The neutron yield measurement system on FTU was initially composed of six BF3 proportional chambers for the start-up phase. The present reliable operations in deuterium, at higher plasma current, produce higher temperature discharges with higher neutron yields, therefore requiring less sensitive detectors: three fission chambers were chosen. We report the results of the new detector system calibration performed in September 1990. The BF3 detectors were also recalibrated as large scattering masses had been added around the machine since the previous calibration (November 1989). The calibration was performed by placing a Cf-252 source (5.81 X 10(7) n/s) at different radial and toroidal positions inside the vacuum vessel; an increased number of points on the torus axis, with the inclusion of some off-port positions, represents the improvement with respect to the previous calibration campaign. Data on the response of the different detectors as a function of the source position are presented. The computation procedures of the relationship between the counts of a single detector and the total neutron yield from an extended plasma are discussed. A comparison between the two detection systems is carried out and, for the BF3 chambers, the data of the '89 calibration are compared with the present ones. Finally, preliminary results of the neutron emission from FTU ohmic discharges (campaign 1991) are presented.
