Radioactivity And The Geiger Counter – Part 2

Doc Mike Finnley, Going Postal
Uranium glazed tiles

At some point in the 1980s a certain Bob Boyd needed to use a lavatory. I suppose that we all do, from time-to-time. And, at some point in the 1800s, someone decided it a very good idea to add uranium to ceramic glaze. It made pretty colours.

Mr Boyd was working as a radiation safety officer for Georgia State University. He was measuring radon gas levels in the university president’s  home using a Geiger counter. The house was about to be renovated.

As Bob was happily taking his measurements the call of nature came knocking. He headed for the nearest bog with his trusty Geiger counter still switched on. Whilst he was preparing himself, for the necessary deed, he noticed that the Geiger counter was ‘clicking’ above the normal background level. In fact, about ten times as much!

He discovered that the bathroom was covered in uranium glazed tiles. The glaze contains uranium oxide.

Walter Wagner, another radiation safety officer based in San Francisco, has taken measurements of uranium ceramic tiles in public buildings: schools, offices etc. He concluded that the main risks would be to young children playing for a long time on such a tiled floor. Or kitchen staff exposed to a wall/floor of uranium tiles for many hours per day.

Uranium glazed pottery was popular throughout the 1920s and 30s and continued to be used until the 1960s. Its use was suspended in 1942 for obvious reasons – we need that bomb!

It was not only used to make tiles but also tableware. That’s one way to keep your tea warm!

Uranium tile

Uranium Hunter Follows Trail of Tiles

Doc Mike Finnley, Going Postal
Doc holding a few pieces of uranium glazed pottery and the little home-made lead box that I keep them in. Radioactive counts-per-minute can be as high as 10000. The real danger comes from the sharp edges. Like razor blades! I’ll be using these in the next part when I test a Geiger tube.

The Geiger-Müller Tube And How It Works

Doc Mike Finnley, Going Postal

The Geiger-Müller tube (shown above) is named after its inventors: Hans Geiger and Walther Müller. It’s very easy to understand.

A high voltage is applied between the anode (+) and the cathode (-), about 400V. When radiation enters the tube it causes the gas inside to ‘ionise’, ie conduct electricity.

Doc Mike Finnley, Going Postal
Left: An electric torch. Right: A Geiger counter

The easiest way to think of a GM-tube is as an electric switch. At school you might have wired a simple lamp circuit – like the circuit above, left. Close the switch and the lamp lights up.

Replace the lamp and switch with a loudspeaker and GM-tube, up the battery voltage to 400 Volts and you turn a simple electric torch into a Geiger counter.

If a radioactive particle enters the GM-tube the argon gas ionises – closing the switch – and a ‘click’ is heard in the loudspeaker.


The Problems…

Where do I buy a 400 volt battery? I want my Geiger counter to run on a few AAA batteries. The signal from a GM-tube is tiny so an amplifier will be needed to hear the clicks.

In the next part we’ll find one solution in a circuit that caused a common failure of the Sinclair Spectrum 48K computer.

Fun fact: Uranium oxide (sodium diuranate) was once used in porcelain dentures to make them look like natural teeth.

Say “Cheese!”.

© Doc Mike Finnley 2019

Audio file