Stabilizing scintillation detector systems with pulsed LEDs: a method to derive the LED temperature from pulse height spectra

Photomultiplier tubes (PMTs) can be stabilized with light emitting diodes (LEDs) used as reference light sources. The LED is supplied with short voltage pulses that produce well-defined light portions if the LED temperature is kept constant. However, if the PMT must be operated in a wide temperature range, the LED light output is no longer a constant but becomes a function of the junction temperature. This problem can be solved at low expense by means of a new method. The LED is operated in two alternating pulse modes distinguished by the pulse voltage. The LED light output depends on the pulse voltage and, therefore, the PMT pulse height spectrum shows two distinct peaks. However, not only is the total amount of light L emitted per pulse but also the shape of the temperature dependence L(T) varies with the pulse voltage. The numerical ratio R of the peak positions corresponding to the two different LED modes is therefore a function of the LED temperature as well. Since it is a ratio, R is independent of the actual PMT gain but characterizes the LED junction temperature. Thus, the pulse height ratio of the different LED signals can serve as an LED thermometer. Moreover, measuring L and R at several temperature points covering the full operational range yields a calibration function L(R). With this knowledge, commercial off-the-shelf LED components can be used as precise reference light sources in a wide range of ambient temperatures