Author Topic: Episode #524  (Read 5749 times)

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Offline dawalters

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Re: Episode #524
« Reply #30 on: September 13, 2015, 02:02:29 AM »

Science or Fiction: Well, I got fooled just like all the XY Rogues.  And, having read a couple of articles about the foam, I still don't understand it, because the articles contain no explanation of the mechanism involved.  I thought blocking radiation relied on just putting enough matter in front of it.  Like with medical X-rays: the denser an object in the body, the less X-ray radiation gets through it, so you can see bones, kidney stones, metal pins, etc. in the body.  Lead shielding is used because it's very dense.  So how the heck does the very low-density foam do it?  Does it involve reflection, refraction, diffusion, an ionic effect, what?  There are times I wish I'd finished college.


Actually, your instincts were good!  The Rogues, it turns out, were on the right track and the so-called "science" answer to this was not justified.  I run radiation transport calculations regularly and can spot hype on magical radiation shielding when I hear it.  The main source for the science or fiction question, from the university https://news.ncsu.edu/2015/07/rabiei-foam-rays-2015/ is a North Carolina State University press release that cleverly does not highlight the test conditions.  Some are stated, though, in the middle of the article: the different types of metal "foam" and solid steel and lead layers were are of the SAME AREAL DENSITY.  Areal density is the material density (in g/cm3) multiplied by the slab thickness (in cm). This means that thicker slabs are needed if you use low-density materials. Layers with the same same areal density and (to shield a certain area, say) the same width and length must weigh exactly the SAME.  Let me repeat: the so-called light-weight foam slab weighed exactly the same as the steel slab and the (thin) lead slab. The foam density is not stated in the source article, but typically to be called a foam the stuff is made with a distribution of voids so that the average density is 25% of the solid density (or less).   Let's say the foam metal is 2 g/cm3 density (compared to solid steel at 8 g/cm3).  And lead is 11.3 g/cm3.  Then the university used a foam metal slab that was 4 times thicker than the steel one and 5.5 times thicker than the lead one.  Weight saved = none. 

I will grant the possibility that the material might make a dandy structural metal, but the radiation shielding feature is clearly produced by just mixing in an even distribution of tiny balls/pellets of tungsten or other high-atomic-number element in order to get almost as good to the x-ray shielding effectiveness of the lead shield and to be "better" than solid steel.  The source article indicates Am-141 as the x-ray emitter, so 59 keV, which interacts with most materials via the photoelectric effect, in which the x-ray is totally absorbed by an inner shell electron.  It appears the researchers used Co-60, a very common source of 1.2 MeV gammas (1200 keV).  The claim that the metal foam is "almost as good as" lead for these energy gammas is amusing, since for 1-5 MeV gammas, all materials with atomic number between 13 (aluminum) and 82 (lead) need the same g/cm2 to have the same shielding effectiveness. Gammas interact with matter via the Compton effect and pair production.  Both mechanisms, to first order, depend on the net number of electrons in the path of the photons, and happens to be proportional to the layer mass, therefore enough g/cm2 is all you need. 

No fancy chemistry or geometry is needed for effective radiation shielding, just a reasonably homogeneous mixture of stuff; the interactions are all one electron and photon at a time because x-rays and gamma rays exhibit their particle nature rather than their wave nature when going through bulk materials.  The bottom line is that the foam metal is nothing special in terms of a radiation shield -- people play different games to pile on layers of different atomic number material to deflect x and gamma rays as well as electrons and neutrons.  It depends on if you want to try to scatter them away from you or absorb them.  Andy you need to make sure that you don't create more radiation in the layers you use (secondary radiation) -- stopping neutrons sometimes creates gamma rays, for example, and stopping energetic electrons generates x-rays.

Online werecow

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Re: Episode #524
« Reply #31 on: September 13, 2015, 05:25:56 AM »
Am-141 as the x-ray emitter

Cool info! But looking at my periodic table, shouldn't that be Am-241?
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Offline dawalters

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Re: Episode #524
« Reply #32 on: September 13, 2015, 08:53:30 AM »
Am-141 as the x-ray emitter

Cool info! But looking at my periodic table, shouldn't that be Am-241?

Oops, yes clearly my tyoi-- stupid fingers! Am-241 is famously used in smoke detectors

Offline UnicornPoop

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Re: Episode #524
« Reply #33 on: September 13, 2015, 11:56:32 AM »

Science or Fiction: Well, I got fooled just like all the XY Rogues.  And, having read a couple of articles about the foam, I still don't understand it, because the articles contain no explanation of the mechanism involved.  I thought blocking radiation relied on just putting enough matter in front of it.  Like with medical X-rays: the denser an object in the body, the less X-ray radiation gets through it, so you can see bones, kidney stones, metal pins, etc. in the body.  Lead shielding is used because it's very dense.  So how the heck does the very low-density foam do it?  Does it involve reflection, refraction, diffusion, an ionic effect, what?  There are times I wish I'd finished college.


Actually, your instincts were good!  The Rogues, it turns out, were on the right track and the so-called "science" answer to this was not justified.  I run radiation transport calculations regularly and can spot hype on magical radiation shielding when I hear it.  The main source for the science or fiction question, from the university https://news.ncsu.edu/2015/07/rabiei-foam-rays-2015/ is a North Carolina State University press release that cleverly does not highlight the test conditions.  Some are stated, though, in the middle of the article: the different types of metal "foam" and solid steel and lead layers were are of the SAME AREAL DENSITY.  Areal density is the material density (in g/cm3) multiplied by the slab thickness (in cm). This means that thicker slabs are needed if you use low-density materials. Layers with the same same areal density and (to shield a certain area, say) the same width and length must weigh exactly the SAME.  Let me repeat: the so-called light-weight foam slab weighed exactly the same as the steel slab and the (thin) lead slab. The foam density is not stated in the source article, but typically to be called a foam the stuff is made with a distribution of voids so that the average density is 25% of the solid density (or less).   Let's say the foam metal is 2 g/cm3 density (compared to solid steel at 8 g/cm3).  And lead is 11.3 g/cm3.  Then the university used a foam metal slab that was 4 times thicker than the steel one and 5.5 times thicker than the lead one.  Weight saved = none. 

I will grant the possibility that the material might make a dandy structural metal, but the radiation shielding feature is clearly produced by just mixing in an even distribution of tiny balls/pellets of tungsten or other high-atomic-number element in order to get almost as good to the x-ray shielding effectiveness of the lead shield and to be "better" than solid steel.  The source article indicates Am-141 as the x-ray emitter, so 59 keV, which interacts with most materials via the photoelectric effect, in which the x-ray is totally absorbed by an inner shell electron.  It appears the researchers used Co-60, a very common source of 1.2 MeV gammas (1200 keV).  The claim that the metal foam is "almost as good as" lead for these energy gammas is amusing, since for 1-5 MeV gammas, all materials with atomic number between 13 (aluminum) and 82 (lead) need the same g/cm2 to have the same shielding effectiveness. Gammas interact with matter via the Compton effect and pair production.  Both mechanisms, to first order, depend on the net number of electrons in the path of the photons, and happens to be proportional to the layer mass, therefore enough g/cm2 is all you need. 

No fancy chemistry or geometry is needed for effective radiation shielding, just a reasonably homogeneous mixture of stuff; the interactions are all one electron and photon at a time because x-rays and gamma rays exhibit their particle nature rather than their wave nature when going through bulk materials.  The bottom line is that the foam metal is nothing special in terms of a radiation shield -- people play different games to pile on layers of different atomic number material to deflect x and gamma rays as well as electrons and neutrons.  It depends on if you want to try to scatter them away from you or absorb them.  Andy you need to make sure that you don't create more radiation in the layers you use (secondary radiation) -- stopping neutrons sometimes creates gamma rays, for example, and stopping energetic electrons generates x-rays.

Such a wonderful post! I think you hit the nail on the head with this post. The UNC press release was misleading (not intentionally, I presume) in that it gave rise to headlines like:

Lightweight Metal Foams Can Effectively Block X-Ray, Neutron Radiation, And Gamma Rays
Metal foams could provide lightweight radiation shielding

It was hard to find any more detailed info other than articles that mostly reproduced the press release and none explaining what Areal Density means. PhysicsWorld provides a more accurate take on the press release.

I think the selling point here really is that metal foam can be tuned to have superior structural and thermal properties and, oh by the way, they can attenuate/block radiation on par with the weight-equivalent of non-foamy high-z steels.

Just one correction for your post: I believe the balls/pellets are hollow steel spheres, not tungsten. The high-Z tungsten is part of the steel matrix in which the spheres are embedded. At least that's how I understand it.

The researchers are continuing to study the properties of foamy steel to try and improve shielding over lead. They have a few variables to play around with, including concentrations of tungsten or other metals, and perhaps sphere outer-wall thickness to diameter. Ultimately, I guess it would be a gradient foam with layered Z steels if the intended use is shielding, taking a page from how best to reduce ionizing radiation. But I'm just guessing at this point.

Anyway, thanks for your post.  It was very insightful.
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