On the Feasibility of Calibrating the Fluctuation Channel for Neutron Diagnostics of Plasma

A relatively simple option of calibrating a fluctuation measurement channel by processing a series of pulses from ionization fission chambers obtained using a low-intensity neutron source is substantiated. The fluctuation channel for measuring the pulse counting rate is an integral part of the ITER Divertor Neutron Flux Monitor, a neutron diagnostics measurement system. The fluctuation measurement method is widely used in control systems for nuclear reactors and has also been repeatedly applied in measuring neutron fluxes at fusion installations with a large neutron yield, TFTR, JET, and JT-60U. The method is based on measuring the dispersion of the signal generated at the output of the signal amplifier of ionization fission chambers. The theoretical basis of the method is Campbell’s second theorem. Regarding the application of the theorem to ionization fission chambers, the signal dispersion is proportional to the repetition rate (counting rate) of the neutron detector pulses, which, in turn, is proportional to the neutron flux. The method is also referred as MSV, which is an abbreviation for mean square value. Calibration of the fluctuation channel consists in determining the proportionality coefficient, which relates the actually measured signal dispersion of the ionization fission chamber and the desired value of the pulse counting rate. The proportionality coefficient depends both on the characteristics of the detector (amplitude distribution of current pulses, collection time, electrical capacitance) and on the parameters of the fluctuation channel measurement system (bandwidth, quality of cable communication lines, amplifiers, filters, etc.). Under the conditions of a real physical experiment, calibration requires an analytical and experimental study of two components of neutron detector signals: the distribution of squared pulse amplitudes to determine the mean square of the amplitude and the type of function describing the “averaged” shape of the signals. The correctness of calculations and estimates is verified using the output values of the reference measurement channel (usually this is a channel for measuring the pulse counting rate). To implement calibration, measurements using intense sources of neutron radiation should be carried out. The feasibility of calibrating a fluctuation measurement channel by recording and processing a series of fission chamber signals from a low-intensity neutron source is substantiated. Analytical expressions are derived for calculating the calibration coefficient of the fluctuation measurement channel. An experimental verification of the method was carried out in measurements of the neutron flux from a neutron generator with an output of 10 11 neutron/s. The possibility of calibrating the fluctuation measurement channel of neutron plasma diagnostics at the ITER site using low-intensity neutron sources is discussed.