Re: Positronium range?

[daleboyce@charter.net]

Hi Mike,



Range seems an odd way to speak of it.  You can get an idea of the range of

velocities of the positronium when it annihilates from the Doppler

broadening of the 511 photopeak.  Checking some quick references the binding

energy of positronium is about 6.8 ev, and the radius about twice that of a

hydrogen atom. I don't have a number in hand for the velocity at

annihilation, but for it to have a noticeable effect in the 511 peak shape

one would tend to think it is of the order of 100 ev.



So from that I would think that you don't just get an atom of positronium

that then propagates as an entity, but rather a series of made and then

broken bonds until the annihilation occurs. Something the size of twice a

hydrogen atom is going to have a lot of overlap with all of the electrons of

every atom it passes. So I'm not sure it even makes sense to talk about a

range for the positronium, but rather a range of the positron after it has

lost enough energy to begin forming positronium.



I'd be really interested to hear an explanation if you get one.



Dale

daleboyce@charter.net





----- Original Message ----- 

From: "Stabin, Michael" <michael.g.stabin@vanderbilt.edu>

To: <radsafe@list.vanderbilt.edu>

Sent: Saturday, September 11, 2004 3:47 PM

Subject: Positronium range?





>

> OK, I heard something new (for me) at a workshop in Europe this week -

> that the positronium has a defined range in matter after formation. We

> always focus on the range of the positron in discussing resolution

> limits in Positron Emission Tomography, and I always had the idea that

> the formation of the positronium 'atom' occurred at the end of the track

> and that was where the two 511 keV photons were formed. But someone

> modeling the physics with Monte Carlo methods was including a fixed

> range for the positronium formed as well, which of course will

> contribute to this resolution issue. Have others heard of this?

>

> Mike

>

> Michael G. Stabin, PhD, CHP

> Assistant Professor of Radiology and Radiological Sciences

> Department of Radiology and Radiological Sciences

> Vanderbilt University

> 1161 21st Avenue South

> Nashville, TN 37232-2675

> Phone (615) 343-0068

> Fax   (615) 322-3764

> Pager (615) 835-5153

> e-mail     michael.g.stabin@vanderbilt.edu

> internet   www.doseinfo-radar.com

>

>

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