Ionizing Radiation
When most people talk about radiation, they are talking about ionizing
radiation. This type of radiation is sometimes referred to as nuclear
radiation. Atoms that are capable of giving off ionizing radiation are
called radioactive.
In ionizing radiation, electrons can be removed from last shell or inner
shells.
In an atom, the number of electrons is equal to the number of protons.
(This is called the atomic number of an element)
Atomic mass number is the sum of electrons and neutrons (or the sum of
protons and neutrons since the number of protons is the same as the number of
electrons)
Isotopes:
Isotopes of an element have the same atomic number but differ in atomic
mass number because, they have different number of
neutrons even though the number of protons and electrons are the same. Examples
are Hydrogen, Uranium and their isotopes. The following is an example for
hydrogen and its isotopes.
|
Hydrogen |
Deuterium |
Tritium |
|
1 electron |
1 electron |
1 electron |
|
1 proton |
1 proton |
1 proton |
|
0 neutrons |
1 neutron |
2 neutrons |
Table 1.
Hydrogen isotopes
Some isotopes are stable and some isotopes are unstable.
Unstable isotopes are also known as radioisotopes.
Stable Isotopes:
These isotopes have enough binding energy in their nucleus to hold the
nuclear particles together.
Unstable Isotopes:
The amount of nuclear binding energy within the nucleus is not enough to
hold the nuclear particles (protons and neutrons) and as a result, part of the
nucleus is lost. This process of nuclear disintegration is called radioactive
decay. The unstable isotopes that show radioactive decay are called radioisotopes.
Half-life:
Radioactivity decays with time. The time required for a radioactive
material to lose half of its activity is its half-life. Examples:
|
Material |
Half-life (years) |
|
Pure Radium |
1620 |
|
Carbon 14 |
5730 |
|
Cobalt 60 |
5 |
|
Uranium 238 |
4.5 billion |
Table 2
Some isotopes have less than 0.50 seconds of half-life. Carbon 14 is
found in the upper atmosphere which is created by cosmic ray bombardment.
Carbon 14 mixes with Carbon12 in atmosphere and becomes part of the plant and
animals.
Most isotopes found in nature are stable. Examples for natural unstable
isotopes are radium, thorium and uranium. Natural radioactive materials are
found in rocks, soil and wood.
Most unstable isotopes are formed artificially which disintegrate
anywhere form couple of seconds to hundreds of years.
Ionization vs. Excitation:
The interaction between an ionizing ray and matter could result in either
ionization or excitation of the atoms. The principal way by which an ionizing
radiation dissipates its energy in matter is by ejection of one or more orbital
electrons. The removal of orbital electrons from an atom is called ionization.
Although atoms are electrically neutral, when one or more electrons are removed
from an atom, the atom becomes positively charged and forms an ion pair with
the removed electron(s).
Not all interactions between an ionizing ray and matter result in
ionization. Sometimes the ionizing ray raises the energy level of certain
electrons, moving them to orbits farther away from the nucleus. This process,
which is less drastic than ionization , is called
excitation. It is estimated that most of the energy of ultraviolet radiation in
human tissue is dissipated by excitation.
|
|
Figure 1.
Ionization and excitation
Types of Ionizing Radiation:
The four most common forms of ionizing radiation are alpha, beta, gamma,
and neutron radiation. They have the following characteristics:
Alpha Radiation
Particle radiation
(largest and most massive of all the ionizing particles particles)
Least penetrating of all
ionizing radiation and can be shielded by a few inches of air, pentrating power can be stopped by a piece of paper or the
outer layer of skin.
Beta Radiation
Particle radiation
smallest
of the particles listed
can
be shielded by several inches of plastic, thin plywood and sheet metal.
Can penetrate up to 1/4
in. into the tissue
Gamma Radiation:
electromagnetic
wave radiation
has
no mass
strong
enough to penetrate into the body
1.5 in. lead, 3 in
.steel, 7 in. thick concrete can reduce the effect of gamma rays to permissible
levels.
During radioactive decay
some materials emit gamma rays from the nucleus of decaying atoms
X-rays:
Similar to gamma rays
Generated by impact from
external electrons
Strong enough to
penetrate into the body
They eject an electron
from an atom upon collision
Neutrons :
particles
high
energy neutrons can penetrate thick lead shields
Produced by nuclear
reactions
Can eject electrons by
colliding with atoms
Can penetrate through
the body
High density materials
containing high levels of hydrogen atoms are necessary to stop them
Figure 2
Radiation Dose and Measurement Units:
Rem: The rem
is a measure of the dose of any ionizing radiation to the body tissue in terms
of its estimated biological effect relative to a dose of one Roentgen. For low doses we use the term millirem. 1/1000 of a rem is
equal to one millirem.
Rad: The Rad is a measure of the dose of ionizing radiation to the
body in terms of energy absorbed per unit mass of the tissue.
Curie: A measure of the activity of the radioactive material. (One
Curie is equivalent of 3.7 x 10 -10 disintegrations per second).
Roentgen: A special unit used for measuring exposure to radiation.
(2.58 x 10-4 coulomb per kilogram of air)
Radiation Sources
We receive approximately 360 millirem/year
from natural background radiation and manufactured sources of radiation.
Exposure to background radiation results in a dose of
300 millirem/year and comes primarily from:
Cosmic rays 30 millirem/year
Radioactive
material in the earth (such as uranium) 30 millirem/year
Ingestion of naturally occurring radionuclides in food (such as potassium-40 in bananas) 40 millirem/year
Inhalation
of radon gas from radioactive decay of uranium, which is naturally present in
the soil . 200 millirem/year
People (carbon-14 and potassium-40 are
naturally present in human-beings)
Manufactured Radiation
Contributes about 60 millirem/year
to the total dose a person receives annually.
More than 90% of this dose comes from
medical procedures such as x rays and diagnostic tests using
radiopharmaceuticals. Less than 10% of this dose comes from consumer products
which contain small amounts of radioactivity (e.g., smoke detectors, lantern
mantels and tobacco products.)
|
Radiation Source |
Radiation Amount (mrem/year) |
|
Cigarette smoking (1 pack/day) |
1300 |
|
Radon in homes |
200 |
|
Medical x-rays and pharmaceuticals |
53 |
|
Direct exposure from naturally occurring radioactivity in soil |
30 |
|
Cosmic radiation |
30 |
|
Continental round trip by air |
5 |
|
Building materials |
3.6 |
|
Smoke detectors (Detector utilizes a small sealed source of Am-241) |
( not measureable) |
Table 3.
Radiation Sources ( millirem
/ year)
Figure 3.
Sources of radiation exposure to the
Measuring Ionizing Radiation and Dosimetry:
Because one cannot sense radioactive material, it is essential to monitor
exposures of people who work with and around such material. Visitors' exposure
is also very important. OSHA and other organizations require such records.
Typically, individuals wear film badges, pocket ionization detectors or other
instruments while in areas where exposures are possible. Exposures are recorded
and the records are retained for each person.
Instruments for measuring ionized radiation typically include a sensing
device and a readout device. Sensors are the most critical, because different
types of sensors are appropriate for different types of radiation. Sensors include
Geiger-Mueller tubes which are used in Geiger Counters, ionization chambers,
luminescent detectors, scintillation detectors and photographic emulsions.
A Geiger -Mueller tube is a gas -filled chamber used to measure alpha,
beta and gamma radiation. Radiation entering the tube ionizes the gas and
creates small currents that the instrument measures.
(See
Appendix A) for Geiger Counters.
Ionization chambers measure beta, gamma and x-rays using a charge placed
on an electrode in a tube. Radiation ionizes the air surrounding the electrode
and allows charge to leak away. The amount of charge lost is related to the
amount of radiation arriving at the chamber. Pocket ionization detectors are
common dosimetry devices.
Radiation sensitive photographic film detects gamma, and x-rays. The
radiation affects the emulsion similar to light. The developed films are
compared to standards to establish the radiation exposure. Film badges are
common dosimetry devices that use this method.
Luminescent detectors measure exposures to neutrons. Arriving radiation
changes the energy content of solids in the detectors. The energy change causes
them to emit light. The amount of light is proportional to the energy change.
(See
Appendix B
and Appendix
C) for Thermo-luminescent detectors
In scintillation detectors incoming radiation strikes a thin layer of
crystals or a solution of organic materials that produce light. Light output is
proportional to the radiation absorbed. These devices measure alpha, beta,
gamma and slow neutron sources.
What Are the Radiation Controls?
Engineered Controls
Administrative Controls
Engineered Controls
Engineered Controls are the primary means of controlling radiation
exposure. Engineered controls include:
Distance
Shielding
Proper ventilation
Containment
devices
Distance : Generally airborne
particulates and gases that are contaminated are diluted with increasing
distance from the radiation source. Particulates that are large enough will
settle out of air. Thus, distance will reduce the likelihood of exposure to
radioactive materials released from an operation. Radiation levels decrease
with the square of distance. As shown in Figure 4, a person at one unit of
distance from a source will have four times more exposure to radiation than a
person who is located at two units of distance from a source of radiation.
Figure 4.
Radiation decreases with the square of the distance from a source
Shielding : Reducing radiation
levels with shielding is another form of protection. Different forms of
radiation are affected by shielding more than others. The ability of a specific
material to attenuate radiation varies with the form of radiation. For example,
to some extent air attenuates low energy beta waves but it has little effect
for other forms of radiation. Also, hydrogen is an effective attenuation medium
for low energy level neutrons.
Proper Ventilation : Generally airborne particulates and
gases that are contaminated are diluted with proper ventilation. However,
exhaust ventilation of radioactive particulates require special methods and equipment .
Containment Devices: Exposure to radiation can be minimized by
using glove boxes and interlocks on enclosures.
Figure 5 .
Glove box
Administrative Controls
Administrative controls supplement engineered controls to provide a safe
workplace. Administrative controls include:
Warning signs
Mass limitations
Safety procedures
Safety Training
Personal
protective equipment
Warning Signs: Warnings should mark locations and equipment where
there are ionizing radiation sources. Figure 6 shows the standard ionizing
radiation symbol. Packaging and labeling of radioactive material requires this
symbol as well. Visual warnings, such as flashing lights,
and audio signals help people in an area recognize
Figure 6.
Ionizing radiation symbol
Mass limitations : The best way to prevent radiation
exposure is to limit the amount of radiation from a source. Limiting the
quantity of ionizing material achieves this goal.
Safety Procedures : Procedures play am important role in
controlling entry or exit of radioactive material from sites. Typical
procedures include security, physical monitoring and manifest systems. Security
procedures and physical security systems can also prevent unauthorized persons
from entering a facility or entering dangerous locations.
Safety Training : people who work with and around
radioactive material need training to understand the hazards of ionizing
radiation. They need to understand how to protect themselves and what
procedures to follow; they need to know the value of protective clothing and
equipment and when to wear it. Also the training must provide the necessary
skills to be developed to perform activities correctly.
Personal Protective Equipment (PPE):
Lab coat
Gloves
Respirator
Figure 7.
Lab coat and gloves used for radiation protection
ALARA:
ALARA is a
philosophy which means that exposures to radiation and chemicals is kept AS LOW
AS REASONABLY ACHIEVABLE.
To achieve this goal:
hazards
must be evaluated before operations begin
appropriate
controls must be designed into the project.
This effort requires the coordinated effort of :
managers
supervisors
hands
on workers
Hazards
Control personnel.
Responsibilities of Employees,
Contractors, and Visitors for Radiation Safety:
Employees and Contractors must:
Wear their dosimeter on
the upper, front portion of their body
Obey all signs and
postings.
NOT enter posted radiation
or contamination areas unless escorted or trained to do so.
Report any unusual
radiological situations.
If the assignment
involves work with radioactive materials or radiation-generating devices, the
person must receive specialized radiation safety training and be familiar with
applicable safety procedures, radiation processes, and the equipment to be
used.
Visitors must:
Obey all signs and
postings.
Attend required
training.
Report any unsafe
conditions to the hosts.
Wear dosimeters if
entering access-controlled areas, where signs indicating notice "Dosimeter
Required" are posted.
Biological Effects of Ionizing
Radiation:
Overexposure to radiation can produce two types of effects in humans:
Somatic Effect
Genetic Effect
The somatic effect is the effect of radiation on organs, tissue, or the
whole body. Somatic effects can vary over a wide range, from rapid death due to
large exposures to reddening of the skin due to minimal exposure. Some somatic
effects of concern, such as cancer and cataracts may be delayed for long
periods. Within the body, cells and tissues respond with varying degrees of
sensitivity to radiation. Partial or whole body radiation and age also are
important factors in determining the somatic effects of overexposure.
Genetic effects of overexposure to radiation are of particular concern
because radiation induced mutations can be carried to subsequent generations
via defective genes.
Potential Risks from Exposure to
Ionizing Radiation
The risks from occupational radiation exposure depend on the amount of
radiation (or dose) received, the time over which the dose is received, and the
part of the body exposed. There are two types of exposures:
(1) Chronic Exposure
(2) Acute Exposure
Chronic Exposure
Occurs when a low dose rate of radiation is received over an extended
period of time (months to years). Examples of chronic radiation exposures
include:
occupational
radiation exposures
exposure
to natural background radiation
Cancer, sterility, aging and cataracts are among the long term effects of
chronic exposure to ionizing radiation.
Acute Exposure
Occurs when a LARGE dose rate of radiation is delivered
in a short period of time, typically in a matter of seconds to days.
Doses of ionizing radiation that brings death within approximately 30 days are
referred to as immediately lethal and the action of that radiation is
classified as acute.
Exposure to increasing doses of acute ionizing radiation over the whole
body results in nausea, vomiting, hair loss, loss of appetite, soreness in the
throat, diarrhea, and weight loss. Extreme level of acute exposure to radiation
causes deaths due to the failure of the vital organ systems.
For example, acute radiation doses were received by many of the fire
fighters responding to the
|
Part of Body |
Annual Maximum Permissible Dose in millirems (mrem) |
|
Whole body |
5,000 |
|
Lens of the eye |
15,000 |
|
Skin |
50,000 |
|
Extremities (Hand, forearms, feet, ankles) |
50,000 |
Table 4.
Annual maximum permissible exposure dose for radiation
Is There A Safe Dose of Ionizing
Radiation?
It is not possible to estimate precisely the risk of cancer induction
from low doses of radiation, because the risk is so low it cannot be measured.
Estimates of the risk from chronic exposure must be inferred from the
effects observed from acute doses. Risk estimates for acute doses were
developed through studies of:
Japanese atomic bomb
survivors
Uranium miners
Radiotherapy patients
The current risk of dying from cancer (all types) in the
If you were to receive the maximum allowable dose of 5000 millirem to the whole body, your assumed risk of dying from
cancer would increase from 20% to 20.20%
Estimated Risk of a Heritable Effect
A heritable effect is an effect that is passed on to the offspring of an
exposed man or woman.
Pregnancy and Radiation
The embryo-fetus is more sensitive to radiation (and other environmental
hazards) than adults.
Any employee who plans, suspects, or has a positive diagnosis of
pregnancy should notify the employer or supervisor as early as possible so the
work environment can be evaluated for potential radiological (and chemical)
exposures. The maximum permissible exposure for a declared pregnant worker
during the gestation period is 500 mrem. There are
relatively few research laboratories where radiation levels are high enough
that a fetus would receive this dose before birth. If a radiation worker is
pregnant, she may notify the Radiation Safety Officer, and then declare the
pregnancy in writing in order for the prenatal exposure limits to take effect.
The pregnant radiation worker will then meet with a health physics staff
member, and a complete assessment of her radiation exposure potential will be
made. The written declaration is made by completing a Declaration of Pregnancy
form.
If notification is not made in writing, the radiation exposure limits
remain at the occupational level, that is, 5 rem per year. An individual may "
undeclare " her pregnancy at any time,
but this also should be documented.
Declared pregnant workers (DPW) are assigned two badges, one for the
whole body, normally worn on the torso and one for the fetus, normally worn on
the abdomen. The badges will be exchanged on a monthly basis. Exposures must be
maintained beneath a cap of 50 mrem per month in
order to prevent exposure spikes.
Workplace modification
Workplace modification is usually not necessary to provide a safe work
environment during pregnancy. However, when appropriate, tasks may be modified
to minimize or eliminate the potential for adverse exposure.
Minors Working With Radioactive Materials
Radiation exposure limits exist for minors, (individuals under 18 years
of age) who work with radioactive materials. These limits are 10% of all of the
occupational limits for adult radiation workers. For these workers, safety
training must be completed prior to work with radioactive materials as with
other occupational workers. It is usually the policy of an institution that an
informed parental consent form must be completed and kept on file for purposes
of liability and risk management.
Last Update:
January 9, 2007
By: Serdar Z. Elgun