1 1 Radiation definition: Radiation, when broadly defined, includes the entire spectrum of electromagnetic waves : radiowaves, microwaves, infrared, visible light, ultraviolet, and x-rays and particles. In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: 1-Electromagnetic radiation, such as heat, radio waves, visible light, x-rays, and gamma radiation (γ) 2-particle radiation, such as alpha radiation (α), beta radiation (β), and neutron radiation (particles of non-zero rest energy) 3-Acoustic radiation, such as ultrasound, sound, and seismic waves (dependent on a physical transmission medium) 4-Gravitational radiation, radiation that takes the form of gravitational waves, or ripples in the curvature of space-time. 1-2 Ionizing Radiation Radiation with sufficiently high energy can ionize atoms; that is to say it can knock electrons off atoms and create ions. Ionization occurs when an electron is stripped (or "knocked out") from an electron shell of the atom, which leaves the atom with a net positive charge. Because living cells and, more importantly, the DNA in those cells can be damaged by this ionization, exposure to ionizing radiation is considered to increase the risk of cancer. Thus "ionizing radiation" is somewhat artificially separated from particle radiation and electromagnetic radiation, simply due to its great potential for biological damage. While an individual cell is made of trillions of atoms, only a small fraction of those will be ionized at low to moderate radiation powers. The probability of ionizing radiation causing cancer is dependent upon the absorbed dose of the radiation, and is a function of the damaging tendency of the type of radiation (equivalent dose) and the sensitivity of the irradiated organism or tissue (effective dose). 1-Ultraviolet radiation Ultraviolet, of wavelengths from 10 nm to 125 nm, ionizes air molecules, causing it to be strongly absorbed by air and by ozone (O3) in particular. 1
2-X-ray X-rays are electromagnetic waves with a wavelength less than about 10 9 m (greater than 3x1017 Hz and 1,240 ev). X-rays are also totally absorbed by the thickness of the earth's atmosphere, resulting in the prevention of the X- ray output of the sun, smaller in quantity than that of UV but nonetheless powerful, from reaching the surface. 3-Gamma radiation Gamma (γ) radiation consists of photons with a wavelength less than 3x10 11 meters (greater than 1019 Hz and 41.4 kev). 4-Alpha radiation Alpha particles are helium-4 nuclei (two protons and two neutrons). They interact with matter strongly due to their charges and combined mass, and at their usual velocities only penetrate a few centimeters of air, or a few millimeters of low density material (such as the thin mica material which is specially placed in some Geiger counter tubes to allow alpha particles in). 5-Beta radiation Beta-minus (β ) radiation consists of an energetic electron. It is more penetrating than alpha radiation, but less than gamma. Beta radiation from radioactive decay can be stopped with a few centimeters of plastic or a few millimeters of metal. 6-Neutron radiation Neutrons are categorized according to their speed/energy. Neutron radiation consists of free neutrons. These neutrons may be emitted during either spontaneous or induced nuclear fission. 7-Cosmic radiation There are two sources of high energy particles entering the Earth's atmosphere from outer space: the sun and deep space. The sun continuously emits particles, primarily free protons, in the solar wind, and occasionally augments the flow hugely with coronal mass ejections (CME). 2
Figure 1: Graphic showing relationships between radioactivity and detected ionizing radiation. 1-3 Non- Ionizing Radiation The kinetic energy of particles of non-ionizing radiation is too small to produce charged ions when passing through matter. For non-ionizing electromagnetic radiation (see types below), the associated particles (photons) have only sufficient energy to change the rotational, vibrational or electronic valence configurations of molecules and atoms. The effect of nonionizing forms of radiation on living tissue has only recently been studied. Nevertheless, different biological effects are observed for different types of non-ionizing radiation. The electromagnetic spectrum is the range of all possible electromagnetic radiation frequenciesthe electromagnetic spectrum (usually just spectrum) of an object is the characteristic distribution of electromagnetic radiation emitted by, or absorbed by, that particular object. 3
Figure 2: The electromagnetic spectrum 1-4 Black-body radiation Black-body radiation is an idealized spectrum of radiation emitted by a body that is at a uniform temperature. The shape of the spectrum and the total amount of energy emitted by the body is a function of the absolute temperature of that body. The radiation emitted covers the entire electromagnetic spectrum and the intensity of the radiation (power/unit-area) at a given frequency is described by Planck's law of radiation. For a given temperature of a black-body there is a particular frequency at which the radiation emitted is at its maximum intensity. That maximum radiation frequency moves toward higher frequencies as the temperature of the body increases. The frequency at which the black-body radiation is at maximum is given by Wien's displacement law and is a function of the body's absolute temperature. A black-body is one that emits at any temperature the maximum 4
possible amount of radiation at any given wavelength. A black-body will also absorb the maximum possible incident radiation at any given wavelength. A black-body with a temperature at or below room temperature would thus appear absolutely black, as it would not reflect any incident light nor would it emit enough radiation at visible wavelengths for our eyes to detect. Theoretically, a black-body emits electromagnetic radiation over the entire spectrum from very low frequency radio waves to x-rays, creating a continuum of radiation. 1 5 Radioactive Elements The atomic structure of most elements contains a nucleus that is stable. Under normal conditions, these elements remain unchanged indefinitely. They are not radioactive. Radioactive elements, in contrast, contain a nucleus that is unstable The unstable nucleus is actually in an excited state that can not be sustained indefinitely; it must relax, or decay, to a more stable configuration. Decay occurs spontaneously and transforms the nucleus from a high energy configuration to one that is lower in energy. 1-6The Nature of Radiation The energy emitted by an unstable nucleus comes packaged in very specific forms. In the years that followed the discovery of radioactivity, determining the kind of radiation emitted from radioactive compounds was of great interest. It was found that these radiations consisted of three types called: alpha, beta and gamma radiations after the first three letters in the Greek alphabet The nuclear emission transforms the element into either a new element or a different isotope of the same element. A given radioactive nucleus does this just once. The process is called a decay or a disintegration. The evidence for the three types of radiation comes from an experiment in which the radiation from radioactive compounds was passed through a magnetic field. rays passed through the field without disturbance, whereas the two other types were deflected from a straight line. Because it was known at that time that charged particles are deflected when they pass through a magnetic field, the conclusion was evident; rays have no charge while and radiations consist of charged particles. The particles, deflected in one direction are positive whereas the particles, deflected in the opposite direction, are negative. 5
1-7Alpha radiation In 1903, Ernest Rutherford (a New Zealander who worked in Cambridge, England most of his life) performed a simple and elegant experiment showing that the particle is the nucleus of the helium atom. Rutherford positioned one glass tube inside a second glass tube. The inner tube contained a radioactive source that emitted particles 1-8Beta and gamma radiation Experiments have shown that the -particle is a fast moving electron, whereas radiation is an electromagnetic wave. Other examples of electromagnetic radiation are ultraviolet (UV), visible light, infrared and radio waves. Electromagnetic radiation is characterized by its wavelength or frequency. The wavelength is the distance from one wave peak to the next and the frequency is the number of waves passing a given point per second. Through quantum mechanics it is known that particles can be described as waves and vice versa. Thus, rays and other electromagnetic radiation are sometimes described as particles and are called photons. 1-9 Photoelectric effect is a process in which a photon interacts with a bound electron. The photon itself disappears, transferring all its energy to the electron and thereby imparting kinetic energy to the electron. This is the most important absorption process for radiation with an energy less than about 100 kev (which is the type of radiation used in medical diagnostics). The photoelectric effect varies dramatically with the electron density of the absorbing medium. Thus material that contains atoms with high atomic 6
numbers, e.g., the calcium in bone, gives strong absorption due to the photoelectric effect. 1-10The Penetration of Radiation When using a gun, the penetration by the bullet depends on the energy of the bullet as well as the composition of the target. For example, a pellet from an air gun will be stopped by a few millimeters of wood but a bullet from a high powered rifle will pass through many millimeters of steel. It is similar with ionizing radiation. There are large differences in penetrating ability depending on the type of radiation. 1-11What is Radioactivity? X-rays and rays will easily penetrate the human body. This property is utilized when x- and rays are used for diagnostic purposes. and particles, on the other hand, lose their energy within a short distance and cannot penetrate the body. Because of these penetration properties, -radiation is easy to observe whereas and -radiation are more difficult to detect. 1-12Biological Half-life The radioactive isotopes that are ingested or taken in through other pathways will gradually be removed from the body via kidneys, bowels, respiration and perspiration. This means that a radioactive atom can be expelled before it has had the chance to decay. The time elapsed before half of the compound has been removed through biological means is called the biological half-life and is usually written tb. If a radioactive compound with physical half-life tp (t1/2) is cleared from the body with a biological half-life tb, the effective half-life (te) is given by the expression: If tp is large in comparison to tb, the effective half-life is approximately the same as tb. 7
The biological half-life is rather uncertain compared to the exact value of the physical half-life. It is uncertain because the clearance from the body depends upon sex, age of the individual and the chemical form of the radioactive substance. The biological half-life will vary from one type of animal to another and from one type of plant to another. Cs-137, having a physical half-life of 30 years, is a good example. It was the most prominent of the radioactive isotopes in the fallout following the Chernobyl accident in the Ukraine. Cesium is cleared rather rapidly from the body and the biological half-life for an adult human is approximately three months and somewhat less for children. Cs-137 has a biological half-life of 2 to 3 weeks for sheep, whereas for reindeer it is about one month. Due to the fact that the biological half-life for animals like sheep is rather short, it is possible to feed down animals, with too high a content of Cs-137, Before slaughtering. 1-13Radio-ecological Half-life Radio-ecological half-life is less precise than the physical and biological halflife. Consider a region which has been polluted by a radioactive isotope (for example Cs-137). Part of the activity will gradually sink into the ground and some will leak into the water table. Each year, a fraction of the activity will be taken up by the plants and subsequently ingested by some of the animals in the area. Radio-ecological half-life is defined as the radioactive half-life for the animals and plants living in the area. It varies for the different types of animals and plants. Knowledge in this area is limited at present, but research carried out after the Chernobyl accident has yielded some information. It is important to note that these ecological half-lifes are significantly shorter than the respective physical half-life, 30 years for Cs-137 and 2 years for Cs- 134. 8