A common problem many of our customers experience is that of excessive background count when monitoring alpha radiation with scintillation detectors. This can be caused by light leaks or contamination of the detector.
Frequently, the problem is caused by light leaking into the detector, which the photomultiplier tube changes into electrical pulses that are counted by the monitoring instrument. Consequently, if the mylar face on the detector is torn or punctured, ambient light may produce an excessive background reading. If the tear or puncture is sizable, a low or even no activity readout may result, which is a result of the instrument's inability to resolve the light pulses. To check the detector for light leaks, simply shade the detector face from the light with an opaque material while monitoring the meter. (Start at one end of the detector and slowly move the shade to the other end while watching the meter face.) When the ambient light is blocked from the detector, a sudden change in the monitoring instrument readout should occur. For small increases in background count, the detector may need to be enclosed in a light-tight bag for a period of time before the cause can be determined. Although torn mylar is usually the cause of light leaks, unwanted light can enter by other means.
Contamination of the detector face itself is another common problem that frequently arises. Even small amounts of alpha contamination may generate an apparent increase in background count. Many people use a thin window GM detector when checking their detector for alpha contamination. The problem with this method of testing originates because the thin window GM detector may not detector small amounts of alpha contamination due to the relatively higher gamma background count. The best way to check for alpha contamination is to use another alpha detector or perform wipe tests to pinpoint the contamination. If you have questions, please contact our Repair Department.