Gamma Radiation
Gamma radiation is a form of electromagnetic radiation characterized by its extremely high energy and short wavelengths, typically defined as radiation with wavelengths less than about 10 picometers or energy levels above 100 keV. It is produced primarily during nuclear decay, specifically through a process known as gamma decay, where an unstable atomic nucleus releases energy in the form of gamma-ray photons. These photons can penetrate materials more deeply than other forms of radiation, such as alpha and beta particles, which makes gamma rays both significant in scientific applications and hazardous to biological systems due to their ionizing properties.
Gamma rays can be generated through various mechanisms, including the deceleration of charged particles (bremsstrahlung) and synchrotron radiation from particles moving in magnetic fields. In astronomy, gamma rays are often linked to cosmic events, such as gamma-ray bursts, which are intense flashes of radiation believed to result from supernovae or other violent cosmic phenomena. The detection of gamma rays is crucial for understanding cosmic events and the fundamental processes of the universe, providing insights into events following the Big Bang. Despite their danger to living organisms, gamma rays also have important applications in medical imaging and treatment due to their penetrating power.
Gamma Radiation
FIELDS OF STUDY: Electromagnetism; Nuclear Physics
ABSTRACT: Gamma radiation is the most energetic form of electromagnetic radiation. This article looks at the nature of gamma rays and how they are generated, touching on detection methods and their role in cosmology. Known sources of gamma radiation include high-energy collisions and nuclear decay.
principal terms
- bremsstrahlung: radiation generated when a particle is slowed via a collision with another particle; from the German word for "braking radiation."
- cosmic rays: extremely high-energy subatomic particles, mainly protons and atomic nuclei, that originate outside of Earth’s atmosphere from largely unknown sources.
- gamma-ray burst: a high-energy flash of gamma radiation produced by a violent explosion in a distant galaxy; believed to be the brightest and most energetic electromagnetic event in the universe since the big bang.
- inverse Compton scattering: a collision between a high-energy electron and a low-energy photon that results in energy being transferred to the photon.
- photon: a massless elementary particle that is the smallest possible unit, or quantum, of light and other electromagnetic radiation.
- radioactive decay: the loss of energy and matter from an unstable nucleus in the form of ionizing radiation.
- radioisotope: an isotope of an element with an unstable nucleus.
- synchrotron radiation: radiation emitted by the acceleration of a charged particle traveling in a magnetic field at near light speed.
- x-rays: high-energy electromagnetic radiation emitted when an excited electron drops back down to a lower energy level.
Gamma Rays
Light is a form of electromagnetic radiation. The word "light" usually refers to the portion of the electromagnetic spectrum that is visible to the human eye. Sometimes it is also used to describe radiation in the infrared and ultraviolet ranges. However, there is much more to electromagnetic radiation than just light.
Electromagnetic radiation is classified according to its wavelength and frequency. The two are inversely related; the shorter the wavelength is, the higher the frequency. Energy is also inversely related to wavelength, meaning that radiation with a shorter wavelength has greater energy. The main types of electromagnetic radiation, in order of decreasing wavelength and increasing frequency and energy, are radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma radiation, or gamma rays.
Gamma radiation is typically the most energetic, with the shortest wavelengths. Historically, the term referred to any electromagnetic radiation below a certain wavelength (about 10−11 meters) or above a certain energy level (about 100 kiloelectronvolts, or keV), while high-energy radiation that fell outside those bounds was considered x-rays. However, the distinction between the two is not quite as clear as was once thought.
Another way of distinguishing gamma rays from x-rays is by their source. While electromagnetic radiation is a wave, it is also composed of small, massless particles called photons. This is possible due to the concept of wave-particle duality, which states that all elementary particles also exhibit properties of waves. When an atom emits gamma rays or x-rays, it is in fact releasing highly energetic photons. Gamma-ray photons are generally released by the nucleus as a result of excess energy or nuclear decay. X-ray photons are released by excited electrons as they drop back down to a lower energy level.
For the most part, these two definitions—energy-based and source-based—correspond with one another. However, as scientists have developed techniques for generating x-rays with energies as high as 25 megaelectronvolts (MeV), the line has become increasingly blurred. As a result, in physics, gamma radiation is defined primarily as radiation produced in nuclear events. In astronomy, this would not be practical, as sources of high-energy radiation in outer space are often unknown. Thus, astronomers define gamma radiation based on its energy level rather than its source.
Radioactive Decay
Radioactive decay is the process by which a radioisotope emits energy from its unstable nucleus in the form of ionizing radiation. Gamma rays are produced by gamma decay, one of the three most common forms of radioactive decay. The other two forms are alpha decay and beta decay, which produce alpha particles and beta particles, respectively. An alpha particle consists of two protons and two neutrons bound together, identical to a helium nucleus. A beta particle is a high-speed, extremely energetic electron or positron (the antiparticle of the electron).
Gamma decay almost always occurs alongside or immediately after another form of decay. When a gamma-ray photon collides with an atom, its energy is transferred to one of the atom’s electrons. Often, the electron absorbs so much energy that it breaks away from the atom, leaving behind a positively charged ion—hence the name "ionizing radiation." Alpha and beta particles are more ionizing, but gamma rays can penetrate more deeply. They are so energetic that they can only be blocked by thick layers of a dense material such as lead. This is what makes gamma rays so dangerous to life-forms; the ionization of molecules in the body inhibits proper cellular function, often resulting in radiation poisoning or cancer.
Generating Gamma Rays
Gamma rays are generated via several mechanisms, most involving the acceleration or deceleration of charged particles. To understand these processes, it is important to remember that photons are the particles that carry the electromagnetic force. As a result, when the motion of a charged particle is affected by electromagnetism, photons are either released or absorbed. When an electron or proton is traveling at near light speed, the amount of energy released in a collision with another particle would be enormous.
This is one way of generating bremsstrahlung, which is electromagnetic radiation produced by the sudden deceleration of a particle. When a particle such as an electron collides with another particle, such as an atom in a sheet of lead, the electron may go from near light speed to a near stop. This collision requires the electron to bleed off a great deal of energy, which it does by converting its kinetic energy into electromagnetic radiation. The more energy the electron must get rid of, the more energetic the photon and the shorter the wavelength.
A similar process is behind synchrotron radiation, which is produced by the radial acceleration of charged particles. It is named after the synchrotron, a type of circular particle accelerator. Any object that travels in a circle is constantly changing its velocity, because velocity has both magnitude and direction. Even if the object’s speed remains the same, the constant change in direction means that the velocity changes, and any change in velocity is a form of acceleration. In the case of an object traveling in a circle, the acceleration is toward the circle’s center. This constant acceleration results in a loss of energy, which the charged particles emit in the form of photons.
It Came from Space
Another source of gamma rays is the highly energetic emissions from black holes. There, high-speed electrons being sucked into the black hole’s accretion disk collide with lower-energy photons generated by heat in the disk. The excess energy transfers from the electrons to the photons. This process is called inverse Compton scattering because the photons "scatter" to higher energy levels. In ordinary Compton scattering, a low-energy charged particle collides with a high-energy photon and absorbs its energy; inverse Compton scattering is, appropriately, the opposite.
Gamma-ray bursts, on the other hand, are still very mysterious. Astronomers know that in deep space, occasional brilliant flashes of electromagnetic energy produce gamma-ray spikes lasting from a fraction of a second to hours in length. These have been observed to occur only at great distances from the Milky Way galaxy, and it is thought that most of them are caused by supernovas.
Compton scattering is actually one of the primary tools for detecting gamma radiation. A detector may consist of massive tanks of water or arrays of sensors laid down on the seabed. When cosmic rays collide with this dense material, they create a flash of less-energetic radiation that can be observed as a water molecule becomes excited and releases the excess energy as a photon. Detection of gamma rays is one way astronomers and physicists can form a more complete picture of the universe, as well as try to look back at the highly energetic events that followed the big bang.
Bibliography
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Kouveliotou, Chryssa, Ralph A. M. J. Wijers, and Stanford E. Woosley, eds. Gamma-Ray Bursts. New York: Cambridge UP, 2012. Print. Cambridge Astrophysics Ser. 51.