17
Efficiency Calibration
The Inspector can be used beyond simple detection of contamination and can also be used to determine the
activity of any beta- or alpha-emitting isotopes present.
Efficiency is the ratio of events detected by the instrument to the number of decays of the particular isotope.
The units are therefore counts per disintegration of c/d. Since efficiencies are generally somewhat less than
one, they are often multiplied by 100 and expressed as a percentage. However, the Observer USB software
requires that values not be entered as percentages. Since efficiency varies based on the energy and abundance
of the particular isotope being measured, it is best to determine efficiency for the specific isotope(s) for which
the activity is to be determined.
The efficiency is also affected by the size of the source and the source to detector distance. To achieve
consistency, efficiencies should be determined using a point source (which means a source with an area
significantly smaller than the detector entrance window) at a fixed distance from the detector face (which
must be specified, but 1 cm will be assumed to be the standard).
Requirements
You will need a PC running Observer USB, Calibration sources, an Inspector Wipe Test Plate (or other constant-
geometry source holder to maintain sources 1 cm from the detector entrance window)
Procedure
(American Units)
1. Attach the Wipe Test Plate to the Inspector. Ensure the Inspector is in Total/Timer mode, displaying
COUNTS.
2. Place the Inspector in the location where the efficiency calibration will be performed and take a count of
at least 10 minutes. Divide the total number counts by the acquisition time (in minutes) to calculate the
background counting rate (RB) in CPM. Record this value.
3. Center the first source to be used into the Wipe Test Holder and take a count of at least 1 minute. Divide
the total number of counts by the counting time (in minutes) to calculate the gross count rate (gcpm or
RG).
4. Calculate the efficiency using the following equation:
Where
Eff is instrument efficiency in counts per disintegration
RG is the gross count rate in counts per minute (cpm)
RB is the background count rate in cpm
A is the activity of the source in disintegrations per minute (dpm)
5. Repeat Steps 3 and 4 for additional radionuclides.
6. If you would like to record efficiency values in the Inspector’s memory for any of the supported
radionuclides (S-35, Sr/Y-90, Cs-137, P-32, C-14, I-131, Co-60, ALPHA), continue with the following steps.
7. Connect the Inspector to the PC running Observer USB using the USB cable. An application window for
the Inspector should open within a few seconds.
8. Display the control panel by clicking View|Cal Panel.
9. Enter the updated efficiencies in the appropriate fields.
10. If you would like the Inspector to display in activity units (i.e., DPM) for a specific source instead of the
count rate (CPM), select it from the Applied Isotope dropdown box. Otherwise, leave as None (cpm/cps).
11. When all relevant values have been updated, click the Update Settings button. After a second or two
the Inspector should beep twice to acknowledge receiving the updated settings.