Neutron methods
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
Neutron Methods link to the German Version
Contents |
Description:
Neutron interrogation techniques involve distinguishing the explosives in landmines from surrounding soil materials by probing the soil with neutrons and/or detecting returning neutrons. Differences in the intensity, energy, and other characteristics of the returning radiation can be used to indicate the presence of explosives. Only three of the many possible reactions involving neutrons or gamma rays have reasonable potential for landmine detection. The first, thermal neutron analysis, is the only nuclear technique that is currently fielded by a military: The Canadian military uses it as a vehicle-mounted confirmatory detector for antitank mines. Size and weight limits imposed by physics preclude it from being person-portable or being able to detect small antipersonnel mines in practical applications. The second method, known as fast neutron analysis, has similar limitations. The third method, neutron moderation, is the only one of the three with the potential to yield a person-portable detector for antipersonnel mines. Neutron moderation discerns buried materials with low atomic numbers (e.g., hydrogen).
Strengths:
The physical properties of neutron moderation allow the
technology to use low-strength source radiation, which reduces
shielding required to protect workers from radiation exposure. Thus,
designing a handheld system may be possible. Costs of a production
imager are expected to be moderate.
Limitations:
Neutron activation methods can, at best, measure relative
numbers of specific atoms but cannot determine what molecular
structure is present. Because neutron moderation is most sensitive to
hydrogen, hydrogenous materials, particularly water, produce many
false alarms. Thus, to detect landmines successfully, it is necessary to
use the response from the neutrons to generate a visual image of the
area under investigation. Simulations show that the method will
work in soil with 10-percent moisture or less and may be usable
when moisture content is as high as 20 percent. Ground-surface
fluctuations and sensor height variation also contribute to false
alarms in nonimaging systems. Imaging can reduce these effects,
although some degradation of the image is expected.
With sources having sufficiently low strength to be practical for
handheld use, a few seconds will be required to acquire an image.
This makes neutron moderation imaging more suitable for confirmation
than for primary detection. Also, there is a perceived (more
than actual) radiation hazard associated with nuclear techniques
that must be overcome by the users. Broad-area, low-power electronic
neutron sources, under development by Defence R&D
Canada, could reduce this perceived risk.
Summary Evaluation:
The majority of neutron technologies have
physical limitations that preclude them from being portable. Only
one technology—neutron moderation imaging—may be useful for
handheld confirmation of antipersonnel landmines. In fielded systems,
images of these mines are likely to appear as fuzzy blobs, but
that will still allow mines to be distinguished from most diffuse or
elongated false alarms generated by moisture. On balance, however,
neutron moderation imaging is very unlikely to yield substantial
improvements in detection speed beyond what is capable with other
confirming detectors.
