Expanded Use of Gamma Gauges for Security Screening. L. Bray

Transcription

1 Expanded Use of Gamma Gauges for Security Screening L. Bray Radiation Safety Department, Science Applications International Corporation, West Bernardo Dr., San Diego CA Abstract. The use of ionizing radiation for non-intrusive inspection of cargo has been in place for several decades. The historical purpose was to screen cargo for contraband, drugs, or incorrectly manifested items. These types of systems free operators from having to undergo the time-consuming, labor-intensive process of physically entering the vehicle and examining its contents, all of which is limited by the range of the human eye. When operated and controlled in a responsible manner, these systems add a tremendous value to their users and the general public. There are many versions of these devices available, however they all typically utilize x-ray generators, liner accelerators or radioactive material sources (gamma gauges) to perform the screening function. A gamma gauge is a radioactive source inside of a mechanical source housing. Recent events have led to the use of this same technology for advanced security screening purposes for both cargo and personnel. Current guidance for use and control of such systems, especially for individual screening has mainly focused on radiation machine based systems (x-ray based). However, with the increased use and requirements for rapid deployment and operation under extreme environmental conditions, the use of gamma gauges is gaining popularity. This paper explains the basic components and theory of operation of the systems, and examines the differences between the radiation fields and controls for both radiation machine and gamma gauge based systems. The purpose of the paper is to familiarize radiation protection personnel with the expanding technology, and discusses radiation standards/criteria for determining when the social benefits of public screening outweigh the risks associated with ionizing radiation exposure. 1. Introduction The use of non-intrusive inspection equipment has increased over the last several decades. These types of inspection systems typically utilize radiation machines, which include x-ray generating devices or linear accelerators, or nuclear/gamma gauges, which include sources of radioactive material. When operated and controlled in a responsible manner, such devices can add a tremendous value to their users and the general public. For non-intrusive inspections systems, the benefit is that you can quickly obtain a sense of what s inside a container without having to open it. This application allows for: cargo to be inspected more quickly than with manual methods, which provides an economic incentive, contraband to be identified more readily, even when hidden inside of secret or shielded compartments, which provides a security related incentive, and operators to view cargo contents remotely without direct handling of the material, which provides a safety incentive. However, the systems do utilize and generate radiation which must be controlled to ensure the safety of the operators and the general public. Balancing that use of radiation requires appropriate regulatory guidance, which is not clearly available for these types of applications at this time. To develop such guidance will require the understanding of the typical system design, operating conditions, radiation fields and specific applications. This paper provides an overview of the primary components of these systems, a general description of how they work, the resulting operational considerations, and some suggested radiation safety criteria for controlling them. 1

2 2. Primary System Components and General Theory of Operation There are really only three main components in a cargo inspection system, the gamma gauge or radiation machine, the radiation detectors, and the communications and analysis system. Systems may include 1 or 2 sources of radiation. The source/machine is used to generate a flux/field of gamma or x-rays, which is directed through the cargo towards a bank of detectors. This field is collimated to a very narrow band, usually less than two to five degrees in the horizontal direction, and typically less than 60 degrees in the vertical direction. The x-ray energies from the radiation machines are usually in the 450 Kev to 9 Mev maximum range, while the gamma sources are usually Cesium 137 or Cobalt 60 with energies in the 0.66 MeV to 1.3 Mev range. The gammas and x-rays interact with the cargo, and are either transmitted, scattered or absorbed in the material. The number of gamma or x-rays that emerge on the other side of the cargo/container is directly related to the type and amount of material in the cargo/container. The more dense the material, or the larger the quantity of material, the less gamma or x-rays actually make it to the detectors. The detector then counts how many gamma rays were transmitted in each section of the container. This process is repeated as the container is moved along between the source and the detectors. The information from the detectors is sent to the main computer system for analysis. The software converts the count data into a visual recreation of the material in the container, much like the images you would see in a typical X-ray. Only, the computer system allows the user to further enhance the image by varying the relative contrast between different densities or shapes of material, as well as allowing color variations. This information can then be compared to what the operator expects to see in the container (i.e. from a manifest, or from historical experience). General Components Sys Control Source Shutter Speed sensors Cameras Interlocks Communications Counts per pixel Data Host Computer Windows PC / Control Console Control and Image Processing Photon counting electronics Detector array NaI-PMTs Shadow cast by vehicle on detectors Vehicle under inspection Gamma ray fan beam Source FIG. 1. General Components

3 The images are normally acquired in standard gray tones, and look very much like a reversed x-ray. The use of analysis tools can further sharpen the image on select items. Typical Images FIG 2. Typical images The cargo inspection systems come in many varieties. However, they all include the three primary components listed previously. The variations occur in how the equipment is mounted, the size and type of cargo that is examined, and the number and type of sources and/detectors used. Some examples include systems which are designed to be set up at a specific location to look at the cargo in trucks, some which are mobile, some which are for Rail-Road Cars, or even separate pallets. 3. Operational Considerations The type of system someone selects depends upon many operational considerations. The most important of which is why the scan is being performed and what are the users trying to accomplish. Answering this question should bound the remaining technical requirements for the system regarding what types of material will be examined, how much space will be available for the inspection, how many locations will the system operate at, what are the throughput requirements, what are the resolution and penetration requirements, and finally what are the controls and costs associated with the operation. Several of these considerations such as penetration and resolution will affect the type and size of source selected, and the resulting exposure/dose rates that will be present. These resulting dose/dose rate issues will impact the type of material that can be safely examined, the area required for operation, the mobility and throughput of the system and the costs/regulatory controls that must be implemented. The resolution is a measure of how fine of a line of detail is required for the analysis and the penetration is based on how much material may be shielding the objects that need to be differentiated. For most users performing bulk cargo inspection, the penetration has become more important that the resolution in many applications. This is because the entire image, which is quite large, is displayed at once, and variations in 3

4 expected cargo are the driving force. (i.e. is there something there that shouldn t be, is there something heavily shielding in one area, that would not be expected). For emergency responders, like bomb technicians who use portable non intrusive inspection systems for smaller packages, the resolution becomes more important, since they usually need to see very fine details of the object for disarmament planning. As a general rule of thumb, the number and size of detectors, and size of source have the most impact on resolution. The energy of the source has the most impact on penetration. Both of these functions have a significant impact on the cost and dose rate profiles of the system. The detector banks are often one of the most expensive items on a system. The smaller the detectors, and the more you need of them for fine resolution, the higher the cost. In addition the larger the source or machine output, the higher the dose rate. The detectors used in the typical radiation machine systems are current based and need a high enough output to ensure the measured flow is above the detectable level. The gamma gauge based systems that can use a photon counting technique, require a lower source output, and hence a lower dose rate field. The harder the gamma or x-ray source, the more shielding is usually required to maintain exposures as low as reasonably achievable, and so the higher the cost and lower the mobility of the device. The harder energy also results in a higher dose rate. It s important to note that gamma gauges are mono-energetic sources, while x-rays are emitted with a spectrum of energies up to the maximum potential of the device. Since only the higher energy x-rays are good for penetration purposes, but the lower ones still produce dose, the dose per scan and dose rate field from a standard (unfiltered ) x-ray system will be much higher than from an equivalent penetration with a gamma gauge source. The dose per scan difference for the currently available accelerator based x-ray and gamma gauge based systems is typically in the range of 100 to 10,000 times higher for the x-rays than the gamma rays. The impacted area due to scattered radiation will also typically be much higher with an x-ray source due to the greater side and backscatter which results from the lower energy contributions. The higher energy and higher flux achievable from radiation machines at greater than 3 MeV will in most cases produce a better resolution and higher penetration than the available gamma gauge based systems. However, the dose/dose rate profiles from these systems will also be significantly higher. For bulk cargo applications with potentially radiosensitive cargo such as film and humans, the improved resolution may not be offset or cost beneficial relative to the impact. The user must optimize the system performance relative to the ALARA concept since the final source type and size that is selected and the resulting dose/dose rate profiles will directly impact the way the system must be regulated and controlled. 4. Radiation Protection Guidance Historically these types of systems have been used to examine cargo for identification of illegal contraband such as drugs or weapons. More recently they have been utilized for validation of manifests and collection of tax revenue. Both of these applications require an evaluation of the radio-sensitivity of the cargo to damage. Excluding film below 400 ISO, and foodstuff, there is limited if any regulatory guidance in this area. However, a new application is rapidly immerging which involves the examination of cargo for illegal immigrants or examination of individuals for security screening purposes. There is some regulatory guidance for incidental exposures to members of the public which specify a limit of 1 msv/year. However, that guidance is directed at licensees performing operations in a given area and does not address the potential for multiple exposures from several different operations/licensees as might be the case for cargo/personnel going though multiple ports and borders. At the present time there is no consistent radiation protection guidance for use of these systems for cargo or security screening applications. As noted above the dose that can result from the scanning applications have a wide range of values depending upon the type and size of source selected. Since the need for and use of such devices is increasing, the radiation protection community should seek to provide some

5 guidance to ensure protection of the public and the operators, as well as the cargo. The public includes those individuals driving the cargo through the system, the illegal immigrants who may be hiding in the cargo and the individuals not directly involved with the operation but who are working in the area surrounding the system. For typical cargo scanning functions the radiation beam is quite narrow, and with the current systems the driver of any vehicle may be positioned outside (in front of) the beam path, or asked to leave the vehicle so they aren t required to be directly scanned. For those cases where the driver remains in the vehicle, the dose to the driver of the scanned vehicle is mostly due to scattered radiation from the cargo itself, and is again dependent upon the source used. Typically lower energy x and gamma rays tend to produce more back and side scatter, which may increase the dose in the cab area, while higher energies tend to have more forward scattering profiles. For comparable system penetration levels, the dose to the driver will typically be much lower for gamma gauge based systems due to the reduced low energy spectrum and emission rate. If the dose to the driver should be maintained at less than 1 msv/year, then the combination of scans per year and dose per scan must be maintained at a level well below this limit. For conservatism a value 25% of this limit (0.25 Sv/year) may be used. Assuming the driver may have to move cargo through the system multiple times per day (1 per hour, or 8 per day), and at least 250 days per year (typical work year) results in a dose per scan limit of < 0.1 usv/scan. This value is consistent with the guidance in ANSI N , Radiation Safety for Personnel Security Screening Systems Using X- rays. 1 This level of exposure should also be safe for radiosensitive material like film, where the current guidance would be < 10 usv/scan (ISO 400 speed). If these same levels are applied to any individual going through the system, including illegal immigrants, or those individuals being scanned for security purposes a consistent and safe level of exposure may be maintained. Regulators and the radiation protection community should seek to provide guidance to those individuals using or planning to use such systems to consider only those systems which meet the criteria of < 0.1 usv/scan, where members of the public may be exposed to the radiation. If higher doses are produced from the systems they should not be used for personnel security screening applications or those applications where members of the public are likely to be present in the cargo. If higher dose systems are still required for specific applications, then prescreening should be done of the cargo to eliminate the potential for personnel to be present. This type of prescreening application should also be implemented for those cargos with radiosensitive material like film, or foodstuff when the system uses x-ray energies in excess of 5 MeV. 2 1 American National Standards Institute, Radiation Safety for Personnel Security Screening Systems Using X-Rays, ANSI N43.27, World Health Organization FAO/WHO Codex Alimentarius Commission, Standard for Irradiated Food, July