All methods of detection of these radiations rely on their ability to ionize. Three
methods are in common use.
- Photographic plate. The fogging of a photographic emulsion led to the
discovery of radioactivity. If this emulsion is viewed under a high-power
microscope, it is seen that beta and gamma rays cause the silver bromide
grains to develop in a scattered fashion. - Scintillation counter. A fluorescent screen (e.g., ZnS) will show the
presence of electrons and X-rays, as already mentioned. If the screen is
viewed with a magnifying eyepiece, small flashes of light, called
scintillations, will be observed. By observing the scintillations, one not only
can detect the presence of alpha particles, but also can actually count them. - Geiger counter. This instrument is perhaps the most widely used at the
present time for determining individual radiation. Any particle that will
produce an ion gives rise to an avalanche of ions, so the type of particle
cannot be identified. However, each individual particle can be detected.
TIP
Know how to use half-life to determine the age of a substance.
See “Radioactive Dating” for more info.DECAY SERIES, TRANSMUTATIONS, AND HALF-LIFE
The nuclei of uranium, radium, and other radioactive elements are continually
disintegrating. It should be emphasized that spontaneous disintegration produces
the gas known as radon. The time required for half of the atoms of a radioactive
nuclide to decay is called its half-life.
For example, for radium, we know that, on the average, half of all the radium
nuclei present will have disintegrated to radon in 1,590 years. In another 1,590
years, half of this remainder will decay, and so on. When a radium atom
disintegrates, it loses an alpha particle, which eventually, upon gaining two
electrons, becomes a neutral helium atom. The remainder of the atom becomes
radon.
Such a conversion of an element to a new element (because of a change in
the number of protons) is called a transmutation. This transmutation can be
produced artificially by bombarding the nuclei of a substance with various
particles from a particle accelerator, such as the cyclotron.
The following uranium-radium disintegration series shows how a
radioactive atom may change when it loses each kind of particle. Note that an