What it shows
How could the fluorescence of the glass in a Crooke's tube generate x-rays? This was the question Henri Becquerel addressed in 1896. His experiments with fluorescence in uranium salts and subsequent discovery of radioactivity are recreated in this demonstration.
How it works
Instead of uranium salts, we use a green glass candy dish—the green glass being uranium glass, a popular consumer item in the 1950's! The green glass fluoresces brilliantly when illuminated by UV (a "black light") and, although not particularly "hot," a Geiger-Mueller counter held next to the glass detects the radioactivity.
Historically, Becquerel's experiments would suggest the following procedure for performing the demonstration. (1) Wrap a plate of unexposed film in black cloth and place the candy dish on top. (2) Shine UV light on the dish to make it fluoresce (the fluorescence was thought to be the origin of the radiation that blackened the film) and relate the famous story of Becquerel's subsequent serendipity. The photographic film wrapped in black cloth is just a prop in this reenactment—a modern radiation detector is substituted in at this point. (3) With the Geiger counter, show that the glass emits radiation even when it's not fluorescing (UV light is off) ... it's radioactive.
Setting it up
Use a 4×5 Polaroid plate for the film. The 18" black light works well for the UV source. Although any of our thin-window Geiger counters will work, the Ludlum model 177 ratemeter with model 44-7 G-M tube is the most convenient to use for this demonstration. The "clicks" of the ratemeter are quite loud if the volume is turned up, eliminating additional audio-visual needs. Note that the end-window of the G-M tube must be held right up to the glass to detect anything. The entire setup occupies very little space on the lecture bench.
The fluorescence of the uranium glass is beautiful. Excellent props to animate a great discovery story.