With nuke plants operating under questionable technical safety, natural events threatening humans and their dangerous energy production artifacts, and other humans disseminating fear through nuclear sovereignty upon the rest of the world, we came a long way from the happy ignorance of distant decades.
As such, in an attempt to further increase the sense of paranoia we readily accept to live in, I took some time to design and build another sensor, in this case a low level radiation detector. Having modularity in mind, I though of creating the device in such way to serve its purpose in more than one context. In order to keep it modular, I have put all the components necessary for it to operate in a stand-alone fashion, and communicate through a standard bus with other modules. Given its adequation to the solution, I have selected the I2C bus. The necessary elements for autonomous operation would have to be:
- Geiger Tube - bought a russian made SI-39G. Known to detect at least hard gamma radiation;
- HV power supply - the tube requires about 400 V DC, and very small current supply. In this case I have used a modified camera flash circuit. Originally would output 300 V. I had to tweak it to generate an extra 100 V without exceding the component ratings;
- Pulse stretching and amplification electronics - the pulse output by GM tube is too short for the human ear to identify (less than a microsecond). At the same time, the pulse signal must be conditioned to be properly handled by digital electronics;
- Pulse counting and I2C interface - a simple 8-bit microcontroller such as the ATMEL ATTiny85 is enough to handle the job. Additionally it also outputs a short tone (about 4 ms long), every time a disintegration is detected (e.g. gamma photon). This tone can then be amplified and fed to a speaker through external electronics;
- Linear regulator - Unlike the rest of the circuitry in the module, which is powered at 5 Volts, the HV power supply only requires 3 Volts. As such a linear regulator had to be added (in this case a LM317 adjusted to produce 3 Volts);
About the testing of this module, it has only been done in respect to the background radiation (that is the radiation that is all around us, coming from several sources such as the trace amounts of radon gas that is in the air, Potassium 40 from our bodies, from the soil, and of course from the outer space). An average of 6 or 7 counts per minute is obtained, which converting to microsieverts (using the tube datasheet) accounts for about 0.100 microsieverts per hour, which is consistent with the expected background radiation for our location and altitude.
Tests with other sources have not yet been carried out, given its scarcity (fortunately :) ). Yet to be tested are common things known to emmit slightly more radiation than background, such as Brazil nuts, granite, bananas, thorium gas mantles (a bit hard to find these days), americium smoke detectors (practically discontinued in the EU), etc. Some of these materials do not release quite many gamma photons, making radioactivity harder to detect. A pancake detector would in this case be far more efficient at detecting a broader range of charged particles.