Valid statistical data on the performance of out-of core neutron and gamma sensors are scarce However it may be inferred from examination of sensor designs and from general knowledge of the field that reliability has been good The most frequently reported difficulties have been from spurious signals attributable to microphomc and electrical effects (These are discussed in Chap 10 ) Since these effects can usually be observed in the debugging period, they can be rectified by modifying the system design

Ionization detectors generally have long lives The life of a neutron detector is, however, limited by consumption of sensitive materials Since the consumption is directly related to the neutron fluence, the loss may be calculated and compensated for by recalibration In early designs, flaking of sensitive material with subsequent deposition in insensitive parts of the chamber volume was occasionally experienced Generally, flaking is not a problem at present As might be expected, gamma detectors have an in­definitely long life since they contain no sensitive materials

In a number of ionization chamber detectors using special gas mixtures, gradual degradation results from radiologically induced changes in gas composition Since this type detector is normally used only for very special purposes, it is expected that the user would be alert to any possible difficulties Certain proportional counters fall in this category and are normally used only at moderate or low radiation levels

Scintillation and solid-state detectors are useful at low radiation levels only and can withstand only a limited total radiation exposure