Radiation
Meters General Information
Interpreting Readings
Health physics, the field that pertains to radiation and its effects
on man, is very complex, and theories and conclusions are constantly being updated as
information becomes available. Data from occupational exposure, animal studies, and events
like Hiroshima and Nagasaki have fairly well established the maximum safe exposure limits
for man. Whether low level radiation causes cancer and birth defects is still being
debated. Delayed effect, which could take years to develop, is difficult to study and
therefore, there are no well-defined lower limits on ionizing radiation. Two publications
entitled "Hormesis with Ionizing Radiation," 1980 and "Radiation
Hormesis," 1991 (CRC Press, Boca Raton) present over one thousand examples of
statistically valid data showing no physiological harm in vertebrates from whole body
exposures to low dose radiation (<20mgy/y).
The units
mR/hr (milli-Roentgen per hour, or 1/1000th of a Roentgen
per hour) pertain only to gamma radiation. Often other units of measurement similar to
mR/hr are used. The term "REM" (Roentgen Equivalent Man) includes the effects of
beta, alpha and neutron radiation. Measurement in REMs is more complete as radiation
affects man, but such measurements are a complicated combination of many measurements each
made with specialized detectors.
It is important to note that the field intensity from a radioactive
object decreases very quickly with distance.
If the object is very small, increasing the distance from the object
by a factor of two decreases the radiation level by a factor of four. This is called a
square law situation, which demonstrates the dependence of proximity on dose for small
radioactive sources. Larger sources, such as a large deposit of radioactive minerals, will
show much less of this effect. In trying to estimate the danger of radioactive materials,
it is important to take into account many aspects of the situation. For instance, the
radiation level at the face of a radium-dial watch may be 3mR/hr, but the measurement
taken from the back of the watch may be 0.3mR/hr.
Another interesting point concerns the energy of the radiation.
Geiger Counters will register one click whenever they detect a ray or particle of
radiation hitting them. These tiny high speed bundles of energy are like short bursts of
light. Some are extremely energetic, while others are not. Geiger Counters cannot
determine the energy of the impinging ray, they only detect its presence. Sper Scientific
models 840007 and 840026
detect Beta and gamma radiation starting at approximately 30KeV and up to 1.5
MeV.
The opposite is the case for cosmic rays, which have enormous
energy—some millions of times more energetic than anything found here on earth. The
compensation figure for radiation of this type is difficult to estimate, due to the
extreme range of energies that have been measured.
Radiation - What is it?
Nuclear physics is a very complex field; however, the basic
principles can be simply explained.
All matter is composed of atoms. Atoms alone and bonded together in
molecules form all the things around us, including ourselves. These atomic units are
extremely small; so small, in fact, that a single grain of table salt contains
approximately 1,000,000,000,000,000,000 atoms (this is not a misprint). It is impossible
to see an atom, except with a sophisticated electron microscope, so many of our present
day theories on the structure and composition of single atoms are based largely on the
study of radiation given off from unstable (radioactive) substances.
Atoms are composed of three basic particles: protons, neutrons and
electrons. Electrons are extremely light, negatively charged particles that exist as a
cloud around the center, or nucleus, of the atom. Sometimes the electrons are said to
occupy orbits around the nucleus. These electrons are attracted to the nucleus because of
the positively charged protons that, along with the neutrons, make up the nucleus. Atoms
bond together in molecules when one atom gives up or shares an electron with another atom.
Chemical reactions utilize this bonding process.
In all atoms, the number of electrons (and therefore the number of
negative charges) equals the number of protons (positive charges). The number of protons
or electrons in an atom determines the chemical nature of the atom, and each element has
its own unique number (example: hydrogen=1, helium=2, etc.). The number of neutrons,
however, may not always be the same in every atom of a particular element. Atoms of an
element with different numbers of neutrons are called isotopes. Every atom of a particular
element has the same atomic number, but different isotopes of a given element have
different atomic weights.
It is the variable number of neutrons in the nucleus of an atom that
leads to a process called nuclear decay that causes radiation. When an atom has too many
or too few neutrons in its nucleus, it will have a tendency to rearrange itself
spontaneously into a new combination of particles that are more stable. In this decay
process, bundles of excess energy are shot out of the nucleus in one of a number of
ways.
When the neutrons are excessive, a neutron can convert itself to a
proton and shoot out an electron at very high speed, known as beta radiation.
A proton may be converted to a neutron to cause an unusual particle
called a positron to be ejected from the nucleus.
In still another process, the nucleus, in a vain attempt to
stabilize itself, kicks out two protons and two neutrons all together as one particle,
called an alpha particle.
The energy released in each decay can be enormous. This decay
process is utilized in atomic reactors and bombs. When certain very heavy isotopes of
uranium or plutonium (or several other isotopes) decay, they may split apart. This process
is called fission. In fission, the entire nucleus splits apart, causing two new atoms and
releasing a very large amount of energy. This process is not very predictable, for the
nucleus can split in many ways, yielding a wide variety of new atoms and even some free
neutrons. The free neutrons, when released, can be absorbed by other fuel atoms, causing
them, in turn, to fission—leading to a continuous or (if not controlled) explosive
chain reaction. Due to the wide range of new atoms produced in the fission process, many
of the daughter products are not stable and will, in turn, decay themselves, leading to
hazardous nuclear waste and fallout.
In all of the above processes, another kind of radiation, gamma, is
almost always released. Unlike the particles previously mentioned, gamma radiation
consists of tiny discrete bundles of energy called quanta. Light, X-rays and gamma rays
can all be described as quanta, the difference being the total energy packed into each
bundle.
In nuclear decay some energy in the unstable nucleus is dissipated
to its surroundings in the form of heat and radiation in the instant that it decays. The
nucleus may remain in its unstable state for billions of years, and then suddenly decay
spontaneously. The time required for half of the atoms of a particular isotope to decay is
called the half-life of that isotope. For an isotope with a half-life of one year, the
pure isotope substance would be only 50% pure after one year, half of the original atoms
having decayed into some other substance. After another year, 25% of the original material
would remain, and so on. Natural radioactive materials in our world are only those with
very, very long half-lives. Uranium-238, for example, has a half-life of 4 billion years,
and exists today only because not enough time has elapsed since its creation for it to
decay away to negligible levels. It is thought that the universe was created from a huge
mass of subatomic particles and energy—the Big Bang Theory.
Of the elements and their isotopes that constitute our planet, the
vast majority are quite stable, the result of billions of years of nuclear decay. The
amount of radiation given off from natural radioactive minerals in the earth's crust is a
major constituent of background radiation. For the most part, it is quite low, due to the
long time required for the remaining radioisotopes to decay. In atomic reactions (either
natural or forced by man) the decay process is sped up by the effect of neutrons given off
in the fission process interacting with more unstable isotopes to cause immediate decay.
While this allows the energy of the isotope to be harvested in a conveniently short time,
the unstable decay products produced generally have short half-lives, on the order of
seconds to centuries, and are very radioactive. As a result of this process, considerable
larger quantities of short half-life (high decay rate) isotopes become a part of the world
we live in. This is the basis for the controversy and concerns on the subject of nuclear
power generation, waste disposal and nuclear weapons.
Interaction of Radiation with Matter
The particles and photons that result from nuclear decay carry most
of the energy released from the original unstable nucleus. The value of this energy is
expressed in electron Volts, or eV. The energy of beta and alpha rays is invested in the
particles' speed. A typical beta particle from Cesium-137 has an energy of about 500,000
eV, and a speed that approaches that of light. Beta energies can cover a wide range, and
many radioisotopes are known to emit betas at energies in excess of 10 million
eV. The
penetration range of typical beta particles is only a few millimeters in human skin.
Alpha particles have even shorter penetration ranges than beta
particles. Typical alpha energies are on the order of 5 million eV, with ranges so short
that they are extremely difficult to measure. Alphas are stopped by a thin sheet of paper,
and in air only travel a few inches at most before coming to a stop. Therefore, alpha
particles cannot be detected without being in close contact with the source, and even then
only the alphas coming from the surface of the source can be detected. Alphas generated
within the source are absorbed before reaching the surface. Due to short range, alpha
particles are not a serious health hazard unless they are emitted from within the body
when their high energy, in close contact with sensitive living tissue, is an extreme
hazard. Fortunately, almost all alpha-emitting substances also emit gamma rays, allowing
for their detection.
Neutrons, having no net charge, do not interact with matter as
easily as other particles, and can drift through great thickness of material without
incident. A free neutron, drifting through space, will decay in an average of 11.7
minutes, yielding a proton and an electron (beta ray). The neutron can also combine with
the nucleus of an atom, if its path carries it close enough. When a neutron is absorbed
into a nucleus, it is saved from its ultimate fate (decay), but may render the nucleus
unstable. This absorption process is used in medicine and industry to create radioactive
elements from non-radioactive ones. Detecting neutrons is specialized and beyond the scope
of typical Geiger counters, but most possible neutron sources also emit gamma and beta
radiation, affording detection of the source.
The highly energetic X-ray and gamma rays lose their energy as they
penetrate matter. X-rays have an energy of up to about 200,000 eV, compared to gamma
radiation which can be as energetic as several million eV. One million eV gamma radiation
can penetrate an inch of steel. Gamma and X-ray radiation are by far the most penetrating
of all common types, and are only effectively absorbed by large amounts of heavy, dense
material of high atomic number, such as lead.
International System of Units
In 1975, the International Commission of Radiation Units and
Measurements (ICRU) recommended that the becquerel be adopted as the standard unit for the
measurement of radioactivity and be included in the International System of Units
(SI). Though many countries have already adopted the becquerel as the standard unit
of radioactivity measurement, ICN has chosen to indicate size quantities with curies
(Ci)
as the measurement standard while indication specific activities in both curies and
becquerels. Furthermore, simple-to-use conversion charts have been provided.
Conversion Table
Curies to Becquerels mCi to kBq mCi
to MBq Ci to GBq |
Curies to Becquerels mCi to MBq mCi
to GBq Ci to TBq |
|
1
|
37
|
35
|
1.29
|
|
2
|
74
|
40
|
1.48
|
|
3
|
111
|
45
|
1.66
|
|
4
|
148
|
50
|
1.85
|
|
5
|
185
|
55
|
2.03
|
|
6
|
222
|
60
|
2.22
|
|
7
|
259
|
65
|
2.4
|
|
8
|
296
|
70
|
2.59
|
|
9
|
333
|
75
|
2.77
|
|
10
|
370
|
80
|
2.96
|
|
15
|
555
|
85
|
3.14
|
|
20
|
740
|
90
|
3.33
|
|
25
|
925
|
95
|
3.51
|
|
30
|
1110
|
100
|
3.70
|
Converting SI
units/non-SI units
|
From
|
To
|
Multiply By
|
|
becquerel
(Bq)
|
curie
|
2.7x10-11
|
|
curie
(Ci)
|
becquerel
|
3.7x1010
|
|
gray
(Gy)
|
rad
|
100
|
|
rad
(rad)
|
gray
|
0.01
|
|
sievert
(Sv)
|
rem
|
100
|
|
rem
(rem)
|
sievert
|
0.010
|
