Geiger Counters

Building a geiger counter is a really simple and fun project. In contrast to other types of detector, geiger tubes are affordable, easy to drive and almost indestructible. Once built, the geiger counter can then be used to go on treasure hunts for radioactive items in local antique shops, forests or mining tailings. I have built multiple geiger counters when I was younger.

Simplistic design

A GM-Tube requires a high voltage supply rail (usually between 400V and 500V) to operate. The supply rail can very easily be generated with minimalistic circuitry by making use of some geiger tube characteristics. If the supplied voltage lies within the geiger-muller tube's plateau range, sensitivity is not strongly affected by voltage changes. Additionally, the current required to run the tube is generally low when measuring moderate levels of radiation.

In this very simple design, I used a NE555 to drive a 6V->230V print transformer. The output voltage from the transformer is then increased using a Cockroft-Waltron multiplier. Because the output voltage neither has to be precise nor particulary clean, I can get away with driving the transformer without closed loop control. The output voltage is mainly limited by the transformers turns ratio. This works suprisingly well. The circuit was built on prototype board using leftover components. Supply noise can be a concern when it is coupled into the detector circuit. In this case the capacitance in the Cockroft-Waltron multplier is quite effective at filtering noise.

The Geiger Muller tube I used in this project is a soviet-made SBM20. The SBM20 tube has becomme rather popular among hobbyists and consumer grade detection equipment. Unused Tubes can easily be obtained online. Regular 6mm fuse holders fit perfectly for mounting the tube.

Compact geiger counter

I built a small geiger counter based on a design I found on the internet. It utilizes a photo flash circuit from a cheap camera to generate the high voltage for the geiger tube. The circuit runs on a single 1.5V battery. I added RC low pass filtering to reduce supply noise. Due to the photo flash circuit's rather high resonance freqeuency, power supply noise would otherwise be coupled into the current amplifier through the geiger tube's capacitance.

Mica-Window Tube

The SBM-20 tube used in the previous project can only measure gamma and beta radiation, as alpha particles are shielded by it's sheet metal case. To allow detection of alpha radiation, some geiger tubes utilize a very thin window. The window is usually constructed from mica.

Since the mica window is transparent, the individual discharges in the geiger muller tube can be seen using the naked eye.