Radiation Detection

Figure 2.18. Photon energy spectrum measured by a scintillation detector.

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Figure 2.18. Photon energy spectrum measured by a scintillation detector.

The interactions of ionising radiation with matter form the basis upon which radiation detectors are developed. The inherent idea in these detectors is to measure the total energy lost or deposited by radiation upon passage through the detector. Typically, radiation detectors convert the deposited energy into a measurable electrical signal or charge. The integral of this signal is then proportional to the total energy deposited in the detector by the radiation. For mono-energetic incident radiation, there will be fluctuations as well as large variations in the total charge collected by the detector (see energy spectrum in Fig. 2.18). The large variations represent incomplete deposition of energy by the incident radiation. For example, in PET some of the incident 511 keV photons may undergo one or more Compton scatter, deposit a portion of their energy and then exit the detector. Multiple Compton scatter could eventually lead to deposition of almost the entire energy by the photon, thereby pushing the event into the photopeak of the energy spectrum. The continuous portion of the energy spectrum (Fig. 2.18) shows the Compton region for this measured energy spectrum with partial deposition of energy. The small fluctuations in the energy spectrum, however, arise due to several processes. The most dominant are the statistical fluctuations in the conversion process of the deposited energy into measurable charge or signal. In Fig. 2.18, the peak position marks the mean energy of the incident radiation (after complete deposition in the detector). The width of this peak (called the photopeak) shows the effect of fluctuations in the measured charge for complete deposition of energy by the mono-energetic photons. The ability of the radiation detector to accurately measure the deposited energy is of paramount importance for most of its uses. This accuracy is characterized by the width of the photopeak in the energy spectrum, and is referred to as the energy resolution of the detector. The energy resolution is a dimensionless number and is defined as the ratio of the full width at half maximum (FWHM) of the photopeak to its centroid position.

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