Final report of the Advisory Committee on Human Radiation Experiments.
- United States. Advisory Committee on Human Radiation Experiments
- Date:
- 1996
Licence: Public Domain Mark
Credit: Final report of the Advisory Committee on Human Radiation Experiments. Source: Wellcome Collection.
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![measuring the movement of sand in an hour- glass; it is more useful to think of it as a continu- ous flow, rather than a series of separate events. The intensity of a beam of ionizing radiation is measured by counting up how many ions (how much electrical charge) it creates in air. The roentgen (named after Wilhelm Roentgen, the discoverer of x rays) is the unit that measures the ability of x rays to ionize air; it is a unit of expo- sure that can be measured directly. Shortly af- ter World War I], a common unit of measure- ment was the roentgen equivalent physical (rep), which denoted an ability of other forms of ra- diation to create as many ions in air as a roent- gen of x rays. It is no longer used, but appears in many of the documents examined by the Advisory Committee. What are the basic types of ionizing radiation? There are many types of ionizing radiation, but the most familiar are alpha, beta, and gamma/x- ray radiation. Neutrons, when expelled from atomic nuclei and traveling as a form of radia- tion, can also be a significant health concern. Alpha particles are clusters of two neutrons and two protons each. They are identical to the nuclei of atoms of helium, the second lightest and second most common element in the uni- verse, after hydrogen. Compared with other forms of radiation, though, these are very heavy particles—about 7,300 times the mass of an elec- tron. As they travel along, these large and heavy particles frequently interact with the electrons of atoms, rapidly losing their energy. They can- not even penetrate a piece of paper or the layer of dead cells at the surface of our skin. But if released within the body from a radioactive atom inside or near a cell, alpha particles can do great damage as they ionize atoms, disrupting living cells. Radium and plutonium are two examples of alpha emitters. Beta particles are electrons traveling at very high energies. If alpha particles can be thought of as large and slow bowling balls, beta particles can be visualized as golf balls on the driving range. They travel farther than alpha particles and, depending on their energy, may do as much 7 f damage. For example, beta particles in fallout can cause severe burns to the skin, known as beta burns. Radiosotopes that emit beta particles are present in fission products produced in nuclear reactors and nuclear explosions. Some beta-emit- ting radioisotopes, such as iodine 131, are ad- ministered internally to patients to diagnose and treat disease. Gamma and x-ray radiation consists of pack- ets of energy known as photons. Photons have no mass or charge, and they travel in straight lines. The visible light seen by our eyes is also made up of photons, but at lower energies. The energy of a gamma ray is typically greater than 100 kilo- electron volts (keV—“k” is the abbreviation for kilo, a prefix that multiplies a basic unit by 1,000) per photon, more than 200,000 times the energy of visible light (0.5 eV). If alpha particles are visualized as bowling balls and beta particles as golf balls, photons of gamma and x-radiation are like weightless bullets moving at the speed of light. Photons are classified according to their origin. Gamma rays originate from events within an atomic nucleus; their energy and rate of pro- duction depend on the radioactive decay process of the radionuclide that is their source. X rays are photons that usually originate from energy transitions of the electrons of an atom. These can be artificially generated by bombarding appro- priate atoms with high-energy electrons, as in the classic x-ray tube. Because x rays are produced artificially by a stream of electrons, their rate of output and energy can be controlled by adjust- ing the energy and amount of the electrons themselves. Both x rays and gamma rays can penetrate deeply into the human body. How deeply they penetrate depends on their energy; higher energy results in deeper penetration into the body. A 1 MeV (“M” is the abbreviation for mega, a prefix that multiplies a basic unit by 1,000,000) gamma ray, with an energy 2,000,000 times that of visible light, can pass completely through the body, creating tens of thousands of ions as it does. A final form of radiation of concern is neu- tron radiation. Neutrons, along with protons, are one of the components of the atomic nucleus. Like protons, they have a large mass; unlike pro- tons, they have no electric charge, allowing them](https://iiif.wellcomecollection.org/image/b32220558_0054.jp2/full/800%2C/0/default.jpg)
