[ RadSafe ] low energy gamma-spikes

[blreider at aol.com]

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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