Gamma Radiation

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Gamma Radiation

Gamma rays are a form of electromagnetic radiation, just like visible light or x rays, but with a much higher energy. The gamma ray spectrum is usually defined as light having a frequency between 10¹⁸ and 10²¹ Hertz. Gamma rays are more energetic than x rays, but are less energetic than cosmic rays. Compared to ultraviolet light (light that corresponds in energy to electronic transitions within molecules and having a frequency of approximately 8 x 10¹⁴ Hertz), a gamma ray photon with of frequency of 10¹⁹ Hertz is approximately 10,000-fold more energetic. Gamma rays are sufficiently energetic to cause nuclear transitions within atoms and, thus, also correspond to the energy release that accompanies the transition of an excited nucleus to its ground (i.e. most stable) state.

The discovery of gamma rays generally is attributed to the French physicist Paul Villard (1860-1934). Following the discover of x rays by the German physicist Wilhelm Röntgen (1845-1923) in 1895, and natural radioactivity for the French physicist Antoine Becquerel (1852-1908) in 1896, Villard worked in this new field of nuclear physics. In studying the rays emitted during the radioactive decay of radium, he observed a new radiation that was unaffected by a magnetic field (was not electrically charged). Moreover, the radiation passed directly through many materials and was capable of penetrating several centimeters into lead. This new radiation, that of a highly energetic photon emitted from the decayed radium nucleus, was called a gamma ray.

Gamma rays are a product of the radioactive decay of an atom. Most commonly, the unstable nucleus of an atom decays to a more stable nucleus of another atom by emitting an alpha particle (the positively charged helium ⁴He nucleus) or beta particle (a negatively charged electron or positively charged positron). When a nucleus decays, through the emission of either an alpha or beta particle, the resulting new nucleus is often left in an excited state. This excited nucleus can discharge its excess energy, and so decay to a lower (more stable) state by emitting a photon of gamma radiation. The energy of this photon corresponds to the difference in energy between the two nuclear states and is generally in the range of 0.1 x 10⁶ to 10 x 10⁶ eV.

The encounter of the highly energetic gamma ray with another molecule can result in the ejection of electrons from that molecule. This process is called ionization, and this capability had led to gamma radiation being termed ionizing radiation. Ionization provides a mechanism for the detection of the gamma ray (for example, by the Geiger counter) and also represents the mechanism by which biological molecules are damaged.

The Geiger counter is named after its inventor, Hans Geiger (1882-1945). In collaboration with his colleague Walther Müller, Geiger devised the Geiger-Müller tube in 1928. The invention of this tube enabled the development and use of the Geiger-Müller counter as a practical method for the detection of radioactivity. This tube consists of two electrical plates, maintained at a potential difference, and surrounded by gas. The passage of radiation through this tube results in ion formation, and these ions then accelerate towards one of the two plates. This results in a measurable voltage change. Unfortunately, gamma radiation is so energetic that most rays pass through the tube with no discernible effect. In this circumstance, scintillation counters are used. In scintillation counting, a fluorescent material absorbs the energy of the photon and emits a new photon as a flash of light (the scintillation) in response. This flash can then be amplified into an electrical pulse, indicating the presence of the radiation.

When the molecules of a living organism are exposed to radiation that results in the loss of an electron from the molecule, reactive intermediates (called free radicals) are formed. While all cells have mechanisms to minimize free radical damage, if the quantity of free radicals that are formed exceeds the cell's detoxification capacity or if the molecular damage is difficult to repair (such as certain DNA lesions), the cells of the organism are damaged. The damage is categorized as either somatic or genetic. Somatic effects are those that directly affect the individual who received the original dose. The short-term effects can include burns, nausea, and hair loss, although some damage is not apparent until years later when it can surface in the form of cancers. It was their exposure to this radiation that led to the premature deaths of several of the first scientists to study radioactivity, including Paul Villard and Marie Curie (1867-1934). Genetic effects of radiation are caused by damage to the DNA of reproductive cells that causes harmful mutations in the children and grandchildren of the exposed individual.

Because of the harmful effects of ionizing radiation, it is important to be able to measure exposure to it. The unit of absorbed dose is call the gray, which is the amount of energy in joules from the radiation absorbed per kilogram of the absorbing material. For biological organisms, however, the effect not only depends on the amount of energy absorbed but also on how it was absorbed. If all the energy was absorbed over a very small distance, on the scale of a cell, the effect is more significant than if the energy is spread over a larger distance. The dose equivalent of radiation is, therefore, the amount of radiation absorbed multiplied by a quality factor (sometimes called the linear energy transfer), which depends on the energy deposited per unit path length. Fast moving gamma photons have a quality factor of about one, whereas alpha particles (which lose all of their energy over a very small path length) have quality factors nearer 20.

Gamma radiation has several uses. Since it created by the decay of a nucleus, it can be used as a nuclear probe, the nature of the emitted radiation giving an insight into the nature of the nuclear state. Gamma rays also are emitted as a result of high-energy processes throughout the universe and can travel large distances before they reach the Earth. They, therefore, provide a useful information source for astronomers on everything from black holes to processes occurring within our own Sun. Gamma rays, such as those emitted from the unstable nuclei of cobalt-60 and cesium-137, are used in industry to assess the metallurgical integrity of, for example, pipes, girders, and turbine blades.

Gamma radiation is also important in the medical sciences. Ionizing radiation is more harmful to cells when they are dividing (replicating their DNA), and this has led to the use of gamma radiation for the treatment of cancer. The rapidly dividing cancer cells are more susceptible to the radiation than the healthy cells and are destroyed selectively without the need for surgery. Molecules that contain very short-lived isotopes are used as tracers to study the distribution of the molecules within the body. Due to the sensitivity with which radiation is detected only minute quantities of the chemical are required. This information is used for both diagnostic and research purposes. For example, it has enabled the three dimensional imaging of the brain as a function of particular stimuli.