A-2 APPENDIX A Biodex Medical Systems, Inc. © 2014
A. Geometric Efficiency (G.E.)
The value for Geometric Efficiency (G.E.) is calculated from the following equations for a well
and probe:
WELL:
I.D. = Inner diameter of the well opening
X = Depth into the well, measured from the top surface
Example: The Biodex Nal well dimensions have an I.D. = 0.75 inches and a well depth of 1.432
inches. When the wipe (or calibration source) is placed in the well, the isotope emitting
radiation will not be at the very bottom. If we take the isotope position to be 1 inch from the
top, then G.E. = 96.82%.
(For X = 1.25 inches deep, G.E. = 97.89%
For X = 0.75 inches deep, G.E. = 94.72%, which are +1.07% and -2.1% respectively, from 1 inch.)
PROBE:
O.D. = Outer diameter of the probe
X = Distance from the probe
Example: The Biodex Nal probe O.D. = 2.00 inches. When the wipe (or calibration source) is
placed .5 inch from the probe, then G.E. = 27.64%.
For X = 0.25, G.E. = 37.9%
For X= 0.75, G.E. = 20.0%
From these two examples, you can see that the well is at least 4 to 5 times more efficient for
wipe test counting than the probe and is less dependent upon wipe positioning errors.
The Atomlab 960 is shipped with the Isotope Efficiency screen set to centimeters (see Figure
4.11 and Figure 4.12). The system can be set to centimeters, inches or any other unit of
measure, as long as it is consistent for the diameter and distance. Please verify that the set-up
in the Isotope Efficiency page is set to match the 5.08 cm diameter tube supplied with this unit.
B. Detector Efficiency (D.E.)
D.E. can be calculated theoretically from system parameters or can be empirically determined
by measuring the count rate from a known activity of each isotope. The empirical method
results in a composite value (G.E. * D.E.) which can be reduced to D.E. by dividing by the
calculated value for G.E. Only the empirical method will be discussed below. The theoretical
(analytic) method is discussed (Item E) following an example calculation.
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