Secondary effects of an antimatter weapon

[Myasishchev]

Something I've been wondering about, here. We all know that the primary effects of an antimatter weapon would be similar but better than a nuclear weapon. But what about the secondary effects?

The secondary effects of fission and fission-fusion bombs are pretty well-understood, but what about antimatter? It seems to be often assumed that an antimatter weapon would be clean, compared to nuclear weapons, since its reactants would perforce be annihilated very soon, especially within an atmosphere. However, I have some specific questions about secondary effects:

Could a sufficiently large antimatter explosion on a planetary surface or in the atmosphere heat the sphere of expanding plasma quickly enough that would form around ground zero to undergo fusion? If so, would neutron flux and subsequent capture produce significant quantities of radioactive material?

Likewise, would gamma ray photodisintegration of matter nuclei contribute any significant radioactive byproducts?

 

[T'Girl]

I believe a matter-antimatter explosion does produce neutrons and other more exotic particles too. So the secondary effect might be simular to a neutron bomb, heavy short duration radiation at ground zero and in the fall-out. Depending on the altutude of the detonation, fall-out would spread down wind like with a regular nuclear bomb, but the irradiation of the fall-out material would be short lived.

I am unsure of any gamma-ray effects.

 

[Chaos Descending]

The primary result of matter/anti-matter annihilation is hard gamma radiation. In some cases you'd have neutrino/anti-neutrino products (or even more massive particles) as well, but still, immense amounts of gamma radiation.

There would be nothing "clean" about it at all.

 

[Christopher]

Could a sufficiently large antimatter explosion on a planetary surface or in the atmosphere heat the sphere of expanding plasma quickly enough that would form around ground zero to undergo fusion? If so, would neutron flux and subsequent capture produce significant quantities of radioactive material?

I doubt it would cause fusion if it were in the open atmosphere. Fusion requires not just heat but confinement. With just heat alone, I think the particles would be dispersed too quickly for any significant fusion to occur.

Likewise, would gamma ray photodisintegration of matter nuclei contribute any significant radioactive byproducts?

You mean transmutation? Yeah, I think there'd be some of that. Not sure how much, though.

 

[Myasishchev]

I mean clean in the sense that the ground cleared by an antimatter detonation would be shortly rehabitable. That is, not contaminated with lingering ionizing radiation sources. The gamma rays themselves are no long-term problem, though I guess you'd want to wait till the real estate resolidified first. Obviously, neutrinos are harmless. That's why I'm interested in whether an antimatter reaction of significant, "photon-torpedo" size, would create problems through other mechanisms: fusion-generated radiogenic products, and radiogenic isotopes created by neutron capture or photodisintegration or proton annihilation.

It occurred to me while I was thinking about an antimatter bomb going off in a reducing environment, in an atmosphere similar to Titan's, but let's say with more methane. Would some amount of monatomic or molecular oxygen be a result? It then occurred to me that if fusion could happen, other nuclear reactions might result, with longer-term consequences than a brief O2/CH4 firestorm.

Anyway, I believe that T'Girl's right about the neutrons--although I was sort of presuming a controlled matter/antimatter reaction with hydrogen and antihydrogen, a quantity of antiprotons simply released in an atmosphere would presumably blast apart any nucleus they came in contact with. I'm not 100% sure it would actually split these atoms, rather than just annihilate a proton, impart great momentum to the nucleus, and maybe spallate some parts of it, but in either event a plasma of Z<7 (sub-nitrogen) elements and a heavy neutron flux would be the result.

I also wonder how quickly an antimatter cloud released into an atmosphere would actually react, due to electric repulsion between the electron shells and the antiprotons. Would it be, in fact, effectively immediate? Or would there be a noticeable period of interaction? Might there be a delayed interaction anyway, since gamma ray production on the reaction surface would tend to separate the reactants?

 

[Myasishchev]

Could a sufficiently large antimatter explosion on a planetary surface or in the atmosphere heat the sphere of expanding plasma quickly enough that would form around ground zero to undergo fusion? If so, would neutron flux and subsequent capture produce significant quantities of radioactive material?

I doubt it would cause fusion if it were in the open atmosphere. Fusion requires not just heat but confinement. With just heat alone, I think the particles would be dispersed too quickly for any significant fusion to occur.

Yeah, I'm not sure it would either. I was suspecting that the compression by the propagating shockwave might be sufficient--they are in a supernova, if I understand correctly, although that's not at all necessarily an analogous situation, since stellar densities, even outside the core, aren't readily comparable to atmospheric ones.

The atmosphere of Venus (or the hydrosphere of Droplet or better yet GJ 1214 ) might respond differently, but even they aren't really comparable to stellar densities either.

You mean transmutation?

I like photodisintegration. It's such a cool word, and more specific (neutron or protons being knocked out of a nucleus or a nucleus fissioned entirely by a γ photon).

Yeah, I think there'd be some of that. Not sure how much, though.

Sweet.

 

[Christopher]

I also wonder how quickly an antimatter cloud released into an atmosphere would actually react, due to electric repulsion between the electron shells and the antiprotons. Would it be, in fact, effectively immediate? Or would there be a noticeable period of interaction? Might there be a delayed interaction anyway, since gamma ray production on the reaction surface would tend to separate the reactants?

Again, the problem is containment. A diffuse cloud of antimatter wouldn't be very destructive at all. If unconfined quantities of matter and antimatter begin to react, the heat and pressure from the initial annihilations will blow apart the other reactants and shut down the reaction. Remember, matter is mostly empty space, particularly when it's gaseous. The antiparticles would react with particles in the air over time, but they'd be spread out so diffusely by the initial aborted blast that they wouldn't do much damage. Basically you'd just get a small rise in the ambient radiation level for some distance around the initial blast site as the antiprotons gradually annihilated one by one.

Yeah, I'm not sure it would either. I was suspecting that the compression by the propagating shockwave might be sufficient--they are in a supernova, if I understand correctly, although that's not at all necessarily an analogous situation, since stellar densities, even outside the core, aren't readily comparable to atmospheric ones.

The atmosphere of Venus (or the hydrosphere of Droplet or better yet GJ 1214 ) might respond differently, but even they aren't really comparable to stellar densities either.

I'm basing this on what I learned a few years back about what would happen if you set off a fusion explosion in a gas giant's atmosphere. I wondered if the fusion reaction would propagate through the hydrogen, but it turned out it would all be dissipated pretty quickly. True, hydrogen gas is less dense than Earthlike air or water, but they're within an order of magnitude or so, and the effects would probably be the same.

Again, it's all about confinement. The hydrogen/helium/whatever in a stellar core is confined by the huge mass of stellar atmosphere pressing down on it from above. The atmosphere higher up, that doesn't have as much stuff pressing down on it, isn't undergoing fusion, because it's just not confined enough. Matter is mostly empty space, so you really have to cram particles together tightly to make them more likely to hit than miss each other. Especially since nuclei are all positively charged and thus tend to repel each other.

 

[Crazy Eddie]

I mean clean in the sense that the ground cleared by an antimatter detonation would be shortly rehabitable. That is, not contaminated with lingering ionizing radiation sources.

Well, it'll irradiate certain materials near the blast site and render THEM radioactive. That's part of the component of fallout, but it wouldn't be as bad as a nuclear device. Neutrons will have this effect too, transmuting some elements into radioactive isotopes that will take a while to decay... again, without vaporized bomb components floating around, less than a conventional nuke.

There's also the fact that antiparticles will only react with their corresponding particle. Neutral particles (hydrogen atoms, for example) don't usually come into contact with each other even in a gas; while the antiprotons would attract other protons, the positrons would repel them (+1 and -1 means zero electrostatic attraction). Positrons in these atoms would eventually attract any free electrons floating around in the environment, though, producing a free-floating antiproton that immediately seeks out another proton and annihilates it, destroying the atom it belongs to in something similar to a fission reaction.

So if we're talking about an antimatter detonation in an atmosphere, the result might be a "bang" and fireball, followed by a short lived glow/fizzle as any leftover antimatter annihilates in the atmosphere and gives off more gamma radiation and fast neutrons.

 

[Chaos Descending]

(+1 and -1 means zero electrostatic attraction).

Ummmmm...

Positrons in these atoms would eventually attract any free electrons floating around in the environment, though, producing a free-floating antiproton that immediately seeks out another proton and annihilates it, destroying the atom it belongs to in something similar to a fission reaction.

Positron + Electron DOES NOT = Anti-Proton

 

[Christopher]

I think he means that if you start out with an antihydrogen atom -- an antiproton orbited by a positron -- the positron would soon enough be annihilated by an electron and leave only a free antiproton.

As for the +1/-1 thing, I don't think that's right. Protons would repel positrons because they're both positive.

 

[Chaos Descending]

Ok, that makes a little more sense.

 

[Myasishchev]

Okay, some goofy math for consideration. Christopher, you have a physics background while mine is law, so if you say it's stupid, I'll believe you.

We begin with a 20,000kg hydrogen/antihydrogen bomb (it's big). We assume a 50% efficiency, due to charged pions becoming useless neutrinos and maybe some very minor reaction inefficiencies. So the 20,000kg annihilation reaction produces 10,000kg worth of usable yield:

E = 10,000 kg X (3X10^8m/s)^2 = 8.98 X 10^20 J in the form of gamma rays.

For simplicity, we assume that the reaction has taken place in a spherical container 100m across, with a surface area 31400m^2. Again for simplicity's sake, the reaction is totally spherical, with products streaming out in a spherical and homogenous fashion. This provides an energy/area ratio of 2.86 X 10^16 J/m^2.

Here's where things get tricky and I suspect I may have faltered somewhere down the line. Firstly, I'm unsure of the relationship between energy and pressure. I know how the derivations of Joules (Nm) and Pascals (N/m^2), but their interaction here may be iffy.

Since a Joule is the action of one Newton across one meter, in order to arrive at a pressure value in Pascals, I introduced a length dimension to the J/m^2, which makes common sense, at least, since the energy is going to be moving through three-dimensional space. I chose 1000m arbitrarily.

This gives:

2.86 X 10^16 J/m2
over
1 X 10^3m

or 2.86 X 10^13 J/m^3. As a Joule is one Newton-meter, this simplifies to 2.86 X 10^13 N/m^2.

However, since the explosion is spherical in nature, it spreads. Any square meter from the initial spherical volume will become a conical section within the expanding sphere. I believe--I stress that I believe, but do not know--this can be corrected by multiplying the above pressure figure by the ratio between the surface area of the original sphere of 100m diameter and the surface area of the new sphere of 1100m diameter. This ratio is, unless I catastrophically fucked something up, 1/121.

This provides a new pressure figure for the edge of the expanded sphere of 2.36 X 10^11 N/m^2. Or, in other words, 236 billion Pascals.

This yields a value that can be plugged into the ideal gas formula in order to figure out a temperature:

temperature = pressure/ density X specific heat capacity

I chose to use the specific heat capacity of nitrogen (.297 Joule/kilograms X Kelvin) and assume a starting density of 1.2kg/m^3, not coincidentally the density of our atmosphere at sea level. Since it is reasonable to assume that by this point the blast has pushed the entire contents of the sphere onto its edge, I reckon that the mass of air on the interface is likely equal to the mass in the conical section of the sphere. This would push up the density a lot more--for ease of calculation, I'll just say the mass in the 1m^3 section of the interface we care about is equal to the 1000m^3 between it and the antimatter. Hence, 1200kg/m^3.

So, finally, we throw all that into the ideal gas formula, which, incidentally, I'm not at all sure is appropriate:

2.36 X 10^11 Pa/(1200kg/m^3 X .297 J/kg X K) =
6.62 X 10^8 K. Or 662 million Kelvin.

This is 40ish times greater than the temp in the sun's core, but the density may not be enough to give a good cross section for a fusion reaction.