Thermal Neutron Absorption Cross Sections by the Pulsed Source Method

The thermal neutron absorption cross sections of twenty-one naturally occurring elements and B¹⁰ have been determined by comparing the time dependence of the neutron flux in water with the time dependence of the neutron flux in a water solution of the sample with the same geometric buckling. After making some small corrections arising largely from the change in the number of hydrogen atoms per cm³ in the solution, the decay constant of the ratio curve gives the macroscopic absorption cross section averaged over the neutron flux spectrum. For a 1/υ cross section the 2200 m/sec cross section can be directly computed. For non-1/υ cross sections the effective 2200 m/sec cross section is obtained.