A Neutron Counting System with a Fission Detector and a Flexible, Balanced Line

Abstract

A neutron counting system containing a fission detector and a pulse transformer driving a balanced transmission cable was investigated to determine its usefulness as an instrument for reactor control. Because of the noise rejection capability of the balanced system input, all active elements can be located away from the hostile environment of the reactor, resulting in longer periods of trouble-free operation. The design criteria of Fowler were used as a guide. The primary objective was to measure the response of the system to electrical interference characteristic of the Oak Ridge Research Reactor (ORR). The results showed no noise pulses in excess of the alpha threshold after six days of operation at the ORR. A detector without 235U was used to make this measurement. The input cable, with a characteristic impedance of 250 ohms, was a 40-ft long double-shielded twisted pair. Its outer jacket and inner dielectric were made of irradiated polyolefin compounds. The cable was enclosed in a flexible, stainless steel conduit for underwater operation. The quality of the shield of this cable and four other types of balanced cables was determined by a measurement of their transfer impedance. To determine the effect of the flexible conduit, a separate measurement was made with the cable-conduit combination. The techniques required to make these measurements are critical and are described. The signal from the detector-cable combination requires a preamplifier with a lownoise input stage. A series-shunt feedback stage using two 2N2857 transistors gave an input noise of approximately 2.