[ RadSafe ] low energy gamma-spikes pranks done with photomultiplier tubes! Blocked air flow causes problem

Thu Sep 30 21:44:12 CDT 2010

Dear Dr Dixon,

In our counting laboratory in Leeds we found an inexplicable shift of a few 
channels in the case of a sensitive gamma spectrometer. The multichannel 
analyser and associated instrumentation belonged to one company (Nuclear 
Diodes??) whereas the detector was from Nuclear Enterprises. Both companies 
defended their turf.

We recorded room temperature continuously. The ambient temperature in the sub 
basement laboratory was strictly controlled within plus or minus a few degrees. 
We spent better part of a month trying to figure out the cause of shift. The 
reason was pretty simple. Some one has inadvertently kept a folder (the typical 
functional one distributed in conferences) over the electronic instruments 
thereby blocking the air flow.
There was another instance in which an instrument behaved quixotically because 
the fibrous filter which removes dust got clogged over the years. We may not get 
such experience from text books

Regards

Parthasarathy

________________________________
From: "Dixon, John E. (CDC/ONDIEH/NCEH)" <gyf7 at cdc.gov>
To: The International Radiation Protection (Health Physics) Mailing List 
<radsafe at health.phys.iit.edu>
Sent: Thu, 30 September, 2010 23:05:54
Subject: Re: [ RadSafe ] low energy gamma-spikes pranks done with 
photomultiplier tubes!

With regard to the energy spectrum shift, I have experienced such a shift in 
solid state detectors due to extreme variances in environmental temperature (the 
temperature of the area being surveyed). I haven't heard of the 'whisker' 
phenomenon.

John E. Dixon

________________________________

From: radsafe-bounces at health.phys.iit.edu on behalf of parthasarathy k s
Sent: Thu 9/30/2010 12:34 PM
To: The International Radiation Protection (Health Physics) Mailing List
Subject: Re: [ RadSafe ] low energy gamma-spikes pranks done with 
photomultiplier 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

________________________________
From: "blreider at aol.com" <blreider at aol.com>
To: radsafe at health.phys.iit.edu
Sent: Thu, 30 September, 2010 17:46:09
Subject: Re: [ RadSafe ] low energy gamma-spikes

I have seen the same effect John has with small PMTs sitting near an equipment
power source.  When I worked for Canberra once our supplier had forgotten to put
the EM shields into the PMTs.   I do not recall at what low energy we saw the
peak but there was another tell-tale sign that this was not a detector
phenomenon and was due a PMT electrode effect:  for the small directional source
of EM in the lab the peak would come and go if you rotated the NAI detector &
PMT 90 degrees with respect to the power source for the system.  So every
quarter turn the peak would come or go because as it turned out the electrodes
inside the PMT were in a plane.  The effect of diffuse EM radiation on PMTs
might not be so easy to identify as was the small power source close by.  BTW
Canberra's techs on the floor quickly recognized this as a problem, got help
(me) and then the batch of detectors was sent back to the mfg. 

I forget who the person is that observed this but perhaps you could try this out
in a lab and see if you see a  peak at the same "energy" as the one  you see in
the field.  Good Luck!

Barbara Reider, CHP

-----Original Message-----
From: Dixon, John E. (CDC/ONDIEH/NCEH) (CDC/ONDIEH/NCEH) <gyf7 at cdc.gov>
To: The International Radiation Protection (Health Physics) Mailing List
<radsafe at health.phys.iit.edu>
Sent: Thu, Sep 30, 2010 7:37 am
Subject: Re: [ RadSafe ] low energy gamma-spikes

Depending upon the physical configuration of your NaI detector (i. e. does it
ave a thin window, etc.), I have seen strong signal responses from X-Rays in
he 2 - 5 Kev range. A high dose rate is not needed to give a response to 25 Kev
-rays. The vulnerable component here is probably the photomultiplier tube
ssociated with the detector. The PMT is susceptible to EM interference.

ohn E. Dixon
________________________________
From: radsafe-bounces at health.phys.iit.edu on behalf of Alston, Chris
ent: Tue 9/28/2010 1:51 PM
o: The International Radiation Protection (Health Physics) Mailing List
ubject: Re: [ RadSafe ] low energy gamma-spikes

Marco
Yes, the doserate would have to be very high for an E-compensated GM detector to

ee something at 25 keV.  Noise from the high tension power lines would be my
irst guess, too.
Cheers
ja
-----Original Message-----
rom: marco bähler [mailto:m.c.baehler at bluewin.ch]
ent: Monday, September 27, 2010 2:51 PM
o: radsafe at health.phys.iit.edu
ubject: [ RadSafe ] low energy gamma-spikes
dear group
I live close to an old bwr and went for a walk with my BNC 935 / 3inch NaI. it
as towards the end of the annual refueling period.
00m downwind, calm wind, on the open field  suddenly I recorded several short
larms, lasting between one and 12 seconds. countrates up to 15000cps. energy
lmost exclusively around 25 keV. I am wondering what this could have been.
nswers from the authorities range from "noise due to high tension lines"
actually 200m away) to "probably x-ray exams somewhere" and  "no hear no see no
ay" or "cosmic particle showers".
ne answer referred to the gm-tube-equipped doserate-monitoring network which
recorded nothing". I think it is quite common for compensated gm tubes not to
ee 25 keV.
s someone of YOU familiar with such spikes (me thinks I can call them clouds)
nd knows possible sources/causes? (Cd109??)
kind regards and thankyou for pondering
marco bähler
euratom level 5b, 2004 utrecht)
etikon
witzerland

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