Neutron 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 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... 

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. 

Common Features of NAA Procedures. In all of the procedures discussed in this article, irradiations are made in a high thermal neutron flux (1011 to 1013 neutrons cm"2 sec 1) simultaneously with the samples and standard(s) sealed in polyethylene containers for a short irradiation or in silica containers for a long irradiation. The standard is a known amount, or solution of known concentration, of a pure compound of the element to be determined. The concentration of the element in the sample is determined by comparing its radioactivity with that of the standard, which is either subjected to the same radiochemical separation as the sample with an inactive matrix or diluted. The radioactivity is counted directly if the sample is measured in solution. The radiochemical yield of precipitated samples is determined directly by weighing and that of solutions of samples by aliquot re-irradiation. 

The analysis scheme for the 10 evaluation samples used two aliquots ( 25 cm2 of filter paper/aliquot). One aliquot was encapsulated in polyethylene and irradiated in a polyethylene rabbit for 5 min in a thermal neutron flux of approximately 1014 n/cm2/sec. This sample was counted at decay times of 5 min, 30 min, and 24 hrs. The other aliquot was encapsulated in high purity synthetic quartz and irradiated in an aluminum rabbit 12-24 hrs. These samples were counted twice, after decay periods of 10 days and 3 wks. Sample counting equipment included one 4096-channel y-ray spectrometer and a Ge(Li) detector. 

The cross sections for (n,y) reactions common in reactor thermal neutron activation generally decrease with increasing neutron energy with the exception of resonance-capture cross section peaks at specific energies. This reaction is, therefore, not important in most 14 MeV activation determinations. However, some thermalization of the 14 MeV flux may always be expected due to the presence of low Z elements in the construction materials of the pneumatic tubes, sample supports, sample vial, or the sample itself (particularly when the sample is present in aqueous solution). The elements Al, Mn, V, Sn, Dy, In, Gd, and Co, in particular, have high thermal neutron capture cross sections and thermal capture products have been observed in the 14 MeV neutron irradiation of these elements in spite of care taken to reduce the amount of low Z moderating materials in the region of the sample irradiation position 25>. 

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. 

At irradiation with thermal neutrons, stable isotopes of mercury and many other elements are converted into radioactive daughter isotopes, that can be identified and quantified by high resolution gamma spectrometry. The irradiation is usually carried out in a nuclear reactor with thermal neutron flux densities of lO -IO cm s NAA is well established as a multi-element technique, and has a reputation of good accuracy. Separation and specification of mercury compounds is, however, not possible, since organic mercury turns into inorganic at irradiation (Rottsohafer et al., 1971). 

Sulfur-35 is the only long-lived radioactive isotope of sulfur and it may be produced by the direct neutron irradiation of elemental sulfur S (n,7)S . However, the specific activity attained is quite low even in high-neutron fluxes. It is preferable to irradiate potassium chloride and take advantage of the reaction CP (n,p)S . Four weeks irradiation of ten grams of potassium chloride in a thermal-neutron flux of 10 neutrons cm. sec. yields about 5 millicuries of which when dissolved in water is isolable as carrier-free sulfate ion. Nevertheless, the isotopes CP , P , K, and Na are concurrently produced in the neutron irradiation of potassium chloride, and a chemical... 

Melt-textured materials (Y-123, Nd-123) with U admixtures represent a highly promising system for high-Jc applications due to the possibihty of forming randomly oriented fission tracks (cf. sect. 6.3). Results on the critical current densities (taken by the flux-profile technique, i.e. in the nearly unrelaxed state) are shown in fig. 40 (Weinstein et al. 1998, Eisterer et al. 1998). 7c following thermal neutron irradiation to the optimal... 

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