[General] background on pair production

[Andrew Meulenberg]

Dear Richard,

I realize that I might not have been clear enough in my statement about the
scattering charge being a lepton rather than a proton or nucleus. And, my
mistake in using the expression for Eγ  certainly did not help the
situation. You (and the reference) were focusing on the minimum energy
threshold for pair production and the difficulties associated with the low
production rates near the threshold. I was looking at the other end of the
question where a light scattering center (e.g., an electron) makes energy
and momentum conservation have a much greater effect.

My memory of photon energy threshold >2 MeV for pair creation from a
collision with an electron is consistent with Eγ ≥ 2 mec (1 + me/mr)  = 4
mec = 2.044 MeV. This may only have been based on theoretical calculation.
I'm not sure that there was any definitive experimental work to support it.
However, the recoiling electron from this interaction  would be energetic
enough to give good confirming information. I'm not sure that Compton
scattering would not interfere with the experiment.

Andrew




On Thu, Jul 26, 2018 at 10:04 AM, <richgauthier at gmail.com> wrote:

> 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  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. I
> 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.ne
>> t/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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