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TOPIC: Hello from Stuttgart Germany

Hello from Stuttgart Germany 3 years 3 months ago #183

  • opengeiger.de
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Hello everybody,

I joined the community a week ago with a station located in Stuttgart (hometown of Mercedes and Porsche) in South-West Germany. The station that feeds the data is a Pin Diode counter made from a Teviso module (see Teviso.com ). For this module I developed a shield that can be placed on an Arduino microcontroller board. I named it Tino (from Teviso + Arduino). On this shield I placed a 7 segment display module and a piezo sound generator to signal registered counts and alarms. I placed the shield on an Ethernet Arduino Board that gets programmed through a FTDI breakout and is removed after programming (see the attached picture). I put the whole into a water tight drainage tube, covered it with a blind plug and put it into the garden. The great advantage of the Pin diode counter is that it does not need a high voltage supply supply. It just needs 5V from the Arduino board a few resistors and blocking caps that's it. The detector module needs less than 1mA and you don't have to bother humidity and even water should not harm it. However, a Pin-diode counter has a much smaller count rate (this one 3.4cpm/uSv/h). So my Arduino translates this into an equivalent value for the SBM-20 tube (by applying the conversion factor of (0.0057uSv/h / cpm). So it appears as an SBM-20 tube but with much more noisy statistics as long as the rain is clean and not contaminated. However, in case there would be a fall out you would notice that the sensor will report values much faster. This is because I reverted the common principle how to calculate the rate. Normally the microcontroller counts the pulses for a given time. In my case the microcontroller counts 10 pulses and measures the time needed in order to calculate the rate. This way the statistics stays always the same (standard dev is mean / sqrt(10)). During normal times 10 pulses need about 15-20 min for a RD2007 module, so I have to wait this time for calculate the rate. But when a contamination would occur 10 pulses need much less time and the rate will be reported much quicker. According to the manufacturer an advantage of the PIN diode counter is that the energy response is almost constant from low energies such as the Am241gamma line up to high energies like the gamma line of Co60. An SBM-20 tube in contrast has some favorite energies in the low keV range which it counts better than other energies. A disadvantage of the Teviso module is that when you place it in front of a mobile phone it will give a high count rate as soon as the phone starts ringing. But if you know this you can avoid it easily and just use it to test the functionality without the need of a radioactive sample.

Aside of this station I already created a lot of of homebrew solutions with tubes and PIN diodes and wrote a couple of articles on the subject of radiation and how to detect it. I also put together of nice places where you can find noticeable radioactivity man-made and natural. Unfortunately most of it is written in German but perhaps I will do some more translations soon (during the dark winter times ;) ) you can find my homepage here:
opengeiger.de

The Tino station is now running for a week more or less stable so I think starting from now on you can rely on it. By the way there is no beta shield placed in front of the detector. So it actually does not measure a gamma dose rate but is sensitive to beta as well. However, as i have seen almost no one is measuring a true gamma dose rate in the strict sense, that would require beta shielding of the tube or sensor used.

Let me know any question or opinion. You'll find my e-mail on my webpage. Or just send me a PM.

Cheers Bernd

TinoEthernet.jpg
Last Edit: 3 years 3 months ago by opengeiger.de. Reason: picture added
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Hello from Stuttgart Germany 3 years 3 weeks ago #575

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Hi Bernd,

An impressive bit of kit!
Would it be possible to use more than one pin-diode to increase the cpm?
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Hello from Stuttgart Germany 3 years 3 weeks ago #581

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Hi tcall,
Yes, you are right, when increasing the number of pin diodes this will increase the count rate. Or more generally speaking, when you somehow increase the active area this will increase the count rate. Meanwhile there are large PIN diodes available e.g. from firstsensor.com that have an active area of about 100 square millimeters whereas a standard BPW34 has 7 square mm. However increasing the area has limits since it increases the capacitance of the diode. Since a photon actually creates some charges per microsecond which is nothing else than a current pulse, this current needs to charge the larger capacitance. The logical consequence is that the voltage at the capacitance gets smaller and is more difficult to be amplified. This is also true if you use a transimpedance amplifier (TIA, or current sensitive amplifier, ISA) or a charge sensitive amplifier (CSA). What limits you there is the noise. Even though you may select an ultra low noise operational amplifier, the capacitance finally dictates the so called noise gain. If it is too large the noise gain will be too large and the only remedy is reducing the bandwidth in the feedback loop. But you have to consider that reducing the bandwidth again reduces the pulse height of the wanted signal. So this is a vicious circle. To my feeling you can operate a 100 square millimeter PIN diode still with one super low noise OP reaching a count rate of about 500cpm / (uSv/h) at a reverse bias of the diode of about 10V (this dramatically reduces the capacitance) but then I would add further diodes only with a separate and individual preamplifier. The different preamplifier signals can then be added digitally (the pulses are small enough) to further increase the count rate.

Cheers Bernd
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Hello from Stuttgart Germany 3 years 3 weeks ago #588

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Hi Bernd,

Many thanks for the detailed explanation. I like the idea of the larger PIN diodes!

Regards,

tcall
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