Radiation is observed via the changes in matter interacting with radiation (Chapter 5). During the detection of radiation, ionization is the most important process. The ionization of matter causes changes in the electric properties. Some of the main detector types (gas-filled and semiconductor detectors) utilize the ioni­zation triggered by radiation. Scintillation of certain substances on the effect of radiation is another characteristic that is frequently used to detect radiation (scintil­lation detectors).

The chemical changes initiated by ionization provide another possibility to detect and measure radiation (autoradiography, chemical dosimeters, etc.). The thermal effects of the radiation, as well as nuclear reactions (e. g., detection of neu­trons), also can be used to detect radiation.

The instruments of radiation detection usually consist of two main parts: the detector and the signal-processing unit. In this textbook, detectors are discussed in detail; the signal-processing units are mentioned as needed in order to understand the operation of the detector and the mechanism of signal formation.

When we talk about detection and measurement of radiation, it includes deter­mining the type, the energy, or the energy distribution of the emitted particle or electromagnetic radiation, as well as the different decay rates (activity or intensity). The type of the particle or electromagnetic radiation and the energy gives informa­tion on the radioactive isotope that emitted the radiation (qualitative analysis). The activity or intensity (see Section 4.1.2) gives the number of radioactive nuclides (quantitative analysis). The detectors are characterized based on the following properties:

• The type (or the energy) of the particles, which can be detected by the detector.

• Dead time: the time needed to detect a novel particle after detecting the previous one. Shorter dead time is better. When the dead time is long, the measured activities or intensities have to be corrected; the correction, however, is the source of uncertainty. The dead time determines the maximum activity or intensity, which can be measured by a detector.

• The signal-to-noise ratio should be optimal. The increase of the sensitivity usually increases the noise too.

• The amplitude of the signals in the detector may be proportional to the energy of the particles or electromagnetic radiation or not. This factor determines whether the

Nuclear and Radiochemistry. DOI: http://dx. doi. org/10.1016/B978-0-12-391430-9.00014-7

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energy of the particles can be measured, i. e., qualitative analysis can be done or not. The amplitude of the signal is proportional to the energy only in the case of some detector types. Other types of detectors produce signals independent of the energy of the radiation. This, however, can be advantageous for situations such as dose measurement.

• The signals should be easily treatable by the signal-processing unit. In the case of the most important detector types (gas-filled tubes, scintillation counters, and semiconductor detectors), this is usually not a problem because in all these detectors, electric impulses are formed under the effect of the radiation, which can be amplified and discriminated easily. The signals with different amplitudes belong to well-determined energies and can be discriminated by using the so-called channels. The number of channels defines how many different groups can be divided from the signals with different amplitudes. When all signals are equal or the total activity has to be measured (no qualitative analysis is needed), the signals are not discriminated and a one-channel analyzer is applied. In the case of the particles or electromagnetic radiations with energy that is not uniform, the detector produces signals with different amplitudes, where the number of channels can be up to several thousand. The increase in the channel numbers obviously makes the instruments more complicated and expensive. Thus, the detectors and the signal­processing units must be constructed in accordance.

• When the detector is able to produce signals with different amplitudes, the resolution is an important property of the detector. The resolution indicates the distance of two signals (energy of radiation), which can be separated from each other. Of course, the high resolu­tion of a detector is useful only when the number of the channels in the signal-processing unit is high enough.

• The resolution is significantly affected by the half-width of the signals (Figure 14.1). The half-width is composed from the natural width of the spectrum lines (Eq. (5.98)) and the interactions of radiation with the matter in the detector.

• The efficiency of the detector indicates the ratio of the particles or photons that are detected.

Since detectors score differently in each of the above categories, the detector has to be chosen carefully and adapted to the given task. In the next sections, the most important types of radiation detectors will be discussed.