An anti-helium balloon

Factual Questions
1

Say I have a balloon…well, not a balloon, but some kind of magnetic trap containing several grams of anti-helium gas. That’s right, 2 anti-protons, 2 anti-neutrons in the nucleus, two positrons orbiting around, and two such anti-atoms bound together just like regular Helium gas. I want to know what happens as I release it into the atmosphere.

Helium is very inert. The electrons are strongly bound. How about the positrons in anti-helium? Are they going to be attracted more to the electrons of nearby nitrogen and oxygen or to their own anti-nucleus?

I can imagine that even if the positrons of anti-helium quickly annihilate it might take longer for the anti-alpha particle nucleus to meet a similar fate. It has a negative charge, so the electrons around normal matter atoms will repel it. But this is just speculation on my part. Can a real physicist give the play-by-play of how this stuff would go boom?

If it makes it more fun you can tell me what supercold liquid anti-helium would dl.

2

I think you’re looking at an Earth shattering kaboom here. I don’t think the fact that Helium is chemically inert is gonna make any difference. Where are all the physicists?

3

IANAPhysicist, but I tend to agree with Small Clanger - the positively-charged entities of your anti-helium atoms are on the outside, where they can readily interact with the negatively charged entities (electrons) of conventional matter.

4

Atoms (or electrons, for that matter) are not tiny classical balls. Atoms “bounce off each other” due to mutual repulsion of their orbiting electrons, generally at much greater distances than we imagine. The orbiting positrons (anti-electrons) of anti-helium would be attracted to the orbiting electrons of any nearby “normal” atom with a ferocity unseen since the Sadie Hawkins Dance. Since nothing prevents them from “coming into contact” (substantially overlapping their waveforms), the positron-electron attraction between interacting atoms and anti-atoms would vastly overwhelm their attraction to their respective nuclei, which are a substantial distance from the orbitals, for quantum reason (good thing - or atoms wouldn’t be stable).

The orbiting positrons (and electrons) would strip from their nuclei almost instantly, and mutually annihilate (convert to a pair of 511 keV gamma rays). The naked nuclei, hampered by their greater inertia, would lag slightly behind behind, because they don’t have quite as favorable a charge/mass ratio - the attractive force ‘felt’ by a Helium nucleus (2n+2p) is only twice that of an electron, but it has almost 8000 times the inertia to overcome. Of course, when it does finally annihilate a micro-jiffy later, it will release 8000 times the energy, in gamma rays that are individually 2000 times as powerful as the ones the electrons gave off.

Several grams of antimatter, regardless of element, is effectively a small nuke. The trickiest part of building an A-bomb are containment and confinement - you need to keep the bomb together long enough for the chain reaction to proceed far enough for a good yield. (A critical mass of an isotope, sitting on a table, will only undergo enough chain reaction to vaporize in a brief messy burst of raditation and vaporized istope that falls far short of an atom bomb.)

The powerful charge attraction anti-particle pairs overcomes that problem easily, and -more importantly- antimatter doesn’t have to be confined to “detonate”. Since antimatter reacts directly and instantly, rather than “positive feedback” chain reaction, it doesn’t matter if it gets scattered by the blast of the first atoms, it will still react with normal matter 100% within some tiny fraction of a second.

Only a fraction of an atom bomb undergoes rapid fission, and only a tiny fraction of the fission products are pure energy, but with antimatter, 100% converts to pure energy. The energy yield is actually 200%, since an equivalent mass of oridinary matter in the air (which we often ignore) will also be converted to energy. [It’s like jets vs. rockets: jets don’t count the air around them as ‘fuel’ but rockets must being their own oxidizer. If you’re doing particle experiments or envisioning a space battle, don’t forget the matter side of the equation.]

A kiloton is 4.2 x 10^12 J. 1 gram of antimatter reacts with one gram of matter to produce 1.8 x 10^14 J (42.85 kT). A 1 megaton bomb is the equivalent of allowing 23.35 g (.82 oz) of antimatter to react with matter.

Don’t do this at home, kiddies

5

Just a nitpick… helium atoms don’t join together to make molecules. Helium gas is composed of individual, unattached atoms floating around.

So while oxygen is O[sub]2[/sub], and nitrogen is N[sub]2[/sub], helium is just He.

6

Ok, I can readily believe that the positrons around the anti-helium will quickly annihilate. However, the remaining nucleus is negatively charged, and it is nearer negative things than positive ones. Plus it is moving slowly (compared to the run-of-the-mill antiproton created an a collision). Thus it seems to me that the anti-He nucleus might take a relatively long time to annihilate. However, I don’t know quantum mechanics. Will the anti-alpha just push aside all electrons as it is attracted to a normal nucleus?

7

You forgot about the newly exposed positively charged nucleus from the normal matter helium atom. So while you are correct that the negatively charged nucleus would repel any other atoms in the vicinity, it doesn’t matter.

8

That would be true if the anti-helium atom actually encountered a normal helium atom (and there are trace amounts of helium in the air) but on the assumption that the anti-helium is encountering mostly nitrogen and oxygen molecules, things get a little tricky.

Annhilating 2 of the outer electrons from an N[sub]2[/sub] or O[sub]2[/sub] molecule will produce a net positive charge, so there shouldn’t be any repulsion between the antihelium nucleus and the ionized nitrogen or oxygen molecule. But, there’s more to it than that - if the force of the explosion overcomes the electrostatic attraction and causes them to fly apart, then the antihelium nucleus would probably eventually collide with a different ion.

9

I mostly agree with KP’s assessment, but I would add that the “explodes as it goes” aspect of the matter-antimatter reaction in an atmosphere in some ways resembles the chemical detonation principle of fuel-air and thermobaric explosives. An antimatter blast in an atmosphere might therefore technically be slightly “slower” than a conventional nuclear detonation, but have a blast effect greater than a nuclear weapon of equivalent yield. :eek: