Search

Gamma ray

Gamma rays (often denoted by the Greek letter gamma, γ) are an energetic form of electromagnetic radiation (see Electromagnetic spectrum) produced by radioactivity or other nuclear or subatomic processes such as electron-positron annihilation.

Gamma rays are often defined to begin at an energy of 10 keV / 2.42 EHz / 124 pm, although electromagnetic radiation from around 10 keV to several hundred keV is also referred to as hard X rays. It is important to note that there is no physical difference between gamma rays and X rays of the same energy -- they are two names for the same electromagnetic radiation, just as sunlight and moonlight are two names for visible light. Rather, gamma rays are distinguished from X rays by their origin. Gamma ray is a term for high-energy electromagnetic radiation produced by nuclear transitions, while X ray is a term for high-energy electromagnetic radiation produced by energy transitions due to accelerating electrons. Because it is possible for some electron transitions to be of higher energy than some nuclear transitions, there is an overlap between what we call low energy gamma rays and high energy X-rays.

Gamma rays are a form of ionizing radiation; they are more penetrating than either alpha or beta radiation (neither of which is electromagnetic radiation), but less ionizing.

Shielding for γ rays requires large amounts of mass. The material used for shielding takes into account that gamma rays are better absorbed by materials with high atomic number and high density. Also, the higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically illustrated by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example, gamma rays that require 1 cm (0.4 inches) of lead to reduced their intensity by 50% will also have their intensity reduced in half by 6 cm (2.4 inches) of concrete or 9 cm (3.6 inches) of packed dirt.

Gamma rays from nuclear fallout would probably cause the largest number of casualties in the event of the use of nuclear weapons in a nuclear war. An effective fallout shelter reduces human exposure at least 1000 times.

Gamma rays are less ionising than either alpha or beta rays. However, reducing human danger requires thicker shielding. They produce damage similar to that caused by X-rays, such as burns, cancer, and genetic mutations.

In terms of ionization, gamma radiation interacts with matter via three main processes: the photoelectric effect, Compton scattering, and pair production.

Photoelectric Effect: This describes the case in which a gamma photon interacts with and transfers all of its energy to an orbital electron, ejecting that electron from the atom. The kinetic energy of the resulting photoelectron is equal to the energy of the incident gamma photon minus the binding energy of the electron. The photoelectric effect is thought to be the dominant energy transfer mechanism for x-ray and gamma ray photons with energies below 50 keV (thousand electron volts), but it is much less important at higher energies.

Compton Scattering: This is an interaction in which an incident gamma photon loses enough energy to an orbital electron to cause its ejection, with the remainder of the original photon's energy being emitted as a new, lower energy gamma photon with an emission direction different from that of the incident gamma photon. The probability of Compton scatter decreases with increasing photon energy. Compton scattering is thought to be the principal absorption mechanism for gamma rays in the intermediate energy range 100 keV to 10 MeV (million electron volts), an energy spectrum which includes most gamma radiation present in a nuclear explosion. Compton scattering is relatively independent of the atomic number of the absorbing material.

Pair Production: By interaction in the vicinity of the coulomb force of the nucleus, the energy of the incident photon is spontaneously converted into the mass of an electron-positron pair. A positron is a positively charged electron. Energy in excess of the equivalent rest mass of the two particles (1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus. The electron of the pair, frequently referred to as the secondary electron, is densely ionizing. The positron has a very short lifetime. It combines within 10-8 seconds with a free electron. The entire mass of these two particles is then converted into two gamma photons of 0.51 MeV energy each. Gamma rays are often produced alongside other forms of radiation such as alpha or beta. When a nucleus emits an α or β particle, the daughter nucleus is sometimes left in an excited state. It can then jump down to a lower level by emitting a gamma ray in much the same way that an atomic electron can jump to a lower level by emitting ultraviolet radiation.

Gamma rays, x-rays, visible light, and UV rays are all forms of electromagnetic radiation. The only difference is the frequency and hence the energy of the photons. Gamma rays are the most energetic. An example of gamma ray production follows.

First cobalt-60 decays to excited nickel-60 by beta decay:

${}^{60}\hbox{Co}\;\to\;^{60}\hbox{Ni*}\;+\;e^-\;+\;\overline{\nu}_e.$

Then the nickel-60 drops down to the ground state (see nuclear shell model) by emitting a gamma ray:

${}^{60}\hbox{Ni*}\;\to\;^{60}\hbox{Ni}\;+\;\gamma.$

Gamma rays of 1.17 MeV and 1.33 MeV are produced.

 Contents

Uses:

In spite of their cancer-causing properties, gamma rays are also used to treat some types of cancer. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed on the growth in order to kill the cancerous cells. The beams are aimed from different angles to focus the radiation on the growth while minimising damage to the surrounding tissues.

Gamma rays are also used for diagnostic purposes in nuclear medicine. Several gamma emitting radioisotopes are used, one of which is technetium-99m. When administered to a patient, a gamma camera can be used to form an image of the radioisotope's distribution by detecting the gamma radiation emitted. Such a technique can be employed to diagnose a wide range of conditions (e.g. spread of cancer to the bones).

History

Gamma rays were discovered by the French Chemist and Physicist, Paul Ulrich Villard in 1900 while he was studying uranium. Working in the chemistry department of the École Normale in rue d'Ulm, Paris with self-constructed equipment, he found that the rays were not bent by a magnetic field.

For a time, it was assumed that gamma rays were particles. The fact that they were rays was demonstrated by the British Physicist, William Henry Bragg in 1910 when he showed that the rays ionized gas in a similar way to X-rays.

In 1914, Ernest Rutherford and Edward Andrade showed that gamma rays were a form of electromagnetic radiation by measuring their wavelengths using crystal diffraction. The wavelengths are similar to those of X-rays and are very short, in the range 10-11m to 10-14m. It was Rutherford who coined the name 'gamma rays', after naming 'alpha' and 'beta' rays; the natures of the different rays were unknown at that time.

Culture

The Marvel Comics character, The Incredible Hulk, who starred in a TV show, as well as a recent movie, was created when Bruce Banner, a scientist, was bombarded by a heavy dose of gamma radiation.