[General] background on pair production

[Richard Gauthier]

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

          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.
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