detector testing with real radioactive sources


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exsafs
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Message 1083 - Posted: 4 May 2012 | 20:01:09 UTC

ok,

as promised i want to share some test results with real radioactive sources with you.
Before i start, one comment - I have access to radioactive sources as i am working in a radioactive laboratory of a Swiss research Institute (http://www.psi.ch) where normal radio protection rules apply. I am working in a controlled zone, have had to pass a training course and i do wear a dosimeter. Every measurement i publish here was done by myself being aware of basic radio-protection rules.

Basically i did a comparison of the readings of a V2.01 device obtained end of March from this project with a certified one available at our laboratory. It is a 6150 AD 6/H dosimeter calibrated at the accredited dosimetry calibration laboratory at PSI, Switzerland. Furthermore, i have had the opportunity to make some preliminary tests in this laboratory with the Radioactive@home device.

First of all - here a picture of the dosimeter at our laboratory:



sorry for the crappy quality, i will try to make better pictures in future.

I tested this device with two radioactive sources from our lab. One is a 373 kBq (01.03.2006) Co-60 source, the other one is 117 kBq Cs-137 of the same date. The measurement was done with both detectors directly attached to the source (not a true scientific approach). The buzzer was on.

Co-60 with readings:



Cs-137 with readings:



you see a clear difference in the values. the readings of the Radioactive@home device are about 30% below the readings of the certified dosimeter.

some more pictures of the sources:




additionally i made a test with both sources and sample of quite high doserate just to see how much the detector may stand:



you see - the device is measuring 0.5 mSv/h where it should read 1 mSv/h. Basically one can say that the device is measuring too low dose rates (Which may not automatically apply to background measurements).
The approach i have chosen is not really meaningful as the radiation field that is generated is not homogeneous if the source has direct contact to the device (The discharge tube of the 6150 device is shorter, so there are some limitation for my measurements just from geometrical considerations).

A much better approach is to generate a homogeneous radiation field from a powerful source at a certain distance. This was possible at the certified dosimeter calibration laboratory at my institute. There, they generated radiation fields between 1 and 100 ┬ÁSv/h with Co-60 and Cs-137 sources of much higher activities.
The outcome of this test is: for Co-60 (mean 1200 keV gamma line) the readings of the V2.01 device where 20% below the expected value, for Cs-137 (600 keV mean) it was already 40%. The operator said that for a self-made device, this thing performs quite well (good background statistics, reasonable results for the dose rate tests).

Congratulations, Team Poland!

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Message 1084 - Posted: 4 May 2012 | 20:09:40 UTC - in response to Message 1083.
Last modified: 4 May 2012 | 20:14:02 UTC

Thank You :)

It's possibility that I send you two or three other detectors for few days for few tests?
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Krzysztof 'krzyszp' Piszczek
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exsafs
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Message 1085 - Posted: 4 May 2012 | 20:26:10 UTC - in response to Message 1084.

Hello Krzysztof,

sure, i may do some tests with other devices too. Just send me a PM.
Concerning beta radiation - it is not possible to directly detect beta radiation with the shown device, neither with your detector. Betas emit Bremstrahlung radiation when they are stopped in matter, and only this radiation may be detected with our devices. It also depends on the beta energy - you will not detect H-3 (roughly 100 keV beta), but you will see P-32, for example (1,7 MeV beta).

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Message 1086 - Posted: 4 May 2012 | 21:50:06 UTC - in response to Message 1085.

Out of curiosity, when we literally nuke these things they read a low.

what about slightly high, or medium doses. say maybe 1 uSV/h which would still be very high as a background and maybe 5 or 10 or even .50

In other words test them at levels we are more likely to see to see how accurate they are. (Ok if we see levels even that high we should be hauling ass out of there, but IF something did happen, this might be more realistic an initial dose to see short of a nuclear burst in which case we better load up on Potassium Iodide and Prussian blue.

Aaron
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Message 1445 - Posted: 6 Nov 2012 | 3:39:03 UTC

Interesting. Can the detector work as a standalone detector not connected to the grid? I would like to experiment with various sources, but I would hate to feed those readings to the grid.

Also, what is the maximum range of detection? Would detector measure close to 0 when over range (like some simple single tube GM detectors?)?

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Message 1448 - Posted: 6 Nov 2012 | 9:26:21 UTC - in response to Message 1445.

The detector works as standalone, you can feed it from USB (5V) charger for example.
The project also supports 'experiment mode' where you can stay connected and watch the data/averages/graphs online, but your sensor is not displayed on the map and the data is not saved in the main database, just stored for 30 days (and marked that it was a test).


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Message 1450 - Posted: 6 Nov 2012 | 16:45:43 UTC - in response to Message 1448.

Thanks TJM!

Any ideas on the upper limit of detection (ie saturation of GM tube)? That could be important in an event of nuclear explosion only though.

Jacek

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Message 1451 - Posted: 6 Nov 2012 | 17:00:08 UTC - in response to Message 1450.

The detector (v2.5, v3) max range is far higher than the tube itself can handle.

AFAIR the tube reaches the limit at about 5k pulses/sec while the sensors were stress-tested at 50kHz and are capable of counting up even higher rates.

I have no idea how the tube reacts to radiation higher than it can measure, it's kind of hard to test :P

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Message 1452 - Posted: 6 Nov 2012 | 20:21:12 UTC

I was able to find these specs for the GM tube:

Parameters: SBM-20 / SBM-20U
Minimum Anode Resistor (meg ohm): 1.0
Recommended Anode Resistor (meg ohm): 5.1
Recommended Operating Voltage (volts) : 400
Operating Voltage Range (volts): 350 - 475
Initial voltage (volts): 260 - 320
Plateau length (volts): at least 100
Maximum Plateau Slope (%/100 volts): 10
Minimum Dead Time (at U=400V, micro sec): 190
Working range (mkR/s): 0.004 - 40
Working range (mR/h): 0.014 - 144
Gamma Sensitivity Ra226 (cps/mR/hr): 29
Gamma Sensitivity Co60 (cps/mR/hr): 22
Inherent counter background (cps): 1
Tube Capacitance (pf): 4.2
Life (pulses): at least 2*1010
Weight (grams): 10 / 9

Looks like 144 mR/hr (1440 uSv/hr) is the limit. That is impressive for a single tube, I guess due to small tube size.

After the saturation event, there should be no counts anymore (DC only), thus counter will show zero. This is how typical GM counters work, unless you measure DC current also to indicate and report over-range.

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Message 1515 - Posted: 14 Dec 2012 | 15:04:39 UTC

Hi guys,
when talking about testing with real sources, I can also help as I work in National radiation protection institute (SURO) in Prague, Czech Republic.

We ordered 2 detectors this August/September and we are going to perform a tests using various sources as soon as we will get them.

Currently we are testing slightly different Libelium Waspmote platform (almost based on Arduino) and in our case we were not able to measure more than about 120 microSv/h using the SBM-20, while with the Chinese J305 we were able to go to about 170 microSv/h.

You can see some of our results here.

Our colleague said that the SBM-20 is simply saturated and that we should have to apply some shielding to shift the measuring range to higher dose rate levels.

We want to try almost the same with radiactive@home detectors and we are very curious. :-)

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