Neutron Coincidence Counting

The He neutron counters detect all neutrons arising from both (a, n) and fission reactions. Sophisticated pulse processing electronics, called neutron coincidence counting circuits (shift register), measure the number of neutrons that are detected within a predefined time interval (gate width), and differentiate between time correlated (coincidence) neutrons emitted from the fission events and single neutrons created as a result of a-particle interactions. The measured coincident neutrons (doubles) are proportional to the mass of the even-even Pu isotopes ( PUefFecdve = 2.43 Pu -I- Pu -I- 1.69 Pu). 

The absolute plutonium mass is determined from the mass of Pu effective and the isotopic abundances. Induced fissions exhibit higher neutron multiplicity than the spontaneous fission events hence, they contribute to the enhancement of coincidence response and introduce nonlinearity in the response for higher amounts. The plutonium content of samples in this type of measurement can typically range from the gram level up to several kilograms. Standard methods have been developed for multiplication corrections. 

The detector system is calibrated using known standards that are subject to DA sampling. 

The neutron measurements are influenced by a number of physical and environmental factors such as filling heights, changes in density, presence of nearby reflectors and additional neutron sources. These factors could change the number of neutrons available to induce fission and hence lead to false coincident events. 

The HLNC is the basic model — a whole femily of instruments exists with various measurement configurations to fit shape and size of the item heing measured (Menlove 1983 Menlove et al. 1994). In most instances, these systems are ladhty resident or integrated into the facility process and can be operated either in attended or unattended mode. The counter is used to measure plutonium in bulk material (e.g., PuOa, mixed Pu02 — UO2 (MOX)) or plutonium in unirradiated MOX fuel assemblies and pins (O Fig. 63.2). 

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C26.10 Cl 207-97 Standard Test Method for Nondestructive Assay of Plutonium in Scrap and Waste by Passive Neutron Coincidence Counting... 

The determination of Pn values is based on the beta saturation counting rate (cP ), the neutron saturation counting rate (C11 ), the beta-neutron coincidence saturation counting rate (dP ), the beta counting efficiency (eg), and the neutron counting efficiency (en). The usual relation for the delayed neutron emission probability is... 

If one could reproduce the neutron end beta efficiencies from one mass to the next, one could use a known Pn value to determine the ratio of beta efficiency to neutron efficiency and then calculate all other Pn values using Eq. 1. With our present apparatus, the neutron efficiency is insensitive to changes from one mass number to the next, but the beta efficiency depends more critically on how the beam is tuned. We thus use Eq. 2 to calculate Pn because the beta efficiency does not appear in the expression. In some mass chains with several precursors (either isomers or isobars) it is possible to determine the beta efficiency for one of the prominent precursors from the ratio of the coincidence counting rate to the neutron counting rate. We then calculate Pn values for the other precursors in that mass chain by use of Eq. 1. In all cases we use the neutron counting efficiency determined for 9 Rb by use of Eq. 2 ond the Pn value of 13.6 0.9 X. 

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

A discussion of the coincidence technique with some general applications has been published by Wahlgren, Wing and Hines 71>. Many of the early applications of the technique made use of the fact that 64Cu is one of the few radionuclides produced by thermal neutron irradiation for which the 0.511 MeV positron annihilation photopeak is a prominent feature of the spectrum. Copper has been determined in meteorites 72> and copper ores 73,74) ]-,y coincidence counting of 04Cu annihilation radiation. The rapid and selective nature of the determination may have important applications in the on-line sorting of copper ores. 

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. 

The analysis of the complex cascade transitions in the [p y) reaction by scintillation spectrometry is simplified by the use of a three-crystal spectrometer (Sect. 14) as in the work of Hird et al.. These authors have also established one particular cascade by coincidence counting. The energy of the main ground state transition has been determined by Carver and Wilkinson by pulse height analysis of the photoprotons from deuterium in a high pressure ionisation chamber. The (pn) reaction with has a high threshold and the neutron resonances lie at a much greater excitation in N than the levels just discussed they have been observed by Bair et al. 

The Universal Fast Breeder Counter (UFBC) is a thermal neutron coincidence counter designed to assay FBR fuel assemblies and other types of plutonium fuel (Menlove et al. 1984). UFBC has 7.0% efficiency using 12 He tubes surrounded by polyethylene and a thin cadmium sleeve. The uniform counting (flat response) region in the detector head (141 cm high and 30.5 cm in diameter) is 105 cm. UFBC can measure assemblies with plutonium loadings of up to 16 kg (24% Pu). 

The waste drum assay system (WDAS) measures the residual small plutonium amounts of in-process wastes in 200 liter drums. The system uses a modified neutron coincidence counter with a counter comprising 60 He tubes ( 20% efficiency) with low background. WDAS applies the add-a-source correction technique that corrects for the effects of the waste matrix on neutrons (Menlove et al. 1993, Menlove 1995). A small Cf source is placed in various positions near the external surface of the sample drum. The changes in the Cf coincidence counting rate provide a matrix correction for the plutonium inside the drum. 

Normal coincidence counting techniques rely on the detection of two coincident neutrons (doubles) and making an assumption based either on the multiplication or on the (a, n) neutron rate. These two analysis methods - passive calibration and known-alpha - require... 

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). 

Passive neutron Total and coincidence counts Content of even Pu Isotopes sensitive to changes 1n geometry corrections for background and (o,n) reactions. Fissile Pu Isotopes not measured directly fuel handling required no HEU fuels. 

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... 

More advanced applications of neutron counting were based on the expectation that spontaneous fission events of superheavy nuclei should be accompanied by the emission of about ten neutrons [41,42], distinctly more than two to four observed for any other spontaneous fission decay. Such neutron bursts can be recognized by recording neutron multiplicities - events with several neutrons in coincidence - with 3He-filled counting tubes [43,44] or large tanks filled with a liquid scintillator sensitive to neutrons [45],... 

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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