## mass energy conversion

I don't know basically anything about nuclear physics, but I do know that mass can be converted to energy. Nuclear reactions, anti-matter, etc. But can energy be converted into mass? If so, where would this happen?

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 Quote by Rubix I don't know basically anything about nuclear physics, but I do know that mass can be converted to energy. Nuclear reactions, anti-matter, etc. But can energy be converted into mass? If so, where would this happen?
In particle colliders, the kinetic energy of the particles is converted into matter and anti-matter. Unless stored, the anti-matter usually finds corresponding matter and annihilates.

In the case of high energy gammas, E > 1.022 MeV, pair production may occur when the gamma interacts with a nucleus. In pair production, an electron-positron pair are formed, and usually the positron finds another electron and they annihilate into 2 gammas of energy ~0.511 MeV.

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 Quote by Astronuc E > 1.022 MeV,
Must the gamma be precisely of that energy for pair production, or would, say, 1.122 MeV also work? In that case, what happens to the remaining 0.1 MeV?

## mass energy conversion

 Quote by mheslep Must the gamma be precisely of that energy for pair production, or would, say, 1.122 MeV also work? In that case, what happens to the remaining 0.1 MeV?
The 1.022 MeV is the minimum energy required to produce the rest mass. AFAIK, the rest of the energy goes into the kinetic energy of the particles. In addition to pair prodcution, a gamma could knock out a neutron, which is know as photoneutron production. This doesn't create mass as much as it liberates a neutron from the nucleus. Basically the gamma energy has to match the binding energy of the last nucleon.

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 Quote by Rubix I don't know basically anything about nuclear physics, but I do know that mass can be converted to energy. Nuclear reactions, anti-matter, etc. But can energy be converted into mass? If so, where would this happen?
Further to everything that Astronuc has stated, energy is converted into mass any time the energy content of a body increases - eg. when a body absorbs radiation. But because of the law of entropy, this process is generally one in which the energy/mass becomes more dispersed over time. So creation of new clumps of matter is not likely to occur. Since the big bang, energy and mass concentration in the universe has been decreasing.

AM

 The idea of mass–energy equality unites the concepts of conservation of mass and conservation of energy, allowing particles which have rest mass to be converted to other forms of energy which have the same mass but require movement.

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Thanks for the response.
 Quote by Astronuc The 1.022 MeV is the minimum energy required to produce the rest mass.
Right, just the enough for the rest mass of the two particles.

 AFAIK, the rest of the energy goes into the kinetic energy of the particles. In addition to pair prodcution, a gamma could knock out a neutron, which is know as photoneutron production.
What factor determines whether the gamma results in pair production, or in a photoneutron, or just excites the nucleus to a higher energy state? If this were the interaction of a photon and electrons I know where to go for a model, but I don't see the analogy here for the nucleus.

 Quote by mheslep What factor determines whether the gamma results in pair production, or in a photoneutron, or just excites the nucleus to a higher energy state? If this were the interaction of a photon and electrons I know where to go for a model, but I don't see the analogy here for the nucleus.
There are two interactions with regard to photons (gamma rays) and atomic electrons: photoelectric effect (photon completely absorbed, and electron ejected from atom), Compton effect (photon scatters and loses energy, and electron is ejected from atom). Pair production involves a photon interaction with the nucleus.

When a relatively low energy neutron is absorbed by a nucleus, the nucleus emits a gamma ray (which is the binding energy). Conversely, a gamma ray with sufficient energy can cause a neutron to be ejected from the nucleus. In the case of a deuteron, a gamma ray of sufficient energy (~2.2 MeV) can cause separation (dissociation) of the proton and neutron (the process is called 'photodissociation').

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 Quote by Astronuc There are two interactions with regard to photons (gamma rays) and atomic electrons: photoelectric effect (photon completely absorbed, and electron ejected from atom),
Or, electron stays in a higher energy orbital.
 Compton effect (photon scatters and loses energy, and electron is ejected from atom). Pair production involves a photon interaction with the nucleus. When a relatively low energy neutron is absorbed by a nucleus, the nucleus emits a gamma ray (which is the binding energy). Conversely, a gamma ray with sufficient energy can cause a neutron to be ejected from the nucleus. In the case of a deuteron, a gamma ray of sufficient energy (~2.2 MeV) can cause separation (dissociation) of the proton and neutron (the process is called 'photodissociation').
My question was, given a incoming gamma of known energy can we say exactly which of the various scenarios will occur? I'm guessing now that the outcome is analogous to an incoming neutron: X% change of absorption, 100-X% chance of fission and so on.

 Quote by mheslep Or, electron stays in a higher energy orbital. My question was, given a incoming gamma of known energy can we say exactly which of the various scenarios will occur? I'm guessing now that the outcome is analogous to an incoming neutron: X% change of absorption, 100-X% chance of fission and so on.
The various reactions compete based on cross-section. For gammas below 1.022 MeV, there will be no pair production, and so only photo-electric effect and Compton scattering occur. There's plenty of experimental data that have been used to develop cross-sections for the different reactions as a function of gamma-ray energy and element.

See figure 3 in this - http://www.physics.uoguelph.ca/~csch...es/highres.pdf

Note the decrease in cross-sections for the photo-electric and Compton scattering.

And just for interest - a thesis on photodisintegration of a deuteron
http://he3.dartmouth.edu/Photodisint...AbbyThesis.pdf

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 Quote by Astronuc The various reactions compete based on cross-section. For gammas below 1.022 MeV, there will be no pair production, and so only photo-electric effect and Compton scattering occur. There's plenty of experimental data that have been used to develop cross-sections for the different reactions as a function of gamma-ray energy and element. See figure 3 in this - http://www.physics.uoguelph.ca/~csch...es/highres.pdf Note the decrease in cross-sections for the photo-electric and Compton scattering. And just for interest - a thesis on photodisintegration of a deuteron http://he3.dartmouth.edu/Photodisint...AbbyThesis.pdf
Thank you

 Recognitions: Gold Member Isn't the result all around us? (Although not a bit self-evident). All the nuclei heavier than that of iron have been created in a process involving energy-mass conversion (although there may have been mass-energy conversion on the way). A lot of it if I remember happened in supernovae. I am a bit vague beyond that, in fact I would be grateful for indications of useful sources of info. books/articles at any level about element creation.