The very large detector

PE Photoelectric absorption CS Compton scattering PP Pair production 

The detail of the manner in which gamma-rays interact with matter determines the size of the detector signal for 

Since for each identical gamma-ray we now expect the same detector response, irrespective of the initial mode of 

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Germanium detectors are characterized by three parameters resolution, peak-to-Compton ratio, and efficiency. The resolution is typically given for the 1332-keV Co line and varies from 1.8 keV for the very best to 2.3 keV for the very large detectors. The peak-to-Compton ratio is measured as the ratio of the number of counts in the 1332-keV peak to the number of counts in a region of the Compton continuum. Values vary from 30 to 90 for the most expensive model. The efficiency is expressed as a relative efficiency compared with the 7.5x7.5-cm Nal(Tl) scintillation detector. Relative efficiencies of HP-Ge detectors vary from 10% up to 150%. The dead time of semiconductor detectors is low, so the count rate is limited largely by the electronic circuit. 

If we go to the opposite size extreme and consider the same interactions in a very small detector - defined as one so small that only one interaction can take place within it -a different picture emerges (Figure 2.9). While the very large detector referred to above is entirely hypothetical, the very small detector now being discussed is not too different from the small planar detectors manufactured for the measurement of low-energy gamma and X-radiation and the necessarily small room-temperature semiconductor detectors that will be discussed in Chapter 3 Section 3.2.5. Again, we can consider various interaction histories for the three modes of interaction. 

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