Detection techniques fission neutrons

Another technique for neutron detection uses a fission chamber. One design contains a stack of alternate anodes and cathodes, one of the electrodes being covered by a thin layer of uranium enriched in The fission fragments produce large ionization even though the gas multiplication is quite low. This detector is more sensitive to fast neutrons than the BF3 counter, and can be used for fast neutron fluxes up to 10 n s with a backgroimd of a few cps. 

A large number of radiometric techniques have been developed for Pu analysis on tracer, biochemical, and environmental samples (119,120). In general the a-particles of most Pu isotopes are detected by gas-proportional, surface-barrier, or scintillation detectors. When the level of Pu is lower than 10 g/g sample, radiometric techniques must be enhanced by preliminary extraction of the Pu to concentrate the Pu and separate it from other radioisotopes (121,122). Alternatively, fission—fragment track detection can detect Pu at a level of 10 g/g sample or better (123). Chemical concentration of Pu from urine, neutron irradiation in a research reactor, followed by fission track detection, can achieve a sensitivity for Pu of better than 1 mBq/L (4 X 10 g/g sample) (124). 

The intention of this work was to measure the concentration of Li in examples of unirradiated reactor steel from which the likely activity concentration could be calculated using suitable values for the neutron flux. Calculated values could then be compared with the activities measured in examples of steel irradiated in a Magnox reactor. From this it was hoped to ascertain whether, given the concentration of Li, the activity could be reliably calculated or whether the effects of formation from ternary fission and subsequent diffusion would prevent this. Whilst the activity in activated steel was measured in a straight forward manner, the measurement of Li in steel was difficult. The conventional analytical techniques of ICP-OES and ICP-MS respectively failed to achieve an adequate detection limit, so alternative techniques were tried. SIMS was sensitive enough to detect Li but is a microscopic technique and so was prone to large uncertainties when used to predict the bulk concentration. NAA followed by radiochemical determination allowed the determination of but initially suffered interference from another activated radionuclide ( S). 

The spent fuel neutron counter (SFNC) is a prototype neutron-detector system that verifies closely packed spent fuel assemblies stored in a spent fuel pond (Ham et al. 2002). The system contains a fission chamber moderated by a polyethylene cylinder housed in a watertight stainless steel enclosure. The SFNC measures total neutron signals from long-cooled spent fuel assemblies while in their storage position, without requiring them to be moved. The technique can detect a missing fuel assembly. These measurements are performed underwater in a gap between four assemblies. 

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