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3.9.7 Neutron activation analysis

Another method which can provide valuable information on samples taken from a contaminated site is based on activation by neutrons. The nuclear track detection techniques for detection of uranium and plutonium particles are neutron induced fission track analysis and alpha track analysis. Both methods are commonly used and are well documented in the literature.

In neutron induced fission track analysis, a subsample of soil, ashed biota or air filter is brought onto a sticky plastic layer spread until all the material has covered the sticking layer. Then the sample is covered with mica film and tightly attached to the fission track detector. The detector is a polycarbonate foil. In this technique, the sample is irradiated with neutrons. A typical integrated flux of 109 n/s/cm2 may be reached during the neutron irradiation; fissile isotopes, such as 235U, 238Pu, 239Pu, 240Pu, etc., will absorb neutrons and thereby generate two fission-product nuclides with a shared kinetic energy of ca. 100 MeV. In the case where the fission product atom penetrates the detector, it will cause radiation damage along its path. In order to make the track visible, the detector is etched (this results in an increased diameter of the track hole). In the detector materials that are used mainly for neutron induced fission track analysis, alpha particles will not create tracks. After the detector has been developed, the distribution of fissile isotopes can be studied with a light microscope. The presence of particulate fissile produced isotopes is observed in a cluster of tracks. With standardization of the method, an estimation can be made of the particle size and the mass of fissile isotopes which are present in the particle. For the instrumented readout of the detectors, automated microscopic systems with an image analyzer are being used.

For the detection of alpha-emitting particles, the use of a material for registering the alpha radiation is required. Such materials are commercially available and the processing of the detectors is comparable to the above described procedure for fission track analysis. With alpha track detection, plutonium is much more effectively detected (on a mass basis) than uranium because of the large differences in specific activities between them. Comparing the number of alpha tracks and fission tracks for a single particle can give an initial indication of the elemental composition of the particle.