I use HPGe detector with digital spectrometer for gamma spectrometry. The resolution of a fix energy changes relatively wide range measurment to measurment (the count rate is permanent).
As I noticed it changes depending on humidity in laboratory room, but I can't explain why it happens :)
The problem is electronic. The first places to look are the cables and connectors. Next is the preamp. Do you have any diagnostic equipment? A pulser and oscilloscope would be useful.
Humidity is a clear indication of an electrostatic problem. Check for proper grounds.
If the power supply to electronics is switched off, this may lead to instability in the electronics used to record the spectrum..resulting in some shifts in the readings.
Another possibility, when the voltage of the detector has been changed, the detector should be left to stabilise for a period of time to avoid such problem. On the other hand, you need to make sure that the detector is operating at the recommended voltage..
Thanks.
The resolution of a HPGe is sensitive to a number of things, particularly electrical noise along with mechanical vibration. Here is an incomplete list of things which can change the resolution
1) Count-rate. The questioner indicated that the count rate is constant but did not indicate if it was a high or low constant. High count-rate causes pileup which can change the resolution.
2) Electrical noise. Sensitive HPGe measurements require clean power. Usually the culprit is grounding problems. A good isolation transformer can be a solution.
3) Microphonics. An HPGe is extremely sensitive to mechanical vibration. If you are performing measurements outside a lab, then this may cause resolution changes.
4) Detector warming. While modern HPGe's use stirling coolers, these operate at fairly high temperature (around 100 K) compared to LN2 and have troubles in hot environments. If you are using LN2, then a hot environment can cause warm-up faster than you realize.
5) My favorite: neutron damage. Neutron damage is reversible only through annealing and so if this is the case, the resolution will continue get worse. If you do not keep your detector cold, the damage will increase through repeated warm-up/cool-down cycles. Unlike electrical noise and microphonics which will symmetrically broaden peaks, neutron damage produces an asymmetric gamma ray line shape with a low energy tail on the Gaussian.
6) Poor detector vacuum. For older HPGes, the o-ring seals can allow humidity to get on the crystal. Like neutron damage, this is an irreversible effect and can only be repaired through annealing or a "pump and bake". This can be detected on systems with transistor reset by the rate of resets per second. On detectors with a large resistor for leakage current, it is more difficult to detect. Again if you do not constantly cool your detector, the damage can increase with time.
My experience has taught me that most of the problems in a standard counting lab are due to external factors such as electrical noise. These are hard problems to solve but like I said earlier, a good isolation transformer can be your friend.
If you are doing measurements around a large nuclear facility like a reactor or in an outdoor environment, your problem is much tougher to solve. You may have to learn to "live" with the bad resolution. There are spectral techniques to handle this if necessary.
Finally I realize that this question is 2 years old but I would not suspect the pre-amp. If it was the preamp, use a test pulser on the test input of the detector. You should be able to very diagnose the problem.
I have two low background HPGe systems. I am noticing, very often, noise in the spectrum in the low energy side of the spectrum. The systems are powered by a good quality uninterrupted power supply system (UPS) with an isolation transformer. Ground to neutral voltage is always zero. The laboratory is always maintained at 22 degrees C. I do not know what would be the problem. Please, anybody, have any suggestions?