Radiation detector

see copyright notice. Page created 15-Mar-2023. Use the button groups above to navigate quickly around the site.

I made this instrument in 1982, during the unsettling time of the "Cold War" revival. It's not really a Geiger counter, as there's no meter or display. All it does is to make the classic clicking sounds, which get faster as radiation levels increase, eventually merging into a sort of spiky white noise. The Geiger-Müller (GM) tube is type CV2247, as used in the grimly-named British Military Meter, Contamination, No. 1. I forget where it came from - most likely Proops, or a similar surplus store in London's Tottenham Court Road.

The main requirement of a GM Tube is for a stable high-tension (350V) power supply, which needs careful design if it's to run from a 9V battery for a reasonably long time. I based this one on a 20kHz oscillator driving something like a TV line output circuit, with a step-up transformer wound on a 35mm ferrite "pot core", followed by a voltage-doubling rectifier. The HT is measured against a 5.1V Zener reference, with feedback to adjust the width of the generated pulses. As the circuit note states, it holds up within 2% over a battery voltage from 6V to 9V and an HT current drain of 0-100µA.

Each time the GM tube is triggered by an ionisation event, it briefly conducts current into a cascaded pair of transistors which drive a 2-inch speaker to produce an audible "click". At high pulse rates the battery draw is limited by a 1k resistor, so the sound becomes (perversely) more peaceful.

I'm slightly concerned to see how I used a dual monostable as the oscillator - such a circuit could theoretically sit in a static condition and fail to "start"! Nevertheless, I'm pleased to say it worked perfectly when powered up in 2023, and registered a background count about the same as I remember it from 40 years ago. Let's hope it stays that way...

One of the alarming discoveries I made in those early days concerned an old magnetic compass with a radioluminescent pointer and dial. I'd carried it around in a jacket pocket for about a decade, supposedly in case I got lost on the way home from a pub! Once I'd heard the cacophony it made with this device, I immediately banished it to a safe place, never to be found again.

I recently acquired an even older compass, when sorting through my late Dad's possessions. It's a "Verners Pattern Mk VII Marching Compass" dating from around 1915. And it, too, has luminous markings made with radium paint. The phosphor in these is a victim of chronic radiation poisoning - the markings are barely discernible now, even in total darkness. But I did manage to take the adjacent (lower) photo, using ISO 6400, a 60-second exposure, and every means of noise-reduction I could throw at it!

Radium has a half-life of more than a thousand years, so I wasn't surprised to hear another lively racket when I fetched the GM detector to the compass. The sound it made is recorded here. The first few seconds are "background" radiation, then the compass is moved in until it's nearly touching the GM tube.

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