The very small detector

If we imagine that this multiple scattering follows an initial event that would have produced a response near to the Compton edge, then we can appreciate that the extra energy absorbed could, in some cases, result in events that would appear in the spectrum between the Compton edge and the full energy peak. These are referred to as multiple Compton events. 

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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. 

This circuit converts the very small detector currents to measurable voltages, and holds the detector bias (nearly) constant. An increase in photon flux will make the current more negative, increasing the output the voltage at point A. If the irradiance is modulated, an AC signal will result at point A in the circuit that voltage may be filtered, amplified, and recorded. 

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