[General] background on pair production

[Andrew Meulenberg]

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>
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
>           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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