Building a radiation detector from scratch 4 - Detector MK II
Since I'd like to keep up with the head-spinning pace of interesting articles, let me present you this year, :) the
In the last episode I described an ion chamber radiation detector. It works very well: stable and sensitive enough for doing simple experiments.
It can be improved though in several ways. Here follows a series of ideas and a circuit diagram. :) I did not build this yet, but I'll build it "soon". In this decade, or the next. :P
High voltage
The voltage potential of the outer shell of the detector (the tin can) could be raised up to several hundred volts to provide more sensitivity.
Teralab did wonderful experiments: http://www.teralab.co.uk/Experiments/Ion_Chamber/Ion_Chamber_Page1.htm.
According to their results, it's well worth increasing the voltage between the transistor's base and the outer shell up to 200V, after that we get diminishing returns.
Raising the voltage could be done in several ways in practice. One could by a DC-DC converter module used to charge flash capactiors, or build a (rather large) charge pump, homebrew a simple boost converter, drive a PCB mains transformer in reverse, etc. Just make sure the voltage source is stable enough at least in the short term. Any noise from the supply will be picked up by the very sensitive detector circuit.
Isolation and safety
Working with voltages in excess of a few dozen volts poses a shock hazard. Careful measures should be taken to minimize the danger of shock during work and use.
It's best to put everyting into a solid metal housing, and don't let any leads or pins out that has dangerous voltages on it.
One can use the internal battery also simply by shorting out the high voltage power supply connector. Since under no condition should any significant current flow through that connector, a simple 50 Ohm off-the-shelf termination resistance can be used also.
There are a lot of possibilities for recording and/or transmitting voltage data. The single-ended output is easily interfaced with and arbitrary device capable of measuring and displaying / logging / transmitting voltages. This can be an analog uamp meter with a suitable resistor in series, a digital voltage meter module, a DVM, and arduino with an SD-card or Bluetooth module, etc.
Next time we'll take a look at the built detector and the measurement results.
In the last episode I described an ion chamber radiation detector. It works very well: stable and sensitive enough for doing simple experiments.
It can be improved though in several ways. Here follows a series of ideas and a circuit diagram. :) I did not build this yet, but I'll build it "soon". In this decade, or the next. :P
High voltage
The voltage potential of the outer shell of the detector (the tin can) could be raised up to several hundred volts to provide more sensitivity.
Teralab did wonderful experiments: http://www.teralab.co.uk/Experiments/Ion_Chamber/Ion_Chamber_Page1.htm.
According to their results, it's well worth increasing the voltage between the transistor's base and the outer shell up to 200V, after that we get diminishing returns.
Raising the voltage could be done in several ways in practice. One could by a DC-DC converter module used to charge flash capactiors, or build a (rather large) charge pump, homebrew a simple boost converter, drive a PCB mains transformer in reverse, etc. Just make sure the voltage source is stable enough at least in the short term. Any noise from the supply will be picked up by the very sensitive detector circuit.
Isolation and safety
Working with voltages in excess of a few dozen volts poses a shock hazard. Careful measures should be taken to minimize the danger of shock during work and use.
It's best to put everyting into a solid metal housing, and don't let any leads or pins out that has dangerous voltages on it.
Because of these, I decided to use optocouplers as the output on a separate battery, and also to separate the high voltage power supply. This makes it possible to build a modular solution, whereby you can choose a separate high voltage (or not so high voltage) power supply that is designed and built separately.
One can use the internal battery also simply by shorting out the high voltage power supply connector. Since under no condition should any significant current flow through that connector, a simple 50 Ohm off-the-shelf termination resistance can be used also.
Below is the schematic of the Mark II Ion Chamber Radiation Detector:
Single-ended output
A simple operational amplifier circuit might be used to deliver a single-ended voltage output. The OPA could use it's own power suply, or utilize the optocoupler's battery. Therefore it is recommended to create three output pins / sockets / leads: "output" (left resistor top lead), "reference" (right resistor top lead), and "Vcc" (optocoupler Vcc).
Resistors should be choosen so the voltages respect the OPA's capabilities. The "SingleOut" output will be our single-ended output. Since this circuit can only output positive voltages, "SingleOut" will peg to zero if "Output" goes below "Reference".
This should't happen in theory, since there always will be a slight background radiation, and therefore some current in the chamber. If the circuit starts to drift however, it might happen, and ruin measurements in case of an unattended setup. Since the last version of the chamber proved to be surprisingly stable, I believe this will hardly be a problem.
Data logging
A simple operational amplifier circuit might be used to deliver a single-ended voltage output. The OPA could use it's own power suply, or utilize the optocoupler's battery. Therefore it is recommended to create three output pins / sockets / leads: "output" (left resistor top lead), "reference" (right resistor top lead), and "Vcc" (optocoupler Vcc).
Resistors should be choosen so the voltages respect the OPA's capabilities. The "SingleOut" output will be our single-ended output. Since this circuit can only output positive voltages, "SingleOut" will peg to zero if "Output" goes below "Reference".
This should't happen in theory, since there always will be a slight background radiation, and therefore some current in the chamber. If the circuit starts to drift however, it might happen, and ruin measurements in case of an unattended setup. Since the last version of the chamber proved to be surprisingly stable, I believe this will hardly be a problem.
Data logging
There are a lot of possibilities for recording and/or transmitting voltage data. The single-ended output is easily interfaced with and arbitrary device capable of measuring and displaying / logging / transmitting voltages. This can be an analog uamp meter with a suitable resistor in series, a digital voltage meter module, a DVM, and arduino with an SD-card or Bluetooth module, etc.
I'm going to use the MK II with a DVM and probably try to hook it up to an Arduino Nano, to interface it with a computer throug USB and log data, and might even set up the Arduino to log the data to an SD card.
Next time we'll take a look at the built detector and the measurement results.
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