How to arrange two GM tubes for a coincidence detector?

Hi Sonarflash, I'll give this a shot. I can only offer my explanation of what I have read/seen on the net. I have never had any experience with a coincidence detector so I can't say I'm qualified but I will try my best to explain. I'm assuming you want to setup a coincidence detector as in the cosmic ray/muon type?

To start, in the setups I have seen, the GM tubes tend to be one on top of the other, parallel to each other, completely aligned. The gap between the two tubes tends to vary from setup to setup. I have read/seen people placing the tubes as close as 1cm right through to 10cm or so. A distance of about an inch, 2.5cm, seems to be fairly common so that would be a good starting point.

I have seen/read various setups using plates of aluminium or lead for two different purposes. One being to shield from background radiation (I'm assuming that muons will pass through thin lead) and the other purpose being to block beta and allow gamma to pass.

From what I can gather the spacing of the GM tubes is relative to the angle the capture area, so the closer together they are will allow for a wider angle of particles to be detected that are travelling in the direction of the GM tubes and thus will detect more coincidences than if they were further apart. Moving them further apart will narrow the coincidence detection and will report more of a single tube detecting. I have seen setups that contain multiple GM tubes such as a 3 by 3 array. On that particular setup there was very little distance between the tubes, 1 or 2mm only. I suspect that setup would be detecting the direction the particles are travelling. there are also setups with three GM tubes mounted one above the other with about 2cm distance between. It looks like the three tube setup would offer less, but more true coincidences as the particle would have to travel through the three tubes and would offer a very narrow detection angle. When I state this detection angle, I am meaning that of a coincidence detection, not a single tube detection.

Now, here goes, some kind of description in text:
For this I am going to assume that you are after detecting muons/cosmic rays. I'm trying to make this as generic as possible so I have looked at a few setups and I am offering notes of the general setup. I this building ths would require some kind of trial and error though no specifics seem to be needed.

The two tubes are mounted one above the other, parallel to each other
The distance between the tubes should be about 2.5cm, so you have the bottom tube, then above that a 2.5cm gap then the top tube in parallel to the bottom one.
A sheet of lead, 1.5mm or so thick, about 5cm wide and a little longer than the tubes is placed between the two tubes. I will refer to this as the middle sheet
An optional sheet of lead, 1.5mm or so thick, about 5cm wide and a little longer than the tubes is placed about 2.5cm above the top tube. I will refer to this as the top sheet.
They are assembled to create a tower like structure and giving the tubes a little space at the bottom would be about 15cm tall, 5cm wide and the length just over that of the tubes
Going from the top down the setup should look like this:
Top sheet right at the top, then a 2.5cm gap below then the top GM tube. 1.25cm below the top tube is the middle sheet, then 1.25cm below that is bottom tube. The rest of the space below the bottom tube is just spare space.

I seems that some setups don't have the top sheet and the middle sheet can vary and may be aluminium. I suspect this is for detecting/shielding from different particles, beta, gamma, muon. If I were to make my own muon coincidence detector I would use both top and middle sheets in lead to stop all beta and gamma but let muons through. One thing I don't understand though is if people use lead sheets to stop beta and gamma, then why not just roll the lead around the GM tubes to create a sleeve. That would surely block gamma from all angles. Maybe I'm missing something with the sheets.

One neat construction I looked at had the whole tower part (both tubes and sheets) attached to a pivot on a base. This allows the entire detector to be angled and aimed at something specific.

Alright Sonarflash, I hope what I have written makes sense. If any part doesn't then please let me know and I will try and explain.

Cheers,
Simon.

Sonarflash, I forgot to add a link (below) to a pdf I found. The pdf contains quite a lot of text that also may be of assistance.
https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0ahUKEwjBwt2Xj4fMAhWCVywKHb6yCg8QFgg0MAQ&url=http%3A%2F%2Fwww.theremino.com%2Fwp-content%2Fuploads%2Ffiles%2FCosmicRaysDetector_ENG.pdf&usg=AFQjCNHVJu_rQ7UTRWdEsDU5ZXqH0-5jug&sig2=F8G3-AUE7CCfw9DDJeSOBA