RE: Peak shift in NaI spectrum of an Oxford Instrument MCA

[Bill Kendall <KENDALL@nucusa.com>]

This may not be your problem, but I had a similar experience with an old PMT
and power supply.  Originally, the system had been used with a 2 MeV
baseline.  When I inherited it, I chose a 1 MeV baseline so that I could get
more resolution in the region of 30 - 35 keV.  To get that much gain from
the PMT, the high voltage had to be increased significantly.

After about a month of chasing a problem very similar to yours, I learned
from an old timer that when you apply excessive HV to a PMT, it becomes a
very sensitive barometer AND a very sensitive room thermometer.

I added a linear amplifier, reduced the HV and never had another problem.

If you need more on this suggestions, call me.

Bill Kendall
(410) 418-9311
kendall@nucusa.com


-----Original Message-----
From: LAM HOI CHING [mailto:lamhc@hkusua.hku.hk]
Sent: Tuesday, September 21, 1999 8:41 PM
To: Multiple recipients of list
Subject: Peak shift in NaI spectrum of an Oxford Instrument MCA


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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