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klugesmith

Thu Mar 03 2016, 10:20PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Excuse me for picking on a minor detail.

Covalent bonds hold carbon atoms together in hydrocarbons etc., fullerenes, and graphene.

Whatever holds graphene layers to other layers in graphite is a much weaker kind of bond. Where's Bored Chemist when we need him?

When pure carbon has covalent bonds in the Z direction, as well as in the XY plane, we call it diamond.

Proud Mary

Thu Mar 03 2016, 10:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Here's a 1974 US patent for a 'Window for radiation detectors and the like.. formed by hot-pressing and annealing pyrolytically deposited graphite and thereafter stripping off layers of sufficient thickness to form the window.'

Conundrum

Fri Mar 04 2016, 04:07AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Hmm. Well done for finding it, wonder if they were ever put into production?
I've not seen anything like this but did once read that some early GM tubes had the inside of their windows coated with tin oxide to increase conductivity.

"Whatever holds graphene layers to other layers in pyrolytic graphite is a much weaker kind of bond. Where's Bored Chemist when we need him?"

I'm led to believe that the bonds might involve shared electrons.

Proud Mary

Fri Mar 04 2016, 09:46AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Conundrum wrote ...

Hmm. Well done for finding it, wonder if they were ever put into production?
I've not seen anything like this but did once read that some early GM tubes had the inside of their windows coated with tin oxide to increase conductivity.

Here is my Centronics B12N, a glass beta-gamma detector where the cathode is a conductive layer deposited on the inner surface of the glass. Working voltage is 675V. I believe it was designed for dipping into liquids.

Graphenic carbon has been used very recently for x-ray transmission windows with superior performance at low energies:

Conundrum

Sat Mar 05 2016, 08:54AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Would this be efficient enough to let through X-ray energy below say 7keV?
I noticed during various experiments that low energy X-rays can adversely affect certain memory cards and can in fact be used for data recovery purposes when a card is registering but cannot be accessed.

there was a faint green glow and counters were both registering above background so pretty sure that that old radio tube was emitting X-rays and UV when pulsed with a 9V gas igniter running at 27V.
Most of them were apparently in the very low keV range but managed to get a faint fluorescence through a Maplins project box and RFID disk was clearly visible.

Proud Mary

Sat Mar 05 2016, 10:35AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Conundrum wrote ...

Would this be efficient enough to let through X-ray energy below say 7keV?

If you mean the Centronics B12N, then no, I wouldn't expect the glass wall with its conductive layer would be transparent to x-rays below about 15 - 18 keV.

GM tubes with mica windows can detect Grenz rays down to about 3 keV, and maybe just a little lower, in my experience.

The energy response of GM tubes is so extremely non-linear with soft x-rays that they can only be used to detect the presence or absence of the rays, rather than provide any meaningful indication of dose rate.

If a GM tube were to be made using commercially available Panasonic 10 micron pyrolytic graphite foil then I'd expect to get down below 2 keV, but of course we also have to take account of absorption by the X-ray source's beryllium exit window - and the air intervening - with these ultra-soft Grenz rays.

Conundrum wrote ...

I noticed during various experiments that low energy X-rays can adversely affect certain memory cards and can in fact be used for data recovery purposes when a card is registering but cannot be accessed.

I have no experience whatever to share with you here.

Conundrum wrote ...

there was a faint green glow and counters were both registering above background so pretty sure that that old radio tube was emitting X-rays

I have found in practice that PDs of 15 - 18kV are required before any measurable quantity of x-rays are transmitted through the glass wall of thermionic valves.

Conundrum wrote ...

Most of them were apparently in the very low keV range but managed to get a faint fluorescence through a Maplins project box and RFID disk was clearly visible.

I can't see how any 'very low keV' rays could be transmitted through the glass of any thermionic valve. Soft rays are certainly generated by field emission in a cold cathode situation when a suitable PD is applied across a valve, but these are blocked by the glass. They could, I suppose, cause the glass to fluoresce without however being able to go all the way through it, though I have no experience of this myself.

I shall certainly get some of the Panasonic 10 micron pyrolytic graphite film, when I have the time, and try it out as a window on a detector tube using butane, or butane-propane, at atmospheric pressure, as the counting gas.

hen918

Sat Mar 05 2016, 11:31AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Conundrum wrote ...

...

kludgesmith wrote:
"Whatever holds graphene layers to other layers in pyrolytic graphite is a much weaker kind of bond. Where's Bored Chemist when we need him?"

I'm led to believe that the bonds might involve shared electrons.

From Wikipedia: "Bonding between layers is via weak van der Waals bonds, which allows layers of graphite to be easily separated, or to slide past each other."

Bored Chemist

Sun Mar 06 2016, 10:19AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

klugesmith wrote ...

Excuse me for picking on a minor detail.

Covalent bonds hold carbon atoms together in hydrocarbons etc., fullerenes, and graphene.

Whatever holds graphene layers to other layers in graphite is a much weaker kind of bond. Where's Bored Chemist when we need him?

When pure carbon has covalent bonds in the Z direction, as well as in the XY plane, we call it diamond.

Bored chemist was wanderinground other bits of the web. Fortunately, someone else had the sense to look on WIKI.
Yes, the layers are held together by (rather weak) Van der Waals forces.
It's hardly going to affect their suitability as radiation windows, though diamond might also work nicely.
I guess it depends if black or colourless fits your preferred colour scheme (and on the size of the budget).

Proud Mary

Sun Mar 06 2016, 12:02PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Bored Chemist wrote ...

... diamond might also work nicely.
I guess it depends if black or colourless fits your preferred colour scheme (and on the size of the budget).

Here is a commercially available diamond x-ray window unit. Diamond windows are available down to a thickness of 1 Âµm which makes them ideal for use with low energy x-rays. They are edge cooled.

Though in theory a Beryllium window should be superior to one made from Carbon, because of its lower atomic number, in practice Be windows are fabricated from an alloy 98% Be/ 2%Cu, which degrades the X-ray spectral purity with copper absorption edges. Beryllium is also a highly toxic environmental hazard, all of which makes carbon/graphene/diamond windows increasingly attractive.

Wastrel

Tue Mar 08 2016, 10:40PM

Registered Member #4095 Joined: Thu Sept 15 2011, 03:19PM
Location: England.
Posts: 122

Many thanks for the PGS window tip. I've been kicking myself for not picking up some BS7's when sovtube had them in stock, they'd have been worth it for the light and vacuum tight beryllium windows alone. This could be trivial money.

I've found the raw 25um PGS for sale at mouser and other places but it looks like the 10um isn't normally sold outside of a more manufactured product like a laminated tape. I'm thinking of ordering some of that too and trying to delaminate or degrade the non graphite layer but 25um would still be a lot better than an aluminium foil window I thought I'd be forced to use.

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