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Author Topic: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?  (Read 2315 times)

Offline McKay

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Does a charged metal plate blocks/ absorbs gamma radiation then a uncharged plate?
From what I gather, it is the increased amount of electrons in dense and/ or heavy (as two materials can be the same density aka atoms per volume, but one of them can be heavier atom per atom) materials that contribute to the absorption, scattering/ blocking of gamma radiation.

Speaking of (negatively) charged metal plates - how much electrons can a piece of metal actually accumulate given perfect insulation?
And speaking of charged plates as a whole - how positively charged can a piece of metal become? I figure after all the so called free electrons are taken out, the next step would be to rip of the stable electrons from deeper shells of atoms, seriously messing up its chemical and physical properties? When a metal is deprived of its free electrons, it would not be a electric conductor anymore, would it?


 

Offline Bored chemist

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Welll, according to this
http://en.wikipedia.org/wiki/Supercapacitor
you can get energy storage up to about 10 W H /Kg
That's 40KJ/Kg or so or 40 J/g
The voltages are a few volts per capacitor- so the charge is a few times less than that, about 10 Coulombs per gram or roughly 10^20 electrons per gram.
The electrodes are made from carbon so lets assume that's where the electrons come from.
A gram of carbon is 1/12 moles.
Each mole is about 6E23 atoms and each atom has 6 electrons so the gram of carbon contains 3E23 electrons

So it looks like the best capacitors you can get only move about 1E20 electrons out of 3E 23
At best, that would change the electron density by about 1 part in 3000.
Not worth the effort.

It's even worse once you realise that, while you have made on plate of the capacitor about 0.3% better at stopping xrays, you have made the other one worse by exactly the same factor.
Also, if you do the calculations for field gradients near charged plates that you might use a shielding, the figures turn out much worse as the separation gets bigger and  the voltages get higher.

It's never going to work.
 

Offline alancalverd

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Lead has 15 times the electron density of carbon and only 5 times the mass density, which is why it is the preferred absorber for gamma radiation. 
 

Offline Bored chemist

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Uranium is even better. You need a coat of paint to stop the alpha particles from the uranium itself, but you would need that to stop it oxidising on exposure to air anyway.
 

Offline alancalverd

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True, but it's a bastard to cut, bend or weld. 
 

Offline McKay

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Hmm, ok. Thanks  for the information, really interesting.
But, anyway - the answer to my question is "yes", to a degree :) Just not practical enough.
 

Offline alancalverd

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On further reflection I don't think that's true anyway!

Absorption of gamma radiation depends on photoelectric absorption by bound electrons, and pair production from gamma-nucleon interaction, both of which degrade the energy of the incoming photon which eventually ends up as heat or mechanical or chemical deformation of the absorber.

Interaction with the free electrons you have added by charging the plate will produce elastic (no energy loss) and near-elastic (Compton) scatter (very little degradation of photon energy).   
 

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