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Re: [Phys-l] Gamma-Gamma Coincidence

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? The figure you linked to is really old.

You might check even Wikipedia under "gamma ray" which correctly states that the energy definition of the difference between x-rays and gamma-rays is no longer used. Wiki says...

"Because of this broad overlap in energy ranges, the two types of electromagnetic radiation are now usually defined by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus (that is, produced by gamma decay), or from other particle decays or annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet and even lower energy photons produced by these processes would also be defined as 'gamma rays'."

Also read the Wiki entry for Electromagnetic Spectrum. Within that listing, read the part under "Types of radiation."

Even back in 1970s when I worked at the National Superconducting Cyclotron Lab at Michigan State University, and then later at Los Alamos National Lab, the Wiki definition was already in use. Indeed, if you look in tables of x-ray data and nuclear data you see x-rays and gamma-rays clearly delineated by their source, not their energy. Furthermore, x-rays are further identified by their electronic transitions as K or L or M, etc. and even subdivided beyond that. And gamma-rays are likewise further divided by the nuclear decay process that generated them such as an E2 transition or M1 transitions, etc.

Although Wiki states annihilation photons fall under the gamma-ray grouping, they are delineated in tables using the gamma(+-) symbol, and some journals will change an author's use of 511-keV gamma-rays to 511-keV annihilation photons. 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.

The Condensed Chemical Dictionary even as old as my 1961 7th edition says (under gamma rays) "Electromagnetic radiation similar to x-rays except that gamma rays originate in the nucleus of an atom whereas x-rays originate in the extranuclear structure."

In the Handbook of Chemistry and Physics it states that gamma-ray energies are given in MeV and that "Ann. Rad. refers to the 511.006 keV photons emitted in the annihilation of positrons in matter. " (Notice the word "photons" after the keV as opposed to the wording "gamma rays." In the handbook the x-rays are all atomic and in a different section from the nuclear data.

In the Table of Isotopes, clear back in my 6th edition from 1967 it clearly delineates x-rays, gamma-rays, and annihilation photons. The entry for 22Na lists only one gamma (the 1.274 MeV gamma) on the page with the decay scheme, but in the tabular data portion at the beginning of book, under photon emission from 22Na, it lists... Ne x-rays, 0.511 (180%, gamma+-), 1.275 (100%). That is, the one true gamma ray is annotated differently from the x-rays and annihilation photons.

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?


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 3:12 PM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
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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