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High purity Germanium detector design related clarifications

Ashish GK

Aug 23, 2017
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Hello everyone,

I am trying to do a case study on some of the design related flaws in a high purity germanium detector. So there is a cylindrical germanium crystal (weighing about a kilogram) that is connected to a vertical copper rod. The setup is shown in the following figure:
upload_2017-8-23_13-6-54.png

There is a clear triangular plate in this setup that I have marked in yellow. This plate cracks even with the most gentlest handling. So I would like to get your ideas on the following:
1) Which one is optimum for the triangular plate: high thermal conductivity or low thermal conductivity?
2) Since it cracks, what material could it have been made?
3) Why would one have chosen this geometry and material?

The crystal must be kept as cold as possible. Hence, the entire setup is connected to a 30 L liquid nitrogen dewar while running. In addition, the detector is under high vacuum to minimize convective heat transfer between the room-temperature
exterior of the detector and the interior cold crystal and copper rods.

So, for the above questions, I feel that as cold environment is more suitable for operation, the plate should have low thermal conductivity, as even the slightest rise in temperature will reduce the efficiency of operation. I am pretty unsure of the material and the reasoning behind this design.

Any suggestions and ideas that would give more insights into the above questions will be very helpful.


Thanks,
Ashish
 

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Alec_t

Jul 7, 2015
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Measure the density of the (cracked) plate. That will be a guide as to what the material is.
 

shrtrnd

Jan 15, 2010
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If it is Plexiglas, and I think davenn is right about that, you should be able to get a machine shop to make you a lot of them pretty inexpensively, and replace them when they crack. You said you're going to check into the properties of the material. Be aware that Plexiglas is only one of many types of this transparent material available. There are many other types available that can withstand more pressure than what you're applying before they crack.
 

KMoffett

Jan 21, 2009
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If the edge is white'ish it's acrylic (Plexiglas). If the edge has a blue'ish cast, it's polycarbonate (Lexan). Where is it cracking? Sharp edges can promote cracks. Are the fasteners tight before cooling? Metal shrinks when cooled, so fasteners can decrease in length and increase pressure on the plastic when cooled. The three point triangle mount can help in axial alignment.

This may help pick a better material for your application:https://www.google.com/search?q=polycarbinate+at+very+low+temperatures&ie=utf-8&oe=utf-8

Ken
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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If it is acrylic, find someone with a laser cutter to produce them very cheaply.

If cut with a laser cutter you won't have edge imperfections that could lead to stress fractures starting. Acrylic can also be annealed, but at liquid nitrogen temperatures I'm not sure it would make any difference.
 
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