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Fwd: Re: FW: GM Detector Response to Positrons
>Date: Thu, 21 Feb 2002 10:23:10 +0100
>To: glen.vickers@exeloncorp.com
>From: Jean Charles ABBE <Jean-Charles.Abbe@irccyn.ec-nantes.fr>
>Subject: Re: FW: GM Detector Response to Positrons
>
>Just a rapid comment about positron, not strictly on GM detection of
positron emitters.
>Inside a gas, an energetic positron ionises the gaz exactly as an
energetic electron and can be detected as such. However, the positron will
encounter an electron, not necessarily an electron from ionisation for
which the probability is low, and either annihilates immediatly or form a
positronium with will also annihilates but with some delay which may be
rather long in gases. In a liquid, the probability of positronium formation
is rather high ( one tird in water) while in solid, the positron will
annihilate promptly with an electron (the electron density is high). Except
if the window is extremely thin, a positron will not enter a GM detector
but preferably annilates in the film.
>Nevertheless, a positron will annilate and thus give birth to two gammas
of .511 MeV which can be detected and which are at the origin of the 0.5
MeV lines in gamma spectra and to escape peaks (line at nominal energies,
En, and at En minus 0.5 and minus 2x0.5 (1;22 MeV)). Interestingly, 0.5 MeV
lines are much broader (a factor of two, approximately) than expected from
the nominal resolution : this reflects the kinetic energy of the
positron/electron pair at the moment of annilition. Nice chemical
information may be derived from the broadness of the peak ...
>Much more to say and to learn about positron annihilation, a nice field of
research ...
>
>At 07:41 20/02/02 -0600, you wrote:
>>
>>
>>> 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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+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
Jean-Charles ABBE
Délégation CNRS
IRCCyN
1, Rue de la Noë
BP 92101
44321 Nantes cedex 3
Tel : 02 51 12 45 16
Fax : 02 51 81 05 77
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