[ RadSafe ] low energy gamma-spikes pranks done withphotomultiplier tubes!

[Cary Renquist]

Clever fix.
The low background detectors probably used lead-free solder and thus are
prone to "tin whisker" growth.
I guess that one needs to use roman-era lead for the solder...

Best regards,
Cary

---
Cary Renquist
cary.renquist at ezag.com


-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of parthasarathy
k s
Sent: Thursday, 30 September 2010 09:34
To: The International Radiation Protection (Health Physics) Mailing List
Subject: Re: [ RadSafe ] low energy gamma-spikes pranks done
withphotomultiplier tubes!

I recall that in our formative years in the Bhabha Atomic Research
Centre, we 
used to all sort of crazy things with photomultiplier tubes. In our
enthusiasm 
one of my colleagues used a magnet of considerable strength and
virtually 
displaced the electrode of a PM tube with disastrous consequences! 


Indeed we studied the response  by removing the electrostatic shields.
We also 
studied influence of earth's magnetic field; if I recall correctly it
was 
significant for large area photomultiplier tubes. 


While on this, let me describe a very interesting experience we had
while using 
large area photomultipliers. 

The Department of Medical Physics, University of Leeds had an
understanding with 
the Nuclear Enterprises by which the company used to pass on low
background 
photomultipliers with sodium iodide (Tl) crystals as integral line
assemblies 
for studies. While working with one such units our routine was to record
Cs-137 
(0.66 MeV) gamma line and TL-208 (2.61 MeV) gamma line at the beginning.
One day 
we found that both these lines have shifted considerably down. It
appeared that 
there was some change in the internal electrode structure which led to
this.

A well experienced member of the technical staff told us that he had
found such 
an event earlier; he went through his old records and located a
remedy.We found 
that in large area PM tubes small whisker like structures can develop
and work 
as a short circuit lowering the voltages between electrodes. The impact
of this 
will be considerable if it happens at the initial stages. The remedy was
to send 
a very low voltage high current pulse between the suspected electrodes
and burn 
off the whisker.  Once this was done the gamma lines appeared in the
same 
channels of the multichannel analyser. 



Regards
Parthasarathy