Purpose of optical coupling gel

Scintillation crystals, PMTs, voltage dividers etc...
tim.hbn
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Purpose of optical coupling gel

Post by tim.hbn » 25 Aug 2021, 22:45

Hi GigaBecquerel

In your post at the following link, you mention that you put optical coupling gel between the crystal and the PMT.

viewtopic.php?f=14&t=841

I would be very grateful indeed if you could explain to me the purpose of the optical coupling gel and what the differece would be if just air was between the crystal and the PMT.

Thank you very much.

Kind regards

Tim

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iRad
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Re: Purpose of optical coupling gel

Post by iRad » 26 Aug 2021, 00:48

Hi Tim, and welcome to the group
Of course, I’m sure you know light needs to travel from the crystal to the PMT. To get there, it must pass through several surfaces. Usually on the crystal there is a glass window, then there is an air gap between the glass window of the crystal and the glass window of the PMT. Every time light passes through another surface including the air, a refractive index of the material must be taken into account. When the refractive index is very different from the previous surface, the light is scattered, and loss also occurs. Ideally, the light should travel in a straight line with little scattering and loss. The coupling compound’s refractive index matches the surfaces between the crystal and the PMT much better than air, thus allowing the light to travel with less scattering and loss. Cheer’s Tom
Cheers, Tom Hall / IRAD INC / Stuart, FL USA
Please check out my eBay Store: http://stores.ebay.com/The-Rad-Lab

tim.hbn
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Re: Purpose of optical coupling gel

Post by tim.hbn » 26 Aug 2021, 03:00

Hi Tom

Thank you very much indeed for your very clear explanation.

To be honest, I thought it might be something vaguely like that but it is great to have it clarified and confirmed by an expert.

Kind regards

Tim

EDIT: Is this the correct stuff?

https://www.scanwel.co.uk/shop/sl-dow-c ... rease.html

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Sesselmann
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Re: Purpose of optical coupling gel

Post by Sesselmann » 26 Aug 2021, 08:23

Tim,

The Vacuum grease is not 100% transparent, it would probably be okay if used in a very thin layer, but optical coupling fluid is clear high viscosity silicone fluid. PMX-200 (100,000 CS)

Not always easy to find, but Tom might actually sell this, not sure, but if you get stuck I could send you a dob in a plastic tube.

Steven

tim.hbn
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Re: Purpose of optical coupling gel

Post by tim.hbn » 26 Aug 2021, 09:20

Hi Steven

Thank you very much for your reply.

Thank you also very much for your offer of sending me some.

Is this the same stuff?

https://www.amazon.co.uk/Team-Losi-Raci ... B01IBDVPKQ

Kind regards

Tim

EDIT: If I use this with a MPPC, is there a risk of a short curcuit?

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Sesselmann
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Re: Purpose of optical coupling gel

Post by Sesselmann » 26 Aug 2021, 09:26

Tim,

That stuff from Amazon looks like it will do the job. Will be enough for many detectors.

Steven

tim.hbn
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Re: Purpose of optical coupling gel

Post by tim.hbn » 26 Aug 2021, 09:31

Hi Steven

Thank you very much for letting me know.

Did you see my edit with the question about the risk of a short circuit?

Thank you very much.

Kind regards

Tim

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Sesselmann
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Re: Purpose of optical coupling gel

Post by Sesselmann » 26 Aug 2021, 10:08

Tim,

The silicone oil is a dielectric, the constant seems to be around 2.5 to 3.5

https://www.vega.com/-/media/pdf-files/ ... nts_en.pdf

Steven

tim.hbn
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Re: Purpose of optical coupling gel

Post by tim.hbn » 26 Aug 2021, 10:11

Hi Steven

Thank you very much indeed for all your help.

Kind regards

Tim

luuk
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Re: Purpose of optical coupling gel

Post by luuk » 28 Aug 2021, 00:48

Hi Tim,
I don't know were you are located but if it is in Europe I can send you some optical gel,
if you are located in the USA it is more easy to contact Tom Hall from Irad.
Luuk

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