What Are Gamma Rays
Gamma rays are a form of high energy electromagnetic (EM) radiation. Other examples of EM radiation are visible light, radio and microwaves, infrared and ultraviolet light, and x-rays. Each of these forms is distinguished in the EM spectrum by their frequency (f and wavelength (A,). In free space (that is, a vacuum) all EM waves travel at the speed of light, c, which is 3 * 108 meters/second, or 186,000 miles/second \ The frequency and wavelength are related to the speed by the equation
While the characterization of EM energy by frequency and wavelength implies wavelike behavior, it is a reality of the microscopic (i.e., quantum mechanical) world that waves of short enough wavelength exhibit particle-like behavior. Such "particles" of light are designated "photons". This particle-like behavior is important in the physics of gamma-gamma logging devices.
Whether or not it is appropriate to treat EM radiation of a particular frequency as a particle is a function of the wavelength of the radiation and how it compares with atomic dimensions. Visible light, which has wavelengths that are a factor of about 107 greater than atomic dimensions, and the even longer wavelength radio waves, are almost always analyzed as waves. But x-rays and gamma rays, which are at the short end of the spectrum of wavelengths, are most often thought of as particles rather than waves. Photons have no mass as do the electron and proton, but they do have energy and momentum. These are dependent on their frequency (or wavelength, since given a knowledge of the speed of light, one can be computed from the other) 2.
It is important to realize that a gamma ray's energy is proportional to its frequency. Thus, as its energy decreases, its speed, c, remains the same (it does not "slow down"); rather, its frequency decreases. Although we speak of gamma rays as particles, they have no mass (in contrast to electrons, protons, and neutrons, which each have their own characteristic mass). However, a fundamental interaction of electrons with gamma rays, that of Compton scattering, is adequately understood by analyzing the interaction as one would a billiard ball type collision between two particles.
Finally, we note that the electron volt (eV) is the unit used to characterize the energy of gamma rays and most other entities in the atomic and nuclear world. The electron volt is the amount of energy gained by an electron when it is accelerated through an electrical potential difference of one volt. It is equal to 1.6 * 10-19 joules of energy (or 4.4 * 10-26 Kilowatt-hours). For comparison, the energy required to ionize a hydrogen atom (separate it into a free proton and electron) is 13.6 eV, and the energy of the gamma rays emitted by the Csm logging source is 662 KeV (that is, 662 * 103 eV).
- The letter "c", which often designates the speed of light mathematically, is chosen thanks to the word "celerity", which denotes rapidity of motion or action.
- The kinetic energy, or energy associated with the motion of a particle, is given by the formula (1/2)mv2, where m and v are mass and velocity respectively. Likewise, the momentum of a particle is the product of its mass and velocity. However, this only applies to particles which have mass. Photons have no mass. The energy of a photon is given by the equation E=hf, where f is the frequency in Hertz, or cycles per second, h is Planck's constant, which is 4.14 * 10-15 eV-s, and E is energy in electron volts (eV). The momentum of a photon is p=h/A=hf/c.
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