[General] background on pair production
Andrew Meulenberg
mules333 at gmail.com
Sun Jul 29 03:08:32 PDT 2018
Dear Richard,
Thank you for looking that up. The words you highlighted in the abstract
are almost exactly like what I remember. I suspect that I read them in "The
Atomic Nucleus" by Evans (1982), which was often a "Bible" for me in my
work; but, I may have encountered the info earlier.
The main point is that the curvature of the photon path during its
"division" in passing by a charge can be quite different for the electron
interaction compared with that from a nucleus. This puts some light (and
limits) on the models for conversion of light to matter.
Andrew
On Sun, Jul 29, 2018 at 2:28 AM, <richgauthier at gmail.com> wrote:
> Hello Andrew (and all),
> The below abstract from http://adsabs.harvard.edu/abs/
> 2006RaPC...75..614H supports your comment about pair production in
> photon-electron interactions.
> Richard
>
> *Title:*
> Electron positron pair production by photons: A historical overview
> *Authors:*
> Hubbell, J. H.
> <http://adsabs.harvard.edu/cgi-bin/author_form?author=Hubbell,+J&fullauthor=Hubbell,%20J.%20H.&charset=UTF-8&db_key=PHY>
> *Affiliation:*
> AA(National Institute of Standards and Technology, Mail Stop 8463,
> Gaithersburg, MD 20899-8463, USA.)
> *Publication:*
> Radiation Physics and Chemistry, Volume 75, Issue 6, p. 614-623.
> *Publication Date:*
> 06/2006
> *Origin:*
> ELSEVIER <http://www.elsevier.com/>
> *Abstract Copyright:*
> (c) 2006 Elsevier Science B.V. All rights reserved.
> *DOI:*
> 10.1016/j.radphyschem.2005.10.008
> <https://doi.org/10.1016/j.radphyschem.2005.10.008>
> *Bibliographic Code:*
> 2006RaPC...75..614H <http://adsabs.harvard.edu/abs/2006RaPC...75..614H>
> AbstractThis account briefly traces the growth of our theoretical and
> experimental knowledge of electron-positron pair production by photons,
> from the prediction of the positron by Dirac [1928a. The quantum theory of
> the electron. Proc. R. Soc. (London) A 117, 610-624; 1928b. The quantum
> theory of the electron. Part II. Proc. R. Soc. (London) A 118, 1928b,
> 351-361] and subsequent cloud-chamber observations by Anderson [Energies of
> cosmic-ray particles. Phys. Rev. 43, 491-494], up to the present time.
> Photons of energies above 2 mec2 (1.022 MeV) can interact with the
> Coulomb field of an atomic nucleus to be transformed into an
> electron-positron pair, the probability increasing with increasing photon
> energy, up to a plateau at high energies, and increasing with increasing
> atomic number approximately as the square of the nuclear charge (proton
> number). *This interaction can also take place in the field of an atomic
> electron, for photons of energy in excess of 4 m**ec2** (2.044 MeV), in
> which case the process is called triplet production due to the track of the
> recoiling atomic electron adding to the tracks of the created
> electron-positron pair.* The last systematic computations and tabulations
> of pair and triplet cross sections, which are the predominant contributions
> to the photon mass attenuation coefficient for photon energies 10 MeV and
> higher, were those of Hubbell et al. [Pair, triplet, and total atomic cross
> sections (and mass attenuation coefficients) for 1 MeV-100 GeV photons in
> elements Z=1-100. J. Phys. Chem. Ref. Data 9, 1023-1147], from threshold
> (1.022 MeV) up to 100 GeV, for all elements Z=1-100. These computations
> required some ad hoc bridging functions between the available low-energy
> and high-energy theoretical models. Recently (1979-2001), Sud and
> collaborators have developed some new approaches including using distorted
> wave Born approximation (DWBA) theory to compute pair production cross
> sections in the intermediate energy region (5.0-10.0 MeV) on a firmer
> theoretical basis. These and other recent developments, and their possible
> implications for improved computations of pair and triplet cross sections,
> are discussed.
>
>
> On Jul 27, 2018, at 3:28 AM, Andrew Meulenberg <mules333 at gmail.com> wrote:
>
> 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.researchga
>> te.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 <http://researchgate.net/>
>>> at https://www.researchgate.net/publication/235335367_The_Mira
>>> cle_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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