Neutron resonance capture

Fig. 1.7. Neutron resonance capture. The probability of capture is greatest when the sum of the binding energy of the neutron and the kinetic energy before the collision (this sum being represented by the line AC) coincides with one of the energy levels of the compound nucleus (dotted lines).

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>. 

Self-shadowing and resonance capture effects. The use of small samples and standards so that the neutron flux is not appreciably attenuated between the exterior and interior of the irradiation unit is to be desired. When large samples are used or appreciable high cross section material is present in the matrix, it is important that the standard be prepared with a matrix physically and chemically similar to that of the sample. 

It was obvious that process 3 was the most normal - a resonance capture of slow neutrons to form 239U, a beta emitter which necessarily decays to element 93. If the Berlin team had been able to detect this element 93 and determine its chemical properties, they would have realized that processes 1 and 2 were incorrect. But they did not do it. In Berlin, as Hahn later wrote, they were not very interested because... 

A maximum specific activity of approximately 850 GBq/mg was achieved when irradiation was carried out at a thermal neutron flux of 1 x 10 n-cni -s for 21 d, which corresponds to around 21% of the maximum achievable specific activity. Tlie specific activity of the Lu obtained was significantly higher than the theoretically calculated value under the irradiation conditions employed (7.9 at.%), accounting for only thermal neutron capture. This could perhaps be attributed to the contribution from epithermal neutrons (resonance integral 1087 b), which is not accounted for in theoretical calculations. 

Because of its neuronic, mechanical, and physical properties, hafnium is an excellent control material for water-cooled, water-moderated reactors. It is found together with zirconium, and the process that produces pure zirconium produces hafnium as a by-product. Hafnium is resistant to corrosion by high-temperature water, has adequate mechanical strength, and can be readily fabricated. Hafnium consists of four isotopes, each of which has appreciable neutron absorption cross sections. The capture of neutrons by the isotope hafnium-177 leads to the formation of hafnium-178 the latter forms hafnium-179, which leads to hafnium-180. The first three have large resonance-capture cross sections, and hafnium-180 has a moderately large cross section. Thus, the element hafnium in its natural form has a long, useful lifetime as a neutron absorber. Because of the limited availability and high cost of hafnium, its use as a control material in civilian power reactors has been restricted. 

Placzek pointed out that uranium and thorium both exhibit a capture resonance for neutrons with medium-range energies of about 25 electron volts. That meant, first of all, that although fission was one behavior uranium could exhibit under neutron bombardment, capture and subsequent transmutation continued to be another. Bohr was not ever to be rid of those inconvenient transuranians. Some of them were real. 

Resonant capture of thermal neutrons occurs only rarely in light nuclei and the radiative widths cannot easily be obtained this is due to the fairly wide... 

Neutron capture y-rays. For practical reasons the neutron capture y-rays which have been studied in most detail are those which are emitted when thermal neutrons are captured. For neutrons of higher energies, the cross sections rapidly become much lower and only very recently have attempts been made to study the y-rays emitted when neutrons are captured at specific resonances. 

R. Schermer and N. Comgold, Resonance capture of neutrons in infinite homogeneous media, Proc. Phys. Sci. vol. 73 (1959) p. 561. 

Thermal Neutron Radiative Capture Cross Sections, y-Factors, and Resonance... 

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