Archive for May, 2011
Well, last week has shown the total lack of any radiation around my home. I have got pretty stable reading for a few days, which moved a little bit higher and lower, but mostly consisted of random background-level noise. The noise (~0.2 clicks per second), which should be expected from this kind of geiger tube at their respective age.
While this kind of picture looks quite dull and boring, I’m kind of cowardly reluctant of ordering any kind of radioactive materials to my home, so I’ve decided to take another approach. I’ve heard the granite stones are supposed to have 0.3-0.5uSv/hour levels, so probably tomorrow would be a good time to visit a stone sculpture factory nearby.
And for that purpose, here comes another board, this time without any USB connection, but equipped with a small LCD screen, so I can attach a pair of batteries and use it to see the results while away from home. Also I’ve got a cheap (relatively) Chinese dosimeter, so there’s a hope I can calibrate the output of my boards and don’t rely anymore on thumb-sucking approach when calculating CPM-to-uSv/h conversion rates.
Here’s the finished board connected to my computer. It works surprisingly well, reading about 20CPM on average — it might be cosmic rays or anything, because I’m too far from Fukushima and don’t really believe anything can be brought over here by wind. The power consumption from the USB bus is less than 10mA, which makes it safe to connect to any USB port, even to a laptop/notebook with a small and weak battery.
But even with this low power consumption, the naked board does not play quite well with bare hands — it still has enough voltage (about 400v) inside and is always ready to zap the finger. Still, the discharge current is very low — about 10uA, so it’s just a bit more annoying than static electricity discharge, like the one you get while getting off the sweater.
And here’s a fresh plot of today’s data. I have no idea what happened between 17:00 and 20:00, maybe my neighbours were microwaving the sausage or we had a solar flare, because it does not look like radiation at all. The radiation usually builds up pretty fast and then slowly decays over time, not the opposite, as seen on the picture. I shall check tomorrow’s graph to see if this phenomena will repeat itself again tomorrow.
The latest events in Japan are more or less related to the results of large earthquake on March, 11th and the radiation outbreak from the damaged units on Fukushima nuclear plant. I’m not equipped or willing to participate in the recovery efforts, but still can provide some help by building and sharing DIY Geiger counters, so people can put their minds at ease.
The design is not very original, first I saw the SparkFun Geiger counter (sold out for a few month already), then I have visited Techlib where I’ve learned a few things here and there about feeding the GM tubes with a proper voltage, and finally I’ve added a few pieces by myself.
The original prototype board was quite messy, but yesterday the first batch of PCBs has arrived and finally I could put all the pieces together.
The core part of the design is the Russian-made Geiger tube SI-3BG, which is not very sensitive, but still can provide good readings if left for a while (30sec-1min interval). The manual said the working voltage is 380-460V, which results in about 15-20 clicks per minute in the calm conditions without any radiation sources around.
The electronics design is quite stable and provides the stabilised high voltage about 400V with the power supply changing from 4V to 9V. The correct output voltage can be selected with the proper collection of Zener diodes and may range from 300V to 500V easily, or maybe even higher with the different transformer and diode/capacitor values.
The tubes I’ve got are from military arsenal, stored and kept in a cold dark place, so most of them work without a single problem. They are sensitive to gamma/beta radiation, well, mostly to the gamma, since beta cannot penetrate very far through the buildings and furnuture. But still, I think these tubes, while difficult to calibrate for a precise measurements, are quite adequate for my purpose — collect click data on the computer and see how it changes with the weather (the wind, and, especially, the rain).
Once in a while the particle or gamma ray enters the tube, and starts the discharge which can be seen as vertical streak on the oscilloscope or the impulse on the microprocessor input. Impulses are counted for a while and then periodically are sent to the computer over an USB connection. Currently I’m just watching the output and thinking about making a small application to build pretty graphs about changes in the background radiation levels around the place where I live.