Definition
Radiation is defined as the propagation of energy through a vacuum or matter. Any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, by interaction with matter is referred to as ionization radiation. Medical uses generally involve X‐rays and gamma rays as well as high‐energy electrons and low‐energy beta particles. The corpuscular emission from radioactive substances or other sources, commonly alpha particles, beta particles, heavy ions, and neutrons, are also forms of ionizing radiation. Radiation in wave form is classified according to wavelength. The relationship of radiation wavelength, velocity, and frequency is shown by
(5.15)
Different Sources of Radiation
The sun is a great nuclear furnace. It makes most of the cosmic radiation that strikes the outside of the Earth’s atmosphere. The atmosphere stops most of the cosmic rays. People who live at high elevations or crews of jet airplanes have less atmosphere above them and they get more cosmic‐ray radiation than other people. People are also exposed to man‐made radiation. Most comes from medical and dental use of X‐rays and gamma rays. These are equal on average to about one‐fourth of the natural background.
We can make uranium atoms split apart (fission) to generate large amounts of energy. Fission also produces large amounts of radioactive isotopes (“fission products”) and neutrons. Stable atoms and neutrons can combine to produce radioactive isotopes. Most fission products give off beta particles and gamma rays. Beta particles are high‐energy electrons. Gamma rays are waves of high energy. Some radioactive atoms give off alpha particles. Alpha particles are high‐energy particles made up of two protons and two neutrons. They are thousands of times heavier than beta particles.
External Exposure and Internal Exposure
External exposure: Gamma rays can penetrate into or through the body. Beta particles can penetrate through the skin. Alpha particles cannot penetrate even the skin’s outer layer. Except for the eye, alpha particles are harmless from outside the body. Beta particles are stopped by thin layers of common light materials like paper or aluminum. Thick layers of common materials like concrete or layers of very dense material like lead are needed to stop gamma rays.
Internal exposure: Any radioactive materials can enter the body with food or drink (ingestion), or by breathing (inhalation), or through an open wound (dermal). If that happens, any kind of radiation can directly harm living cells. There are differences in the damage a particular amount of radioactive material can do. That depends on how quickly it decays and the kind of radiation it produces. It also depends on the kind of element it is. Some elements decay quickly, while some elements are eliminated by normal body functions. But some are long‐lived and attach to tissues. They can stay in the body for a long time. Some stay in the body indefinitely.
Radionuclide Decay
All radionuclides, i.e., alpha, beta, etc. are characterized by a specific physical or radiological half‐life, the time required for half of the radioactive nuclei to decay. Radioactive decay is an exponential process and is described by the fundamental decay law, which is expressed mathematically as
where A refers to the activity at any time t, and A0 refers to the activity at zero time. The decay constant λ is expressed in units of reciprocal time and is an indication of the rate or probability of decay. The larger the value of λ, the shorter the half‐life, T½, which is expressed by
A rule of thumb is that after 7 half‐lives, the activity is reduced to less than 1% of the original amount, and after 10 half‐lives, to less than 0.1%. Activity refers to the number of radioactive transformation per unit of time. In the old system of units, activity was expressed in curie (Ci) units. One Ci was defined as 3.7 × 1010 disintegration per second. The comparable unit in the SI unit is Becquerel (Bq), which is simply one disintegration per second.
EXAMPLE 5.18 RADIOACTIVE DECAY
How long will it take for 750 Bq of a radionuclide to decay to 200 Bq if the decay constant is 0.086/day? What is the half‐life?
SOLUTION
- A0 = 750 Bq
- A = 200 Bq
- Λ = 0.086/day
Time to decay, using Eq. (5.16):
Half‐life, using Eq. (5.17)
Radiation Dose
Fundamentally, the harmful consequences of ionizing radiations to a living organism are due to the energy absorbed by the cells and tissues of the organism. The absorbed energy or dose produces chemical decomposition of the molecules present in the living cells. The mechanism of the decomposition appears to be related to ionization and excitation interactions between the radiation and atoms within the tissue. The amount of ionization or number of ion pairs produced by ionizing radiations in the cells or tissues provides some measure of the amount of decomposition or physiological damage that might be expected from a given quantity or dose. The ideal basis for radiation dose measurement could be, therefore, the number of ion pairs (or ionizations) taking place within the medium of interest. For certain practical reasons, the medium chosen for defining exposure does is air.
Radiation Dose Units
Exposure Dose: The Roentgen
The exposure dose of X‐rays or gamma radiation within a unit used for expressing the exposure to X‐rays or gamma radiation is the roentgen (R). Its merit lies in the fact that the magnitude of the exposure does in roentgens can usually be related to the absorbed dose, which is of importance in predicting or quantifying the expected biological effect (or injury) resulting from the radiation.
The roentgen is an exposure dose of X‐rays or gamma radiation such that the associated corpuscular emission per 0.001 293 g of air produces, in air, ions carrying one electrostatic unit of quantity of electricity of either sign. Note that 1 cm3 of air at STP has a mass of 0.001 293 g.
Absorbed Dose: The Gray
The absorbed dose of any ionizing radiation is the energy imparted to matter by ionizing radiation per unit mass of irradiated material at the place of interest. The SI unit of absorbed dose is the gray (Gy). One gray is equivalent to the absorption of 1 J/kg (Joule per kilogram). The former unit of absorbed dose was the rad. One rad is equivalent to the absorption of 100 ergs/g. 1 Gy – 100 rads. The Seivert (Sv) is obtained by multiplying the absorbed dose in GY by a radiation weighting factor.
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