Electronics for radiation detectors

Radiation monitoring is essential for the control of nuclear reactors and reprocessing plants. In power reactors, nuclear fission is controlled by measuring the neutron flux emitted by the boiler, and in reprocessing plants, the quantities of nuclear materials are controlled throughout the process by measuring the neutrons or gamma photons emitted. These measurements are either flux or energy measurements. In experimental systems associated with particle gas pedals, it is also necessary to determine the trajectories of the particles emitted. All these measurements are carried out by very different experimental set-ups. What they have in common, however, is the use of the information contained in the series of current pulses delivered by the detector. This article focuses solely on the instrumentation of nuclear reactors and reprocessing plants, and on installations for monitoring sites and people.

Two classes of measurement can be identified: the first is the set of methods for estimating the number of pulses over a given time or measuring a current, since a current measurement is in fact the counting of a large number of elementary pulses; the second is the set of techniques for measuring a particular characteristic of each pulse. The charge of the current pulse is a relatively frequent example of such a measurement, since it is proportional to the energy deposited in the detector. Energy measurements are widely used in reactor and process plant control, and will be the only measurements of this second class to be studied in this article. They are also known as spectrometric measurements.

Nuclear instrumentation has necessitated the development of a large number of specific detectors. It is legitimate to wonder about the specific nature of the electronics, processing methods and computer architectures encountered in the nuclear instrumentation of reactors and plants. Many electronic functions and a number of processing methods are borrowed from general electronics. Examples include amplification and filtering functions, counters, analog-to-digital converters, samplers, processors and so on. However, nuclear instrumentation has two specific features which have led to the development of specific methods and products: the need to ensure a good signal-to-noise ratio for random pulse flows, and the importance attached to operating safety.

Measurement chains are particularly sensitive to the signal-to-noise ratio, which is a decisive factor in ensuring high measurement quality. Detectors deliver low signal values and pulse charges typically in the picocoulomb range. The measurement accuracies required in energy measurements are high: for gamma photons, for example, it is necessary to cool the detector and front-end electronics to reduce noise...