A portable, sensitive, low power, analog Geiger counter
A portable, sensitive, low power, analog Geiger<br>counter
You may already have<br>seen my<br>previous page about a basic Geiger counter.<br>As described there, building and playing with Geiger counters has<br>accompanied me throughout my life as an electronician. I first read<br>about Geiger-M�ller counter tubes in an old German book that introduced<br>all sorts of electronic devices, when I was 14 years old or so. When<br>the Chernobyl disaster happened, I bought a small old Geiger tube and<br>built my first counter. Now, quite a few decades older, I'm sort of closing<br>this<br>chapter, by building a highly sensitive and fast-reacting, yet<br>extremely simple and power-efficient analog portable Geiger counter.
When<br>it comes to Geiger tubes, size does matter. The larger the tube is, the<br>more counts per minute it will give for a given radiation intensity,<br>simply because of its larger capture area. At normal background<br>radiation levels, a very small Geiger tube might produce only a very<br>few counts per minute, requiring averaging over several minutes to get<br>a meaningful measurement. A large tube will give many more<br>counts per minute, allowing to average it over a much shorter time to<br>get the same quality of measurement.
Instead at high<br>radiation levels, smaller tubes are better, because a large tube would<br>give so many counts that it will lose linearity, since too often a<br>count would be missed because the tube is still recovering from the<br>previous count.
Since I'm interested only in measuring very low<br>radiation levels, like the background level and weak radiation sources<br>that are at most a few times stronger than the background, I needed a<br>large Geiger tube. I bought a Chinese J306β tube, the glass<br>version, which is rated to give 88 counts per minute at<br>average<br>background radiation, or 8 counts per second at a radiation intensity<br>of 1�Sv/h. This tube is nearly 20cm long, and about 18mm in diameter.
took quite some time to decide whether to use a microcontroller to<br>count the pulses and display various processed measurements on an LCD,<br>or choose the classical way and use dumb circuitry and an analog meter.<br>Each method has its own specific advantages: A microcontroller can<br>integrate the counts over a long time, achieving greater accuracy when<br>measuring stable radiation levels, and also measuring the total dose<br>over a long time. Instead an analog meter is far more convenient<br>when scanning an area for radiation sources, because it's so<br>much<br>easier to watch a needle move up, than to read and interpret dancing<br>numbers on a display. It was sort of a tie between the two approaches.<br>The decisive factor, in the end, was that with this large tube I could<br>easily do enough averaging by simply using a basic analog low-pass<br>circuit, a feat that is not really practical when using small tubes. So<br>the rule is: Small tubes used to measure low-level radiation need to be<br>used with digital counters, while large tubes can use analog circuits<br>even at low radiation levels. At high levels, any size of tube can work<br>with analog circuitry. My decision to built an analog Geiger counter<br>was further reaffirmed by the fact that many Chinese digital Geiger<br>counters are widely available. Instead of building my own, it would be<br>more practical and even less expensive to just buy a ready-made one from China. So,<br>to bring any sense into building a Geiger counter at home these days, I<br>went analog.
My counter has two ranges. In the low range,<br>full scale is 1�Sv/h, while the high range goes up to 10�Sv/h. If<br>anything I find pegs that scale, I prefer to run, instead of measuring<br>exactly how much radiation there is! So I don't need a higher scale<br>than 10�Sv/h.
The averaging time is 4.4 seconds in the low<br>range, and 0.44s in the high range. This change of averaging time<br>combines with the higher count rate obtained at higher radiation<br>intensity, to produce the same degree of needle stability in both<br>ranges. In the high range, the reaction time is almost real-time,<br>allowing very quick scanning of areas, while in the low range it's<br>still fast enough to allow scanning, even if at a slower pace.
included a speaker, so that I can hear a click for each count. But my<br>practical experience is that hearing the clicks is a bit of an<br>overrated feature. When scanning an area, it's hard to clearly notice by ear a<br>moderate increase of the count rate, given the irregular spacing<br>that the clicks of a Geiger counter always have. One really has to look<br>at the meter. Anyway, the clicking sounds cool...
While<br>developing the circuit I was in a power-saving mood. I started basing<br>the circuit on the use of a 9V battery, and after some tweaking brought<br>the current consumption down to 5mA. Commenting about this to an<br>electronician friend, he dryly replied that 0.5mA would be better. He<br>was right, of course, so I began working seriously to reduce power<br>consumption. After a few days the consumption was down to 0.21mA at 9V.<br>But then the question...