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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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