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Photon Detector Categories Relationship of Relative Efficiency to Active Volume As the volume of a coaxial detector increases, so does its relative efficiency (measured at 1.33 MeV). However, there is not a simple relationship between volume and relative efficiency. The efficiency increases faster with detector radius than with detector length. An approximate (not dimensionally correct) relationship is:
Since the density of germanium is 5.33 g/cc, ~23 g of Ge in the finished detector is required for each "percent" of efficiency. A more recent empirical formula relating volume to efficiency is the following (courtesy of Dr. T.L. Khoo of Argonne National Lab):
where D = active crystal diameter, L = crystal length, K = 2.4321, a = 2.8155, and b = 0.7785. (Diameter and length in decimeters.) This formula illustrates how detectors of the same % relative efficiency (IEEE 325) can have very different dimensions. |
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