A-10 APPENDIX A Biodex Medical Systems, Inc. © 2014
E. Analytically Determined (Theoretical) Detector Efficiencies
The following table of 25 isotopes contains analytically determined efficiency values for use
with this instrument. Stated with the efficiency value is lower and upper region of interest for
which the efficiency is valid. In the wipe test program, these ROI’s are fixed and cannot be
adjusted. Therefore, these efficiency values may be used in the wipe test program of this
instrument instead of empirically determining the efficiency for each isotope.
NOTE: It is recommended that you empirically determine the detector efficiency of your system,
the analytically determined values are approximations.
The detector efficiency “D.E.” is a composite number which allows conversion from detector
counts to disintegrations for a given isotope. Three factors affect it:
1. The photon intensity in the isotope decay scheme defines the number of photons which are
emitted per 100 disintegrations of the isotope. This number can be less than or greater than
100% as exhibited by Cr-51 (~10%) and Co-60 (~200%). The photon energy and percent
abundance in the decay scheme can be found in “Table of Radioactive Isotopes” by Edgardo
Browne and Richard B. Firestone, pub John Wiley & Sons, 1986, ISBN 0-471-84909-X.
2. The photon interaction in the detector will produce counts which integrate to a fraction of
the total number of photons passing through the detector. This will always be less than
100% and will depend upon the window thickness which the photon detector must pass
through, detector crystal geometry and the photon energy. This photon interaction has been
calculated using “Nal (Tl) SCINTILLATION DETECTORS”, published by Bicron, manufacturer of
the probe and well detectors. This publication contains two sets of curves which were used
in the calculation of detector efficiency: Figure 14 “Absorption Efficiency of Nal (Tl)” for
various thicknesses of Nal, and Figure 17 “X-Ray and Gamma Transmission Through Bicron
Detector Windows” for various window thicknesses.
3. The ROI setting in the MCA determines the fraction of the MCA counts which are
accumulated. Normally the ROI is adjusted around the photo peak, however, there can be
several photon energies which are not included in this ROI because they may have a low
emission intensity or their energies may cover a range too broad to be practical for
background subtraction. The photo peak contribution to the photon calculation in the Nal
crystal was calculated using “GAMMA-RAY ABSORPTION COEFFICIENTS FOR ELEMENTS 1
THROUGH 100 DERIVED FROM THE THEORETICAL VALUES OF THE NATIONAL BUREAU OF
STANDARDS”, published by Los Alamos Scientific Laboratory of the University of California,
Los Alamos, New Mexico, Pub # LA-2237.
The Detector Efficiency (D.E.) has been calculated for both probe and well detectors. There is a
difference because the photon path length through the Nal detector is different in the two
detector configurations. There are some isotopes that the well efficiency has not been
analytically determined due to gamma ray summing in the well. You must use the empirical
method for these isotopes. You may have to create a custom version of the isotope with a
different ROI to properly count the isotope in the well.
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