Neutron irradiation, high flux

Os-191 is produced by neutron irradiation of isotopically enriched 0s-190 (isotopic composition 0s-190, 97.8 o Os-188, 0.47 o Os-192, 1. 02 o). Irradiations are currrently performed at the Oak Ridge National Laboratory in the High Flux Isotope Reactor (HFIR) at a neutron flux of 2.5 x 10 n/cm -s. The routes to the various nuclides produced during irradiation of the 0s-190 target and the neutron cross-section values (2 ) are summarized below (Scheme I). 

The use of radioactivated discs of aluminum and steel to estimate fragment erosion in solid targets at very high velocities was found to be feasible. Activation of discs was accomplished by slow neutron irradiation in a nuclear reactor at a flux of 8 x 1012 neutrons per cm2 per sec for 3 days for the aluminum discs (4g, 2.5cm diam x 0.3cm thick), and for 4 hrs for the steel discs (5g, 2.5cm diam x 0.15cm thick). Gamma-ray spectrometry indicated the presence of 59Fe (half-life 46 days) and 51Cr (half-life 28 days) in ratios 0.5 for aluminum and 1.3 for steel. The radioactivities in the aluminum arose solely from impurities, whereas in the steel they were contributed by the major component, iron, and only supplemented by the chromium impurity. The radioactivity was found by successive acid soln determinations to be distributed evenly in both metals... 

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. 

The straightforward way to obtain light actinides is by neutron irradiation of elements of lower atomic number. For example the production of Pa has been produced by the transmutation of Th with neutron produced in a high-flux nuclear reactor. 

Bowden and Singh [37,38] utilized an Sb-Be source with a slow neutron flux of about 10 n/cm /sec, and a cyclotron for fluxes up to 3 X 10 n/cm /sec. Explosives were irradiated for 1 hr so that the maximum total slow neutron dose was 1.08 X lO n/cm. In most cases a large number of high-velocity recoil atoms are formed during the irradiation of the explosives with slow neutrons (Table IX). In no case did an explosive (even nitrogen iodide) detonate as a result of the slow neutron irradiation. 

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