Peak shift in NaI spectrum of an Oxford Instrument MCA

[LAM HOI CHING <lamhc@hkusua.hku.hk>]

Ask for advice for an old piece of MCA:
I'm puzzled what problem is inside my Oxford Instrument MCA (version PCA3).
Every time when I check the spectrum energy with Cs137(662keV) , the energy
reads 600~630keV, the same percentage drop happens with channel readings
too. When the hardware fine gain is increased by 5% to 10%. The peak
becomes 662keV again but after 2 hours, the peak raises to 680keV. This
pattern has repeated for many weeks. I have excluded the room temperature
effect to the device. This MCA does not have any abnormalities in FWHM
shapes and efficiency counts. What could possibly be the problem inside the
hardware ?

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