Capture cross section, thermal

Cf = capture cross section = thermal velocity = trap density... 

Gadolinium has the highest thermal neutron capture cross-section of any known element (49,000 barns). 

Isotope CAS Registry Number Occurrence, % Thermal neutron capture cross section, 10-" ... 

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

The capture cross section of Li for this reaction using thermal neutrons is 930 x 10 (930 b) (43). AU of the experimental data available to the end of... 

Production in Heavy Water Moderator. A small quantity of tritium is produced through neutron capture by deuterium in the heavy water used as moderator in the reactors. The thermal neutron capture cross section for deuterium is extremely small (about 6 x 10 consequendy the... 

Nickel-manganese-palladium brazes are resistant to attack by molten alkali metals and And applications in sodium-cooled turbine constructions. Their freedom from silver and other elements of high thermal neutron-capture cross-section allows them to be used in liquid-metal-cooled nuclear reactors. 

Niobium (also known as columbium) is a soft, ductile, refractory metal with good strength retenti on at high temperature, and a low capture cross-section for thermal neutrons. Itis readily attacked by oxygen and other elements above 200°C. CVD is used to produce coatings or free standing shapes. The properties of niobium are summarized in Table 6.8. 

There are a number of papers in the open literature explicitly reporting on the properties of boron cluster compounds for potential neutron capture applications.1 Such applications make full use of the 10B isotope and its relatively high thermal neutron capture cross section of 3.840 X 10 28 m2 (barns). Composites of natural rubber incorporating 10B-enriched boron carbide filler have been investigated by Gwaily et al. as thermal neutron radiation shields.29 Their studies show that thermal neutron attenuation properties increased with boron carbide content to a critical concentration, after which there was no further change. 

Nuclear and magneto-hydrodynamic electric power generation systems have been produced on a scale which could lead to industrial production, but to-date technical problems, mainly connected with corrosion of the containing materials, has hampered full-scale development. In the case of nuclear power, the proposed fast reactor, which uses fast neutron fission in a small nuclear fuel element, by comparison with fuel rods in thermal neutron reactors, requires a more rapid heat removal than is possible by water cooling, and a liquid sodium-potassium alloy has been used in the development of a near-industrial generator. The fuel container is a vanadium sheath with a niobium outer cladding, since this has a low fast neutron capture cross-section and a low rate of corrosion by the liquid metal coolant. The liquid metal coolant is transported from the fuel to the turbine generating the electric power in stainless steel... 

YeUow metal face centered cubic crystals lattice constant, a at 25°C 4.0786A density 19.3 g/cm hardness 2.5-3.0 (Mohs), 18.5 (BrineU) melts at 1,064°C vaporizes at 2,856°C electrical resistivity 2.051 microhm-cm at 0°C and 2.255 microhm-cm at 25°C Young s modulus 11.2x10 psi at 20°C (static) Poisson s ratio 0.52 thermal neutron capture cross section 98.8 barns insoluble in almost all single acids or hydroxide solutions dissolves in aqua regia. 

Grayish, soft metal with a white luster on polished surfaces ductile and very malleable at room temperature also highly ductile at cryogenic temperatures body-centered cubic crystals density 8.66 g/cm at 20°C melts at 2,468+10°C vaporizes at 5,127°C electrical resistivity 13.2 microhm-cm at 20°C becomes superconducting at 9.15K thermal neutron-capture cross section 1.1 barns insoluble in water insoluble in hydrochloric acid, nitric acid and aquaregia soluble in hydrofluoric acid soluble in fused alkah hydroxide. 

Of all the properties of the rare earths that contribute to their many and varied applications one that ranks of special interest is the extremely high thermal neutron capture cross-section associated with the elements gadolinium, samarium, europium and dysprosium, see Table IV. 

It is also necessary to note that the success of TSR techniques to obtain information on trapping states in the gap depends on whether or not the experiment can be performed under conditions that justify equation (1.2) to be reduced to simple expressions for the kinetic process. Usually, the kinetic theory of TSR phenomena in bulk semiconductors—such as thermoluminescence, thermally stimulated current, polarization, and depolarization— has been interpreted by simple kinetic equations that were arrived at for reasons of mathematical simplicity only and that had no justified physical basis. The hope was to determine the most important parameters of traps— namely, the activation energies, thermal release probabilities, and capture cross section— by fitting experimental cnrves to those oversimplified kinetic descriptions. The success of such an approach seems to be only marginal. This situation changed after it was reahzed that TSR experiments can indeed be performed under conditions that justify the use of simple theoretical approaches for the determination of trapping parameters ... 

All TSRs involve the release of trapped charge carriers into either the conduction band or valence band and their subsequent capture by recombination centers and recapture by other traps (retrapping). Their experimental investigation is undertaken with the goal of determining the characteristic properties (parameters) of traps cap-tnre cross sections, thermal escape rates, activation energies, concentration of traps. 

The excess free carriers (and excitons) do not represent stable excited states of the solids. A fraction of them recombine directly after thermahzation either radiatively or by multiphonon emission. In most materials, nonradiative transitions to defect states in the gap are the dominant mode of decay. The lifetime of free carriers T = 1/avS is determined by the density a of recombination centers, their thermal velocity v, and the capture cross section S, and may span 10-10 s. Electrons, captured by states above the demarcation level, and holes, captured by states below the hole demarcation level, may get trapped. The condition for trapping is given when the occupied electron trap has a very small cross section for recombining with a free hole. The trapping process has, until recently, not been well understood. 

The thermally activated emission rates are proportional to a Boltzmann factor, and by use of the principle of detailed balance can be related to the capture cross section (a ) ... 

Example Problem Calculate the average thermal neutron capture cross section and the mean free path for LiF, a solid crystalline material at room temperature with a density of 2.635 g/cm3 and a molar mass of 25.94 g/mol. Lithium has two stable isotopes 6Li (7.5%) and 7Li (92.5%) with thermal neutron capture cross sections of ct thermal = 39 mb and 45 mb, respectively. Fluorine is monoisotopic, 19F, with (Tthermal = 9.6 mb. 

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