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From: crandell@ut-sally.UUCP (Jim Crandell)
Newsgroups: net.physics
Subject: Re: M = E/C^2 ??? How??? - (nf)
Message-ID: <302@ut-sally.UUCP>
Date: Wed, 2-Nov-83 02:36:48 EST
Article-I.D.: ut-sally.302
Posted: Wed Nov  2 02:36:48 1983
Date-Received: Sun, 6-Nov-83 05:13:37 EST
References: <548@ucbcad.UUCP>, <3375@umcp-cs.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 27

Here's an example that's very common, and it doesn't require anything
nearly as exotic as a high-frequency laser that emits x-rays (a spot
of Cs-137 will do nicely).  It's been a few years, but I vaguely recall
from working with a NaI scintillation detector that the spectra I obtained
were often rendered faintly misleading by -- though one learns to work
around it -- an irrelevant response peak at about 511 kev.  That energy
level just happens to be that of the photons emitted from electron/positron
annihilations.  Where did the positrons come from?  Well, some of those
1+ Mev photons that the Cs-137 pours out sorta get caught in the web of
that darned ol' NaI crystal, and they sorta lose some energy.  Sure, a
lot of them do it by kicking valence electrons out of their orbits or by
undergoing funny kinds of collisions with all those massive particles,
but a few manage to turn about 511 kev of it (there's that number again)
directly into mass in the form of a positron and an electron; you have
to have one of each, because you don't want the crystal taking on a net
charge.  Naturally, with all those charged particles around, neither the
positron nor the electron gets very far, and right away (on the average)
you get annihilation, with its attendant "ann. rad.".

By the way, the materials used to make proportional radiation detectors
are certainly NOT the only ones in which pair production occurs; however,
you're not likely to notice it elsewhere, for reasons which I hope are
obvious.
-- 
   Jim ({ihnp4,kpno,ut-ngp}!ut-sally!crandell or crandell@ut-sally.UUCP)