Oct. 13, 2003 Metal detectors have become so commonplace that you might think we know all we need to about them. However, the law enforcement community must continually update performance standards for metal detectors to ensure that new products purchased in the marketplace operate at specified minimum levels. Further-more, they must know if exposure to the magnetic fields generated by metal detectors affects the functioning of personal medical electronic devices (such as cardiac defibrillators, infusion pumps, spinal cord stimulators, etc.)
With funding from the U.S. Department of Justice's National Institute of Justice, researchers at the National Institute of Standards and Technology (NIST) develop and revise such standards as new technologies become available. One project concentrated on finding better materials to mimic the human body's response to the magnetic fields generated by metal detectors. By using such biologic "phantoms," researchers can create more realistic testing scenarios without subjecting medical patients to exposure.
Since about two-thirds of the human body is made of water, conventional phantoms utilize liquids and salts. However, the liquids are subject to evaporation that changes both the salinity and the electrical conductivity, making it difficult to model human body components consistently.
The NIST researchers came up with an improved phantom material, a polymer mixed with carbon powder. By varying the amount of carbon powder used, the materials can mimic blood, bone, fat and skin. The researchers chose carbon black--a fine powder made almost entirely of elemental carbon--because of its electrical conductivity and low cost. The impregnated polymers can be formed in a variety of shapes and sizes. A recent NIST publication* discusses the material and its low-frequency electrical properties in detail.
* NIST Technical Note 1529, Carbon-Loaded Polymer Composites Used as Human Phantoms: Theoretical Models for Predicting Low-Frequency Dielectric Behavior. R.G. Geyer, J. Baker-Jarvis, M.D. Janezic, and R.K. Kaiser.
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