The late unpleasantness in Japan has recently focused our attention on homemade radiation detectors. These are often lumped together under the term “Geiger counter,” but in fact there is more than one way to skin that particular cat. A “Geiger counter,” formally, is a radation-sensitive instrument that uses a Geiger-Müller tube to detect the tiny electrical charges produced when radiation in the environment knocks electrons off atoms of a low-pressure gas sealed inside a glass tube…
A scintillation detector, like that inside the military surplus DT-590A/PDR-56F probe shown above, works by an entirely different principle—that is, optically, rather than electronically. Some transparent solids, called scintillators, emit tiny flashes of visible light when their molecules interact with environmental radiation, and though these flashes are usually too dim to see with the naked eye, if you seal a chunk of scintillator in a dark case, behind a window that passes high-energy radiation but not visible light, you can see them with a photomultiplier tube. The associated electronics have to A) power the photomultiplier and B) read its output into audible “clicks” or other discrete, countable signal events.
Because the detecting volume in a scintillator is a solid, rather than a rareified gas, scintillation detectors are much more sensitive than Geiger counters, especially when it comes to detecting gamma rays. While a typical Geiger-Müller tube reacts to less than 1% of the gamma rays that penetrate it, the comparatively dense atomic lattice of a scintillator crystal will interact with practically all of them.
Multi-talented engineer, scientist, and author David Prutchi has designed an excellent DIY method for converting these common military surplus scintillation probes, which were designed for detecting the very specific energies of gamma rays from plutonium fallout, into general-purpose gamma ray detectors that, in David’s words, “will outperform virtually any handheld Geiger counter in the detection of 100keV to 1.3MeV photons.” He teaches homemade power-supply and signal-processing circuits for the mlitary probe, as well as the necessary trim adjustments for the probe circuitry itself. It’s fantastically informative, well-documented work. [Thanks, David!]