The Effect of Non-1/v Absorbers on Thermal-Neutron Transport in Finite Size Assemblies of Water and Heavy Water

The pulsed neutron problem is studied in small but finite sized assemblies of liquid water and heavy water poisoned with such non-1/v neutron absorbers as samarium, cadmium, and gadolinium. It is found that the asymptotic neutron density is markedly dependent on the nature of the non-1/v absorber. For a given buckling, one can find a critical concentration of gadolinium that will give a Maxwellian asymptotic neutron distribution. Thus, a finite assembly in the presence of a definite concentration of gadolinium acts as an infinite assembly. This is not the case with samarium or cadmium.