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Re: [Phys-l] Gamma-Gamma Coincidence



In John Denker's first post on this topic he said,

"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:

This was the main point in John's post that I was objecting to in my second post. His energy-based distinction between gamma rays and x-rays is clearly incorrect. It's interesting that in his next post he didn't mention this at all. Rather, he focused the discussion only toward the annihilation discussion. John, are you conceding that your x-ray versus gamma-ray statements were incorrect?

With regard to annihilation radiation and a few other sub-atomic processes I have acknowledged that one can find reference books and Internet sites where these are referred to as "gamma-rays" without further clarification. Yes, people use that wording (without further distinction), but that usage occurs primarily in casual conversation. In "formal" nuclear physics dialogue and publications it is customary to sub-designate the processes the created the photon(s). In all the standard nuclear data reference sources the distinction is made between nuclear-transition-generated gamma rays and positron-annihilation-generated photons. For example, look at the Chart of the Nuclides. For those nuclides that decay by positron emission, the chart never lists the 511-keV annihilation photons in the list of gamma rays observed from those nuclides. Likewise in the Table of Isotopes and the Table of Radioactive Isotopes and the Handbook of Chemistry and Physics... if the 511-keV photons are listed, they are listed as gamma(+-) rather than gamma-rays.

When a nuclear physicist states that he or she is doing a gamma-ray coincidence experiment, he or she does not mean an experiment to look for coincidence of 511-keV annihilation photons. We already know these are coincident. The 22Na source is great for basic demonstration of how one can observe two photons that are "generated at the same time." I put "generated at the same time" in quotation marks because in true gamma-ray coincidence experiments one is elucidating which nuclear-generated gamma-rays are in the same nuclear-transition cascade. Hence, the typical gamma-ray coincidence experiment is more complicated than detecting the coincidence of annihilation photons. When a student performs a laboratory experiment that detects coincidence of annihilation photons, that's a good experiment, and it can teach things such as basic nuclear electronics, and something about true coincidence versus chance coincidence, and of course something about annihilation radiation. But it's not a gamma-ray coincidence experiment. I'm a little rusty with respect to some of the timing methods used today, but a traditional gamma-ray coincidence experiment would use three multichannel analyzers... one for each gamma detector, and a third for a time-to-amplitude converter. Events that occur in "coincidence" within a certain time window would be recorded as to the energy of each gamma plus the time lapse between them. During analysis, a coincidence spectrum is generated from all those gammas in spectrum two that occurred within a particular time window of a specific gamma-ray energy in spectrum one. This can be repeated for each gamma-ray energy observed in spectrum one, and can be repeated for differing time windows. This is also often done at varying detector angles so that one can look for angular correlations. These are very time consuming experiments both for data acquisition and data analysis. The energy coincidence and timing information help determine the ordering of nuclear energy levels and which decay cascades occur. The angular correlation data helps determine whether the nuclear transitions generated the gamma-rays by electric or magnetic dipole or quadrupole (etc.) mechanisms.

In an overall summary to the overall dialogue so far, I would say that...

(1) I strongly object to a distinction between x-rays and gamma-rays based upon energy.

(2) I moderately object to saying that observing coincidences between annihilation photons is a "gamma-ray coincidence" experiment.

(3) I mildly object to reference to annihilation photons as gamma-rays without further annotation of (+-), especially in formal dialogue and writing.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu


--------------------------------------------------
From: "John Denker" <jsd@av8n.com>
Sent: Sunday, March 28, 2010 9:30 PM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] Gamma-Gamma Coincidence

On 03/28/2010 04:19 PM, Michael Edmiston wrote:
John, before you said...

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

... did you do any serious checking?

I did.

The figure you linked to is really old.

But as I said, there were many others, and all I
could find were consistent. I chose one that was
scanned at a high resolution and had a variety of
units: wavelength, frequency, and energy.

You might check even Wikipedia under "gamma ray"

I did. Unless it has changed in the last few minutes,
the second sentence of that article says
"They are produced by sub-atomic particle interactions
such as electron-positron annihilation, ...."

directly contradicting the "nucleus only" definition.

As I have previously pointed out in other contexts,
it is not unusual for wikipedia articles to be self-
contradictory (in addition to just plain wrong).

I carelessly let the wording 511-gammas slip into a draft of my
thesis (in just one place), and my whole committee noticed it, and I was
asked to change it to 511-keV (gamma)(+-) , which I did, with embarrassment.

<snip>

I have a whole shelf of reference books and nuclear physics books that say
the same things, and this is the way all my "nuclear colleagues" talk and
write. What more do you need?

I stand by my assertion that "nuclear only" is not the
only definition. I checked earlier, and I checked again
just now, and there are many, many instances of annihilation
radiation being called gamma rays.

My one-time colleague recently wrote:
A. P. Mills, Jr., D. B. Cassidy and R. G. Greaves
"Prospects for making a Bose-Einstein-condensed Positronium
Annihilation Gamma Ray Laser"

I very much doubt that his choice of words was sloppy,
ignorant, or out-of-date.

A search for
http://www.google.com/search?q=gamma+annihilation
turns up millions of hits, and instances of "gamma ray"
vastly outnumber instances of "(gamma)(+-)". I consider
this an unbiased sampling of recent usage.

A search of scientific papers via

http://arxiv.org/find/all/1/ANDNOT+ti:+AND+gamma+annihilation+ti:+dark/0/1/0/all/0/1
produces the same overwhelming statistics in support
of what I said. Again I consider this an unbiased
sampling of recent usage.

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