Geometric efficiency

The sensitivity of a PET scanner is defined as the number of counts per unit time detected by the device for each unit of activity present in a source. It is normally expressed in counts per second per microcurie (or megabecquerel) (cps/pCi or cps/kBq). Sensitivity depends on the geometric efficiency, detection efficiency, PHA window settings, and the dead time of the system. The detection efficiency of a detector depends on the scintillation decay time, density, atomic number, and thickness of the detector material that have been discussed in Chap. 2. Also, the effect of PHA window setting on detection efficiency has been discussed in Chap. 2. The effect of the dead time on detection efficiency has been described in Chap. 3. In the section below, only the effects of geometric efficiency and other related factors will be discussed. 

If a detector with a circular window of radius 5 is at a distance r from a source of radius d, the geometrical efficiency is given by... 

The activity of the smear sample may be measured either with a portable contamination monitor or in a standard counting castle. Care is necessary in converting the count rate to surface activity as a number of factors such as counting efficiency, geometrical efficiency, counter calibration and the fraction of contamination removed from the surface to the smear sample will affect the final result. 

In many cases, the fiill detector efficiency can be factorized into intrinsic and geometric efficiencies. The intrinsic efficiency is the ratio of the number of recorded pulsestothe number of gamma rays incident on the detector surface, which depends on the interactions between the gamma photons and the material of the detector, as discussed above, while the geometric efficiency equals the fraction of the gamma rays reaching the detector relative to the emitted ones. 

The detection efficiency for unattached fractions is obtained from the product of the geometric efficiency (11.6%), the collection efficiency on the screen (86.1%) with a face velocity 37.1 cms (Thomas and Hinchliffe, 1972 Cheng et al., 1980) and the counting efficiency for unattached fractions on the up-stream side of the screen (80.1 %). The 50% cutoff diameter... 

Fig. 36 Decay chains attributed to Cn observed in the experiments to study the chemical property of Cn (from [136]). Indicated is the deposition temperature at which the decay of the Cn-atoms was observed. The first four decay chains were observed on the Au surface, whereas the last one occurred in the region where the detectors were covered by a thin layer of ice. t is the lifetime, i.e., the measured time difference between the a decay of the mother nucleus and the SF event. n.d. not detected due to incomplete geometrical efficiency of the detector array...

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