anti-protons and helium nuclei

[idobox]

A question that bothers me with matter-antimatter reaction that I am totally unable to anwer:
If an antiproton was to collide with a normal atom, let's say helium, what would happen?

It would be great if antiprotons could react only with protons, and not with larger nuclei, since it would be rather easy to store them. Since storing antimatter seems very difficult, I assume it's not the case.

Assuming a reaction happens, what happens to the energy? do we have most of the enrgy released as pions? Do we end up with a tritium nucleus or with free proton and neutrons? Is a significant part of the energy converted to kinetic energy of the resulting tritium nucleus/baryons?

[tooyoo]

You should find quite a lot of information on this when looking for proton-antiproton annihilation.

In either case - there are some people on here who know this better - but the following should happen: You're essentially colliding
2 Proton + 2 Electron + 1 Antiproton
Since it's a quantum process, the result is statistical. That is, there are many possible outcomes, all that is known are the probabilities for each possible product.

Concerning possible products, relativistic kinematics tells you the following: The result of the reaction has to conserve the overall charge(s) of the above (+2-2-1 = -1 for electrical charge) as well as the energy and momentum. Momentum conservation will not be a problem, since you've got enough "stuff" carrying momentum to balance equations. Assuming that the helium atom is at rest, the total energy will be:
rest mass helium + rest mass antiproton + kinetic energy antiproton (of course with relativistic corrections, etc, etc)
So if the kinetic energy is large enough, you can create many, many particles.

To get a bit more concrete: You're essentially looking at the physics of proton-antiproton annihilation (except that you have a bit more stuff floating around). An example of that is given by the Tevatron (the world's premier accelerator until the LHC came online). Most of the time you'll produce pions, but in general you'll be able to generate all sorts of particles, as long as charges are conserved.

[Tass]

I am at the moment sitting in geneva because I am getting a guided tour on CERN along with a bunch of fellow physicists. Yesterday we the experiment where they study antihydrogen. A side project of theirs is actually using antiprotons for radiation therapy. This is advantageous because they become much more likely to interact strongly with other matter as they slow down, therefore one can make most of them annihilate in precisely the right depth where the tumor is. Presently normal protons are being used for this purpose since they still deposit most of their energy where they stop, rather than what they went trough before. But antiprotons are better because when they stop they find a nucleus and annihilate, blowing the rest of the nucleus apart, doing a lot of secondary damage around. Basically they work as depth charges against cancer.

So the answer is yes, an antiproton will annihilate with any proton in any atom, and the energy will usually blow the nucleus apart.

Actually even an antiproton an a neutron will annihilate when the get to close. You have both quarks and anti-quarks which can annihilate each other, and the total baryon number is zero so the end result will only be a bunch of leptons and photons.

[legend]

The total energy you get from annihilation of a proton-antiproton pair is 1877 Mev, the total binding energy of the nucleons in a He4 atom is 28 Mev, so basically negligible it the process. The stuff that you get in the end thus should looks more or less the same as you would get from mixing together one part protons, one part antiprotons and two parts thermal neutrons. Also as pointed before, it's more likely that you'll get antiproton-neutron annihilation that antiproton-proton, simply because you have more neutrons.

[idobox]

Thanks for your knowledge.

Tass, I knew they used positrons for that. What is the advantage of anti-protons for this task?

I've read that antiprotons were not as good a fuel as positrons because a large part of the energy is lost as neutrinos. But if you make them react with heavy nuclei, a significant part of the energy should be converted to kinetic energy. no?

[legend]

Sure, but the kinetic energy includes the kinetic energy of the neutrinos. Since you can't absorb the neutrinos, you can't really reuse the energy for something useful, so it's "lost". If you really want to use antimatter as fuel, I think antihydrogen is still your best bet, simply because it's really difficult to store large amounts of charged particles in a reasonable amount of space.

[mfb]

Tass, I knew they used positrons for that. What is the advantage of anti-protons for this task?

You can shoot anti-protons from outside, which makes it easier to focus them on a spot where you want to have them. In addition, positron annihilation gives high-energetic gamma rays, which do not deposit most of their energy locally. Slow (<<c) nucleons from an antiproton/proton annihilation in an atom are much more effective.

A part of the energy goes to neutrinos and is lost, but if you compare "input electric power" to "power delivered to the cancer", antiprotons are worse than anything else by many orders of magnitude anyway, as their production and trapping is very inefficient.

[Diadem]

But you don't really care about 'input electric power'. You care about 'power delivered to the cancer' compared to 'power delivered to the rest of the body. Electricity cost is not one of the major problems with radiation theory.

[thoughtfully]

But you don't really care about 'input electric power'.

You care when it takes a major facility like CERN. The therapy is essentially subsidized by existing scientific funding. Someday, antimatter might be mass produced for spaceflight, but that's a long ways off. As far as I'm aware, CERN is the only place in the world that makes them now, since the tevatron got shut down. It's not like an MRI machine you can install at any major hospital.
http://lhc-machine-outreach.web.cern.ch ... mption.htm

[mfb]

The GSI and the planned FAIR facility should be quite good in producing and controlling antiprotons, too.
Regular ion therapy is quite expensive already, and carbon/oxygen/... atoms are easy to get, compared to antiprotons .

>> But you don't really care about 'input electric power'.
This is right, but you care about "total cost for the facility".
Well, my argument was just that "neutrinos remove a part of the energy" does not matter.

[idobox]

Sure, but the kinetic energy includes the kinetic energy of the neutrinos. Since you can't absorb the neutrinos, you can't really reuse the energy for something useful, so it's "lost". If you really want to use antimatter as fuel, I think antihydrogen is still your best bet, simply because it's really difficult to store large amounts of charged particles in a reasonable amount of space.

It seems to me that storing large amount of uncharged antimatter is even more difficult.

Well, my argument was just that "neutrinos remove a part of the energy" does not matter.

For radiation therapy, yes. For rocket fuel, it matters quite a lot.

You can shoot anti-protons from outside,

I can't find it on google, but I vaguely remember the CERN organising some sort of contest to decide who they would give an old particle accelerator, that an hospital won, and that they were using it to emit positrons with the right energy, so that they would disintegrate inside the tumor. I read that years ago, and don't remember the details, so true story might be quite different. Also a quick google search didn't give any results

[Tass]

Sure, but the kinetic energy includes the kinetic energy of the neutrinos. Since you can't absorb the neutrinos, you can't really reuse the energy for something useful, so it's "lost". If you really want to use antimatter as fuel, I think antihydrogen is still your best bet, simply because it's really difficult to store large amounts of charged particles in a reasonable amount of space.

It seems to me that storing large amount of uncharged antimatter is even more difficult.

Storing small amounts is easier when it is charged. You just use a storage ring. For the large amounts needed for a spaceship that would be completely impractical, it scales terribly. Tons of magnetic equipment for micrograms of antimatter.

Uncharged solid antimatter storage would scale much better. One big block magnetically levitated in a vacuum. Not easy to do however you turn it, but not much harder to do with a ton than with a gram. Especially not in space.

How to get it solid? I didn't say it was easy, but fortunately hydrogen does freeze in contrast to helium. I am not going to propose anti-graphite, storage would be easier, production - not so much.