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How to Choose the Right
Photon Detector
"Well" Detectors
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Figure 14. GWL Series
well
detector cut away. |
The Well detector (GWL Series) has been around for some
years. A Well detector has the highest geometric efficiency
possible, but can only contain small samples and introduces
some other complexities. The geometry is so "close" that
coincident summing complicates calibration. Software can
correct for coincident summing (TCC), but you must remember
this if you are considering a Well detector. Well detectors
are for use in cases where the quantity of sample is very small,
e.g., in some forensics applications.Making the correct choice not only means you get the best
result, but can also save money. Look at the two detector
efficiency plots shown in Fig. 15. comparing an 81% and 181%
efficiency detector.
Because both detectors greatly exceed the diameter of the
source, the sample-detector solid angle is large and there
is no difference in efficiency at medium to low energies. At low
energies, the thickness of the endcap and the dead layer
make the 81% more efficient than the 181%. At medium energies,
because the 81% detector is already quite long, the 181%
detector has little advantage. At high energies, the
larger
detector will start to show real advantage
(>1 MeV). Despite
very similar performance in this geometry and over the
energy range of 50 to 1000 keV, the purchase price of these two
detectors will differ by a factor of two.
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Figure 15. 81% and 181% relative efficiency detectors
compared with point source geometry.
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GEOMETRY "Rules of Thumb"
- The closer the sample "activity center of gravity" is to
the Ge, the higher the absolute efficiency, and the better the MDA
- The higher the absolute efficiency and the more sample
counted, the better the MDA
- If all of the sample can be made into a small diameter
disk on endcap, the only better geometry is a well detector
- For a disk on endcap, the crystal diameter should be at
least 1.2 times the filter diameter for best efficiency
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