ORP/EERF 73-1
SUITABILITY OF GLASS-ENCAPSULATED
CaF2:Mn THERMOLUMINESCENT DOSIMETERS
FOR ENVIRONMENTAL RADIATION SURVEILLANCE
U.S ENVIRONMENTAL PROTECTION AGENCY
Offics ol Radiation Programs
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OFFICE OF RADIATION PROGRAMS
EASTERN ENVIRONMENTAL RADIATION FACILITY
TECHNICAL REPORTS
Technical reports of the Eastern Environmental
Radiation Facility are available from the National Technical
Information Service, Springfield, Virginia 22151, when
a PB number is indicated after the title. Microfiche
copies are $0.95; prices for paper copies are indicated
after the PB number. Bulk order prices are available from
NTIS. The PB number should be cited when ordering.
Title
Radiological Survey of Major California
Nuclear Ports (April 1967) (PB 178 728
$6.00)
Radiological Survey of Hampton Roads,
Virginia (January 1968) (AD 683 208 $6.00)
BRH/SERHL 70-1
RO/EERL 71-1
EERL 71-2
ORP/EERF 73-1
ORP/EERF 73-2
ORP/EERF 73-3
Laboratory Examination of a Ruptured 50-mg
Radium Source (May 1970) (PB 191 810 $3.00)
Development of Ion Exchange Processes for
the Removal of Radionuclides from Milk
(January 1971) (PB 198 052 $0.50)
Investigation of Tritiated Luminous
Compounds (June 1971)
Suitability of Glass-Encapsulated CaF2:Mn
Therjnoluminescent Dosimeters for Environ-
mental Radiation Surveillance (June 1973)
Construction and Operation of an Ion
Exchange Cartridge for Monitoring Radio-
nuclides in the Environment (June 1973)
Portable Annealer for Thermoluminescent
Dosimeters (June 1973)
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ORP/EERF 73-1
SUITABILITY OF GLASS-ENCAPSULATED
CaF2:Mn THERMOLUMINESCENT DOSIMETERS
FOR ENVIRONMENTAL RADIATION SURVEILLANCE
J.E. Partridge
S.T. Windham
Eastern Environmental Radiation Facility
P.O. Box 61
Montgomery, Alabama 36101
and
J.L. Lobdell, M.S.P.M.
J.A. Oppold, Ph.D.
Tennessee Valley Authority
River Oaks Building
Muscle Shoals, Alabama 35660
June 1973
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Radiation Programs
Waterside Mall East
401 M. Street, S.W.
Washington, D.C. 20460
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The mention of commercial products,
their source, or their use in
connection with material reported
herein is not to be construed as
either an actual or implied endorse-
ment of such products by the U. S.
Environmental Protection Agency.
11
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FOREWORD
The Eastern Environmental Radiation Facility participates
in the identification of solutions to problem areas as
defined by the Office of Radiation Programs. The Facility
provides laboratory capability for evaluation and assess-
ment of radiation sources through environmental studies
and surveillance and analysis. The EERF provides technical
assistance to the State and local health departments in
their radiological health programs and provides special
laboratory support for EPA Regional Offices and other
federal government agencies as-.recuiested.
CRarles'R. Porter
Acting Director
Eastern Environmental Radiation Facility
111
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CONTENTS
Page
Foreword ii
List of Tables and Figures iv
Abstract v
Introduction 1
Physical Characteristics 1
Calibration Procedures 2
Annealing 4
Calibration and Precision 4
1. Individual dosimeters 4
2. Dosimeters as a group 6
Internal Background 6
Fading as a Function of Temperature 8
Sensitivity to Light 9
Shock and Vibration Effects 9
Electromagnetic and Ultrasonic Effects .... 9
Aging Effects 9
Operating Experience 10
Formula for the Determination of Exposure
Rates in the Environment 10
Conclusion 11
v
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TABLES
Page
1. Calibration factors ..... 6
2. Internal backgrounds for groups of
dosimeters 7
3. Minimum detectable exposure rate
4. Fading of reading with time . . ,
FIGURES
1. EG&G model TL-15 dosimeter 3
2. EG&G model TL-3 dosimeter reader. ... 3
VI
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ABSTRACT
The suitability of glass-encapsulated
thermoluminescent dosimeters for environmental radiation
surveillance was investigated. More than two hundred
dosimeters were subjected to extensive laboratory and
field tests. Various parameters such as accuracy,
precision, sensitivity, self dosing, and fading were
investigated.
Selected dosimeters of this type can be used for
accurate determination of environmental radiation levels
if certain precautions are taken. Such precautions
are proper calibration and determination of self-dosing
characteristics for individual dosimeters.
VI1
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SUITABILITY OF GLASS-ENCAPSULATED
CAF2:MN THERMOLUMINESCENT DOSIMETERS
FOR ENVIRONMENTAL RADIATION SURVEILLANCE
ORP/EERF-73-1
Jennings E. Partridge, M.S., Sam T. Windham, M.S.,
*John L. Lobdell, M.S.P.H., *James A. Oppold, Ph.D.
INTRODUCTION
The need for a small, accurate dosimeter to measure
the natural gamma background radiation around nuclear
facilities and to quantitate the increase in exposure
due to plant operations has been recognized for some
time. The proposed exposure rates in 10 CFR Part 50,
Appendix I, require that this dosimeter be sensitive
enough to measure increases on the order of 10 mR/yr.
Since thermoluminescent dosimetry is rapidly becoming
one of the most common techniques for measuring ionizing
radiation, it is logical that it be considered for environ-
mental measurements. Glass-encapsulated CaF2:Mn thermo-
luminescent dosimeters are presently being used at many
nuclear facilities for environmental monitoring purposes.
This report, a result of a joint study by the Tennessee
Valley Authority (TVA) and the Eastern Environmental Radia-
tion Facility (EERF) of the U. S. Environmental Protection
Agency (EPA), investigates the characteristics and suita-
bility of this type dosimeter for environmental radiation
measurements.
PHYSICAL CHARACTERISTICS
The dosimeter studied was the EG&G Model TL-15 (figure
1). The thermoluminescent material employed in the dosimeter
is Ca?2, manganese activated, and is in the form of two
hot-pressed chips which are held on either side of a flat
*Tennessee Valley Authority, River Oaks Building, Muscle
Shoals, Alabama
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heating element. The chips and the heating element are
enclosed in a glass envelope which is filled with CO2 at
atmospheric pressure. When in use, the glass envelope
is placed in an energy compensating shield made of tin,
aluminum, and lead. The shield reduces the over-response
of CaF2:Mn to low energy gamma radiation.
The reader employed was the EG&G Model TL-3 equipped
with a low noise photomultiplier (PM) tube (figure 2).
Dosimeters are read when a constant current of 6.5 amperes
is passed through the heating element of the dosimeter.
The heated CaF2:Mn emits light which is proportional to
the magnitude of the exposure. The resulting glow curve
which depicts light intensity vs. time or temperature, is
plotted on a 5-inch strip chart recorder. The magnitude
of the exposure is determined not by integrating the glow
curve but by measuring the height of the peak.
CALIBRATION PROCEDURES
Special calibration equipment was constructed that is
capable of delivering accurate and reproducible exposure
to the dosimeters. The facilities were especially con-
structed to provide reproducible positioning of dosimeters
and to minimize errors introduced by scatter.
At EERF several NBS calibrated radium-226 sources were
used to expose the dosimeters. Radium was chosen because
its energy spectrum nearly simulates the radiation energies
encountered in environmental exposures. Throughout the
study the dosimeters were given exposures ranging from
3 mR to 11 mR. This is the range of exposure expected in
environmental monitoring situations for periods of a month.
At TVA, a calibrated 0.1 Ci Cs source was employed
to irradiate the dosimeters. An exposure ring was con-
structed that positions the dosimeters in a circular ring
with the source at the center. Exposures from 1 to 5,000
mR were used in these investigations.
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Figure 1. EG&G Model TL-15 Dosimeter
Figure 2. EG&G Model TL-Z Dosimeter Reader
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ANNEALING
Prior to using a dosimeter for making a measurement,
it is necessary to anneal the dosimeter to eliminate the
stored signal from past exposures. Exposures up to 5 R
were made and different annealing techniques were used to
determine the most satisfactory. An annealing method was
determined to be satisfactory if following an exposure
the dosimeter was annealed and read and no detectable
reading was obtained. For exposures up to 100 mR, which
are possible in environmental surveillance, complete
annealing can be obtained by simply putting the dosimeter
through an instrument readout cycle as used when making
a normal measurement. For exposures higher than 100 mR,
the following oven annealing procedures were determined
to be satisfactory.
Exposure (mR) Annealing Procedure
100 - 500 5 minutes at 350° C or 2 readout cycles
500 - 3,000 10 minutes at 350° C
3,000 - 5,000 15 minutes at 350° C
CALIBRATION AND PRECISION
1. Individual Dosimeters
The first part of the investigation was to determine
calibration factors for each dosimeter, and from this
information the precision and reproducibility for the
group of dosimeters was quantitated. Since the reading
provided by each dosimeter is in arbitrary units, it is
necessary to obtain a calibration factor to convert these
units to exposure in mR. Therefore, each dosimeter was
exposed a minimum of five times to known quantities of
radiation and read so that calibration factors might be
obtained. From these data, a mean calibration factor
and one sigma standard deviation were calculated for each
dosimeter. The mean calibration factors for the dosimeters
in the group varied from 0.23 to 0.34 mR/reader unit. This
variation dictates that, for precise work, each dosimeter
be well calibrated. The standard deviations associated
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with the measurement of the calibration factors were
averaged for the group of 162 dosimeters and was 2.9
percent. This represents the precision or reproducibility
of this dosimeter-reader system at typical environmental
radiation levels (1 to 50 mR) under laboratory conditions.
The major factors which account for the variations in
individual response were found to be the errors in reader
adjustments. These adjustments are the strip chart zero,
the PM tube high voltage, and the heater current.
The zero adjustment is accomplished by inserting the
empty read head into the reader and manipulating the
control until a zero reading is obtained on the recorder.
To adjust the high voltage applied to the PM tube
a reference light source with an equivalent mR reading
is supplied by the manufacturer. The adjustment is made
by inserting the light source into the reader and adjusting
the high voltage until the equivalent mR reading is obtained
on the chart. Considerable differences (20 percent) in
the mR equivalence assigned by the manufacturer were found
to exist among several light sources. This would prohibit
the use of dosimeters on different readers adjusted with
different light sources unless the dosimeters were calibrated
on each system. With a single reader the ability to reset
the high voltage to a known mR equivalent is satisfactory.
The adjustment of heater current to 6.5 amps is accom-
plished with the aid of a built-in ammeter. Variation
in this adjustment will effect a -3 percent error per 0.1
amp between 6.1 amps and 6.8 amps. Other minor sources
of error were due to timing of the calibration exposures
and operator error in reading the strip chart recorder.
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2. Dosimeters as a Group
To quantitate the variation in response from one
dosimeter to another due to the manufacturing process,
an average calibration factor for three groups of
dosimeters was determined. The three groups were from
three production batches—Group F (53 dosimeters), Group
G (34 dosimeters), and Group L (75 dosimeters).
The results, summarized in table 1, show the variation
in relative response from batch to batch. The variation
in relative response of individual dosimeters within each
group is indicated by the standard deviation associated
with the mean calibration factor for the group. These
data illustrate the essentialness of individual calibration
factors for each dosimeter.
Table 1
Calibration factors
Group F (53) Group G (34) Group L (75)
Range .259 - .339 .239 - .313 .230 - .268
Mean ± a (%) .280 ± 5.4% .269 ± 6.7% .247 ± 3%
INTERNAL BACKGROUND OR SELF DOSING
The most significant aspect studied was the internal
background or self-dosing properties of the dosimeters.
The internal background is due to the irradiation of the
thermoluminescent material by the radioactivity in the
materials used to construct the dosimeter.
The internal background was determined by placing
annealed dosimeters in a storage shield with a known back-
ground. This background exposure rate was determined
using a calibrated high pressure ionization chamber.
The dosimeters were left in the shield for a period of
72 to 120 hours before being read. The reading obtained
was a result of a combination of the internal background
of the dosimeters plus the natural background in the shield,
By subtracting the natural background from the dosimeter
reading the internal background was determined. A minimum
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of five measurements was performed with each individual
dosimeter. From these determinations a mean internal
background and one sigma standard deviation were calculated
for individual dosimeters and for the dosimeters as a
group. The results are summarized in table 2. The wide
range of internal backgrounds for the various groups
indicates that this parameter should be determined for
each dosimeter.
Table 2
Internal Backgrounds for Groups of Dosimeters
yR/hr
Group F Group G Group L
Range 7.34 - 33.93 11.10 - 61.00 2.34 - 15.69
Mean ± a 13.45 ± 5.52 25.37 ± 9.95 3.78 ± 2.58
It is concluded that the primary contributor to the
internal backgrounds is K in the glass envelopes. A
sample of the glass used to construct the envelopes for
Groups F and G was examined and found to contain 20 pCi
of 4^K per gram of glass. Glass for the L Group was found
to contain 0.5 pCi of K per gram of glass.
NBS Handbook 80 defines the minimum detectable level
as three times the standard deviation associated with the
background measurement. Therefore, the standard deviation
associated with the measurement of the internal background
for each dosimeter is important. As an example, assume
a typical dosimeter from Group F which has an internal
background alone. Therefore, the minimum detectable exposure
for a 1-month monitoring period using this dosimeter would
be three times 0.44 mR or 1.32 mR, which is below the
monthly natural radiation exposure levels.
The average of the individual dosimeter standard deviations
from the mean internal background for the dosimeters in
each group is given in table 3. An indication of the
average minimum detectable exposure for each group is
three times the average standard deviation for the group.
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Table 3
Minimum Detectable Exposure Rate
Group F Group G Group L _
Average a 0.52 yR/hr 1.33 yR/hr 0.31 yR/hr
M.D.E. (3a) 1.56 yR/hr 3.99 yR/hr 0.93 yR/hr
FADING AS A FUNCTION OF TEMPERATURE
After an exposure, the reading produced by the dosimeter
fades with time. An investigation was conducted to quantitate
the degree of fading at three environmental temperatures,
18° F, 75° F, and 130° F. It has been observed that essentially
all of the fading occurs within the first 24 hours. The
maximum fading at the three temperatures is shown in table
4.
Table 4
Fading of Reading with Time
Relative Reading with Relative Reading with
No Time Delay Between 24-hour Delay Between
Temperature Exposure and Readout_ Exposure and Readout
18° F 1.00 0.97
75° F 1.00 0.96
130° F 1.00 0.95
Since fading is a significant factor, it must be
considered for all measurements. Two methods of quantitating
the effect are possible. The first is, during dosimeter
calibration, to fix the time between exposure and readout
to less than one hour. In this manner, the correction
factors shown above need to be applied when making field
measurements. In contrast, if during dosimeter calibration
the time between exposure and readout is fixed at 24 hours
or greater, fading and correction factors need not be
considered for experimental exposures.
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SENSITIVITY TO LIGHT
A dosimeter which has been exposed to radiation will
be affected by sunlight and office fluorescent lights,
and will cause the dosimeter reading to be less than if
it had not been subjected to light. When exposed to
sunlight, a dosimeter will lost 40 percent of its reading
in 5 minutes and 90 percent in 10 minutes. When exposed
to office fluorescent light, a 5 percent reduction is seen
in 20 minutes and a 20 percent loss in two hours. Therefore,
following an exposure to radiation, the dosimeters must
remain in the shield until they are read so as to prevent
loss of response.
For dosimeters that have been annealed and not exposed
to radiation, sunlight and office fluorescent lights do not
cause a buildup of energy within the dosimeter that would
produce a reading that could be interpreted as having been
an exposure to radiation.
SHOCK AND VIBRATION EFFECTS
Exposed and unexposed dosimeters were subjected to
sudden shock (drop on a hard concrete floor) and steady
vibration (60 Hz) and no effect on the anticipated
dosimeter readout was seen.
ELECTROMAGNETIC AND ULTRASONIC EFFECTS
Exposed dosimeters were subjected to electromagnetic
(2450 MHz, 200 mW/cm^ for 30 minutes) and ultrasonic
fields of varying intensity (ultrasonic cleaner for 30
minutes). These studies were performed with the dosimeters
inside and outside their protective shields. No effect
could be observed with either electromagnetic or ultrasonic
fields.
AGING EFFECTS
Preliminary results indicate an aging effect on the
dosimeters. This aging effect causes an increase in the
calibration factor.or a decrease in the sensitivity (less
reading for the same exposure). The effect appears to be
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10
time-dependent and not a function of use. This was
established by exposing and reading a group of new
dosimeters six times over a period of three days. These
dosimeters showed no change in their calibration factors.
Another group which had been exposed and read six times
over a period of approximately one year showed an 8 per-
cent increase in calibration factors.
This aging effect might be due to a darkening of the
crystal which is observed in the older (over one year)
dosimeters. This darkening could cause a decrease in the
light output, thereby reducing the response for a given
exposure .
OPERATING EXPERIENCE
The dosimeters have been used for up to four years
to measure the natural background gamma exposure rates in
the vicinity of nuclear power plants under construction.
The dosimeters are placed on metal stakes so that the
exposure rate is monitored one meter from the ground. The
stakes are positioned on a 500-foot grid surrounding the
plants. Fifty- four dosimeters are used at one site
while forty-four are employed at the other. The dosimeters
are exposed in the field for three months.
As a result of the work described herein, the exposure
rate is calculated by using the following formula:
Formula for the Determination of Exposure Rates
in the Environment
(Dosimeter reading) (calibration factor) . . . , ,
baCkgr°Und
amg facor (hours exposed)
environmental exposure rate
where the fading factor is 0.96, and the internal background
rate is dependent upon the dosimeter.
When dosimeters are used to monitor the levels of
radiation around a plant site, experience has shown that
at least one year's data are necessary to accurately
quantitate the annual exposure levels. This period would
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11
allow sufficient time to quantitate any seasonal trends
and provide enough information so that an accurate annual
average exposure rate might be obtained through appropriate
data reduction.
CONCLUSION
Selected glass-enaapsulated CaF2 dosimeters can be used
for environmental radiation monitoring with reasonable
accuracy if certain conditions are met. Determination
of internal backgrounds and calibration factors for
individual dosimeters is necessary. Fading characteristics
should be typical for all the dosimeters in the group.
The possibility of an aging effect warrants a reevaluation
of calibration factors on an annual basis.
In order to detect an increase of 10 mR/yr above
background as proposed in 10 CFR Part 50, Appendix I, it
is imperative that measurements be made with great care and
accuracy. A monitoring program that utilizes CaF2 glass-
encapsulated dosimeters should be initiated at least one
year prior to plant operation, and detailed statistical
analysis of these data will be required to determine if
the radiation exposure rate prior to plant operation is
different than after plant startup.
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IJ. E. Partridge, S. T. Windham,
• J. L. Lobdell, and J. A.
I Oppold: SUITABILITY OF GLASS-"
ENCAPSULATED CaF2:Mn THERMOLUMINESCENT
DOSIMETERS FOR ENVIRONMENTAL RADIATION
SURVEILLANCE.
Environmental Protection Agency, Office of
Radiation Programs Publication No. ORP/EERF
73-1 (June 1973) 11 pp. (limited distribution).
ABSTRACT: The suitability of glass-encapsulated
CaF^iMn thermoluminescent- dosimeters for
environmental radiation surveillance was
investigated. More than two hundred dosimeters
were subjected to extensive laboratory and
field tests. Various parameters such as
(over)
Accession No.
Ij. E. Partridge, S. T. Windham,
J. L. Lobdell, and J. A.
Oppold: SUITABILITY OF GLASS-
ENCAPSULATED CaF2:Mn THERMOLUMINESCENT
DOSIMETERS FOR ENVIRONMENTAL RADIATION
SURVIELLANCE.
Environmental Protection Agency, Office of
Radiation Programs Publication No. ORP/EERF
73-1 (June 1973) 11 pp. (limited distribution).
ABSTRACT: The suitability of glass-encapsulated
CaFoiMn thermoluminescent dosimeters for
environmental radiation surveillance was
investigated. More than two hundred dosimeters
were subjected to extensive laboratory and
field tests. Various parameters such as
(over)
I J. E. Partridge, S. T. Windham,
' J. L. Lobdell, and J. S.
I Oppold: SUITABILITY OF GLASS^"
ENCAPSULATED CaF :Mn THERMOLUMINESCENT
| DOSIMETERS FOR ENVIRONMENTAL RADIATION
• SURVEILLANCE.
I Environmental Protection Agency, Office of
I Radiation Programs Publication No. ORP/EERF
' 73-1 (June 1973) 11 pp. (limited distribution).
I ABSTRACT: The suitability of glass-encapsulated
CaF^:Mn thermoluminescent dosimeters for
| environmental radiation surveillance was
investigated. More than two hundred dosimeters
I were subjected to extensive laboratory and
field tests. Various parameters such as
(over)
Accession No.
-------
accuracy, precision, sensitivity, self-dosing, I
and fading were investigated.
Selected dosimeters of this type can be used |
for accurate determination of environmental ,
radiation levels if certain precautions are I
taken. Such precautions are proper calibration
and determination of self-dosing characteristics
for individual dosimeters.
KEYWORDS: Thermoluminescent Dosimeters,
Environmental Surveillance.
accuracy, precision, sensitivity, self-dosing, j
and fading were investigated.
Selected dosimeters of this type can be used |
for accurate determination of environmental ,
radiation levels if certain precautions are I
taken. Such precautions are proper calibration i
and determination of self-dosing characteristics '
for individual dosimeters.
KEYWORDS: Thermoluminescent Dosimeters, j
Environmental Surveillance.
~f
I
accuracy, precision, sensitivity, self-cosing,
and fading were investigated. J
Selected dosimeters of this type can te used ,
for accurate determination of environmertal I
radiation levels if certain precautions are j
taken. Such precautions are proper calibration '
and determination of self-dosing characteristics I
for individual dosimeters.
I
KEYWORDS: Thermoluminescent Dosimeters,
Environmental Surveillance.
<.
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ERRATA
The paragraph beginning "NBS Handbook 80 defines..." on
page 7, should read:
NBS Handbook 80 defines the minimum detectable level as
three times the standard deviation associated with the
background measurement. Therefore, the standard deviation
associated with the measurement of the internal background
for each dosimeter is important. As an example, assume a
typical dosimeter from Group F which has an internal
background of 9.08 ± 0.58 yR/hr was placed in the field
for a one month monitoring period. At the end of the 750-
hour monitoring period the dosimeter would have accumulated
6.81 ± 0.44 mR from internal background alone. Therefore,
the minimum detectable exposure for a 1-month monitoring
period using this dosimeter would be three times 0.44 mR
or 1.32 mR, which is below the monthly natural radiation
axposure levels.
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