[Geezer]

SeanB brought up the high energy neutrons, and that was the context he used for saying iron was the best shielding.  So when you asked why, I answered in the context of neutron shielding.  I don't know why he specifically brought up neutrons, but they certainly will be a risk to someone getting very close to a reactor.

Ah, right! Thanks!

[yor_on (OP)]

"Ionizing radiation comes in several further flavors. The ones of concern in the context of this post are alpha, beta, gamma, and neutron radiation. Alpha and beta radiation are similar in that they are a byproduct of radioactive decay in which high energy, short-ranged (milimeters or less) particles are emitted. Alpha decay releases alpha particles, which are identical to helium nuclei. Beta decay releases electrons and certain antimatter particles. Alpha radiation is less dangerous than beta radiation, as an external source does not penetrate the skin. An external source of beta radiation can cause burns if it makes contact with skin. Both can be harmful or fatal if the sources of radiation are in some way ingested, as the radioactive material can become concentrated in the tissues of vital organs.

Gamma radiation is different from Alpha and beta radiation in that it is not a particle per se, but a wave frequency in the electromagnetic spectrum, similar to x-rays. In nuclear bombings and accidents, gamma radiation is released after radioactive materials have undergone alpha or beta decay. High energy gamma radiation is more penetrating than alpha and beta radiation, which means that the whole body and its tissues can be irradiated by an external source. High energy neutrons can make other matter into sources of radiation via a process called neutron activation, which can turn otherwise well-intentioned matter into unstable, radioactive matter. They also have the ability to ionize atoms. Neutron radiation is more penetrating than alpha and beta radiation. It is sometimes more penetrating than gamma radiation as well. " From
Radiation: How it Works and What it Does.

"The earthquake, followed by the tsunami, absolutely wiped out their infrastructure, causing the cooling pumps to lose power. They have also experienced fires in the pumps providing cooling water to the spent fuel pools. Without a flow of cooling water, the water in the core or spent fuel pool eventually boils. With no water to remove the decay heat, the temperature of the fuel rises, causing the pressure to increase. In the limit, it can increase to the point that the fuel cell ruptures, which would release radioactive fission products. Chernobyl is the only example where this has happened."

And "When a uranium atom fissions, or splits, it releases energy as it splits into two other midweight atoms. ... These unstable radioactive atoms decay as they seek a stable state, emitting radiation in the process. Fissions still occur when the reactors are shut down -- just not at the rate when the reactor is "critical" or producing power. The shutdown fission rate is 8-10 or more orders of magnitude lower than during reactor operation.

Two types of radiation -- gamma rays (think of X-rays) and neutrons -- emitted from these decay products generate heat by mechanical interaction. Some of the neutrons are absorbed in the uranium fuel, causing more fission and more heat. So while the shutdown reactor doesn't require anywhere near the cooling needed when it is operating, it still requires cooling. Even the spent fuel removed from the reactors still generates heat. Water is the most prevalent coolant.

You may have heard that boric acid or borated water is being pumped into the cores. (Editor's note: South Korea has agreed to send some of its boron reserve to Japan.) Boron has a tremendous affinity for neutrons. It prefers to absorb them rather than the fuel, reducing the heat generated." from
foolish FAQ: The Fukushima Nuclear Crisis.

[Bored chemist]

OK,
here are a few facts.
Lead is, on a weight for weight basis a much better attenuator of gammas than, for example, iron
The scattering is proportional to the square of the atomic number.
OK, lead bismuth and, ironically, uranium are very good gamma blockers.
However, you need inches of lead to screen against high energy gammas and so there's just no way of making that portable, never mind wearable.
Gamma-blocking clothing is a non starter.

Iron isn't a particularly good neutron absorber. It has a capture cross section ( for thermal neutrons) of just 2.5 barns.
One of the metals used as a neutron shield is cadmium which as a capture cross section roughly a thousand times greater than iron. If you want to try something more exotic gadolinium tops the league with 49000 barns. With isotopically enriched Gd you can do even better but that's not practical
On a practical note the sea water being used to cool the reactors in Japan has a lot of salt in it; chlorine has a high cross section too (35.5 barns) so the sea water cannot act as a neutron moderator and produce an accidental criticality in the way that fresh water (possibly) might.

The other material used as a neutron sink is boron.

Since they scrammed the reactors before the tsunami hit and they are dumping salt water and borate onto the reactors there is very little chance of any significant neutron production anyway.

[Geezer]

Thanks BC. So if you square the atomic number and divide by the atomic weight of an element, you get a rough idea of its relative effectiveness as a gamma shield, weight for weight, against other elements.

On that basis, lead is way up there at around 32. Uranium is even better at around 35 and silicon, for example, is around 7.

[burning]

My thanks also, BC.  I allowed myself to get drawn into "reverse engineering" another's post, but I wasn't careful enough to think of the broader picture.  I appreciate you setting the facts straight.

[CliffordK]

This is why I think some basic science education should be mandatory in the US.

There was a piece on the TV this morning about this that was, frankly, awful reporting. The "Doctor" who is flogging this "stuff" did a demo where he shielded a Geiger counter from a radiation source with his amazing "wonder cloth".

Wow! The Geiger counter almost stopped counting. Did the reporter ask any questions at all - nope! If he had at least a basic understanding of science, he might have suggested substituting a sheet of paper, or a chunk of drywall. Alas, no.

So the good "Doctor" was able to get a huge free plug for his product. No doubt a bunch of scientifically challenged Americans will now rush out and buy these things. Probably the same ones that have bought up all the available supplies of potassium iodide.

Paper...  or water is good at blocking alpha and beta.  Not effective with gamma.

Hopefully the demo wasn't done with an alpah/beta emitter or sensor.

I was going to ask about cosmic rays...  but the discussion seems to indicate that the benefit would likely be minimal, but perhaps the material would augment spacecraft construction materials.

[CliffordK]

so they really need suicide volunteers for the most dangerous jobs.

Usually what OSHA does is gives you an annual dose limit.
They don't care if you get that dose in 15 minutes, or 365 days.

So, the really hot work is done with a relay team in very short shifts.

The other alternative is robotics. 

[yor_on (OP)]

So, no studies known then?

That I can't accept, I will have to look some more, there should be some at least. And I agree, above possibly 2 MeV (Compton scattering) the suits seems to give little protection for Gamma radiation. Still, it seems the best there are, for the moment. It seems also able block all kinds of bacterias, virus etc, according to some sources, so yeah, it should definitely be watertight. Still, having a gamma-protection up to that level seems better than using nothing?  

[wolfekeeper]

Here's the old puzzle. You've got an alpha source, a beta source, a gamma source and a neutron source. You've got to throw one away, swallow one, put one in your pocket and hold the other at arms length. Which one do you do what to?

[Geezer]

Hopefully the demo wasn't done with an alpah/beta emitter or sensor.

It sure looked like one! It was sitting on a table in the middle of a factory, had no apparent shielding, and nobody was wearing any sort of protective gear at all.

As far as I could tell, the only thing that seemed to be missing was a big label saying "P. T. Barnum".

Oh yes - I forgot to mention - the "Doctor" also threw in some nanoparticle mumblespeak too. Of couse, we know that Florida is a also major center of resarch in that field.

[Geezer]

Still, it seems the best there are, for the moment.

No it ain't. If the level of protection is provides is little better than a plastic bunny suit, but it gives people a false sense of security, it's worse than a plastic bunny suit, and even worse, it's making somebody rich.

[yor_on (OP)]

Geezer if you trust the research it seems to take care of the gamma radiation at the normal x-ray level for hospitals at least. So they should be better than nothing. Also they seem very impregnable for all other biological substances, saw that a lot of different forces was buying them in the States.

I totally agree in that they won't protect, as a guess that is, at all over 2 MeV and so won't allow people any closer than they already can come maybe. But there they will protect better than what they had I think.

[Geezer]

On the TV clip I saw no evidence that they are able to block any kind of X-rays. The fact that the guy demonstrated its "effectiveness" with an alpha or beta source makes me highly skeptical.

Are there any test reports available from reputable independent test agencies?

[syhprum]

Radiation is a very loosely used term in relation to nuclear mishaps, to my mind radiation means electromagnetic energy such as gamma rays but it is used for Alpha and Beta particles and even microscopic particles of Plutonium etc.

[yor_on (OP)]

Radiation testing on Demron fabric samples was performed by the US Department of Energy at Lawrence Livermore National Laboratory (2003?).

Here. 

" Demron is effective as a radiation shield, comparable to lead in terms of g/cm2 and tantalum according to the mass attenuation coefficient, against gamma, x-ray and beta emissions. For example, for 100 keV photon radiation, the mass attenuation coefficient is about 3.8 cm2/g, which means that the transmission will be down to the l/e point for a thickness of 1/33 = 0.26 g/cm*.

For Demron, with a density of 3.14 g/cm3, the thickness would be 0.8 mm corresponding to 2 layers for the present sample. For lead with a density of 11.3 g/cm3, the thickness would be 0.2 mm."

But it is old, I would have liked to find a newer one.

[Geezer]

Gee whizz! Those boys at Lawrence Livermore really know their stuff.

"Demron is effective as a radiation shield, comparable to lead in terms of g/cm2**."

Perhaps the fact that the material is infused with elements with "high atomic numbers" has something to do with it.

 
Well, he's certainly been busy on the patent front.

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=0&f=S&l=50&TERM1=Meridian+Research+and+Development+&FIELD1=ASNM&co1=AND&TERM2=&FIELD2=&d=PTXT

Claims from the most recent patent -

3. The article of clothing of claim 1 wherein the nano-material is formed from one or more nano-sized substances selected from the group of tungsten, barium, boron, tantalum, bismuth, silver, gold, platinum, aluminum, copper, depleted uranium, cerium oxide (CeO.sub.2), yttrium oxide (Y.sub.2O.sub.3), lanthanum oxide (La.sub.2O.sub.3) and neodymium oxide (Nd.sub.2O.sub.3).

4. The article of clothing of claim 1 wherein the nano-material is formed from one or more radiopaque substances including nano-sized lead or tin.

Hmmmm? Nano-sized lead or tin....



** WTbleep does g/cm2 (grams per square centimeter presumably) mean?

[Bored chemist]

g/cm ^2 is what's called areal density.
since the suit has to have enough area to cover a person that area is pretty much fixed.
So, for a given areal density it's a measure of how heavy the suit is.
As explained earlier lead is better shielding on that basis than most things.
If the suit was better than lead I'd be surprised.
As it turns out, it's comparable with lead.
Gosh! That's because of the lead in it.

[yor_on (OP)]

So, there should be a be newer test somewhere. Why not find a test to prove your views? After all, I was the one asking

I do not find Lawrence Livermore National Laboratory incompetent, or this test incomprehensible, but I would like to see another, newer one too, or was the material already in its final state 2003?

[Geezer]

The latest patent was filed in 2004, so they were probably still developing it.

There is nothing wrong with the Lawrence Livermore report. But it seems to conclude that, if the material is as heavy as lead, it will provide the same protection as lead, which is hardly surprising if the material is full of lead. However, the stuff I saw on the TV obviously did not have much lead in it, if it had any lead in it at all.

The patents tell us that Meridian is combining various elements with polymers to produce a flexible fabric, but when it comes to X rays, they really have to use lead or something with an even higher atomic number. I took a quick look to find out why nano-sized lead would be any better than any other variety of lead, but I didn't see much, so I'm inclined to think it's largely hype, although I did not crawl through all the patents in depth, so I may have missed something.

My basic objection to all of this is that Meridian would like us to believe that they have invented a material with special shielding properties, but it seems the bottom line is, if you want to block gamma radiation, you better get behind a bunch of lead, whether it's built into a suit or not.

[Bored chemist]

Finely powdered lead might affect the flexibility of the garment less and blend into the polymer more easily.