Antimatter residual radiation Q

[bombNumber20]

I've written a SF story where a twenty million ton metallic asteroid is moved by antimatter propulsion. Apparently antimatter engines emit gamma rays. At the moment I'm planning to stick to pure antiproton annihilation 'cause it's easier to explain (so not a hybrid engine). For plot purposes (and because I've already written this in) I'm hoping a huge sustained gamma emission (i.e. annihilating the ludicrous amount of antimatter needed to propel such a large mass) might result in some residual contamination. I'm hoping the engine blast cones will become (at least slightly) radioactive. Could this happen?

 

[Joskoplas]

I'm struggling to answer this, because I'm having a hard time figuring out what sort of material could be used for the hypothetical engine bell. For 'pure' annihilation, the material has to be able to reflect gamma rays ... darn hard to do. So if you're going to posit some new technology that can produce and store such large quantities of antimatter, it's a short leap to say that there's also some strange substance that can do this. But speaking of strange: my guess might be material made from strangelets -- high mass, very dense, but possibly prone to make all sorts of trouble when promoted to excited internal states by gamma rays. I don't think anyone has a good enough idea of the properties of strange matter to argue against whatever you decide :)

 

[bombNumber20]

Thanks for your prompt answer Joskoplas.
It's my understanding that the blast cone need not reflect gamma emissions per se, simply redirect thrust. I'm hoping the gamma rays (or any another product resulting from an antimatter reaction) will interact to some extent with the blast cone/ engine materials. Since the reaction would be sustained any such effect could become significant over time.

 

[bombNumber20]

Okay I've done some more searching (would have been far more sensible to do this before I wrote it). I'm no physicist & the story's theme is resource depletion. Gamma rays are all you get? So much for an Orion like spacecraft . However I have found mention of "pions and other charged particles" resulting from antimatter reactions. Another thing I don't understand (so many things...) is why descriptions of antimatter bombs tend to sound like fusion/fission explosions. Perhaps I should go with antimatter induced fusion. Any help appreciated...

 

[Joskoplas]

Gamma rays are all you'd get from the non-relativistic collision of lighter particles, like electrons and positrons. Higher mass particles ... well, think if it this way. One of the guidelines in particle physics is that if it's not forbidden, it's mandatory. When a proton annihilates with an antiproton, there's a lot of energy suddenly available to make new particles. Now, gamma rays are a good option, but the energies are so high that it's unlikely you'll just see two Really Big Photons. Any pair of particles lighter than a pair of protons could be produced.

Increase the energy of the collision, and you even have the somewhat wacky result where a particle-antiparticle pair could collide and produce two particle-antiparticle pairs! (Of course, conservation of mass / energy still applies, so this is no way to get free antimatter fuel.)

I'm bowing out of this now, because if particle physics were a game of cards, you're just talking to the six of diamonds. Any minute now an Ace will come by and set us all straight ;-)

 

[Astronuc]

Anti-matter hydrogen would involve anti-protons and positrons. Larger nuclei would involve anti-neutrons in the nuclei.

In annhilation, positrons and electrons annihilate to produce two gamma rays of approximately 0.511 MeV each.

Anti-protons and protons annhilate into pions, which then decay to muons and neutrinos, which decay to electrons and neutrinos. Neutral pions sometimes decay to gammas.

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/hadron.html#c2
http://hyperphysics.phy-astr.gsu.edu/hbase/particles/lepton.html#c3


Refer to this for some history - http://inpa.lbl.gov/pbar/talks/F4_Goldhaber.pdf

http://science.nasa.gov/newhome/headlines/prop12apr99_1.htm

Some PSU papers
http://www.matter-antimatter.com/antimatter_propulsion.htm

 

[Vanadium 50]

First, it's your story - you can do whatever you want. You'll surely have to invent some new physics to get all this antimatter in the first place, so deciding to make the "nozzle" (to give it a name) radioactive or not is your choice.

Second, the thing that will (or will not) make the nozzle radioactive is not the prompt particle production. It's the interaction with these particles on the nuclei that make up the nozzle. You'll be chipping off pieces of nuclei (a process called spallation) and the nuclei that are left behind will likely be radiaoctive to some degree.

 

[bombNumber20]

Thanks very much for your replies Astronuc and Va 50 (and Joskoplas, of course).
To summarize (please correct me if I’m wrong):
Electrons + positrons -> gamma rays
Protons + antiprotons -> pions -> muons -> gamma rays
So no residual radiation. Spallation, as the name suggests, would lead to the creation of lighter elements from heavier ones. Also no light show, or heat for that matter, in a vacuum.
However gamma rays are absorbed by matter (chance of absorption is dependant on atomic number) the energy transferred to electrons which, in turn, transfer the energy to the rest of the mass, producing ionization, bond dissolution and eventual heating of the medium… thus an antimatter explosion within Earth’s atmosphere could appear similar to a nuclear bomb detonation (gamma ray emission would be extreme).
My story is in tatters but, I think, salvageable.
Apparently there’s a postulated engine (beamed core) which electromagnetically funnels the charged versions of the reaction intermediaries to generate thrust. Since the setting is the distant future I can use that design.

One thing I’m still confused about. Although the original design concept doesn’t mention this (http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/13840/1/00-0212.pdf ) according to (many) others “The spacecraft will have to be designed to be very long as the annihilation products travel close to the speed of light.” I don’t see why (unless the later pion -> muon -> gamma ray transitions generate further thrust).

Does anyone have any thoughts on this?

 

[bombNumber20]

Apologies Vanadium 50, it seems there’s more to spallation than my first enquiries showed. I’m guessing stray protons could chip neutrons from surrounding materials and deposit them elsewhere. As I’ve stated before I’m no physicist.

Still hoping for a reply to the ‘long spacecraft ’ question, any takers?

 

[mheslep]

First, it's your story - you can do whatever you want...

Well a rule of thumb for good SF is to make only one extra physical assumption; everything else should obey the rules or follow logically from the first assumption.