Tests were performed on two parallel plate and two cylindrical fission detectors in pulse and dc mode. The effect of gamma background on neutron sensitivity was studied in thermal neutron flux from 30nv to 60nv over which gamma field intensity ranging from 2.30kR/h to 3.7MR/h was superposed. In the case of one of the parallel plate detectors the fall in neutron sensitivity was observed to be 3.7% at IMR/h and negligible below IMR/h. In the case of one of the cylindrical counters t he fall in neutron sensitivity was negligible below 500kR/h and 37% at IMR/h. The data was used to derive the design parameters for a wide range fission detector to be procured for PFBR instrumentation for operation at 600^˚C and gamma background of IMR/h.