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Atomic Radiation: Types & Relationships
May 1969 Electronics World

May 1969 Electronics World

  Table of Contents 

Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

Nuclear energy was a big topic in the 1960s and 1970s as it was believed to be the future of electrical power generation for the world (at least up until the 3 Mile Island and Chernobyl incidents occurred). Ships and submarines were being powered by reactors that allowed them to run for months at a time without refueling, atmospheric emissions were practically zero, and the fuel source was abundant (albeit not simple to obtain). Medical and space applications were increasingly dependent on a greater knowledge of radiation and its effects on humans, plants, animals, and electronics. Many people by that time were working with and around radiation sources, so having knowledge of which is and is not safe was paramount to responsible activities. Proper operation of many types of equipment depend on adequate shielding from the effects of radiation. Probably the two major discriminators between safe and not safe are the level of intensity at the point of interest and whether the radiation is ionizing or non-ionizing.

Author Joseph Wujek published a 3-part article in Electronics World in 1969 to address the issues. Here is Part 1 - Types & Relationships, Part 2 - Detection Methods, and Part 3 - Measuring Techniques.

Atomic Radiation: Types & Relationships - Part 1

By Joseph H. Wujek, Jr.

Not knowing what radiation is all about is as dangerous as the phenomenon itself. This article explains some fundamental concepts.

Editor's Note: This is the first of a three-part series of articles which attempts to disperse the fog of misunderstanding surrounding radioactivity. Part 2, scheduled to appear in the June issue, will cover radiation detection processes, while Part 3, which will appear in the July issue, covers radiation measurements.

Conceptual model of the helium atom - RF Cafe

Fig. 1 - Conceptual model of the helium atom. The nucleus is in the center and the electrons orbit in shells around orbit.

The public understands little of atomic energy; and, in general, neither does the technical community (except, of course, those actively engaged in atomic physics work). The purpose of this article is to briefly discuss some of the common types of atomic radiation and, hopefully, remove some of the mystery which surrounds them.

However, before exploring the radiation process, it would be helpful if we first discussed the makeup of an atom.

The Building Blocks of Atoms

Fig. 1 is a simple conceptual model of a helium atom. The nucleus, which is composed of neutrons and protons each having a mass approximately 1.7 X 10-27 kilogram, is in the center and the electrons revolve in orbital shells about it. An electron has a mass of about 9.1 X 10-31 kilogram, or about 1/1900th that of a proton or neutron. (One kg equals 2.2 pounds in a 1-g gravitational field.)

The neutron, as the name suggests, is electrically neutral. But the electron carries a negative charge of 1.6 X 10-19 coulomb; the proton has the same charge but it is of opposite polarity. An atom which is not ionized has an equal number of electrons and protons, and thus a net charge of zero, the "+" nuclear charge neutralizing the "-" electron charge.

The number of protons in the nucleus determines the element. There are over 100 elements known to scientists, some of which are extremely rare. The sum of the protons and neutrons in the nucleus determines the atomic mass of the element. Atoms of the same element having different masses are called isotopes. The most abundant isotope of oxygen, for example, has eight protons and eight neutrons - a mass of 16. But isotopes having masses of 17 (8 protons, 9 neutrons) and 18 (8 protons, 10 neutrons) exist. Except for properties due to the mass difference, these oxygen atoms behave (chemically) in identical fashion. Incidentally, many isotopes are unstable, decaying and disintegrating spontaneously. More than 1300 natural and artificial radioisotopes have been identified.

When the chemical symbol for an element is written, a subscript is used for the number of protons (the "+" charge, on the nucleus, also called atomic number Z) and a superscript for the nuclear mass (sum of protons and neutrons). Thus an oxygen atom of 8 protons and 9 neutrons is written: "8O17," the "O" being the chemical symbol for oxygen.

Symbols for elements can be found in physics and chemistry texts and other scientific handbooks.

Particle Energy

The standard unit of energy used in atomic physics is the electron-volt, abbreviated eV. The familiar prefixes "k" for thousand and "M" for million are also used, as keV and MeV. One electron-volt is the energy imparted to an electron when accelerated through a potential of one volt. For energies W, below about 10 keV, the relation ½Mv2 is sufficiently accurate, where M0 is the "rest mass" of the particle (kilograms), v is the velocity (meters/second), e is the electron charge (1.6 X 10-19 coulomb), and V is the potential difference in volts. For particles of higher energy, the Einstein relation must be used: Relativistic Equation - RF Cafe, where c = 3 X 108 meters per second, or the velocity of light in vacuum. Therefore, we see that we cannot accelerate a particle to the speed of light for, if we try, v = c, and the denominator goes to zero, yielding an undefined value of W.

One other equation is useful, the expression W = hv. Here, h is Planck's constant, or 6.62 X 10-34 joule-second. The Greek letter v is the frequency of a wave. The French physicist, de Broglie, detailed the relationship between particles and waves but, for our purposes, it is sufficient to recognize that v = c / λ where λ is the wavelength of the radiation. Thus, for an x-ray of λ = 1.24 X 10-12 meter v = 3 X 108 meters per sec/1.24 X 10-12 meter or 2.42 X 1020 hertz. Then the energy is simply W - hv = 6.62 X 10-34 joule-second X 2.42 X 1020 per second = 1.6 X 10-13 joule, or in electron-volts (dividing by the electron charge) 106 e V, or MeV. It is useful to recognize that a volt is equivalent to one joule per coulomb.

Having thus prepared ourselves with a few simple relationships, we next examine the principal kinds of atomic radiation encountered in the laboratory and in nature.

Atomic Radiation

The four levels of atomic radiation which we will discuss are: alpha (α) particles, beta (β) particles, neutrons, and gamma (γ) and x-rays. We consider γ and x-rays as one level of radiation since they are both, essentially, high-energy rays.

The least damaging (from a biological standpoint) and the easiest to shield are α particles. Alpha particles are helium ions which have lost two electrons and thus have a charge of + (2 X 1.6 X 10-19) coulomb. A moderate energy a beam may be attenuated by placing a barrier, such as aluminum foil, in the path of the particles. Paper and cloth also provide shielding from α radiation.

Free electrons, or β rays, are more difficult to shield than α particles. They interact with the bound electrons of atoms and produce x-rays, thus creating a secondary source of radiation.

Neutron radiation, in general, requires a thicker shield than either α or β radiation. Because of the relatively high mass and volume of neutrons, interactions occur when these particles bombard matter. Materials which provide good shielding against neutron radiation are termed moderators. Moderator material in the form of rods is commonly used to control neutron flux levels in reactors. By inserting or removing control rods from a reactor core, more or fewer neutrons are permitted to interact and the reactor heat (power level) is regulated.

Gamma radiation and x-rays require the heaviest shielding. Lead is the most common material used for this application. It is convenient to think of γ and x-rays as electromagnetic energy of very short wavelength. However, all radiation (and all matter) exhibits the dual properties of particle and wave phenomena, but α, β and neutrons are considered particles, while γ and x-rays are thought of as waves. These "visualizations" are useful models, but we should not forget that they are both particle and wave.

Radiation can be generated in various ways. Some elements have isotopes which are naturally radioactive and emit radiation, while other elements have radioactive isotopes which are artificially created, or both kinds may exist. Artificial radio-isotopes are produced by the high energy bombardment of elements by particles. Usually particle accelerators are used to provide scientists with artificial isotopes.

The half-life of an isotope refers to the time required for one-half of the radioactive material to change into another elemental form. Half-lives vary with the particular isotope, and may be as short as picoseconds (10-12 second) or as long as 1012 years and more. As an example of radioactive decay consider the reaction 92U238 --> 90U234 + α + γ.

This reaction is read, "Uranium 238 (Z of 92) decays to Thorium 234 (Z of 90), yielding an α particle (2He4) ++ and γ-ray energy." Notice that the subscripts and superscripts balance, since the α particle is a helium (He) ion. The half-life of this reaction is 4.5 X 109 years, meaning that if we start today with a specific quantity of 92U238, in 4.5 X 109 years half of the U238 would still be reacting, while the other half would have degenerated into the stable atom Th234. We have omitted the subscripts the last writing since the meaning is clear. It is this kind measurement of carbon-14 content that allows geologists estimate ages of rocks.

Biological Hazards

Even the briefest of discussions of nuclear radiation must point out the safety hazards associated with these emanations. Since the human body is a complex of chemical compounds, atomic radiation interacts with body molecules to produce chemical changes. Some of these changes may beneficial, as in the case of radiological treatment of cancerous tissue. But, in general, excessive radiation causes detrimental effects in body chemistry. Often permanent changes in-cell structure result, causing, among other things, mutations in the offspring of the victim. Such mutations may take several generations to become evident.

Other serious interactions can occur in blood cells, leading a condition not unlike leukemia in symptoms. And while radiation can be used to treat cancer, radiation can also produce cancerous growths. Damage to the body organs is another biological hazard which must be avoided. Tests conducted over long periods of time have led to safety standards for radiation dosage, which we will examine after we define me of the units used in this work.

Radiation Units

The principal units used in biological radiation work are: the roentgen, the rep, the rad, and the rem.

The roentgen, named after the German physicist who at the end of the 19th century discovered x-rays, is the quantity of x or gamma radiation which will generate 2.08 X 109 ion-pairs in one cubic centimeter of air, measured at standard conditions (approximately 14.7 lbs. per square inch of atmospheric pressure and a temperature of 32°F). Ion-pairs refers to the stripping of electrons from atoms by incident radiation energy, giving rise to one ion and one electron. The roentgen (abbreviated R) does not take into account exposure time. Time is important because the longer the exposure the more damaging the radiation burn. Thus, the roentgen as a measurement unit has limited use in biological radiation work.

Making an Isotope

Several processes are used to create radioactive isotopes. The most common is the (n, γ) process where a neutron is captured by a target atom and a gamma photon emitted immediately. Since there is no change in the atomic number, the resultant element remains the same as the target material. In the (n, p) process, the neutron entering the target material has sufficient energy to cause a proton to be released. Therefore, the atomic number is changed by 1 and the affected atom transmuted into a different element. On the other hand, the capture of a high-energy neutron in the (η, α) process causes an alpha particle to be emitted and the atomic number reduced by 2. In the fission process, several isotopes of an element can be produced. Typically, these are fragments of uranium atoms which have undergone fission (radioactive atoms from atomic numbers 30 through 64). -Editor

The rep, or roentgen equivalent physical, relates radiation to ionization in tissue and yields a measurement which is more meaningful in human exposure. The rep is approximately equivalent to 1.1 times the energy intake of tissue as compared to air. Hence, if in a given radiation flux, the air absorbs x energy units, tissue will absorb x times 1.1 energy units. But, again, time is not included.

The rad, or radiation absorbed dose, is a measure of energy absorption in any material and is equivalent to approximately 1.2 times the energy intake of the medium as compared to air.

The rem, or roentgen equivalent man, is the quantity of any type of radiation which will produce the same biological action in man as the absorption of 1 roentgen of x-irradiation. The rem may be calculated by multiplying the roentgen level by certain constants which depend upon the type of radiation involved. These factors, called the relative biological effectiveness (RBE) factor, vary from one to 20 or more. Thus 1 roentgen of x-ray or gamma radiation has an RBE of 1, while a 5 MeV neutron has an RBE of 10. So 1 R of x-ray radiation is equivalent of 1 rem, while 1 R of the 5 MeV neutron radiation is 1 X 10 = 10 rem.

Since safety levels or radiation depend upon time exposure as well as the area of the body exposed, there is some variation in the level of absorbed radiation permitted. For safety sake, those who work in radiation areas should be cognizant of the Atomic Energy Commission's Standards for radiation protection.

 

 

Posted January 11, 2018

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