Neutron absorption coefficient

Neutron economy in graphite occurs because pure graphite has a neutron capture cross section of only 0.0032 0.002 x lO " cm. Taking into account the density of reactor grade graphite (bulk density 1.71 g/cm ), the bulk neutron absorption coefficient is 0.0003/cm. Thus a slow neutron may travel >32 m in graphite without capture. 

Zirconium is used for structural parts in the core of water moderated nuclear reactors to this end Zr has several good properties and especially it has low thermal neutron cross-section. Hf, on the contrary, has a high thermal neutron absorption coefficient, so it is necessary to be able to prepare Hf-free zirconium. On the other hand, in some cases the Hf properties too may be useful in nuclear technology, in the control rods of submarine reactors. 

Table 3.1. Neutron coherent scattering cross-sections b and neutron absorption coefficients fi/o (A = 1.08 A) for elements found in biological molecules. [183]...

Neutron absorption coefficient (cm ) Boric acid, boron carbide 3.08 0.06 Encapsulation of nuclear materials... 

Atomic number Atomic weight Crystal structure Melting Density Thermal Electrical resistivity (at 20°C) Temperature coefficient of resistivity Specific Thermal Standard electrode potential Thermal neutron absorption cross-section. 

We have limited our investigations to the action of gamma-rays and fast neutrons on aromatic, alicyclic, aliphatic, and ionic compounds. The absorption coefficients for these types of radiation have an order of magnitude of lO /cm which is particularly adequate. Shallow penetrating radiations would only alter the superficial layers and would obviously not be able to affect the intensity of the quadrupole line substantially. This is for instance the case for ultraviolet light which has been shown to be unable to produce any effect on the resonance line of iodoform (CHI3 3Sg)... 

The process of absorption of x-rays or neutrons is not affected by the state of bonding of the atoms in the material, and therefore for a material consisting of more than one element, the overall absorption coefficient depends only on the relative numbers of various atoms present in it. Thus the linear absorption coefficient of a sample can be calculated by... 

The thickness of sample cell holders is optimally given by the reciprocal of the absorption coefficient. While the scattered intensity increases linearly with thickness, the sample absorption, however, increases exponentially. The scattered intensity reaches its maximum value when the incident beam is weakened to /e = 0.37, and this means that 1 mm thick samples are usual in X-ray and neutron work in H2O buffers. For neutron work in H20 buffers, 2 mm thick samples are usual, even though the optimal thickness is now greater than 10 mm. Samples that are too thick may lead to curve artefacts from multiple scattering events. Allowance for... 

Fig. 1. The average absorption coefficient of uranium for neutrons as a function of energy under assumptions a and 6 described in the text.

Neutron absorption in uranium is believed to be principally of two general types an absorption by U-238, chiefly without fission, in the thermal region and in a resonance region and absorption by U-235 which leads to fission. A detailed knowledge of these absorption processes is of interest in calculating the optimum size and spacing of the spheres to be used in the experiments on the chain reaction. The object of the present series of experiments is the study of the absorption processes in uranium and in particular is the accumulation of data from which can be calculated absorption coefficients for thermal and resonance neutrons. 

The absorption coefficient is a macroscopic property characteristic of the material of the sphere. It depends not only upon the absorption cross sections of the constituent atoms and upon their number per cubic centimeter but also upon their scattering cross sections. From the calculated absorption coeflBcient for thermal neutrons and the measurements of resonance absorption already reported for UaOs and in progress for uranium metal and compressed UsOg, it is the intention to deduce the optimum dimensions for the typical cell in the proposed lattice. 

One sees in Heitler s book that the absorption coefficient of water for 7-rays of about 5 MeV (which are hardest to absorb) is. 03. A shield of 110 cm gives a 6 = 3 for (2), permitting an about 6.5 cm decrease of the thickness of the Pb to be used. On the other hand, the lead shield also guards against neutrons. According to Friedman s report (C-99), the total cross section of Pb is about 6 X 10 cm for fast neutrons, of which 3.6 is due to inelastic scattering. This gives. 19 for the macroscopic absorption coefficient of lead for fast neutrons and the presence of 13.5 cm lead should permit one to decrease the thickness of the water layer to 105 cm. 

In addition to 1 /p2 it is interesting to note the factor by which the absorption in the carbon is increased by having the uranium concentrated in lumps, rather than spread out uniformly. Since the U has a higher absorption coefficient than the carbon, the neutron density will be larger in the carbon than in the U and we shall call the ratio of these densities the disadvantage factor d. It can be calculated from p2 as follows. 

Surrounding the reaction tank 1 we provide means to withdraw heat therefrom, such as by a heat exchanger system, all components of which are of materials such as steel having relatively low neutron absorption danger coefficients, so that particles abraided therefrom will not materially affect the chain " reaction. We have shown six heat exchangers 5 in the drawings although it will be ap-... 

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