Will you be 'mine'? Physics key to detection
October 25, 2000
RICHLAND, Wash. –
Creating a technology that can quickly and easily detect landmines can be as daunting a challenge as removing the deadly weapons. But a promising detector being built at the Department of Energy's Pacific Northwest National Laboratory promises to do just that, with the help of physics.
PNNL has called on 40-plus combined years of experience in radiation detection and arms control treaty verification to develop an affordable, easy-to-use method to detect landmines. Called the Timed Neutron Detector, the prototype offers benefits not available from many of today's existing detection technologies-low price, quick assessment, ease of use and, most importantly, the ability to detect mines containing little or no metal. The system can scan a 100-square-foot area-about the size of a child's bedroom-at a walking pace. Also, the system is portable and lightweight, which makes it more appealing for scanning large areas.
"If a detection system is easy to use and affordable, there's a greater chance Third World countries will use it," said Richard Craig, PNNL principal investigator. "We have focused on building a system that meets the requirements of those countries, because they have the greatest need for clearing mines."
The laboratory field-tested the system in September and is pursuing further development. Craig will make a presentation on the Timed Neutron Detector Nov. 14 as part of the international meeting of the American Nuclear Society and the European Nuclear Society in Washington, D.C.
The United Nations' Landmine Database estimates that at present rates it would take 1,100 years to clear the world's existing inventory of 110 million landmines hidden in soils of nearly 70 countries.
The Timed Neutron Detector appears similar to a metal detector yet applies physics to discover signs indicative of a landmine's presence. Specifically, the system detects hydrogen, which is present in casings and explosives found in plastic or metal landmines, and its interactions with neutrons.
The PNNL-developed system consists of a neutron source-about the size of a personal pager-and a detector built into one system that a person sweeps over the ground, much like a metal detector.
PNNL physicists designed the neutron source to hold a small amount of californium-252. During californium-252's natural decay, the fission events produce three or four neutrons. Electronics built into the neutron source record the time fission occurred. This process, called time tagging, allows the system to differentiate between neutrons that have interacted with hydrogen in landmines and those that are simply scattered from the soil.
This fission reaction shoots the neutrons out of the source thousands of times faster than a bullet exits a rifle and many of them go into the soil. If the neutrons encounter a buried landmine-plastic or metal-they will interact with any hydrogen found in the casing or explosive material. During this interaction, hydrogen removes part of the neutron's energy and reflects those neutrons-now called slow neutrons because they don't have as much energy-back toward the handheld system.
The slow neutrons return to the detector. Once inside, nonradioactive helium-3 stored in low-pressure pipes collects a neutron then emits an electron. A tube with high-voltage wire collects those electrons and translates them into a simple determination of whether a landmine exists.
PNNL physicists chose to target hydrogen because it removes more energy from neutrons than other elements, making the change in energy of neutrons easier to identify. Also, hydrogen is a common element in both metal and plastic mines. A person operating the Timed Neutron Detector for one day would be exposed to about the same radiation received during a dental x-ray or a cross-country airplane flight.
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Tags: Energy, National Security, Radiation Detection