Absolute efficiency: it is the ratio of the number of
counts recorded by the detector to the number of
gamma rays emitted by the source (in all directions).
(ii) Intrinsic efficiency: it is the ratio of the number of
pulses recorded by the detector to the number of
gamma rays hitting the detector.
(iii) Full-energy peak (or photopeak) efficiency: it is the
efficiency for producing full-energy peak pulses only,
rather than a pulse of any size, for the gamma ray.