|<< Back to Ionizing Radiation
Robert A. Curtis - 1/99
- Basic Model of a Neutral Atom.
Electrons(-) orbiting nucleus of protons(+) and neutrons.
Same number of electrons as protons; net charge = 0. Atomic number
(number of protons) determines element. Mass number (protons +
sneutrons) gives mass in terms of 1/12th mass of Carbon atom.
- Definition of Ionizing Radiation.
Ionization vs. Excitation: Excitation
transfers enough energy to an orbital electron to displace it further away from the
nucleus. In ionization
the electron is removed, resulting in an ion
pair (the newly freed electron(-) and the rest of the atom(+))
Ionizing Radiation: Any electromagnetic or
particulate radiation capable of producing ion pairs by interaction with
matter. Scope limited to X and gamma
rays, alpha particles, beta particles (electrons), neutrons,
and charged nuclei.
Particularly important biologically since media can be
altered (e.g., ionized atom in DNA molecule may be altered, thereby causing cell death, or
a change in cell reproduction, division, or mutation).
- TYPES OF IONIZING RADIATIONS
- General Characteristics
Particulate vs. Electromagnetic Radiations: Particulate
Radiations are sub-atomic particles with mass (e.g., alpha and
Beta particles, electrons, neutrons). EM Radiations (X-rays and gamma
rays) have no mass and no charge.
High vs. Low Energy Radiation: Absorption of
radiation is the process of transferring the energy of the radiation to the atoms of the
media through which it is passing. Higher energy radiation of the same
type will penetrate further. Usually expressed in KeV or MeV (103
or 106 electron Volts). 1 eV = 1.6 x 10-19 Joules = 1.6 x 10-12 ergs
High vs. Low Linear Energy Transfer (LET) to absorbing
material: LET is measured by the ionization density (e.g., ion pairs/cm of tissue)
along the path of the radiation. Higher LET causes greater biological impact and
is assigned a higher Quality Factor(QF). Example QF values: X, gamma, and beta
have QF = 1; alpha QF=20; thermal neutrons QF=3; "fast" neutrons (>10 KeV) QF
= 10; fission fragments QF>20.
- Characteristics of Common Radiations
Alpha Particles (or Alpha Radiation):
Helium nucleus (2 neutrons and 2 protons); +2 charge; heavy (4
AMU). Typical Energy = 4-8 MeV; Limited range (<10cm in air;
60µm in tissue); High LET (QF=20) causing heavy damage
(4K-9K ion pairs/µm in tissue). Easily shielded (e.g., paper, skin) so
an internal radiation hazard. Eventually lose too much energy to ionize;
speed electron ejected from nucleus; -1 charge, light 0.00055
AMU; Typical Energy = several KeV to 5 MeV; Range approx. 12'/MeV in air, a few mm in
tissue; Low LET (QF=1) causing light damage (6-8 ion
pairs/µm in tissue). Primarily an internal hazard, but high beta can be an external
hazard to skin. In addition, the high speed electrons may lose energy in the
form of X-rays when they quickly decelerate upon striking a heavy material. This is called
Bremsstralung (or Breaking) Radiation. Aluminum
and other light (<14) materials and organo-plastics are used for shielding.
Note: Beta particles with an opposite (+) charge are called
positrons. These quickly are annihilated by combination with an electron, resulting in
gamma radiation (see Pair Production below).
ejected from a nucleus; 1 AMU; 0 Charge; Free neutrons are unstable and decay by Beta
emission (electron and proton separate) with T½ of approx. 13 min. Range and
LET are dependant on "speed": Slow (<10 KeV), "Thermal" neutrons, QF=3,
and Fast (>10 KeV), QF=10.
Shielded in stages: High speed neutrons are
"thermalized" by elastic collisions in hydrogenous materials (e.g., water,
paraffin, concrete). The nuclei which are "hit" give off the excess energy as
secondary radiation (alpha, beta, or gamma). Slow neutrons are captured by secondary
shielding materials (e.g., boron or cadmium).
X- and Gamma Rays:
X-rays are photons (Electromagnetic or EM radiations) emitted from
electron orbits, such as when an excited orbital electron "falls" back
to a lower energy orbit; Gamma rays are photons emitted from
the nucleus, often as part of radioactive decay. Gamma rays typically
have higher energy (Mev's) than X-rays (KeV's), but both are unlimited.
No mass; Charge=0; Speed = C; Long
range (km in air, m in body); light damage (QF = 1); An external
hazard (>70 KeV penetrates tissue); Usually shielded with lead or concrete
(see equation for shielding effectiveness).
Photon Interactions: Three types of indirect ionization caused by EM radiation.
Photoelectric effect: Can occur at
low energies ( < .5 MeV); incoming photon ejects an electron. Compton effect:
Occurs at medium energies (.5 - 5 MeV); incoming photon ejects an electron and a photon
with longer wavelength. Pair production: Requires high energies (
> 1.02 MeV, usually > 5 MeV); incoming photon ejects an electron and a positron, but
positron quickly encounters an electron and annihilates to two 0.51 MeV gamma rays (E=Mc2).
- Radioactive Decay
Matter transforms from unstable to stable
energy states. Radioactive materials are substances which spontaneously
emit various combinations of ionizing particles (alpha and beta) and gamma
rays of ionizing radiation to become more stable. This process is called radioactive
decay. Radioisotopes are isotopes (same number of protons
but different numbers of neutrons) which are radioactive.
Alpha Decay: Atomic mass reduced by 4; protons reduced by 2.
Radium → alpha particle + Radon
226Ra → 4He +2 + 222Rn
Beta Decay: No change in atomic mass;
protons increase by 1. (Note: Consider a neutron as a proton embedded with an
electron; net charge = 0. When the electron is ejected, a proton is
"created", thus increasing the atomic number.)
Strontium → Beta electron + Y
90Sr → Beta electron + 90Y
Series Decay: Radioactive parent
decays to a "daughter" which may also be radioactive,
therefore, is also simultaneously decaying. Resulting exposure is to the combination of
both decays (and possibly additional daughters). Radon daughters are an important example
of series decay exposure in uranium mines and basements.
- QUANTIFICATION OF RADIATION
- Quantifying Radioactive Decay
Measurement of Activity in disintegrations
per second (dps); 1 Becquerel (Bq) = 1 dps; 1 Curie (Ci)
= 3.7 x 1010 dps; Activity of substances are expressed as activity per weight
or volume (e.g., Bq/gm or Ci/l).
Simple Decay Equations (see example problems 1,2)
| At = Aoe(- x t) = Activity after time t
| Nt = Noe(- x t) = Number of atoms ater time t
| = 0.693/T½ = disintegration
Ao = Original activity
No = Original atoms
T½ = Half-Life = time to reduce to half the original
- Quantifying Exposure and Dose
1 Roentgen (R) = amount of X or gamma
radiation that produces ionization resulting in 1 electrostatic unit (esu) of charge in 1
cm3 of dry air at STP. Instruments often measure
exposure rate in mR/hr.
Absorbed Dose: rad
1 rad (Roentgen absorbed dose) = absorption of 100 ergs of
energy from any radiation in 1 gram of any material; 1 Gray
(Gy) = 100 rads = 1 Joule/kg; Exposure to 1 Roentgen approximates 0.9 rad in air.
Biologically Equivalent Dose: rem
Dose (in rads) = 0.869(f)(Roentgens) where
the f-factor is the ratio of mass energy-absorption coefficient of medium, such as bone,
compared to air.
Rem (Roentgen equivalent man) = dose in rads x QF,
where QF = quality factor. 1 Sievert (Sv) = 100 rems.
- Exposure Limits
Regulatory Agencies: OSHA, personnel exposures (29
CFR 1910.96, 1910.120); Nuclear Regulatory Commission, (10 CFR 19, 20, and 71); Dept of
Transportation, (49 CFR). Most advocate ALARA - As Low As Reasonably
OSHA Limits: Whole body limit = 1.25 rem/qtr or 5
rem (50 mSv) per year (approx. 2.5 mrems/hr for all work hours). Hands and feet
limit = 18.75 rem/qtr. Skin of whole body limit = 7.5 rem/qtr. Total life
accumulation = 5 x (N-18) rem where N = age. Can have 3 rem/qtr
if total life accumulation not exceeded. Restricted areas at 200 mrem/hr. Posting at 200
and 100 mrem/hr. Note: New recommendations reduce the 5 rem to 2 rem.
Working Level Month(WLM): Unit of exposure to Radon
progeny in uranium mines. 1 Working Level Month (WLM) = exposure to 1
Working Level (1.3 x 105 MeV of alpha energy) for one month; roughly 100 pC/l.
Hazardous Waste Sites: Radiation above background
(0.01-0.02 mrem/hr) signifies possible presence which must be monitored. Radiation above 2
mrem/hr indicates potential hazard. Evacuate site until controlled.
- HEALTH EFFECTS
Generalizations: Biological effects are due to the
ionization process that destroys the capacity for cell reproduction or division or causes
cell mutation. The effects of one type of radiation can be reproduced by any other type. A
given total dose will cause more damage if received in a shorter time period. A fatal
dose (600 R) causes a temperature rise of only 0.001 C, and ionization of 1 atom
in 100 million.
Acute Somatic Effects: Relatively immediate effects
to a person acutely exposed. Severity depends on dose. Death usually results from damage
to bone marrow or intestinal wall. Acute radiodermatitis is common in
radiotherapy; chronic cases occur mostly in industry.
ACUTE DOSE(RAD) EFFECT
| No observable effect.
Minor temporary blood changes.
Possible nausea and vomiting and reduced WBC.
Increased severity of above and diarrhea, malaise, loss of appetite. Some death.
Increased severity of above and hemorrhaging, depilation. LD50 at 450-500 rads.
Symptoms appear sooner. LD100 approx. 600 rads.
Delayed Somatic Effects: Delayed effects to exposed
person include: Cancer, leukemia, cataracts, life shortening from organ failure (not
directly observed in man), and abortion. Probability of an effect is proportional to dose
(no threshold). Severity is independent of dose. Doubling dose for cancer is approximately
Genetic Effects: Genetic effects to off-spring of
exposed persons are irreversible and nearly always harmful. Doubling dose for mutation
rate is approximately 50-80 rems. (Spontaneous mutation rate is approx. 10-100 mutations
per million population per generation.)
Critical Organs: Organs generally most
susceptible to radiation damage include: Lymphocytes, bone marrow, gastro-intestinal, gonads, and other
fast-growing cells. The central nervous system is resistant. Many nuclides concentrate in
certain organs rather than being uniformly distributed over the body, and the organs may
be particularly sensitive to radiation damage, e.g., isotopes of iodine concentrate in the
thyroid gland. These organs are considered "critical" for the specific nuclide.
RBE Relative Biological Effectiveness: Ratio
that compares the effect on standard cells to the effect of test cells.
General Population Exposure: Average Annual Dose is
approx. 200-300 mrem per year from Medical(100 mrem), Radon(100+),
Terrestrial(55), Cosmic(30), Fallout(4), Industrial(<1). Internal (e.g., K40,
Ra226, Pb210, Rn22, C14) varies with target
organs, typically < 20mrem (K40 alone
accounts for most).
Occupational Exposures (Examples): Naturally
occurring radioactive materials, such as radon in mining, but many are in sealed sources
for protection (see below); industrial and medical radioisotopes, such as tracer elements;
High Voltage devices such as X-ray machines, Radar generators, VDT's and TV's; nuclear
Sealed Source: Radioactive material that
is permanently bonded or fixed in a capsule or matrix designed to prevent release of the
material under the most severe conditions of normal use and handling.
High Voltage Devices: The electric fields of high
voltage (> 10 KV) devices, such as television and radar klystron tubes, can accelerate
electrons to a high speed such that they escape their orbitals, resulting in an ion pair.
Nuclear Reactions: Nuclear fission (splitting atoms,
e.g., nuclear weapons) and fusion (combining atoms, e.g., the sun combines two H's into
He) both result in the release of energy (radiation).
Neutron bombardment is used to inject an extra neutron into
an otherwise stable atom, thus producing an unstable radionuclide. Neutron
usually occurs in industry when a light weight element is bombarded with alpha particles
or gamma rays, as shown:
|With alpha = 9 Be + 4 He +2 → 1 neutron + 12 C
|With photon = 9 Be + photon → 1 neutron + 8 Be
- RADIATION CONTROLS
- Basic Control Methods for External Radiation
Time: Minimize time of exposure to
minimize total dose. Rotate employees to restrict individual dose.
Distance: Maximize distance to
source to maximize attenuation in air. The effect of distance can be estimated from the
- Estimating Intensity near the Source
||D = Exposure rate (R/hr)
Γ = Specific gamma ray constant in units of R · cm2/mCi · hour
d = distance from source in cm
A = Activity in mCi of the isotope
- Estimating Attenuation in Air Using Inverse Square
||I2, I1 = Intensity
at d2, d1
d2, d1= Distance to source
Shielding: Minimize exposure by
placing absorbing shield between worker and source. The following estimates the
effectiveness of shielding.
|I = IoBe-µx = intensity on other side of shield
|| Io = original
µ = linear absorption coefficient for material
x = shield thickness (same units as #u)
B = radiation scatter "build up" factor (Assume B = 1)
Half-Value Layer (HVL): The shield
thickness necessary to reduce intensity by half.
HVL = 0.693 / µ . Similarly, a tenth-value layer reduces intensity by 10.
Personal Dosimeters: Normally they do not prevent
exposures (no alarm), just record it. They can provide a record of accumulated
exposure for an individual worker over extended periods of time (hours, days or
weeks), and are small enough for measuring localized exposures (e.g., ring badges). Common
types: Film badges; Thermoluminescence detectors (TLD); and pocket dosimeters.
Direct Reading Survey Meters and Counters: Useful in
identifying source of exposures recorded by personal dosimeters, and in evaluating
potential sources, such as surface or sample contamination, source leakage,
inadequate decontamination procedures, background radiation. Some can be used to evaluate
IH samples (e.g., air, bulks, wipes).
| Proportional or
Geiger-Mueller or Proportional counters
Continuous Monitors: Continuous direct reading
ionization detectors (same detectors as above) can provide read-out and/or alarm to
monitor hazardous locations and alert workers to leakage, thereby preventing
Long-Term Samplers: Used to measure average
exposures over a longer time period. For example, charcoal canisters or electrets are set
out for days to months to measure radon in basements (should be <4 pCi/L).
- Example Elements of Radiation Protection Program
Identification and inventory of sources. (Note:
Isotopes are classified as to relative radiotoxicity from Class 1, Very high, to
Class 4, slight toxicity.) Including analysis of reports of inspections and
Monitoring of exposures: Personal, area, and
screening measurements; Medical/biologic monitoring. (Note: NRC requires reporting of
exposures > PEL within 30 days, > 5 rem within 24 hrs, and > 25 rem immediately.)
Task-Specific Procedures and Controls: Initial,
periodic, and post-maintenance or other non-scheduled events. Engineering (shielding) vs.
PPE vs. administrative controls. Including management and employee commitment and
authority to enforce procedures and controls.
Emergency procedures: Response,
"clean-up", post clean-up testing.
Training and Hazard Communications including signs,
warning lights, lockout/tagout, etc. Criteria for need, design, and information given.
Material Handling: Receiving, inventory control,
storage, and disposal.
| Note: DOT Shipping Limits:
|| Label Color
|| 1 Meter
|| White Rad I
Yellow Rad II
Yellow Rad III
- Example from Requirements of 29 CFR 1910.120 Related to Ionizing Radiation
Monitoring with direct reading
instruments; Risk identification; Employee notification
prior to work; Engineering controls, work
practices, and PPE for employee protection; Drums and containers
containing radioactive wastes shall not be handled until their hazard to employees is
properly assessed; and Decontamination.
Attachment 1. Ionizing Radiation Equations
Attachment 2. Example Problems
Attachment 3. Ionization
Attachment 4. EM Interaction
Attachment 5. Radio Active Decay Series
Attachment 6. Maximum Permissible Dose Equivalent for Occupational Exposure
Attachment 7. X-Ray Tube
Attachment 8. Radioactive Isotopes
Attachment 9. Radioactive Cobalt
Attachment 10. Tank Level Detector
Attachment 11. Principal Isotopes in Sealed Sources
Attachment 12. Example of Half-Value Layers
Attachment 13. Shielding Layer Examples
Attachment 14. Gas ionization (Regions of Instrument Response)
Attachment 15. Radiation Detection Instruments