Detector sensitivity - efficiency
The detector sensitivity or efficiency is a value for the relation of incident photons to measured photons. If differs from detector to detector and with photon energy. There are quite some different definitions for efficiency, they are described in the following.
The Incident intrinsic efficiency is the ratio of all photons which cause a measurable impulse within the detector to all photons incident to the detector. The Source intrinsic efficiency is the ratio of all photons which cause a measurable impulse within the detector to all photons emitted by the radiation source. In difference to the incident intrinsic efficiency the source intrinsic efficiency depends additionally on the measurement setup as it includes also a geometry factor of F/4pr². The Incident full energy peak efficiency is the ratio of all photons which are measured with their full energy to all photons incident to the detector. So in contrast to the intrinsic efficiency only photons which appear in the photo peak are counted. The Source full energy peak efficiency includes here again the geometry factor.
For spectroscopic measurements the full energy peak efficiency is interesting, the intrinsic efficiency is more relevant for measurements without resolving energy (integral counting measurements). In HPGe (High Purity Germanium) detector data sheets often the term relative efficiency is found. This term means the ratio of efficiency at 1333 keV from the described detector to a 3*3" standard NaI detector. For CdZnTe-detectors there is sometimes found an effective detector area at 662 keV. This is a kind of integral yield of the detector. It can be thought that every photon incident in this area is found in the photo peak
Source: Dr. Joerg Brutscher, Identify manual