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.
The 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 1,018 cycles per second, or hertz (Hz), and wavelengths of less
than 100 picometers (pm), or 4 x 10^9 inches. (A picometer is one-trillionth of a meter.)
Gamma-rays and hard X-rays overlap in the EM spectrum, which can make it hard to
differentiate them. In some fields, such as astrophysics, an arbitrary line is drawn in the
spectrum where rays above a certain wavelength are classified as X-rays and rays with
shorter wavelengths are classified as gamma-rays. Both gamma-rays and X-rays have enough
energy to cause damage to living tissue, but almost all cosmic gamma-rays are blocked by
Earth's atmosphere.
Discovery of gamma-rays
Gamma-rays were first observed in 1900 by French chemist Paul Villard when he was
investigating radiation from radium, according to the Australian Radiation Protection and
Nuclear Safety Agency (ARPANSA). 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 that are created during a nuclear reaction
— and the name stuck.
Gamma-ray sources and effects
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=mc^2, 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 then nuclei of lighter elements. In this process, which involves collisions
with other particles, heavy nuclei, such as uranium and plutonium, are broken into smaller
elements, such as xenon and strontium. 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=mc^2, 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.
Artist impression of gamma ray burst.
(Image credit: NASA)
Gamma-ray therapy
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 CyberKnife
and Gamma Knife therapies.
Gamma Knife radiosurgery uses specialized equipment to focus close to 200 tiny beams of
radiation on a tumor or other target in the brain. Each individual beam has very little effect on
the brain tissue it passes through, but a strong dose of radiation is delivered at the point where
the beams meet, according to Mayo Clinic.
Gamma-ray astronomy
One of the more interesting sources of gamma-rays are gamma-ray bursts (GRBs). These are
extremely high-energy events that last from 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.
Gamma-ray bursts are "the most energetic form of light," according to NASA. They shine
hundreds of times brighter than a typical supernova and about a million-trillion times as
bright as the sun.
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 Earth's atmosphere blocks most gamma-rays, they're seen only with
high-altitude balloons and orbiting telescopes.
