The original in-pile collimator was retained (Figure 1), with the exception that a 25 cm crystalline bismuth filter, sealed in two close fitting aluminium containers, was inserted into the rotating lead shutter at the reactor face. The purpose of this crystal was to decrease the transmitted gamma flux, and to act as a neutron filter, attenuating the fast neutron component of the beam. A thermocouple attached to the filter indicated there was no significant temperature rise due to nuclear heatingo The area of the beam at the exit of the filter was 2" horizontal by l 1/4" vertical.nIn initial experiments a twin rotor system was used, with rotor tanks and spinning heads from the original multi-chopper system together with new rotors designed for slow neutron work. It quickly became apparent that considerable effort would need to be devoted to fast neutron shielding to minimize the fast neutron background at the detectors, with particular care being given to the region surrounding the rotor tanks and spinning heads.
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