SBM-20 and STS-5 (CTC-5) Sensitivity Comparison

I wanted to see if there was much difference between the sensitivity between a SBM-20 and STS-5 tubes. For a background test I had an SBM-20 running for a while then swapped it for a STS-5. I think the graph says it all. (Note - there was a discrepancy with the initial background test (below). The test was repeated later and the results are shown towards the end.)

I took this a step further and compared the two tubes using 13 different sources. In hindsight I should have tested a J305 at the same time. It took a surprisingly long time, so I might set up some kind of testing jig and program an ESP to automate a lot of the task. I would also like to do tests at 1cm, 5cm and 10cm, so it would definitely need some kind of automation. I was actually a little surprised to see the STS-5 get a higher CPM for every source, yet the background is less for the STS-5.

Here are the results. There is also a PDF to download if you wish.

Source                                  STS-5      SBM-20    Difference

    1  Uraninite                        9559        8380     14.07%
    2  Uranium Glass                    856         805       6.34%
    3  Uraninite with Gummite           2478        2301      7.69%
    4  Thorium Dioxide (Powder)         1117        958      16.60%
    5  Potassium Chloride               246         232       6.03%
    6  Thorium Dioxide (In Solution)    244         187      30.48%
    7  Autunite                         21871       19834    10.27%
    8  Amerecium 241                    886         785      12.87%
    9  Strontium 90                     69219       64654     7.06%
    10 Radium Watch Hands               181         174       4.02%
    11 Uranium Glaze                    5724        5130     11.58%
    12 Thorium Gas Mantle               10413       9361     11.24%
    13 Depleted Uranium                 3195        3002      6.43%

I was asked by Juzzie over at radmon.org if I used the same Geiger counter for the background test (1st graph above) and the comparison. The answer is no, they were two different counters. The background was from a GK Radmon, and the source testing was done with my breadboard dev counter, which is effectively a GK v5.5 (older version.) However, I have checked the voltages of both counters and both are correctly set at 390-400v. It wouldn't hurt to repeat the test though using the same counter as the source testing, so I have thrown a SBM-20 on it for a day or so, and then I will swap out to the STS-5 for a day or so. I also think it might be worth repeating the tests I have already done with some averaging - 1 minute, 5 minutes, 15 minutes and maybe 30 minutes. When I was testing the sources with both tubes, the way I decided the CPM was a little fluffy. I let it sit for just over a minute, waited until it peaked and fell, and then used the CPM reading of it's next peak. It might also be worth getting a good background reading for each tube to be tested so that can be subtracted from the resulting CPM to give a more accurate result. I think that might be necessary for doing testing over a distance of 5/10cm.

I am considering actually building some kind of test rig and specific code so semi-automated testing. That way I can just set it up and swap things then push a button and let the Arduino/MCU do it's stuff. I have a few less sensitive tubes (SI3BG/Dextray 3G8B/DOB-50/DOB-80. It would be nice to get some kind of sensitivity data on these too as well as the more common tubes.

I compared the STS-5 and SBM-20 again on the same counter (breadboard dev counter) I used with the sources. Interesting result.... Hardly any difference.

It does beg the question though; why on the first counter (GK Radmon Basic) is there such a difference, yet on the second counter (breadboard GK v5.5) there is hardly any difference. I have doubly checked the voltage on the breadboard counter and its reading 404v right now. Strange. I shall have to have a look at the GK Radmon and see why.....

So the GK Radmon now is showing the same background for both tubes. The graph is from radspod_two. I just grabbed this graph, that's why the timing is off the other from SimoTester. It's currently sitting on a shelf in my office and I have been swapping the tubes from that. Radspod_two has SBM-20, breadboard counter has STS-5, then radspod_two has STS-5, breadboard counter gets SBM-20. There was obviously something amiss with the first background reading with both tubes. I know the voltage was high on the GK Radmon, but I can't remember when I changed it back down to 400v. It wasn't when I changed tubes... Or maybe. I dunno. Can't remember. Too many counters around me at the moment. I really do think a purpose built semi-automated system would be a good idea for this. Something that logs every few seconds, checks voltage, or at least logs it and does some decent averaging.

Anyway, the graph from radspod_two, that shows no change in background between tubes, this time....

Archived from radmon.org - originally posted 20/10/2023