Isotope, Radioactivity and Half-life
Isotope is one of two or more species of atom having the same atomic number, hence constituting the same element, but differing in mass number. As atomic number is equivalent to the number of protons in the nucleus, and mass number is the sum total of the protons plus the neutrons in the nucleus, isotopes of the same element differ from one another only in the number of neutrons in their nuclei.
Research
Experiments carried out early in the 20th century indicated that radioactive substances
that were chemically inseparable might differ from each other only in the structure of
their nuclei. The British physicist Sir Joseph Thomson demonstrated in 1912 the existence
of stable isotopes by passing neon through a discharge tube and deflecting the neon ions
by means of magnetic and electric fields; this showed that the stable element neon exists
in more than one form. Thomson found two isotopes of neon, one of mass number 20 and
another of mass 22. Later experiments showed that naturally occurring neon contains 90% of
neon-20 (the isotope with mass 20), 9.73% neon-22, and 0.27% neon-21. Research on isotopes
was continued by many scientists, notably the British physicist Francis William Aston;
their work in detecting and studying isotopes was accelerated by the development of the
mass spectrograph.
It is now known that most elements in the natural state consist of a mixture of two or
more isotopes. Among the exceptions are beryllium, aluminum, phosphorus, and sodium. The
chemical atomic weight (atomic wt.) of an element is the weighted average of the
individual atomic weights, or mass numbers, of the isotopes. For example, chlorine, atomic
wt. 35.457, is composed of chlorine-35 and chlorine-37, the former occurring with an
abundance of 76% and the latter of 24%. All the isotopes of elements with atomic numbers
higher than 83 (above bismuth in the periodic table) are radioactive, and a few of the
lighter isotopes, such as potassium-40, are radioactive. About 280 naturally occurring
stable isotopes (not radioactive) are known.
Artificial radioactive isotopes, known also as radioisotopes, were produced for the first
time in 1933 by the French physicists Irène and Frédéric Joliot-Curie. Radioisotopes
are prepared by the bombardment of naturally occurring atoms with nuclear particles, such
as neutrons, electrons, protons, and alpha particles using particle accelerators.
Separation
The separation of isotopes of the same element from each other is difficult. Full
separation in one step by chemical methods is impossible, because isotopes of the same
elements have the same chemical properties; physical methods are generally based on the
extremely small differences in physical properties caused by the differences in mass of
the isotopes. Electrolytic separation and various exchange procedures for isotope
separation, however, depend on chemical rate or equilibrium differences that are based
primarily on the difference in energy of chemical bonds, which are a function of isotope
mass. The isotopes of hydrogen, deuterium (hydrogen-2) and ordinary hydrogen (hydrogen-1)
were the first to be separated in appreciable quantities. This accomplishment is credited
to the American chemist Harold Urey, who discovered deuterium in 1932.
Before 1940 many methods were used for the separation of small amounts of isotopes for
research purposes. Some of the most successful were the centrifuge method, fractional
distillation, thermal diffusion, electrolysis, gaseous diffusion, and electromagnetic
separation. Each of these methods depends on the small difference in weight of the
isotopes to be separated, and is most effective with the hydrogen isotopes, where the
difference in mass between the two substances amounts to 100%; by contrast, the difference
in mass between the carbon isotopes carbon-12 and carbon-13 or between the neon isotopes
neon-20 and neon-22 amounts to only about 10%, and between the uranium isotopes
uranium-235 and uranium-238 to only a little over 1%. This factor of 10 to 1 or 100 to 1
makes the separation far more than 10 or 100 times as difficult. In all processes except
the electromagnetic, which is the sole one-stage procedure, isotope separation involves a
series of production stages. The net result of any single stage is the separation of the
original material into two fractions, one of which contains a slightly higher percentage
of the heavy isotope than the original mixture and the other contains slightly more of the
light isotope.
To obtain an appreciable concentration, or enrichment, in the desired isotope, it is
necessary to separate further the enriched fraction. This process is usually carried out
by means of a cascade, comprising a large number of stages. The enriched fraction from any
stage becomes the raw material for the next stage, and the depleted fraction, which still
contains a considerable percentage of the desired isotope, is mixed with the raw material
for the preceding stage. Even the depleted material from the original stage is stripped in
additional stages when the raw material (for example, uranium) is scarce. Suitable
apparatus is designed to make the flow from stage to stage automatic and continuous.
Such a cascade is extremely flexible, and units can be shifted from one stage of the
separation to another as desired. For example, in the separation of uranium, a large
amount of material must be handled at the beginning, where the desired uranium-235 is
mixed with about 140 times as much uranium-238; at the end of the process the uranium-235
is almost pure, and the volume of material is much smaller. Furthermore, by merely
changing the piping, it is possible to shift stages to compensate for addition at an
intermediate stage of material that results from preliminary enrichment by a different
process.
Centrifuge and Distillation
In the centrifuge method the apparatus is so arranged that vapor flows downward in the
outer part of the rotating cylinder and upward in the central region of the cylinder. The
centrifugal force acts more strongly on the heavy molecules than on the light ones,
increasing the concentration of the heavy isotopes in the outer region. In separation by
fractional distillation a mixture containing various isotopes is distilled. The molecules
of the fraction having the lower boiling point (the lighter isotopes) tend to concentrate
in the vapor stream and are collected.
Thermal Diffusion
This method utilizes the tendency of lighter molecules of a liquid or gas to concentrate
in a hot region and for heavier molecules to concentrate in a cold region. A simple form
of thermal-diffusion apparatus consists of a tall vertical tube with a wire electrically
heated to about 500° C (932° F) running down its center, producing a temperature
gradient between the center and wall of the tube. The heavier isotopes tend to concentrate
in the outer portions of the tube, and the lighter isotopes, to concentrate toward the
center. At the same time, because of thermal convection, the gas or liquid near the wire
tends to rise, and the cooler outer gas or liquid tends to fall. The overall effect is
that the heavier isotopes collect at the bottom of the tube and the lighter at the top.
Electrolysis
The electrolytic method of separation is of historical as well as current interest,
because it was the first method used to separate practically pure deuterium. This method
depends on the fact that when water undergoes electrolysis, the lighter hydrogen isotope
tends to come off first, leaving behind a residue of water that is enriched in the heavier
isotope.
Gaseous Diffusion
This and the electromagnetic method of separating isotopes of uranium afforded the first
large-scale separation ever achieved. The problem of separating uranium-235 from
uranium-238 arose in 1940 after the demonstration of the susceptibility of the 235 isotope
to fission by neutrons. Uranium-235 exists in naturally occurring uranium to the extent of
7 parts to 1000 of uranium-238. Under the auspices of the atomic bomb project, the various
methods for separating isotopes were considered, and the gaseous-diffusion and
electromagnetic methods were put into large-scale operation for the production of about 1
kg (2.2 lb) per day of uranium-235 to be used in atomic bombs. See NUCLEAR WEAPONS.
The gaseous-diffusion method exploits the different rate of diffusion of gases of
different molecular weight. The rate of diffusion of a gas is inversely proportional to
the square root of the mass; light atoms diffuse through a porous barrier faster than
heavier atoms. In the separation of uranium isotopes, the only gaseous compound of
uranium, the fluoride of uranium, UF6, is used. The uranium hexafluoride is pumped
continuously through porous barriers. The difference in weight between uranium-235 and
uranium-238 is slightly greater than 1%, but the difference in weight between the
fluorides is slightly less than 1%. The enrichment factor, which depends on the square
root of the above difference, is theoretically 0.43% for an instantaneous process or 0.30%
for a continuous process, but in practice an enrichment factor of only about 0.14% per
stage has been achieved. To produce 99% uranium-235 from natural uranium, which contains
about 0.7% uranium-235, 4000 such stages are required. The process requires the use of
thousands of miles of pipe, thousands of pumps and motors, and intricate control
mechanisms.
Electromagnetism
Although the gaseous-diffusion method yields large amounts of uranium-235, the first
comparatively large amounts of the isotope were produced by electromagnetic means at Oak
Ridge, Tennessee. A series of separator units was built in which an ionic beam obtained
from a uranium compound was passed through a magnetic field. Because the radius of the
curvature of the path of the ions deflected by the beam depends on the mass of the ion,
ions of different mass complete their path at different positions, and the uranium
isotopes are appreciably separated. Only a small amount of material, however, can be
treated in one operation. Because of this limitation on production, the use of the
electromagnetic process for large-scale isotope separation was abandoned after the war in
favor of the gaseous-diffusion process.
Laser Beam
The concept of laser separation and enrichment of isotopes arose soon after the invention
of the laser in 1960. It gained further incentive six years later with the development of
the tunable dye laser, which provides photon beams in a selectably narrow range of
infrared to ultraviolet wavelengths. According to this concept, if an element is first
vaporized its atoms can then be selectively excited and ionized by an accurately tuned
laser beam to separate out the desired isotope. Isotopes can also be separated in
molecular form by selective laser-beam dissociation of those molecules of the compound
that contain the desired isotope. Since 1972 such processes have been under development,
particularly for uranium and plutonium enrichment—for nuclear power and nuclear
weapons, respectively. Much of the work in the U.S. is classified, but a pilot plant may
be operational by the later 1980s. The method is costly and technically difficult, but
only a few stages are required for production of highly enriched material.
Radioactivity, spontaneous disintegration of atomic nuclei by the emission of subatomic particles called alpha particles and beta particles (see ALPHA PARTICLE; BETA PARTICLE), or of electromagnetic rays called X rays (see X RAY) and gamma rays. The phenomenon was discovered in 1896 by the French physicist Antoine Henri Becquerel when he observed that the element uranium can blacken a photographic plate, although separated from it by glass or black paper. He also observed that the rays that produce the darkening are capable of discharging an electroscope, indicating that the rays possess an electric charge. In 1898 the French chemists Marie Curie and Pierre Curie deduced that radioactivity is a phenomenon associated with atoms, independent of their physical or chemical state. They also deduced that because the uranium-containing ore pitchblende is more intensely radioactive than the uranium salts that were used by Becquerel, other radioactive elements must be in the ore. They carried through a series of chemical treatments of the pitchblende that resulted in the discovery of two new radioactive elements, polonium and radium. Marie Curie also discovered that the element thorium is radioactive, and in 1899 the radioactive element actinium was discovered by the French chemist André Louis Debierne. In that same year the discovery of the radioactive gas radon was made by the British physicists Ernest Rutherford and Frederick Soddy, who observed it in association with thorium, actinium, and radium.
Radioactivity was soon recognized as a more concentrated source of energy than had been
known before. The Curies measured the heat associated with the decay of radium and
established that 1 g (0.035 oz) of radium gives off about 100 cal of energy every hour.
This heating effect continues hour after hour and year after year, whereas the complete
combustion of a gram of coal results in the production of a total of only about 8000 cal
of energy. Radioactivity attracted the attention of scientists throughout the world
following these early discoveries. In the ensuing decades many aspects of the phenomenon
were thoroughly investigated.
Types of Radiations
Rutherford discovered that at least two components are present in the radioactive
radiations: alpha particles, which penetrate into aluminum only a few thousandths of a
centimeter, and beta particles, which are nearly 100 times more penetrating. Subsequent
experiments in which radioactive radiations were subjected to magnetic and electric fields
revealed the presence of a third component, gamma rays, which were found to be much more
penetrating than beta particles. In an electric field the path of the beta particles is
greatly deflected toward the positive electric pole, that of the alpha particles to a
lesser extent toward the negative pole, and gamma rays are not deflected at all.
Therefore, the beta particles are negatively charged, the alpha particles are positively
charged and are heavier than beta particles, and the gamma rays are uncharged.
The discovery that radium decayed to produce radon proved conclusively that radioactive
decay is accompanied by a change in the chemical nature of the decaying element.
Experiments on the deflection of alpha particles in an electric field showed that the
ratio of electric charge to mass of these particles is about twice that of the hydrogen
ion (see ION). Physicists supposed that the particles could be doubly charged ions of
helium (helium atoms with two electrons removed). This supposition was proved by
Rutherford when he allowed an alpha-emitting substance to decay near an evacuated
thin-glass vessel. The alpha particles were able to penetrate the glass and were then
trapped in the vessel, and within a few days the presence of elemental helium was
demonstrated by use of a spectroscope. Beta particles were subsequently shown to be
electrons (see ELECTRON), and gamma rays to consist of electromagnetic radiation of the
same nature as X rays but of considerably greater energy.
The Nuclear Hypothesis
At the time of the discovery of radioactivity physicists believed that the atom was the
ultimate, indivisible building block of matter. The recognition of alpha and beta
particles as discrete units of matter and of radioactivity as a process by means of which
atoms are transformed into new kinds of atoms possessing new chemical properties because
of the emission of one or the other of these particles brought with it the realization
that atoms themselves must have structure and that they are not the ultimate, fundamental
particles of nature. In 1911 Rutherford proved the existence of a nucleus within the atom
by experiments in which alpha particles were scattered by thin metal foils (see ATOM AND
ATOMIC THEORY). The nuclear hypothesis has since grown into a refined and fully accepted
theory of atomic structure, in terms of which the entire phenomenon of radioactivity can
be explained. Briefly, the atom is thought to consist of a dense central nucleus
surrounded by a cloud of electrons. The nucleus, in turn, is composed of protons (see
PROTON) equal in number to the electrons (in an electrically neutral atom), and neutrons
(see NEUTRON). An alpha particle, or doubly charged helium ion, consists of two neutrons
and two protons, and hence can be emitted only from the nucleus of an atom. Loss of an
alpha particle by a nucleus results in the formation of a new nucleus, lighter than the
original by four mass units (the masses of the neutron and of the proton are about one
unit each). An atom of the uranium isotope of mass 238, upon emitting an alpha particle,
becomes an atom of another element of mass 234. Each of the two protons that form part of
the alpha particle emitted from an atom of uranium-238 possesses a unit of positive
electric charge. The number of positive charges in the nucleus, balanced by the same
number of negative electrons in the orbits outside the nucleus, determines the chemical
nature of the atom. Because the charge on the uranium-238 nucleus decreases by two units
as a result of alpha emission, the atomic number of the resultant atom is 2 less than that
of the original, which was 92. The new atom has an atomic number of 90 and hence is an
isotope of the element thorium.
Thorium-234 emits beta particles, which are electrons. According to current theory, beta
emission is accomplished by the transformation of a neutron into a proton, thus resulting
in an increase in nuclear charge (or atomic number) of one unit. The mass of the electron
is negligible, thus the isotope that results from thorium-234 decay has mass number 234
but atomic number 91 and is, therefore, a protactinium isotope.
Gamma Radiation
Gamma emission is usually found in association with alpha and beta emission. Gamma rays
possess no charge or mass; thus emission of gamma rays by a nucleus does not result in a
change in chemical properties of the nucleus but merely in the loss of a certain amount of
radiant energy. The emission of gamma rays is a compensation by the atomic nucleus for the
unstable state that follows alpha and beta processes in the nucleus. The primary alpha or
beta particle and its consequent gamma ray are emitted almost simultaneously. A few cases
are known of pure alpha and beta emission, however, that is, alpha and beta processes
unaccompanied by gamma rays; a number of pure gamma-emitting isotopes are also known. Pure
gamma emission occurs when an isotope exists in two different forms, called nuclear
isomers, having identical atomic numbers and mass numbers, but different in nuclear-energy
content. The emission of gamma rays accompanies the transition of the higher-energy isomer
to the lower-energy form. An example of isomerism is the isotope protactinium-234, which
exists in two distinct energy states with the emission of gamma rays signaling the
transition from one to the other.
Alpha, beta, and gamma radiations are all ejected from their parent nuclei at tremendous
speeds. Alpha particles are slowed down and stopped as they pass through matter, primarily
through interaction with the electrons present in that matter. Furthermore, most of the
alpha particles emitted from the same substance are ejected at very nearly the same
velocity. Thus nearly all the alpha particles from polonium-210 travel 3.8 cm through air
before being completely stopped, and those of polonium-212 travel 8.5 cm under the same
conditions. Measurement of distance traveled by alpha particles is used to identify
isotopes. Beta particles are ejected at much greater speeds than alpha particles, and thus
will penetrate considerably more matter, although the mechanism by means of which they are
stopped is essentially similar. Unlike alpha particles, however, beta particles are
emitted at many different speeds, and beta emitters must be distinguished from one another
through the existence of the characteristic maximum and average speeds of their beta
particles. The distribution in the beta-particle energies (speeds) necessitates the
hypothesis of the existence of an uncharged, massless particle called the neutrino, and
neutrino emission is now thought to accompany all beta decays.
Gamma rays have ranges several times greater than those of beta particles and can in some cases pass through several inches of lead. Alpha and beta particles, when passing through matter, cause the formation of many ions; this ionization is particularly easy to observe when the matter is gaseous. Gamma rays are not charged, and hence cannot cause such ionization directly, but when they interact with matter they cause the ejection of electrons from atoms; the atoms minus some of their electrons are thereby ionized. Beta rays produce @ to A of the ionization generated by alpha rays per centimeter of their path in air. Gamma rays produce about @ of the ionization of beta rays. The Geiger-Müller counter and other ionization chambers , which are based on these principles, are used to detect the amounts of individual alpha, beta, and gamma rays, and hence the absolute rates of decay of radioactive substances. One unit of radioactivity, the curie, is based on the decay rate of radium-226, which is 37 billion disintegrations per sec per g of radium.
Modes of radioactive decay, other than the three above mentioned, exist. Some isotopes are
capable of emitting positrons, which are identical with electrons but opposite in charge.
The positron-emission process is usually classified as a beta decay and is termed
beta-plus emission to distinguish it from the more common negative-electron emission.
Positron emission is thought to be accomplished through the conversion, in the nucleus, of
a proton into a neutron, resulting in a decrease of the atomic number by one unit. Another
mode of decay, known as K-electron capture, consists of the capture of an electron by the
nucleus, followed by the transformation of a proton to a neutron. The net result is thus
also a decrease of the atomic number by one unit. The process is observable only because
the removal of the electron from its orbit results in the emission of an X ray. In recent
years it has been shown that a number of isotopes, notably uranium-235 and several
isotopes of the artificial transuranium elements, are capable of decaying by a
spontaneous-fission process, in which the nucleus is split into two fragments. In the
mid-1980s a unique decay mode was observed, in which isotopes of radium of masses 222,
223, and 224 emit carbon-14 nuclei rather than decaying in the usual way by emitting alpha
radiation.
Half-life of Uranium
Half-life is the length of time required for half of a given number of initial number of atoms of that isotope to decay.
Example of three different isotopes of uranium and its half-life period is given below.
uranium-234: half life = 244 thousand years, 0.0055% of all uranium.
uranium-235: half life = 704 million years, 0.72% of all uranium.
uranium-238: half life = 4.5 billion years, 99.28% of all uranium.
The decay of some substances, such as uranium-238 and thorium-232, appears to continue
indefinitely without detectable diminution of the decay rate per unit mass of the isotope
(specific-decay rate). Other radioactive substances show a marked decrease in
specific-decay rate with time. Among these is the isotope thorium-234 (originally called
uranium X), which, after isolation from uranium, decays to half its original radioactive
intensity within 25 days. Each individual radioactive substance has a characteristic decay
period or half-life; because their half-lives are so long that decay is not appreciable
within the observation period, the diminution of the specific-decay rate of some isotopes
is not observable under present methods. Thorium-232, for example, has a half-life of 14
billion years.
Radioactive Decay Series
When uranium-238 decays by alpha emission, thorium-234 is formed; thorium-234 is a beta
emitter and decays to form protactinium-234. Protactinium-234 in turn is a beta emitter,
forming a new isotope of uranium, uranium-234. Uranium-234 decays by alpha emission to
form thorium-230, which decays in turn by alpha emission to yield the predominant isotope,
radium-226. This radioactive decay series, called the uranium-radium series, continues
similarly through five more alpha emissions and four more beta emissions until the end
product, a nonradioactive (stable) isotope of lead (element 82) of mass 206 is reached.
Every element in the periodic table between uranium and lead is represented in this
series, and each isotope is distinguishable by its characteristic half-life. The members
of the series all share a common characteristic: Their mass numbers can be made exactly
divisible by four if the number 2 is subtracted from them, that is, their mass numbers can
be expressed by the simple formula 4n + 2, in which n is a whole number. Other natural
radioactive series are the thorium series, called the 4n series, because the mass numbers
of all its members are exactly divisible by four, and the actinium series, or 4n + 3
series. The parent of the thorium series is the isotope thorium-232, and its final product
is the stable isotope lead-208. The actinium series begins with uranium-235 (named
actinouranium by early investigators) and ends with lead-207. A fourth series, the 4n + 1
series, all the members of which are artificially radioactive, has in recent years been
discovered and thoroughly characterized. Its initial member is an isotope of the synthetic
element curium, curium-241. It contains the longest-lived isotope of the element
neptunium, and its final product is bismuth-209.
An interesting application of knowledge of radioactive elements is made in determining the
age of the earth. One method of determining geologic time is based on the fact that in
many uranium and thorium ores, all of which have been decaying since their formation, the
alpha particles have been trapped (as helium atoms) in the interior of the rock. By
accurately determining the relative amounts of helium, uranium, and thorium in the rock,
the length of time during which the decay processes have been going on (the age of the
rock) can be calculated. Another method is based on the determination of the ratio of
uranium-238 to lead-206 or of thorium-232 to lead-208 in the rocks (that is, the ratios of
concentration of the initial and final members of the decay series). These and other
methods give values for the age of the earth of between 3 billion and 5 billion years.
Similar values are obtained for meteorites that have fallen to the surface of the earth,
as well as samples of the moon brought back by Apollo 11 in July 1969, indicating the
possibility that the entire solar system could be about the same age as the earth.
Artificial Radioactivity
All the naturally occurring isotopes above bismuth in the periodic table are radioactive
and in addition naturally radioactive isotopes of bismuth, thalium, vanadium, indium,
neodymium, gadolinium, hafnium, platinum, lead, rhenium, lutetium, rubidium, potassium,
hydrogen, carbon, lanthanum, and samarium exist. In 1919 Rutherford carried out the first
nuclear reaction when he bombarded ordinary nitrogen gas (nitrogen-14) with alpha
particles and found that the nitrogen nuclei captured alpha particles and emitted protons
very rapidly, forming a stable isotope of oxygen, oxygen-17. This reaction can be written
symbolically as wN + nHe ± xO + eH
in which the atomic numbers of the participating nuclei are conventionally written below
and to the left of the chemical symbols and their mass numbers above and to the left. In
the above reaction the alpha particle is shown as a helium nucleus and the proton as a
hydrogen nucleus.
Not until 1933 was it demonstrated that such nuclear reactions could sometimes result in
the formation of new radioactive nuclei. The French chemists Irène and Frédéric
Joliot-Curie prepared the first artificially radioactive substance in that year when they
bombarded aluminum with alpha particles. The aluminum nuclei captured alpha particles and
then emitted neutrons with the consequent formation of an isotope of phosphorus, which
decayed by positron emission with a short half-life. They also produced an isotope of
nitrogen from boron and one of aluminum from magnesium. Since that time a great many
nuclear reactions have been discovered, and the nuclei of elements throughout the periodic
table have been bombarded with different particles, including alpha particles, protons,
neutrons, and deuterons (ions of the hydrogen isotope of mass 2). As a result of this
intensive investigation, more than 400 artificial radioactivities are now known. This
research has been aided immeasurably by the development of particle accelerators that
accelerate the bombarding particles to enormous speeds, thus in many cases increasing the
probability of their capture by the target nuclei.
The vigorous investigation of nuclear reactions and the search for new artificial
radioactivities, especially in connection with the search for such activities among the
heavier elements, was responsible for the discovery of nuclear fission and the subsequent
development of the atomic bomb. The investigations have also resulted in the discovery of
several new elements that do not exist in nature. The development of nuclear reactors has
made possible the production on a large scale of radioactive isotopes of nearly all the
elements of the periodic table, and the availability of these isotopes is an incalculable
aid to chemical research and to biological and medical research. Of great importance among
the artificially produced radioactive isotopes is an isotope of carbon, carbon-14, which
has a half-life of about 5730 ± 40 years. The availability of this substance has made
possible the investigation of numerous aspects of life processes, such as the process of
photosynthesis, in a more fundamental manner than hitherto considered possible.
Scientists have recently shown that a very minute but unchanging amount of carbon-14 is
present in the atmosphere of the earth and that all livIng organisms assimilate traces of
this isotope during their lifetime. After death this assimilation ceases and the
radioactive carbon, constantly decaying, is no longer maintained at a steady
concentration. Estimation of the ages of a number of objects, such as bones and mummies,
of historical and archaeological interest has been made possible by carbon-14
measurements.
In neutron-activation analysis, a sample of a substance is made radioactive in a nuclear
reactor. A number of impurities that cannot be detected by other means can then be found
by detecting the particular types of radioactivity that are associated with radioisotopes
of these impurities. Other applications of radioactive isotopes are in medical therapy,
industrial radiography, and specific devices such as phosphorescent light sources, static
eliminators, thickness gauges, and nuclear batteries.
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