Summary:
The sources of radiation are the body receives a radiation externally from the three principal sources: natural background radiation, medical radiation, and the radioactive wastes.
Objectives:
To balance nuclear reactions
To be aware of the units of radiation and of the methods used to detect radiation
The Discovery of Radioactivity
In 1896 a French physicist, Henry Becquerel (1852-1908), found that uranium crystals had the property of “fogging” a photographic place that had been placed near those crystals. This fogging took place even though the photographic plate was wrapped in black paper. By placing crystals of uranium on a photographic plate covered with black paper and then developing the plate, he obtained a self-picture of the crystals. Becquerel concluded that the uranium gave off some kind of radiation of rays that affected the photographic plate.
Sometimes like uranium that spontaneously gives off radiation are said to be radioactive. Radioactivity is the property that causes the element to emit radiation. This radiation comes from the nucleus of the atom.
Types of Radiation Produced by a Radioactive Substance
The following experiment was performed to study the radiation and produced by a radioactive element. A piece of radium was placed at the bottom of a thick lead well. The purpose all the lead was to absorb all the radiation except that going directly upward. The escaping radiation was allowed to fall on a photographic plate. When the radiation was passed through a strong electrostatic field, three different areas showed up on the photographic plate. This indicated that were actually three different kinds of radiation. These were called alpha, beta, and gamma.
Alpha Particles
Alpha particles (α particle) are attracted toward the negative electrostic field, which indicates that they are positively charged. Alpha particles consist of positively charge of helium nuclei; that is, they consist of the nuclei of helium atoms (each of which contains two protons and two neutrons) and so they have a charge of +2. Alpha particles have a very low penetrating power. They can be stopped by a piece of paper or by a thin sheet of aluminum foil. Alpha particles are relatively harmless when they strike the body because they do not penetrate the outer layer of the skin. However, if the source of alpha particles is inhaled or ingested or gets into the body through an open wound, then those particles can cause damage to the cells and to the internal organs.
Alpha particles result from the radioactive decay of heavy elements such as uranium and radium.
Beta Particles
Beta particles (β particles) are attracted toward the positive electrostatic field, which indicates that they consist negatively charged particles. Beta particles consist of high-speed electrons that travel in excess of 100,000 miles per second. Note that beta particles (electrons) are produced in the nucleus by the transformation of the neutron into a proton and an electron. The electron is emitted as a beta particle and a proton remains in the nucleus. Beta particles (electrons) have a charge of -1.
Beta particles have a slight penetrating power. They pass through a sheet of paper but can be stopped by heavy clothing. When beta particles strike the body, they penetrate only a few millimeters and do not reach any vital organs. If a source of beta particles should be inhaled or ingested, those particles could only cause the internal damage to the body cells and the organs.
Beta particles are deflected by the electrostatic field to a much greater extent than are the alpha particles. This indicates that the beta particles have much smaller mass than the alpha particles.
Gamma Rays
Gamma rays (γ rays) are not affected by the electrostatic field because they have no charge. They are not particles at all; they have no mass. Gamma rays are the form of electromagnetic radiation similar to x-rays. Gamma rays are very penetrating; they will pass through the body, causing cellular damage as they travel through. Gamma rays are often emitted along with alpha or beta particles. Gamma rays are originated from unstable atoms releasing energy to gain stability. The “gamma knife” focuses a dose of gamma radiation to a precise target point to the brain. It is used to treat deep-seated brain tumors that were previously considered inoperable. There is no incision in the scalp and no need for general anesthesia.
Other Types of Radiation
X-rays are a form of electromagnetic radiation usually produced by machines, whereas gamma rays are emitted by radioactive substances.
Neutrons are released from elements that undergo spontaneous fission. Their relatively large mass gives them great energy, and because they have no charge they readily penetrate the body. Neutrons are used in the treatment of cancer.
Units of Radiation
Radiation is measured in terms of several different units, depending on whether the measurement relates to a physical or a biological effect.
The physical unit of radiation is a measure of the number of nuclear disintegrations occurring per second in a radioactive source. The standard unit is the *curie, which is defined as the number of the nuclear disintegrations occurring per second in 1 g of radium; 1 curie (1 Ci) equals 37 billion disintegration per second. Smaller units are the mill curie (1 mCi=37 million disintegrations per second) and the micro curie (1 μCi= 37,000 disintegrations per second). These smaller units are frequently used in describing the amount of radioactive fallout. The curie is not useful in biologic work because it simply indicates the number of disintegrations per second regardless of the type of radiation and regardless of the effect of that radiation upon tissue.
The *roentgen (abbreviated R) is a unit of radiation generally applied to x-rays and gamma rays only. X-rays and gamma rays produce ionization in the air and also in tissue. The roentgen is defined as the intensity of X-rays or gamma rays that produces 2 billion ion pairs in 1 mL of air. This is not the same of tissue as it is for air so that the roentgen does not accurately indicate the amount of radiation on tissue.
The *rad (radiation absorbed dose) refers to the amount of radiation energy absorbed by the tissue that has been radiated. One rad corresponds to the absorption of 100 ergs of energy per gram of tissue. An erg is a very small unit of energy. More than 40 million ergs are required to the equal 1 cal. However, even if the erg is an extremely small unit of energy, the effect of 1 rad (100 ergs per gram) is important because of the ionization that the radiation produces in the cells. The Sl unit for the absorbed dosage of radiation is the gray.
Different types of radiation cause different biological damage to the cells. This difference in biologic effectiveness may be expressed in terms of the relative biologic effectiveness (RBE) of radiation.
The standard for RBEs is the gamma radiation from Cobalt 60.
Detection and Measurement of Radiation
The problem of detecting and measuring radiation is very important in medical work, particularly in the protection of personnel. One device used to detect radiation is the Geiger counter. This device consists of a glass tube containing a glass at low pressure through which runs a wire connected to a high-voltage power supply. When the device is brought close to a radioactive substance, the radiation causes a momentary current to flow through the tube. A speaker is usually placed in the cirl counting device is connected to the tube to indicate the amount of radiation.
Scanners use another type of device, called a crystal of sodium iodide containing a small amount of thallium iodide. When the crystal is hit by radiation, it gives off a flash of light, scintillations, and the result is produced a “scan.”
X-ray technicians and others who work around radiation usually are required to wear film badges. These badges indicate the accumulated amount of radiation to which they have been exposed. They contain a piece of photographic.
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