Re: [Phys-l] Gamma-Gamma Coincidence

[Bill Nettles <bnettles@uu.edu>]

Michael is correct if you can identify the source.  Electron-positron annihilation photons will produce a much broader peaks in GeLi/HPGe detector system than will a 511 keV gamma ray emitted by a nucleus.  It is true that the detector itself doesn't know the source, so a gamma ray detector will also detect x-rays and annihilation photons and SiLi detectors, designed primarily for x-rays, will detect low-energy gammas which originate from the nucleus, not from electron transitions.  If you know the source, use the correct name.

Bill Nettles

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Sunday, March 28, 2010 2:13 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Gamma-Gamma Coincidence

On 03/28/2010 11:40 AM, Michael Edmiston wrote:

> The 511-keV 
> photons from B+ annihilation are not gamma rays.  Gamma rays come from the 
> nucleus and these definitely are not coming from the nucleus. 

"From the nucleus" is not the only possible definition.  
AFAICT it's not even the usual definition.

The usual definitions revolve around energy aka frequency
aka wavelength;  anything below 100 keV is an X-ray
while anything above 100 keV is a gamma:
  http://img20.imageshack.us/img20/9160/1260675741893.jpg
  and many others.

The energy-based definition makes sense to me.  In
particular, if I'm building a gamma detector, I don't 
care whether it was produced by a nucleus, by a 
positron, by a pulsar, or whatever.  The gamma is
a gamma, even after it is long separated from its
original source.

511 keV is not even close to the "soft" end of the
"soft" gamma rays.
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