Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: McKay on 06/09/2014 10:05:59

Title: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: McKay on 06/09/2014 10:05:59
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?
Title: Re: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: Bored chemist on 06/09/2014 11:54:47
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.
Title: Re: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: alancalverd on 06/09/2014 12:38:40
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. 
Title: Re: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: Bored chemist on 06/09/2014 14:03:30
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.
Title: Re: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: alancalverd on 06/09/2014 18:08:00
True, but it's a bastard to cut, bend or weld. 
Title: Re: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: McKay on 06/09/2014 18:25:32
Hmm, ok. Thanks  for the information, really interesting.
But, anyway - the answer to my question is "yes", to a degree :) Just not practical enough.
Title: Re: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: alancalverd on 06/09/2014 21:07:17
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).   
Title: Re: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: syhprum on 09/11/2019 08:45:38
Sorry to be pedantic but arithmetic accuracy is important in scientific discussions so I will point out that 1 part in 3000 is 0.033%.
Title: Re: Charged metal plate blocks/absorbs gamma radiation bette then a uncharged plate?
Post by: Bored chemist on 09/11/2019 11:48:34
Sorry to be pedantic but arithmetic accuracy is important in scientific discussions so I will point out that 1 part in 3000 is 0.033%.
Oops!
Good catch.
Better late than never