Chapter 1 Radiation and Atom PDF

CHAPTER 1 RADIATION AND ATOM CHAPTER ONE: RADIATION AND ATOM CHAPTER CONTENTS 1.1 The Atom 2 1.1.1 Fundamental Particles 2 1.1.2 Atomic Structure 3 1.1.3 Binding Energy 3 1.2 Wave-Particle Duality 4 1.3 Radiation 6 1.3.1 Non-Ionizing Radiation 8 1.3.2 Ionizing Radiation...

CHAPTER 1 RADIATION AND ATOM CHAPTER ONE: RADIATION AND ATOM CHAPTER CONTENTS 1.1 The Atom 2 1.1.1 Fundamental Particles 2 1.1.2 Atomic Structure 3 1.1.3 Binding Energy 3 1.2 Wave-Particle Duality 4 1.3 Radiation 6 1.3.1 Non-Ionizing Radiation 8 1.3.2 Ionizing Radiation 8 1.4 Types of Ionizing Radiation 8 1.4.1 Particle Radiation 9 1.4.1.1 Alpha Particles 9 1.4.1.2 Beta Particles 9 1.4.1.3 Neutron Radiation 10 1.4.2 Types of Electromagnetic Ionizing Radiation 10 1.4.2.1 Gamma Rays 11 1.4.2.2 X- Rays 11 1.4.2.3 Ultraviolet 11 1.5 Inverse Square Law for Radiation 11 1.6 Properties Considered When Ionizing Radiation Measured 13 1.7 Radiologic Units 13 1.7.1 Roentgen 13 1.7.2 Rad 13 1.7.3 Rem 14 1.7.4 Curie 14 1.7.5 Electron Volt 14 1.8 Practical units 15 1.8.1 Absorbed dose 15 1.8.2 Equivalent dose 15 1.8.3 Effective dose 17 1.1 The Atom Atoms are far too small to see directly, even with the most powerful optical microscopes. But atoms do interact with and under some circumstances emit light in ways that reveal their internal structures in amazingly fine detail. It is through the "language of light" that we communicate with the world of the atom. This section will introduce you to the rudiments of this language. 1.1.1 Fundamental Particles Diagnostic imaging employs radiations – X, gamma, radiofrequency and sound – to which the body is partly but not completely transparent, and it exploits the special properties of a number of elements and compounds. As ionizing radiations (X-rays and gamma rays) are 2  CHAPTER 1 RADIATION AND ATOM used most, it is best to start by discussing the structure of the atom and the production of X- rays. Table 1.1: Fundamental properties of particulate radiation Relative Mass Approximate Energy particle Symbol charge (amu) Equivalent (MeV) Neutron n0 0 1.008982 940 1 + Proton P, H +1 1.007593 938 − − Electron (beta minus) e ,β −1 0.000548 0.511 + + Positron (beta plus) e ,β +1 0.000548 0.511 4 2+ Alpha α, H +2 4.0028 3727 1.1.2 Atomic Structure An atom consists mainly of empty space. Its mass is concentrated in a central nucleus which contains a number A of nucleons, where A is called the mass number as shown in figure 1.1. Figure 1.1: Electron shells in a sodium atom. The nucleons comprise Z protons, where Z is the atomic number of the element, and so (A-Z) neutrons. A nuclide is a species of nucleus characterized by the two numbers Z and A. The atomic number is synonymous with the name of the element. The electron limit per shell can be calculated from the expression: 2n2 where n is the shell number. In each atom, the outermost or valence shell is concerned with the chemical, thermal, optical and electrical properties of the element. X-rays involve the inner shells, and radioactivity concerns the nucleus. 1.1.3 Binding Energy Binding energy, amount of energy required to separate a particle from a system of particles or to disperse all the particles of the system. Binding energy is especially applicable to 3  CHAPTER 1 RADIATION AND ATOM subatomic particles in atomic nuclei, to electrons bound to nuclei in atoms, and to atoms and ions bound together in crystals. Nuclear binding energy is the energy required to separate an atomic nucleus completely into its constituent protons and neutrons, or, equivalently, the energy that would be liberated by combining individual protons and neutrons into a single nucleus. The hydrogen-2 nucleus, for example, composed of one proton and one neutron, can be separated completely by supplying 2.23 million electron volts (MeV) of energy. Conversely, when a slowly moving neutron and proton combine to form a hydrogen-2 nucleus, 2.23 MeV are liberated in the form of gamma radiation. The total mass of the bound particles is less than the sum of the masses of the separate particles by an amount equivalent (as expressed in Einstein’s mass– energy equation) to the binding energy. Electron binding energy, also called ionization potential, is the energy required to remove an electron from an atom, a molecule, or an ion. In general, the binding energy of a single proton or neutron in a nucleus is approximately a million times greater than the binding energy of a single electron in an atom. An atom is said to be ionized when one of its electrons has been completely removed. The detached electron is negative ion and the remnant atom a positive ion. Together they form an ion pair. The binding energy depends on the shell , and on the element, increasing as the atomic number increases. An atom is excited when an electron is raised from one shell to another farther out. 1.2 Wave-Particle Duality There are two aspects for Electromagnetic radiation can be regarded as a stream of ‘packets’ or quanta of energy, called photons (i.e. quantum aspects), traveling in straight lines. The photon is the smallest possible packet (quantum) of light; it has zero mass but a definite energy. Electromagnetic radiation can also be regarded as sinusoidally varying electric and magnetic fields (i.e. wave aspects), traveling with light velocity when in vacuum. They are transverse waves: the electric and magnetic field vectors point at right angles to each other and to the direction of travel of the wave. Einstein is most famous for saying "mass is related to energy". Of course, this is usually written out as an equation, rather than as words: or Energy Mass Speed of Light in Joules in kg ( ) Because of the wave-particle duality of light, the energy of a wave can be related to the wave's frequency by the equation: 4  CHAPTER 1 RADIATION AND ATOM Energy planck's Constant Frequency (Joules) ( ) (Hz or s-1) There are three measurable properties of wave motion: amplitude, wavelength, and frequency, the number of vibrations per second. The relation between the wavelength λ (Greek lambda) and frequency of a wave (Greek nu) is determined by the propagation velocity ; Wavelength frequency velocity (constant) This relation is true of all kinds of wave motion, including sound; although for sound the velocity is about a million times less. More usefully, since frequency is inversely proportional to wavelength, so also is photon energy: E (in keV) =1.24/λ (in nm) For example: Blue light λ=400 nm E=3 eV Typical X- and gamma rays λ=0.1 nm E=140 keV At any point, the graph of field strength against time is a sine wave, depicted as a solid curve in Figure 1.3. The peak field strength is called the amplitude (A). The interval between successive crests of the wave is called the period (T). The frequency is the number of crests passing a point in a second, and. The dashed curve refers to a later instant, showing how the wave has travelled forward with velocity c. At any instant, the graph of field strength against distance is also a sine wave. The distance between successive crests of the wave is called the wavelength (λ). Wavelength (λ) or Period (T) Amplitude (A) Propagation Velocity Mahmood & Haider Figure 1.2: Electromagnetic wave. 5  CHAPTER 1 RADIATION AND ATOM The types of radiation are listed in Table 1.2, in order of increasing photon energy, increasing frequency, and decreasing wavelength (see figure 1.3. When the energy is less than 1 keV the radiation is usually described in terms of its frequency, except that visible light is usually described in terms of its wavelength. It is curious that only radiations at the ends of the spectrum, radio waves and X- or gamma rays, penetrate the human body sufficiently to be used in transmission imaging. Table 1.2: Electromagnetic spectrum Electromagnetic Wavelength Frequency Energy Radio waves 30-6 m 10-50 MHz 40-200 neV Infrared 10-0.7 µrn 30-430 THz 0. 2-1.8 eV Visible light 700-400 nm 430-750 THz l.8-3eV Ultraviolet 400-100 nm 750-3000 THz 3-12 eV X- and gamma 60-2.5 pm 5× I06– 120× 106THz 20-500 keV 1.3 Radiation Radiation is a fact of life: all around us, all the time. Radiation is energy moving in the form of waves or streams of particles. Understanding radiation requires basic knowledge of atomic structure, energy and how radiation may damage cells in the human body. There are many kinds of radiation all around us. When people hear the word radiation, they often think of atomic energy, nuclear power and radioactivity, but radiation has many other forms. Sound and visible light are familiar forms of radiation; other types include ultraviolet radiation (that produces a suntan), infrared radiation (a form of heat energy), and radio and television signals. Figure 1.3 presents an overview of the electromagnetic spectrum. Electromagnetic radiation is a form of energy. Electromagnetic energy is the term given to energy traveling across empty space and used to describe all the different kinds of energies released into space by stars such as the Sun. All forms of electromagnetic radiation (which includes radio waves, light, cosmic rays, etc.) moves through empty space with the same velocity at the speed of 299,792 km per second (very close to 3×108 ms-1) and not significantly less in air. These kinds of energies include some that you will recognize and some that will sound strange. They include:  Radio Waves  TV waves  Radar waves  Heat (infrared radiation)  Light  Ultraviolet Light (This is what causes Sunburns)  X-rays (emitted by X-ray tubes)  Short waves  Microwaves, like in a microwave oven 6  CHAPTER 1 RADIATION AND ATOM  Gamma Rays; gamma rays (emitted by radioactive nuclei) have essentially the same properties of X-rays and differ only in their origin. All these waves do different things (for example, light waves make things visible to the human eye, while heat waves make molecules move and warm up, and x rays can pass through a person and land on film, allowing us to take a picture inside someone's body) but they have some things in common. They all travel in waves. The fact that electromagnetic radiation travels in waves lets us measure the different kind by wavelength or how long the waves are. That is one way we can tell the kinds of radiation apart from each other. Although all kinds of electromagnetic radiation are released from the Sun, our atmosphere stops some kinds from getting to us. For example, the ozone layer stops a lot of harmful ultraviolet radiation from getting to us, and that's why people are so concerned about the hole in it. We humans have learned uses for a lot of different kinds of electromagnetic radiation and have learned how to make it using other kinds of energy when we need to. Figure 1.3: The electromagnetic spectrum In general, the radiation is a descriptor for energy (in the form of either particles or waves) travelling through space or another medium. The energy is emitted from the source and radiates in straight lines and in all directions. If the radiation is an electromagnetic wave, it will travel at the speed of light. Because of the way the energy is radiated, radiation is relatively straightforward to detect and measure and inferences can be made about its source. The properties of the energy emitted will determine the way it interacts with matter (and living tissue) and therefore its measurement technique and requirements for regulation. Not all radiation interacts with matter in the same way. There are two forms of radiation: non-ionizing and ionizing. 7  CHAPTER 1 RADIATION AND ATOM 1.3.1 Non-Ionizing Radiation Non-ionizing radiation is the radiation that has enough energy to move atoms in a molecule around or cause them to vibrate, but not enough to remove electrons. That mean it does not possess enough energy to produce ions. Non-ionising radiation consists of parts of the electromagnetic-spectrum (Figure 1.3), which includes radio waves, microwaves, infra-red, visible and ultraviolet light, together with sound and ultrasound. Cellular telephones, television stations, FM and AM radio, and cordless phones use non-ionizing radiation. Other forms include the earth’s magnetic field, as well as magnetic field exposure from proximity to transmission lines, household wiring and electric appliances. These are defined as extremely low-frequency (ELF) waves and are not considered to pose a health risk. The electromagnetic spectrum also includes ionising electromagnetic radiation (x and gamma rays). 1.3.2 Ionizing Radiation Ionizing radiation is a special type of radiation (in the form of either particles or waves) that has enough energy to remove tightly bound electrons out of their orbits around atoms, thus creating ions. In other words, Ionizing radiation is any kind of radiation capable of removing an orbital electron from an atom with which it interacts, the atom is said to be ionized. This process is called ionization. Ionizing radiation includes the radiation that comes from both natural and man-made radioactive materials. Examples of this kind of radiation of interest for the purpose of this chapter are gamma (γ) and x-rays. Gamma radiation consists of photons that originate from within the nucleus, and X-ray radiation consists of photons that originate from outside the nucleus, and are typically lower in energy than gamma radiation. We take advantage of its properties in diagnostic imaging, to kill cancer cells, and in many manufacturing processes. Ionization is the process by which a stable atom or a molecule loses or gains an electron(s), thereby acquiring an electric charge or changing an existing charge. An atom or molecule with an electric charge is called an ion, which may behave differently, electrically and chemically, from a stable atom or molecule. The altered behaviour may lead to new possibly undesired molecules, a change in the conductive properties of the material in the vicinity of the ion, a release of energy, or a combination of these effects. In the human body, these effects may lead to changes in the structure or behaviour of cells. Therefore, ionizing radiation has sufficient energy to be able to displace an electron from its orbit around an atom and, conversely, non-ionizing radiation does not have sufficient energy to displace electrons. Ionizing radiation can occur in one of two forms: particulate or electromagnetic. Particulate ionizing radiation is emitted when components of the structure of an atom are ejected, artificially or naturally. Ionizing radiation includes the radiation that comes from both natural and man-made radioactive materials. 1.4 Types of Ionizing Radiation Photon radiation can penetrate very deeply and sometimes can only be reduced in intensity by materials that are quite dense, such as lead or steel. In general, photon radiation can travel 8  CHAPTER 1 RADIATION AND ATOM much greater distances than alpha or beta radiation, and it can penetrate bodily tissues and organs when the radiation source is outside the body. Photon radiation can also be hazardous if photon-emitting nuclear substances are taken into the body. An example of a nuclear substance that undergoes photon emission is cobalt-60, which decays to nickel-60. There are several types of ionizing radiation. 1.4.1 Particle Radiation Particle radiation consists of a stream of charged or neutral particles, both charged ions and subatomic elementary particles. This includes solar wind, cosmic radiation, and neutron flux in nuclear reactors. 1.4.1.1 Alpha Particles Alpha particles (α), helium nuclei, are the least penetrating. Some unstable atoms emit alpha particles. Alpha particles are positively charged and made up of two protons and two neutrons from the atom’s nucleus. Alpha particles come from the decay of the heaviest radioactive elements, such as uranium, radium and polonium. Even very energetic alpha particles can be stopped by a single sheet of paper. They are so heavy that they use up their energy over short distances and are unable to travel very far from the atom. The health effect from exposure to alpha particles depends greatly on how a person is exposed. Alpha particles lack the energy to penetrate even the outer layer of skin, so exposure to the outside of the body is not a major concern. Inside the body, however, they can be very harmful. If alpha-emitters are inhaled, swallowed, or get into the body through a cut, the alpha particles can damage sensitive living tissue. The way these large, heavy particles cause damage makes them more dangerous than other types of radiation. The ionizations they cause are very close together--they can release all their energy in a few cells. This results in more severe damage to cells and DNA. 1.4.1.2 Beta Particles Beta particles (β) are fast-moving particles with a negative electrical charge. Beta particles (electrons) are emitted from an atom’s nucleus during radioactive decay with more penetrating, but still can be absorbed by a few millimeters of aluminum. However, in cases where high energy beta particles are emitted shielding must be accomplished with low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite). They travel farther in air than alpha particles, but can be stopped by a layer of clothing or by a thin layer of a substance such as aluminum. In the case of beta+ radiation (positrons), the gamma radiation from the electron-positron annihilation reaction poses additional concern. These particles are emitted by certain unstable atoms such as hydrogen-3 (tritium), carbon-14 and strontium-90. Beta particles are more penetrating than alpha particles but are less damaging to living tissue and DNA because the ionizations they produce are more widely spaced. Some beta particles are capable of penetrating the skin and causing damage such as skin burns. However, as with alpha-emitters, beta-emitters are most hazardous when they are inhaled or swallowed. 9  CHAPTER 1 RADIATION AND ATOM 1.4.1.3 Neutron Radiation Neutron radiation is not as readily absorbed as charged particle radiation, which makes this type highly penetrating. Neutrons are absorbed by nuclei of atoms in a nuclear reaction. This most-often creates a secondary radiation hazard, as the absorbing nuclei transmute to the next- heavier isotope, many of which are unstable. Apart from cosmic radiation, spontaneous fission is the only natural source of neutrons. A common source of neutrons is the nuclear reactor, in which the splitting of a uranium or plutonium nucleus is accompanied by the emission of neutrons. The neutrons emitted from one fission event can strike the nucleus of an adjacent atom and cause another fission event, inducing a chain reaction. The production of nuclear power is based upon this principle. All other sources of neutrons depend on reactions where a nucleus is bombarded with a certain type of radiation (such as photon radiation or alpha radiation), and where the resulting effect on the nucleus is the emission of a neutron. Neutrons are able to penetrate tissues and organs of the human body when the radiation source is outside the body. Neutrons can also be hazardous if neutron-emitting nuclear substances are deposited inside the body. Neutron radiation is best shielded or absorbed by materials that contain hydrogen atoms, such as paraffin wax and plastics. This is because neutrons and hydrogen atoms have similar atomic weights and readily undergo collisions between each other. Figure 1.4 summarizes the types of radiation discussed in this chapter, from higher-energy ionizing radiation to lower-energy non-ionizing radiation. Each radiation source differs in its ability to penetrate various materials, such as paper, skin, wood and lead. Figure 1.4: Penetration abilities of different types of ionizing radiation 1.4.2 Types of Electromagnetic Ionizing Radiation In general, electromagnetic radiation consists of emissions of electromagnetic waves, the properties of which depend on the wavelength. Ionizing radiation has more energy than non- ionizing radiation such that it can cause chemical changes by interacting with an atom to remove tightly bound electrons from the orbit of the atom, causing the atom to become charged or ionized. The types of ionizing electromagnetic radiation are categorized according to their wavelength. 10  CHAPTER 1 RADIATION AND ATOM 1.4.2.1 Gamma Rays Gamma rays (γ) are weightless packets of energy called photons. Gamma-rays have the smallest wavelengths and but have much higher energy of any other wave in the electromagnetic spectrum. Unlike alpha and beta particles, which have both energy and mass, gamma rays are pure energy. Gamma rays are often emitted along with alpha or beta particles during radioactive decay and in nuclear explosions. Gamma rays are a radiation hazard for the entire body. They can easily penetrate barriers, such as skin and clothing that can stop alpha and beta particles. Gamma rays have so much penetrating power that several inches of a dense material like lead or even a few feet of concrete may be required to stop them. Gamma rays can pass completely through the human body easily; as they pass through, they can cause ionizations that damage tissue and DNA or kill living cells, a fact which medicine uses to its advantage, using gamma-rays to kill cancerous cells. 1.4.2.2 X-Rays Because of their use in medicine, almost everybody has heard of x-rays. X-rays are similar to gamma rays in that they are photons of pure energy. X-rays and gamma rays have the same basic properties but come from different parts of the atom. X-rays are emitted from processes outside the nucleus, but gamma rays originate inside the nucleus. They also are generally lower in energy and, therefore, less penetrating than gamma rays but have higher energy than ultraviolet waves. As the wavelengths of light decrease, they increase in energy. We usually talk about X-rays in terms of their energy rather than wavelength. This is partially because X- rays have very small wavelengths. It is also because X-ray light tends to act more like a particle than a wave. X-rays can be produced naturally or artificially by machines using electricity. Literally thousands of x-ray machines are used daily in medicine. Computerized tomography, commonly known as CT or CAT scans, uses special x-ray equipment to make detailed images of bones and soft tissue in the body. Medical x-rays are the single largest source of man-made radiation exposure. X-rays are also used in industry for inspections and process controls. 1.4.2.3 Ultraviolet The dividing line between ionizing and non-ionizing radiation in the electromagnetic spectrum falls in the ultraviolet portion of the spectrum and while most UV is classified as non-ionizing radiation, the shorter wavelengths from about 150 nm (UV-C or ‘Far’ UV) are ionizing. UV-C from the sun is nearly all absorbed by the ozone layer. 1.5 Inverse Square Law for Radiation Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius is the source strength divided by the area 11  CHAPTER 1 RADIATION AND ATOM of the sphere. Being strictly geometric in its origin, the inverse square law applies to diverse phenomena. Point sources of gravitational force, electric field, light, sound or radiation obey the inverse square law. When light is emitted from a source such as the sun or a light bulb, the intensity decreases rapidly with the distance from the source. X-rays exhibit precisely the same property. The intensity of the radiation is inversely proportional to the square of the distance from a point source (see figure 1.6). Figure 1.6: The inverse square law applying to a point source. As intensity is the power per unit area, Intensity = Power /Area , It naturally decreases with the square of the distance as the size of the radiative spherical wave front increases with distance. So, the luminous intensity on a spherical surface a distance from a source radiating a total power P is: As P and remain constant, the luminous intensity is proportional to the inverse of distance: 12  CHAPTER 1 RADIATION AND ATOM Thus, if I double the distance to a light source the observed intensity is decreased to of its original value. Generally, the ratio of intensities at distances and are 1.6 Properties Considered When Ionizing Radiation Measured Ionizing radiation is measured in terms of:  the strength or radioactivity of the radiation source,  the energy of the radiation,  the level of radiation in the environment, and  the radiation dose or the amount of radiation energy absorbed by the human body. From the point of view of the occupational exposure, the radiation dose is the most important measure. Occupational exposure limits like the ACGIH TLVs are given in terms of the permitted maximum dose. The risk of radiation-induced diseases depends on the total radiation dose that a person receives over time. 1.7 Radiologic Units There are five units accustomed for measure radiation: 1.7.1 Roentgen (R) The Rontgen (R or r) is the unit of dose of electromagnetic radiation exposure or intensity. It is equal to the radiation intensity that will create 2.08×109 ion pairs in a cubic centimeter of air that is: 1R = 2.08 ⨉ 109 ion pairs/cm3 The official definition, however, is in terms of electric charge per unit mass of air: 1R=2.58 ⨉ 10-4 C/kg The charge refers to the electrons liberated by ionization. The output of x-ray machines is specified in roentgens or sometimes millroentgens (mR). The roentgen applies only to x-rays and gamma rays and their interactions with air. 1.7.2 Rad The Rad (radiation absorbed dose) is used to measure the amount of radiation absorbed by an object or person which reflects the amount of energy that radioactive sources deposit in materials through which they pass. The radiation-absorbed dose (rad) is the amount of energy (from any type ofionizing radiation) deposited in any medium (e.g., water, tissue, air). Biologic effects usually are related to the radiation absorbed dose, and therefore the rad is the unit most often used when describing the radiation quantity received by a patient or an experimental animal. The rad is used for any type of ionizing radiation any exposed matter, 13  CHAPTER 1 RADIATION AND ATOM not just air. An absorbed dose of 1 rad means that 1 gram of material absorbed 100 ergs of energy (a small but measurable amount) as a result of exposure to radiation. 1Rad=100 ergs/g (10-2 Gy) Where the erg (joule) is a unit of energy, and the gram (kilogram) is a unit of mass. The related international system unit is the gray (Gy), where 1 Gy is equivalent to 100 rad. 1.7.3 Rem The rem (Roentgen equivalent man) is the traditional unit of dose equivalent (DE) or occupational exposure. It is used to express the quantity of radiation received by radiation workers. Some types of radiation produce more damage than x-rays. The rem accounts for these differences in biologic effectiveness. This is particularly important to persons working near nuclear reactors or particle accelerator. 1.7.4 Curie Curie (Ci) the original unit used to express the decay rate of a sample of radioactive material. The curie is equal to that quantity of radioactive material (not the radiation emitted by that material) in which the number of atoms decaying per second is equal to 37 billion (3.7×1010). In other words, One Curie is that quantity of material in which 3.7x1010 atoms disintegrate every second (3.7x1010 Becquerel, Bq). It was based on the rate of decay of atoms within one gram of radium. It is named for Marie and Pierre Curie who discovered radium in 1898. The curie is the basic unit of radioactivity used in the system of radiation units in the United States, referred to as "traditional" units. Becquerel (Bq) or Curie (Ci) is a measure of the rate (not energy) of radiation emission from a source. 1.7.5 Electron Volt Electron Volt (eV) is the amount of energy by the charge of a single electron moved across an electric potential difference of one volt. A more fundamental unit of energy is the Joule (J). That means, a particle with charge q has energy after passing through the potential V. Therefore, one electron volt is equal to J. The energy of an x-ray is measured in electron volts or, more often, thousands of electron volts (KeV). An electron that is accelerated by an electric potential of one volt will acquire energy to one eV. Most x-ray used in diagnostic radiology have energy up to 150 KeV, where as those in radiotherapy are measured in MeV. Other radiological important energies, such as electron and nuclear binding energies and mass-energy equivalence, are also expressed in eV. Note: because diagnostic radiology is concerned primarily with x-rays, for our purposes we may consider: 1R equal to 1rad equal to 1rem With other types of ionizing radiation this generalization is not true. 14  CHAPTER 1 RADIATION AND ATOM Table 1.3: The special quantities of radiologic science and their-associated special units Customary unit SI unit Quantity Name Symbol Name Symbol Exposure Roentgen R Coulomb per kilogram C/kg Absorbed dose rad rad gray Gy Dose equivalent rem rem Seivert Sv Radioactivity Curie Ci Becquerel Bq 1.8 Practical Units The dose of radiation is when radiation’s energy is deposited into our body’s tissues. The more energy deposited into the body, the higher the dose. For the purpose of radiation protection, dose quantities are expressed in three ways: absorbed, equivalent, and effective. The practical units in everyday use are described below. 1.8.1 Absorbed Dose When ionizing radiation penetrates the human body or an object, it deposits energy. The fundamental units do not take into account the amount of damage done to matter (especially living tissue) by ionizing radiation. This is more closely related to the amount of energy deposited rather than the charge. The energy absorbed from exposure to radiation is called an absorbed dose. The absorbed dose is measured in a unit called the gray (Gy). The gray (Gy), with units J/kg, is the SI unit of absorbed dose, which represents the amount of radiation required to deposit 1 joule of energy in 1 kilogram of any kind of matter. The rad (radiation absorbed dose), is the corresponding traditional unit, which is 0.01 J deposited per kg. 100 rad = 1 Gy. 1.8.2 Equivalent Dose When radiation is absorbed in living matter, a biological effect may be observed. However, equal doses of different types or energies of radiation cause different amounts of damage to living tissue. For example, 1 Gy of alpha radiation causes about 20 times as much damage as 1 Gy of X-rays and more harmful to a given tissue than 1 Gy of beta radiation. Therefore, the equivalent dose was defined to give an approximate measure of the biological effect of radiation. To obtain the equivalent dose, the absorbed dose is multiplied by a specified radiation weighting factor (WR), which is different for each type of radiation. This weighting factor is also called the Q (quality factor), or RBE (Relative Biological Effectiveness of the radiation). The equivalent dose provides a single unit that accounts for the degree of harm that different types of radiation would cause to the same tissue. The equivalent dose is expressed in a measure called the sievert (Sv). The weighted absorbed dose is called equivalent dose. 15  CHAPTER 1 RADIATION AND ATOM Where  The sievert (Sv) is the SI unit of equivalent dose. Although it has the same units as the gray, J/kg, it measures something different. For a given type and dose of radiation(s) applied to a certain body part(s) of a certain organism, it measures the magnitude of an X- rays or gamma radiation dose applied to the whole body of the organism, such that the probabilities of the two scenarios to induce cancer is the same according to current statistics.  1 sievert = 100 rem. Because the rem is a relatively large unit, typical equivalent dose is measured in millirem (mrem), 10−3 rem, or in microsievert (μSv), 10−6 Sv. 1 mrem = 10 μSv.  A unit sometimes used as a measure of low level radiation exposure is the BRET (Background Radiation Equivalent Time, BRET). This is the number of days of an average person's background radiation exposure the dose is equivalent to. That mean, One BRET is the equivalent of one day worth of average human exposure to background radiation. This unit is not standardized, and depends on the value used for the average background radiation dose. For comparison, the average 'background' dose of natural radiation received by a person per day makes BRET 6.6 μSv (660 μrem). However local exposures vary, with the yearly average in the US being around 3.6 mSv (360 mrem), and in a small area in India as high as 30 mSv (3 rem). The lethal full-body dose of radiation for a human is around 4–5 Sv (400–500 rem). The health hazards of low doses of ionizing radiation are unknown and controversial, because the effects, mainly cancer and genetic damage, take many years to appear, and the incidence due to radiation exposure can't be statistically separated from the many other causes of these diseases. The purpose of the BRET measure is to allow a low level dose to be easily compared with a universal yardstick: the average dose of background radiation, mostly from natural sources, that every human unavoidably receives during daily life. Background radiation level is widely used in radiological health fields as a standard for setting exposure limits. Presumably, a dose of radiation which is equivalent to what a person would receive in a few days of ordinary life will not increase his rate of disease measurably. 16  CHAPTER 1 RADIATION AND ATOM The BRET corresponding to a dose of radiation is the number of days of average background dose it is equivalent to. It is calculated from the equivalent dose in sieverts by dividing by the average annual background radiation dose in Sv, and multiplying by 365: The definition of the BRET unit is apparently unstandardized, and depends on what value is used for the average annual background radiation dose, which differs in different countries and regions. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2000) estimate for worldwide background radiation dose is 2.4 mSv (240 mrem). Using this value each BRET unit equals 6.6 μSv. BRET values range from 2 BRET for a dental x-ray to around 400 for a barium enema study. 1.8.3 Effective Dose Different tissues and organs have different radiation sensitivities (see Figure 1.7). For example, bone marrow is much more radiosensitive than muscle or nerve tissue. To obtain an indication of how exposure can affect overall health, the equivalent dose is multiplied by a tissue weighting factor (WT) related to the risk for a particular tissue or organ. This multiplication provides the effective dose absorbed by the body. The unit used for effective dose is also the sievert. Effective dose is not a real physical quantity, but is a "manufactured" quantity invented by the International Commission on Radiological Protection (an international scientific group). It is calculated by multiplying actual organ doses by "risk weighting factors" (which gives each organ's relative radiosensitivity to developing cancer) and adding up the total of all the numbers - the sum of the products is the "effective whole-body dose" or just "effective dose." These weighting factors are designed so that this "effective dose" supposedly represents the dose that the total body could receive (uniformly) that would give the same cancer risk as various organs getting different doses. If several tissues, T1, T2, T3 etc., individually receive equivalent doses , , , etc. then the total risk to individual should not exceed that resulting from the stipulated dose limit to uniform whole body irradiation. Depending on the extent to which the risk from stochastic effects in a tissue/ organ may contribute to the total risk from stochastic effects, a weighting factor, WT is assigned to each tissue/ organ. Thus, the effective dose, E is defined as: Where WT is tissue weighting factor, HT is equivalent dose to tissue T. 17  CHAPTER 1 RADIATION AND ATOM For practical puposes, one gray and one sivert are essentially equal and Roentgen, rad and rem are equivalent Figure 1.7: Tissue weighting factors For example, if someone’s lungs and thyroid are exposed separately to radiation, and the equivalent doses to the organs are 2 mSv (they have a weighting factor of 0.12) and 1 mSv (it has a weighting factor of 0.05) respectively. The effective dose is: (2 mSv × 0.12) + (1 × 0.06) = 0.3 mSv. The risk of harmful effects from this radiation would be equal to a 15.5 mSv dose delivered uniformly throughout the whole body. This model says that the cancer risk from the whole body getting 0.3 mSv uniformly is the same as the lungs getting 2 mSv and the thyroid getting 1 mSv (and no other organ getting a significant dose). Figure 1.8 presents an overview of the relationship between effective, equivalent and absorbed doses. 18  CHAPTER 1 RADIATION AND ATOM For each organ or tissue estimate the ABSORBED DOSE Energy "deposited" in a kilogram of a substance by radiation in mGy Multiply by the RADIATION WEIGHTING FACTOR "WR" for the radiation used Absorbed dose weighted for susceptibility to effect of different radiations And so obtain the EQUIVALENT DOSE to the organ or tissue in mSv Multiply by the TISSUE WEIGHTING FACTOR "WT" for the tissue or organ concerned Equivalent dose weighted for susceptibility to effect of different tissues Sum over all the organs and tissues irradiated And so obtain the EQUIVALENT DOSE to the patient in mSv Figure 1.8: Relationship between effective, equivalent and absorbed doses 19