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Air Force Researching Antimatter Weapons 1062

Posted by CmdrTaco
from the build-a-bigger-gun dept.
mlmitton writes "The San Francisco Chronicle is reporting that the Air Force is actively pursuing antimatter weapons. Such weapons would easy eclipse nuclear weapons in power, e.g., 1 gram of antimatter would equal 23 space shuttle fuel tanks of energy. Perhaps more interesting, after an initial inquiry by the Chronicle in the summer, the Air Force issued a gag order that prohibits any Air Force employee from discussing antimatter research or funding."
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Air Force Researching Antimatter Weapons

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  • by wowbagger (69688) on Monday October 04, 2004 @04:05PM (#10432658) Homepage Journal
    1948 units, 71 prefixes, 28 functions

    You have: grams*c^2
    You want: tonnes-tnt
    * 19487.022
    / 5.1316205e-05

    So 1 gram antimatter + 1 gram matter is about 39 kT of TNT. Hiroshima was about 20 kT, Nagasaki was 13 kT, so one gram antimatter would release just a scosh more than both devices.

    So let us use a bit more sensible units than "shuttle fuel tanks".

    However, the costs of manufacturing the antimatter, and the size of the containment system, and the fail-null mode of antimatter vs. the fail-safe mode of a nuke (a nuke may leak, but it will not detonate without everything going just right), would lead me to wonder about the utility of an antimatter weapon.
  • by sczimme (603413) on Monday October 04, 2004 @04:07PM (#10432687)

    1 gram of antimatter would equal 23 space shuttle fuel tanks of energy.

    I thought the standard unit of explosive power was the ton of dynamite...

    Perhaps more interesting, after an initial inquiry by the Chronicle in the summer, the Air Force issued a gag order that prohibits any Air Force employee from discussing antimatter research or funding

    This isn't really that interesting or even unusual: Uncle Sam frequently limits what military folks can say about ongoing projects. There is a classification called "Sensitive But Unclassified", or SBU, whcih means the info is not classified as such (Secret, TS, etc.) but it is still not for public disclosure. (Years ago SBU was called "For Official Use Only" or FOUO.) Budgets are generally considered at least SBU, so it should be no suprise that the budget is not publicized.

    /spent six years in the Air Force
  • by koreth (409849) on Monday October 04, 2004 @04:11PM (#10432760)
    As energy storage, maybe. But right now it takes millions of times more energy [] to produce a unit of antimatter than you get by annihilating that unit afterwards.
  • by Trespass (225077) on Monday October 04, 2004 @04:11PM (#10432766) Homepage
    The most common sci-fi containment system is holding the antimatter in a vacuum while suspending it in a powerful magnetic field to keep it from contacting the walls of vessel holding it. I understand something similiar is done with plasma in experimental fusion reactors. It doesn't sound very portable.
  • by harrkev (623093) <> on Monday October 04, 2004 @04:11PM (#10432769) Homepage
    Anit-matter does not produce anti-gravity. Experiments have confirmed this.
  • Re:Energy Conversion (Score:5, Informative)

    by MoonBuggy (611105) on Monday October 04, 2004 @04:13PM (#10432802) Journal
    I present to you the Antimatter Calculator [] - it actually releases less energy than I would have thought since an entire kilo is slightly less powerful than the most powerful nuke ever detonated (although still a helluva lot of power, ~40 kilotons/gram)
  • by abcho (320284) on Monday October 04, 2004 @04:13PM (#10432804) Homepage

    For a balanced view, it is important to realize that anti-matter physics have yielded substantial medical and non-military benefits already. Many people probably already encountered various applications of this technology without realizing it.

    For example, Positron Emission Tomography (PET) is a very useful clinical and medical research tool for brain and cardiac functional imaging. See: Positron Emission Tomography []

  • Re:Energy Conversion (Score:5, Informative)

    by abb3w (696381) on Monday October 04, 2004 @04:13PM (#10432808) Journal
    1 kg antimatter mixed with equal matter yields about 42 megatons, half from matter conversion to energy, half from antimatter conversion to energy. The energy would initially be in the form of gamma rays at the neighborhoods of .5 and 900 MeV, but the latter would self-scatter (? correct term ?) due to electron positron emission/annihilation, and head down REAL fast towards the .5 MeV.

    Still a hell of a chest X-ray to give the planet.

  • by peculiarmethod (301094) on Monday October 04, 2004 @04:15PM (#10432848) Journal
    You would store it with magnetic fields, presumably. We know a lot about those. Pretty good at making them efficiently now, as well. Radiation? We're pretty good at testing radiation safely as well, but I suspect we'll just use it on some poor unsuspecting country first. Measure later. If you get a warning that we're under attack by the same type of device, just duck and cover like Tommy the Turtle.

    Anti-matter can't touch matter.. but you can build anti-matter containments.. even devices, but I suspect we are very far from this, as it takes a lot of anti-matter to create something on such a large scale. We'll probably use a form of particle trickery, directing the resulting anti-matter towards matter. Viola. Weapon.
  • Re:Really... (Score:4, Informative)

    by Mysticalfruit (533341) on Monday October 04, 2004 @04:16PM (#10432862) Journal
    Here's the thing.

    We've got the bomb. In fact we've got nuclear submarines so fucking quiet you wouldn't even know their in your harbor just chuck full of the little bastards. However, much like spitting into the wind, using said weapons means we get to glow in the dark as well.

    Anti-matter weapons don't have this spit in face problem. We could drop a anti-matter bomb on Iran and flatten the whole country to within an inch of sea level and nobody is going to be dying of cancer from the nuclear fallout.

    It's like the bomb, only much better.
  • by pz (113803) on Monday October 04, 2004 @04:19PM (#10432912) Journal
    Uh, basic physics, people. The Universe is comprised of matter, not anti-matter. You can make anti-matter, but it takes a heapload of energy (recall that E=mc^2 applies to anything that has mass), and you cannot go out and mine anti-matter. Why? Mostly because if there were any antimatter around, it would have a nasty tendency to interact with all that matter and be converted to energy.

    So, you can use it to create a nice bomb, but it's equivalent to pumping up a pressurized bottle with a lot of air -- the only energy that's going to come out is the energy that you've put in to create the anti-matter. You make some anti-matter, find a way to confine it and later release it in a controlled fasion and you get a very nice bomb which is incredibly powerful given the mass of the active ingredients. But you cannot use it as an energy source because unlike coal, oil, natural gas and uranium, it isn't freely available: you have to make it.

    This is in stark contrast with nuclear fusion and fission: there is lots of available material lying around in the ground and in the seas, just waiting to be extracted and used. While you can find ways of generating anti-matter without putting too much energy into the process (eg, by triggering nuclear decay) you just don't get that much mass very quickly. Unless, of course, you've got a right raging nuclear reaction going, and, then, well, your problems of bomb making are pretty well solved.
  • by egomaniac (105476) on Monday October 04, 2004 @04:32PM (#10433108) Homepage
    Would you have to store the anti-matter, or create it as you need it? The first seems impossible, unless you has some kind of containment where the anti-matter doesn't actually touch anything. The other requires a massive amount of energy. Is this even plausible?

    The only mechanisms we know of to create antimatter are UNBELIEVABLY power-hungry. The technology to manufacture even a mere gram of antimatter does not exist. So, the answer to your question -- we really have no idea. We can't manufacture meaningful amounts of antimatter at all, so the question of when it would get manufactured is something of a moot point.

    What about the radiation involved? We've measured the rays that result from minor, single-atom collisions, but what happens when the collision is actually big enough to damage something?

    IANANP (I am not a nuclear physicist), but I don't believe it would be significant. Nuclear weapons have two major sources of residual radiation (fallout): fission byproducts and induced radioactivity caused by neutron bombardment. Antimatter bombs wouldn't produce either. The radiation produced by a matter-antimatter reaction is high-energy gamma rays -- the explosion's extreme energy levels would probably manage to split or fuse a few atoms, and probably create very small amounts of radioactive material, but without fission byproducts or neutron flux you shouldn't see any large-scale radioactivity. The explosion would essentially look and behave just like a nuclear explosion (thermal pulse, mushroom cloud, shock wave, etc.) but without the fallout.

    How do you propel something like this? Magnets? Or am I wrong in assuming anti-matter can't touch anything?

    You are correct -- matter-antimatter collisions are bad news, and you can't allow the antimatter to touch any matter until the desired moment of explosion. Fortunately, antiprotons and antielectrons (positrons) are both electrically charged, and can therefore be magnetically contained in a vacuum to keep them from contacting any matter. A (very simple and dangerous) bomb design might be as simple as a containment shell with antimatter inside. You drop it on the target, the bomb ruptures and releases antimatter, BOOM.

    The real problem is that the failure mode of antimatter weapons (at least ones that relied on pre-manufactured antimatter) is so damned dangerous. If the circuitry in a nuclear weapon fails, no biggie -- the bomb just doesn't detonate. Even in the worst case all that happens with a nuke is leakage of radioactive material. In fact, even an accidental critical mass isn't enough to produce a large-scale explosion -- unless you contain everything just right it just doesn't give you a big blast.

    With an antimatter bomb, the opposite is true. You have to contain everything just right, because the second you don't, BOOM.
  • by sexylicious (679192) on Monday October 04, 2004 @04:51PM (#10433373)
    USAF is still FOUO. The other levels are the same. There is also an unofficial classification that is used called sensitive. Basically, anything that reveals personal info is sensitive and treated as FOUO, even if it's not marked that way.
  • Re:Energy Conversion (Score:1, Informative)

    by Anonymous Coward on Monday October 04, 2004 @04:55PM (#10433443)
    Antimatter isn't likely to be low cost, efficient or safe for a long time. There is no cheap, abundant source of antimatter available to us. Antimatter has to be created, typically through radioactive decay of certain isotopes to positrons. You can also create antimatter with high energy collisions in cyclotrons. A small percentage of the outcome of collisions will be antimatter. You can only obtain a few particles of antimatter at a time this way and the cost is enormous. Once you have antimatter, you need to contain it, which is also very expensive. Finally, when you want to use your antimatter to get power you have to annihilate it with normal matter which produces gamma rays. Unlike visible light, most materials are transparent to gamma rays so trying to convert gamma rays to usable energy (electricity) will not be a very efficient process. You can think of visible light as a wiffle ball, its limited in energy but its pretty easy to transfer it. Gamma rays are more like bullets, lots of energy but they won't bounce off of you.
  • Actually (Score:2, Informative)

    by Genady (27988) <[] [at] []> on Monday October 04, 2004 @04:57PM (#10433478)
    you cannot go out and mine anti-matter. Why? Mostly because if there were any antimatter around, it would have a nasty tendency to interact with all that matter and be converted to energy.

    Ummmm... actually... there's plenty of anti-matter around. It's everywhere actually, poping into an out of existance all the time on a quantum scale. The tricky part is botteling it before it annihalates with the virtual matter particle that was spontaneously created with it.

    Still, I bet you could get more bang out of evaporating quantum black holes. You just need a Tevatron to make them.
  • Re:Energy Conversion (Score:1, Informative)

    by Anonymous Coward on Monday October 04, 2004 @05:03PM (#10433555)
    Because, barring the possibility that you locate a large natural source of antimatter, you'll always expend far more energy making it than you will get back by using it.
  • by NichG (62224) on Monday October 04, 2004 @05:03PM (#10433556)
    No neutrons. Matter and antimatter are opposite in all of the conserved (well, jointly conserved in some cases) quantities except mass-energy and spin, so all that you're left with its a pair of photons that have the net energy, momentum, and angular momentum of the original particles. To get neutrons you'd need to have a net baryon number, which is positive for 'normal' matter and negative for antimatter. Neutrons have positive baryon number and have an antiparticle version (antineutrons).

    You could of course get more complicated results, but you'd need to put in extra energy to start with so that you can create other particle/antiparticle pairs (since all the numbers have to balance out in the end), and since we're usually talking about something like electron-positron collisions, there's not much out there with lower mass-energy to be formed.

    The perfect annihilation is why anything involving antimatter is so attractive from an energetic point of view: its a 100% efficient (since we're talking about individual annihilations, not an ensemble of them with someone trying to extract energy from the whole mess, we're not violating any thermodynamic laws) process that converts one form of energy (that is, the mass-energy of the particle-antiparticle pair) to another (the resultant photons). And it has no byproducts.

    Of course, you could never use it as an energy 'generation' scheme since there's not really any antimatter out there to go and harvest thanks to that weird breaking of the symmetry between matter and antimatter. There are processes that do not have that symmetry (i.e. CP violation, ...), but to the best of my recollection they've been shown to be too weak to produce the current state of the universe (i.e. the particular ratios of matter/energy we have). At best it'd be an extremely efficient form of storage (a major leap from those hydrogen fuel-cells).

    As a consequence, it's also the classic bomb-like weapon: 'pack a lot of metastable or unstable energy into something and drop it on your target'.

    However, given the difficulty involved in making macroscopic quantities of this stuff, we could probably make a couple of those carbon nanotube space ladders with the budget to build a single significantly-sized bomb.
  • by blueg3 (192743) on Monday October 04, 2004 @05:04PM (#10433578)
    Last I checked, and feel free to correct me, antimatter is not radioactive. Antiparticles are viable ground-state particles that do not spontenously decay, which is what radioactivity is.

    Or perhaps you're thinking that antimatter would be an energy weapon, much like a thermonuclear device, that liberates large amounts of electromagnetic radiation.

    Fortunately, the classic problems with radioactive materials -- particularly, long-term storage and environmental effects of their byproducts (whether in cannisters or in the form of fallout) -- should not exist with antimatter weapons.
  • by weedenbc (719416) on Monday October 04, 2004 @05:06PM (#10433592)
    Anti-matter weapons are radioactive in the same way as neutron bombs - a burst of gamma radiation. But they are NOT like fission bombs in releasing radioactive particles.

    In a fission reaction the fallout comes from two sources. The first is the by-products of the fission reaction. I believe it is radioactive isotopes of Cesium and Potassium. This radioactive particles combine with the uranium/plutonim that did not fission and get distributed as fallout.

    A pure fusion bomb, e.g. neutron bomb, has only a fusion reaction and thus theoretically produces no radioactive fallout. However in practice a fission reaction is used to create the pressure and heat needed to start the fusion reaction.

    See the Special Weapons Primer at [] for more info.

  • slight correction... (Score:5, Informative)

    by Dr. Zowie (109983) < minus city> on Monday October 04, 2004 @05:07PM (#10433610)
    whoops -- I mistyped the comparison with the Sun. That should read "The surface of the golf ball would appear 10^11 times brighter than sunlight". The surface of the golf ball would "only" be 2 million times brighter than the surface of the Sun.
  • Re:Energy Conversion (Score:4, Informative)

    by Dyolf Knip (165446) on Monday October 04, 2004 @05:07PM (#10433622) Homepage
    For those interested in doing the math...

    A kilo of antimatter reacting with a kilo of matter releases 2kg x (300,000,000 m/s)^2 = 1.8e17 Joules. The specific combustion energy of TNT is 4.6e6 J/kg, hence 1 kt TNT = 4.6e12 J, 1 Mt TNT = 4.6e15 J. Therefore 1.8e17 J / 4.6e15 J ~= 40 MT of TNT.

    You get about 70 times as much energy as you would from fusing 2kg of hydrogen into helium. But then fusion is a viable power source, whereas antimatter is at best a battery. Unless we find a way to make antimatter without having to make matter as well; then the reaction would be a net gain in usable energy (at the expense of matter in the universe, of course)

  • by Anonymous Coward on Monday October 04, 2004 @05:12PM (#10433685)
    You are both correct and way off base. The 5.56, while technically not as powerful as the 7.62 round has one distinct advantage over it's larger competitor. The 5.56 round "tumbles" and shreds on impact, thus causing a considerable amount more damage to the target. A 7.76 round at medium range will, if it misses a bone, pass right through the target leaving them with a good chance of living via a clean wound.

    However, someone shot with a tumbling 5.56 will be more likely to sustain tremendous internal injuries that will require immediate medical attention or otherwise result in death.

    The 5.56 is smaller, but much much nastier.
  • Re:Energy Conversion (Score:5, Informative)

    by SeanTobin (138474) * < minus pi> on Monday October 04, 2004 @05:14PM (#10433706)
    You had to ask didn't you? Well, I asked google how many burning libraries of congress(es?) in one gram of antimatter... And google was stumped :(

    So, here we go... 1 gram of antimatter -> burning libraries of congress(es?):

    For the sake of argument, lets assume that the Library of Congress is entirely non-flamable and only the books contribute to the heat. Furthermore, lets assume that all the books are made of 100% wood or equivilant.

    Now, 1 gram of wood when completely burned produces 3000 calories [].

    The Library of Congress contains approximately 128 million [] items. Again, some of these are recordings of various natures and will not burn as well as books... so to compensate we'll deviate from our initial assumptions and assume that the burning of the 530 miles [] of bookshelves compensate for any lack of flamability of the old records.

    So... our average paperback weighs under 1lb [] and our average hardcover book weighs between 1 and 2lbs []. Seems reasonable enough. Lets assume a distribution between hardcover and paperbacks so as the average book weight in the LOC is 1lb.

    Now, Google can help us some more here. Our friendly search engine lets us know that one pound is 453.59237 grams []. We'll round that off to 453 grams, since we're averaging book weight anyway.

    So, the LOC has (453*128,000,000) or 57,984,000,000 grams worth of books. At 3000 calories per gram, burning down the LOC would produce 173,952,000,000,000 calories of energy. For the sake of sanity, lets convert that to joules. Google says that 173 952 000 000 000 calories = 7.27815168 × 10^14 Joules []

    Now, our space shuttle main tank (and engines, NOT including boosters which are more powerful) produce 1,987,500,000 Watts [] of energy, and burn for 8.5 minutes []. That's (510*1,987,500,000) 1013625000000 Watt/seconds of energy. Converted to joules, that is remarkably 1013625000000 Joules [].

    So.. One space shuttle fuel tank of energy is 1013625000000 Joules. 23 space shuttle tanks of energy is 23313375000000 Joules. For convienence, one space shuttle tank is 0.23313375x10^14 joules.

    So... it comes down to one burning LOC is 7.27815168 × 10^14 joules. 23 space shuttle fuel tanks are 0.23313375*10^14 joules. So, one gram of antimatter combining with one gram of matter is approximately 0.032 Burning Libraries of Congress(es?). I actually expected it to be more.

    Now how do I get Google to include space shuttle fuel tanks and burning libraries of Congress(es?) as acceptable measurements?

  • by kc_cyrus (759211) on Monday October 04, 2004 @05:31PM (#10433870)
    Actually, antimatter does not make good bombs. Even more ordinary nuclear bombs can "fizzle" unless carefully designed: the reaction gets going but too slowly, so the bomb blows itself apart before the reaction can proceed very far.

    With antimatter this problem is far worse, because while fission and fusion occur throughout the reaction volume, the matter-antimatter reaction occurs only on a contact surface.

    It's exceedingly difficult to get a major explosion with antimatter.(Tiny ones are not hard, since the square-cube law gives you more surface area per volume as the scale shrinks.)

    Also, with production technology we can reasonably foresee, antimatter is impossibly expensive for weapons applications.
    Even the US military has finite budgets. The cost of burning a city down with conventional weapons is large but not infinite. We won't get the price down below US$ 60.e6/mg using foreseeable Earth-based technologies and, at 43 kT/gm of antimatter, we're talking roughly US$ 1.4e9 per kiloton !!!!!!!!! Even the Pentagon's budget isn't THAT large...

  • by qeveren (318805) on Monday October 04, 2004 @05:32PM (#10433890)
    Not quite true. The neutron bomb was actually developed as an anti-armor technology, because tanks and their ilk are very resilient when it comes to 'normal' nuclear blasts. The fast neutron radiation from neutron bombs is excellent at penetrating armor and killing the crews of such vehicles. Unfortunately, the blast radius is roughly equivalent to the lethal radiation radius, so infrastructure still tends to be creamed by the blast. :)
  • by hchaos (683337) on Monday October 04, 2004 @05:35PM (#10433916)
    Actually, making antimatter *can* be a net plus in energy. Synthesizing the antimatter out of thin air takes MC^2 energy, but reacting it with matter releases 2*MC^2 -- you don't need to synthesize the regular matter, but you still get the energy from reacting it!
    To my knowledge, it is theorectically impossible to synthesize antimatter without creating an equal amount of matter.
  • by BlueUnderwear (73957) on Monday October 04, 2004 @05:36PM (#10433930)
    Last I checked, and feel free to correct me, antimatter is not radioactive. Antiparticles are viable ground-state particles that do not spontenously decay, which is what radioactivity is.

    True, antimatter alone is as stable as normal matter. However, problems arise if you bring anti- and normal matter together. And in our world made up of normal matter, this is almost unavoidable unless some elaborate containment devices are used...

  • by Durandal64 (658649) on Monday October 04, 2004 @05:39PM (#10433958)
    A particle of anti-matter colliding with its matter counterpart will produce an annihilation of 100% efficiency. And yes, there will be resulting gamma-ray photons. But this reaction will not produce radioactive materials, like a nuclear fission reaction would.

    And the article didn't mention the chief problem with storing anti-matter. You can't allow it to touch anything. At all. It has to be in a vacuum container and make no contact with the edges. Otherwise, you'll get an explosion.
  • by Anonymous Coward on Monday October 04, 2004 @05:47PM (#10434026)
    Don't look if you ate recently.
  • by jfdawes (254678) on Monday October 04, 2004 @05:59PM (#10434131)
    Or course, the following quote is from the article that only some people who click the link actually get:

    Another problem is the terribly unruly behavior of positrons whenever physicists try to corral them into a special container. Inside these containers, known as Penning traps, magnetic fields prevent the antiparticles from contacting the material wall of the container -- lest they annihilate on contact. Unfortunately, because like-charged particles repel each other, the positrons push each other apart and quickly squirt out of the trap.

    If positrons can't be stored for long periods, they're as useless to the military as an armored personnel carrier without a gas tank. So Edwards is funding investigations of ways to make positrons last longer in storage.
  • by Blethrow (208830) on Monday October 04, 2004 @06:13PM (#10434246)
    A neutron bomb will also generate a ton of local radioactivity by neutron capture activation of nuclei in the immediate environment. Most of this will be pretty short lived.
  • by exp(pi*sqrt(163)) (613870) on Monday October 04, 2004 @06:16PM (#10434276) Journal
    Matter-anitmatter is in fact quite effective. If such a weapon accidentally blows itself apart it still continues to function because antimatter will react with any conventiently available matter, whereas if you misfire a conventional nuclear weapon then you can end up with bits of relatively inert plutonium sitting around.
  • Re:Energy Conversion (Score:3, Informative)

    by kesuki (321456) on Monday October 04, 2004 @06:29PM (#10434373) Journal
    what the calulation doesn't take into consideration, is the massive massive flood of gamma radiation released by an antimatter matter conversion. the reason that it it ignored, is because we have no current technology capable of using the massive amount of gamma radiation released... oh hey and as for laws of conservation someone brought up... technically the matter 'destroyed' still exists... as energy and gamma radiation... it has neither been destroyed nor violated any laws of conservation. However reconstituting gamma radiation into matter is far beyond our technology level. but converting matter into energy and gamma radiation is just barely within our capabilities.

    Do we need any more proof this is a type 13 planet in it's final stages? I think not ;)
  • Re:Plutonium (Score:5, Informative)

    by Muhammar (659468) on Monday October 04, 2004 @06:54PM (#10434605)
    Here you mixed details from 2 related stories: Daghlian accident during the war and Slotin accident after the war. The accidents happened with the same plutonium sphere. Slotin was boss of Daghlian and saw him dying. He had very similar accident and died in the same hospital room 9 months later. This Pu sphere was stored for safety reasons as 2 separate hemispheres and these were put together before experiment. The accidents were caused not by combining the Pu hemispheres but by surrounding them with neutron reflector which turned the system critical.

    Daghlian was trying to find the practical (=just barely subcritical) arrangement of cube of tamper material (tungsten carbide) which would be completely surrounding a solid 6.2 kg sphere of delta-phase of Pu239. The carbide bricks functioned as neutron reflector also. Daghlian was working very slowly as he was getting close to critical configuration (neutron reflection increased reactivity). One of the heavy bricks felt out of his hand - on top of the Pu sphere and the system went critical. Daghlian trew the brick quickly away and disasembled the system into more strable configuration, etc. He got just above letal dose so he was dying very slowly.

    Slotin was demonstrating for his colleagues reactivity of Pu depending on reflection of neutrons from berylium cover (Be holow hemispheric cover surrounding Pu sphere which was sitting half-embeded within another large Be hemispheric stand). The Be cover slipped, enclosed the Pu sphere, the system went critical, there was flash, Slotin took it apart with his bare hands (to save his colleagues) and got huge dose which killed him few days later.
  • by blueg3 (192743) on Monday October 04, 2004 @07:02PM (#10434661)
    Sure, but that just makes it volatile, not radioactive.
  • by Anonymous Coward on Monday October 04, 2004 @07:56PM (#10435105)
    But you make a good point- and if there's *any* supra-uranium metals nearby, the gamma ray could trigger fission.
    Electron-positron annihilation releases a pair of 511 keV photons. Medical x-ray photons are typically in the range 15-100 keV. So the radiation isn't terribly exotic. It might have some potential for directly stimulating nuclear reactions, but I wouldn't expect it to be a major concern, especially with the energetic stuff like uranium and thorium.

    The thing to worry about is fusion bombs. In the classic designs, the trigger is a fission bomb, which makes it difficult and expensive. With antimatter for the trigger, a fusion bomb would become fairly easy. You might be able to build it without any radioactives at all, which would be really scary, although less clever designers should probably design it around a slightly-enriched uranium spark plug to guarantee ignition.

    Here's a question to ponder: How much antimatter take, and how deep in does it need to be delivered, to ignite thermonuclear burning in the surface of a star?

  • by Xilman (191715) on Monday October 04, 2004 @08:32PM (#10435357) Homepage Journal
    Yes you can conserve momentum with a single photon except in head on collison. It is true however that there are always two photon.

    Consider that the electron is moving in south east direction and positron in north east. When they collide their north - south momentum will cancel out. Now the only remaining momentum is in the east direction. This momentum will be the momentum of the photon moving in the east direction. The reason the momentum cant be conserved during head on collison is that the resultant momentum has to be zero and single photon will have a mometum.

    Errm, no you can't.

    Einstein told us that as far as the laws of physics are concerned, one reference frame is as good as another. Now imagine you are sitting near to the the center of mass of the electron and positron and moving along with it. In your frame you see the two particles moving towards a head-on collision and with no net momentum. After the collision you will see at least one photon (by conservation of mass-energy) with zero net momentum (by conservation of momentum). The only way to get net zero momentum is by having at least two photons, travelling in suitable directions, since every photon carries momentum.

    If you bring conservation of angular momentum into play, there is another wrinkle. The electron and positron each have a half-unit of intrinsic angular momentum, called "spin" for brevity. A photon has unit spin. So the two initial particles can have either net zero spin (the two angular momenta are equal, opposite and cancel) or net unit spin. After the collision, the photons each have unit spin and so the net spin is either zero or two, depending on whether the spins are opposite or aligned respectively.

    Spot the problem?

    If the initial total spin of the two particles is unity, three photons have to be created to ensure that all the conservation laws are satisfied.

    In the center of mass frame of reference, the three photons have equal momenta (and so have equal energy and are directed at an angle of 120 degrees from each other) and two of them have spins which are oppositely aligned.


  • by museumpeace (735109) on Monday October 04, 2004 @08:33PM (#10435368) Journal
    I couldn't think of it when I posted, but I found a very readable article [] from someone at SLAC about the mystery of m/am asymmetry
  • by Bendebecker (633126) on Monday October 04, 2004 @09:02PM (#10435568) Journal
    No, the character played by John Cusack was Doctor Louis Slotin who had on the 21st of May 1945 been involved in an accident:

    Slotin had been instructing a colleague, Alvin C. Graves, who was to replace him at the Omega Site. Also present was S. Allan Kline, a 26-year-old graduate of the University of Chicago, who had been called over to observe the procedure. Five other colleagues were close by as Slotin, a Canadian physicist from Winnipeg who had been part of the team that created the atomic bomb, performed the action that would bring into close proximity the two halves of a beryllium-coated sphere and convert the plutonium to a critical state.

    With his left thumb wedged into a cavity in the top element, Slotin had moved the top half of the sphere closer to the stationary lower portion, a micro-inch at a time. In his right hand was a screwdriver, which was being used to keep the two spheres from touching. Then, in that fatal moment, the screwdriver slipped. The halves of the sphere touched and the plutonium went supercritical.

    The chain reaction was stopped when Slotin knocked the spheres apart, but deadly gamma and neutron radiation had flashed into the room in a blue blaze caused by the instantaneous ionization of the lab's air particles. Louis Slotin had been exposed to almost 1,000 rads of radiation, far more than a lethal dose. Kline, who had been three or four feet away from Slotin, received between 90 and 100 rads, while Graves, standing a bit closer, received an estimated 166 rads.

    The accident involving Daghlian occured in August, oddly enough again on a 21st.
  • by Fran_P (740315) on Monday October 04, 2004 @10:18PM (#10436009)
    But you make a good point- and if there's *any* supra-uranium metals nearby, the gamma ray could trigger fission.

    Correct me if I'm wrong, but I thought that fission was caused by low energy neutrons. Gamma rays, and even high energy massive particles would not result in fission in any nearby fissile material.

A sheet of paper is an ink-lined plane. -- Willard Espy, "An Almanac of Words at Play"