Anti-Matter and gamma rays

[Garrettguy457]

I believe that when a proton and an anti-proton connect they turn into energy yet produce powerful gamma rays that are very dangerous. When a positron and a electron combine i believe that the gamma ray "let off" is less that when the anit-proton and the proton combine. IF one was to combine a full anti-hydrogen with hydrogen would it give off a lot of gamma rays or less. MY question is basically was is more efficient and less dangerous: proton/anti-proton, positron/electron, or anti-atom/atom?

[swansont]

The energy of the photons will be related to the mass of the particles annihilating. e-/e+ has 1.02 MeV of mass energy, so you will (usually) get two photons of 512 keV. p+/p- has about 1878 MeV, so you get 939 MeV photons. Hydrogen/antiHydrogen will basically be the sum of the two.

[Gilded]

What you're apparently missing here is that gamma ray photons aren't a byproduct of annihilation; they are pretty much the only product. All the mass of the reacting particles is converted into gamma rays, regardless the type of these particles.

 

As for efficiency all annihilations are theoretically 100% efficient. You'll get the same amount of energy from a kilogram of positrons and electrons as you get from a kilogram of hydrogen and antihydrogen.

[timo]

What you're apparently missing here is that gamma ray photons aren't a byproduct of annihilation; they are pretty much the only product. All the mass of the reacting particles is converted into gamma rays, regardless the type of these particles.

It is not true that matter-antimatter reaction have to turn into photons only. Depending on the type of matter and anti-matter I think it is usually not even the case that the majority of reactions does. Past and future particle physics experiments like Tevatron and ILC look at the reactions that are not purely photonic (for Tevatron they should be mostly hadronic, for ILC they probably are mostly leptonic with potentially a sufficiently high rate of supersymmetric particles and Higgs bosons.).

There is only one example where the statement with "photons only" is true: Electron-positron annihilation with both particles being slow (meaning their kinetic energies are small compared to their rest masses). The reason you can only get photons there is that to create new particles you need the energy for their rest mass. Since electrons are -with the exceptions of photons that actually are produced and gluons that cannot exist as free particles- the lightest particles and the kinetic energy was supposed to be negligible this reaction simply does not have the energy for any other reaction product than photons. Admittedly this only example is also the most important one as it is -to my knowledge- the only particle-antiparticle reaction that is used in applications, namely positron emission tomography.

 

In short: The statement that matter and anti-matter must react to photons only is wrong. I think this misconception is so widespread because there is one prominent special case where it is true.

 

EDIT: My statement why e+ e- is photon-only above is incomplete/wrong. I forgot the neutrino-antineutrino mode which, however, should be strongly suppressed by the weakness of the weak interaction at the energy considered.

Edited September 7, 2008 by timo

[Gilded]

It is not true that matter-antimatter reaction have to turn into photons only.

 

Yes, that's why I said "pretty much" which is of course a rather ambiguous expression as far as physics goes. I assumed the main focus was just energy production from antimatter, in which case there isn't that much of a need for higher energy collisions where other products than photons are favored.

Edited September 7, 2008 by Gilded