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Question

Dear Colleagues!
Could somebody make clear for me the following question?
I am Dr.Yuriy B. Nabyvanets from National Yang Ming University, Taipei,
Taiwan. I investigate the contents of radionuclides (Cs-137 and gamma
emitting radionuclides from Th-232 series) in soil. I use the EG&G ORTEC
Nomad Plus Portable Spectroscopy System, HPGe Coaxial Detector System
(GEM Series) and Gamma Vision Software for gamma spectrum analysis.
About two months I had not any problems with my device and it worked
perfectly. But approximately 2 weeks ago I got some problems. As the
contents of radionuclides in soil samples rather low, I need 8 hours to
measure each sample.  And here approximately in 5 hours of measurements
I have become to notice that the spectrum of samples begins to change.
It means, that near to, for example, peak of K-40 (from the left side to
the region of low energies) occurs the second and third peaks. That
occurs also for peak of Cs-137 but in this case shift goes to the region
of high energies. It seems, that during measurements  energy calibration
varies. I check energy calibration every day and really peak of Cs-137
>from standard source changes location on a spectrum. The difference
sometimes reaches 10 KeV.
So, what the possible reason of this phenomenon? I hope for your help.
Sincerely  Yuriy B. Nabyvanets
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