Carbon, neutron absorption cross section

Hard silvery-white metal hexagonal close-packed crystal structure density 12.41 g/cm3 at 20°C melts at 2,334°C vaporizes at 4,150°C electrical resistivity 7.1 microhm-cm at 0°C hardness (annealed) 200-350 Vickers units Young s modulus 3.0x10 tons/in magnetic susceptibility 0.427 cm /g thermal neutron absorption cross section 2.6 barns insoluble in water, cold or hot acids, and aqua regia can be brought into aqueous phase by fusion of finely divided metal with alkaline hydroxides, peroxides, carbonates and cyanides. 

Silvery-white lustrous metal when pure or dark gray amorphous powder orthorhombic crystals hardness 2.3 Mohs density 6.25 g/cm melts at 452°C vaporizes at 990°C modulus of elasticity 6.0x10 psi thermal neutron absorption cross section 4.7 0.1 barns insoluble in water, carbon disulfide, and benzene also insoluble in HCl soluble in sulfuric acid, nitric acid, and aqua regia also soluble in caustic potash and in solutions of alkali metal cyanides. 

Grayish-white metal hody-centered cubic crystalline structure density 19.3 g/cm3 melts at 3,422°C vaporizes at 5,555°C vapor pressure 1 torr at 3,990°C electrical resistivity 5.5 microhm-cm at 20°C modulus of elasticity about 50 to 57 x lO psi (single crystal) Poisson s ratio 0.17 magnetic sus-ceptibilty +59 x 10-6 thermal neutron absorption cross section 19.2 + 1.0 barns (2,200m/sec) velocity of sound, about 13,000 ft/sec insoluble in water practically insoluble in most acids and alkabes dissolves slowly in hot concentrated nitric acid dissolves in saturated aqueous solution of sodium chlorate and basic solution of potassium ferricyanide also solubibzed by fusion with sodium hydroxide or sodium carbonate in the presence of potassium nitrate followed by treatment with water... 

The SDP is an average value over the epithermal neutron energy region. A good neutron moderator should divert few neutrons from the fission process, i.e. the neutron absorption cross section must be small. In this respect both heavy water (i.e. D2O) and carbon are... 

For gas-cooled reactors, the coolant which has been most widely used is carbon dioxide, which is readily available and has a low neutron absorption cross section. The poor heat transfer capability of a gas as compared to a liquid coolant requires the gas-cooled reactor to be operated at a high pressure (about 600 psi) in order to provide the required heat removal capacity. 

Carbon dioxide has the advantages of low cost and low neutron absorption cross section. The use of gas as coolant permits much higher core... 

Again, (4) and (5) contain two terms. The first corresponds to fast neutrons and in this c(E) = 1/( E) where is the slowing down power of the material (.158 for carbon). The second term refers to thermal neutrons. In this / c E) dE is the ratio of total scattering cross section and total absorption cross section per unit cell, multiplied by the thermal disadvantage factor (about 1.4 in the usual arrangement). (This means, of course, that the f c(E)dE of this term is the number of collisions a neutron suffers while it is thermal.)... 

Boron [7440-42-8] B, is unique in that it is the only nonmetal in Group 13 (IIIA) of the Periodic Table. Boron, at wt 10.81, at no. 5, has more similarity to carbon and siUcon than to the other elements in Group 13. There are two stable boron isotopes, B and B, which are naturally present at 19.10—20.31% and 79.69—80.90%, respectively. The range of the isotopic abundancies reflects a variabiUty in naturally occurring deposits such as high B ore from Turkey and low °B ore from California. Other boron isotopes, B, B, and B, have half-Hves of less than a second. The B isotope has a very high cross-section for absorption of thermal neutrons, 3.835 x 10 (3835 bams). This neutron absorption produces alpha particles. 

Essentially, the absorption of neutrons occurs in two distinct stages. Fast neutrons are slowed down by elastic and inelastic scattering processes with nuclei, particularly light nuclei like carbon and hydrogen. The slowed-down neutrons are then captured, as the capture cross-section for low-energy neutrons is high for most elements. 

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