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Re: GM Detector Response to Positrons
Glen,
Firstly apologies, as I can't contribute much on the detection of
positron emitters. However having checked The Radiological Health Handbook
I found that Co-58 decays mainly by Electron Capture (EC), 85% abundance
rather than positron emission,15% abundance. I can hopefully contribute
something on the issue of detection of isotopes that decay through EC.
In the gas reactors in the UK the issue of detecting and measuring isotopes
that decayed through EC was first identified in the early 1980's (although I
can't recall what first prompted this). Since then a number of isotopes
have been identifed as significant sources of potential transferrable
contamination. These include:
Cobalt-58
Manganese-54
Chromium-51
and Iron-55.
These are produced by activation of the stainless steel cladding inside the
reactor (in contact with the carbon dioxide coolant). This oxide can become
quite thick and spall of into the coolant.
In addition to the above (if I remember correctly) water cooled reactors can
also contain, in addition to those already listed, the following activation
products that also decay through EC.
Nickel-56 (although this only has a half life of approximately 6 days)
Zinc-65
and Copper-64 (although this decays be EC 43% of the time, Beta decay 38% of
the time and positron emission 19% of the time).
In my experience in the UK the most commonly used instrument for the
detection of isotopes that decay through EC is a Type 44B probe fitted to a
series 900 ratemeter manufactured by Mini-Instruments in the UK. The
following summarises the information that the manufacturer provides on this
type of probe.
SCINTILLATION PROBE TYPE 44B
The sodium iodide crystal (32mm dia by 2.5mm thick) is mounted on the front
surface of the probe to facilitate monitoring of large contaminated areas.
This probe is fitted with a beryllium end window and extends the sensitivity
down to approximately 5keV making it useful for 55Fe. 3.7kBq (0.1µCi) of
55Fe at 10mm approx 180 counts s-1
Window weight 47mg cm-2.
Mini-Instruments is a division of St Gobain Crystals and Detectors (who also
own Bicron-NE). According to their web site the following is the US
distributor for mini-instruments-
Dosimeter, a division of Artisan Electronics
Beth Finnegan
Sales Manager
5 Eastmans Road
PARSIPPANY, NJ 07054
USA
Phone: (1) 973 887 7100
Phone Ext: 287
Fax: (1) 973 887 4732
eMail: bfinnegan@dosimeter.com
Web site: www.dosimeter.com
I don't know what success if any you might by going through Bicron NE (or is
it NE Bicron?) in the US instead.
As of 4 years ago this type of probe was in use in both the UK Advanced Gas
Reactors and the single UK PWR (owned and operated by British Energy,
partners in Amergen in the US) for the detection the isotopes identified
above. This is in addition to the standard (NE Technology BP4 scintillation
probe used on gas reactors, and the HP260/EP15 pancake GM probes used at the
PWR for the detection of other beta emitting isotopes).
Best Regards
Julian Ginniver
----- Original Message -----
From: <glen.vickers@EXELONCORP.COM>
To: <radsafe@list.vanderbilt.edu>
Sent: Wednesday, February 20, 2002 1:41 PM
Subject: FW: GM Detector Response to Positrons
>
>
> > What do you suppose the GM pancake probe response would be to a positron
> > emmitter like Co-58? I would think that if the positron made it into
the
> > sensitive volume of the detector, that it would cause some form of
> > ionization that would be counted.
> >
> > Would the positron ever make it out of the phyical-form crud it is
> > deposited in? Once generated, there are "clouds" of electrons nearby
for
> > the positron to interact with.
> >
> > The positron will have kinetic energy imparted from being accelerated
from
> > the positive charges in the nucleus, but then again the cathode in the
GM
> > tube will try to prevent the positron from entering the GM tube.
> >
> > I've never seen anything in a text book about detection. Any
references?
> >
> > Power reactors make a significant amount of Co-58 and it's important to
be
> > more knowledgeable in this area...
> >
> > Glen Vickers
> > Braidwood Nuclear Station
>
>
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