A mild environment (low temperature and noncorrosive media) poses few problems Sensors are available with cable fittings, and the cable choice can be made easily Over extended periods of time, the nuclear radiation field can interact with the gaseous medium in the vicinity of a sensor and cause corrosion If the medium is air, elimination of moisture during installation and prevention of moisture accumulation after installation is advisable (see Chap 10) In a severe environment, a detector with an integral cable should be used In this case, the electrical parts of the sensor are effectively separated from the medium

2-5.2 Hardware

Radiological and mechanical requirements pose dif­ferent installation problems Frequently, some radiation conditioning must be provided, і e, gamma or neutron shielding at the detector location If the exact conditions are known, permanent shielding or neutron thermahzers can be built Usually, some permanent shielding is installed, and flexibility is provided by adjusting the detector position In this case, all possible locations of the detectors must be considered and shielded as needed

It is often possible to provide some gamma shielding or neutron thermahzation in the detector package A detector assembly is then needed The detector assembly must be positioned as a unit, which requires some provision for storing excess cable in or around the reactor shield This is particularly difficult for a detector with integtal cable For counters a preamplifier must also be mounted as close to the detector as possible Frequently, the excess cable and preamplifier space can be combined

For a detector that is positioned vertically, gravits can usually be relied on A load-bearing cable is then required to support the detector, relieving the electrical connections of any possible stress If positioning is horizontal or at some angle to the vertical, constraints m both directions must be provided This can be done by using a fixed or a temporary rigid member If the position might be disturbed, for example, by moving a second detector in the same general location, a rigid connection or possibly a hne-and pulley arrangement may be needed

2-5.3 Circuits

The more specialized circuits are covered m other chapters, particularly Chap 5, and the effects of reactor safety requirements are covered in Chap 12 Some special points are considered here

Interlocking circuits art customarily provided with each detector These interlocks provide a scram signal if the high voltage or signal connections to a detector are interrupted regardless of the cause of the interruption

Also it is generally desirable to prevent the establish­ment of a ground loop, і e, to prevent ground currents from flowing in the return or ground lead This is done by preventing the exposure of any bare metallic members of the return path Thus, cable connectors and the chamber itself are sometimes insulated The signal circuits are sensitive and of high impedance, so ground loops are a frequent source of trouble (see Chap 10)