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Author: Admin | 2025-04-28
Of an accident such as an explosion.[109] These sources emit fast neutrons with an energy of 14.1 million electron volts (MeV) from the neutron tube and 0–13 MeV from californium-252. If low-energy (thermal) neutrons are needed, they must be passed through a moderator.[10]In one method, thermal neutron analysis (TNA), thermal neutrons are captured by a nucleus, releasing energy in the form of a gamma ray. One such reaction, nitrogen-14 captures a neutron to make nitrogen-15, releasing a gamma ray with energy 10.835 MeV.[103] No other naturally occurring isotope emits a photon with such a high energy,[108] and there are few transitions that emit nearly as much energy, so detectors do not need high energy resolution.[103] Also, nitrogen has a large cross section for thermal neutrons.[108] The Canadian Army has deployed a multi-detector vehicle, the Improved Landmine Detection System, with a TNA detector to confirm the presence of anti-tank mines that were spotted by other instruments.[108] However, the time required to detect antipersonnel mines is prohibitively long, especially if they are deeper than a few centimeters, and a human-portable detector is considered unachievable.[103]An alternative neutron detector uses fast neutrons that enter the ground and are moderated by it; the flux of thermal neutrons scattered back is measured. Hydrogen is a very effective moderator of neutrons, so the signal registers hydrogen anomalies.[110] In an antipersonnel mine, hydrogen accounts for 25–35% of the atoms in the explosive and 55–65% in the casing. Hand-held devices are feasible and several systems have been developed.[108] However, because they are sensitive only to atoms and cannot distinguish different molecular structures, they are easily fooled by water, and are generally not useful in soils with water content over 10%. However, if a distributed pulsed neutron source is used, it may be possible to distinguish wet soil from explosives by their decay constants. A "Timed Neutron Detector" based on this method has been created by the Pacific Northwest National Laboratory and has won design awards.[103][111][112]Acoustic/seismic methods involve creating sound waves above the ground and detecting the resulting vibrations at the surface. Usually the sound is generated by off-the-shelf loudspeakers
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