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A Radiation Detector

A "Geiger counter" usually contains a metal tube with a thin metal wire along its middle, the space in between them sealed off and filled with a suitable gas, and with the wire at about +1000 volts relative to the tube.

An ion or electron penetrating the tube (or an electron knocked out of the wall by X-rays or gamma rays) tears electrons off atoms in the gas, and because of the high positive voltage of the central wire, those electrons are then attracted to it. In doing so they gain energy, collide with atoms and release more electrons, until the process snowballs into an "avalanche" which produces an easily detectable pulse of current. With a suitable filling gas, the flow of electricity stops by itself, or else the electrical circuitry can help stop it.

The instrument was called a "counter" because every particle passing it produced an identical pulse, allowing particles to be counted (usually electronically) but not telling anything about their identity or energy (except that they must have sufficient energy to penetrate the walls of the counter). Van Allen's counters were made of thin metal, with insulating plugs at the ends. http://www-istp.gsfc.nasa.gov/Education/wgeiger.html

geiger counter

Common Radioactive Elements

All elements beyond bismuth (Bi, element 83), but only a few before it, are unstable. The unstable ones include of course uranium (U, element 92). They spontaneously decay into lighter elements, which may in turn still be heavy enough to decay into yet lighter ones forming an interesting decay chain. The decay chain continues until a stable element, often lead (Pb, element 82), is reached. The decays are accompanied by alpha, beta and gamma radiation and the beta and gamma rays are easily picked up by a Geiger counter. An alpha ray (helium nucleus) knocks an element two positions down to a new element in the periodic table while a beta ray (electron) bumps it back up one position to a new element. A gamma ray (photon) doesn't change the element, it's just the nucleus settling down.

The radiation that they emit is a big help in identifying even very small traces of these heavy elements.

Radium 226 (element 88) discovered in 1898 by Mme. Curie was long used (in combination with zinc sulfide) for many "glow in the dark" applications, such as the hands on alarm clocks.
It is a good alpha emitter and the alpha particle makes the zinc sufide fluoresce. The combination is called radioluminescence.

Thorium 232 (element 90) is a naturally occuring unstable element that was commonly used between 1890 and 1940 to provide the bright white light of gas lighting (the Welsbach mantle). During that time gas lighting was competing with electric lighting.
See Limelight, gas light, incandescent light.
Today thorium salts can still be found in some camping gas lantern mantles.
The thorium decay chain

Uranium 238 (element 92) is also naturally occuring and unstable. Its uses in power generation is well known.
The uranium decay chain
gamma (photon) spectrum ( cached image).
Uranium is easiest found in Uranium ore or Uranium glass. Uranium glass was quite popular during the art deco period (1910-1930), and contains Uranium oxide that gives it an eerie green glow. This type of glass is not only radioactive but also fluoresces nicely: it glows bright under ultraviolet light. No wonder green has become a favorite color for depicting aliens!

Three uranium oxides ("salts") have been extensively used in ceramic and glass coloring:
UO2 (uranium dioxide or uranous oxide) for black and grays,
U3O8 (uranium uranate) for greens,
UO3 (uranium trioxide or uranyl) oxide for reds.

The red oxide provides the vivid color in pre-1976 orange fiesta dinnerware.

Americium 241 (element 95) no longer occurs naturally. It was first produced in 1940 during research on nuclear fission.
It is a good beta emitter and can be found in many smoke detectors.
gamma (photon) spectrum (cached image)

On The Web

Radioactive Products and Other Sources Of Radiation a nice list from Black Cat Systems.