Archived from radmon.org - originally posted 12/08/2019
In reply to a nzoomed's question on radmon.org:
Im wanting to build an outdoor monitoring station with an SBM20 and not sure if its good to use a PVC pipe to place the tube inside?
Obviously im wanting something as thin as possible to let as many particles through as possible.
Are there some materials I should avoid?
Are beta particles typically encountered much in the environment? Should I only be worried about cosmic rays? Im not sure how much background radiation is from other sources, but im expecting there is a great deal from alpha decay from the likes of radon etc which will never be picked up anyway?
What does everybody else do? Im wanting to get the highest possible outdoor readings if possible.
I mounted my counter in an ABS enclosure outside under a canopy on my workshop. I cut a slot in the front where the tube is and covered that with Kapton tape to keep it reasonably air tight. I have seen people use PVC pipe for housing their tubes. I used some 20mm plastic (PVC?) electrical conduit to make a house for a SBM-20 tube that I can fix externally to a window on my car with a sucker. It works fine but it is ever so slightly less sensitive. For background monitoring though I think this would be fine. As you mention, use something with as thin a wall as possible and also keep it watertight if used outside. Airtight would be better but it is hard to achieve whilst maintaining accessibility for service etc. I have a couple of large bags of desiccant behind the board in my counter that I have replaced once in 3 years and has done a fine job of keeping it dry and corrosion free. I live by the sea so we have quite humid air at most times.
When it comes to detection and detecting alpha/beta/gamma it depends how far down the rabbit hole you want to go. The SBM-20 tube is capable of detecting gamma and hard beta OK. It however, will not detect alpha at all. For that you will need a tube such as the LND712 (great for portable hand held counters) or something like the SBT-10A (very sensitive to alpha, beta & gamma - great for surveying). The issue with detecting alpha outdoors is that the tube itself would need to be open to the elements. Over time the tube window would need cleaning which in most cases would ruin the tube itself as you are not supposed to touch the alpha window at all. I have a SBT-10A that mysteriously destroyed itself but does show how fragile they are. I believe it is possible by using mica to create a window for the tube as that apparently lets alpha pass through but I have never experimented with so can't say for sure although a lot of alpha tubes use mica themselves. It is possible to detect beta and gamma using SBM-20 tubes but this requires two tubes, one shielded with 3-4mm of aluminium and the necessary counting electronics. Basically two separate Geiger counters, one with the shielded tube. The unshielded tube will detect gamma and beta whereas the shielded tube will detect gamma only thus if there is a noticeable difference in counts, math can be applied and the difference in counts would be the beta detection.
Going back to your SBM-20 housed outdoors there are some things to be aware of. Make sure the cable between the tube and the counter is no more than about 1m in length. If you are going to mount the tube separate to the counter it would be prudent to place the anode resistor at the tube itself and not at the counter. This will reduce interference causing inaccurate readings. I like to keep my tube as close to the counter as possible which rules out any interference a cable may pick up. It would be perfectly fine to place the counter on the inside of a wall and the tube directly outside using a small cable (20cm or so). I would still put the anode resistor at the tube though. But in doing that you will have two things to house and mount so putting the tube in a larger enclosure with the tube may mean a lot less work.
With regard to Radon gas, it is pretty much impossible to detect outdoors as it dilutes in the air fast and with just a gentle breeze it is gone. Radon is an alpha emitter but also produces alpha and beta decay products during it's decay. The beta decay products would be detectable using an SBM-20 tube but their half life is so small it would be very hard unless you had a monumental release of Radon. Radon outside won't harm you as it is diluted by the air and swept away in the wind but it can be potentially very harmful indoors if exposed to over time. The best places to detect Radon gas at home would be in a basement or under the floor boards on the ground floor as it emanates out of the ground itself. If you were conscious about checking for radon there are specially made units for detecting Radon gas but I don't believe any outdoor units exist except for very expensive specialized detectors.
Are beta particles typically encountered much in the environment?
Most likely (Radon decay products produce beta), but without the equipment to differentiate between gamma and beta there is no way to know, unless you knew what the actual radioactive source isotope was. Using the two tube method it is possible to gain a feel for whether a detection was beta or gamma but bear in mind that an average Geiger counter isn't very accurate. I believe my NET-IO GC10 with SBM-20 is about +-25% accurate so what it tells me could actually be 25% less or 25% more radioactive than what the counter lets on. You can increase the accuracy by using more sensitive tubes like the SBT-10A or an SI-22G. The SI-22G is about 9 times more sensitive (and about 4 times larger) than the STM-20 so over time this would give a better resolution but not necessarily more accurate, although a greater resolution would offer a greater accuracy over time. Over time you will get to know your counter and it's results and you should get a good feel for whether it is detecting anything other than background.
Should I only be worried about cosmic rays?
Nope. You shouldn't worry about cosmic rays (muons) at all. They are subatomic particles and will literally go straight through you. Every square meter on earth is hit by thousands of these every second so there is a good chance you are getting hit with thousands of them right now, and they are passing right through you. The occasional one might bump into one of the atoms in your body but they are few and far between and the body will naturally get rid of any damaged cells and replace them as it sees fit, providing you are relatively fit and healthy. I suspect a percentage of background radiation that is detected is caused by muons and a muon detector can be made using multiple GM tubes using electronics to detect coincidences (simultaneous detections) with the tubes. With enough tubes in a matrix you could even see what direction they are traveling in. But you shouldn't worry about them at all. The sun will cook you far faster than muons could cook you, unless our sun goes supernova! ;) I often get spikes with my counter, sometimes going from ~20CPM to ~60CPM or more and sometimes in clusters over a few minutes/hours. I think this is due to a flurry of muons hitting my counter as a given time.
I'm not sure how much background radiation is from other sources, but I'm expecting there is a great deal from alpha decay from the likes of radon etc which will never be picked up anyway?
It is hard to say. There are lots of radioactive sources on this planet besides Uranium, Plutonium and the usual suspects. Granite and Lead (some isotopes) are radioactive but they are so slow at decaying they are hard to detect, but they are there and probably account for a certain amount of background. The tubes themselves have their own internal background count which I believe is about ~14CPM for the SBM-20. So if my SBM-20 has an internal background of 14 and my average is about 21CPM that would give about 7 counts per minute actual background, which is pretty much nothing in the big scheme of things. As I have mentioned you will need a tube capable of alpha for detecting Radon but would be pretty useless outside.
Im wanting to get the highest possible outdoor readings if possible.
No, you're not wanting this, far from it! You want the lowest results possible from your counter unless you want to be irradiated! :P I think what you mean is that you want the most accurate results? It's doable at the relevant expense. The more accurate the equipment, the higher the cost and the SBM-20 is a toy compared to some other detection equipment. If you wanted real accuracy then you should go down the route of scintillation and gamma spectroscopy. The SBM-20 is a great tube for background monitoring though. I think most people here that have an external Geiger counter use the SBM-20. It offers modest detection and is super cheap and reliable. Once you have your counter up and running you will get a feel for it and after getting a baseline of a few days/weeks/months you will know if there is elevated radiation being detected. It is hard to work out dosage for a given tube as you need to know what the radioisotope is and what the CPM/CPS would be for that particular isotope with the particular tube. Tubes have a conversion factor from CPM to uSv/hr (dose) but that will only apply to a particular isotope say Cobalt-60. A different conversion factor would have to be applied for Caesium-137 or Strontium-90. It is a mine field when it comes to getting an accurate reading and does require very expensive equipment. An SBM-20 for a domestic background radiation detector is suitable for just that but it is only going to tell you that there is an increase in radiation, not what.
You will probably find that (if your tube/counter is outdoors) when it rains occasionally you may get a slight increase in counts. This is due to the radioactive matter being caught in the rain and coming down to the ground. It won't last long though as most is caused by Radon decay products and last for minutes at most before decaying into their final elemental isotope which would always be non-radioactive and stable.
Something fun to do outdoors (which I have yet to do myself!) is to get something to filter air. Just some cloth or sponge or whatever you have to hand and set it up outside with a fan so it constantly pulls the outside air through the filter. Leave that running for about a week or so then get the filter (cloth/sponge/whatever) and place it close to your counter. You may see an increase in CPM as there would be radioactive particles captured in the filter. Also wiping the rain off a car outside (whilst still wet and has been raining for a bit) and checking that with a counter as the particles that come down with the rain tend to get stuck on the cars surface. Don't worry though as most of these isotopes have a half life of seconds to minutes only so aren't harmful.
I hope this has been helpful to you and if not helpful interesting at least. :)
My background monitor at home (mounted outside under a canopy on my workshop)
