Surface effects
A surface is distinct from bulk matter in that it constitutes the physical interface with the environment. Whether or not a metal will corrode in salt water, for example, or how much resistance to wear is inherent in the design of a bearing are concerns that relate primarily to the physical condition of surfaces. The latter, in turn, may be selectively modified by the application of coatings or by the action of radiation, or by both. Three of the most common examples of surface modification by radiation—ultraviolet curing, ion implantation, and sputtering—are considered here.
Ultraviolet curing is a process in which polymers, generally employed as coatings, are irradiated by ultraviolet light. Such action produces electronic excitation and ionization of the long chain molecules that make up the polymer, either directly or through the mediation of imbedded, light-sensitive “activators.” This results in intermolecular bonding, a process called cross-linking. The entire polymeric coating, typically on the order of tenths of millimetres thick (depending on the application), becomes so highly cross-linked as to take on the character of a single giant molecule. The major effects of ultraviolet irradiation of polymers include reduction of friction, increased resistance to wear, increased hardness, and increased resistance to attack by acids and other corrosive agents. Ultraviolet curing is employed for diverse purposes ranging from the formation of “no-wax” coatings on floor tiles to application in the photolithographic process integral to the fabrication of solid-state electronic devices.
Ion implantation involves the irradiation of solids by beams of energetic ions emanating from particle accelerators. Typical energies employed are on the order of 100 keV (100,000 electron volts). Typical depths of penetration are on the order of several thousand angstroms, depending on energy, ion type, and target material. In ion implantation, virtually any atomic species can be embedded to predetermined depths and with predetermined concentration profiles in any target material so as to modify the surface characteristics without affecting desirable bulk properties. A typical example is the implantation of titanium in iron alloys to reduce wear of bearings and gears. A particularly promising technique was developed by physicists Michael W. Ferralli and Luntz, in which vacuum deposition of polymeric coatings on metallic substrates and simultaneous ion-beam irradiation act to produce implanted hydrocarbon films. The latter can be made to vary in carbon-to-hydrogen ratio from very high values—with the implanted region having some characteristics of diamond—to values on the order of unity and corresponding polymeric characteristics. This is accomplished by a process called preferential sputtering (see below). The films so produced are highly resistant to corrosion and appear to possess important bio-compatibility properties, making them suitable for applications in, for example, the treatment of the surfaces of surgical implants such as artificial hip joints. Such effects of ion implantation result in part from structural changes induced by radiation damage (e.g., implantation of boron or phosphorus in steel can render the surface amorphous so as to eliminate grain boundaries and other corrosion-sensitive sites), and in part from chemical changes arising from bonding of the implanted species with constituents of the substrate.
Sputtering is a process in which atoms, ions, and molecular species in the surface of a target material are ejected under the action of ion-beam irradiation. Energies typical of ion implantation are employed and, while any ion type may be used, noble (or rare) gases such as argon and neon are most common. The latter avoid unwanted chemical interactions between the ions of the beam and the substrate. Sputtering results from several interaction mechanisms. Conceptually, the simplest is rebound sputtering, in which an incident ion strikes an atom on the surface, causing it to recoil into the target. The recoiling atom promptly collides with a neighbouring atom in the target, rebounds elastically, and is ejected from the surface. A similar but somewhat more complex mechanism is recoil sputtering, in which a struck, recoiling surface atom undergoes a random sequence of elastic scatterings in the target material, ultimately migrating back to, and through, the surface. Yet another mechanism is prompt thermal sputtering, in which energized atoms in thermal spikes created close to the surface escape through the surface before annealing occurs. Certain materials (e.g., crystalline alkali halides) are prone to electronic sputtering, in which energy associated with electronic excitations induced by the incident ion is transformed into atomic recoil kinetic energy, often sufficient to cause the ejection of ions through the surface. By means of any of these various mechanisms, several atoms may be sputtered for each ion incident on the target. The number of atoms sputtered per incident ion is called the sputtering yield.
Surface modifications caused by sputtering are characterized as structural (e.g., phase conversion from crystalline to amorphous and vice versa), topographical (e.g., alteration of the shape of surface protrusions such as grain boundaries, development of facets, and the removal of surface contaminants), electronic (e.g., radiation-induced chemical changes), and compositional (e.g., preferential sputtering of a particular atomic species resulting in changes in the composition of alloys).
Biological effects of ionizing radiation
The biomedical effects of ionizing radiation have been investigated more thoroughly than those of any other environmental agent. Evidence that harmful effects may result from small amounts of such radiation has prompted growing concern about the hazards that may be associated with low-level irradiation from the fallout of nuclear weapons, medical radiography, nuclear power plants, and other sources.
Assessment of the health impact of ionizing radiation requires an understanding of the interactions of radiation with living cells and the subsequent reactions that lead to injury. These subjects are surveyed in the following sections, with particular reference to the principal sources and levels of radiation in the environment and the different types of biologic effects that may be associated with them.
Historical background
Within weeks after Röntgen revealed the first X-ray photographs in January 1896, news of the discovery spread throughout the world. Soon afterward, the penetrating properties of the rays began to be exploited for medical purposes, with no inkling that such radiation might have deleterious effects.
The first reports of X-ray injury to human tissue came later in 1896. Elihu Thomson, an American electrical engineer, deliberately exposed one of his fingers to X rays and provided accurate observations on the burns produced. That same year, Thomas Alva Edison was engaged in developing a fluorescent X-ray lamp when he noticed that his assistant, Clarence Dally, was so “poisonously affected” by the new rays that his hair fell out and his scalp became inflamed and ulcerated. By 1904 Dally had developed severe ulcers on both hands and arms, which soon became cancerous and caused his early death.
During the next few decades, many investigators and physicians developed radiation burns and cancer, and more than 100 of them died as a result of their exposure to X rays. These unfortunate early experiences eventually led to an awareness of radiation hazards for professional workers and stimulated the development of a new branch of science—namely, radiobiology.
Radiations from radioactive materials were not immediately recognized as being related to X rays. In 1906 Henri Becquerel, the French physicist who discovered radioactivity, accidentally burned himself by carrying radioactive materials in his pocket. Noting that, Pierre Curie, the co-discoverer of radium, deliberately produced a similar burn on himself. Beginning about 1925, a number of women employed in applying luminescent paint that contained radium to clock and instrument dials became ill with anemia and lesions of the jawbones and mouth; some of them subsequently developed bone cancer.
In 1933 Ernest O. Lawrence and his collaborators completed the first full-scale cyclotron at the University of California at Berkeley. This type of particle accelerator was a copious source of neutrons, which had recently been discovered by Sir James Chadwick in England. Lawrence and his associates exposed laboratory rats to fast neutrons produced with the cyclotron and found that such radiation was about two and a half times more effective in killing power for rats than were X rays.
Considerably more knowledge about the biologic effects of neutrons had been acquired by the time the first nuclear reactor was built in 1942 in Chicago. The nuclear reactor, which has become a prime source of energy for the world, produces an enormous amount of neutrons as well as other forms of radiation. The widespread use of nuclear reactors and the development of high-energy particle accelerators, another prolific source of ionizing radiation, have given rise to health physics. This field of study deals with the hazards of radiation and protection against such hazards. Moreover, since the advent of spaceflight in the late 1950s, certain kinds of radiation from space and their effects on human health have attracted much attention. The protons in the Van Allen radiation belts (two doughnut-shaped zones of high-energy particles trapped in the Earth’s magnetic field), the protons and heavier ions ejected in solar flares, and similar particles near the top of the atmosphere are particularly important.
Units for measuring ionizing radiation
Ionizing radiation is measured in various units. The oldest unit, the roentgen (R), denotes the amount of radiation that is required to produce 1 electrostatic unit of charge in 1 cubic centimetre of air under standard conditions of pressure, temperature, and humidity. For expressing the dose of radiation absorbed in living tissue, the principal units are the gray (Gy; 1 Gy = 1 joule of radiation energy absorbed per kilogram of tissue) and the rad (1 rad = 100 ergs per gram of tissue = 0.01 Gy). The sievert (Sv) and the rem make it possible to normalize doses of different types of radiation in terms of relative biologic effectiveness (RBE), since particulate radiations tend to cause greater injury for a given absorbed dose than do X rays or gamma rays. The dose equivalent of a given type of radiation (in Sv) is the dose of the radiation in Gy multiplied by a quality factor that is based on the RBE of the radiation. Hence, one sievert, defined loosely, is that amount of radiation roughly equivalent in biologic effectiveness to one gray of gamma rays (1 Sv = 100 rem). Because the sievert and the rem are inconveniently large units for certain applications, the milligray (mGy; 1 mGy = 1/1000 Gy) and millisievert (mSv; 1 mSv = 1/1000 Sv) are often substituted.
For expressing the collective dose to a population, the person-Sv and person-rem are the units used. These units represent the product of the average dose per person times the number of people exposed (e.g., 1 Sv to each of 100 persons = 100 person-Sv = 10,000 person-rem).
The units employed for measuring the amount of radioactivity contained in a given sample of matter are the becquerel (Bq) and the curie (Ci). One becquerel is that quantity of a radioactive element in which there is one atomic disintegration per second; one curie is that quantity in which there are 3.7 × 1010 atomic disintegrations per second (1 Bq = 2.7 × 10-11 Ci). The dose that will accumulate over a given period (say, 50 years) from exposure to a given source of radiation is called the committed dose, or dose commitment.
Sources and levels of radiation in the environment
Natural sources
From the beginning, life has evolved in the presence of natural background ionizing radiation. The principal types and sources of such radiation are: (1) cosmic rays, which impinge on the Earth from outer space (Table 3; Figure 4); (2) terrestrial radiations, which are released by the disintegration of radium, thorium, uranium, and other radioactive minerals in the Earth’s crust (Table 4; Figure 4); and (3) internal radiations, which are emitted by the disintegration of potassium-40, carbon-14, and other radioactive isotopes that are normally present within living cells (Table 5; Figure 4). The average total dose received from all three sources by a person residing at sea level is approximately 0.91 mSv per year (Table 6); however, a dose twice this size may be received by a person residing at a higher elevation such as Denver, Colo., where cosmic rays are more intense (Table 3), or by a person residing in a geographic region where the radium content of the soil is relatively high (Table 4). In the latter type of region, the radioactive gas radon, which is formed in the decay of radium, may enter a dwelling through its floor or basement walls and accumulate in the indoor air unless the dwelling is well ventilated periodically; occupants of such a dwelling may therefore receive a dose as high as 100 mSv per year in their lungs from inhalation of the entrapped radon and its disintegration products (Table 5; Figure 4).
| Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. population | |
| source of radiation | average dose rates (mSv/year) |
| Natural | |
| environmental | |
| cosmic radiation | 0.27 (0.27–1.30)* |
| terrestrial radiation | 0.28 (0.30–1.15)** |
| internal radioactive isotopes | 0.36 |
| subtotal | 0.91 |
| Man-made | |
| environmental | |
| technologically enhanced | 0.04 |
| global fallout | 0.04 |
| nuclear power | 0.002 |
| medical | |
| diagnostic | 0.78 |
| radiopharmaceuticals | 0.14 |
| occupational | 0.01 |
| miscellaneous | 0.05 |
| subtotal | 1.06 |
| total | 1.97 |
| *Values in parentheses indicate range over which average levels for different states vary with elevation. **Range of variation (shown in parentheses) attributable largely to geographic differences in the content of potassium-40, radium, thorium, and uranium in the Earth's crust. | |
| Average dose due to natural radioactivity deposited internally | ||||
| isotope | radioactivity in millibecquerel (mBq)* | radiation | dose in mSv (per year) | critical organ |
| carbon-14 | 2.2(10−7) per kilogram | beta rays | 0.016 | gonads |
| potassium-40 | 3.9(10−7) per kilogram | beta rays | 0.165 | gonads |
| potassium-40 | 5.6(10−8) per kilogram | gamma rays | 0.023 | gonads |
| radium and daughters | 3.7(10−9) in body | alpha, beta, gamma rays | 7.6 | bones |
| radon and daughters | 1.2(10−2) per 1 in inhaled air | alpha, beta, gamma rays | 20 | lungs |
| *Millibecquerel is a unit of radioactive disintegration rate; it corresponds to thatquantity of a radioactive element in which there is one disintegrationevery 1,000 seconds. | ||||
| External dose due to natural radioactivity in soil or rock | |
| source | dose in mSv per year |
| ordinary regions | 0.25-1.6 |
| active regions | |
| granite in France | 1.8-3.5 |
| houses in Switzerland (alum shale) | 1.58-2.2 |
| monazite alluvial deposits in Brazil | mean 5; max 10 |
| monazite sands, Kerala, India | 3.7-28 |
| Cosmic-radiation exposure | |
| location | mean dose in millisievert (mSv)* per year |
| sea level, temperate zone | 0.20-0.40 |
| 1,500 metres | 0.40-0.60 |
| 3,000 metres | 0.80-1.20 |
| 12,000 metres | 28 |
| 36-600 kilometres | 70-150 |
| interplanetary space | 180-250 |
| Van Allen radiation belt (protons) | <15,000 |
| single solar flare (protons and helium) | <10,000 |
| *Millisievert is a radiation dose-equivalent unit: it corresponds to adose equivalent in biologic effectiveness to 10 ergs energy of gammaradiation transferred to one gram of tissue. | |
