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

[Bob Hearn <rah@america.net>]

If you are using a scintillator detector such as sodium iodide, you should 
check the resistor chain of the voltage divider in the photomultiplier base.
A deteriorated component or solder joint can result in such an intermittant
effect.
Any variation in the output of the high voltage supply to the photomultiplier 
will also displace the spectrum as a gain shift.

Of course check for line voltage (mains) fluctuations if you do not have a
good 
line conditioner or voltage regulator, or if the unit could be failing.

A pulser may be used to check the stability of the amplifier and ADC
(PCA3).  If
you are using an external preamp with a test input, use the pulser to check
this 
component as well.  The preamp power inputs should be checked (along with
NIM bin 
voltages if used).

Bob Hearn
rah@america.net

At 08:42 PM 9/21/99 -0500, LAM HOI CHING wrote:
>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 ?
>
>************************************************************************
>The RADSAFE Frequently Asked Questions list, archives and subscription
>information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
>
>
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html