The Electromagnetic Spectrum by year 12 physics students

Gamma Rays

Sulaxan Shanmulgalingam 13B

Gamma radiation, also known as gamma rays, and denoted by the Greek letter g, refers to electromagnetic radiation of extremely high frequency and therefore high energy per photon. They typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometres (less than the diameter of an atom). However, this is not a hard and fast definition, but rather only a rule-of-thumb description for natural processes. There is in fact a large overlap with X-rays and what distinguishes it from X-rays is the method of production. X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus. Because X-rays are extremely high frequency waves and carry a large amount of energy they pass through most materials, and are quite difficult to stop – you need lead or concrete in order to block them out.

Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes such as potassium-40, and also as a secondary radiation from various atmospheric interactions with cosmic ray particles. Some rare terrestrial natural sources that produce gamma rays that are not of a nuclear origin are lightning strikes and terrestrial gamma-ray flashes, which produce high energy emissions from natural high-energy voltages. Gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth’s atmosphere and must be detected by spacecraft. Notable artificial sources of gamma rays include fission such as occurs in nuclear reactors, and high energy physics experiments, such as neutral pion decay and nuclear fusion.

A Little History

The first gamma ray source to be discovered historically was the radioactive decay process called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately upon formation. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium.

image         image

Paul Ulrich Villard (28 September 1860 – 13 January 1934) was a French chemist and physicist, born in Saint-Germain-au-Mont-d’Or, Rhône, 28 September 1860.


Unlike optical light and x-rays, gamma rays cannot be captured and reflected by mirrors. Gamma-ray wavelengths are so short that they can pass through the space within the atoms of a detector. Gamma-ray detectors typically contain densely packed crystal blocks. As gamma rays pass through, they collide with electrons in the crystal. This process is called Compton scattering, where a gamma ray strikes an electron and loses energy, similar to what happens when a cue ball strikes an eight ball. These collisions create charged particles that can be detected by the sensor.


Uses of Gamma Rays

As gamma rays kill living cells, they are used to kill cancer cells without having to resort to difficult surgery. This is called Radiotherapy and works because cancer cells can’t repair themselves like healthy cells can when damaged by gamma rays. There’s also targeted radiotherapy where a radioactive substance is used to kill cancer cells – but it’s a substance that’ll be taken up by a specific part of the body, so that the rest of the body only gets a low dose. Radioactivity is particularly damaging to rapidly dividing cells, such as cancer cells.

Doctors can put slightly radioactive substances into a patient’s body, and then scan the patient to detect the gamma rays and build up a picture of what’s going on inside the patient. This is very useful because they can see the body processes actually working.



Gamma rays kill microbes and are used to sterilise food so that it will keep fresh for longer. It is also used to sterilise medical equipment. This is very useful for developing countries that don’t have access to electricity for running sterilising equipment like autoclaves.


The above picture is an Image of the entire sky in 100 MeV or greater gamma rays as seen by the EGRET instrument aboard the CGRO spacecraft. Bright spots within the galactic plane are pulsars while those above and below the plane are thought to be quasars.

Other uses of gamma rays include providing information about some of the most energetic phenomena in the universe, molecular changes to alter the properties of semi-precious stones, non-contact industrial sensors for measuring levels, densities and thicknesses and scanning of ships for stowaways.


Gamma-ray image of a truck with two stowaways taken with a VACIS (vehicle and container imaging system)

Dangers of Gamma Radiation

Gamma rays are very penetrating, causing diffuse damage throughout the body (e.g. radiation sickness, cell’s DNA damage, cell death due to damaged DNA, increasing incidence of cancer) rather than burns. External radiation exposure should also be distinguished from internal exposure, due to ingested or inhaled radioactive substances, which, depending on the substance’s chemical nature, can produce both diffuse and localized internal damage

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