The key difference between gamma rays and X-rays is how they are produced.
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A gamma ray is a packet of electromagnetic energy (photon) emitted by the nucleus of some radionuclides following radioactive decay. Gamma photons are the most energetic photons in the electromagnetic spectrum. Their emission commonly occurs within a fraction of a second after radioactive decay but sometimes occurs several hours later.
Paul Villard, a French physicist working in Paris at the same time as Marie and Pierre Curie, is credited with discovering gamma rays. In 1895, Roentgen discovered X-rays and shortly thereafter Becquerel discovered radioactivity of uranium salts.
Paul Villard's main interest was in chemistry, which guided him into his studies of cathode rays, X-rays, and "radium rays." His experiments in radioactivity led to the unexpected discovery of gamma rays in 1900. Villard recognised them as being different from X-rays because the gamma rays had a much greater penetrating depth. He had discovered they were emitted from radioactive substances and were not affected by electric or magnetic fields.
Gamma rays are a form of electromagnetic radiation (EMR). They are the same as x-rays, distinguished only by the fact that they come from the nucleus. Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each traveling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. Gamma-ray photons have the highest energy in the EMR spectrum and their waves have the shortest wavelength.
Scientists measure the energy of photons in electron volts. Ultraviolet radiation falls in the range from a few electron volts (eV) to about 100 eV. X-ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-ray photons generally have energies greater than 100 keV. The high energy of gamma rays enables them to pass through many kinds of materials, including human tissue. Very dense materials, such as lead, are commonly used as shielding to slow or stop gamma photons
The key difference between gamma rays and X-rays is how they are produced. Gamma rays originate from the nucleus of a radionuclide after radioactive decay whereas X-rays are produced when electrons strike a target or when electrons are rearranged within an atom. Cosmic rays also include high-energy photons and these are also called gamma-rays whether or not they originated from nuclear decay or reaction.
Gamma emitting radionuclides are the most widely used radiation sources. The penetrating power of gamma photons has many applications. However, while gamma rays penetrate many materials, they do not make them radioactive. The three radionuclides by far most useful are cobalt-60, cesium-137, technetium-99m and americium-241.
Uses of Cesium-137:
- measure and control the flow of liquids in numerous industrial processes
- investigate subterranean strata in oil wells
- measure soil density at construction sites
- ensure the proper fill level for packages of food, drugs and other products.
Uses of Cobalt-60:
- sterilise medical equipment in hospitals
- pasteurise certain foods and spices
- gauge the thickness of metal in steel mills.
Uses of Technetium-99m:
- TC-99m is the most widely used radioactive isotope for diagnostic studies. It has a short half-life and as a result it doesn't remain in the body for long. Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies.
Uses of Americium-241:
- fluid-level gauges
- fluid-density gauges
- thickness gauges
- aircraft fuel gauges
- distance-sensing devices, all of which utilise its gamma radiation.
A mixture of americium-241 and beryllium provides a neutron source for industrial devices that monitor product quality. Two examples are devices for nondestructive testing of machinery and gauges for measuring the thickness of glass and other products.