Instability of photomultiplier tubes and consequences for accuracy of radioactivity measurements.

The gain instability of some types of small photomultiplier tubes has been studied. The variation of gain with time was measured with the aid of a pulse-height-variation meter especially developed for this investigation. It was found that a stepwise change in the count rate led to a time-dependent change in the gain of the tube; the over-all change in gain sometimes amounted to several percent. This change in gain leads to a change in the measured count rate and hence to errors in the measurement of radioactivity. The magnitude of these errors depends on the energy resolution of the detector, which in turn depends on the energy of the radiation. The gain instability of the photomultiplier tube can lead to serious errors in dynamic tracer studies. It is recommended that the stability of each tube should be checked by means of a device like that used in this study.