Many of our customers inquire about using scintillation and GM detectors interchangeably with the same instrument. To make this possible, a few extra steps must be taken because of the difference in operating voltages of these two types of detectors.
Most scintillation detectors contain a 1.13 in. or 1.5 in. photomultiplier tube (PMT) that is usually operated between 600 to 1000 volts, while most GM detectors operate at approximately 900 volts. Therefore, the scintillation detector must be changed to operate at 900 volts. Usually, the simplest way to change the high voltage plateau on a scintillation detector is to change the existing end cap on the detector to an adjustable gain cap. The adjustable gain cap allows the customer to "shift" or adjust the operating voltage of the scintillator by simply adjusting the recessed control in the end of the cap. This control will affect the voltage operating point over a range of approximately 150 volts.
Another alternative is to remove the original detector end cap and place a resistor between the tube socket and the end cap. The resistor in series with the photomultiplier tube and the connected instrument effectively forms a voltage divider network consisting of the resistor and the total input impedance of the instrument. Increasing the resistance will decrease the pulse amplitude from the photomultpilier tube requiring an increase in operating voltage to obtain the same detector operating point before the resistor was added. Resistor values will vary for each detector and instrument combination. Typical values for a 50 to 150 volt increase is 200k to 1M ohms, respectively. Discretion should be used in placement of the resistor due to the high operating voltage (use heat-shrink over the resistor).
Please note for both of the above procedures that the detector operating voltage can only be increased and never lowered below the initial operating point. A detector which originally operates at 1000 volts cannot be lowered to an operating point of 900 volts. (Exception: A resistor has previously been added as described above, and therefore can be changed.)
Both procedures will vary the operating point approximately 150 to 200 volts above the initial detector operating voltage. A detector operating at 600 volts may not conceivably be shifted to a 900 volt operating point.
Please contact us if you have any questions.