[General] background on pair production

[richgauthier at gmail.com]

Hi Andrew and all,
  Below is a pdf copy of the article https://www.researchgate.net/publication/235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold <https://www.researchgate.net/publication/235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold>  with the abstract (below) you are quoting from. Definitely the minimum incoming photon energy is much less than 2 MeV and much nearer to the quoted value. It turns out that it’s very hard (as explained in the article) to experimentally confirm the minimum photon energy value for a particular recoil nuclear mass, given by the formula, so there’s surprisingly much experimental (and perhaps theoretical also) work still needed on this relatively straightforward conversion process of a photon to an electron-positron pair.
    Richard



> On Jul 25, 2018, at 9:36 PM, Andrew Meulenberg <mules333 at gmail.com> wrote:
> 
> 
> Note that the threshold energy for pair production "...  given by the relation Eγ ≥ 2 mec (1 + me/mr), where mr is the mass of the recoiling particle," gives > 1 MeV for an electron or positron. My memory said that a >2 MeV photon was required. It may be related to the angle of recoiI. don't have time to look it up.
> 
> Andrew
> 
> On Tue, Jul 24, 2018 at 1:35 PM, Richard Gauthier <richgauthier at gmail.com <mailto:richgauthier at gmail.com>> wrote:
> Hi Chip and all,
>   Here's a little background on experimental pair production from the abstract to an article on Researchgate.net at https://www.researchgate.net/publication/235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold <https://www.researchgate.net/publication/235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold> 
>           Richard
> 
> 
> Pair production was first observed in 1932, which led to two early Nobel prizes in physics, to Carl Anderson for the discovery of positrons (1936) and to Paul Dirac for the theory of anti particles (1933). Science textbooks state that the production of electron-positron pairs is possible at photon energies above 1.022 MeV, which is the sum of the rest masses of the particles involved. Measurements at the threshold require a selectable photon energy in the range above 1 MeV, high-energy resolution to scan the onset, and high intensities. Due to the need of simultaneous energy and momentum conservation, pair production needs a recoiling particle, and thus it can be observed most easily in solid matter. More exactly, the minimum energy required for pair production is given by the relation Eγ ≥ 2 mec (1 + me/mr), where mr is the mass of the recoiling particle [1]. With the particle rest energy of me = 511 keV/c , in heavy atoms we get mr >> me, and thus in a good approximation photon energies Eγ ≥ 2·mec = 1.022 keV allow the creation of electron-positron pairs. However, for a proton as recoil particle the calculated threshold energy is increased by 557 eV, for a copper target by 9 eV, and even for the very heavy element 111Roentgenium by about 2.1 eV. Thus pair production cannot take place at exactly 2ámec. 
> 
> _______________________________________________
> If you no longer wish to receive communication from the Nature of Light and Particles General Discussion List at mules333 at gmail.com <mailto:mules333 at gmail.com>
> <a href="http://lists.natureoflightandparticles.org/options.cgi/general-natureoflightandparticles.org/mules333%40gmail.com?unsub=1&unsubconfirm=1 <http://lists.natureoflightandparticles.org/options.cgi/general-natureoflightandparticles.org/mules333%40gmail.com?unsub=1&unsubconfirm=1>">
> Click here to unsubscribe
> </a>
> 
> 
> _______________________________________________
> If you no longer wish to receive communication from the Nature of Light and Particles General Discussion List at richgauthier at gmail.com
> <a href="http://lists.natureoflightandparticles.org/options.cgi/general-natureoflightandparticles.org/richgauthier%40gmail.com?unsub=1&unsubconfirm=1">
> Click here to unsubscribe
> </a>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.natureoflightandparticles.org/pipermail/general-natureoflightandparticles.org/attachments/20180726/f502d358/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Dirk  Miracle_of_Pair_Production_Threshold_SRN.pdf
Type: application/pdf
Size: 2355404 bytes
Desc: not available
URL: <http://lists.natureoflightandparticles.org/pipermail/general-natureoflightandparticles.org/attachments/20180726/f502d358/attachment.pdf>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.natureoflightandparticles.org/pipermail/general-natureoflightandparticles.org/attachments/20180726/f502d358/attachment-0001.html>