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.
X
APPENDIX A A-6
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