RADIATION QUANTITIES: DEFINITIONS AND UNITS
Radiation can be defined as energy that is transmitted in the form of waves or particles. Radiation is also refers to energies or particles given off by radioactive matter.
There are many different quantities and units used to quantify radiation, because there are a number of different aspects of an x-ray beam or gamma radiation that can be used to express the amount of radiation. The selection of the most appropriate quantity depends on the specific application. The primary objective is to help the student to develop a conceptual understanding of the various radiation quantities and units and gain sufficient factual knowledge to support the usage.
Radiation is generally classified as ionizing radiation or non-ionizing radiation. Ionizing radiation has sufficient energy to remove an electron from an atom. It includes the radiation that comes from both natural and man-made sources. Non-ionizing radiation has less energy than ionizing radiation and cannot remove an electron from an atom. Examples of non-ionizing radiation include radio waves and microwaves.
Background radiation is the radiation constantly present in the environment. It is emitted by natural and artificial sources. Four main types of ionizing radiation include;
- alpha
- beta
- photon (X-rays and gamma rays)
- neutron.
Alpha and beta radiation may be emitted while a nucleus undergoes radioactive decay. Alpha and beta particles are often also accompanied by the release of additional energy, in the form of photon radiation. Neutron radiation can be produced from nuclear fission, which occurs only for certain nuclear substances with a high atomic number, such as uranium and plutonium. Except for several fission fragments with very short half-lives, and californium-252, which undergoes spontaneous fission, there are no other radioisotopes that emit neutrons. Other neutron sources depend on nuclear reactions for the emission of neutrons. Regardless of the source, each of these radiation types (alpha, beta, photon, and neutron) is capable of penetrating the human body to a varying degree and delivering a radiation dose.
Alpha radiation (α)
Alpha radiation (α) consists of alpha particles that are made up of two protons and two neutrons each and that carry a double positive charge. Due to their relatively large mass and charge, they are extremely limited in their ability to penetrate matter. Alpha radiation can be stopped by a piece of paper or the dead outer layer of the skin. Consequently, alpha radiation from nuclear substances outside the body does not present a radiation hazard. However, when alpha-radiation-emitting nuclear substances are taken into the body (for example, by breathing them in or by ingesting them), the energy of the alpha radiation is completely absorbed into bodily tissues. For this reason, alpha radiation is only an internal hazard. An example of a nuclear substance that undergoes alpha decay is radon-222, which decays to polonium-218.
Beta radiation (β)
Beta radiation (β) consists of charged particles that are ejected from an atom’s nucleus and that are physically identical to electrons. Beta particles generally have a negative charge, are very small and can penetrate more deeply than alpha particles. However, most beta radiation can be stopped by small amounts of shielding, such as sheets of plastic, glass or metal. When the source of radiation is outside the body, beta radiation with sufficient energy can penetrate the body’s dead outer layer of skin and deposit its energy within active skin cells. However, beta radiation is very limited in its ability to penetrate to deeper tissues and organs in the body. Beta-radiation-emitting nuclear substances can also be hazardous if taken into the body. An example of a nuclear substance that undergoes beta emission is tritium (hydrogen-3), which decays to helium-3.
Photon radiation (gamma [γ] and X-ray)
Photon radiation is electromagnetic radiation. Two types of photon radiation are of interest for the purpose of dosimetry: gamma (γ) and X-ray. Gamma radiation consists of photons that originate from within the nucleus, and X-ray radiation consists of photons1 that originate from outside the nucleus. While gamma radiation is a familiar term for electromagnetic radiation, here the term photon is used to denote electromagnetic radiation – so as to include X-rays generated from nuclear substances. 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 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.
Neutron radiation (n)
Apart from cosmic radiation, spontaneous fission is the only natural source of neutrons (n). 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.
UNIT SYSTEM
A complicating factor is that American society is undergoing a slow change in the units used to express a variety of physical quantities. In everyday life we see this as a change from the conventional British unit system (feet, pounds, miles) to the metric system (meters, kilograms, kilometers). In radiology we are experiencing a change not only to the general metric units but also to the proposed adoption of a set of fundamental metric units known as the International System of Units (SI units). The adoption of SI radiation units is progressing rather slowly because there is nothing wrong with our conventional units, and SI units are somewhat awkward for a number of common applications. Throughout this text we use the units believed the most useful to the reader. In this chapter both unit systems are discussed and compared.
The table below is a listing of most of the physical quantities and units encountered in radiology. It is a useful reference especially for the conversion of one system of units to another.
| Radiation Units and Conversion Factors | |||
| Exposure | Conventional Unit | SI Unit | Conversions |
| Exposure | roentgen (R) | coulomb/kg of air (C/kg) | 1 C/kg = 3876 R |
| 1 R = 258 uC/kg | |||
| Dose | rad (R) | gray (Gy) | 1 Gy = 100 rad |
| Dose equivalent | rem | sievert (Sv) | 1 Sv = 100 rem |
| Activity | curie (Ci) | becquerel (Bq) | 1 mCi = 37 mBq |
RADIATION QUANTITIES
There are many different physical quantities that can be used to express the amount of radiation delivered to a human body. Generally, there are advantages as well as disadvantages and limitations for each of the quantities. Radiation quantities used to describe a beam of x-radiation fall into two general categories. One category comprises the quantities that express the total amount of radiation delivered to a body, and the other comprises the quantities that express radiation concentration at a specific point.
Radiation Concerntration
- Photon fluence
- Energy fluence
- Exposure
- Dose
- Dose equivalent
Total Radiation
- Total photons
- Total energy
- Integral exposure
- Integral dose
Photon fluence
Photon is a particle of light defined as a discrete bundle of electromagnetic waves (EM) or light energy. Since an x-ray beam and gamma radiation are showers of individual photons, the number of photons could, in principle, be used to express the amount of radiation. In practice, the number of photons is not commonly used, but it is a useful concept in understanding the nature of radiation and distinguishing between concentration and total radiation. The different ways the concentration of radiation delivered to a small area on a patient’s body could be expressed can be examine. For instant, if 1cm2 area is draw on the surface of the patient and then the number of photons passing through the area during a radiographic procedure is counted, the indication of the concerntration of radiation delivered to the patient is known. During a single abdominal radiographic exposure, it could be observed that close to 1010 photons would have passed through the square centimeter.
TOTAL PHTONS
If the number of photons entering the total exposed area is counted, an indication of the total amount of radiation energy delivered to the patient is known. This quantity depends on the size of the exposed area and the radiation concentration. If the radiation is uniformly distributed over the exposed area, the total number of photons entering the patient can be found by multiplying the concentration (fluence) by the exposed area. Changing the size of the exposed area does not affect the concentration entering at the center of the beam. However, reducing the exposed area does reduce the total number of photons and radiation entering the patient.
EXPOSURE
Exposure can be defined as the quantity most commonly used to express the amount of radiation delivered to a point. The conventional unit for exposure is the roentgen (R), and the Sl unit is the coulomb per kilogram of air (C/kg):
1 R = 2.58 x 10-4 C/kg
1 C/kg = 3876 R
The exposure is such a widely used radiation quantity because it can be readily measured. All forms of radiation measurement are based on an effect produced when the radiation interacts with a material. The specific effect used to measure exposure is the ionization in air produced by the radiation.
Exposure is generally measured by placing a small volume of air at the point of measurement and then measuring the amount of ionization produced within the air. The enclosure for the air volume is known as an ionization chamber. The concept of exposure and its units can be developed from the figure above. When a small volume of air is exposed to ionizing radiation (x-ray, gamma, etc.), some of the photons will interact with the atomic shell electrons. The interaction separates the electrons from the atom, producing an ion pair. When the negatively charged electron is removed, the atom becomes a positive ion. Within a specific mass of air the quantity of ionizations produced is determined by two factors: the concentration of radiation photons and the energy of the individual photons.
An exposure of 1 roentgen produces 2.08 x 109 ion pairs per cm3 of air at standard temperature and pressure (STP); 1 cm3 of air at STP has a mass of 0.001293 g. The official definition of the roentgen is the amount of exposure that will produce 2.58 x 10-4 C (of ionization) per kg of air. A coulomb is a unit of electrical charge. Since ionization produces charged particles (ions), the amount of ionization produced can be expressed in coulombs. One coulomb of charge is produced by 6.24 x 1018 ionizations. Exposure is a quantity of radiation concentration. For a specific photon energy, exposure is proportional to photon concentration or fluence.
Exposure and photon concentration is related by;
The relationship between exposure and photon concentration is shown below; the relationship changes with photon energy because both the number of photons that will interact and the number of ionizations produced by each interacting photon is dependent on photon energy. If we assume a photon energy of 60 keV.
1R exposure is equivalent to a concentration of approximately 3 x 1010 photons per cm2.
AIR KERMA (KINETIC ENERGY RELESED PER UNIT MASS OF AIR)
Air kerma is another radiation quantity that is sometimes used to express the radiation concentration delivered to a point, such as the entrance surface of a patient’s body.
AIR kerma is a measure of the amount of radiation energy, in the unit of joules (J), actually deposited in or absorbed in a unit mass (kg) of air. Therefore, the quantity, kerma, is expressed in the units of J/kg which is also the radiation unit, the gray (G) . Air kerma is just the Absorbed Dose in air.
The quantity, air kerma, has two things going for it and is beginning to replace the quantity, exposure, for expressing the concentration of radiation delivered to a point, like the entrance surface to a human body.
1. It is easy to measure with an ionization chamber. Since the ionization produced in air by radiation is proportional to the energy released in the air by the radiation, ionization chambers actually measure air kerma as well as exposure. An ionization chamber can be calibrated to read air kerma, or a conversion factor can be used to convert between air kerma and exposure values.
2. It is expressed in a practical metric SI unit. Air kerma is expressed in the units of joule per kilogram, J/kg. This is also the unit gray, Gy, used for absorbed dose. Therefore, if air kerma measured or calculated at a point where soft tissue is located is known, the absorbed dose in the tissue will be just about equal to the air kerma.