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CHAPTER 4 Radiation Quantities and Units
67
TABLE 4.3 Organ or Tissue Weighting
Factors
Organ or Tissue Weighting Factor (WT)
Gonads 0.20
Red bone marrow 0.12
Colon 0.12
Lung 0.12
Stomach 0.12
Bladder 0.05
Breast 0.05
Liver 0.05
Esophagus 0.05
Thyroid 0.05
Skin 0.01
Bone surface 0.01
Remainder*
*The remainder takes into account the following additional
tissues and organs: adrenals, brain, small intestine, large intestine, kidney, muscle, pancreas, spleen, thymus, and uterus.
†
In extraordinary circumstances in which one of the remainder
tissues or organs receives an equivalent dose in excess of the
highest dose in any of the 12 organs for which a weighting
factor (WT) is specified, a WT of 0.025 should be applied to
that tissue or organ and a WT of 0.025 to the average dose in
the other remainder tissues or organs.
Data from National Council on Radiation Protection and Measurements (NCRP): Limitation of exposure to ionizing radiation,
Report No. 116, Bethesda, Md, 1993, NCRP. Reprinted with
permission of the National Council on Radiation Protection and
Measurements, http://NCRPonline.org.
†
0.05
the risk to the entire organism brought on by all types
of irradiation of individual tissues and organs. The
ICRP originally introduced the WT concept because
uniform, whole-body irradiation seldom occurs, causing different organs and body tissues to vary considerably in the amount of D received and, consequently, the
intensity of their response.
To determine EfD, an D is multiplied by a WR to
obtain EqD and that product is multiplied by a tissue
weighting factor (WT) to give:
EfD 5 D 3 WR 3 W
T
EfD is expressed in sieverts or millisieverts. An
example of determining and expressing EfD in Sv is
provided in Box 4.8.
EfD can be used to compare the average detrimental
amount of radiation received by the entire body from a
specific radiologic examination with that from natural
BOX 4.8 Determining and Expressing
Effective Dose in Sievert
Example: The WR for alpha particles is 20 (see Table 4.2),
and the W
lungs receive an absorbed dose (D) of 0.5 Gy
exposure to alpha radiation, what is the effective dose
(EfD) in Sv?
Answer:
5 0.5 3 20 3 0.12
5 1.2 Sv
for the lung is 0.12 (see Table 4.3). If the
T
EfD 5 D 3 W
3 W
R
T
from
t
TABLE 4.4 Typical Values for Radiation
Doses Associated With an Anteroposterior
Lumbar Spine Examination
Absorbed dose to skin at entrance surface 6.4 mGy
Absorbed dose to bone marrow 0.6 mGy
Absorbed dose to a fetus 3.5 mGy
Equivalent dose to a fetus 3.5 mSv
Effective dose to a fetus 3.3 mSv
background radiation (see Table 1.1). By using the background equivalent radiation time (BERT) method
as discussed in Chapter 1, it is possible to describe
the examination’s significant radiation dose in terms
of the length of time it would take to acquire a comparable amount from environmental sources.
Table 4.4 gives some typical values for radiation
doses that are associated with a radiographic examination of the lumbar spine, and it illustrates some of the
principles of the different ways to specify radiation dose.
The dose to the patient is highest at the “entrance skin
surface,” the surface of the patient that is toward the
x-ray tube. This surface will be exposed to the unattenuated primary beam of x-rays. Absorbed doses to various
organs may be calculated from standard tables. Two
organ absorbed doses are given in Table 4.4, namely,
bone marrow and fetus. The EqD to the fetus is also
given and is the same as the D to the fetus because
the WR is 1. Finally, the EfD to the fetus is given. It was
calculated from the various tissue weighting factors
and organ absorbed doses for fetal organs in the field
of view of this examination.
68
CHAPTER 4 Radiation Quantities and Units
BOX 4.9 Determining Collective Effective
Dose Using the Radiation Unit PersonSievert
Example: If 200 people receive an average effective
dose of 0.25 Sv, the collective effective dose (ColEfD) is
200 3 0.25 5 50 person-sieverts.
Collective Effective Dose
In addition to EqD and EfD, another dosimetric quantity has been derived and implemented for use in radiation protection. It takes into account both internal and
external dose measurements. Collective Effective Dose
(ColEfD) represents an attempt to describe the radia-
tion exposure of a population or group from low doses
of different sources of ionizing radiation. It is determined as the product of the average EfD for an individual belonging to the exposed population or group
and the number of persons exposed. The radiation unit
for this quantity is person-sievert. An example using this
unit is provided in Box 4.9. With respect to the validity
of this concept, the ICRP states: “Collective effective
dose is an instrument for optimization, for comparing
radiological technologies and protection procedures.
ColEfD is not intended as a tool for epidemiological
studies, and it is inappropriate to use it in risk projections. This is because the assumptions implicit in the
calculation of ColEfD (e.g., when applying the LNT*
model) conceal large biological and statistical uncertainties. Specifically, the computation of cancer deaths
based on collective effective doses involving trivial
exposures to large populations is not reasonable and
should be avoided.”
4
Total Effective Dose Equivalent
Total Effective Dose Equivalent (TEDE) is a radiation
dosimetry quantity that was defined by the Nuclear
Regulatory Commission (NRC) to monitor and control
human exposure to ionizing radiation. Essentially, as
described by NRC regulations, it is the sum of EfD
equivalent from external radiation exposures and a
quantity called Committed Effective Dose Equivalent
*LNT, which stands for linear nonthreshold, is a dose model
that implies there is no dose value below which there is no risk
of biologic damage and that the degree of risk is directly proportional to the dose at any level.
TABLE 4.5 SI Unit Equivalents
1 SI exposure unit
equals
1 coulomb equals 1 ampere-second
1 coulomb per kilo-
gram of air equals
1 gray equals 1 J/kg
1 sievert equals 100 centisievert (cSv)
1 joule equals 1 newton-meter
1 joule equals 6.24 3 10
1
R
4
2 58 10
( . )
1 SI unit of exposure
C/kg R
100 cGy
1000 mGy
1 J/kg (for x-radiation, Q 5 1)
1000 mSv
1 J/kg
7
10
1
eV
4
2 58 10
( . )
erg/kg 5 104 erg/gm
18
(CEDE)* from internal radiation exposures. Thus
TEDE is designed to take into account all possible
sources of radiation exposure. It is a particularly useful
dose monitor for occupationally exposed personnel
such as nuclear medicine technologists and interventional radiologists, who are likely to receive possibly
significant radiation exposure during the course of a
year. Traditionally, the whole-body TEDE regulatory
limit is 0.05 Sv for occupationally exposed personnel
and 0.001 Sv for the general public. Radiation monitoring services can provide annual TEDE values for
individuals.
Table 4.5 and Table 4.6 summarize radiation quanti-
ties, units, and equivalents. An additional table emphasizing relationship between traditional units and SI
units is also found in Appendix A.
*The “committed dose” in radiation protection is a measure of
the probabilistic health effect on an individual as a result of an
intake of radioactive material into the body. A “committed dose”
from an internal source takes into account the total amount of
radiation dose delivered by radioactive material in the body until
it is eliminated (exhaled, excreted, physically decays away). This
is the origin of the name “committed effective dose equivalent.”
For nuclear medicine technologists who, through certain procedures (e.g., thyroid ablations, using iodine-131), have a possibility of radioisotope absorption and consequent internal exposure, committed dose is certainly an appropriate measure.
CHAPTER 4 Radiation Quantities and Units
TABLE 4.6 Summary of Radiation Quantities and Units
Measuring
Type of Radiation Quantity SI Unit
X-radiation or
gamma radiation
All ionizing radiations Absorbed dose (D) Gray (Gyt) Any object Amount of energy per unit
All ionizing radiations Equivalent dose (EqD) Sievert (Sv) Body tissue Biologic effects
All ionizing radiations Effective dose (EfD) Sievert (Sv) Body tissue Biologic effects
Exposure (X) Coulombs per
kilogram (C/kg)
Air kerma Gray (Gy-a) Air Kinetic energy deposited in air
Medium Radiation Effect Measured
Air Ionization of air
mass absorbed by objectAir kerma Gray (Gyt)
S U M M A R Y
69
• German physics professor Wilhelm Conrad Roentgen discovered “x-rays” on November 8, 1895, during
an experiment investigating the nature of cathode
rays and fluorescent materials.
• Many individuals who were exposed to substantial
doses of x-rays in the early years after their discovery
developed somatic damage from the exposure.
• Skin erythema dose was used from 1900 to 1930 as
the unit for measuring radiation exposure. Eventually a tolerance dose was established for occupationally exposed individuals that could be regarded as a
threshold dose. MPD replaced the tolerance dose in
the early 1950s. In 1977 dose equivalent or effective
dose equivalent replaced the MPD. In 1991 the ICRP
replaced effective dose equivalent with the term effec-
tive dose, which is still in use today.
• Effective dose is based on the energy deposited
in biologic tissue by ionizing radiation. It takes into
account both the type of radiation and the variable
sensitivity of the tissues exposed to the radiation.
EfD is expressed in the SI unit sievert (Sv) or in subunits of the sievert.
• In 1980 the ICRU adopted SI units for use with
ionizing radiation. Many developed countries, particularly in Europe, have already made a complete
transition to SI units. In the United States, this transition is not as yet fully complete, and some conventional units are still broadly in use.
• SI radiation units are preferred for specifying radiation quantities because the traditional system of
units does not fit into the metric system that provides “one unified system of units for all physical
quantities.”
2
• Coulomb per kilogram (C/kg) is used for specifying
x-ray or gamma ray exposure in air only. This exposure
unit is equal to an electrical charge of 1 coulomb produced in a kilogram of dry air by ionizing radiation.
• Air kerma is an SI quantity that is used to express
how energy is transferred from a beam of radiation
to air.
• DAP is the sum total of air kerma multiplied by the
exposed area of the patient’s surface.
• Absorbed dose (D) is the amount of energy per unit
mass absorbed by an irradiated object.
• The gray (Gy) is used for measuring absorbed
dose in air (Gya) or for measuring absorbed dose in
tissue (Gyt).
• The number of gray times 1000 equals the number
of milligray. The number of gray times 100 equals
the number of centigray.
• LET is the amount of energy transferred on average
by incident radiation to an object per unit length
of track, or passage, through the object and is
expressed in units of kiloelectron volts per micrometer (keV/µm).
• Equivalent dose (EqD) and effective dose (EfD) are
the quantities of choice for measuring biologic effects when all types of radiation must be considered.
• EqD specifies how the potential for biologic damage
from different types and doses of radiation will be
equivalent if correct weighting factors are included.
To calculate equivalent dose: EqD 5 D 3 WR.
• EfD describes the total biologic damage to a human
that is caused by equivalent doses received by specific organs. To calculate effective dose: EfD 5 D 3
WR 3 WT.
70
CHAPTER 4 Radiation Quantities and Units
• In the SI system, sievert (Sv) or the subunits millisievert and microsievert are used to specify EqD
and EfD. These units are used for occupational radiation exposure.
• ColEfD represents an attempt to describe the radiation exposure of a population or group from low
doses of different sources of ionizing radiation.
Person-sievert is the radiation unit used to calculate
this quantity. According to the ICRP it is not a valid
method for computing the potential number of
deaths from cancer.
• The radiation dosimetry quantity, TEDE, is designed
to take into account all possible sources of radiation
• The CEDE in radiation protection is the total effec-
G E N E R A L D I S C U S S I O N Q U E S T I O N S
1. Why should diagnostic imaging personnel be famil-
iar with standardized radiation quantities and units?
2. When, where, and how did Wilhelm Conrad Roent-
gen discover x-rays?
3. What types of medical problems did early radiation
workers develop as a consequence of their occupational exposure?
4. What is the benefit of using the International System
of Units of measurement for ionizing radiation?
5. What is a threshold dose?
6. In 1991 the International Commission on Radiologi-
cal Protection (ICRP) revised tissue weighting factors.
On what data was this revision based?
7. What radiation quantities are currently in use, and
8. What instrument can be calibrated to read air
9. What factors determine the amount of x-ray energy
10. When a person receives exposure from various
11. How is centigray converted to gray?
12. How were radiation dose limits calculated and
exposure and is used for dose monitoring for occupationally exposed personnel who are likely to receive possibly significant radiation exposure during
the course of a year. The whole-body TEDE regulatory limit for exposed personnel is 0.05 sievert and
0.001 sievert for the general public.
tive dose to an individual resulting from an intake of
radioactive material into the body. Such material
may remain in the body for some time, resulting in a
dose that accumulates over time.
what SI units are used to relate these quantities?
kerma?
absorbed by a human anatomic structure?
types of ionizing radiation, what radiation quantity
and what SI unit should be used to specify this
exposure?
established?
R E V I E W Q U E S T I O N S
1. Which of the following was used as the first measure
of exposure for ionizing radiation?
A. Air kerma
B. Skin erythema
C. Sievert
D. Roentgen
2. A radiation weighting factor (WR) has been estab-
lished for each of the following ionizing radiations:
x-rays (WR 5 1), fast neutrons (WR 5 20), and
alpha particles (WR 5 20). What is the total equivalent dose (EqD) in sieverts for a person who has
received the following exposures: 0.2 Gyt of x-rays,
0.07 Gyt of fast neutrons, and 0.3 Gyt of alpha
particles?
A. 9.4 Sv
B. 7.6 Sv
C. 4.3 Sv
D. 1.9 Sv
3. Which of the following is the unit of collective
effective dose (ColEfD)?
A. Coulombs per kilogram-sievert
B. Gray-sievert
C. Person-sievert
D. Rad-sievert
4. The concept of tissue weighting factor (WT) is used
to do which of the following?
A. Account for the risk to the entire organism
brought on by irradiation of individual tissues
and organs
B. Eliminate the need for determining effective dose
C. Measure absorbed dose from all different types of
ionizing radiations
CHAPTER 4 Radiation Quantities and Units
71
D. Modify the radiation weighting factor for different
types of ionizing radiation
5. To convert the number of gray into milligray, the
number of gray must be:
A. Divided by 100
B. Divided by 1000
C. Multiplied by 100
D. Multiplied by 1000
6. What is the SI radiation unit coulomb per kilogram
used to specify?
A. Equivalent dose
B. Absorbed dose in biologic tissue
C. Radiation exposure in air only
D. Speed at which x-ray photons travel
7. Which of the following radiation quantities accounts
for some biologic tissues being more sensitive to
radiation damage than other tissues?
A. Absorbed dose
B. Exposure
C. Equivalent dose
D. Effective dose
8. The radiation weighting factor for alpha particles
is 20, and the tissue weighting factor for the lungs
is 0.12. If the lungs receive an absorbed dose of
0.2 Gyt from exposure to alpha particles, what is
the effective dose in sievert?
A. 0.48 Sv
B. 4.8 Sv
C. 48.0 Sv
D. 480.0 Sv
9. If 100 people received an average effective dose
of 0.35 Sv, what is the collective effective dose?
A. 17.5 person-sieverts
B. 35 person-sieverts
C. 70 person-sieverts
D. 285 person-sieverts
10. How is the SI unit for dose area product (DAP)
usually specified?
A. Coulomb
B. Erg-sec
C. mGy-cm
2
D. Sievert
5
Radiation Monitoring
O B J E C T I V E S
After completing this chapter, the reader will be able to
perform the following:
• Define all key terms.
• Discuss the requirement for a personnel dosimeter
and explain the function and characteristics of such
devices.
• Identify the appropriate location on the body where
the personnel dosimeter(s) should be worn during
the following procedures or conditions: (1) routine
radiographic procedures, (2) fluoroscopic procedures, (3) special radiographic procedures, and
(4) pregnancy.
• Identify the sensing material in the thermoluminescent dosimeter ring badge.
• Describe the various components of the optically
stimulated luminescence (OSL) dosimeter, and the
personnel direct ion storage dosimeter (DSI) and
explain the use of each of these devices as personnel
monitors.
• Explain the function of radiation survey instruments.
• List three gas-filled radiation survey instruments.
• Explain the requirements for radiation survey
instruments.
• Recognize the operating regions of the following
instruments: (1) ionization chamber–type survey
meter (cutie pie), (2) proportional counter, and
(3) Geiger–Müller (GM) survey meter.
• Identify the radiation survey instrument that can
be used to calibrate radiographic and fluoroscopic
x-ray equipment.
C H A P T E R O U T L I N E
Personnel Monitoring
Requirements for Personnel Monitoring
Purpose of Personnel Dosimeters
Placement of Personnel Dosimeters
Extremity Dosimeter
Record of Radiation Exposure
Personnel Dosimeters for Occupational Monitoring
Characteristics
K E Y T E R M S
control monitor
extremity dosimeter
Geiger–Müller (GM) survey
meter
glow curve
ionization chamber-type survey
meter (cutie pie)
72
optically stimulated lumines-
cence (OSL) dosimeter
personnel direct ion storage
dosimeter (DSI)
personnel dosimeter
personnel dosimetry
personnel monitoring reports
Types
Radiation Survey Instruments for Area Monitoring
Radiation Detection and Measurement
Types of Instruments
Requirements
Gas-Filled Radiation Survey Instruments
Instruments Used to Measure X-Ray Exposure
Summary
proportional counter
radiation survey instruments
thermoluminescent ring
dosimeter
CHAPTER 5 Radiation Monitoring
73
To ensure that occupational radiation exposure levels
are kept well below the annual effective dose (EfD)
limit, some means of monitoring personnel exposure
must be employed. The radiographer and other occupationally exposed persons should be aware of the
various radiation exposure monitoring devices and
their functions. This chapter provides an overview of
both personnel and area monitoring. In addition,
because radiation dosimetry reports still specify radiation exposure for workers in traditional units and
subunits, traditional units’ numerical values are identified in parentheses after International System (SI)
units’ numerical values. However, facilities may request
that dosimetry reports employ SI units.
PERSONNEL MONITORING
Requirement for Personnel Monitoring
Personnel dosimetry refers to the monitoring
of equivalent dose to any person occupationally
exposed on a regular basis to ionizing radiation,
which is recommended. It is required, however, whenever radiation workers are likely to risk receiving
10% or more of the annual occupational EfD limit
of 50 millisievert (mSv) (5 rem*) in any single year
as a consequence of their work-related activities. In
keeping with the as low as reasonably achievable
(ALARA) concept, most health care facilities issue
dosimetry devices when personnel could receive approximately 1% of the annual occupational EfD limit
in any month, or approximately 0.5 mSv (50 mrem).
Radiation exposure monitoring is accomplished by
wearing personnel dosimeters.
• Does not protect the wearer from exposure because
the instrument is only capable of detecting and measuring the amount of ionizing radiation to which it
has been exposed
Placement of Personnel Dosimeters
During Routine Radiographic Procedures. A person-
nel monitoring device records only the exposure received
in the area where the device is worn. During routine
radiographic procedures, when a protective apron is
not being used, the primary personnel dosimeter should
be attached to the clothing on the front of the body
at collar level to approximate the location of maximal
radiation dose to the following (Fig. 5.1):
• Thyroid
• Head
• Neck
Purpose of Personnel Dosimeters
The personnel dosimeter:
• Provides an indication of the radiation exposure
working habits and working conditions of diagnostic
imaging personnel
• Determines occupational exposure by detecting and
measuring the quantity of ionizing radiation to
which the dosimeter has been exposed over a period
of time
*Rem (radiation equivalent man) is the traditional unit for the
quantity, equivalent dose (EqD).
Fig. 5.1 To approximate the maximum radiation dose to the
thyroid and the head and neck during routine radiographic
procedures, the primary personnel monitor should be attached
to the clothing on the front of the body at collar level.
74
CHAPTER 5 Radiation Monitoring
BOX 5.1 Most Current Personnel
Monitoring Devices
1. Optically stimulated luminescence (OSL) dosimeter
2. Direct ion storage dosimeter (DIS)
3. Extremity dosimeter (thermoluminescent ring dosim-
eter [TLD])
Consistency of location in wearing the dosimeter is
necessary and is the responsibility of the individual
wearing the device. A list of the types of personnel
monitors available to diagnostic imaging personnel is
found in Box 5.1. A discussion of each of the personnel monitoring devices will be provided later in this
chapter.
When a Protective Apron Is Worn. Fluoroscopy,
surgery, and special radiographic procedures produce
the highest occupational radiation exposure for diagnostic imaging personnel. When a protective lead apron
is worn during such procedures, the dosimeter should
be placed outside the apron at collar level on the anterior surface of the body since the unprotected head,
neck, and lenses of the eye receive 10 to 20 times more
exposure than the protected body trunk. Located at
collar level, the dosimeter provides a reading of the
approximate equivalent dose to the exposed thyroid
gland and eyes of the occupationally exposed person.
If the lead apron’s shielding integrity is not compromised, a dosimeter reading that is within acceptable
limits outside of the apron ensures a minimal reading
under the apron.
As a Second Monitor When a Protective Apron Is Worn. During lengthy interventional fluoroscopy pro-
cedures (e.g., cardiac artery patency investigations),
some health care facilities may prefer to have diagnostic imaging personnel wear two separate monitoring
devices. As mentioned previously, the first, or primary, dosimeter is to be worn outside the protective
apparel at collar level, whereas the second dosimeter
should be placed beneath a wraparound-style lead
apron at waist level to monitor the approximate
equivalent dose to the lower body trunk. Commercially available lead aprons typically have either 0.5mm or 0.25-mm lead equivalent shielding. Another
version is also available with 0.35-mm lead equivalent
in the front and 0.25-mm shielding in the back. For
those occupationally exposed personnel who utilize
two radiation dosimeters, it is useful to have some
knowledge of the difference in equivalent dose
readings between the two dosimeters.
As a Monitor for the Embryo-Fetus. In addition to
a primary dosimeter worn at collar level, pregnant diagnostic imaging personnel are typically issued a second
monitoring device (also worn beneath the protective
apron, at waist level) to record the approximate radiation dose to the abdomen during gestation. This monitor, therefore, can provide an estimate of the equivalent
dose to the embryo-fetus.
Extremity Dosimeter
An extremity dosimeter, typically a thermolumines-
cent ring dosimeter (TLD) (Fig. 5.2), should be worn
by an imaging professional as a second monitor when
performing fluoroscopic procedures that require the
hands to be near the primary x-ray beam. Ring dosimeters are most commonly utilized by nuclear medicine
technologists, due to the need for occupational handling of unsealed radioactive sources. Even though ring
dosimeters are worn under gloves to avoid contamination, such extremity monitors have a laser-etched cover
to ensure the retention of permanent identification
of the wearer. The TLD element of the dosimeter is
encapsulated within its engraved cover.
The TLD ring is a light-free device that contains a
crystalline form (powder or, more frequently, small
chips) of lithium fluoride (LiF), which functions as
the sensing material of the dosimeter. When irradiated, some of the electrons in the crystalline lattice
Fig. 5.2 An extremity dosimeter (thermoluminescent ring do-
simeter [TLD]) can be used to monitor the equivalent dose to
the hands. (From Landauer, Inc., Glenwood, IL.)
CHAPTER 5 Radiation Monitoring
TABLE 5.1 Occupational Exposure Values for a Typical Year
NUMBER OF WORKERS
(THOUSANDS)
Category
Medicine 584 277 0.7 1.5
Industry 350 156 1.2 2.4 380
Nuclear power 151 91 3.6 5.6 550
Flight crews, flight attendants 97 97 1.7 1.7 165
†
Other
*See NCRP Report No. 101, p 60.
†
Includes workers in the US government (Department of Energy, US Public Health Service), uranium mining, well logging, miscellaneous workers, visitors to facilities, and so forth.
Data from National Council on Radiation Protection and Measurements (NCRP): Exposure of the U.S. population from occupational
radiation, Report No. 101, Bethesda, MD, 1989, NCRP, pp 65–70.
AVERAGE ANNUAL
EFFECTIVE DOSE (mSv)
Total 2300
Collective Effective
Dose (Person-Sv)*All Exposed All Exposed
416
789
75
structure* of the LiF molecules absorb energy and are
“excited” to higher energy levels or bands. The presence of impurities in the crystal causes electrons to
become trapped within the bands. When the LiF crystals are passed through a special heating process for
dosimeter reading purposes, the trapped electrons
receive enough energy to rise above their present locations into a region called the conduction band. From
there, the electrons can return to their normal state,
with the emission of energy in the form of visible
light. The energy emitted is equal to the difference
between the electron-binding energies of the two
orbital levels.
Radiation dose determination is accomplished
through the use of an electronic instrument known as
a TLD analyzer. After the LiF crystals are heated to
free the trapped, highly energized electrons, this instrument records the amount of light emitted by the
crystals as the electrons return to their ground state.
This amount is proportional to the dosimeter exposure. A graphic plot is constructed to demonstrate the
relationship of light output, or emitted thermoluminesence intensity, to temperature variation of the LiF
crystals. The plot, known as a glow curve, represents
a unique signature of the exposure received by the
TLD ring dosimeter.
Advantages of the TLD Ring Dosimeter. For the
purpose of monitoring radiation to the hands the TLD
ring dosimeter is relatively accurate and reliable. The
TLD ring dosimeter is small, light-weight and the LiF
crystals interact with ionizing radiation in the same
manner as does human tissue.* Exposures as low as
1.3 3 1026 C/kg (5 mR)** can be measured precisely.
Because the dosimeter is not affected by humidity, pressure, and normal temperature changes, it can be worn
up to 3 months, and is reusable after a reading has
been obtained. Therefore, on-going use is reasonably
cost-effective.
Disadvantages of the TLD Ring Dosimeter. Thermo-
luminescence readings will be lost if not carefully
recorded. The readout process destroys information
stored in the TLD, thus preventing the “read” TLD from
serving as a permanent legal record of exposure. Calibrated dosimeters must be prepared before-hand and
read with each group of TLDs as they are processed.
Record of Radiation Exposure
A record of radiation exposure should be included in
the employment record of all radiation workers. Table
5.1 provides occupational exposures (gathered from
personnel dosimeter readings) for a typical year. The
*A geometric arrangement of the points in space at which the
atoms, molecules, or ions of a crystal occur. Also called space
lattice.
*The effective atomic number of LiF is 8.2, which is similar to
that of human soft tissue (Z 5 7.4).
**1 R 5 2.58 3 (10)
24
C/kg by definition
76
CHAPTER 5 Radiation Monitoring
listed values represent the corresponding average annual EfD to the whole body and the related collective
effective dose.
PERSONNEL DOSIMETERS FOR OCCUPATIONAL MONITORING
Characteristics
A personnel dosimeter should be lightweight, easy to
carry and constructed of materials durable enough to
tolerate normal daily use. The dosimeter must be able
to detect and record both small and large exposures in
a consistent and reliable manner. Outside influences
such as very warm weather, humidity, and ordinary
mechanical shock should not affect the performance
of the instrument. The monitors should be reasonably
inexpensive to purchase and maintain.
Types
Two types of personnel dosimeters are predominantly
used to measure individual exposure of the whole body
to ionizing radiation:
• Optically stimulated luminescence (OSL) dosimeters
• Direct ion storage dosimeter (DIS)
Optically Stimulated Luminescence Dosimeter. The
optically stimulated luminescence (OSL) dosimeter for
personnel monitoring offers excellent features (Fig. 5.3)
while eliminating many of the disadvantages of its predecessors. The OSL is the most common type of device used
for monitoring of occupational exposure in diagnostic
imaging and radiation therapy. The OSL dosimeter has
replaced earlier devices for personnel monitoring in essentially all health care facilities.
The OSL dosimeter shown in Fig. 5.3 contains an
aluminum oxide (Al2O3) thin layer detector. An exposed
dosimeter is “read out” by using laser light at selected
frequencies. When such laser light is incident on the
sensing material, the material becomes luminescent
in proportion to the amount of radiation exposure
received by the detector.
Although the OSL dosimeter can be worn continuously for up to one year, it is common practice for it to
be worn without a reading, for a period of one to three
months. OSL dosimeters are typically shipped to a
monitoring company for analysis and dose determination, a task that requires some time for the report of a
reading to be communicated. An in-house reader may
be purchased from a vendor. With the in-house reader,
Fig. 5.3 Optically stimulated luminescence (OSL) dosimeter.
Disassembled OSL dosimeter demonstrating components of the
monitor: sensing material holder, preloaded packet incorporating
an Al2O3 strip sandwiched within a three-element filter pack,
which is heat sealed within a light-tight black paper wrapper that
has been laminated to the white paper label. The front of the
white paper packet may also be color-coded to facilitate correct
usage and placement of the dosimeter on the body of occupationally exposed personnel. (All components are sealed inside a tamperproof plastic blister pack.) (From Landauer, Inc., Glenwood, IL.)
occupational exposure doses can be determined on the
day of occurrence.
Energy discrimination. As seen in Fig. 5.3, three dif-
ferent filters are incorporated into the detector packet of
the OSL dosimeter. The filters are, respectively, made of:
• Aluminum (Al)
• Tin (Sn)
• Copper (Cu)
Each filter blocks a portion of the radiation-sensitive
aluminum oxide and causes a different degree of attenuation for any radiation striking the dosimeter, depending on its energy. The aluminum filter offers the least
absorption, whereas the copper filter attenuates the
most. When the exposed aluminum oxide layer of the
OSL dosimeter is read out by a laser, the degree of luminescence detected in the areas from beneath the filters is