TABLE OF CONTENTS
-
INTRODUCTION
-
RADIATION FUNDAMENTALS
-
INTERACTION OF RADIATION WITH MATTER
-
ACTIVITY, EXPOSURE, AND DOSE
-
BIOLOGICAL EFFECTS OF IONIZING RADIATION
-
RADIATION DOSIMETRY PROGRAM
-
RADIOACTIVE MATERIAL HANDLING AND
LABORATORY SAFETY
-
RADIATION SURVEY METERS
-
RADIOACTIVE WASTE DISPOSAL
-
RADIATION SAFETY FOR X-RAY UNITS
-
ACCELERATOR SAFETY
APPENDICES
-
DOSE CONCEPTS
-
RADIATION RULES OF THUMB
-
SI UNITS AND CONVERSION FACTORS
-
GLOSSARY OF TERMS
-
REFERENCES
INTRODUCTION
RADIATION FUNDAMENTALS
INTRODUCTION
For the
purposes of this manual, we can use a simplistic model of an atom. The
atom can be thought of as a system containing a positively charged nucleus
and negatively charged electrons which are in orbit around the nucleus.
The
nucleus is the central core of the atom and is composed of two types
of particles, protons which are positively charged and neutrons which
have a neutral charge. Each of these particles has a mass of approximately
one atomic mass unit (amu). (1 amu = 1.66E-24 g)
Electrons
surround the nucleus in orbitals of various energies. (In simple terms,
the farther an electron is from the nucleus, the less energy is required
to free it from the atom.) Electrons are very light compared to protons
and neutrons. Each electron has a mass of approximately 5.5E-4 amu.
A
nuclide is an atom described by its atomic number (Z) and its mass
number (A). The Z number is equal to the charge (number of protons)
in the nucleus, which is a characteristic of the element. The A number
is equal to the total number of protons and neutrons in the nucleus.
Nuclides with the same number of protons but with different numbers
of neutrons are called isotopes. For example, deuterium (2,1H) and
tritium (3,1H) are isotopes of hydrogen with mass numbers two and
three, respectively. There are on the order of 200 stable nuclides
and over 1100 unstable (radioactive) nuclides. Radioactive nuclides
can generally be described as those which have an excess or deficiency
of neutrons in the nucleus.
RADIOACTIVE DECAY
Radioactive
nuclides (also called radionuclides) can regain stability by nuclear
transformation (radioactive decay) emitting radiation in the process.
The radiation emitted can be particulate or electromagnetic or both.
The various types of radiation and examples of decay are shown below.
ALPHA
(a)
Alpha particles
have a mass and charge equal to those of helium nuclei (2 protons +
2 neutrons). Alpha particles are emitted during the decay of some very
heavy nuclides (Z > 83).
226,88Ra
--> 222,86Rn + 4,2a
BETA
(B-, B+)
Beta particles
are emitted from the nucleus and have a mass equal to that of electrons.
Betas can have either a negative charge or a positive charge. Negatively
charged betas are equivalent to electrons and are emitted during the
decay of neutron rich nuclides.
Positively
charged betas (positrons) are emitted during the decay of proton rich
nuclides.
22,11Na
--> 22,10Ne + 0,1B + g
GAMMA
(g)
Gammas (also
called gamma rays) are electromagnetic radiation (photons). Gammas are
emitted during energy level transitions in the nucleus. They may also
be emitted during other modes of decay.
99m,43Tc
--> 99,43Tc + g
ELECTRON
CAPTURE
In certain
neutron deficient nuclides, the nucleus will capture an orbital electron
resulting in conversion of a proton into a neutron. This type of decay
also involves gamma emission as well as x-ray emission as other electrons
fall into the orbital vacated by the captured electrons.
125,53I
+ 0,-1e --> 125,52Te + g
FISSION
Fission
is the splitting of an atomic nucleus into two smaller nuclei and usually
two or three neutrons. This process also releases a large amount of
energy in the form of gammas and kinetic energy of the fission fragments
and neutrons.
235,92U
+ 1,0n --> 93,37Rb + 141,55Cs + 2(1,0n) + g
NEUTRONS
(n)
For a few
radionuclides, a neutron can be emitted during the decay process.
17,7N
--> 17,8O* + 0,-1B (*excited state)
7,8O*
--> 16,8O + 1,0n
X-RAYS
X-rays are
photons emitted during energy level transitions of orbital electrons.
Bremsstrahlung
x-rays (braking radiation) are emitted as energetic electrons (betas)
are decelerated when passing close to a nucleus. Bremsstrahlung must
be considered when using large activities of high energy beta emitters
such as P-32 and S-90.
CHARACTERISTICS OF RADIOACTIVE DECAY
In addition
to the type of radiation emitted, the decay of a radionuclide can
be described by the following characteristics.
HALF-LIFE
The half-life of a radionuclide is the time required
for one-half of a collection of atoms of that nuclide to decay. Decay
is a random process which follows an exponential curve. The number of
radioactive nuclei remaining after time (t) is given by:
N(t) = N(0) x exp(-0.693t/T)
where
N(0) = original number of atoms
N(t) = number remaining at time t
t = decay time
T = half-life
ENERGY
The basic
unit used to describe the energy of a radiation particle or photon
is the electron volt (eV). An electron volt is equal to the amount
of energy gained by an electron passing through a potential difference
of one volt. The energy of the radiation emitted is a characteristic
of the radionuclide. For example, the energy of the alpha emitted
by Cm-238 will always be 6.52 MeV, and the gamma emitted by Ba-135m
will always be 268 keV. Many radionuclides have more than one decay
route. That is, there may be different possible energies that the
radiation may have, but they are discreet possibilities. However,
when a beta particle is emitted, the energy is divided between the
beta and a neutrino. (A neutrino is a particle with no charge and
infinitesimally small mass.) Consequently, a beta particle may be
emitted with an energy varying in a continuous spectrum from zero
to a maximum energy (Emax) which is characteristic of the radionuclide.
The average energy is generally around forty percent of the maximum.
INTERACTION OF RADIATION WITH MATTER
ENERGY ABSORPTION
The transfer
of energy from the emitted particle or photon to an absorbing medium
has several mechanisms. These mechanisms result in ionization and excitation
of atoms or molecules in the absorber. The transferred energy is eventually
dissipated as heat.
Ionization
is the removal of an orbital electron from an atom or molecule, creating
a positively charged ion. In order to cause an ionization, the radiation
must transfer enough energy to the electron to overcome the binding
force on the electron. The ejection of an electron from a molecule can
cause dissociation of the molecule.
Excitation
is the addition of energy to an orbital electron, thereby transferring
the atom or molecule from the ground state to an excited state.
ALPHA PARTICLES
Interactions
between the electric field of an alpha and orbital electrons in the
absorber cause ionization and excitation events. Because of their
double charge and low velocity (due to their large mass), alpha particles
lose their energy over a relatively short range. One alpha will cause
tens of thousands of ionizations per centimeter in air. The range
in air of the most energetic alpha particles commonly encountered
is about 10 centimeters (4 inches). In denser materials, the range
is much less. Alpha particles are easily stopped by a sheet of paper
or the protective (dead) layers of skin.
BETA PARTICLES
Normally,
a beta particle loses its energy in a large number of ionization and
excitation events. Due to the smaller mass, higher velocity and single
charge of the beta particle, the range of a beta is considerably greater
than that of an alpha of comparable energy. Since its mass is equal
to that of an electron, a large deflection can occur with each interaction,
resulting in many path changes in an absorbing medium.
If
a beta particle passes close to a nucleus, it decreases in velocity
due to interaction with the positive charge of the nucleus, emitting
x-rays (bremsstrahlung). The energy of the bremsstrahlung x-rays has
a continuous spectrum up to a maximum equal to the maximum kinetic energy
of the betas. The production of bremsstrahlung increases with the atomic
number of the absorber and the energy of the beta. Therefore, low Z
materials are used as beta shields.
A
positron will lose its kinetic energy through ionizations and excitations
in a similar fashion to a negative beta particle. However, the positron
will then combine with an electron. The two particles are annihilated,
producing two 511 keV photons called annihilation radiation.
PHOTONS
Gammas and
x-rays differ only in their origin. Both are electromagnetic radiation,
and differ only from radio waves and visible light in having much shorter
wavelengths. They have zero rest mass and travel with the speed of light.
They are basically distortions in the electromagnetic field of space,
and interact electrically with atoms even though they have no net electrical
charge. While alphas and betas have a finite maximum range and can therefore
be completely stopped with a sufficient thickness of absorber, photons
interact in a probabilistic manner. This means that an individual photon
has no definite maximum range. However, the total fraction of photons
passing through an absorber decreases exponentially with the thickness
of the absorber. There are three mechanisms by which gammas and x-rays
lose energy.
The
photoelectric effect is one in which the photon imparts all its energy
to an orbital electron. The photon simply vanishes, and the absorbing
atom becomes ionized as an electron (photoelectron) is ejected. This
effect has the highest probability with low energy photons (< 50
keV) and high Z absorbers.
Compton
scattering provides a means for partial absorption of photon energy
by interaction with a 'free' (loosely bound) electron. The electron
is ejected, and the photon continues on to lose more energy in other
interactions. In this mechanism of interaction, the photons in a beam
are scattered, so that radiation may appear around corners and in front
of shields.
Pair
production occurs only when the photon energy exceeds 1.02 MeV. In pair
production the photon simply disappears in the electric field of a nucleus,
and in its place two electrons, a negatron and a positron, are produced
from the energy of the photon. The positron will eventually encounter
a free electron in the absorbing medium. The two particles annihilate
each other and their mass is converted into energy. Two photons are
produced each of 0.511 MeV. The ultimate fate of these two photons is
energy loss by Compton scattering or the photoelectric effect.
SECONDARY IONIZATIONS
The electrons
from ionizations and pair production will themselves go on to cause
more ionization and excitation events in the same way as described
for betas.
ACTIVITY, EXPOSURE, AND DOSE
DEFINITIONS
Activity
Activity is the rate of decay (disintegrations/time) of a given
amount of radioactive material.
Dose
Dose is a measure of energy deposited by radiation in a material,
or of the relative biological damage produced by that amount of energy
given the nature of the radiation.
Exposure
Exposure is a measure of the ionizations produced in air by
x-ray or gamma radiation. The term exposure (with its 'normal' definition)
is sometimes used to mean dose. (e.g. 'He received a radiation exposure
to his hand.')
UNITS
ACTIVITY
1 Curie
(Ci) = 3.7E10 disintegrations per sec (dps). The Becquerel (Bq) is
also coming into use as the International System of Units (SI){XE
"International System of Units (SI)"} measure of disintegration
rate. 1 Bq = 1 dps, 3.7E10 Bq = 1 Ci, and 1 mCi = 37 MBq.
EXPOSURE
The unit
of radiation exposure in air is the roentgen (R). It is defined as
that quantity of gamma or x-radiation causing ionization in air equal
to 2.58E-4 coulombs per kilogram. Exposure applies only to absorption
of gammas and x-rays in air.
DOSE
The rad
is a unit of absorbed dose. One rad is equal to an absorbed dose of
100 ergs/gram. (1 erg = 6.24E11 eV) The SI unit of absorbed dose is
the Gray (Gy). 1 Gy = 1 joule/kilogram = 100 rad. An exposure of 1 R
results in an absorbed dose of 0.87 rad.
A
quality factor (Q) is used to compare the biological damage producing
potential of various types of radiation, given equal absorbed doses.
The effectiveness of radiation in producing damage is related to the
energy loss of the radiation per unit path length. The term used to
express this is linear energy transfer (LET). Generally, the greater
the LET in tissue, the more effective the radiation is in producing
damage. The quality factors for radiations frequently encountered
are:
Beta particles & electrons 1
Alpha particles & fission fragments 20
The
rem is a unit of dose equivalent. The dose equivalent in rem is equal
to the absorbed dose in rad multiplied by the quality factor. Dose equivalent
determinations for internally deposited radioactive materials also take
into account other factors such as the non-uniform distribution of some
radionuclides (e.g. I-125 in the thyroid). The SI unit for dose equivalent
is the Sievert (Sv). 1 Sv = 100 rem.
CALCULATION OF ACTIVITIES
The half-life
of a radionuclide is the time required for one-half of a collection
of atoms of that nuclide to decay. This is the same as saying it is
the time required for the activity of the sample to be reduced to one-half
the original activity. This can be written as:
A(t) = A(0) x exp(-0.693t/T)
where
A(0) = original activity
A(t) = activity at time t
t = decay time
T = half-life
EXAMPLE
P-32 has
a half-life of 14.3 days. On January 10, the activity of a P-32 sample
was 10 uCi. What will the activity be on February 6? February 6 is 27
days after January 10, so
A(Feb
6) = A(Jan 10) x exp[-0.693(27/14.3)] = 2.7 uCi
A
quick estimate could also have been made by noting that 27 days is about
two half-lives. So the new activity would be about one-half of one-half
(i.e. one-fourth) of the original activity.
CALCULATION OF EXPOSURE RATES
Gamma exposure
constants (G) for some radionuclides are shown below. G is the exposure
rate in R/hr at 1 cm from a 1 mCi point source.
An
empirical rule which may also be used is
6 x Ci x n x E = R/hr @ 1 foot,
where:
Ci = source strength in curies.
E = energy of the emitted photons in MeV.
n = fraction of decays resulting in photons with an energy
of E.
It
should be noted that this formula and the gamma constants are for
exposure rates from gammas and x-rays only. Any dose calculations
would also have to include the contribution from any particulate radiation
that may be emitted.
INVERSE SQUARE LAW
Exposure
rate varies inversely with the square of the distance from a point source
of radiation. This is often referred to as the inverse square law.
ER2 = ER1 x (D1/D2)^2
where
ER2 = exposure rate at distance 2
ER1 = exposure rate at distance 1
D1 = distance 1
D2 = distance 2
For
example, from the table above, the G for Co-60 is 13.2. Therefore, the
exposure rate at 1 cm from a 1 mCi source would be 13.2 R/hr. At 30
cm from the same source, the exposure rate would be
(13.2
R/hr)(1/30)^2 = 0.0147 R/hr = 14.7 mR/hr.
BETA DOSE RATES
For a beta
emitter point source, the dose rate can be calculated using the empirical
equation
300
x Ci = rad/hr @ 1 foot, where Ci = source strength in curies.
This
calculation neglects any shielding provided by the air, which can
be significant. For example, the maximum range in air for a beta from
S-35 is less than one foot, so the dose rate at one foot is zero for
any size S-35 source.
SKIN DOSE
For energies
above 0.6 MeV, the dose rate to the skin from a uniform deposition of
1 uCi/cm^2 of a beta emitter on the skin is about 9 rem/hr.
INTERNAL DOSE CALCULATIONS
See Appendix
1 for methods and examples of internal dose calculations.
BIOLOGICAL EFFECTS OF IONIZING RADIATION
RADIATION HAZARDS
The hazards
associated with the absorption of radiation in mammalian systems and
tissue are related to both the type of radiation and the nature of
the absorbing tissue or organ system.
ALPHA
Alpha particles
will be stopped by the dead layers of skin, so they are not an external
hazard. However, many alpha emitters or their daughters also emit
gammas which are penetrating and therefore may present an external
hazard. Internally, alphas can be very damaging due to their high
linear energy transfer (LET). That is, they deposit all of their energy
in a very small area. Based on their chemical properties, alpha emitters
can be concentrated in specific tissues or organs.
BETA
Externally,
beta particles can deliver a dose to the skin or the tissues of the
eye. Many beta emitters also emit gammas. A large activity of a high
energy beta emitter can create a significant exposure from bremsstrahlung
x-rays produced in shielding material. Internally, betas can be more
damaging, especially when concentrated in specific tissues or organs.
PHOTONS
Externally,
the hazard from low energy (< 30 keV) gammas and x-rays is primarily
to the skin or the tissues of the eye. Higher energies are more penetrating
and therefore a whole body hazard. Internally, gamma emitters can
effect not only the tissues or organs in which they are deposited,
but also surrounding tissues.
MECHANISMS OF DAMAGE
As discussed
earlier, radiation causes atoms and molecules to become ionized or
excited. These ionizations and excitations can result in:
-
Production of free radicals.
-
Breakage of chemical bonds.
-
Production of new chemical bonds and cross-linkage between
macromolecules.
-
Damage to molecules which regulate vital cell processes
(e.g. DNA, RNA, proteins).
TISSUE SENSITIVITY
In general,
the radiation sensitivity of a tissue varies directly with the rate
of proliferation of its cells and inversely with the degree of differentiation.
EFFECTS OF ACUTE HIGH RADIATION DOSES
A whole
body radiation dose of greater than 25 to 50 rem received in a short
time results in the clinical 'acute radiation syndrome.' This syndrome,
which is dose related, can result in disruption of the functions of
the bone marrow system (>25 rem), the gastro-intestinal system
(>500 rem), and the central nervous system (>2000 rem). An acute
dose over 300 rem can be lethal.
EFFECTS OF LOW RADIATION DOSES
There is
no disease uniquely associated with low radiation doses.
Immediate
effects are not seen below doses of 25 rem. Latent effects may appear
years after a dose is received. The effect of greatest concern is the
development of some form of cancer.
The
National Academy of Sciences Committee on Biological Effects of Ionizing
Radiation (BEIR) issued a report in 1990 entitled Health Effects
of Exposure to Low Levels of Ionizing Radiation, also known as
BEIR V. The following is an excerpt from the Executive
Summary of the report:
On the basis of the available evidence, the population-weighted
average lifetime risk of death from cancer following an acute dose equivalent
to all body organs of 0.1 Sv (0.1 Gy of low-LET radiation) is estimated
to be 0.8%, although the lifetime risk varies considerably with age
at the time of exposure. For low LET radiation, accumulation of the
same dose over weeks or months, however, is expected to reduce the lifetime
risk appreciably, possibly by a factor of 2 or more. The Committee's
estimated risks for males and females are similar. The risk from exposure
during childhood is estimated to be about twice as large as the risk
for adults, but such estimates of lifetime risk are still highly uncertain
due to the limited follow-up of this age group.
. .
The Committee examined in some detail the sources of uncertainty in
its risk estimates and concluded that uncertainties due to chance sampling
variation in the available epidemiological data are large and more important
than potential biases such as those due to differences between various
exposed ethnic groups. Due to sampling variation alone, the 90% confidence
limits for the Committee's preferred risk models, of increased cancer
mortality due to an acute whole body dose of 0.1 Sv to 100,000 males
of all ages range from about 500 to 1200 (mean 760); for 100,000 females
of all ages, from about 600 to 1200 (mean 810). This increase in lifetime
risk is about 4% of the current baseline risk of death due to cancer
in the United States. The Committee also estimated lifetime risks with
a number of other plausible linear models which were consistent with
the mortality data. The estimated lifetime risks projected by these
models were within the range of uncertainty given above. The committee
recognizes that its risk estimates become more uncertain when applied
to very low doses. Departures from a linear model at low doses, however,
could either increase or decrease the risk per unit dose.
Example
whole body dose limit for planned exposures is 500 mrem/year (5 mSv/yr).
If a worker were to receive the maximum allowable planned dose each
year for twenty years, the total dose received would be 10 rem (0.1
Sv). According to the BEIR V report, the worker's chance of death from
cancer would increase by approximately 0.4%. This is fairly small compared
to the normal chance of death from cancer in the U. S. of about 20%.
RADIATION DOSIMETRY
EXTERNAL DOSIMETRY
Film badge
dosimeters, thermoluminescent dosimeters (TLDs), pocket dosimeters,
“chirpers” are some of the devices for determining a persons exposure
to radiation.
FILM
BADGE
The film
badge is used to measure whole body dose and shallow dose. It consists
of a film packet and a holder. The film is similar to ordinary photographic
film but will be exposed by radiation. (It will also be exposed by light,
so if the packet is opened or damaged, the reading will be invalid.)
The holder has several filters which help in determining the type and
energy of radiation. The badge will detect gamma and x-rays, high energy
beta particles, and in certain special cases, neutrons. It does not
register radiation from low energy beta emitters such as H-3, C-14,
and S-35, since their betas will not penetrate the paper covering on
the film packet.
The
badge is usually worn at the collar or chest level to measure the radiation
dose received by the trunk of the body. When not in use, the badge should
be left in a safe place at the facility away from any radiation sources.
(Use the film badge rack if one is provided.) Be sure the badge is available
for the film packet exchange which should be done monthly.
TLD
RING
The TLD
ring is used to measure dose to the hand. They are issued to individuals
who may use millicurie amounts of a gamma or high energy beta emitter.
The TLD is a small crystal which absorbs the energy from radiation.
When heated, it releases the stored energy in the form of visible light.
The crystal is mounted in a ring which should be worn on the hand which
is expected to receive the larger dose. Wear the ring inside your glove
with the label facing towards your palm.
PERSONNEL
(POCKET OR PEN) DOSIMETERS
These are
pen like devices which can give a more immediate indication of exposure
to radiation then the two above methods.
It is usually worn in a shirt pocket or collar and is viewed
by looking through one end pointed up to a light source. A meter like needle will display the exposure amount and the
unit is discharged to 0 through another device. See picture below.
CHIRPERS
OR BEEPERS
These are
devices that are also worn in a shirt pocket or on the collar and will
emit a “chirp” or other warning sound audibly of radiation exposure. Limits can be set for specific thresholds.
The obvious advantage is that it gives and immediate indication
of radiation exposure. See picture below.
PRECAUTIONS
The radiation
doses recorded by your dosimeters should be part of your occupational
radiation dose record and such procedures should be implemented if
not already. Make sure that this record is valid and accurate by observing
the following precautions:
-
Always wear your badge when using radioactive materials
or radiation producing machines. Wear your ring when using gamma
or high energy beta emitters.
-
Keep your dosimeters away from radiation sources when
not in use. Do not deliberately expose a dosimeter to radiation
or wear your badge when receiving medical or dental x-rays.
-
Do not tamper with the film packet or remove it from the
holder.
-
Never wear someone else's dosimeter or let someone else
wear yours.
-
Avoid subjecting the badge to high temperatures or getting
it wet.
Notify
the safety officer or appropriate administrative official if your badge
or ring has been damaged or lost, or if you have reason to believe that
you or your dosimeter has received an accidental high dose.
RADIOACTIVE MATERIAL HANDLING AND LABORATORY SAFETY
REDUCTION OF DOSE TO PERSONNEL
The following
are ways in which radiation doses can be reduced.
TIME
Carefully
plan your activities in order to minimize the time spent handling
or in the vicinity of radiation sources.
DISTANCE
Increasing
the distance from a radiation source by the use of handling devices
will reduce the dose received, since exposure rate decreases as 1/r^2,
where r is the distance from a point source. For example:
At
10 cm, a 5 mCi I-125 source has an exposure rate of 75 mR/hr. Moving
to 30cm would reduce the exposure rate to
(75
mR/hr)(10/30)^2 = 8.3 mR/hr
Note:
The 1/r^2 formula (also known as the inverse square law) does not take
into account shielding provided by air. This can be significant for
particulate radiation. Even the most energetic alpha paricles commonly
encountered have a range in air of about 4 inches. A beta from the decay
of S-35 has a maximum range in air of about 12 inches.
SHIELDING
As gammas
and x-rays pass through an absorber their decrease in number (by the
processes discussed in chapter 3) is governed by the energy of the radiation,
the density of the absorber medium, and the thickness of the absorber.
This can be expressed approximately as
I = Io exp(-ux)
where
Io is the intensity of the initial radiation,
I is the radiation intensity after it has passed through the
absorber,
u is a factor called the linear absorption coefficient (The
value of u depends on the energy of the
incident radiation and the density of the absorbing medium.),
and
x is the thickness of the absorber.
TVL & HVL
The thickness
of an absorber needed to reduce the radiation intensity by a factor
of two and by a factor of ten are called the half-value layer (HVL)
and the tenth-value layer (TVL), respectively. Approximate lead TVL's,
HVL's, and linear attenuation coefficients for some radionuclides are
listed below.
Nuclide Gamma Energy(MeV) HVL(mm) TVL(mm) u(cm^-1)
I-125 0.035 0.05 0.16 150
Am-241 0.060 0.14 0.45 51
Co-60 1.17 & 1.33 12 40 0.58
Example:
At 30 cm, a 10 mCi Co-60 source produces an exposure rate of about 150
mR/hr. How much lead shielding is needed to reduce the rate to 4 mR/hr?
40
mm (one TVL) will reduce the rate to 15 mR/hr. Adding 12 mm (one HVL)
will make it 7.5 mR/hr. One more HVL will put the rate at about 4 mR/hr.
So the total lead shielding needed is 40 + 12 + 12 = 64 mm.
SHIELDING CONCERNS
When designing
shielding there are several points to be kept in mind.
-
Persons outside the shadow cast by the shield are not
necessarily protected.
-
A wall or partition may not be a safe shield for people
on the other side.
-
Radiation can be "scattered" around corners.
BREMSSTRAHLUNG
The absorption
of high energy beta radiation (e.g. P-32 and Sr-90) in high Z materials
such as lead and tungsten may result in the production of electromagnetic
radiation (bremsstrahlung) which is more penetrating than the beta
radiation that produced it. Low Z materials such as plastics and glass
minimize the production of bremsstrahlung.
HANDLING PRECAUTIONS
Here are
some of the radiological characteristics of and special precautions
associated with some radionuclides commonly used on campus. In addition
to the specific precautions for each nuclide, the following general
precautions should always be followed when applicable to your work.
-
Whenever practical, designate specific areas for radioactive
material handling and use. Clearly label the area and all containers.
Minimize and confine contamination by using absorbent paper and
spill trays. Handle potentially volatile materials in certified
fume hoods.
-
Do not smoke, eat, or drink in rooms where radioactive
materials are used. Do not store food or drink in refrigerators,
freezers, or cold rooms used for radioactive material storage.
-
Use an appropriate instrument to detect radioactive contamination.
Regularly monitor the work area. Always monitor yourself, the
work area, and equipment for contamination when your experiment
or operation is completed. Decontaminate when necessary.
-
Use appropriate shielding when handling millicurie or
greater amounts of gamma emitters or high energy beta emitters.
-
Wear the dosimeters issued to you while using radioactive
materials.
-
Wash your hands before leaving the lab, using a telephone,
or handling food.
P-32 INFORMATION
Radioactive half-life 14.3 days
Decay mechanism Beta emission
Contamination monitoring Thin window Geiger-Mueller detector
Dosimetry Film badge, TLD ring, urinalysis
-
The dose rate on contact on the side of a 1 mCi delivery
vial will be on the order of 1000 mrem/hr. If possible, avoid
direct hand contact with vials and sources. When working with
100 uCi or more of P-32, work should be done behind a 1 cm lucite
shield.
-
One microcurie of P-32 in direct contact with 1 cm^2 of
bare skin gives a dose rate to the skin of about 8 rem/hr. Always
protect your skin when handling unsealed materials. Wear gloves,
lab coats, and shoes.
-
An thin window G-M survey meter should always be available.
A survey should be made immediately after use and any 'hot spots'
should be decontaminated.
-
Film badges must be worn for all P-32 work. TLD rings
should be worn for all P-32 work, and are required when handling
1 millicurie or more.
-
Handle and store your radioactive waste carefully. The
one gallon polyethylene bottles for liquid waste should be placed
in a secondary container (e.g. a bucket or tray) to contain spills
or leaks. When more than a millicurie is involved, place 1 cm
lucite in front of the container for shielding. The metal barrels
for dry waste provide sufficient shielding but be sure to keep
the lid on.
S-35 INFORMATION
Radioactive half-life 87.4 days
Decay mechanism Beta emission
Contamination monitoring Thin window Geiger-Mueller detector,
liquid scintillation counter for wipe surveys
-
Radiolysis of S-35 labeled amino acids may lead to the
release of S-35 labeled volatile impurities. Delivery vials should
therefore be opened in a fume hood.
-
The addition of stabilizers (buffers) will reduce, but
not eliminate, the evolution of S-35 volatiles from tissue culture
media. Incubators should be checked for contamination after using
S-35 methionine or other volatile compounds.
-
S-35 may be difficult to distinguish from C-14. If both
nuclides are being used in the same laboratory, establish controls
to ensure they are kept separate. If 'unknown' contamination is
found, treat it as C-14.
I-125 INFORMATION
Radioactive half-life 59.6 days
Decay mechanism Electron capture (gamma and x-ray emission)
Contamination monitoring Thin crystal NaI detector,
liquid scintillation counter for wipe surveys
Dosimetry Film badge, TLD ring,
-
The dose rate at 1 cm from a 1 mCi point source is about
1.5 rem/hr The dose rate is inversely related to the square of
the distance from the source. Thus while a small amount of I-125
held for a short time can result in a significant dose to the
hands, a relatively short separation distance reduces the dose
rate to an acceptable level.
-
The volatility of iodine requires special handling techniques
to minimize radiation doses. Solutions containing iodide ions
(such as NaI) should not be made acidic or be frozen. Both lead
to formation of volatile elemental iodine. Once bound to a protein,
the volatility of the radioiodine is tremendously reduced.
-
Always work in a fume hood with a minimum face velocity
of at least 125 linear feet per minute when working with NaI.
The sash should be below the breathing zone.
-
Use shoulder length veterinary gloves with short vinyl
gloves on top to minimize skin absorption.
-
Avoid opening the septum on delivery vials. It is preferable
to remove radioiodine using a hypodermic needle and syringe.
A radiation survey instrument should be available in the immediate
area. A low energy scintillation detector is preferable to a G-M detector.
You should do a wipe survey in your work areas after each use.
-
Film badges must be worn for all radioiodine work, and
finger rings are required when handling 1 mCi or more of I-125.
-
Use lead to shield quantities of 1 mCi or more. 1 mm of
lead will essentially absorb all of the radiation emitted from
I-125.
-
Contact the safety officer to schedule a thyroid assay
after using 1mCi or more of NaI, or in cases of suspected accidental
contamination.
-
Waste should be kept in appropriate waste containers
and stored in a fume hood.
-
H-3 (TRITIUM) INFORMATION
Radioactive half-life 12.4 years
Decay mechanism Beta emission
Contamination monitoring Liquid scintillation counter for wipe surveys
Because
the beta emitted has a very low energy, tritium can not be detected
with the usual survey meters found in the lab. Therefore, special care
is needed to keep the work area from becoming contaminated. Tritium
can be detected by doing a wipe survey and counting the wipes in a liquid
scintillation counter.
Many tritiated
compounds readily penetrate gloves and skin. Wearing two pairs of
gloves and changing the outer pair every fifteen or twenty minutes
will reduce the chances of cross contamination and absorption through
the skin.
C-14 INFORMATION
Radioactive half-life 5730 years
Decay mechanism Beta emission
Contamination monitoring Thin window Geiger-Mueller detector,
liquid scintillation counter for wipe surveys
-
Some C-14 labeled compounds can penetrate gloves and skin.
Wearing two pairs of gloves and changing the outer pair every
fifteen or twenty minutes will reduce the chances of absorption
through the skin.
-
C-14 may be difficult to distinguish from S-35. If both
nuclides are being used in the same laboratory, establish controls
to ensure they are kept separate. If 'unknown' contamination is
found, treat it as C-14.
RADIATION SURVEY METERS
INTRODUCTION
There are
several types of portable radiation survey instruments. Various types
have different qualities and can therefore have very different detection
capabilities.
As
a user of radioactive materials or radiation producing machines, you
are expected to be able to use the survey meters in your laboratory.
During your initial training, you will learn how to operate the instruments
in your lab. You should know their capabilities and limitations and
be able to interpret the meter readings. See sample picture below.
GEIGER-MUELLER DETECTOR
The Geiger-Mueller
(G-M) counter is the most common radiation detection instrument on campus.
In this type of meter, an ionization in the detector results in a large
output pulse that causes meter and audio responses. Because of the inherent
characteristics of the detector, all initial ionizing events produce
the same size output pulse. Therefore, the meter does not differentiate
among types or energies of radiation.
Most
G-M detectors have a thin mica film 'window' at one end. This window
is very fragile. Always use the thin end window for detecting pure beta
emitters and low energy photons (e.g. P-32, S-35, C-14, Fe-55, I-125,
and x-rays less than 40 keV). The aluminum side wall should be used
only for the detection of penetrating x-rays and gamma radiation.
Very
low energy beta emitters such as H-3 and Ni-63 are not detectable since
their betas do not have enough energy to penetrate the window. They
are best detected by using liquid scintillation counting techniques.
C-14 and S-35 emit betas energetic enough to pass through the thin window.
However, covering the window with plastic wrap or paraffin film will
stop most or all of their betas from entering the detector.
The
efficiency of a meter for a specific source of radiation is given
by the ratio of the meter count rate to the actual disintegration
rate of the source (cpm/dpm). Some examples of approximate G-M efficiencies
through the end window at 1 inch from a point source are given below:
· Not detectable if the detector window is covered with paraffin film, plastic wrap, or other material.
Example:
Your G-M counter reads 5000 cpm at one inch from a small spot of P-32
contamination on the bench. What is the total activity of the contamination?
actual
disintigration rate = (5000 cpm)/(0.05 cpm/dpm) = 100,000 dpm = 1700
dps = 1700 Bq = 45 nCi
Because
of the randomness of radioactive decay, the meter reading at low count
rates often fluctuates widely. For this reason, the audio speaker is
sometimes a better indicator of small amounts of radioactivity than
the meter reading. At higher count rates, the speaker response is often
faster than the meter reading. It is better, therefore, to have the
speaker on when using a G-M counter.
Very
high radiation fields may temporarily overload the detector circuit
resulting in a partial or complete loss of meter or audio response.
If this happens, remove the meter and yourself from the area and push
the reset button or turn the meter off then back on. The meter should
resume normal operation. Always turn on a survey meter before entering
an area that might have high radiation fields.
SCINTILLATION DETECTOR
Scintillation
detectors which incorporate a sodium iodide crystal are used in some
laboratories for the detection of low energy gamma emitters such as
I-125. Some survey meters allow the use of either a G-M detector or
a scintillation detector. The efficiency of a low energy scintillation
probe for the detection of I-125 is about 5% at one inch -- over a
hundred times better than a G-M probe.
ION CHAMBER
Ionization
chambers are suitable for measuring radiation exposure rate or cumulative
radiation exposure at high radiation intensities. They are not especially
useful at low radiation intensities or for detecting small quantities
of radioactive material.
CALIBRATION
Most survey
meters have scales that read in milliRoentgen per hour (mR/hr) and/or
counts per minute (cpm) or counts per second (cps). After detector efficiency
is taken into consideration, the cpm or cps scales give an indication
of the quantity of radioactivity. The mR/hr scales give an indication
of the radiation exposure rate. There is an important difference in
these measurements. Exposure rate measurements are only valid for electromagnetic
radiation.
Calibration
of the portable radiation survey instruments should be performed by
a qualified individual. Two general types of calibration procedures
-- one for meters that are used for detection and measurement of particulate
radiation, and another for meters used for detection and measurement
of penetrating electromagnetic radiation. The two procedures are explained
briefly below so that you will know what to expect.
Survey
meters used in biology and chemistry research labs are calibrated for
the detection and measurement of particulate radiation. These meters
are calibrated using a pulse generator so that the cpm or cps scales
read correctly (i.e. one pulse in = one meter count). If the meter reads
only in cpm or cps, we may place an additional calibration tag on the
instrument giving the mR/hr equivalent of the count rate reading for
penetrating electromagnetic radiation. If the meter also reads in mR/hr,
those readings may not be accurate for the measurement of electromagnetic
radiation. We will indicate a correction factor.
Survey
meters that are used for radiation exposure measurements are calibrated
with a comparable radiation source. The mR/hr scale will read correctly
when the detector is exposed to electromagnetic radiation greater than
100 keV.
RADIOACTIVE WASTE DISPOSAL
WASTE MINIMIZATION
Due to limitations
in waste disposal minimization of radioactive waste should be adhered
to. Some ways to minimize waste are listed below.
-
Design experiments to use as little radioactive material
as possible.
-
Use proper handling techniques. This will reduce the chance
of contamination.
-
When practical, use techniques which do not involve radioactive
materials. There are many new techniques and products available
which can be used in place of radioactive materials.
-
Monitor for contamination and dispose of as little as
possible. If there is a spot of contamination on a piece of absorbent
paper, cut out that spot and dispose of it rather than the whole
piece. Don't automatically place your gloves in the radioactive
waste. Monitor them. If there is no detectable contamination,
throw them in the regular trash.
-
Liquid radioactive waste includes the radioactive material
and the first rinse of its experimental container. After the first
rinse, the container can be washed in the sink.
SEGREGATION BY HALF-LIFE
All radioactive
waste must be segregated according to radionuclide half-life. The
three categories for segregation are:
-
Half-life less than 15 days (P-32)
-
Half-life between 15 and 90 days (S-35, Cr-51, I-125)
-
Half-life greater than 90 days (H-3, C-14, Ca-45)
Waste
containers should be marked with the category of waste they are intended
for. It is very important that waste is placed in the proper container.
If
waste contains two different radionuclides, place it in the container
appropriate for the longer half-life.
DISPOSAL TAGS
Radiation
Safety should provide a tag for each liquid and solid radioactive
waste container. The top portion of the tag must be filled out completely
with the following information:
PROHIBITED ITEMS
All radioactive
labels, markings, and tape must be defaced or removed before being put
in a waste container.
Solid
waste can not be picked up by Radiation Safety if it contains any
of the following:
-
Hazardous material (e.g. lead, toxins)
-
Biohazard bags or other hazardous material markings
-
Radioactive markings
-
Sharps (e.g. needles, razor blades)
Liquid
radioactive waste must be readily soluble or dispersible in water.
It must not contain any hazardous materials such as solvents or scintillation
fluid.
LEAD PIGS/SHIELDING
Lead shipping
containers and other lead shielding should not be disposed of as ordinary
trash or placed in solid radioactive waste containers. Lead which
is boxed and identified will be picked up by Radiation Safety when
requested.
GELS
If a gel
is very solid at room temperature, it may be disposed of as solid waste.
If it is soft or semi-solid at room temperature, use a solubilizer to
liquefy it and dispose of it as liquid waste.
RADIATION SAFETY FOR X-RAY UNITS
NATURE OF ANALYTICAL X-RAYS
Analytical
x-ray machines produce intense beams of ionizing radiation that are
used for diffraction and fluorescence studies. The most intense part
of a beam is that corresponding to the K emission of the target material
and is called characteristic radiation. In addition to the characteristic
radiation, a continuous radiation spectrum of low intensity is produced
ranging from a very low energy to the maximum kV-peak setting. This
is referred to as 'bremsstrahlung' or white radiation. Undesirable wavelengths
may be filtered out using a monochromator.
X-ray
diffraction wavelengths (w) are selected so as to roughly correspond
to the inter-atomic distances within the sample, and to minimize fluorescence.
Wavelengths commonly used are 1.54 Å (Cu targets), 0.71 Å
(Mo targets), 0.56 Å (Ag targets), and 2.3 Å (Cr targets).
The relationship between wavelength and x-ray photon energy is determined
by the equation
E = hc/w
where
E = energy in ergs (1eV = 1.6E-12 erg)
h = Planck's constant = 6.614E-27 erg-sec
c = velocity of light = 3E10 cm/sec
w = wavelength in cm (1Å = 1E-8 cm)
X-rays
emitted from an open, uncollimated port form a cone of about 30 degrees.
The x-ray flux can produce a radiation field at one meter on the order
of 10,000 R/hr. A collimator reduces the beam size to about 1 millimeter
diameter.
X-RAY HAZARDS AND BIOLOGICAL EFFECTS
X-rays produced
by diffraction machines are readily absorbed in the first few millimeters
of tissue, and therefore do not contribute any dose to the internal
organs of the body. However, the lens of the eye can receive a dose
from x-rays of this energy. Overexposure of lens tissue can lead to
the development of lens opacities and cataracts.
Absorbed
doses of a few hundred rad may produce a reddening of the skin (erythema)
which is transitory in nature. Higher doses -- 10,000 rad and greater
-- may produce significant cellular damage resulting in pigment changes
and chronic radiation dermatitis. Exposure to erythema doses may not
result in immediate skin reddening. The latent period may be from several
hours to several days.
(Note:
X-rays used for medical diagnosis are about one order of magnitude
shorter in wavelength. Diagnostic rays are designed for tissue penetration
and are carefully filtered to avoid x-ray damage to the skin caused
by the longer, more readily absorbed wavelengths).
SOURCES OF IONIZING RADIATION
The primary
beam is not the only source of ionizing radiation. Any high voltage
discharge is a potential source of x-rays. Faulty high-voltage vacuum-tube
rectifiers may emit x-rays of twice the voltage applied to the x-ray
tube. Other sources of ionizing radiation are:
-
Secondary emissions and scattering from the sample, shielding
material, and fluorescent screens.
-
Leakage of primary or scattered x-rays through gaps and
cracks in shielding.
-
Penetration of the primary beam through or scattering
from faulty shutters, beam traps, or collimator couplings.
SAFETY PRECAUTIONS AND NOTES
The shielding,
safety equipment and safety procedures prescribed for x-ray diffraction
equipment are applicable only for up to 75 kV-peak x-rays. Additional
or greater precautions are necessary for machines operating at higher
voltages.
The
PI has the basic responsibility for providing a safe working environment
by ensuring that equipment is operationally safe and that users understand
safety and operating procedures.
The
equipment operator is responsible for his own safety and the safety
of others when using an analytical x-ray machine.
Prior
to removing shielding or working in the sample area, the operator must
check both the warning lights and the current (mA) meter on the console.
Never trust a warning light unless it is on! Always use a survey meter
to check that the shutters are actually closed if current is still being
supplied to the tube. It is possible for a shutter to be stuck partially
open even when the indicator shows that it is shut. The best way to
avoid an accidental exposure is to turn the machine off before working
in the sample area.
Never
put any part of the body in the primary beam. Exposure of any part of
the body to the collimated beam for even a fraction of a second may
result in damage to the exposed tissue.
A
person not knowledgeable about x-ray equipment should not attempt to
make repairs or remedy malfunctions. If you suspect a machine is malfunctioning,
turn it off or unplug it. Place a note on the control panel and inform
the PI or his designated representative.
Repairs
to the high voltage section must not be made unless the primary leads
are disconnected from the high voltage transformer and a signed and
dated notice is posted near the x-ray ON switch. Turning off a circuit
breaker is not sufficient.
Bare
feet are not permitted in the laboratory or around electrical equipment.
Even slightly moist skin is an excellent electrical conductor and contact
with faulty, ungrounded equipment may result in severe injury or death.
Do
not attempt to align x-ray cameras without first consulting an experienced
person. Alignment procedures require special training and knowledge.
Special
care is required when one power supply is connected to more than one
x-ray tube.
EYE PROTECTION
The use
of safety glasses or prescription lenses is encouraged when working
with analytical x-rays. While glasses cannot be depended upon to provide
complete protection to the eyes, they can reduce x-ray exposure. Glass
provides about 10 times the protection of plastic. Neither, however,
will adequately protect the eye from direct exposure to the primary
beam.
FLUORESCENT SCREENS
It is unsafe
to inspect an x-ray beam with a fluorescent screen without special
precautionary measures. Notify the Safety Office before performing
a procedure using a fluorescent screen.
TUBE STATUS INDICATORS
There must
be a visual indication located on or near the tube head to indicate
when x-rays are being produced This is usually an assembly consisting
of two red bulbs, wired in parallel and labeled X-RAYS ON. If one
of the lights is burned out, the operator should either replace it
before leaving the room, or leave a note on the light assembly indicating
that the bulb is burned out. An unlit warning bulb does not necessarily
mean that x-rays are not being produced. Always check the control
panel.
SAFETY DEVICES
Interlock
switches are used to prevent inadvertent access to the beam. They should
not be bypassed. Interlocks should be checked periodically to insure
that they are functioning properly.
Interlocks
and other safety devices and warning systems are not foolproof or fail-safe.
A safety device should be used as a back-up to minimize the risk of
radiation exposure -- never as a substitute for proper procedures and
good judgment.
ACCELERATOR SAFETY
Accelerator
facility designs, operating procedures, and safety protocols (including
requirements for installed safety devices) should be reviewed and approved
by the director.
In
research environments such as the Sloan, Kellogg, and Steele accelerator
facilities, a major responsibility for safety is placed on the users.
Whenever you are in one of the accelerator facilities, you must be aware
of and follow the safety protocols of that facility.
Before
you start an operation or enter the accelerator or target rooms, be
sure to have planned what you are going to do, and understand the
safety precautions you will need to take. In particular:
-
Know the location of the installed x-ray/gamma and neutron
detectors and how to interpret their associated radiation meters
adjacent to the control console.
-
Portable Geiger-Mueller, ion chamber, and fast neutron
detectors are available. Know their capabilities and limitations
and how to interpret their readings. If a detector is not working
or if you have doubts about its accuracy, report it to a responsible
person so that it can be repaired or calibrated as soon as possible.
-
Before you enter the accelerator or target room, always
check the console to see if terminal voltage is being generated
and if the meters from the installed detectors indicate the presence
of radiation.
-
If you need to enter the accelerator or target room when
there is voltage on the accelerator high voltage terminal:
-
During accelerator conditioning, hazardous radiation fields
may exist near the high voltage terminal. You should avoid being
in the vicinity of the tank at any time during the conditioning
period.
-
In addition to radiation hazards, accelerators generally
involve high voltage power supplies and various kinds of moving
machinery that may constitute serious hazards for electrical shock
or injury.
-
Safety devices and installed radiation detectors must
be maintained in an operating condition. Inoperative equipment
affecting safety must be reported to the Principal Investigator
immediately.
APPENDICES
DOSE CONCEPTS
INTRODUCTION
This discussion
is provided as an additional source of information to those who desire
a more in-depth understanding of radiation dose concepts.
TOTAL DOSE CONCEPT
Previously,
the radiation doses received from external radiation sources and internally
deposited radioactive materials were treated separately. Limits on internal
uptake of radioactive materials were based on the dose to a "critical
organ" and could not be compared to the 'whole body' dose received
from an external source.
The
external dose number was and still is related to the risk of stochastic
effects (primarily cancer). For a stochastic effect, the higher the
dose received, the greater the chance of developing the effect. The
new regulations have a mechanism for determining the increased risk
of stochastic effects from an intake of radioactive material. The
dose calculated is based on a variety of factors such as the biological
half-life of the material, the distribution of the material in the
body, and the type and energy of the radiation. The result is that
both the external dose and the internal dose are related to the risk
of stochastic effects and thus can be added to obtain a total dose.
ORGAN DOSE
For a few
radionuclides, the limits on intake are based on nonstochastic effects
rather than stochastic effects. For a nonstochastic effect, the higher
the dose received, the more severe the effect. However, unlike stochastic
effects, there is a threshold dose, i.e. a certain dose, below which
the effect will not occur. Limits on the internal intake of radioactive
materials are set to keep organ doses well below the thresholds. Even
in these cases, however, the additional risk of stochastic effects must
also be determined.
The
dose limit for external exposure of the lens of the eye is also based
on prevention of a nonstochastic effect (lens opacities).
DEFINITIONS
Absorbed
Dose
The energy imparted by ionizing radiation per unit mass of
irradiated material.
Dose Equivalent
The product of the absorbed dose in tissue, quality factor,
and all other necessary modifying factors at the location of interest.
Deep-dose
Equivalent (DDE)
The dose equivalent at a tissue depth of 1 cm. (Applies to
external whole-body exposure.)
Shallow-dose
Equivalent
The dose equivalent at a tissue depth of 0.007 cm. (Applies
to external exposure of the skin or an extremity.)
Eye Dose
Equivalent
The dose equivalent at a tissue depth of 0.3 cm. (Applies to
the external exposure of the lens of the eye.)
Committed
Dose Equivalent (CDE)
The dose equivalent to organs or tissues of reference that
will be received from an intake of radioactive material by an individual
during the fifty-year period following the intake.
Weighting
Factor
For an organ or tissue, the proportion of the risk of stochastic
effects when the whole body is irradiated uniformly.
Committed
Effective Dose Equivalent (CEDE)
The sum of the products of the weighting factors applicable
to each of the body organs or tissues that are irradiated and the CDE
to these organs or tissues.
Total Effective
Dose Equivalent (TEDE)
The sum of the deep-dose equivalent (for external exposures)
and the committed effective dose equivalent (for internal exposures).
TEDE = DDE + CEDE
Total Organ
Dose Equivalent (TODE)
The sum of the DDE and the CDE to an organ or tissue.
Annual Limit
on Intake (ALI)The derived limit for the amount of radioactive material
taken into the body of an adult worker by inhalation or ingestion
in a year. ALI is the smaller value of intake of a given radionuclide
in a year by the reference man that would result in a CEDE of 5 rem
or a CDE of 50 rem to any individual organ or tissue.
EXAMPLE DOSE CALCULATIONS
Fortunately,
the NRC has already determined the ALIs for all of the radionuclides
and listed them in a table. This makes calculating CEDEs and CDEs fairly
simple. Table 2 shows the ALIs for several of the radionuclides used
at Caltech.
EXAMPLE
1
P-32 in
most chemical forms has an ALI for ingestion of 600 uCi. This is listed
as a stochastic ALI, which means that ingesting 600 uCi of P-32 would
result in a CEDE of 5 rem.
If
a worker accidentally ingests 10 uCi of P-32, the CEDE would be (10
uCi)(5 rem/600 uCi) = 0.083 rem = 83 mrem.
EXAMPLE
2
I-125 has
a nonstochastic ALI for inhalation of 60 uCi. This means that inhaling
60 uCi of I-125 would result in a CDE to the thyroid of 50 rem. The
stochastic ALI for inhalation of I-125 is 200 uCi.
If
a worker accidentally inhales 3 uCi of I-125, the CDE to the thyroid
would be (3 uCi)(50 rem/60 uCi) = 2.5 rem. The CEDE would be (3 uCi)(5
rem/200 uCi) = 0.075 rem.
Suppose
this worker also received an external dose from working with a high
energy gamma emitter. Evaluation of his film badge showed a DDE of 50
mrem. The TEDE would then be 50 mrem + 75 mrem = 125 mrem.
EMBRYO/FETUS DOSE
The dose
limit to the embryo/fetus of a declared pregnant woman is 0.5 rem. Efforts
must also be made to avoid a dose substantially higher than 0.06 rem
in one month. A declared pregnant woman means a woman who has voluntarily
informed the Safety Office, in writing, of her pregnancy and the estimated
date of conception.
The
dose to an embryo/fetus is the sum of the deep-dose equivalent to
the declared pregnant woman and the dose from internally deposited
radionuclides in the embryo/fetus and in the woman.
DOSE REPORTING
Each worker
who is monitored must be advised annually of his or her dose.
DOSE LIMITS
A summary
of dose limits implemented in several research facilities that planned radiation doses shall not
exceed ten percent of the limits for adult radiation workers.
The
dose limit for an individual member of the public is 0.1 rem/year TEDE.
Table
1
Revised Occupational Dose Limits
Dose Category Adult Occupational Dose Limit
Total Effective Dose 5 rem/year*
Total Organ Dose 50 rem/year to any individual organ or tissue except the
Equivalent (TODE) lens of the eye*
Eye Dose Equivalent 15 rem/year*
Shallow Dose Equivalent 50 rem/year*
Embryo/Fetus Dose 0.5 rem for the entire gestation period
*Occupational dose limit for minors is 10% of the adult limit
Table
2
Annual Limit on Intake (ALI) for Radionuclides Commonly Used at
Caltech
Radionuclide Form ALI for ALI for
S-35 most 8E3 stochastic 2E4 stochastic
Compounds 1E4 nonstochastic
I-125 all 4E1 nonstochastic 6E1 nonstochastic
Compounds 1E2 stochastic 2E2 stochastic
RADIATION RULES OF THUMB
ALPHA PARTICLES
An alpha
energy of at least 7.5 MeV is required to penetrate the protective layer
of the skin (0.07mm).
BETA PARTICLES
A beta energy
of at least 70 keV is required to penetrate the protective layer of
the skin (0.07mm).
The
average energy of a beta-spectrum is approximately one-third the maximum
energy.
The
range of beta particles in air is about 12 ft per MeV. (e.g. The maximum
range of P-32 betas is 1.71 MeV x 12 ft/MeV = 20 ft).
The
skin dose rate from a uniform thin deposition of 1 uCi/cm^2 is about
9 rem/hr for energies above 0.6 MeV.
For
a beta emitter point source, the dose rate in rem/hr at one foot is
approximately 300 x Ci where Ci is the source strength in curies. This
calculation neglects any shielding provided by the air, which can be
significant. For example, the maximum range in air for a beta from S-35
is less than one foot, so the dose rate at one foot is zero for any
size S-35 source.
GAMMAS AND X-RAYS
For a point
source gamma emitter with energies between 0.07 and 2 MeV, the exposure
rate in R/hr at 1 foot is approximately 6 x C x E x n, where C is the
activity in curies; E is the energy in MeV; and n is the number of gammas
per disintegration.
Gammas
and x-rays up to 2 MeV will be attenuated by at least a factor of 10
by 2 inches of lead.
SI UNITS AND CONVERSION FACTORS
SI
Units for Radioactive Materials
Prepared
by
U.S.
Council for Energy Awareness
Committee
on Radionuclides and Radiopharmaceuticals
SI
(Systeme International) units are now being used in many countries as
the primary measurement system, including measurement of radioactivity,
and the system is coming into use in the United States. Many journals
(including those published by the American Medical Association) now
require the use of SI units, and U.S. regulatory agencies are beginning
to use SI units as well as conventional units in regulations. It is
the policy of the United States Government that regulations should not
impede the transition to SI units.
The
U.S. Trade Act of 1988 includes a provision establishing federal policy
to designate the metric system as the preferred measurement system for
U.S. trade and commerce. It also requires all federal agencies to adopt
the metric system for business-related activities by 1992, except where
it proves impractical.
The
SI unit for radioactivity is the becquerel (Bq), and is defined as one
nuclear transformation per second. It is a small unit when compared
to the curie (Ci), and it is convenient to use multiples of the unit
(see listing later in this brochure). It does have the convenience however
of relating directly to count rate once corrections have been made for
counting efficiency.
Most
suppliers of radioactive materials including the National Institute
of Standards Technology (NIST-formerly NBS) have been using dual units
(curies and becquerels) in catalogs, product literature and labelling
for some time and plan to do so for the foreseeable future. The European
Economic Community (EEC) has stated that it will accept only SI units
for radioactivity after 1999, and it is anticipated that all suppliers
of radioactive products will be using only SI units at that time. In
Canada, Atomic Energy Control Board documents produced since 1985 have
been in SI units only, and conversion of regulations is in progress.
Other
SI radiation measurement units are as follows:
Exposure and Exposure Rate
The roentgen
(R) is the traditional unit of measurement for exposure, the charge
produced in air by gamma or x-rays. The SI unit of exposure is coulombs
per kilogram (C/kg) of air.
1
C/kg = 3876 R
1
R = 2.58E-4 C/kg
No
special name has been given to this SI unit (C/kg) and since there is
no convenient conversion to other SI units, it is seldom used. Instead,
the observed dose rate in air, that is the air kerma rate, is typically
being used as the SI measurement to replace exposure rate. An example
of the use of air kerma rate is to define the radiation output from
a sealed radioactive source in SI units. The SI units usually used to
express air kerma rate are grays/second. In traditional units, exposure
rate from a sealed source has typically been expressed in roentgens/hour
at a distance of 1 meter from the source.
Charge
as defined in exposure (charge produced in air by gamma and X-radiation)
does not include ionization produced by bremsstrahlung arising from
absorption of electrons (beta particles). Apart from this difference,
which is significant only with high energy beta particles, exposure
is the ionization equivalent of air kerma. For a further discussion
of air kerma see ICRU (International Commission on Radiation Units and
Measurements) Report 33, 1980.
Absorbed Dose
This is
the amount of energy imparted to matter, and the rad has been the unit
of measurement. The SI unit for absorbed dose is the gray (Gy).
1
Gray (Gy) = 100 rad
1
rad =
0.01 Gy
One
roentgen of X-radiation in the energy range of 0.1-3 MeV produces 0.96
rad in tissue.
Dose Equivalent
The dose
equivalent is the absorbed dose multiplied by modifying factors such
as a quality factor (accounts for the biological effect of different
types of radiation) and the dose distribution factor. The rem is the
unit of measurement that has been used, and the SI unit is the sievert
(Sv).
1
Sv = 100 rem
1
rem = 0.01 Sv
CONVERSION TABLE FOR RADIOACTIVITY
Curie Units Becquerel Units
Curie Units Becquerel Units
To
convert from one unit to another, read across from one column to the
other ensuring the units are in the same line of the column headings.
For example:
From
the first table:
0.1
mCi =
3.7 MBq
0.1
Ci =
3.7 GBq
From
the second table:
50
mCi =
1.85 GBq
3.7
MBq = 100
uCi
SI Units
1 becquerel
(Bq) = 1 disintigration/second
1
becquerel = 2.7027E-11 curie or about 27 picocuries (pCi)
To
convert becquerels to curies, divide the becquerel figure by 37E9 (alternatively
multiply the becquerel figure by 2.7027E-11)
1
curie (Ci) = 3.7E10 disintigrations/second or 37 gigabecquerels (GBq)
To
convert curies to becquerels, multiply the curie figure by 37E9
Curie
units that are frequently used:
1
Curie (Ci) = 1000 mCi
1
millicurie (mCi) = 1000 uCi
1
microcurie (uCi) = 1000 nCi
1
nanocurie (nCi) = 1000 pCi (picocuries)
Becquerel
units that are frequently used:
1
kilobecquerel (kBq) = 1000 Becquerels (Bq)
1
megabecquerel (MBq) = 1000 kBq
1
gigabecquerel (GBq) = 1000 MBq
1
terabecquerel (TBq) = 1000 GBq
1
Ci = 37 GBq
1
mCi = 37 MBq
1
uCi = 37 kBq
1
nCi = 37 Bq
GLOSSARY OF TERMS
ABSORBED
DOSE:
The energy imparted by ionizing radiation per unit mass of
irradiated material.
ABSORPTION:
The process by which radiation imparts some or all of its energy
to any material through which it passes.
ACTIVITY:
The rate of decay (disintegrations/time) of a given amount
of radioactive material.
ALARA:
An acronym for As Low As Reasonably Achievable. The principal
that radiation doses should be kept as low as reasonably achievable
taking into account economic and social factors.
ALPHA PARTICLE
(a):
A strongly ionizing particle emitted from the nucleus during
radioactive decay which is equivalent to a helium nucleus (2 protons
and 2 neutrons).
ANNIHILATION
RADIATION:
The two 511 keV photons produced when a positron combines with
an electron resulting in the annihilation of the two particles.
ANNUAL LIMIT
ON INTAKE (ALI):
The derived limit for the amount of radioactive material taken
into the body of an adult worker by inhalation or ingestion in a year.
ALI is the smaller value of intake of a given radionuclide in a year
by the reference man that would result in a CEDE of 5 rem or a CDE of
50 rem to any individual organ or tissue.
ATOMIC MASS
UNIT (amu):
One-twelfth the mass of a neutral atom of C-12. (1 amu = 1.66E-24
g)
ATOMIC NUMBER
(Z):
The number of protons in the nucleus of an atom.
ATTENUATION:
Process by which a beam of radiation is reduced in intensity
when passing through material -- a combination of absorption and scattering
processes.
AUTORADIOGRAPH:
Record of radiation from radioactive material in an object,
made by placing the object in close proximity to a photographic emulsion.
BACKGROUND
RADIATION:
Ionizing radiation arising from sources other than the one
directly under consideration. Background radiation due to cosmic rays
and the natural radioactivity of materials in the earth and building
materials is always present.
BECQUEREL
(Bq):
The SI unit of activity equal to one disintegration per second.
(1 Bq = 2.7E-11 Ci).
BETA PARTICLE
(B):
A charged particle emitted from the nucleus of an atom, having
a mass equal to that of the electron, and a single positive or negative
charge.
BIOLOGICAL
HALF-LIFE:
The time required for the body to eliminate by biological processes
one-half of the amount of a substance which has entered it.
BREMSSTRAHLUNG:
X-rays produced by the deceleration of charged particles passing
through matter.
CARRIER
FREE:
An adjective applied to one or more radionulcides of an element
in minute quantity, essentially undiluted with stable isotope carrier.
COMMITTED
DOSE EQUIVALENT (CDE):
The dose equivalent to organs or tissues of reference that
will be received from an intake of radioactive material by an individual
during the fifty-year period following the intake.
COMMITTED
EFFECTIVE DOSE EQUIVALENT (CEDE):
The sum of the products of the weighting factors applicable
to each of the body organs or tissues that are irradiated and the CDE
to these organs or tissues.
COMPTON
SCATTERING:
The elastic scattering of a photon by an essentially free electron.
CONTAMINATION:
The deposition of radioactive material in any place where it
is not desired, particularly in any place where its presence may be
harmful.
COUNT:
The external indication of a device designed to enumerate ionizing
events.
CURIE (Ci):
The unit of activity equal to 3.7 x 1010 disintegrations per
second.
DEEP-DOSE
EQUIVALENT (DDE):
The dose equivalent at a tissue depth of 1 cm from external
radiation.
DOSE:
A general term denoting the quantity of radiation or energy
absorbed in a specified mass.
DOSE EQUIVALENT:
The product of the absorbed dose in tissue, quality factor,
and all other necessary modifying factors at the location of interest.
EFFECTIVE
HALF-LIFE:
Time required for a radioactive nuclide in the body to be diminished
fifty percent as a result of the combined action of radioactive decay
and biological elimination.
EFFICIENCY:
The ratio of the count rate given by a radiation detection
instrument and the actual disintegration rate of the material being
counted.
ELECTRON
CAPTURE:
A mode of radioactive decay involving the capture of an orbital
electron by its nucleus resulting in conversion of a proton to a neutron.
ELECTRON
VOLT (eV):
A unit of energy equal to the amount of energy gained by an
electron passing through a potential difference of 1 volt.
ERYTHEMA:
An abnormal reddening of the skin due to distention of the
capillaries with blood.
EXPOSURE:
A measure of the ionizations produced in air by x-ray or gamma
radiation. Sometimes used to mean dose.
EYE DOSE
EQUIVALENT:
The dose equivalent at a tissue depth of 0.3 cm from external
radiation at the eye.
FILM BADGE:
A packet of photographic film in a holder used for the approximate
measurement of radiation dose.
GAMMA:
Electromagnetic radiation (photon) of nuclear origin.
GEIGER-MUELLER
(G-M) COUNTER:
A radiation detection and measurement instrument.
GRAY (Gy):
The SI unit of absorbed dose equal to 1 Joule/kilogram.
HALF VALUE
LAYER:
The thickness of any specified material necessary to reduce
the intensity of an x-ray or gamma ray beam to one-half its original
value.
HEALTH PHYSICS:
The science concerned with the recognition, evaluation, and
control of health hazards from ionizing radiation.
ION:
Atomic particle, atom, or chemical radical bearing an electrical
charge, either negative or positive.
IONIZATION:
The process by which a neutral atom or molecule acquires either
a positive or a negative charge.
IONIZATION
CHAMBER:
A radiation detection and measurement instrument.
IONIZING
RADIATION:
Any electromagnetic or particulate radiation capable of producing
ions, directly or indirectly, by interaction with matter.
ISOTOPES:
Nuclides having the same number of protons in the nuclei, and
hence having the same atomic number, but differing in the number of
neutrons, and therefore in mass number. Almost identical chemical properties
exist among isotopes of a particular element.
LABELLED
COMPOUND:
A compound consisting, in part, of radioactive nuclides for
the purpose of following the compound or its fragments through physical,
chemical, or biological processes.
LINEAR ENERGY
TRANSFER (LET):
Average amount of energy lost per unit track length by the
individual particles or photons in radiation passing through an absorbing
medium.
MASS NUMBER
(A):
The number of protons and neutrons in the nucleus of an atom.
NUCLIDE:
An of atom characterized by its mass number, atomic number,
and energy state of its nucleus.
POSITRON:
A particle having a mass equal to that of an electron and a
charge equal to that of an electron, but positive.
QUALITY
FACTOR (Q):
The LET-dependant modifying factor that is used to derive dose
equivalent from absorbed dose.
RAD:
The unit of absorbed dose equal to 100 erg/gram (or 0.01 Joule/kilogram).
RADIATION:
Energy propagated through space or a material medium.
RADIOACTIVE
DECAY:
Disintegration of the nucleus of an unstable nuclide by the
spontaneous emission of charged particles, neutrons, and/or photons.
RADIOACTIVE
HALF-LIFE:
The time required for a radioactive substance to lose fifty
percent of its activity by decay.
RADIOACTIVITY:
The property of certain nuclides of spontaneously disintegrating
and emitting radiation.
RADIONUCLIDE:
An unstable (radioactive) nuclide.
RADIOTOXICITY:
The potential of a radioactive material to cause damage to
living tissue by radiation after introduction into the body.
REM:
The unit of dose equivalent equal to the absorbed dose in rad
multiplied by any necessary modifying factors.
ROENTGEN
(R):
The unit of radiation exposure in air equal to 2.58E-4 coulombs/kilogram.
SCINTILLATION
COUNTER:
A radiation detection and measurement instrument in which light
flashes produced in a scintillator by ionizing radiation are converted
into electrical pulses by a photomultiplier tube.
SHALLOW-DOSE
EQUIVALENT:
The dose equivalent at a tissue depth of 0.007 cm from external
exposure of the skin or an extremity.
SIEVERT
(Sv):
The SI unit of dose equivalent equal to 1 Joule/kilogram.
SPECIFIC
ACTIVITY:
Total activity of a given radionuclide per unit mass or volume.
SYSTEME
INTERNATIONAL (SI):
A system of units adopted by the 11th General Conference on
Weights and Measurements in 1960 and used in most countries of the world.
THERMOLUMINESCENT
DOSIMETER (TLD):
A dosimeter made of a crystalline material which is capable
of both storing energy from absorption of ionizing radiation and releasing
this energy in the form of visible light when heated. The amount of
light released can be used as a measure of absorbed dose.
TOTAL EFFECTIVE
DOSE EQUIVALENT (TEDE):
The sum of the deep-dose equivalent (for external exposures)
and the committed effective dose equivalent (for internal exposures).
TEDE = DDE + CEDE
TOTAL ORGAN
DOSE EQUIVALENT (TODE):
The sum of the DDE and the CDE to an organ or tissue.
WEIGHTING
FACTOR:
The proportion of the risk of stochastic effects for an organ
or tissue when the whole body is irradiated uniformly.
X-RAY:
Electromagnetic radiation (photon) of non-nuclear origin having
a wavelength shorter than that of visible light.
REFERENCES
Basic
Radiation Biology. D. J. Pizzarello and R. L. Witcofski;
Lea & Febiger, 1967.
Basic
Radiation Protection Criteria. National Council on Radiation Protection and Measurements
(NCRP) Report No. 39; NCRP, 1971.
The
Dictionary of Health Physics & Nuclear Sciences Terms. R. J. Borders; RSA Publications,
1991.
The
Effects on Populations of Exposure to Low Levels of Ionizing Radiation:
1980. Committee on the Biological Effects
of Ionizing Radiations (BEIR); National Academy Press, 1980.
Health
Effects of Exposure to Low Levels of Ionizing Radiation. Committee on the Biological Effects
of Ionizing Radiations (BEIR); National Academy Press, 1990.
The
Health Physics and Radiological Health Handbook, Revised Edition. B. Shleien; Scinta, 1992.
The
Health Physics and Radiological Health Handbook. B. Shleien; Nucleon Lectern Associates,
1984.
Influence
of Dose and its Distribution in Time on Dose-Response Relationships
for Low-LET Radiations.
NCRP Report No. 64; NCRP, 1980.
Introduction
to Health Physics.
H. Cember; Pergamon Press, 1969.
The
Physics of Radiology.
H. E. Johns, and J. R. Cunningham; Thomas, 1978.
Radiation
Biophysics.
H. Andrews; Prentice-Hall, 1961.
Radiation
Protection Design Guidelines for 0.1 - 100 MeV Particle Accelerator
Facilities. NCRP Report No. 51; NCRP, 1977.
Radiation
Protection Training Manual. A. Zea; University of Southern California.
