The OpenRelief project is building “open, modular information solutions for disaster relief”, and as part of which they're developing a range of network-enabled sensors. I offered to help out by prototyping a radiation detector, opting to use an ionisation chamber in favour of a Geiger-Muller tube and constructing this from an old treacle tin. In this post I describe the basic principles of operation, before going on to cover early experiences and next steps.
The Geiger counter has become synonymous with radiation detection but there are many ways to achieve this other than using a Geiger-Muller (GM) tube. The ionisation chamber works on similar principles to the GM tube, but is an incredibly simple design that can be constructed from an old tin can and which does not require the use of high voltages and an inert gas and halogen fill. You don't get something for nothing and there are trade-offs, but we'll come to those later.
The inspiration for building an ionisation chamber-based detector came last year around the time of the Fukushima Daiichi nuclear disaster. As fears rose that insufficient information was available to track the spread of leaked radioactive materials, hackers around the world worked on designs for DIY Geiger counters, and those based in Japan made use of the Cosm (formerly Pachube) web service to publish real-time data online. As one might expect there was a run on GM tubes and supplies of those at the cheaper end of the market dried up. Which led me to thinking: why not use an ionisation chamber instead? This is essentially little more than an air filled vessel which is open at one end, and with two electrodes across which the flow of current is measured.
Ionisation chamber illustration by Michael Schönitzer [CC-BY-SA-2.5]
As radiation such as alpha or beta particles or gamma rays enters the chamber it strips the electrons from air molecules, and the ions and dissociated electrons are attracted to electrodes of the opposite polarity. Tiny currents in the order of nano/picoamps are measured flowing between the electrodes, and this provides an indication of the level of ionising radiation entering the chamber.
In this prototype an empty steel treacle tin was used for the chamber and to provide an outer electrode, with a thick copper wire used for a central electrode. Given the incredibly small currents that are being measured the use of extremely good insulating material is essential, and the wire is fed into the can through a PTFE (a.k.a. TeflonTM) bush and supported at the other end by PTFE rod.
An electrometer grade PN4117 JFET is used to amplify the current in order that we might then measure this using a microcontroller ADC. So as to reduce spurious readings the amplifier is screened in its own enclosure. The PTFE bushes that lead the wires out from the diecast aluminium box are not strictly necessary as insulation requirements are not as demanding after the JFET.
The ionisation chamber assembly is connected to a Nanode — an Arduino-compatible microcontroller that makes use of through-hole components (for ease of DIY assembly) and integrates Ethernet, along with optional extras of 868MHz wireless, an RTC and microSD. The idea being that the detector can log data directly to the Internet if an Ethernet connection is available, or alternatively it may broadcast readings via 868MHz wireless. In the case of the latter there would either be ground-based wireless infrastructure, or UAV drones flying overhead that have wireless and a GSM/3G modem or some other means of relaying data to monitoring stations.
The Arduino code that is responsible for taking readings is extremely simple and the steps involved are as follows:
Briefly pull the JFET source low to discharge the chamber wire
Take a voltage reading
Take a second voltage reading
Calculate the voltage drift
For those that are curious the code can be found at GitHub, but please note that at the time of writing this is alpha quality and it's very much a work in progress.
At this point I must give due credit to Charles Wenzel, who not only published the design that I've made use of here, but has also been extremely generous with his time in providing advice. One of the really great thing about his approach is that, aside from the microcontroller, power supply and chamber, the bill-of-materials consists of 1 transistor, 1 capacitor and 2 resistors. All I've done so far is opted to use an Arduino-compatible board over a PICAXE microcontroller.
The OpenRelief infrastructure for collecting together and making sense of data has not been worked out yet, but in the meantime I've been publishing to a test feed hosted by the real-time open data web service, Cosm. This has been a great help with debugging and getting the Nanode to publish to Cosm is quite easy to do.
Note that the test feed will not be very interesting as the detector is disconnected for most of the time, and also the units are not calibrated.
I mentioned at the start of this post that there are some trade-offs and the biggest challenge is going to be to make the design reliable and easily repeatable. Even then it's not going to be calibrated, but its highly unlikely that low end Geiger counters, which includes DIY designs, are either. Although something that uses a professionally manufactured GM tube is going to be more reliable than an old treacle tin with a wire running down the middle! So, what can be done to increase reliability? Well, current chamber design considerations include:
A sturdier chamber of “standard” proportions
Aluminium instead of steel (latter may emit some alpha particles)
Chamber window to keep air inside dry (humidity = lower resistance = bad)
Placing the chamber outside so that it's both better able to detect pollution and away from radon progeny common to buildings
It will also be necessary to design a custom Arduino shield to generate the bias voltage for the chamber, and this may be increased from 36v to e.g. 50v if it results in higher sensitivity.
It's important to note that the ionisation chamber is not simply a crude design that was abandoned many years ago in favour of things such as Geiger and scintillation counters. It does have certain benefits besides cost, still finds much use today, and provides the basis of most smoke alarms.
The Radiac No. 2, a radiation survey meter which was in use by the British military from the 1950s up until the 1990s.
The primary objective is to come up with a solution that is low cost, reliable and that will provide a warning when something is clearly not right, I.e. there is a significant increase in background radiation. And this will be easily possible with a very simple ionisation chamber design. Although there is nothing to say that once this has been achieved the design could not be evolved to the point where it can be calibrated, should a cost-effective means of calibrating sensors be found.
Should anyone be interested in helping out with this project, please join the OpenRelief developer mailing list and introduce yourself. And with thanks once again to Charles Wenzel for his support.
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