The beam aperture consists of two boron carbide sheets (each 3-mm thick)to give an aperture diameter of 25 mm. A hollow graphite block 15-cm thick separates theaperture from the moderator block in order to obtain a uniform neutron beam, and the outerdiameter of the divergent beam collimator is 30 mm. Streaming of the radiation is eliminatedby using bismuth and lead as beam stoppers that intercept all the radiation coming from thecore of the reactor, gamma rays that arise from radiative capture of the neutrons, and scatteredradiation from the sample and sample holder.The beam shutter ensures the safe operation of the facility while positioning the sample. Thisshutter system consists of two parts:(a) first segment is made from borated paraffin, cadmium and boron carbide, andcadmium sheets, and is enclosed in aluminium casing - thermalized neutrons areattenuated and absorbed by the borated paraffin, cadmium and boron carbide sheets
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