[en] The thermal neutron cross section of the ⁹⁰Sr(n, γ)⁹¹Sr reaction was measured in a D₂O moderated reactor with an activation technique. The Cd ratio of the neutron flux at the location of the irradiation is about 30. For reduction of the ⁹⁰Y high energy beta-ray background strontium-specific columns (Sr. Spec(TM)) were used to chemically separate the Y from Sr before irradiation and again after irradiation prior to commencement of counting the ⁹¹Sr activity. The amount of ⁹⁰Sr in a counting sample was determined from the beta activity of ⁹⁰Y with an end-window beta-ray proportional counter. This system was calibrated with a known ⁹⁰Sr standard sample. The neutron flux at the irradiation position in the D₂O moderated reactor was determined from a Co/Al alloy monitor irradiated simultaneously with a ⁹⁰Sr sample. The ⁹¹Sr activity was measured with a high resolution Ge spectrometer. The sensitivity to beta induced bremstrahlung was reduced by the use of lucite, cadmium, copper, and lead absorbers around the sample and the Ge detector. The overall efficiency of the spectrometer was determined with a set of calibrated ¹³⁷Cs, ¹³⁴Cs, ¹³³Ba, ⁸⁸Y, ⁶⁰Co and ⁵⁴Mn sources. These measurements yielded a value of (9.7±0.7 mb) for the D₂O moderated neutron cross section of ⁹⁰Sr(n, γ)⁹¹Sr. (orig.)