X-ray backscatter
From Wiki Against Mines
The text on this site is published with permission of RAND and taken from "Alternatives of Landmine Detection" Jacqueline MacDonald et.al, RAND report, ISBN 0-8330-3301-8, Document Number: MR-1608-OSTP, Year: 2003[1]
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Description
Traditional x-ray radiography produces an image of an object by passing photons through the object. X rays have a very small wavelength with respect to mine sizes, so in principle they could produce high-quality images of mines. Although pass-through x-ray imaging of the subsurface is physically impossible, the backscatter of x rays may still be used to provide information about buried, irradiated objects. X-ray backscatter exploits the fact that mines and soils have slightly different mass densities and effective atomic numbers that differ by a factor of about two. There are two basic approaches to using backscattered x rays to create images of buried mines. Methods that collimate (i.e., align) the x rays employ focused beams and collimated detectors to form an image. The collimation process increases size and weight and dramatically reduces the number of photons available for imaging. Thus, high-power x-ray generators must be used as sources. The large size, weight, and power requirements of such systems are not amenable to person-portable detectors. Alternatively, uncollimated methods illuminate a broad area with x rays and then use a spatial filter to deconvolve the system response. They may be suitable for person-portable detection.
Strengths
To readily distinguish mines from soils, it is necessary to use low-energy incident photons (60–200 keV). In this energy range, cross sections are roughly 10 or more times larger than is possible with most other nuclear reactions that would be applicable to mine detection. In addition, because of the reduced shielding thickness needed to stop low-energy photons, uncollimated systems can be made small and relatively lightweight. Largely because of the medical imaging industry, compact x-ray generators are now obtainable. Low-energy isotopic sources have been readily available for a long time. Practical imaging detectors are becoming more widespread, although it may be necessary to custom build for mine detection purposes. The medical imaging industry is likely to drive further advances in x-ray imaging hardware.
Limitations
In the required energy range, soil penetration of x-ray backscatter devices is poor. This limits detection to shallow mines (less than 10 cm deep). If source strengths are kept low enough to be safe for a person-portable system, the time required to obtain an image may be impractically long. In addition, the technology is sensitive to source/detector standoff variations and ground-surface fluctuations. Further, to image antipersonnel mines, high spatial resolution (on the order of 1 cm) is required. This may be difficult to achieve in the field. Finally, the technology emits radiation and thus will meet resistance to use because of actual or perceived risks.
Summary/Evaluation
X-ray detection using the uncollimated imaging approach may be useful for handheld confirmatory detection of antipersonnel landmines. In fielded systems, images of mines are likely to be fuzzy but should still allow mines to be distinguished from most diffuse or elongated false alarms. On the whole, however, x-ray backscatter does not offer particular innovations or likely avenues of improvement relative to other technologies and is unlikely to yield substantial improvement in detection capabilities.
Links / References
- ↑ Jacqueline MacDonald et.al: Alternatives of Landmine Detection, RAND report, ISBN 0-8330-3301-8, Document Number: MR-1608-OSTP, 2003
