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Craig
USA
22 Posts |
 Posted - 12/03/2014 : 21:47:28
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* I'm taking a wild guess that the single, greatest obstacle to sales of GQ's GMC series of instruments is the lack of a conventional gamma sensitivity specification --normalized to a standard, beta shielded, cesium-137 source.
* Since resorting to "first principals" and analyzing the geometry of a test/calibration setup is beyond me, I "cut to the chase" and simply compared my GMC-200 to a known to be in good calibration instrument --in what my survey indicated was a fairly homogenous, non-directional field of local gamma "background radiation" here, a meter and more off the ground.
* Briefly (perhaps too briefly): in a 0.10 uSv/hr field, the long term average of my GMC-200's M4011 tube comes out to about an even 16cpm --same as the SBM-20 tube in my Radex-1503. The less-than-optimized SBM-20 that I retrofitted into a Medcom "Inspector" clicks at about 15cpm. The Inspector's original LND-7317 pancake tube would click at about 35cpm in that same field.
** UN-briefly: I must break down and buy a cesium-137 source, block its beta with an aluminum plate, then let it shine in an open area at a GMC Geiger counter --and then my comparison Geiger counters --at the same distance. I'd orient non-windowed, small tube Geiger counters with their tubes broadside to the source.
~ The reason for this redo is to check the GMC and comparison Geiger counters at a level that's significantly above "background", which several G-M tube manufacturers seem to believe is largely due to the internal noise level of their tubes (something they don't like to talk about --understandably --since they sell so many tubes that are destined for low level background monitoring --like what we do).
~ Some authorities say that maybe half of a low (ie: 0.10 uSv/hr) background at sea level is due to "muons" --which reach us as a consequence of cosmic rays --while others say that muons barely register via a G-M tube --"less than 10 percent".
~ It seems for sure that muon detection is about equal to background counts (detector/sensor exposed profiles being about equal), but those muon counts are typically done with a high energy capable scintillator setup --not Geiger-Mueller tubes. (The detection rate is pretty low --I'm told-- with a two G-M tube, coincidence circuited, "cosmic ray telescope".)
** Complicating this issue further: I've lowered my SBM-20 equipped Medcom Inspector to two meters of depth in our local bay, using a sealed pressure vessel off the end of a dock. That little stunt cut the "background" CPMs in half --which should not have happened if the component from the sky was highly penetrating muons --AND:
** That rather belies the ridiculously high noise level ("inherent counter background") of the SBM-20's specification.
** When I get my hands on a GMC-320 that has a self-timer function, I'll send it down and see what happens to the CPMs --since the M4011's noise spec ("own background") is fairly high as well --at 12cpm --which translates to 0.075 uSv/hr by my reckoning. Maybe I can cut that count in half --would be nice.
Then, perhaps we can start talking about _actual_ background radiation.
(This posted comment will be reworked and put to the "RadWatch" group in Berkeley. Hopefully, they'll be able to clear these questions up for us.)
Craig |
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| Reply #1
Craig
USA
22 Posts |
Posted - 12/13/2014 : 12:13:53
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I re-submitted my question to the Berkeley RadWatch team, with a copy to the RadViews forum --per:
> h**p://radviews.com/forums/topic/is-background-monitoring-useful/
--for which you might care to offer an answer.
Craig |
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| Reply #2
Alchemy2
Canada
89 Posts |
Posted - 12/20/2014 : 08:50:30
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Hi Craig!
I have four GMs so far now: Two GQ 320+ units, one equipped with a SBT-11A tube on it (call this unit #1), one with a SBT-20 wrapped in Indium 100um thick foil (Call this unit#2), an Atomic Dave Titan which has a LND7317 tube, sim. to the Inpector series GM units, and the fourth is a MyGieger kit with another SBT-11A on it.
For background, before replacing the M4011 tubes that were stock, I found my early BG counts here to be for both units around 0.10 +/- 0.03 uSv/h, which for the 4011s is around 15CPM +/- 5CPM. Some variance to this was seen before I changed out the tubes, and in general I'd say my BG is 18CPM +/- 6CPM with the modified (In)SBT-20 now. More on that later. The SBT-11A equipped 320+ unit has the lowest BG and I have seen counts in the single digits on occasion, but typically it is at about 18-22CPM. My Titan with its 7317 runs around 32-38CPM on average, sometimes going higher or lower. I find that it and the SBT11A tubes can also detect weaker beta and some alpha radiation off higher levels of Radon 222 that sometimes plagues my area. Typical BG tracks well with Ra222 levels. When I had a spike at ~450Bq/m3 this fall for two days, the BG on the Titan was between 50-70CPM! If I went outside, down go the counts, as low as 24-28CPM!
This was proven indirectly by measuring counts on my furnace filter which after running the fan for even just 10 minutes, I'd get almost 500 CPM on the GQ unit #1, and nearly 800CPM on the Titan. Laying paper between the tubes and the filter didn't change that too much, counts maybe dropped by ~10%, but Al foil and steel shielded the some of the betas off the Pb214 and Bi214 daughters caught in the HEPA filter as dust particulates!
I think that NORM and Ra222 soil/scriptir levels have a HUGE impact on BG readings, and many folks do not always take this into account. Best way to get a closer to true BG reading would be to get a 2-3" thick lead box or castle, and have the unit sit in there for readings. Temperature and even humidity can affect things a bit too. I found that a microfiber shirt I own had enough latent static to run in the 2uSv/h reading with the Titan and its very sensitive 7317 tube, yet I was nowhere near a contaminated place. Taking it off and sprinkling water on it and voila! nothing but nominal BG counts. plastic casings can buildup some static on low humidity situations, so watch for that!
Whether muons are much of an issue, not too sure. I think that solar flux and geomagnetism play a more significant part in radiation BG all other things like NORM and Radon being removed. While cosmic BG can vary as well, its tough to generally account for this with simple gear, unless you really shield well.
Detector geometry and response also affect BG. If I were measuring BG, I'd run two pancakes aimed 180 apart, one up, one down. That would yield a 4pi fully spherical field response. Hot dog or cylindrical tubes have a dipolar response, with one axis at null. You'd need at least 2 of these to fully read a field.
The SBT-5 and 20 tubes seem to be set up to detect the core nuclides from Uranium production and use. These are not sensitive to soft beta, and in general, anything below about 180keV seems very inefficiently detected. Other tubes seem hot for Cs137. and so on.
Stuff to think about...
Darren
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The more I learn, the more I realize I do not know! |
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| Reply #3
Alchemy2
Canada
89 Posts |
Posted - 12/20/2014 : 10:47:37
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I forgot to mention about my Indium foil coated SBT-20 tube:
Indium has a large cross section for neutrons larger than cadmium for non-thermal neutrons. Upon absorbing a neutron, Indium nuclei gain a neutron, converting the In115 to In116m1 which decays to emit three gammas ranging from 350keV to 1200 keV, and a 1MeV beta. T1/2 is 54min. Thus this is a somewhat delayed response, but the basic idea is that In will permit the tube to detect some neutrons as well. Little reduction in standard gamma response is observed, and some beta is shielded by the presence of In. Anything below around 200keV is gone from this coated tube's response.
But, it should make a good general indicator of neutron flux for thermal, epithermal (moderate) and some faster neutrons, if moderated. I see slightly elevated dose readings using this tube. So, some fast neutrons must be interacting with this tube. Events come is small spurts over a period of 5s or so. Kind of neat...
D |
The more I learn, the more I realize I do not know! |
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| Reply #4
Craig
USA
22 Posts |
Posted - 12/20/2014 : 16:25:18
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Hi back athcha, Alchemy2.
* My take-home from your interesting equipment run-down is that you've equated 0.10 uSv/hr with about 15cpm out of an M4011 equipped Geiger counter. I was hoping someone doing careful work would corroborate the similar sensitivity range I cam up with (closer to 16cpm here, but we both have an error box).
* The delayed detection of neutrons by mediating them with Indium foil is something I wasn't aware of --nor was I aware of the 3 flavors of neutrons. I wish those "Sky Calculus" reports at Space Weather (per:
> h**p://www.spaceweather.com/scriptrchive.php?view=1&day=20&month=12&year=2014
--wasn't so closed-mouthed about the equipment and neutron mediation that they're using. (Perhaps they're fishing for some brand name to sponsor their activities, before they do show-n-tell.) Hopefully: I won't have to worry about neutrons here at MSL+90 feet.
* There might be more beta and alpha coming out of your air filters than is indicated, since follow-on gamma gets generated (via bremsstrahlung), especially when hard beta gets stopped.
** The intended take-homes from what I wrote (above):
~ When I took a Geiger counter out over the water in our bay here, the background level didn't drop by much, so since the radiation wasn't from below, it must have been coming from above.
~ When I submerged the Geiger counter to a depth of 2 meters, I got about half the CPMs, so what's coming from above isn't muons (right?).
~ When I packed my Geiger counter in 40 pounds of lead (here in the office), the CPMs dropped to about 75%. (Of course: lead can actually amplify muons by creating showers.)
Craig |
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| Reply #5
Craig
USA
22 Posts |
Posted - 12/20/2014 : 16:28:11
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| Correction: "so THAT PART of what's coming from above isn't muons". |
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| Reply #6
Alchemy2
Canada
89 Posts |
Posted - 12/20/2014 : 20:10:44
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Hi Craig, Darren here...
Interesting. First off, my understanding is that most muons tend to decay into beta particles and neutrinos. However, it is also my understanding that since these muons are usually in the >10MeV range for energy, they often pass through a GM tube and do not get registered much. Cloud/bubble ohambers of even a scintillation counter is used in lieu.
Having placed the detector below the water effectively shields it from most beta, neutron and protons. NORM (naturally Occurring radioactive materials) from soil, sea, etc. are probably the biggest contributor to BG counts.
Indium is similar to cadmium in its ability to interact with neutrons (called a neutron cross section). Cd gold (Au) and In foils have been used in the nuclear industry for many years. It is common practice to wrap a mixed service gamma-neutron tube in Cd foil to assist in having it detect them.
I am in the process of making a neutron generator using a Am-241 1uCi source from a smoke detector, and some Be (beryllium) foil - the 5.4 MeV alpha particles interact with the Be atoms and generate neutrons. Since ~37000Bq = 1uCi, that means that since this combination makes around 40+ n/1000000 disintegrations, the system should produce around 90+ n/min, or about 3-5 every 2s. Many of these will get scattered, and I will use paraffin wax as a moderator to slow them down. Even if only 15-20% of these get through, that should show as a count increase of 6-10CPM over general background. Not a lot, but enough to be seen.
I have a slow neutron tube (SNM-13) that runs on 500V, and once I think I have neutrons made, I'll wrap it in In foil (100um thick) and see if I can get counts off the GY detector kit, as the SNM-13 will have some gamma response too. I use lead to shield the gamma (1/4") above the source/foil. That way, the neutrons pass through the lead, and into the moderator.
I am just waiting for the Be foil to arrive! Fun playing with this stuff...
Oh! the NOAA group probably uses either scintillation detectors, or He3 filled tubes, with paraffin or HDPE encasing multiple tubes to get thermalization of the neutrons. Fun stuff! |
The more I learn, the more I realize I do not know! |
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| Reply #7
Craig
USA
22 Posts |
Posted - 12/21/2014 : 00:06:59
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G-M tubes are said to be good muon detectors by so many sources:
> h**p://www.cybermeme.net/muon1.html ("muons are 5 to 10 percent of background")
> h**p://www.ncbi.nlm.nih.gov/pubmed/21147787
> h**p://www.hardhack.org.au/geiger_muller_issues ("The Geiger–Müller tube is a very good detector")
> h**p://www.academia.edu/2520925/Building_a_simple_cosmic_muon_detector_to_verify_special_relativity
> h**p://www.ergotelescope.org/scriptbout-ergo/ergo-pixels/construction-of-pixels/
> h**ps://www.physicsforums.com/threads/create-a-home-made-muon-or-gamma-ray-detector.328632/
> h**p://quarknet.fnal.gov/toolkits/new/crdetectors.html (Fermi National Accelerator Laboratory)
--that I have to believe it's true. What I don't know is what percent of a low (10 uR/hr, say) background count is typically due to muons. Others say it's 10% or less, and my tests suggest it's low.
* Certain types of plastic sheet scintillators are used (if I recall correctly) because they don't respond well to low energy terrestrial gamma, and therefore more reliably detect high energy muons.
* US Navy tests have indicated that gamma rays travel poorly in water. Either for that reason, and/or because there wasn't much radiation in the bay water I tested in, the CPMs were cut in half with submersion.
Either way, I concluded that much less radiation was coming up out of the water --than was raining down from above --whatever it was and whatever the source, and that the from-above stuff was on par with background levels.
*** I want to carefully repeat that submersion test, step by step. Maybe others would like to try it as well, since I think there's much to learn/surmise.
* The submersion test also suggested that the noise level in that SBM-20 tube was no more than 7cpm.
Good luck with your neutron project. (Be sure your americium-241 pellet is in a well labeled container and locked away from the grand-kids.)
Craig |
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| Reply #8
Alchemy2
Canada
89 Posts |
Posted - 12/21/2014 : 08:16:18
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Thanks Craig,
I have several sources: Cs-137, 0.25uCi (9.3kBq), Co-60, 1uCi (37kBq), Ba-133, 1uCi (37kBq), 3x Am-241, 1uCi, 0.9uCix2;
The Am-241 sources were removed from flame and smoke detectors, and two are mounted in 25mmx5mm acrylic disks (like the other sources from labs I have Cs/Co/Ba). All fit into a lead castle, but gamma is still leaking from the Co-60 (~0.5uSv/h at 6"). I have these stored high and out of reach, no significant nominal increase in domestic BG. I also have several steel sealed (coffee and tea style containers with banded glass lids) which hold leaded steel tins and have several ore samples, Thorium mantles, etc.
I have one ore sample that's over 150kCPM, another is around 75kCPM! Red hot... get your red hot! Must be high grade U ore. (got on eBay to boot!)
All marked and placed in same area. Safety first!
What did surprise me is how much Bremstrallung and soft Xrays come off one of those beta light key chain accessories! I get 130+CPM over BG with my SBT-11A/GMC-320+, and 200-240 CPM with my LND7317 equipped Titan counter. (Atomic Dave)
Craig, I am guessing, but are you trying to do a background monitoring program in the SF Bay area, to follow Fukashima?
Water does moderate/shield gamma, and neutrons FYI (and beta of course)
If I were trying to do more cosmic ray work, I'd set up a series of SBT-11A tubes, and have them encased in a nearly solid block of acrylic, or HDPE (high density polyethylene); cover the tube windows carefully with some In foil, and then you can get neutron counts too. Inter-disperse some Cd or Pb rods to interact with the muons as well. Might be a fun project!
Nice chatting with you. My daughter flies in from Edmonton today, for Christmas. Looking forward to seeing her and having fun over the holidays.
Darren |
The more I learn, the more I realize I do not know! |
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| Reply #9
Craig
USA
22 Posts |
Posted - 12/21/2014 : 12:50:19
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* Happy holidays, Darren.
* I post to:
> h**p://radviews.com/map/
--and that's my station icon, close to the Oregon coast.
* Of course I'm first in line for stuff drifting in from Fukushima, but we have mining, shipping, plus "NORM" from our foundations/soils, beaches and sand dunes.
* My interest in cosmic rays/showers/muons is mainly to account for them in my/our CPMs.
* Just as we get contradictory information about whether muons amount to 5%, 10%, 50% --or whether they even ring up on our Geiger counters, I also see contradictions about the biological hazard they present. Officially (IAEA) they count like gamma, but some say they're harmless ("don't interact"). At 200x the mass of an electron, carrying an ionizing charge, energy at 10s of MeV, and traveling at relativistic speeds, you'd think they're boring itty-bitty smoking holes through us from head to foot.
Ho-Ho-Ho,
Craig |
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Calibration
