Irradiation, high flux thermal

The availability of high flux thermal neutron irradiation facilities and high resolution intrinsic Ge and lithium drifted germanium (Ge(Li)) or silicon (Si(Li)) detectors has made neutron activation a very attractive tool for determining trace elemental composition of petroleum and petroleum products. This analytical technique is generally referred to as instrumental neutron activation analysis (INAA) to distinguish it from neutron activation followed by radiochemical separations. INAA can be used as a multi-elemental method with high sensitivity for many trace elements (Table 3.IV), and it has been applied to various petroleum materials in recent years (45-55). In some instances as many as 30 trace elements have been identified and measured in crude oils by this technique (56, 57). 

The DT reactor needs several kg tritium as starting material. A likely technique involves the irradiation of a Li-Al alloy in a high flux thermal fission reactor which produces both tritium and He (17.43) These can be separated on the basis of their different vapor pressures, different permeability through palladium, or through their different chemical reactivities. 

The track density (number of fission tracks per cm ) in a mineral is a function of the concentration of U and the age of the mineral. For the purpose of dating, a sufficient number of tracks must be counted, which means that the concentration of U or the age (or both) should be relatively high. Usually, first the fission track density due to spontaneous fission of is counted, then the sample is irradiated at a thermal neutron flux density in order to determine the concentration of U in the sample by counting the fission track density due to neutron-induced fission of The age t of the mineral is calculated by the formula... 

Reactor neutrons are most frequently used for activation analysis, because they are available in high flux densities. Moreover, for most elements the cross section of (n,y) reactions is relatively high. On the assumption that an activity of lOBq allows quantitative determination, the lower limits of determination by (n,y) reactions at a thermal neutron flux density of lO cm s are listed in Table 17.2 for a large number of elements and two irradiation times (1 h and 1 week). Detection limits of the order of 10 to g/g are, in general, not available by other analytical methods. 

Neutron sources include nuclear reactors, accelerators, and isotopic sources. Nuclear reactors are, by far, the most frequently used irradiation facilities. They provide high fluxes [upper limit 10 neutrons/(m s)] of mostly thermal neutrons E < I eV). Fast neutrons in the keV range are also available, but at lower flux levels. 

The most practical neutron source for NAA is a nuclear reactor, which produces neutrons via the nuclear fission process (see Chap. 57 in Vol. 6). Many research reactors are equipped with irradiation facilities that provide a stable, well-tailored, isotropic neutron field with sufficiently high flux. Low-energy (thermal) neutrons comprise the most important part of the reactor spectrum hence the degree of moderation is an important parameter. The irradiation channels are usually created in moderator layers, such as a thermal column or a Be reflector blanket. 

NTD wafers were produced by irradiating natural ultra pure Ge crystals by means of a flux of thermal neutrons (see Section 15.2.2). To realize the electrical contacts, both sides of the wafers (disks, 3 cm in diameter, 3 mm thick) were doped by implantation with B ions to a depth of 200nm. The implanted layers are doped to such a high concentration that the semiconductor becomes metallic. Then a layer of Pd (about 20 nm) and Au (about 400 nm) was sputtered onto the both sides of the wafers. Finally, the wafers were annealed at 200°C for 1 h. The wafers are cut to produce thermistors of length 3 mm between the metallized ends (3x3x1 mm3 typical size) the electrical contacts are made by ball bonding with Au wires. 

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