Neutron Absorption in Pile Neutron Analysis

Abstract

As a basis for the discussion of the absorption effects in pile neutron activation analysis, this report begins with an expression for the pile neu tron-induced activity in a nuclide by an (n,y)-reaction under the assumption of no absorption effects The induced activity is expressed in terms of a thermal neutron density and a pile neutron absorption cross-section or a reso nance integraL The absorption effects of pile neutrons on the neutron induced activity are discussed, and the activity is expressed in terms of an average thermal neutron density and an effective pile neutron absorption cross—section or an effective resonance integraL The absorption effect of the thermal neutrons is measured by the ratio of the average thermal neutron density inside the sample to the unperturbated thermal neutron density at the site of the sample prior to its insertion into the neutron flux. A comparison between theoretical and experimental values for this ratio for spherical samples is given.The effect of the absorption of the epithermal neutrons is shown to de pend on the cadmium ratio and in a complex manner on the composition of the sample A calculation of this effect on the pile neutron activation analysis of copper and gold in silver is given as an illustration of the calculations involved The results are compared with experimental results Methods by which the effects of neutron absorption can be eliminated or experimentally determined are reviewed, and their limitations are discussed