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Exposure Rate Calibration
An exposure rate calibration correlates the number of counts in a given time to the exposure rate present in
that field. This is in many ways similar to efficiency but we refer to it in this case as gamma sensitivity. Although
it is possible to express this value in several different units, the Inspector uses a formula of CPM/mR/hr (counts
per minute per milliroentgen per hour).
The instrument is placed in gamma radiation fields of different (but known) intensities that span the working
range of the instrument, and the settings are adjusted (if needed) such that all measurements are within ±10%
of the true value. The relevant settings include both the sensitivity and the dead time.
The gamma field is usually generated by a relatively strong Cs-137 source. Exposure rates are calculated to the
center of the detector’s volume.
Requirements
You will need a PC running Observer USB and a Gamma calibration range
Procedure
1. Power on the instrument.
2. Connect the instrument to the PC running Observer USB using the USB cable. An application window for
the attached instrument should open within a few seconds.
3. Display the calibration panel by clicking View|Cal Panel.
4. Record the displayed values for Dead Time and Sensitivity.
5. Begin checking the instrument’s Sensitivity. Place the instrument in a relatively low field, approximately 5
mR/hr, and allow it 30 seconds or so to stabilize.
6. If the indicated response is off by more than 3% at this level, adjust the Sensitivity to a lower value if the
instrument is reading too low, or to a higher value if the instrument is reading too high. After changing
the value on the Calibration Panel, click the Update Settings button. After a second or two the instrument
will beep to indicate that the new value has been received.
7. Repeat Step 5 as needed to bring the instrument indication as close as possible to the true value.
8. Next, expose the instrument to a field near the top of its range of 100 mR/hr. Allow the reading to
stabilize for at least 30 seconds. In the preferences tab, you can adjust the time in the gird to show a
selected average count.
9. If the indicated response is off by more than 10%, adjust the Dead Time to a lower value if the instrument
reading is too low, or to a higher value if the instrument reading is too high. After changing the value on
the Calibration Panel, click the Update Settings button. After a second or two the instrument will beep
twice to indicate that the new value has been received.
10. Repeat step 9 as needed to bring the instrument’s indication within ±10% of the true value.
11. Once the values for Sensitivity and Dead Time have been verified to be correct, expose the instrument
to a series of values spanning the range of the instrument. All indications should be within ±10% of the
true value.
