Many of us have experienced the problem with punctures or "bad" seals (commonly referred to as "light leaks") in the thin mylar face of an alpha or beta scintillator. The problem symptom occurs with a detector that has been functioning properly and then suddenly will not respond to normal background count or a check source. The counting instrument appears to have "died", so it usually becomes the first suspect because of no response on the meter except maybe in the BAT position.
Commonly it is "checked out" by substituting the questionable detector with a known "good" detector or by connecting a pulse generator and confirming count and HV operation. The detector has now been isolated as the problem and is closely inspected and found to have a small hole in the mylar face. Why doesn't the instrument respond to this light? After all, it is light that the photomultiplier (PMT) sees from the scintillation medium. The problem is the "light lead" which saturates the PMT. The overall gain of the PMT is decreased, thus reducing the incoming light pulses far below the threshold of the pulse counting instrument. This phenomenon will cause the instrument to appear "dead".
Another occurring problem with scintillation and GM detectors is when the radioactive field intensity is increased until a decrease in count rate is observed. If field intensity is increased too much, once again no response or a "dead" instrument may be observed. The overall PMT gain has again decreased from both GM and scintillation type detectors below the threshold of the pulse counting instrument due to count saturation.
The "dead" response of the counting instrument from the above phenomenons can be overcome by using another condition that occurs with the saturated detector. Even though the gain decreases from the detector, the direct current (DC) drain through the detector increases as it is saturated. The DC increase can be monitored within the instrument and used to activate a circuit to drive the analog meter to full end scale, illuminate an LED, etc. These circuits are referred to as detector overload, saturation, detector paralysis, or detector overrange with the first being the most commonly used terminology. This detector overload circuit (DOC) is usually provided with an adjustable control for activating the circuit at a finite point. Example: In the detector light leak condition, the DOC would be activated to drive the ratemeter to full end scale thus indicating the problem. The DOC activating point would be adjusted typically for 2.5 times full scale reading from the detector instead of at the "light leak" condition. This would insure that the meter would not decrease due to count saturation. It would also insure that even a very non-visible light leak would not lower the meter reading.
For GM type detectors, the adjustment of the activating point should be made in between the full scale reading of the counting instrument and the point where the meter drive circuit no longer increases when the field intensity is increased. Care must be used in substituting different types of detectors with the same DOC. A scintillation detector is typically adjusted to trigger the DOC setting from 0.5 to 1 µA while a GM pancake detector typically draws 2 to 3 µA at 10 mRad/hr to activate the overload circuit.
Caution must also be taken in the type of load connected to the instrument's overload circuitry. Example: the Ludlum Model 500 pulser has a DC impedance of approximately 2 GΩ (2 x 109 Ω). If a DOC is set to activate at 0.5 µA at 1000 V, the pulser DC impedance would activate its circuit (1000 divided by 2 x 109 = 0.5 µA). In this example, the circuit would be adjusted or disabled during calibration with the pulser, then adjusted again with the detector.
If you have questions about these procedures, please contact us.