Proper tables must be used for interconnecting nuclear instrumentation to reduce noise and to transmit the best possible signal to the readout equipment. See Chap. 10 for
additional information on grounding, shielding, and selec­tion of cables

Noise-free cables must be used in the start-up instru­mentation Noise pulses will be amplified and counted as neutron pulses. Besides the inaccuracies in counting, noise bursts can cause period and level scrams at low reactor power levels

When a vacuum-tube preamplifier is used, the sensor and preamplifier are usually connected together as a unit or with a few feet of coaxial cable The cable used must have an impedance that matches the preamplifier input. The signal cable used between the vacuum-tube amplifier and the LCRM should be coaxial and match the impedances of the two units If the cable is routed through high-noise areas, a tnaxial cable should be used with the outer braid connected to ground.

Power-supply cables between the power supply and vacuum-tube preamplifier should be shielded conductors to minimize the noise level The power-supply ripple voltage should not exceed 10 mV for satisfactory operation of the sensor and attached preamplifier.

The greater the distance between the detector and the preamplifier, the more important is the cable quality. For

LOCAL

METER

OUTPUT

RECORDER

OUTPUT

COMM.

REMOTE

METER

-15V +15V

ТС TRIM

HIGH TRIP

Fig. 5.12—Solid-state alarm (trip) circuit.

solid-state preamplifiers the importance of the cable cannot be overemphasized. There is one positive procedure for noise reduction complete the installation, and, using one or more of the methods outlined in Chap. 10, experiment until the noise has been reduced to a minimum. Neutron sensors for use at high temperatures are under development Cables and connectors for use with these sensors must operate at high temperature without producing noise pulses Radiation-resistant cables should be used to increase the time between replacement, to reduce maintenance costs, and to improve availability. Cables and connectors are available for high-temperature use, but extreme care must be exercised in their use Avoid use of high-tempera­ture components if at all possible, they are subject to changes in resistance which adversely affect both signals and high-voltage cables

A triaxial cable should be used to minimize the noise introduced between the sensor and solid-state preamplifier The outer braid of the triaxial cable should be tied at both ends to the inner braid. The impedance of the cable must match the input impedance of the preamplifier to reduce pulse reflections on the cable.