radioactivity

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radioactivity

 [ra″de-o-ak-tiv´ĭ-te]
the emission of particulate or electromagnetic radiation as a result of decay of the nuclei of unstable elements, a property of all chemical elements of atomic number above 83, and possibly inducible in all other known elements. The chemical elements are made up of atoms, each consisting of a nucleus around which orbits a cloud of negatively charged electrons. The nucleus itself is made up of two kinds of particles: neutrons (which have no electrical charge) and protons (each of which has a single positive charge). A neutral atom has an equal number of protons and electrons and no electric charge. The atomic number of an element is the number of protons in the nucleus of each of its atoms. The mass number of an element is the sum of the number of protons and neutrons in the nucleus. 

All of the atoms of a particular element have the same atomic number, but they can have different numbers of neutrons (i.e., different mass numbers). isotopes of a chemical element consist of atoms having different numbers of neutrons. When an atomic nucleus is unstable it decomposes or decays spontaneously, emitting high-energy particles. The emissions from radioactive decay, called radiation, can consist of electrons (beta particles), or electromagnetic energy in the form of photons, or helium ions (alpha particles). The process of decay can produce a product that is itself unstable, in which case it too will decay. The process continues until a stable nuclide is finally formed.

The radioactivity of a substance can be measured by finding the rate at which atoms decay in a given period of time. The basic unit of measurement of radioactivity is the curie (Ci), which equals 37 billion disintegrations per second. One thousandth of a curie is a millicurie and one millionth is a microcurie; these units are used to calculate the dosage of radioactivity administered for therapeutic procedures in much the same way that units such as the gram and milligram are used to measure dosages of medications. The curie may eventually be replaced by a new SI unit of radioactivity, the becquerel (Bq), which is equal to 1 disintegration per second. One microcurie equals 37 kilobecquerels.

The half-life of an element is the time necessary for one half of a given amount of the isotope to decay. Half-lives can range from billions of years to fractions of a second. The rate at which atomic decay occurs in a particular isotope cannot be altered by any outside force such as temperature, pressure, or chemical reaction. The knowledge of the half-life of a particular isotope is essential to the proper handling of the substance for the protection of the medical staff and the patient who is receiving some form of radiation therapy.

Both particulate and electromagnetic radiations are capable of penetrating matter and interacting with it by indiscriminately knocking out the electrons from atoms and molecules. The process of ionizing radiation produces ions (charged particles) and complex radicals (a group of charged atoms), which may combine to form different molecules. Radiation damage to the nuclei of cells interferes with their reproduction by changing their genetic structure. This ability to penetrate matter and change the basic structure and function of cells is used beneficially in the treatment of malignant tumors. (See also radiation therapy.)

Rendering an element radioactive by artificial means does not alter its chemical behavior within the body; therefore, the body's systems respond to a radioactive element as if it were stable. This phenomenon makes it possible to use radioactive elements for diagnostic as well as therapeutic purposes. For example, the iodine isotope 131I is readily taken up by the thyroid gland, permitting evaluation of iodine uptake of the gland by measuring the amount of radiation emitting from it. An isotope of technetium, 99mTc, is attached to many different compounds that localize in various organs. The organ or structure can then be imaged with a scintillation camera.

ra·di·o·ac·tiv·i·ty

(rā'dē-ō-ak-tiv'i-tē),
The property of some atomic nuclei of spontaneously emitting gamma rays or subatomic particles (α and β rays) by the process of nuclear disintegration and measured in disintegrations per second (dps). One dps is equal to 1 becquerel, and 3.7 × 1010 dps equals 1 curie.

radioactivity

Radiation The spontaneous transformation of an unstable atom, often resulting in the emission of radiation and a lower energy state. See Airborne radioactivity, Radioactive decay.

ra·di·o·ac·tiv·i·ty

(rā'dē-ō-ak-tiv'i-tē)
The property of some atomic nuclei of spontaneously emitting gamma rays or subatomic particles (alpha and beta rays).

radioactivity

1. Spontaneous emission of RADIATION.
2. The radiation emitted by unstable atomic nuclei or in the course of a nuclear reaction. Radioactivity includes alpha particles (helium nuclei), beta particles (high speed electrons), neutrons and gamma rays.

ra·di·o·ac·tiv·i·ty

(rā'dē-ō-ak-tiv'i-tē)
The property of some atomic nuclei of spontaneously emitting gamma rays or subatomic particles (alpha and beta rays).
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