Use a Cheap PIN Diode as a Geiger Counter

After the Fukushima nuclear power plant disaster, radiation measurement became newly relevant for a lot of people. Geiger-Müller tubes, previously a curiosity, became simultaneously important and scarce. (English-language version here) has complete instructions for making a Geiger counter without a Geiger-Müller tube. Instead, this counter uses a PIN photodiode and some carefully chosen operational amplifiers. The total cost of such a device is significantly cheaper than the alternative: under $1 for the diode and around $5 for the rest. And since the PIN photodiode in question is used in many other devices, it’s not a niche component like a Geiger tube is.

The secret sauce is in component selection and tuning. Opengeiger uses the BPW34 diode because it is relatively common and has a large surface area, but also because it has a very low capacitance when reverse-biased. The first-stage opamp choice is also fairly critical. Considering that an average gamma radiation event produces only around 10 nanoamps for about 50 microseconds, a lot of amplification (100,000x), low noise, and high bandwidth are a must.

If you want to get started with this project, you could first browse through the explanation (PDF) to get an overview of the project’s goals, read up on all the technical considerations (PDF) or just head straight for the DIY instructions for the “Stuttgarter Geigerle” (PDF, schematic is on the last page). All of the documentation is chock-full of relevant references and totally worth the read.

Digital Geiger Counter

Worried about radiation levels? I’m not, but I still want to build one of these. Mr. Fission here was built by the same guy that’s behind the OpenTracker project. [Scott] based it on the Bargraph Geiger counter built by [Russel E. Cliff]. Both use a standard Geiger tube like the LND712. The tube works with high voltages – easy enough with older electronics, but it’s a slightly tougher challenge with todays low voltage gear. [Scott] used the high voltage power supply from the original project, and built the rest around a good ol’ Motorola HC86 series processor. [Scott]’s idea of using an inverter supply designed for a cold cathode lamp is definitely an interesting one.