On “Death Rays”

One bad thing about SteelyKid’s preschool graduation yesterday was that it drained my phone battery, causing me to miss an interview request from a local TV station looking for somebody to talk about a a couple of local guys arrested for a plot to build a “death ray” from X-ray components. This is pretty far from my area of expertise, but, hey, I’d be willing to go on tv and talk about just about anything physics-related. And, as always, I promise to be at least 90% less wrong than Michio Kaku.

Of course, on another level, I’m kind of glad that I didn’t get the message in time, because when I decided to write something about it this morning for the blog, I was reminded that I absolutely hate dealing with medical physics. I work in the world of lasers and optics in the visible-ish region of the spectrum, where the language we use to talk about optical phenomena is pretty straightforward: we talk about energy in terms of either the total energy delivered to a given area or the number of photons delivered to a given area. Medical physics, on the other hand, is a sucking swamp of cryptic units for exposure, dose, absorbed dose, effective dose, dese and dose, and all kinds of other crap. this page is about the best I’ve found for connecting these things together, and it’s still more unit conversion than I want to worry about. On top of that, it’s damnably difficult to get straightforward information about the numbers you would need to talk sensibly about any of this stuff– everything is either proprietary or custom, and thus specs aren’t available on the Web, or it’s a resource aimed at panicky laypeople afraid that they’re going to sprout a second head because their dentist x-rayed their teeth, and God knows, including actual numbers would only freak those people out.

But let’s see what we can come up with based on the little information given in that news story, and see what can be done to assess the plausibility of this plot. The information given in the article suggests that they were planning to use a “powerful industrial X-ray machine that would be powered by a makeshift, 2,000-watt battery,” that would fit in a van, and aim a “high-energy lethal beam of radioactivity at human targets.” The object was to have victims “mysteriously die from radiation poisoning within days.”

Now, given that these guys were apparently trying to shop this to both Jewish groups (for “killing Israel’s enemies while they slept”) and the Ku Klux Klan, they’re fairly clearly a few fries short of a Happy Meal ™. But that aside, is it physically possible to do this?

In order to kill someone “within days,” the Wikipedia page on radiation sickness suggests you would need to deliver a dose of around 30 Gray. This is a measure of the amount of radiation delivered to a body, which is a function of both time and intensity. You don’t necessarily need a “high-energy lethal beam” to do this– enough time spent in front of a more modest source, like a medical scanner, would do the trick.

So, how long would this take? Well, the data provided by a radiology association suggest that the largest effective dose of any of the procedures they list is around 20 milli-Sieverts, another of the weird units used in medical physics. The actual conversion from one to another is potentially messy, but a passing mention on this page is going to serve as justification for claiming that a Gray is a Sievert and vice versa. We’re doing order-of-magnitude here, anyway. So, the highest medical scan dose would clock in at 0.02 Gy, where you need 30Gy to kill somebody quickly. That suggests you’d need to spend (appropriately enough) 666 times the duration of a typical scan inside the medical x-ray machine in order to kill yourself with radiation poisoning. Not having had such a scan, I can’t really convert that to a time with any great accuracy, but 10 seconds seems like a reasonable order-of-magnitude for the time they run such a scanner, in which case it would take a bit more than 100 minutes to kill somebody with a CT scanner.

That seems kind of improbable, but they’re talking about a “powerful industrial x-ray machine,” so maybe the flux is higher. You’d need it to be 100 times more powerful to get down to a minute of exposure to kill, which still seems like a long time to keep your target in the beam, but if you’re killing Israel’s enemies in their sleep, maybe that works. I’m not sure I believe you can run that off 2kW of batteries in the back of a van, though.

But there’s another problem, here, which has to do with the distance. The x-rays from a typical source spread out as they travel, which means that the same beam covers a larger area as you move away from the device. This area increases as the square of the size of the beam, so doubling the working distance reduces the intensity, and thus the dose, to a quarter of its initial value. If you’re getting a medical x-ray, you’re probably on the order of 0.1m away from the source, but if you’re trying to poison sleeping enemies, I’d imagine your van would need to be at least 10m away. That’s a factor of 100 in distance, or 10,000 in the dose. If it took one minute to cook somebody at 0.1 m, you would need 10,000 minutes to kill them at a distance of 10m, or about a week.

Ah, but what about the “beam” aspect of this? They were planning to focus the x-rays on target, weren’t they? Doesn’t that take care of the expanding beam issue? Well, maybe, but I doubt it. You see, it’s damnably difficult to focus x-rays. If you were trying to cook somebody with visible light, you could just make a big curved mirror to focus the beams at a particular distance, and take care of that. X-rays aren’t like visible light, though– they either pass through or are absorbed by most of the materials you would normally make a mirror out of.

Focusing of x-rays can be done, but it’s a very daunting project. Something like the Chandra X-ray observatory uses an array of metal “lenses” that reflect x-rays at a glancing angle (for reasons that don’t really matter here, it’s really easy to get light of any frequency to bounce off things at a glancing angle– this is part of the reason why asphalt, not ordinarily noted for its high reflectivity, produces enough glare to justify polarized sunglasses for driving). The resulting array is pretty big– a meter or more across– and requires quite a bit of precision machining. And then there’s a significant issue of targeting– getting the tightly focused beam on the target (who presumably is not sleeping out in the open) is a non-trivial problem, even before you factor in that they planned to trigger this thing remotely, from half a mile away.

So, given the limited specs available, the “Hiroshima on a light switch” idea that these guys were pitching to a selection of mutually incompatible groups seems pretty far-fetched on the basis of physics. I don’t think there was very much danger of this succeeding.

(Of course, you don’t even really need to invoke physics to see the implausibility of this scheme: if it were possible to make a van-mounted death ray that ran off a 2kW battery with parts that you could scavenge from an industrial machine shop, every military in the world would be arming themselves with these. The fact that we don’t see these in general use suggests that these guys were just whack jobs.)

Comments

  1. #1 Eric Lund
    June 21, 2013

    if it were possible to make a van-mounted death ray that ran off a 2kW battery with parts that you could scavenge from an industrial machine shop, every military in the world would be arming themselves with these

    Not to mention every terrorist group in the world.

    Of course, some of those X-ray photons get absorbed or scattered in air. For photos of 100 keV, a calculator I found by Googling estimates the loss to be a bit less than 2% per meter. (More energetic photons travel further.) For a soft target in a medium density setting, that’s going to be a relatively small (factor of order 1) correction. A target in a building where your truck isn’t allowed to get close (say, a US embassy, or a medieval city with streets too narrow for your truck) requires significantly more energy in the beam.

  2. #2 Chad Orzel
    June 21, 2013

    Physics Buzz offers another approach to the problem, going from the total energy. They also have the criminal complaint text, with some slightly different information than used above.

  3. #3 Lurker #753
    June 21, 2013

    Hmmm….. those crazy astronomers who put x-ray telescopes in orbit seem to spend a great deal of effort (a) getting their surfaces as close to single-atom smoothness as possible, and (b) coating them with the densest metal they can find (e.g. gold or iridium).

    Even if they could focus the beam, if 100keV loses 2% per metre in air, how much is it going to lose going through the side of a house (even for small values of house)?

  4. #4 Lurker #753
    June 21, 2013

    Oh, okay. PhysicsBuzz says unfocused, but asks for 100MJ at a distance of …. 2m! with a target area of …. 1m^2!

    I think there’s a mistake here – setting mass to 80kg & area to 1m^2, then looking only at bone absorption. Absorption mass should be x15-20% for dry skeleton, area should be …. not sure, maybe about the same fraction, so maybe this works out (though I’d like to halve it for somebody lying down – not face on).

    Even so, the 2m is ridiculous…. as would be the 1GJ required at 20 ft (6.3m). 1GJ = a 1 kW heater running for 11.5days, 30litres of gasoline, or (drumroll) 200kg of dynamite.

  5. [...] Principles is skeptical a super KKK ray gun would work as apparently [...]

  6. #6 Alex
    June 22, 2013

    Order of magnitude correction: Having had a few x-rays, I’m pretty sure it’s way less than 10 seconds in the beam. 1 second would be a good guess.

  7. #7 Shwell Thanksh
    June 22, 2013

    Three words might give the “ha ha impossible lolz” crowd some egg on their faces: tabletop xray laser.

    http://phys.org/news/2013-06-particle-tabletop-chapter-science.html

  8. #8 Chad Orzel
    June 22, 2013

    Three words might give the “ha ha impossible lolz” crowd some egg on their faces: tabletop xray laser.

    I think I missed the part where the “tabletop x-ray laser” was run off batteries in the back of a van…

    Don’t get me wrong, it’s impressive research. But they’re producing high-energy electrons in sub-picosecond bursts using the Texas Petawatt Laser, which is housed in a clean room with approximately the same square footage as my entire house, on a 34-foot long optical table. And maybe, sometime in the next decade or so, they hope they might be able to generate beams as intense as the current state-of the art technology, which involves gigantic synchrotron sources.

    A portable “death ray” this is not.

  9. [...] Principles is skeptical a super KKK ray gun would work as apparently [...]

  10. #10 eyesoars
    June 22, 2013

    A while back, there was a device that accidentally killed several patients from radiation overdose. (See http://en.wikipedia.org/wiki/Therac-25 )

    In that case, the device that could irradiate either beta rays (electrons) or X-rays, the latter being generated by jumping the power of the former substantially and interposing a metal target, which emitted X-rays in response to electron bombardment.

    Through a programming goof, it could fail to interpose the metal target when programmed for X-rays, and did, giving several people radiation poisoning and killing at least three.

    I can’t find any description of the machine’s power draw, but on several occasions it emitted many-times-fatal doses of beta radiation within a few seconds.

  11. #11 ColonelFazackerley
    June 24, 2013

    Radiotherapy Physicist here…

    To get this to work you want to throw MeV electrons at a bremstrahlung target, not keV. then you get KE to X-ray conversion efficiency of 30-50% instead of 1%.

    These are linacs, of course, not simple static potentials.

    Some therapy X-ray machines can produce 24Gy per minute at one metre over 20×20 cm field.
    http://www.variantruebeam.com/pdf/TrueBeam_Brochure.pdf

    There are also (lower dose rate) therapy machines in vans
    http://www.itnonline.com/article/tomotherapy-mobile-radiation-therapy-system

    These will need to be hooked up to an industrial power supply from a building, though…

  12. #12 William Hendrixson
    June 24, 2013

    I’m not in a position to dispute the plausibility, but every military in the world is explicitly not interested in a weapon with an effective range of 10m. Not for the last 1000+ years, anyway.

  13. #13 Ron
    June 26, 2013

    May I humbly request a follow-up article on the requirements to generate a lethal dose of neutrinos in a death ray? (Nothing can stop it, bwahahahahaha!)

  14. #14 crd2
    June 29, 2013

    @12 Sure they are. It would be the equivalent of invisible arrows that kill yet leave no puncture holes or blood behind. And this wouldn’t be a direct assault weapon. This would be a tool for covert assassinations. Although I would have to agree that a 10m isn’t ideal. Trade off: Range vs. Stealth

  15. #15 Rick Pikul
    July 1, 2013

    Actually, the bit where they shopped it around to seemingly incompatible groups does make sense: They were in it for the money and why restrict yourself to just the one customer? So they could have been aiming to sell one to each of the groups or they could have been planning on telling them “we only have the one and some of the other bidders might just be targeting you.”

    If they were after the money, the whole thing would make sense even if the death ray didn’t work: What’s the KKK going to do, go to the police and report the fraud?[1] Add in a cooked demonstration with someone you want dead, (or who wants to commit suicide or can fake a non-lethal dose), or just go for the deposit(s)[2] you insisted on before handing it over for them to try.

    [1] I’ll assume that the scammers are smart enough to skip town and hide in order to avoid answers like ‘tire iron’.

    [2] Another distraction: You ‘sell’ it to the various groups and give them the same time and place to pick it up. Hilarity ensues.