Choice of detectors

Choosing a detector for the application depends on the desired information needed and the operating conditions how the sensor should be used. We will discuss two types of detector systems: semi-conductor detectors and scintillation crystals. The principle of operation of both detector types is similar in that sense that in both systems:

1. The energy of the photons is transferred to electrons by photo-electric absorption or Compton scattering.
2. The number of electrons is measured.

As described in the previous sections, the amount of absorbed 𝛾-rays (process 1)) depends highly on the density and of the volume of the material. The type of interaction (photo-electric effect or Compton scattering) that causes the absorption is a function of the elemental composition of the material (parameterised in Z^eff) and energy of the photons. A higher density of the detector material results generally in a higher amount of absorbed 𝛾-rays: the response is high. The same holds for the volume of the detector: the larger the volume, the higher the probability that a photon will interact with the detector. Thus, increasing the density and volume of a detector system increases the response of the detector. For a determination of the energy of a photon, the photons should interact with the detector via the photo-electric effect (). Therefore, increasing the Z of the detector material will increase the sensitivity of the detector system.

In process 2), the measurement of the kinetic energy, the difference in detector type is of importance. In a scintillation crystal, the electrons that are generated in the processes of photo-electric absorption or Compton scattering will generate excited molecular states in the scintillation crystal. The photons that are emitted in the de-excitation of these molecular states are measured with a photo-multiplier tube (PMT). The chain of events that must take place in converting the incident photons to an electrical signal from the PMT, involves many inefficient steps. Therefore, the energy to produce one information carrier (light pulse) is large and the number of generated light pulses is relatively small. The statistical fluctuation in such a small number of information carriers places an inherent limitation on the resolution of a scintillation detector system and photo peaks will be rather broad (). The only way to increase the statistical limit on the energy resolution after detection, and to decrease the width of the photo-peak described in , is to increase the number of information carriers. In semi-conductor detector systems, the binding energies of electrons are small. Therefore, a semi-conductor detector allows the photo-electric interactions to be measured with a high resolution.

A 𝛾-ray detector cannot detect all photons that are emitted from the sediment. The efficiency of the detector system depends on the geometry of the detector and the detector response. For sea-borne and airborne detectors, only half of the detector “faces” the radiation coming from the sediment and the “effective surface” of the detector will be more or less half the actual detector surface. In a laboratory set-up, other geometries can be used to increase the efficiency of a detector system.  When a Marinelli beaker with sediment is placed over the detector, the detector is almost completely covered with sediment and the efficiency is increased compared to field mapping conditions.

In general, scintillation counters have a higher Z value and density than semi-conductor detectors and can be produced with larger volumes. Therefore, scintillation crystals have a higher response, which can reduce measuring time. However, when a high resolution is required (e.g. for spectroscopy), semi-conductors are most suitable. For laboratory measurements where the focus is on determination of specific 𝛾-rays, the increased resolution outweighs the large measuring time of a High Purity Germanium (HPGe) semi-conductor detector. In measurements on the sea floor, measuring time is an important parameter determining the spatial resolution of the mapping. The high-efficiency combined with the well-known composition of the radionuclide suite and the absence of the need to cool the detector (a semi-conductor detector must be cooled with liquid nitrogen), a scintillation crystal is preferred for field measurements. 

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