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Lightweight Radiation-proof Fabric? 92

Posted by michael
from the you-go-first dept.
kramer writes "New Scientist is reporting the creation of a lighweight radiation-proof fabric called Demron. Demron is being touted by its inventors as comparable to lead shielding at a fraction of the weight. Could be very useful for any future interplanetary space missions where the amount of radiation absorbed by the crew is a significant concern."
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Lightweight Radiation-proof Fabric?

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  • "The molecules are lined up to give the illusion of the presence of large atoms," says Hefler."

    And hence the illusion of being protected...
  • Wasn't this story posted on Tuesday?
  • Volume (Score:5, Funny)

    by moc.tfosorcimgllib (602636) on Thursday November 14, 2002 @03:06PM (#4670598) Journal
    I'm sure you meant the fabric takes up less volume, as I would hope that weight is not an issue in space.

    • No, weight (Score:3, Informative)

      by Grotus (137676)
      Weight might not be a big issue once you are actually in space, however, it is a huge issue when it comes to getting there. The currently used number for launch price is around $10000 a pound to get into low earth orbit.
    • But weight is a problem getting into space.
    • Re:Volume (Score:3, Insightful)

      by chaidawg (170956)
      Weight may not be an issue in space, but mass still is. This fabric would also have less mass then a lead coat. Easier to start it moving (by the wearer) and, more importantly, easier to stop it moving.
    • Re:Volume (Score:1, Redundant)

      by tswinzig (210999)
      I'm sure you meant the fabric takes up less volume, as I would hope that weight is not an issue in space.

      Ummm... heavier stuff requires more energy to get it off Earth into space, so yes, weight is an issue.
  • Where can I get underwear made out of this stuff.
    • This stuff may really be a godsend to alien abduction victims. You could make hats out of it, and they wouldn't look silly like the tinfoil variety. It could also work for disabling the V-chip. [] How soon can I start a business selling this stuff?
    • hey, if you got underwear (and other clothes) made out of thise stuff i would feel a little more safe using those nuclear powered laptops they talked about yesterday
  • No worries then (Score:3, Interesting)

    by Syncdata (596941) < minus math_god> on Thursday November 14, 2002 @03:14PM (#4670704) Journal
    So I guess we no longer have to worry about losing our magnetic field [] then?
  • Testing (Score:5, Informative)

    by justanyone (308934) on Thursday November 14, 2002 @03:16PM (#4670728) Homepage Journal
    It seems reasonable that that gamma radiation (high energy photons) could be intercepted by a 'cloud of electrons' in the polymer in this material.

    However, high energy neutrons would not interact with the electrons due to their high velocities. The relativistic effect of width/length contraction applies to these neutrons. This was the fundamental problem of early fission reactors - they had to moderate (reduce the speed of) the neutrons in order for them to appear big enough to interact with a nucleus. An electron is 1,836 times less massive than a proton. Thus you'd need 1836 electrons to equal one proton-with of neutron blocking power. I doubt they have that many electrons in the polymer's cloud!

    However, any material that more effectively screens high energy photons is a welcome material. It would also be highly useful in creating X-Ray and Gamma-ray telescopes, methinks! NASA, you listening?

    The article should have gone into which radiation types besides Alpha particles that it would block (Alphas I think are just Hydrogen nucleii - or is it Helium...).

    Cosmic ray shielding would be useful on the ISS as well, but it would not stop relativistic particles, and it might break down under repeated insults of high energy collisions occurring regularly in space.

    Any nuclear engineers out there who can comment better? go Navy?!?

    • Re:Testing (Score:1, Informative)

      by Anonymous Coward
      Alpha particles are Helium nuclei. Hydrogen nuclei on the other hand, are commonly referred to as protons :)
    • Re:Testing (Score:5, Informative)

      by Imabug (2259) on Thursday November 14, 2002 @06:24PM (#4672604) Homepage Journal
      anybody who's studied radiation interactions with matter will tell you that for gamma/X rays, it's not the density (g/cm^3) of the material but the electron density (electrons/cm^3) that determines the attenuation properties of a material. Generally, metals with a higher atomic number (Z) have more electrons, so they usually have better attenuation properties.

      As it turns out, sometimes you can pack lighter weight nuclei close enough together that you get a similar electron density to metals (or even higher sometimes). this is probably what they've done with this 'polymer'. Althogh on a mailing list I'm on, someone mentioned that the material was just fabric that had been soaked in a barium solution and dried. Whether that was true or not was never establised though.

      There is one incorrect statement in the article though.

      "The helium nuclei that make up alpha radiation have so little energy that almost any physical barrier can stop them."

      Alpha particles can be emitted with extremely high energies, upwards of 10 MeV. What makes alpha particles so non-penetrating is their charge. Their +2 charge causes them to interact very strongly with neighbouring electrons, which causes them to lose energy quickly. That's also the reason why there isn't any special shielding necessary for alpha particles. Most alpha particles will be stopped in the dead layer of your skin, and therefore pose little external hazard. What makes alpha particles dangerous is if they become internalized (inhaled or ingested). The fact they lose energy quickly means that energy is transferred to whatever material it happens to collied with, causing much damage in the process.
      • Re:Testing (Score:2, Informative)

        by esonik (222874)
        With respect to gamma/X-ray shielding who depend on electron number, I don't think that large improvements in total weight of shielding can be made. The reason is of course that for charge neutrality reasons each electron has to be balanced by one proton in a core plus some amount of neutrons. It is only the amount of additional neutrons that can be varied by chosing different materials and as it happens to be the percentage of neutrons in nuclei increases with Z, with the extreme being the hydrogen 1H atom that has no proton at all. From this reasoning it would be best to use materials with high hydrogen amount (polymers for example) to reduce weight. But then volume is a concern too, esp. with polymers who have a low density (compared to high Z metals).
  • Question, (Score:5, Funny)

    by 3-State Bit (225583) on Thursday November 14, 2002 @03:18PM (#4670752)
    The researchers say it's comparable to lead shielding in terms of shielding radiation, but this does not quite address an important concern. Is it like lead shielding only with respect to nuclear radiation? Or x-rays and such as well?

    I'm only asking because I'm trying to figure out whether Superman can see through it. Any help is appreciated.

    • by lbrt (625194)

      *** NEWS FLASH ***

      Breast cancer rates alarmingly high in the city of Metropolis. Former reporter Lois Lane tells us how the strange burning sensation turned into a nightmare ... I felt like somebody constantly watched me ...

      Jimmy Olsen, Daily Planet News

  • Metal detectors (Score:3, Interesting)

    by diesel_jackass (534880) <> on Thursday November 14, 2002 @03:19PM (#4670755) Homepage Journal
    Anyone know if a pair of pants made of this will set off the Airport metal detectors?

    Can I stuff a duffle bag full of guns and make them not show up on that little thing?

    For the cost it probably wouldn't be worth it though.
    • duh, then they would just open the bag to search it by hand.
    • Don't metal detectors work precisely because the x-rays are reflected back to the x-ray device when they hit metal? Therefore your entire duffle bag would set the alarm off and be searched, because it would reflect all the x-rays back (if it does indeed reflect x-rays). So either way I think you're screwed.
      • i hope you're just joking.

        metal detectors work by generating an electromagnetic field, and sensing any disruptions caused by the movement of metal objects through the field.
        • Yeah, but they're actually talking about the airport baggage scanners, which do use x-rays iirc. At least i assume they're talking about baggage scanners, because you can't walk through one of the frame type metal detectors with your bag.
  • by 0x69 (580798) on Thursday November 14, 2002 @03:23PM (#4670792) Journal
    Low-energy alpha & beta are easily blocked with century-old technology. Their explanation of how they block X and gamma sounds like smoke & mirrors to me.

    BTW, '0.00% of gamma gets through' may be worse than nothing, since the secondary shower (from whatever the gamma ray hit) is often worse than the gamma.
  • However, if this does pan out, it will probably help with the radiation issues they are currently having aboard the ISS. I would like to see lightweight fabrics, and other lightweight materials for that matter, become more prevalent in the future - as long as they are durable and protective! This is one important way we will be able to increase fuel efficiencies and with any luck see long-range space travel in our kids' lifetime.
    • This is basically the first thing that popped into my head... I was thinking more for radiation protection on a Mars mission, however. Until they actually show some real-world/space test results, I'll remain skeptical.
    • I've given up on the idea of traditional humans ever traveling into deep space. There may not even be much of a point to try to survive on a local planet like Mars. As depressingly limiting as it might be, we have evolved for life on Earth. By the time technology ever gives us an answer to the many problems associated with our reliance on Earthly gravity, atmospheric conditions, etc., we will no longer be human...and that's OK.

      Post-humans, if they are ever allowed to exist, will be free to take their silicon bodies wherever they want to go, without limits.

      Bottom line: The stars are not for man.
      • they might not necessarily be silicon, though... we're already delving into electronic repair if not outright augmentation; as well as genetic work and whatnot [though apparently things like test-tube babies can be very risky for the resulting child according to NPR this morning and shows just how little we know about these intricate things].
    • > However, if this does pan out, it will probably
      > help with the radiation issues they are currently
      > having aboard the ISS.

      I doubt it. Those problems are due to high-energy particles striking the alumimum hull and producing cascades of high-energy photons. The best solution would be to replace the aluminum with plastic.

      I don't understand why they didn't see this coming.
  • by m_chan (95943) on Thursday November 14, 2002 @03:32PM (#4670899) Homepage
    I can finally look fashionable while thwarting the cia's mind readers instead of that baked potato look I had going for me.
    • Sorry, you'll still need the reynolds wrap. X-rays and gamma rays are the so called "death rays". So while we won't need red dust to kick their prissy little xenomorphic asses back to alpha centari, they'll still be able to know where you hide your porn, how many gigs you have, and how it's sorted.
  • other applications (Score:3, Interesting)

    by tps12 (105590) on Thursday November 14, 2002 @03:35PM (#4670926) Homepage Journal
    Interplanetary space missions? Uh, that's great. I'll be looking for a cell phone case made out of this stuff. And boxer briefs, of course.
    • DAMMIT! (Score:1, Interesting)

      by Anonymous Coward
      There are NO (as in ZERO) peer-reviewed studies showing that cell phone radiation is harmful! NONE!
      • Tell that to the 5 inch tumor on my head. *POP* Oh wait, it was just a pimple.

        Moderation totals: -1 (Disgusting)
    • Cell phones emit microwave radiation if I recall correctly (non ionizing radation regardless), aluminum will stop that. But thats not the point, the radiation if coming from the antenna. If you block that you cant transmit (or receive for that matter.

      If you are really that woried, just wear your tin foil helmet. It will "protect" you.
  • So when are they going to start making tighty whiteys outa this stuff I have to drive past a nuke plant everyday and I still want to have kids!
    • Re:Underwear (Score:3, Insightful)

      If you're that concerned about having kids, then why are you still wearing tighty-whiteys? Your body heat is doing more to drop yer counts than that power plant!
    • Re:Underwear (Score:3, Informative)

      by theedge318 (622114)
      If you are worried about radiation, you might want to kick your wife out of bed, b/c she is exposing you to 0.1 millirems of radiation per year. Which incidentily is the same amount of radiation you experience by living within 50 miles of a nuclear power plant. Before you kick your wife out of bed, which will have its own adverse affect on having children, try doing the following to reduce your radiation exposure:
      • Throw away your alarm clocks and wrist watch, the glowing hands give you 0.6 mrem/year.
      • Or try giving up bananas, most people recieve about 40 mrems/year per year in Potassium-40 radiation
      • But the absolute worst source of radiation, cigarettes- 1.5 packs per day = 8000 mrem/year
      • And move away from coal fired plants; Uranium occurs naturally in coal, which when burned exposes you to 1-4 mrems/year
      All thanks to having a girlfriend at a national lab (which probably means she radiates more than your wife ... so I am going out and buying lead blankets) For more radiation doses: Lawrence Berkeley Labs []
  • Demron consists of a polyethylene and PVC-based polymer fused between two layers of a woven fabric. The polymer molecule has been designed so that incoming radiation will meet a large electron cloud, which will deflect or absorbed it.

    From what I remember of polymers, they are essentially composed of long string-like molecular structures. With such a structure, it is easy to orient the molecules into specific directions. Add a little vulcanizing effect for strength; voila! Strength, high density of electrons, flexibility (similar to tire rubber), and wearable?
    • > Strength, high density of electrons, flexibility
      > (similar to tire rubber), and wearable?

      Just like a plastic bag. And just as effective.
  • ...confuse the filters so that one could pour hot grits down them and still post such nonsense?

  • by Renraku (518261)
    We also have some other new technologies to stop alpha particles. Even objects which may be in our houses today. Paper, wood, walls, a layer of skin or two..this is all it takes to stop all but the strongest burst of alpha or beta radiation. But take off your paper, wooden, or drywall outfits! Now you can look great while defending yourself from harsh radiation!
  • by weglian (626186) on Thursday November 14, 2002 @04:46PM (#4671720)
    I was a Navy nuke, so I know a thing or two about radiation. What people think of as "radiation suits" are not designed to shield radiation. They are used to prevent contamination. The suit prevents radioactive material from getting on your skin and clothes, so that you don't take it home with you. There are several kinds of radiation. Alpha particles are helium nuclei (He4). These particles are not dangerous unless you ingest or breathe them into your lungs. The dead layer of skin on your hand is enough to stop them. If they get into your body, they cause serious harm, as there is no "dead layer" of anything to stop them. Beta particles are electrons. They are light and fast, so they penetrate more. They will give you a dose to your skin, but not much deeper than that. A contamination suit can probably stop Betas. Even the old ones. Gammas are photons/electromagnetic radiation. Whatever you want to call them. X-Rays, microwaves, and radio waves all fall into this general category. The real biological hazard comes from the high energy particles/waves. These will pass right through your skin and can affect your internal organs. A contamination suit is not expected to reduce the dose from gammas. neutrons are produced in fission and some rare decay events, mostly shortly after a fission event. Neutrons are the only thing that can make something ELSE radioactive. Neutrons have no charge, so shielding them is difficult. It takes LOTS of lead or iron or water, etc. I don't care what their material is made of, if it's light enough to wear, it will not shield neutrons well. The web site said that it shileds low energy gammas, alphas, and betas. Low energy gammas don't cause much harm. Unless you work with a low energy gamma source on a routine basis, this material is not going to change your life. Or your life expectancy. This is my first post. Is it WAY too long? Sorry. -John
    • Good post, just need to break it up into paragraphs a little. One clarification though, you said:

      These particles are not dangerous unless you ingest or breathe them into your lungs.

      As you said, it's not as much about protecting against the radiation as it is protecting against contamination. Along that line, you wouldn't want to ingest a material that was emitting alpha particles, you don't have to worry about ingesting the particles themselves. :)
    • No, longer posts are better. If I had some mod points I'd give them to you :)
    • This is my first post. Is it WAY too long? Sorry.

      Not to long, just use a bit of whitespace between paragraphs. Use <p> or <br> if you are posting using html format.

      Also, mentioning first posts is being frowned upon here :)

    • Eletromagnetic radiation tends to reflect of any conductive surface, like a faraday cage.

      Faraday cages are effective if the radiation wavelength is bigger than the cage net. As you increase radiation energy the wavelenght gets smaller. You can still reflect it by decreasing the aparent net size as seen from the photon.

      That's why a X-ray telescope uses special mirrors that reflect radiation at shallow angles (1 or 2 degrees). Those pthtons see the atom lattice from shallow angle, therefore it looks smaller.

      Perhaps this kind of polymer has an high eletronic density as seen from most angles, therefore being effective against most low energy X-rays for example.

      I doubt it has any effect against hard X-rays, or gamma rays, witch are generally deflected by direct interaction with the nucleus charge, and not with the eletron clouds, but it could be effective against low energy X-rays (1-2 Kev or so).

  • Flight crew (Score:3, Funny)

    by bckspc (172870) on Thursday November 14, 2002 @04:52PM (#4671775) Homepage
    I hope it comes in all kinds of gawdawful pastel colors so airlines can make nifty new uniforms for their flight crew.
  • Secondary emissions? (Score:4, Informative)

    by dpilot (134227) on Thursday November 14, 2002 @05:10PM (#4671926) Homepage Journal
    The article doesn't mention anything about the secondary emission characteristics of this fabric.

    Secondary emissions are a very real bane of shielding for interplanetary travel, due to the extremely high energies of cosmic rays. Even if you could launch lead shielding for a spacecraft, manageable thicknesses would cause secondary emissions that were even more dangerous to the occupants than the original cosmic rays.

    They've tried a new type of radiation shielding on the ISS made of polyethylene that is supposed to block without creating secondaries, and I see that's part of this fabric. Unfortunately the new shields don't seem to be as effective as hoped.

    Radiation is anticipated to be a big problem on a Mars mission. IIRC, the radiation exposure will have a cancer risk equivalent to a lifetime of smoking.

    But you really know to be scared of cosmic rays if they make rapping noises on your spacecraft, and upon return to Earth you start stretching, bursting into flame, become invisible, or turn into a pile of muscular orange bricks.
    • > The article doesn't mention anything about the
      > secondary emission characteristics of this
      > fabric.

      Because it has essentially none. Heavy high-energy particles such as cosmic rays will zip right through it as if it wasn't there.

      > They've tried a new type of radiation shielding
      > on the ISS made of polyethylene that is supposed
      > to block without creating secondaries,...

      It isn't new and it isn't supposed to block without creating secondaries: it is supposed to block secondaries. Better to get rid of the metal that is creating them in the first place.

      > ...and I see that's part of this fabric.

      Yup. They'll sell lots of their expensive plastic bags.
    • Mars mission. IIRC, the radiation exposure will have a cancer risk equivalent to a lifetime of smoking.

      Yes, but most astronauts would gladly take up smoking for an entire lifetime if it meant they could go to Mars :)

  • To make clothing out of for those of us who live in areas that could potentially be hit by dirty bombs!!!!!

  • Does this make anyone else think of Z for Zacharia []?!
  • Perhaps I can finally stop wearing this dismal fashion statement of an aluminum foil hat..

    Perhaps even get a date in my trendy new radation proof pub cap.

    Perhaps not.
    • Perhaps I can finally stop wearing this dismal fashion statement of an aluminum foil hat.

      Upgrade man, upgrade. I wear these []. The ladies are all over me in my baseball cap, underwear, t-shirt, and scarf.

      I'm joking of course. I'd have to leave the house.
  • by Hadlock (143607) on Thursday November 14, 2002 @05:40PM (#4672184) Homepage Journal
    huzzah! now when the magnetic field of earth changes polarity and it dissapears for some time, we'll all be safe wearing our Demron jackets. I'll be the first to sell Demron Letter Jackets for highschool football teams!
  • by ljhiller (40044) on Thursday November 14, 2002 @06:18PM (#4672557)
    I checked out their website a few days ago and as you've noticed, it has no details. You can get an idea of the technology from their granted patent (amended) 6,459,091 here []. The idea in the patent is to impregnate cloth with a heavy metal such as barium salt, with a metal film layer or cloth woven from metal thread for additional protection. The technology mentioned in the New Scientist article sounds like a newer version that is still pat. pending. There's still no hard information about how well the suits actually work for the different radiation sources, e.g. Medical x-rays, airline cosmic rays, nuclear warfare. It's all marketing.
  • by Peter T Ermit (577444) on Thursday November 14, 2002 @08:38PM (#4673657)
    I'm not even going to talk about the ludicrous "science" behind the fabric, and I'm not going to talk about how it tends to be heavy nuclei, not electrons, that stops ionizing radiation. Even without using those arguments, I can still show that they're full of shit.

    Check out their "test results." [] (PDF file) Go to page 3 to see the blocking power of their fabric.

    Error #1: There is no such unit as a Kv. They mean KeV, as in Po-210 emits an alpha particle with 5,300 KeV of energy. But this is minor.

    Error #2: X-rays and gamma rays are both photons; they're only distinguished by their energies. But the X-ray results and gamma ray results (which overlap in energy, which, in itself, makes no sense) contradict each other. For example, why does the fabric block only 52% of 60KeV gamma rays, but 82% and 72% of 50 and 70KeV X-rays respectively? Makes no sense, but this is nothing compared to...

    Error #3: The real killer, and what makes me suspect this is fraud, rather than mere incompetence. They use 0.5 mm of lead as the comparison for their gamma emitter tests. The radiation-absorbing properties of lead are well known, and easily accessible in handy-dandy tables. For example, the half-value layer (the amount of material needed to block 50% of incoming radiation) for a Co-57 source is 0.15 mm of lead (ref here [] ), so 0.5 mm of lead should block more than 90% of the radiation, not just 52%. Proof positive that they're full of shit.

    Fuck them, and fuck New Scientist. (Pardon my French.)

    • Heavy nuclei tend to stop particles, like alphas, betas, and neutrons, but electrons are opaque to em radiation like x-rays and gamma rays. So the "heavy nuclei, not electrons" is incorrect. But the argument remains the same.
    • and I meant "...why does the fabric block 100% of 60 KeV gamma rays, but only 82% and 72% of 50 and 70 KeV X-rays respectively?"
      • The mistake they're making is not mentioning that their x-ray source is probably polyenergetic (getting a mono-energetic x-ray source is somewhat difficult) and contains a wide spectrum of x-ray energies.

        Most gamma ray sources tend to be mono-energetic, so if you have a source of 60 keV (kiloelectronVolts) gamma rays, that's all you have. On the other hand, if you have a 60 kV (kiloVolts) x-ray source (the tube operating potential) you have x-rays anywhere from 10-60 keV, with most of them probably around 25-40 keV. The effective energy of this beam will be around 30 keV.

        Note the distinction in units. Subtle but very different.

        Getting back to the difference in absorption, since the x-ray beam has a much higher proportion of low energy photons compared to the gamma ray source, more of these will be absorbed by the material. Thus more of the x-ray beam would be absorbed, compared to the gamma ray source.

        Their results aren't bogus...they're just comparing apples and oranges (which may be just as bad).
        • and of course after i've written the above and gone to read the PDF again, i see where the strangeness is.

          0.5 mm lead should be more than sufficient to stop all of a 50 kV x-ray beam and most of a 70 kV beam.

          I certainly wouldn't expect 0.5 mm lead to stop all of a 60 keV gamma ray source though. Most of it definitely.

          However, the document doesn't go into any of the details of how the material was tested. But still, it does raise some questions.

          then again, the x-ray part of the table mentions broad beam apparatus, which means scatter radiation was included. including scatter radiation makes it look like more radiation reached the detector, decreasing the apparent blocking ability.

          this would probably account for why their 50 kV beam was only partially attenuated instead of fully, and why the other x-ray energies aren't blocked as much as they should have been.
          • Yeah... there are definite questions about how this was done, but there shouldn't be. They're working with relatively low-energy sources that should be easy to control and collimate and direct at a well-defined target without trouble from edge effects, etc. Even an undergrad should be able to compare the two materials better than these guys -- whose "scientific" conclusion is that their material works as well as lead, and contradicts known cross-section data. The only question in my mind is: are they incompetent or are they frauds?
        • My first thought was along these lines -- that Kv was kV and meant some sort of potential across an x-ray tube. But when I saw that the table had radioisotopes with the proper energies, it became obvious that they meant eV and not V, at least for the first part of the table. If they switched terminologies midway down the table, that's really bad, but it might be an explanation for the x-ray results.

          However, this doesn't address my main objection, and I contend that the results are bogus. Are my calculations wrong about 0.5 mm of lead blocking more than just 52% of the Co-57 emissions?

          And re: gamma rays vs. x-rays: obviously different fields of physics have slightly different definitions. The one I'm familiar with is just based upon energy; there's a not-too-well defined border between x-rays and gamma rays at about 0.1 MeV. Your definition, on the other hand, works in almost all cases, but I can give a few counterexamples; e.g. x-rays from astrophysical sources can be thermal/black body rather than electronic in origin, and gamma rays need not come from a nuclear process, but from, say, the annihilation of an electron with an anti-electron. But since yours is a reasonable definition, I withdraw my parenthetical objection in the original posting.

    • Error #2: X-rays and gamma rays are both photons; they're only distinguished by their energies

      The distinction between x and gamma rays is their origin.

      gamma rays are nuclear in origin, resulting from the decay of radionuclides.

      X-rays are electronic origin, generated when an electron loses energy in some fashion (i.e. moving from a higher energy level to a lower energy level).

      aside from that, there is no way to tell a gamma photon from an x-ray photon.
    • As a working physicist, I routinely run radiation transport codes: both photons and particles with mass(neutrons and ions). I just found out about the Demron claims late last Friday and bumped into these posts while trying to find some sign of outrage on the Internet. Lots of pseudoscience and bogus hi-tech gets on the market but rarely is it so uncritically touted in the "mainstream" news like this. Yes, this stuff is bogus IF the claim is that an equivalent thickness of low-Z material(low atomic number, like plastic) attenuates x rays (soft gammas) in the 20 to 200 keV range as well as lead or other high-Z materials. Your calculation of the attenuation of Co-57 photons (at 122 keV) by 0.5mm lead is about right -- you say it should be about 90% and I calculate 84% with my simple x-ray transport code. If Demron is essentially PVC, the highest Z in it is Chlorine. But there's not enough to do much good. My code says that the absorption from 0.43mm of PVC, would be about 0.5% -- Demron should be terrible at absorbing 122 keV gammas! The reason it must be THAT terrible is something that Einstein got a Noble Prize for: the photoelectric effect. X rays (soft gammas) below a few hundred keV interact with matter primarily through the photoelectric effect. Photons at or just above the binding energy of electrons are absorbed by a quantum mechanical effect of the atom+electron+photon system. The cross section for the interaction is very dependent on the Z of the material. For photons around 100 keV, you need to introduce the x rays to material with a very tightly bound inner shell electron -- gold or lead for example -- in order to get a very large cross section. Well, lead has a 100x larger cross section than PVC at 122 keV. A beam of photons gets absorbed by this factor: exp(-cross section in cm^2/g * density in g/cm^3 * thickness in cm). So differences in DENSITY and THICKNESS are also important! Now, if you go above 1 MeV, cross sections for absorptions for all materials all start to converge. The photoelectric effect is no longer important -- now the Compton effect takes over, and over 10 MeV, the dominant process is pair production. The high energy photons mainly care about how many electrons per cubic cm are in their way, and as you folks already have pointed out, that tends to be proportional to material density. So fine, for a 2 MeV gamma, 10 g/cm^2 of Demron works just as well as 10 g/cm^2 lead, and note I said g/cm^2, so you need a 5x thicker Demron layer. So big deal -- Demron should be useless for absorbing 20-200 keV photons. Dental x rays run about 60 keV, as an example of the "medical" x rays that are supposed to be blocked. I think I still prefer that lead apron, if you don't mind. I don't care what goes into arranging the "electron cloud" in the Demron fabric, since any games with the chemistry of the material are irrelevant. The properties that determine absorption are the atomic number (Z) of the material and the density (how many of those atoms are crammed into the material per unit volume). Since I can estimate the probable mix of Z's in Demron and they state the density at less than 3 g/cm^3, it is not possible for this stuff to efficiently absorb 20 to 200 keV x rays. The only question is how fast Demron customers wise up.
      • Very nice and informative post. Case closed. I hope this company goes the way of ZeoSync, which claimed to have universal lossless compression. (They seem to have fallen off the face of the earth.)

        As for 90% vs 84%, my calculation was nothing more complicated than 1 - (0.5)^(0.5/0.15), so I obviously defer to the numbers of an expert. :)

        And as for other outrage on the internet, Bob Park's What's New column at mentioned it, which is always the kiss of death. But it's been pretty quiet other than one thread in a physics-related newsgroup, as far as I can tell.

  • Did anyone notice that these suits are only rated for 2-3 uses? That would preclude using them in space or almost any situation but emergencies.

    Maybe they can sell a bunch to India and Pakistan,

"Regardless of the legal speed limit, your Buick must be operated at speeds faster than 85 MPH (140kph)." -- 1987 Buick Grand National owners manual.