GAMMA (γ) RAYS

As the wavelength of EM rays becomes shorter and shorter, their penetrating power increases until focusing is impossible. The cutoff point where the x-ray region ends and the gamma-ray region begins is approximately 0.01 nm (10^-11 m). Gamma rays can, in theory, get shorter than this without limit.
The gamma classification represents the most energetic of all EM fields. Short-wavelength gamma rays can penetrate several centimeters of solid lead or more than a meter of concrete. They are even more damaging to living tissue than x-rays. Gamma rays come from radioactive materials, both natural (such as radon) and human-made (such as plutonium).

Radiation counters are the primary means of detecting and observing sources of gamma rays. Gamma rays can dislodge particles from the nuclei of atoms they strike. These subatomic particles can be detected by a counter. One type of radiation counter consists of a thin wire strung within a sealed cylindrical metal tube filled with alcohol vapor and argon gas. When a highspeed subatomic particle enters the tube, the gas is ionized for a moment. A voltage is applied between the wire and the outer cylinder so that a pulse of electric current occurs whenever the gas is ionized. Such a pulse produces a click in the output of an amplifier connected to the device.
A simplified diagram of a radiation counter is shown in Fig. 1. A glass window with a metal sliding door is cut in the cylinder. The door can be opened to let in particles of low energy and closed to allow only the most energetic particles to get inside. High-speed subatomic particles, which are tiny yet massive for their size, have no trouble penetrating the window glass if they are moving fast enough. When the door is closed, gamma rays can penetrate it with ease.

Fig. 1. Simplified diagram of a radiation counter.