Neutron counting techniques

NDA based on neutron measurements plays an important role in the qualitative and quantitative analysis of nuclear material, in particular plutonium in bulk and item form. Plutonium samples have a high rate of spontaneous fission neutrons, while uranium samples are typically interrogated using an induced fission neutron signature. 

Neutrons are primarily emitted from nuclear material in three ways  

Spontaneous fission of uranium, plutonium (in particular involving even isotopes of plutonium), and curium (in spent fuel). 

Induced fission from fissile isotopes of uranium and plutonium, typically by means of a low-energy neutron source. 

Qt-particle induced reactions, involving light elements such as oxygen and fluorine. 

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Nass et al. [238] used a delayed neutron counting technique to determine down to 50 ng of 235uranium in soils. Steam digestion has been employed in the preparation of soil samples for the determination of uranium (and thorium). 

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). 

Using this fast neutron moderation technique, relative count rates vs % H20 are shown in Fig 15 for four different powdered solids with sand included as a basis for comparison. The lowest exptl point on each curve represents the lowest water content at which an increase in count rate of at least 500 counts/min was obtained over the completely dry solid... 

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

The amount of spurious is small because of the comparative complexity of process (i), but considerable amounts of " As can be produced. The interference of " As has been overcome by the use of proportional counting techniques which distinguish between the different /9-energies, 0.7 MeV for As and 3.1 MeV for " bVs. Choice of suitable time of irradiation and neutron flux can minimise the yield of simrious " As, making possible the determination of as little as 0.01 ppm As in Ge02. 

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 thermal neutron activation technique has been applied by Borsaru and Mathew (1980) to find the concentration of AI2O3 in coal measuring the 1.78 MeV y-ray produced from the reaction Al(n, y) A1. In another study, these authors (Borsaru and Mathew 1982) applied the thermal neutron activation technique to bulk samples (=11 kg) of Australian black coal for the determination of alumina, silica, and ash. The determination of alumina was based on the reaction Al(n, p) Mg and coimted the 0.844 MeV peak (ti/2 = 9.4 min). Silica was determined by means of the reaction Si(n, p) Al and counting the 1.78-MeV peak ti/2 = 2.3 min) applying correction for the interference from alumina. 

In NAA, the sample in a suitable container, often a pure PE tube, is bombarded with slow neutrons for a fixed time and standards are bombarded in parallel with the samples. Transmutations convert analyte elements into radioactive elements, which are either different elements or isotopes of the original analyte. After removal from the reactor the product is subjected to various counting techniques and various forms of spectrometry to identify the elements present and their concentrations. 

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