Beyond the Radio Spectrum Help (page 3)

By — McGraw-Hill Professional
Updated on Apr 25, 2014

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. 18-8. 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.

Forms of Radiation Beyond the Radio Spectrum Cosmic Particles


Fig. 18-8 . Simplified diagram of a radiation counter.

Cosmic Particles

If you sit in a room with no radioactive materials present and switch on a radiation counter with the window of the tube closed, you’ll notice an occasional click from the device. Some of the particles come from the Earth; there are radioactive elements in the ground almost everywhere (usually in small quantities). Some of the radiation comes indirectly from space. Cosmic particles strike atoms in the atmosphere, and these atoms in turn eject other subatomic particles that arrive at the counter tube.

In the early 1900s, physicists noticed radiation apparently coming from space. They found that the strange background radiation increased in intensity when observations were made at high altitude; the radiation level decreased when observations were taken from underground or underwater. This space radiation has been called secondary cosmic radiation or secondary cosmic particles . The actual particles from space, called primary cosmic particles , usually do not penetrate far into the atmosphere before they collide with and break up the nuclei of atoms. To observe primary cosmic particles, it is necessary to ascend to great heights, and as with the UV and x-ray investigations, this was not possible until the advent of the space rocket.

While the radiation in the EM spectrum—the radio waves, IR, visible light, UV, x-rays, and gamma rays—consists of photons traveling at the speed of light, primary cosmic particles are comprised of matter traveling at almost, but not quite, the speed of light. At such high speeds, the protons, neutrons, and other heavy particles gain mass because of relativistic effects, and this renders them almost immune to the effects of the Earth’s magnetosphere. Such particles arriving in the upper atmosphere come to us in nearly perfect straight-line paths despite the magnetic field of our planet. By carefully observing the trails of the particles in a device called a cloud chamber aboard a low-orbiting space ship, it is possible to ascertain the direction from which they have come. Over time, cosmic-particle maps of the heavens can be generated and compared with maps at various EM wavelengths.

Beyond the Radio Spectrum Practice Problem


What is the energy contained in each photon of a barrage of gamma rays whose wavelength is 0.00100 nm?


To solve this problem, use the formula for energy in terms of wavelength λ the speed of EM propagation in free space c , and Planck’s constant h:

e = hc

For EM rays in free space, the product he is approximately equal to 1.9865 × 10 −25 . The wavelength 0.00100 nm is equivalent to 1.00 × 10 −12 m. Therefore, the energy e , in joules, contained in each photon of the gamma ray is

e = (1.9865 × 10 −25 )/(l.00 × 10 −12 )

= 1.99 × 10 −13 J

Practice problems of these concepts can be found at: Forms of Radiation Practice Test

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