Active Neutron Coincidence Counting

The fissile isotopes of uranium ( U) and plutonium ( Pu, Pu) can be measured using active neutron counting techniques. This technique uses an external neutron source to induce fission in the fissile plutonium and manium content of the sample. The multiple induced fission neutrons are then measured using standard coincidence counting methods. The technique is mainly applied to determine the mass of in uranium-bearing samples (from LEU to HEU) in powder, metal, pellets, fresh fuel elements, and waste drums. It can be operated either with or without a cadmium liner (fast or thermal mode). 

NDA methods are presently the only feasible means of verifying spent fuel. Several instruments are available for measuring spent fuel assemblies stored underwater. Direct measurement of plutonium and uranium in spent fuel is normally impossible because the primary neutron and gamma signals come firom higher actinides (e.g., spontaneous fission neutrons from the Cm and Cm isotopes) and fission products. The measurement of gamma and neutron radiation from spent fuel assemblies can be used to confirm various attributes and to estimate burnup, which can be correlated to the quantity of plutonium in the assembly. The variability of the fuel 

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Table 2. Thermal neutron activation determinations using gamma-gamma coincidence counting techniques...

Fujii et al. 7 8> have developed a rapid method for the determination of praseodymium, using 14 MeV neutron activation and gamma-gamma coincidence counting. 

One of the earlier and still one of the most interesting applications of coincidence counting in activation analysis is the isotopic determination of 6Li by Coleman 84>. In this method aqueous solutions of LiOH are irradiated with thermal neutrons to produce tritons by means of the 6Li(w,f)4He reaction. The tritons then react with the oxygen in the aqueous solution by the 160(f, )18F reaction. The 18F is a positron... 

B) Greenland, L. P. Application of Coincidence Counting to Neutron Activation Analysis. Geological Survey Prof. Paper 600-B, pp. B 76—78. Washington, D. C. U. S. Geological Survey 1968. 

Measurement methods for I, 40 Coincidence counting, 42 Comparison of methods, 42, 43 Gamma-ray spectrometry, 40 Laser extinction, 43 Liquid scintillation, 40 Low-level beta counting, 40 Mass spectrometry, 42 Neutron activation, 42 x-ray spectrometry, 40... 

The NAA method can be divided into NAA (Instrumental NAA) and RNAA (Radiochemical NAA). In the latter, the various neutron-induced products are separated chemically to minimize interferences. There are several comprehensive review papers on INAA published in the literature 1,2,3,4). Briefly, the basic parameters controlling sensitivity for a multi-element determination are neutron flux, irradiation time, delay interval prior to counting, half-life and gamma-ray energy of the induced activity, and eflBciency and resolution of the detector. Table I outlines the irradiation parameters used for each of the two sequential irradiations. The final count occurring 40-50 days after the second irradiation is performed on an anti-coincidence-shielded Ge(Li) system developed recently in our laboratory. 

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