AbstractThe perturbation in a thermal neutron spectrum near an absorber immersed in a moderator has been studied using activation techniques. The results show two separate effects for pure thermal spectra, a‘transmission hardening’depending on the energy variation of the absorber cross section and a‘boundary effect’depending on the energy variation of the moderator scattering cross section. These results are in agreement with energy-dependent Milne problem calculations for a mass one, free-gas moderator and with another experimental result when a third‘source hardening’effect in reactor spectra is considered.