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APPENDIX D
Mass, charge, and velocity of a
particle, as well as the electron density of the material with which it
interacts, all affect the rate at which ionization occurs. The higher the charge of the particle and the lower
the velocity, the greater the propensity to cause ionization. Heavy, highly charged particles, such as alpha
particles, lose energy rapidly with distance and, therefore, do not penetrate deeply. The result of these
interaction processes is a gradual slowing down of any incident particle until it is brought to rest or
"stopped" at the end of its range.
D.2.4 Characteristics of Emitted Radiation
D.2.4.1 Alpha Emission.
In alpha emission, an alpha particle consisting of two
protons and two
neutrons is emitted with a resulting decrease in the atomic mass number by four and reduction of the
atomic number of two, thereby changing the parent to a different element. The alpha particle is identical
to a helium nucleus consisting of two neutrons and two protons. It results from the radioactive decay of
some heavy elements such as uranium, plutonium, radium, thorium, and radon. All alpha particles
emitted by a given radioisotope have the same energy. Most of the alpha particles that are likely to be
found have energies in the range of about 4 to 8 MeV, depending on the isotope from which they came.
The alpha particle has an electrical charge of +2. Because of this double positive charge and their size,
alpha particles have great ionizing power and, thus, lose their kinetic energy quickly. This results in very
little penetrating power. In fact, an alpha particle cannot penetrate a sheet of paper. The range of an
alpha particle (the distance the charged particle travels from the point of origin to its resting point) is
about 4 cm in air, which decreases considerably to a few micrometers in tissue. These
properties cause
alpha emitters to be hazardous only if there is internal contamination (i.e., if the radionuclide is inside the
body).
D.2.4.2 Beta Emission.
A beta particle () is a high-velocity electron ejected from a disintegrating
nucleus. The particle may be either a negatively charged electron, termed a negatron (-) or a positively
charged electron, termed a positron (+). Although the precise definition of "beta emission" refers to
both - and +, common usage of the term generally applies only to the negative particle, as distinguished
from the positron emission, which refers to the + particle.
D.2.4.2.1 Beta Negative Emission.
Beta particle (-) emission is another process by which a
radionuclide, with a neutron excess achieves stability. Beta particle emission decreases the number of
neutrons by one and increases the number of protons by one, while the atomic mass number remains
unchanged.
1
This transformation results in the formation of a different element. The energy spectrum of
beta particle emission ranges from a certain maximum down to zero with the
mean energy of the
spectrum being about one-third of the maximum. The range in tissue is much less. Beta negative
emitting radionuclides can cause injury to the skin and superficial body tissues, but mostly present an
internal contamination hazard.
D.2.4.2.2 Positron Emission.
In cases in which there are too many protons in the nucleus, positron
emission may occur. In this case a proton may be thought of as being converted into a neutron, and a
positron (+) is emitted.
1
This increases the number of neutrons by one, decreases the number of protons
by one, and again leaves the atomic mass number unchanged. The gamma radiation resulting from the
annihilation (see glossary) of the positron makes all positron emitting isotopes more of an
external
radiation hazard than pure emitters of equal energy.
D.2.4.2.3 Gamma Emission.
Radioactive decay by alpha, beta, or positron emission, or electron
capture often leaves some of the energy resulting from these changes in the nucleus. As a result, the
1
Neutrinos also accompany negative beta particles and positron emissions
PLUTONIUM
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APPENDIX D
nucleus is raised to an excited level. None of these excited nuclei can remain in this high-energy state.
Nuclei release this energy returning to ground state or to the lowest possible stable energy level. The
energy released is in the form of gamma radiation (high energy photons) and has an energy equal to the
change in the energy state of the nucleus. Gamma and x rays behave similarly but differ in their origin;
gamma emissions originate in the nucleus while x rays originate in the orbital electron structure or from
rapidly changing the velocity of an electron (e.g., as occurs when shielding high energy beta particles or
stopping the electron beam in an x ray tube).
D.3 ESTIMATION OF ENERGY DEPOSITION IN HUMAN TISSUES
Two forms of potential radiation exposures can result: internal and external. The term exposure denotes
physical interaction of the radiation emitted from the radioactive material with
cells and tissues of the
human body. An exposure can be "acute" or "chronic" depending on how long an individual or organ is
exposed to the radiation. Internal exposures occur when radionuclides, which have entered the body (e.g.,
through the inhalation, ingestion, or dermal pathways), undergo radioactive decay resulting in the
deposition of energy to internal organs. External exposures occur when radiation enters the body directly
from sources located outside the body, such as radiation emitters from radionuclides on ground surfaces,
dissolved in water, or dispersed in the air. In general, external exposures are from material emitting
gamma radiation, which readily penetrate the skin and internal organs. Beta and alpha radiation from
external sources are far less penetrating and deposit their energy primarily on the skin's outer layer.
Consequently, their contribution to the absorbed
dose of the total body dose, compared to that deposited
by gamma rays, may be negligible.
Characterizing the radiation dose to persons as a result of exposure to radiation is a complex issue. It is
difficult to: (1) measure internally the amount of energy actually transferred to an organic material and to
correlate any observed effects with this energy deposition; and (2) account for and predict secondary
processes, such as collision effects or biologically triggered effects, that are an indirect consequence of
the primary interaction event.
D.3.1 Dose/Exposure Units
D.3.1.1 Roentgen.
The roentgen (R) is a unit of x or gamma-ray exposure
and is a measured by the
amount of ionization caused in air by gamma or x radiation. One roentgen produces 2.58x10
-4
coulomb
per kilogram of air. In the case of gamma radiation, over the commonly encountered range of photon
energy, the energy deposition in tissue for a dose of 1 R is about 0.0096 joules (J) /kg of tissue.
D.3.1.2 Absorbed Dose and Absorbed Dose Rate.
The absorbed dose is defined as the energy
imparted by the incident radiation to a unit mass of the tissue or organ. The unit of absorbed dose is the
rad; 1 rad = 100 erg/gram = 0.01 J/kg in any medium. An exposure of 1 R results in a dose to soft tissue
of approximately 0.01 J/kg. The SI unit is the gray which is equivalent to 100 rad or 1 J/kg. Internal and
external exposures from radiation sources are not usually instantaneous but are distributed over extended
periods of time. The resulting rate of change of the absorbed dose to a small volume
of mass is referred
to as the absorbed dose rate in units of rad/unit time.
D.3.1.3 Working Levels and Working Level Months.
Working level (WL) is a measure of the
atmospheric concentration of radon and its short-lived progeny. One WL is defined as any combination
of short-lived radon daughters (through polonium-214), per liter of air, that will result in the emission of
1.3x10
5
MeV of alpha energy. An activity concentration of 100 pCi radon-222/L of air, in equilibrium
with its daughters, corresponds approximately to a potential alpha-energy concentration of 1 WL. The
WL unit can also be used for thoron daughters. In this case, 1.3x10
5
MeV of alpha energy (1 WL) is
released by the thoron daughters in equilibrium with 7.5 pCi thoron/L. The potential alpha energy