May 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
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,
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 -
Relationships, Part 2 -
Methods, and Part 3 -
Atomic Radiation: Types & Relationships - Part 1
Fig. 1 - Conceptual model of the helium atom. The nucleus
is in the center and the electrons orbit in shells around orbit.
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.
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
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:
, 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.
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,
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
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
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.
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
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.
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 June 8, 2023
(updated from original post