<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class=""></div><div class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Jul 25, 2018, at 9:36 PM, Andrew Meulenberg <<a href="mailto:mules333@gmail.com" class="">mules333@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">Note that the threshold energy for pair production "... given by
<span style="color:rgb(17,17,17);font-family:Roboto,Arial,sans-serif;font-size:14px;text-align:left;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline" class=""> the relation Eγ ≥ 2 mec (1 + me/mr), where mr is the mass of the recoiling particle</span>," 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.<br class=""></div><div class=""><br class=""></div><div class="">Andrew<br class=""></div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Tue, Jul 24, 2018 at 1:35 PM, Richard Gauthier <span dir="ltr" class=""><<a href="mailto:richgauthier@gmail.com" target="_blank" class="">richgauthier@gmail.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="">Hi Chip and all,<div class=""> Here's a little background on experimental pair production from the abstract to an article on <a href="http://Researchgate.net" class="">Researchgate.net</a> at <a href="https://www.researchgate.net/publication/235335367_The_Miracle_of_the_Electron-Positron_Pair_Production_Threshold" target="_blank" class="">https://www.researchgate.<wbr class="">net/publication/235335367_The_<wbr class="">Miracle_of_the_Electron-<wbr class="">Positron_Pair_Production_<wbr class="">Threshold</a> </div><div class=""> Richard</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">
<span style="color:rgb(17,17,17);font-family:Roboto,Arial,sans-serif;font-size:14px;text-align:left;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline" class="">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.</span>
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