aluminium tube for PMT, why ?

[invader]

Hello,

I think that all spectroscopy tubes I have seen, all have aluminium tubes, my question is, why aluminium ?

I have some obvious reasons:
- conductive metal , that enables electrical grounding, for EMC shielding.
- low Z, that means less contamination of spectrum from secondary x-ray
- mecanical hardness
- low price

but I see a disadvantage
- even with low Z, radiation shielding

my question is, why not the front part be some other material, maybe plastic ? maybe berilium ? something with lower radiation shielding tham aluminium.

best regards
Paulo

[Jim Kovalchick]

Some scintillation detectors for lower energy indeed do use beryllium windows. They are usually more expensive. Aluminum is a good choice for electrical grounding and limited attenuation.

[iRad]

Hi Paulo
Your question and stated reasons cause me to think you are questioning the housing design for the entire detector, not just the PMT (Photomultiplier Tube).

Your reasonings mostly fit both uses, the PMT housing and the Crystal can portion of a detector. Often these are made of the same material, and there may not be separation between those portions of the detector housing with some builds. This is often true for detectors that have a single diameter and are considered integral, meaning that the crystal is directly coupled to the PMT without the use of a sperate “canned” crystal. For the reasons you stated (and others), canned crystals are almost always in aluminum cans. For tapered detectors (usually 3” and larger detectors), the crystal housing is almost always separate from the PMT housing, but may be the same material, especially if it’s aluminum. As Jim stated, sometimes beryllium or other window materials are used, mostly when low energy detection is desirable.

As for the PMT housing itself, these are often made of some type of steel. When they are made of steel, the steel is often treated in a process that makes it resistant to magnetic influence of the PMT inside. These are often called mu-shield, or reference by the brand name Mu-Metal. When PMT housings use aluminum, they almost always have a thin piece of steel mu-metal coiled around the PMT for magnetic (and RF) shielding (should be grounded). Some detectors use both a stainless-steel mu-shield, plus a mu-metal sleeve around the PMT. The popular Scionix Holland 38B57 is an example of this type of detector and happens to also have an aluminum can surrounding the crystal.

The use of RF and magnetic shielding, as well as grounded housing, is not necessary to make a functioning detector. I sell very popular and inexpensive plastic detectors and DIY kits that are perfect for counting applications. However, for serious spectrometry use, well designed and built housing are essential for best results.

[invader]

I'm changing my tubes configuration, continue to use aluminium tubes but the front parte if I use aluminium I will use a diferente thickness , but I'm also consedering use Berilium for that portion of the tube.

[iRad]

Beryllium can be problematic (even hazardous) to work with. It is also prone to corrosion, very expensive, and difficult to find. It is rarely used for more than "window" material at the front of a probe used for very low energy. It would only be useful if you had a PMT and crystal assembly capable of low energy, which is not as easy as one might think. I recommend just sticking with a thin aluminum can for the crystal on most builds...

[invader]

Dont worry about my safety, we are all handling radioactive materials :)

I dont need to work with beryllium, I buy the metal already cut in a round shape, just have to put it on the end of the aluminium tube.

I know that it probably dont do much diference using berilium window, but I like to try diferent things.

.