ST365 Radiation Counter Technical Manual
May 2020 – Revision 2.0
Spectrum Techniques, LLC
www.spectrumtechniques.com
Page 10 of 14
Step 3
SET A PRESET TIME, COUNT, AND RECORD
Once the start voltage is determined, set a preset time of 10 seconds and start counting. At the end of
the 10 second count, record the results.
Step 4
INCREASE THE HIGH VOLTAGE AND REPEAT
Increase the high voltage by 10 or 20 volts, count again, and record the results. Repeat this step until the
upper limit voltage of the ST365 is reached.
Step 5
REDUCE HIGH VOLTAGE
Once finished, reduce the high voltage and power off the ST365.
Step 6
CREATE THE GM PLATEAU CURVE
With the collected data, create an X-Y graph with the X-axis being the high voltage (volts) and the Y-axis
being the counts (counts). Plot the data points and draw the curve. The plotted data should resemble the
graph below.
GEIGER PLATEAU CURVE
Notice the ST365 started counting at a point corresponding to the Geiger Threshold Voltage; from there follows the
Geiger Plateau with little change in the counts detected as the voltage increases. Finally, a point is reached where
the counts rapidly increase until the ST365/GM tube moves into continuous discharge – This is called the break down
region.
A flat, long Geiger Plateau is a desirable characteristic in a counter. In practice, most counters have a slightly sloping
Geiger Plateau due to GM tube defects such as gas filling.
Cou
nt
s (c
oun
ts)
Operating Voltage
High Voltage (volts)
Starting Voltage
Geiger
Threshold
Voltage
GEIGER PLATEAU
Geiger
Breakdown
Voltage
Upper Limit
Voltage for the
ST365
