GB2424065A

GB2424065A - Radiation detection apparatus

Info

Publication number: GB2424065A
Application number: GB0504951A
Authority: GB
Inventors: Edward Marsden
Original assignee: Corus UK Ltd
Current assignee: Corus UK Ltd
Priority date: 2005-03-11
Filing date: 2005-03-11
Publication date: 2006-09-13
Also published as: WO2006095188A1; GB0504951D0

Abstract

An apparatus for the detection of radiation associated with bodies under investigation, the apparatus comprises: ```a plurality of radiation detectors configured for detecting one or both of gamma and neutron radiation, the radiation detectors being arranged around a portal through which bodies under investigation may pass; ```an integrated interface unit for receiving and displaying levels of gamma and/or neutron radiation detected by the radiation detectors; ```a processor unit associated with the interface unit, which processor unit is configured to perform the following method steps; <SL> <LI>a) analysing the gamma total count rate and energy spectrum for each detector; <LI>b) detecting an increased gamma count rate across one or more detectors; <LI>c) altering the count rate across each detector to compensate for background radiation; <LI>d) combining energy spectra associated with each detector detecting an increased count rate, to provide a single combined spectrum; <LI>e) analysing the spectrum to identify peaks associated with specific isotopes; <LI>f) analysing the peak distribution to identify a point source; and <LI>g) alerting the user when a point radiation source is detected. </SL>

Description

RADIATION DETECTION APPARATUS

This invention relates to the detection of radiation associated with bodies under investigation. In particular the invention has application in security systems for detecting the unauthorised transport or importation of radioactive materials or devices which may present a security risk to the public.

It will be appreciated that there is a naturally occurring background : **.* level of gamma radiation in the atmosphere and therefore any detection system must be set to ignore this. Levels of background radiation in the environs of a radioactive source can be influenced by, for example, heavy masses of metal which may absorb the background levels. It will be realised that this reduction in the background level measured can result in some well shielded radioactive materials and devices passing through a system undetected, if the detection system is set to pass small bodies S...

which do not carry such materials or devices. Thus, there is a need for a detection system which is accurate and reliable at detecting the presence of radioactive materials and devices in bodies of a range of sizes and with different levels of shielding.

The applicant's European patent no. EP 0746760 Bi discloses a method for the detection of the presence of radioactive impurities in an agglomeration of scrap metal, the method including the following steps; a) measuring in each of a plurality of discrete gamma ray energy bands the background gamma ray emission without the agglomeration in the measuring apparatus, b) measuring the combined emission from the agglomeration and background emission with the agglomeration present, c) determining for each measurement the ratios of the measured emissions in each of the energy bands, and d) using the divergences of the ratios between the measurements as an indicator of the presence of radioactive impurities in the agglomeration.

The inventors have found that in suitably adapted apparatus, a similar method can be beneficial in detecting the presence or otherwise of radioactive material or devices concealed about the bodies of persons, : **s vehicles, baggage and the like. Similarly, the method can be adapted for S...

use in monitoring the levels of radiation present in, for example, fumes or waste products emitted by processing plants or in any articles passing through a suitably configured detection apparatus on a conveyor belt.

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Emissions monitored may be from non-metallic contaminants, and S..

: the energy bands may be selected from those characteristic of potassium, S...

actinium, bismuth, lead and thallium. The method may include measurement by scanning by a matrix of gamma ray detectors disposed around the body under investigation. The level of the emissions measured with the body present may be compared with a pre-selected level to estimate additionally the quantity of radioactive impurities present.

One problem associated with security aspect of radiation detection relates to false alarms. The number of vehicles that travel through the security detection apparatus is such that the number of false alarms can be quite substantial. There is an increasing demand to reduce the number of false alarms in such detection systems.

The apparatus and method of the present invention seeks to reduce and, if possible minimise the number of false alarms by providing a system that first detects the presence of a radiation source and then cross-checks the detected peak distribution against a relevant part of an energy spectrum to conclude whether the radiation is emitted from a specific point source or whether it is part of the natural background.

In accordance with the present invention, there is provided an apparatus for the detection of radiation associated with bodies under investigation, the apparatus comprising; a plurality of radiation detectors configured for detecting one or both of gamma and neutron radiation, the radiation detectors being : ** arranged around a portal through which bodies under investigation may pass; * * S...

an integrated interface unit for receiving and displaying levels of gamma and/or neutron radiation detected by the radiation detectors; a processor unit associated with the interface unit, which processor unit is : configured to perform the following method steps; S...

a) analysing the gamma total count rate and energy spectrum for each detector; b) detecting an increased gamma count rate across one or more detectors; c) altering the count rate across each detector to compensate for

background radiation;

d) combining energy spectra associated with each detector detecting an increased count rate, to provide a single combined spectrum; e) analysing the spectrum to identify peaks associated with specific isotopes; f) analysing the peak distribution to identify a point source; and g) alerting the user when a point radiation source is detected.

Preferably, the method further includes the method of analysing the neutron radiation detected by each detector.

Preferably, the method further includes the step of comparing the increased gamma count rate with a pre-determined value based on the probability of the increased count rate occurring, whereby the count rate is only analysed in accordance with steps (e) to (h) if the count rate is in excess of the pre-determined value.

Although the value is pre-determined at the point of a specific incident, the value is continually computed from prior background levels and all foreground data.

: ** Preferably still, the method further includes the step of recording a picture of a body to which the an alert if generated. S...

Preferably, the method further includes identifying the location of the radiation source on the body by analysing the relative time for each detector to detect an increased count and the relative distances between : each detector. * S S...

Optionally, the detectors are plastic scintillation detectors. Desirably, the radiation detectors are inorganic scintillation detectors. The latter are advantageous in that they have improved resolution in the detection of gamma energy (around 10% resolution) and an enhanced sensitivity and ability to categorize different sources. Furthermore, they are compact in size and thus allow flexible and easy installation around the portal.

Suitable detector configurations include but are not strictly limited to the following: 1. A compact CsI gamma radiation only detector (G360) 2. Combined CsI gamma and He3 neutron detector (GN500) 3. Large combined CsI gamma and He3 neutron detector (GN 1000) 4. Hybrid CsI/plastic gamma and He3 neutron detector (GN 1000/P).

The combined detector configurations are desirably arranged such that a gamma crystal is sunk into a surface of the neutron moderator.

Such an arrangement provides background shielding to the gamma crystal and optimises the overall size of the combined detector.

In addition to the radiation detectors, the apparatus may be provided with one or more of, a speed detection sensor, an image capture device, a vehicle occupancy detector and a vehicle number plate recognition device. Other optional additional features include; networking and modem interfaces permitting remote monitoring and control of the : ** apparatus, a touch screen interface and printer for printing out ratios and divergences determined by the apparatus. I...

The portal size and shape may be adapted to specific applications.

For example, the portal may be designed to be floor mounted and suitably proportioned to allow pedestrians to walk through. Alternatively, the : portal may be configured to allow large motor vehicles to pass through. In another option a smaller scale portal may be positioned around a conveyor on which objects such as packages or luggage are placed. In yet another option, the portal may be located about an exit conduit from a processing plant so as to detect unexpectedly high levels of radioactive material in fluids released from the plant.

For the purposes of exemplification, an embodiment of the invention will now be described in more detail with reference to the following Figures in which; Figure 1 illustrates schematically an embodiment of the invention configured for detecting the presence of radioactive materials or devices in motor vehicles. Figure la) shows the portion of the apparatus through which a motor vehicle passes, Figure ib) shows the user interface part of the apparatus Figure 2 illustrates a combined gamma and neutron radiation detector suitable for use in the embodiment of Figure 1; and Figure 3 illustrates schematically the operation of the sensors and processor of the embodiment of Figure 1 as a lorry passes through the portal.

As can be seen from Figure la) the apparatus comprises a substantially square shaped portal frame 1 positioned over a gangway 2 along which a vehicle 3 can travel. Beneath the gangway 2 and along each * ** side of the portal frame 1 are provided a plurality of gamma and/or neutron radiation sensors 4. A camera 5 is also fixed to the portal frame 1. The camera 5 captures an image of the vehicle and occupants for recording and incorporates a an alphanumeric character recognition system by means of which vehicle registration plate numbers can be cross checked and validated with a central database. Is. * S S ** S *SS.

As can be seen from Figure lb), the interface of the apparatus basically comprises a personal computer 6 terminal including a processor 7 connectable with a keyboard 8, mouse 9, display monitor 10, audio speakers 12 and a printer 13. The processor is provided with a plurality of digital inputs 14 for receiving data from the plurality of sensors 4, one or more inputs 11 for receiving image data from the camera 5 and is powered by an uninterruptible power supply 15 which may be a mains supply or a dedicated supply. The processor is optionally also provided with a network/modem interface to permit remote access to remote data bases and/or transmission of data to other locations. The display monitor is optionally provided with a touch screen interface (not shown).

Figure 2 illustrates in more detail a combined gamma and neutron radiation detector suitable for use in the embodiment of Figure 1. As can be seen, the device detector comprises a moderator 21 (typically of polyethylene), in to which are set a pair of He3 neutron tubes 22a, 22b.

In a top surface of the moderator 21, there is provided a recess 23 which is positioned about midway between the two He3 neutron tubes 22a, 22b.

The recess 23 is lined with a lead screen 24, behind which is located a CsI gamma radiation detector 25. The compact design of the detector permits improved, directional shielding resulting in a reduced background radiation count without a reduction in the count rate detected from the body under investigation.

In order to provide reliable and consistent gamma energy measurements the detectors are automatically stabilised. This is achieved * *. by applying variable gain based on detector temperature and on the ::::: energy peak produced by 40K (1.46 MeV) emission. In most circumstances sufficient 40K is present with normal background measurement for this purpose. Should this not be the case a small amount of this isotope can be encapsulated within the detector. **

: Figure 3 illustrates the radiation detection and analysis operation performed by the embodiment of Figure 1. The mode of operation is further described below.

The processor is controlled by a software package which is configured to run under a Windows TM operating system on a standard personal computer. The processor when installed with the software performs the following functions; a) Data collection from the plurality of radiation detectors (typically four to eight or more).

b) Analysis of the gamma radiation total count, the energy spectrum for each detector independently and the energy spectrum for one or more combinations of the detectors.

c) Analysis of neutron radiation levels for each detector independently and for one or more combinations of the detectors.

d) Monitoring of the condition of the detectors.

e) Detection of vehicles and speed measurement of vehicles as they pass through the portal.

f) Capture of images viewed by one or more cameras directed at the portal.

* g) Database storage of records of alarms triggered by the system and configuration settings. **** * * ****

h) Interfacing via a network to a central server. ***

As previously discussed, naturally occurring background radiation I..

: presents a limitation to sensitivity of a radiation detection system, more particularly, fluctuations in the background radiation level due to the screening effect of vehicles, pedestrians or other bodies passing through the portal can mask the presence of a concealed radioactive material or device. The processor mitigates this problem by relating the measured gamma radiation total count to a datum which is retrospectively altered in order to compensate for any reduction in background radiation level.

Suspected sources are initially identified by repeated calculation of the statistical probability of a given count rate with respect to the background datum or gamma rareness'. i.e., a count rate that is significantly above the datum will have a low probability of occurring or a high rareness'. Only when a pre-defined rareness is exceeded is the data point identified as suspect'. By using this rareness' measurement, the user can effectively select an acceptable false alarm rate which is preferable to an arbitrary threshold which might typically be used in prior art systems. Data adjacent the suspect' data point in time and space is successively combined with the data at the suspect' point and the degree of rareness' recalculated. This process is repeated until the rareness' measurement is maximised.

Energy information associated with all the data points is combined to produce a definitive energy spectrum for the suspect' area. The energy is fed into a peak-fitting algorithm, which approximates the energy data presented to it with a series of energy peaks shaped to the performance of the detector. A look up table is then used to identify the most likely isotope (or combination of isotopes) to match to the identified suspect' peaks.

With each isotope identification is stored a category to which is associated an appropriate alarm message. For example, the isotope may be identified as an industrial isotope, a special nuclear material (SNM), naturally occurring radioactive material (NORM) or a medical isotope. * Is I. -

* During analysis, the peak distribution should be considered. If, for example, the peak is not a point source then the radiation detected may well be natural. A wide distribution of the energy spectra is also likely to indicate NORM.

The relationship of all detectors must also be considered. If all the detectors detect similar patterns then the radiation detected then this may indicate NORM rather than a point radiation source.

The position of a suspect' area in time and space is used to predict the location of the source in the body. This can be displayed pictorially on the display monitor. A suspect' area that extends over a significant portion of a vehicle length and/or cross section is indicative of NORM; this information can be combined with the isotope results to assist in identifying discrete sources of NORM.

A similar rareness measurement can be taken for neutron radiation, though in this case the datum need not be dynamically adjusted. Neutron sensitivity is optimised by combining counts from adjacent detectors, in a master slave arrangement.

The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to falling within the scope of the invention.

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Claims

1. An apparatus for the detection of radiation associated with bodies under investigation, the apparatus comprising; a plurality of radiation detectors configured for detecting one or both of gamma and neutron radiation, the radiation detectors being arranged around a portal through which bodies under investigation may pass; an integrated interface unit for receiving and displaying levels of gamma and/or neutron radiation detected by the radiation detectors; a processor unit associated with the interface unit, which processor unit is configured to perform the following method steps; a) analysing the gamma total count rate and energy spectrum for each detector; b) detecting an increased gamma count rate across one or more detectors; c) altering the count rate across each detector to compensate

for background radiation;

2. ApparatLis according to claim 1, the method further including the step of analysing the neutron radiation detected by each detector.

3. Apparatus according to claim 1 or claim 2, the method further including the step of comparing the increased gamma count rate with a predetermined value based on the probability of the increased count rate occurring, whereby the count rate is only analysed in accordance with steps (e) to (g) if the count rate is in excess of the pre-determined value.

4. Apparatus according to any preceding claim, the method further including the step of recording a picture of a body in respect of which an alert is generated.

5. Apparatus according to any preceding claim, the method further including the step of identifying the location of the radiation source on the body by analysing the relative time for each detector to detect an increased count and the relative distances between each detector.

6. Apparatus according to any preceding claim, wherein the detectors are plastic scintillation detectors.

7. Apparatus according to any of claims 1-5, wherein the detectors are inorganic scintillation detectors.

8. Apparatus for the detection of radiation associated with bodies under investigation substantially as described herein or as shown in the drawings.

for background radiation;

2. Apparatus according to claim 1, the method further including the step of analysing the neutron radiation detected by each detector.