purpose computer, the display screen may also be a part of the general purpose computer.

In the example shown in Figure 3, the processing unit 106 contains associated readout electronics and signal processing hardware for the spectrometers. The processing unit 106 may also contain a neutron detector such as a high-pressure He-3 proportional counter. This will allow sources of neutrons to also be detected. In Figure 3 each detector body 105 is shown to contain only a scintillation body (with associated photomultiplier and light guide) and all of the associated processing hardware has been grouped together in the processing unit 106.

In use, the PMTs 203 provide respective output signals to the transmission lines 204, that are indicative of the amount of energy deposited in each of the scintillation bodies 201. The output signals are received by the processing unit 106.

The processing unit 106 is configured to generate energy loss spectra based on the signals from the PMTs and to perform further processing.

The spectral information in the energy loss spectra could be used to identify the nature of the source in the usual way (e.g., by identifying features in the spectra which are characteristic of a given radioactive material). This could be done using conventional techniques, for example spectrum deconvolution such as described in WO 02/031536 [1]. The technique described in WO 02/031536 [1] takes account of the modelled responses of the individual scintillation bodies, and so in these cases it may be preferable for the spectral processing to be performed separately for each detector. This approach also allows gain-stabilisation and energy-calibration to be managed separately.

The processing unit 106 may provide the ability to stabilise the spectrum using conventional techniques even when the system is used in locations where the ambient temperature can range widely. The data may be processed in such a way that the isotopic analysis of any radiation is made available with a spatial resolution of between 0.25 and 1 m along the full length of the vehicle for each height-range covered by the individual detector bodies.

During the scanning process the screening system 102 collects data as the screening system 102 is moved along the length of the stationary vehicle 101. A predetermined spatial resolution for the isotropic analysis may be set by an operator of ) the screening system 102. The scanned data is divided into discrete time intervals, depending on the required spatial resolution and the speed at which the screening system 102 is driven. For example, if the screening system 102 is driven at a velocity of 1 ms, with a spatial resolution of I m, the energy loss spectra will be divided up into I second intervals in time. These energy loss spectra of I second intervals in time will be processed in the usual way, as described above, and will typically be mapped to the position-in-time data along the length of the scanned vehicle.

The data from the plurality of detection bodies 105 could be combined or summed in such a way as to maximise the signal-to-background ratio in order to form the optimum composite spectrum before the isotope analysis routines are applied.

Summing can be useful if the overall count rates are low because summing can help to reduce the statistical noise. Summing is typically most effective where the responses of each of the scintillation bodies are first normalised. Normalisation may help to ensure the summed energy loss spectrum is close to that which would be obtained from a spectrometer comprising a single scintillation body with a volume comparable to that of all of the plurality of scintillation bodies together. This may help to optimise the ability to identify isotopes from the spectrum.

The data received from the plurality of detection bodies 105 may be processed during the scanning process as described above. For example, while a section of the vehicle is being scanned, the scan data from the previous section of the vehicle is processed. Alternatively, the entire vehicle could be scanned and then the processing of the data, as described above, is carried out to determined the nature of any radioactive material contained in the vehicle.

The scanning procedure described above could be used to provide an immediate indication of the seriousness of the threat as soon as the scan has been completed (i.e. both sides of the vehicle have been scanned). As described previously, the operator may also be provided with an audio signal that conveys the radiation intensity detected at every point during the scan, or the results of real-time isotope identification analysis. A detailed log of the isotopic composition of the cargo as a function of time may be obtained. The isotope composition data is combined with the position-in-time data, such that the isotope composition can be obtained at discrete positions along the length of the scanned vehicle. )

The isotope composition data and the position-in-time data (from the wheel revolution counter), along the length of each scanned vehicle, could be stored for the duration of the operator's shift on a hard-drive integral to the processing unit 106, which could also be relayed to the office that controls the security at the border-crossing point or to another location via an appropriate data communication link. For example, to provide a comprehensive log of activity through the screening system.

Each of the plurality of detector bodies 105 typically have a sensitivity of approximately 40 times greater than that of a hand-held unit that is currently in use at locations where vehicles are scanned for radioactive material (because of their larger size). In total, one screening system comprising a plurality of detector bodies can typically provide the sensitivity of more than 150 such hand-held units.

The use of multiple detector bodies, configured one on top of the other, could provide a uniform coverage of the full height of the vehicle. Contrastingly, a hand-held unit can only be used to typically scan half the height of a vehicle, a lorry for example, even if the operator of the hand-held unit is working at full-stretch. If an operator is working at "full-stretch" the isotope measurement is likely to be unreliable.

Furthermore, it may be difficult for the operator of the hand-held unit to view the read-out display of such a hand-held unit while scanning the vehicle. Accordingly, the constant low speed of the slow moving screening system 102 in Figure 2 enables a thorough and impartial scan of the vehicle to be made, with uniform sensitivity, with the possibility of allowing the operator to view any output screen throughout the scanning process. The improved secondary screening system may provide a report on the isotopic origin of any radiation detected at regular intervals of, e.g., between 0.25 and I m along the length of the vehicle and for the full height of the vehicle, e.g. in 0.25 to lm width bands.

The overall sensitivity of the secondary-screening system described above is likely to be better than that of an ASP. This is because reliable slower scan speeds are possible which means more data can be collected in a given scan, and so statistical accuracy can be improved. Such low speeds are not reliably achievable with "drive- through" systems. Thus, a more precise isotope-source identification and isotope-source location within the scanned vehicle could be obtained. )

Such a system could also be used on its own to provide a high level of security at little-used border crossing points without the need to first carry out a primary scan using a primary screening portal of the kind discussed above.

The screening system 102 is powered by a battery 104. The battery 104 is of the type that can be easily changed and recharged. Accordingly the screening system 102 may be used continuously.

Figure 4 shows a screening system according to a second embodiment of the present invention. In Figure 4 a screening system 301 comprises a moveable platform 304 (e.g. a pallet), which may have slots 310 for inserting forks of a typical fork-lift truck. Mounted on the pallet is a plurality of detector bodies 302 and a processing unit 303. The detector bodies 302, are similar in construction to those shown in Figure 3 (102). The screening system 301 is operable to be moved using a fork-lift truck 305 or any other similar machine, for example a counter balance. The fork-lift 305 is a typical fork-lift which comprises two forks that extend from the body of the fork-lift that can be inserted in to the slots 310 in the moveable platform 304, to allow the platform to be moved in a number of directions. The fork-lift 305 comprises a drive mechanism, wheels, operator controls and a vertical-drive mechanism 306. The fork-lift is powered by a battery 307. The screening system may also be powered by the fork-lift's battery 307.

The revolutions of the wheels of the fork-lift 305 may be counted using a revolution-counter (not shown) that is coupled to the wheels of the fork-lift 305 or any other position sensor may be used to determine the position of the screening system relative to the target during a scan. The output of the revolution counter is connected to the processing unit 303 using a data transmission line (not shown).

The plurality of detection bodies 302 and the processing unit 303 could be configured in any of the same configurations as those which are described for other embodiments of the present invention. The screening system 301 may be operated in broadly the same manner as that which is described above, except in this embodiment the height of the screening system 301 may also be readily altered, using the vertical- drive mechanism 306.

In the example shown in Figure 4, a plurality of containers are stacked one on top of the other. This is a typical arrangement that might be found at a port that provides transit for containers, which could be used to transport cargo containing radioactive material.

The scanning procedure for the lower container 309 is broadly similar to that which is described above, whereby the screening system 301 is moved along the length of the container in a slow and controlled manner at speeds typically in the range of 0.25 to 1 ms. Once the lower container 309 has been scanned, possibly on both sides, the height of the screening system 301 is increased using the vertical-drive mechanism 306. The height of the screening system 301 is increased to a height whereby the screening system 301 is operable to scan the entire height of the upper container 308. For example the base of the lower detection body is adjacent the base of the upper container 308. The scanning procedure is carried out, as described above, to scan the upper container 308.

A high-lift fork-lift truck could typically be used to raise the detector system up to approximately 7m, for example, so that the forks are level with the base of a third layer of containers (not shown), to carry out a scan of a third container stacked on top of two other liked-sized containers.

It will be appreciated that the operator could scan one side of each of the containers 308 and 309, before moving the screening system to the opposite side of the containers. This is because all of the data from the detection bodies 302 and the position of the fork-lift with respect to the container are recorded. The processing unit could then compute an appropriate isotope analysis based upon the fact that the two sides of the two containers were scanned first, followed by the opposite two sides of the two containers.

Furthermore, if the screening system is mounted on a moveable platform, it could be exchanged from one fork-lift truck to another, allowing for the first fork-lift to be recharged while the second fork-lift is in use and vice-versa. This could allow for the mounted-screening system 301 to be continuously used.

Furthermore still, the mounted-screening system 301 could be transferred between ports, boarder crossings or even to inspect vehicles at a road-side location, and operated using a fork-lift.

Figure 5 shows a third embodiment of the present invention. The screening system 401 is mounted on a mechanised platform 405 operable to move forwards and backwards along a horizontal direction. The mechanised platform comprises a platform for mounting the screening system 401, a motor for driving the platform and wheels 403. The screening system 401 comprises broadly the same elements as those of the screening system 102 shown in Figure 2, namely a plurality of detector bodies, a battery and processing unit mounted on the mechanised platform. However, the wheels 403 on the mechanised platform 405 are the type that will engage with a pair of parallel tracks 402, to allow the screening system to move along the tracks 402.

A vehicle 404 is directed to the designated scanning bay and the procedure to scan the vehicle 404, as that described above is carried out. The screening system 401 is driven in a first direction, parallel to a stationary vehicle 404, along the fixed tracks 402 and the screening system 401 scans the vehicle 404. The vehicle 404 is then turned around, such that it is again parallel to the tracks 402, but the front of the vehicle is now facing in the opposite direction. The opposite side of the vehicle 404 is then scanned using the screening system 401, by driving the screening system 401 back to the same position that it was in at the beginning of the scanning process.

An embodiment of the present invention shown in Figure 5 could be automated such that the screening system 401 can be programmed to move forward and backward along the fixed tracks 402 and scan the vehicle 404. An appropriate trigger (not shown) could be used to start the scan cycle. For example, the presence and the length of a vehicle to be scanned could be detected by moving-coil sensors, mounted at the designated inspection bay.

Although it is not shown, another screening system could be positioned on parallel tracks on the opposite side of the designated inspection bay, such that the two screening systems are operable to move simultaneously along the length of a vehicle, thus only requiring one scan cycle to scan both sides of a vehicle.

The embodiments of the present invention are not limited by the number of detector bodies that are used. More or less detector bodies could be used depending on the height of the vehicles or the containers that are scanned using the screening system descried above in various embodiments.

Thus there has been described a screening system for scanning a target for radioactive isotopes, wherein the screening system comprises a gamma-ray spectrometer comprising a scintillation body for generating photons in response to incident gamma-rays, a photo-detector coupled to the scintillation body for detecting the photons and generating output signals therefrom, and a processor for receiving the output signals from the photo-detector, calculating energy loss spectra for incident gamma-rays and for identifying radioactive isotopes present in the target based on the energy loss spectra. The screening system further comprises a drive mechanism coupled to the gamma-ray spectrometer, which is arranged to move the gamma-ray spectrometer relative to the target to perform a scan. Thus a full spectroscopic analysis can be obtained for the target.

Claims (15)

1. A screening system for scanning a target for radioactive isotopes, the screening system comprising: a gamma-ray spectrometer comprising: a scintillation body for generating photons in response to incident gamma-rays; a photo-detector coupled to the scintillation body for detecting the photons and generating output signals therefrom; and a processor for receiving the output signals from the photo-detector and calculating energy loss spectra for incident gamma-rays, and for identifying radioactive isotopes present in the target based on the energy loss spectra; the screening system further comprising a drive mechanism coupled to the gamma-ray spectrometer and arranged to move the gamma-ray spectrometer relative to the target to perform a scan.

2. A screening system according to claim 1, wherein the drive mechanism is operable to move the gamma-ray spectrometer at a fixed speed relative to the target.

3. A screening system according to claim I or 2, wherein the drive mechanism is operable to move the gamma-ray spectrometer at a speed less than 1, 0.5 or 0.25 ms1 relative to the target.

4. A screening system according to any proceeding claim, wherein the drive mechanism comprises an electrically powered vehicle arranged to carry the gamma-ray spectrometer.

5. A screening system according to claim 4, wherein the electrically powered vehicle is a fork-lift truck.

6. A screening system according to any proceeding claim, further comprising rails for guiding the gamma-ray spectrometer as it moves relative to the target.

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7. A screening system according to any proceeding claim, wherein the drive mechanism further comprises a controller configured to automatically move the gamma-ray spectrometer relative to the target in a pre-determined manner.

8. A screening system according to any proceeding claim, wherein the processor is configured to calculate an energy loss spectrum and identify radioactive isotopes present in the target based on the spectrum at intervals as the gamma-ray spectrometer moves relative to the target.

9. A screening system according to claim 8, wherein the processor is operable to calculate the energy loss spectra and identify radioactive isotopes at intervals corresponding to a movement of 1, 0.5, 0.25 m of the gamma-ray spectrometer relative to the target.

10. A screening system according to any proceeding claim, wherein the gamma-ray spectrometer comprises at least one further scintillation body coupled to at least one further photo-detector.

11. A screening system according to claim 10, wherein the processor is operable to generate respective energy loss spectra from output signals from the respective photo-detectors.

12. A screening system according to claim 10, wherein the processor is operable to generate energy loss spectra from combined output signals from the respective photo-detectors.

13. A method for scanning a target for radioactive isotopes, the method comprising: providing a gamma-ray spectrometer comprising: a scintillation body for generating photons in response to incident gamma-rays; a photo-detector coupled to the scintillation body for detecting the photons and generating output signals therefrom; and a processor for receiving the output signals from the photo-detector and calculating energy loss spectra for incident gamma-rays, and for identifying radioactive isotopes present in the target based on the energy loss spectra; and and moving the gamma-ray spectrometer relative to the target using a drive mechanism coupled to the gamma-ray spectrometer.

14. A screening system for scanning a target for radioactive isotopes substantially as hereinbefore described with reference to Figures 2 to 5 of the accompanying drawings.

15. A method for scanning a target for radioactive isotopes substantially as hereinbefore described with reference to Figures 2 to 5 of the accompanying drawings.