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Gamma ray: the story on HearLore | HearLore
Gamma ray
In 1900, a French chemist named Paul Villard was studying the radiation emitted by radium when he noticed something that defied all previous expectations. While Henri Becquerel had identified beta rays and Ernest Rutherford had cataloged alpha rays, Villard observed a third type of radiation that was far more powerful and penetrating than anything known to science. He did not name it, nor did he fully understand its nature, but his discovery marked the beginning of a new era in physics. This radiation, later named gamma rays by Rutherford in 1903, possessed an energy range from 10 kiloelectronvolts to 10,000 kiloelectronvolts, and in some cases, ultra-high-energy gamma rays exceeded 10 to the power of 11 kiloelectronvolts. The name gamma was chosen because Rutherford had already used the first three letters of the Greek alphabet to classify radiation by their ability to penetrate matter, placing gamma rays at the top of the list as the most penetrating form of electromagnetic radiation. Unlike alpha and beta rays, which were deflected by magnetic fields, gamma rays passed through magnetic fields undeflected, a property that initially led Rutherford to believe they might be extremely fast beta particles. It was not until 1914 that gamma rays were observed to be reflected from crystal surfaces, proving they were electromagnetic radiation with wavelengths shorter and frequencies higher than X-rays. This discovery fundamentally changed the understanding of atomic structure and the forces that govern the universe.
Nuclear Secrets Unveiled
The story of gamma rays is deeply intertwined with the behavior of atomic nuclei, which often remain in an excited state after undergoing other forms of decay. When a radioactive nucleus emits an alpha or beta particle, the resulting daughter nucleus is frequently left in an excited state, and it can take as little as 10 to the power of minus 12 seconds to decay to a lower energy state by emitting a gamma ray photon. This process, known as gamma decay, is the primary mechanism by which gamma rays are produced in radioactive materials. However, some excited nuclear states are more stable than average, taking 100 to 1000 times longer to decay, and are termed metastable excited states or nuclear isomers. These isomers can remain in their excited state for minutes, hours, days, or even longer before emitting a gamma ray, making their decay rates much easier to measure. The decay of nuclear isomers is a form of isomeric transition, which differs from standard gamma emission because it involves intermediate metastable states with high nuclear spin. The rate of gamma decay is also slowed when the energy of excitation of the nucleus is small, and the process may require a change in spin of several units or more. This complexity in nuclear decay schemes is what allows scientists to identify decaying radionuclides using gamma spectroscopy, a technique that analyzes the energy spectrum of gamma rays to determine the identity of the source. For example, the decay scheme of cobalt-60 involves beta decay followed by the emission of gamma rays in succession of 1.17 megaelectronvolts and 1.33 megaelectronvolts, a path followed 99.88 percent of the time.
Who discovered gamma rays and when was the discovery made?
Paul Villard discovered gamma rays in 1900 while studying radiation emitted by radium. Ernest Rutherford later named the radiation gamma rays in 1903.
What is the energy range of gamma rays and how do they compare to X-rays?
Gamma rays possess an energy range from 10 kiloelectronvolts to 10,000 kiloelectronvolts and can exceed 10 to the power of 11 kiloelectronvolts. Gamma rays have wavelengths shorter and frequencies higher than X-rays and are distinguished by their origin from the atomic nucleus rather than electrons outside the nucleus.
How are gamma rays produced in radioactive materials and what is gamma decay?
Gamma rays are produced when a radioactive nucleus emits an alpha or beta particle and the resulting daughter nucleus decays to a lower energy state by emitting a gamma ray photon. This process known as gamma decay is the primary mechanism by which gamma rays are produced in radioactive materials and can occur in as little as 10 to the power of minus 12 seconds.
What materials are used to shield against gamma rays and how effective is lead?
Gamma rays require shielding made from dense materials such as lead or concrete because they pass through magnetic fields undeflected and penetrate matter more deeply than alpha or beta particles. A lead shield is 20 to 30 percent better as a gamma shield than an equal mass of a low-Z shielding material such as aluminum or concrete due to lead having a high atomic number and high density.
What are the health effects of gamma rays and how are they used in medicine?
High doses of gamma rays produce deterministic effects such as acute tissue damage and can cause symptoms including nausea vomiting hair loss and hemorrhaging. Gamma rays are used to treat cancer through gamma-knife surgery and for diagnostic purposes in PET scans using the nuclear isomer technetium-99m.
What are gamma-ray bursts and how far away can they be detected?
Gamma-ray bursts are long-duration bursts that last 20 to 40 seconds and release energy as much as the Sun will produce in its entire lifetime. These bursts can be detected even at distances of up to 10 billion light years if the beam of particles moving at relativistic speeds is pointed toward Earth.
While gamma rays originate from radioactive decay on Earth, the most intense sources of gamma rays in the universe are cosmic events that dwarf any human-made phenomenon. Gamma-ray bursts, the most intense sources of gamma rays known, are long-duration bursts that last only 20 to 40 seconds but release a total energy output of about 10 to the power of 44 joules, which is as much energy as the Sun will produce in its entire lifetime. These bursts are rare compared to other sources, and the leading hypotheses for their production involve inverse Compton scattering and synchrotron radiation from high-energy charged particles. The beam of particles moving at relativistic speeds is focused for a few tens of seconds by the magnetic field of an exploding hypernova, and if the beam happens to be pointed toward Earth, it can be detected even at distances of up to 10 billion light years. Short gamma-ray bursts, lasting two seconds or less, are thought to produce gamma rays during the collision of pairs of neutron stars or a neutron star and a black hole. Pulsars, which are neutron stars with magnetic fields that produce focused beams of radiation, are another source of gamma rays, and they are far less energetic, more common, and much nearer sources than quasars or gamma-ray bursts. Quasars and active galaxies produce powerful gamma rays through mechanisms similar to a particle accelerator, where high-energy electrons produced by the quasar are subjected to inverse Compton scattering, synchrotron radiation, or bremsstrahlung. The power of a typical quasar is about 10 to the power of 40 watts, a small fraction of which is gamma radiation, and these sources fluctuate with durations of a few weeks, suggesting their relatively small size of less than a few light-weeks across.
The Shielding Challenge
The penetrating nature of gamma rays makes them a formidable challenge for radiation protection, requiring large amounts of shielding mass to reduce them to levels that are not harmful to living cells. Unlike alpha particles, which can be stopped by a sheet of paper, or beta particles, which can be shielded by a thin aluminum plate, gamma rays require shielding made from dense materials such as lead or concrete. A lead shield is 20 to 30 percent better as a gamma shield than an equal mass of a low-Z shielding material, such as aluminum, concrete, water, or soil, because lead has a high atomic number and high density. The thickness of the shielding required depends on the energy of the gamma rays, and materials for shielding gamma rays are typically measured by the half-value layer, which is the thickness required to reduce the intensity of the gamma rays by one half. For example, gamma rays that require 1 centimeter of lead to reduce their intensity by 50 percent will also have their intensity reduced in half by 1.5 centimeters of granite rock, 6 centimeters of concrete, or 9 centimeters of packed soil. Depleted uranium is sometimes used for shielding in portable gamma ray sources due to its smaller half-value layer compared to lead and its cheaper cost compared to tungsten. In a nuclear power plant, shielding can be provided by steel and concrete in the pressure and particle containment vessel, while water provides a radiation shielding of fuel rods during storage or transport into the reactor core. The loss of water or removal of a hot fuel assembly into the air would result in much higher radiation levels than when kept under water, highlighting the critical importance of proper shielding in nuclear facilities.
Healing and Destruction
Gamma rays are a double-edged sword, capable of causing severe damage to living tissue while also being used to save lives through medical treatments. Low levels of gamma rays cause a stochastic health risk, which is defined as the probability of cancer induction and genetic damage, and high doses produce deterministic effects, which is the severity of acute tissue damage that is certain to happen. An acute full-body equivalent single exposure dose of 1 sievert, or 1 gray, will cause mild symptoms of acute radiation sickness, such as nausea and vomiting, and a dose of 2.0 to 3.5 sieverts causes more severe symptoms, including nausea, diarrhea, hair loss, hemorrhaging, and inability to fight infections. A dose of 3 to 5 sieverts is considered approximately the LD50, or the lethal dose for 50 percent of the exposed population, even with standard medical treatment. Despite their cancer-causing properties, gamma rays are used to treat some types of cancer through a procedure called gamma-knife surgery, where multiple concentrated beams of gamma rays are directed to the growth to kill the cancerous cells. Gamma radiation is also used to kill living organisms in a process called irradiation, which includes the sterilization of medical equipment, the removal of decay-causing bacteria from many foods, and the prevention of the sprouting of fruit and vegetables to maintain freshness and flavor. In nuclear medicine, gamma-emitting radioisotopes are used for diagnostic purposes, such as in a PET scan, where a radiolabeled sugar called fluorodeoxyglucose emits positrons that are annihilated by electrons, producing pairs of gamma rays that highlight cancer. The most common gamma emitter used in medical applications is the nuclear isomer technetium-99m, which emits gamma rays in the same energy range as diagnostic X-rays.
The Energy Spectrum Defined
The distinction between gamma rays and X-rays has evolved over time, shifting from a definition based on wavelength to one based on origin. Originally, the electromagnetic radiation emitted by X-ray tubes almost invariably had a longer wavelength than the radiation emitted by radioactive nuclei, and older literature distinguished between X and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10 to the power of minus 11 meters, defined as gamma rays. However, since current artificial sources are now able to duplicate any electromagnetic radiation that originates in the nucleus, as well as far higher energies, the wavelengths characteristic of radioactive gamma ray sources versus other types now completely overlap. Thus, gamma rays are now usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus. Exceptions to this convention occur in astronomy, where gamma decay is seen in the afterglow of certain supernovas, but radiation from high energy processes known to involve other radiation sources than radioactive decay is still classed as gamma radiation. For example, modern high-energy X-rays produced by linear accelerators for megavoltage treatment in cancer often have higher energy, from 4 to 25 megaelectronvolts, than do most classical gamma rays produced by nuclear gamma decay. One of the most common gamma ray emitting isotopes used in diagnostic nuclear medicine, technetium-99m, produces gamma radiation of the same energy, 140 kiloelectronvolts, as that produced by diagnostic X-ray machines, but of significantly lower energy than therapeutic photons from linear particle accelerators. In physics and astronomy, the only naming convention that is still universally respected is the rule that electromagnetic radiation that is known to be of atomic nuclear origin is always referred to as gamma rays, and never as X-rays.