On the Utilization of Thermal Neutrons — I
The present report contains calculations on the proportion of the thermal neutrons which are absorbed, in the usual lattice arrangement, in U rather than in the damper (C). It is assumed that the thermal neutrons are created, with a uniform density, throughout the carbon and none created in the U or U3O8. In case of the metal, this assumption is well realized, and even in-the case of the oxide the number of neutrons slowed down in the U3O8 remains small (.0875 Mo/Mq where Mo and Mq are the masses of oxide and of carbon per cell). It will be further assumed that one can attribute to the thermal neutrons definite absorption and scattering cross sections both in the damper and the U or U3O8, just as if they had one definite energy. This assumption is hardly justifiable, since Maxwell’s energy distribution covers a wide energy range. This is particularly significant for the metal, in which there hardly is any energy exchange between neutrons and scatterer. Thus the low energy neutrons, which are most easily absorbed in the outer layers of the U, will not be replenished as the neutrons penetrate further inward and one must expect, even on the average, a higher than thermal energy for the neutrons in the inside of the metal. However, it would be very difficult to take the polyenergetic nature of the thermal neutrons into account rigorously and, at least for the small spheres which seem to offer most prospects, assuming a single energy value for all thermal neutrons will not introduce a great error. This may be particularly true if the constants entering the calculation are obtained under experimental conditions closely approximating those in the arrangement in question.
Unable to display preview. Download preview PDF.