What Are Gamma-Rays?
This all-sky image, constructed from two years of observations by NASA's Fermi Gamma-ray Space Telescope, shows how the sky appears in gamma-ray light.
Credit: NASA/DOE/Fermi LAT Collaboration

Gamma-rays are a form of electromagnetic radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Gamma-rays can be used to treat cancer, and gamma-ray bursts are studied by astronomers. 

Electromagnetic (EM) radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. That spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays. 

Gamma-rays fall in the range of the EM spectrum above soft X-rays. Gamma-rays have frequencies greater than about 1018 cycles per second, or hertz (Hz), and wavelengths of less than 100 picometers (pm), or 4 × 10-9 inches. (A picometer is one-trillionth of a meter.) They occupy the same region of the EM spectrum as hard X-rays. The only difference between them is their source: X-rays are produced by accelerating electrons, whereas gamma-rays are produced by atomic nuclei. 

This diagram shows the entire spectrum of electromagnetic waves. The scale at the bottom indicates representative objects that are equivalent to the wavelength scale. The atmospheric opacity determines what radiation reaches the Earth's surface.
This diagram shows the entire spectrum of electromagnetic waves. The scale at the bottom indicates representative objects that are equivalent to the wavelength scale. The atmospheric opacity determines what radiation reaches the Earth's surface.
Credit: UC Regents.

Gamma-rays were first observed in 1900 by French chemist Paul Villard when he was investigating radiation from radium, according to NASA. A few years later, New Zealand-born chemist and physicist Ernest Rutherford proposed the name "gamma-rays," following the order of alpha-rays and beta-rays — names given to other particles observed from nuclear radiation — and the name stuck.

Gamma-rays are produced primarily by four different nuclear reactions: fusion, fission, alpha decay and gamma decay. Nuclear fusion is the reaction that powers the sun and stars. It occurs in a multistep process in which four protons, or hydrogen nuclei, are forced under extreme temperature and pressure to fuse into a helium nucleus, which comprises two protons and two neutrons. The resulting helium nucleus is about 0.7 percent less massive than the four protons that went into the reaction. That mass difference is converted into energy according to Einstein's famous equation E = mc2, with about two-thirds of that energy emitted as gamma-rays. (The rest is in the form of neutrinos, which are extremely weakly interacting particles with nearly zero mass.) In the later stages of a star's lifetime, when it runs out of hydrogen fuel, it can form increasingly more massive elements through fusion up to and including iron, but these reactions produce a decreasing amount of energy at each stage. 

Another familiar source of gamma-rays is nuclear fission. Lawrence Berkeley National Laboratory defines nuclear fission as "the splitting of a heavy nucleus into two roughly equal parts (which are nuclei of lighter elements), accompanied by the release of a relatively large amount of energy in the form of kinetic energy of the two parts and in the form of emission of neutrons and gamma-rays." In this process, heavy nuclei, such as uranium and plutonium, are broken into smaller elements, such as xenon and strontium, in collisions with other particles. The resulting particles from these collisions can then impact other heavy nuclei, setting up a nuclear chain reaction. Energy is released because the combined mass of the resulting particles is less than the mass of the original heavy nucleus. That mass difference is converted to energy according to E = mc2 in the form of kinetic energy of the smaller nuclei, neutrinos and gamma-rays. 

Other sources of gamma-rays are alpha decay and gamma decay. Alpha decay occurs when a heavy nucleus gives off a helium-4 nucleus, reducing its atomic number by 2 and its atomic weight by 4. This process can leave the nucleus with excess energy, which is emitted in the form of a gamma-ray. Gamma decay occurs when there is too much energy in the nucleus of an atom, causing it to emit a gamma-ray without changing its charge or mass composition. 

Gamma-rays are sometimes used to treat cancerous tumors in the body by damaging the DNA of the tumor cells. However, great care must be taken because gamma-rays can also damage the DNA of surrounding healthy tissue cells. One way to maximize the dosage to cancer cells while minimizing the exposure to healthy tissues is to direct multiple gamma-ray beams from a linear accelerator, or linac, onto the target region from many different directions. This is the operating principle of the CyberKnife and the Gamma Knife. According to the Mayo Clinic website, "In Gamma Knife radiosurgery, specialized equipment focuses close to 200 tiny beams of radiation on a tumor or other target. Although each beam has very little effect on the brain tissue it passes through, a strong dose of radiation is delivered to the site where all the beams meet."

One of the more interesting sources of gamma-rays is gamma-ray bursts (GRBs). These are extremely high-energy events that last only a few milliseconds to several minutes. They were first observed in the 1960s, and they are now observed somewhere in the sky about once a day. 

"For a long time, it was believed that GRBs must come from within our own galaxy," the University of California, Berkeley website states. "It seemed impossible that they could be much more distant — for a gamma-ray burst to have come from a distant galaxy, it would have to be incredibly powerful to explain its observed brightness." We now know that most GRBs actually do come from galaxies that are more than 100 million to billions of light-years away.  

According to Robert Patterson, a professor of astronomy at Missouri State University, GRBs were once thought to come from the last stages of evaporating mini black holes. They are now believed to originate in collisions of compact objects such as neutron stars. Other theories attribute these events to the collapse of supermassive stars to form black holes. In either case, GRBs can produce enough energy that, for a few seconds, they can outshine an entire galaxy. Because the Earth's atmosphere blocks most gamma-rays, observations are typically conducted using high-altitude balloons and orbiting telescopes. 

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