Section 7.1.6.1.1. Detection efficiency

The detection efficiency of a detector is determined in the first instance by the fraction of the number of incident photons transmitted by any necessary window or inactive layer, multiplied by the fraction usefully absorbed in the active region of the detector. This product, which is often called the absorption efficiency or the quantum efficiency, should be somewhere between 0.5 and 1.0 since the information loss due to incident photons not absorbed in the active region cannot be retrieved by subsequent signal amplification. The useful efficiency is best described by the so-called detective quantum efficiency (DQE), (Rose, 1946; Jones, 1958). For our purposes, this can be defined as where N is the number of quanta incident upon the detector and σ is the standard deviation of the analogue output signal of amplitude S. For a photon counter with an absorption efficiency q, S = qN, , and = q. An analogue detector with a DQE thus behaves like a perfect counter that only detects a fraction of the incident photons.

Under favourable conditions, the DQE of analogue detectors for X-rays is in excess of 0.5, but varies with counting rate and is lower for detectors with a very large dynamic range, as shown below.

The DQE of CCD- and vidicon-based X-ray detectors has been discussed by Stanton, Phillips, Li & Kalata (1992a).