Answer to Question #12194 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
The reference light source reading from our Harshaw 5500 thermoluminescent dosimeter (TLD) reader has been increasing over the years—from approximately 400 nanocoulombs (nC) in 2009 to about 530 nC now (September 2017). The photomultiplier tube (PMT) noise is fairly steady at approximately 0.08 nC. Is this a usual characteristic of the PMT/reference light, or is it an indication that the PMT performance is deteriorating?
The greater than 30% change in the reference light source reading that you cite is significant and does deserve investigation. The first question that comes to mind is whether you have done any investigation of this at an earlier time. One significant consideration would be whether the reader calibration factor has also shown a commensurate change during the time that the light source reading has changed. If only the light source reading is changing, I would suspect a change in the light source itself or a change in its operating environment that could affect it.
The Model 5500, to my understanding, uses a light-emitting diode (LED) as the reference light source. With usage over time, an LED operating in a stable environment will expectedly degrade, normally yielding a reduced light output with increasing age. The fact that your readings have increased leads me to believe something else is going on.
One characteristic of LEDs is that the light output generally decreases with increasing environmental temperature and increases with decreasing temperature. The Model 5500 does incorporate a temperature stabilization system for the LED reference light. I do not know the details of how the temperature stabilizing system works, but it is possible that the system has been malfunctioning by producing lower temperatures over time.
You should also confirm that the power supply to the LED has not changed; i.e., that both the applied voltage and current are within acceptable limits. The brightness of LEDs is determined by the current flowing through them; the current can increase with increased voltage. The LEDs usually incorporate a current-limiting resistance but degradation over time could affect both current and voltage.
It is also possible that some other degradation in the system outside of the light source is resulting in a decrease in temperature in the operating environment. Light leaks, resulting from possible mechanical damage, can have obvious effects on measured light, but these are more often sudden consequences associated with the event(s). The bialkali photocathodes typically used in the PMTs are quite stable and are thermoelectrically cooled to reduce noise. Changes in photocathode sensitivity through aging or any changes in the PMT response from faulty temperature control or electronic problems would also be reflected in the dosimetric response, as would be evident from the reader calibration factor.
You may be able to isolate the cause of the increased LED reading, but you may well require the assistance of the manufacturer to resolve the problem.
George Chabot, PhD