Monday, June 18, 2012

Geiger Tubes

Geiger counters are a relatively old technology, a common sight and sound in sci-fi films or thrillers, and their parts are commonly available on the surplus market due to soviet stockpiles. Naturally I had to buy a tube to play around with. They are relatively simple devices, a tube is filled with a special low pressure mixture of gases and a high voltage (400 to 900 volts) is applied across two electrodes inside the tube. When ionizing radiation passed through the tube, it triggers a discharge that briefly causes the tube to conduct, causing a detectable current pulse that can be counted by integrator circuitry, and creating the recognizable click sound that most people are familiar with from movies and television. Most tubes are sensitive to beta and gamma radiation, but not alpha since it is so easily stopped and cant make it through the tube's glass or metal walls. some, however are given a thin mica end window to allow the alpha radiation into the tube and allowing it to sense alpha radiation as well. I was able to find one of these mica window tubes on eBay and ordered it from a seller in Ukraine. I also built the high voltage supply for it from the schematic at this website. It used a 555 timer to create a boost converter to create the high voltage, and a hex inverter to create the click sound. I intended to make this into a single handheld unit, with the tube and supply on board, as well as the battery, charger circuitry, microcontroller and both an analog panel meter and digital LCD display to show the current reading. I never finished building the whole thing but I did get the tube and supply working together and took apart a cheap smoke detector to get a alpha radiation test source. Most smoke detectors use an extremely tiny amount of radioactive Americium, a byproduct of maintaining our plutonium based nuclear arsenal. This element is a powerful alpha emitter, and even the tiny amount of it present in the smoke detector is enough to overwhelm the Geiger tube, creating a constant screeching sound instead of a series of discrete clicks. However, as an alpha emitter it is relatively harmless as its radiation is easily stopped by a single piece of paper, your skin, or a few inches of air. It produces no detectable radiation when stored inside a small piece of aluminum foil. I am still interested in completing this project and will likely pick it back up sometime in the near future.

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