Tests of Fission Chambers Upgraded to Register Thermonuclear Neutrons

The ITER vertical neutron camera (VNC) is a multichannel neutron collimator designed to measure the neutron source profile in the poloidal cross section of the tokamak plasma. In order to increase the direct neutron signal, it was previously proposed to use rectangular collimators with an aperture extended in the toroidal direction. This optimization improved the signal-to-background ratio by several times without deteriorating the spatial resolution of the system. This entailed a change in the design of the VNC detector units, in particular, the electrode system of the fission ionization chamber. Fission chambers (FCs) with rectangular electrode plates matching the shape of the VNC collimator channels were developed and manufactured. The article presents the results of tests of three FC modules under irradiation by D–D and D–T neutron generators (NG-14 and NG-24M, respectively). The sensitivities were measured, and changes in the FC count rate were investigated when the detector unit was rotated around its axes. Neutron fluence control at the location of FC placement was provided by 115In and 27Al activation samples. On the basis of the results, it was concluded that the use of the new rectangular electrodes for the ITER VNC is fully justified.