The effective delayed neutron fraction, βeff has been measured in the three cores 3, 5 and 8 of the fast zero-power reactor FRO. The variance-to-mean method, in which the statistical fluctuation of the neutron density in the reactor is studied, was used. A 3He-gas scintillator was placed in the reflector and used as a neutron detector. It was made more sensitive to fast neutrons by surrounding it with polythene. Its efficiency, expressed as the number of counts per fission in the reactor, was determined using fission chambers with known efficiency placed in the core. The space distribution of the fission rate in the core was determined by foil activation technique.
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