Dont worry about alpha particles, the SBM-20 is made from metal, so alpha particles can't reach the inside of the tube. Alpha particles are usally blocked by a piece of paper or a sufficient space of air. The SBM-20 tube you have is not alpha sensitive, so don't worry about them.
For an accurate measurement, i would record the background counts for like 1hr, and then place the tubes there for one hr, this will get you a better average CPM than short-time samples. The middle metal part of the tube is GND, so it would be enough to cover the terminals, or cut out a long-hole in the case. Or take a plastic pipe and drill a lot of holes in it. For shielding i would use lead, have a look at "gamma spectroscopy lead shielding" - it will work for gm tubes as well. Since quality lead will not be available for free, it might be a better idea to get a more sensitive tube. LND Inc will sell pancake tubes LND 7314 (alpha,beta,gamma) and similar tubes that opperate at 500V, they will work at 450 too. The price for a LND7314 is around 115$,and is very similar to the one that is inside the rate meter i have in the picture. And here you can find a comparison between common geiger tubes: Link! <https://9ab62263-a-62cb3a1a-s-sites.googlegroups.com/site/diygeigercounter/gm-tubes-supported/Chart%20-%20GM%20all%20tubes.png?attachauth=ANoY7cplmIWWH23tu8tPiPLu7eKvS0EVilyG63LF3EKI_ToJAwn7Wgy7PXzU5Kf-HWKtC7vhHuKSZnF-lOjYbMwOh06xQuRQvIYkt2U2fYJ9yatFfdNfTB2f92fIrbCY5FJ6WuOrzetACCH_WkgAibmaX2YPmIEMQGMvGT7NIpWemy8rUWGMBFRNBeQCKICLYMmO6892kLXwXCttC3utcbiBCO2MFhU8eRePUV74o92SMJqGuG3KvHkeq4rwAGZwWO78hhh_fgso&attredirects=0> -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/5245dc16-3af7-4a18-9e09-a8106874f0e8%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
