K, 42 K / Neutron activation / Thermal cross section / Resonance integral / Gamma ray spectrometrySummary. We measured the 41 K thermal neutron absorption and resonance integral cross sections after the irradiation of KNO 3 samples near the core of the IEA-R1 IPEN pooltype research reactor. Bare and cadmium-covered targets were irradiated in pairs with Au-Al alloy flux-monitors. The residual activities were measured by gamma-ray spectroscopy with a HPGe detector, with special care to avoid the 42 K decay β − emission effects on the spectra. The gamma-ray self-absorption was corrected with the help of MCNP simulations. We applied the Westcott formalism in the average neutron flux determination and calculated the depression coefficients for thermal and epithermal neutrons due to the sample thickness with analytical approximations. We obtained 1.57(4) and 1.02(4) b, for thermal and resonance integral cross sections, respectively, with correlation coefficient equal to 0.39.