Does crystal shape matter ?

Scintillation crystals, PMTs, voltage dividers etc...
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Steven Sesselmann
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Does crystal shape matter ?

Post by Steven Sesselmann » 31 Oct 2018, 12:10

Someone here might be able to answer my question...

I was just pondering if there is such a thing as an optimal shape for a crystal. We understand that Compton scatter results from photon energy lost through the sides of the crystal, and we know that generally larger crystals show less Compton scatter. This all makes sense because the surface area to volume is less in a larger crystal than in a small one.

Assuming the surface area to volume affects lost energy it seems reasonable that the optimum shape ought to be a sphere, which we know has the smallest surface to volume ratio.

Has anyone experimented with spherical NaI(Tl) crystals or at the very least tried rounding off the corners rather than shaping it like a straight cylinder ?

Maybe Luuk can shed some light on this question ?

Steven
Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | rephopper.com | beejewel.com.au |

luuk
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Re: Does crystal shape matter ?

Post by luuk » 01 Nov 2018, 05:46

Hi Steven,

We build several spherical NaI(Tl) detectors for customers over the years used for all kind of experiments mostly used in universities, but unfortunately I have no data to show you!
When I did some standard test with one of them I was not really impressed, not much better than a standard 2x2” detector in my eyes.
A big problem with these type of detectors is the mechanical construction, it is very complicated to make and therefore these detectors very expensive.
Also it is not simple to make the crystal spherical it is hard to do, and there is a big chance that they break during this process.
Here a picture of two nice detectors the small one is about 1”diameter the bigger one 3”.

Luuk
Steven I just see that I cannot attach. any pictures here, that is a pity they are really nice!

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Steven Sesselmann
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Re: Does crystal shape matter ?

Post by Steven Sesselmann » 01 Nov 2018, 07:24

Luuk,

Not sure why you can not add pictures, did you click on the full editor/preview button?

I understand spherical detectors can be difficult to make, but possibly putting a small 5 mm radius in the normal 2" cylindrical might slightly reduce compton?

Just a thought.

Steven
Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | rephopper.com | beejewel.com.au |

luuk
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Re: Does crystal shape matter ?

Post by luuk » 02 Nov 2018, 06:19

Hi Steven,
Been a long time when I tested a ball detector, but like it said before I was not impressed with the results
If we make a ball detector in the future I shall see if I can get some time to do some tests, but that is not always possible many time detectors are shipped out a.s.a.p. so there is no time to play with it, but if I can I shall post the results.
But it may take some time because we build maybe only one every 5 years so it is not a very popular product.
A detector from just 5mm is not simple to make it is to small to handle the rounding is handmade!
I managed to add a picture from two nice ball detectors the small one is 1” the big one 3”.
Luuk
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iRad
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Re: Does crystal shape matter ?

Post by iRad » 03 Nov 2018, 02:53

Forget the performance, they look really cool... Cheers, from Tom (in the middle of the Atlantic Ocean)
Please check out my eBay Store: http://stores.ebay.com/The-Rad-Lab

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