United States
Environmetal Protection
Agency
Office of Radiation Programs
National Air and Radiation
Environmental Laboratory
1504 Avenue A
Montgomery, AL 36115-2601
EPA 520/5-90-032
November 1990
Radiation
NAREL Standard Operating
Procedures for Radon-222
Measurement Using Diffusion
Barrier Charcoal Canisters
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EPA 520/5-90-032
NAREL STANDARD OPERATING PROCEDURES FOR
RADON-222 MEASUREMENT USING DIFFUSION BARRIER CHARCOAL CANISTERS
David J. Gray
Sam T. Windham
November 1990
U.S. Environmental Protection Agency
Office of Radiation Programs
National Air and Radiation Environmental Laboratory
1504 Avenue A
Montgomery, Alabama 36115-2601
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CONTENTS
I. Introduction 1
II. Materials and Equipment 2
A. Canisters 2
B. Counting System 2
C. Calibration Chamber 4
D. Standards 7
III. Laboratory Methods 9
A. Canister Preparation 9
B. Calibration 9
C. Calculation of Radon 16
D. Calculation of Minimum Detectable Activity . . 20
E. Quality Assurance 22
IV. Survey Methods 25
A. Shipping of Canisters 25
B. Deployment 25
C. Forms 26
D. Time Restrictions 26
E. Reporting 29
11
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NAREL Standard Operating Procedures for
Radon-222 Measurement Using Diffusion Barrier Charcoal Canisters
I. Introduction
Activated charcoal has a strong affinity for several gases
and vapors, including radon-222 (hereafter referred to as radon).
Radon is sorbed onto the charcoal grains and decays to several
particulate decay products: radium A (polonium-218); radium B
(lead-214); radium C (bismuth-214); radium C' (polonium-214); and
radium D (lead-210). Radon concentration is determined by
counting the gamma ray emissions of both lead-214 (295 KeV and
352 KeV) and bismuth-214 (609 KeV). This is possible due to the
relatively short half-lives of these progeny. Within three hours
the progeny are in equilibrium with radon-222.
The passive nature of activated charcoal allows both
adsorption and desorption of radon, and, since the adsorbed radon
undergoes radioactive decay during the exposure period, the
ability of the non-diffusion barrier (open-faced) canister to
uniformly integrate over the entire exposure period can be
impaired. To help alleviate this problem, the EPA open-faced
canister was modified by inserting a polyethylene membrane
between the retaining screen and the carbon bed. This decreased
the water vapor and radon adsorbed by the carbon, reduced the
rate of adsorption/desorption between the carbon and the
environment, and improved integration capability.
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II. Materials and Equipment
A. Charcoal Canister
The diffusion barrier charcoal canister used by EPA consists
of an 8 ounce metal can with lid (4 inch diameter by 1 1/8 inch
deep); 70+1 grams of 6 X 16 mesh activated charcoal; a metal
screen with an openness of at least 30 to 50 percent; a 1.25 mil
polyethylene membrane containing twenty 0.091" diameter holes;
a removable, internally expanding retaining ring; a pad material
attached to the inner surface of the lid; and a 13-inch strip of
pliant vinyl tape. The materials are assembled as seen in
Figure 1.
The activated charcoal must have low radioactivity content.
Specifically, gamma emitters, except for natural potassium-40,
must be less than 0.2 picocuries per gram.
B. Counting System
The following components make up the counting system used by
EPA's National Air and Radiation Environmental Laboratory
(NAREL)*: 3" by 3" sodium-iodide detector and photomultiplier
tube inside counting shield; a high voltage power supply;
In March of 1990, the Eastern Environmental Radiation
Facility was renamed the National Air and Radiation Environmental
Laboratory, coinciding with the move to the new laboratory on
Gunter Air Force Base in Montgomery, Alabama.
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Polyethylene membrane (4" diameter)
inserts under stainless steel screen.
Pad attached to lid to retain
screen during shipping
Internal expanding
stainless steel
retainer ring
8 ounce metal can
13" strip of vinyl tape
\
Stainless steel screen,
30-50 percent open
70-1-1 grams of 6 ±16 mesh charcoal
FIGURE 1
Charcoal canister assembly
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a pre-amplifier and amplifier; a single channel analyzer and
sealer; a timer; a time of day clock; and a printer.
Four sodium iodide detectors are served by a single high
voltage power supply, timer, printer, and time of day clock.
Each detector has its own pre-amplifier, amplifier, single
channel analyzer, and sealer. The four detectors are arranged
together inside a shield that has steel walls 8 inches thick.
Within the shield the detectors are separated from each other by
4 inches of lead. A wooden jig is used on each detector to
assure consistent counting geometry for the canisters on the
detector. The arrangement of detectors in the shield and the
electronics are shown in Figures 2 and 3.
The counting system is operated in the single channel mode
with a lower discriminator setting of 270 KeV and an upper
discriminator setting of 720 KeV. All counts in this region are
summed during the counting period. A multi-channel analyzer is
used once a week to assure there has been no shift of the peaks
in the region of interest.
C. Radon Calibration Chamber
NAREL uses three radon calibration chambers to expose
charcoal canisters and other instruments to known concentrations
of radon and radon decay products in controlled environmental
conditions. Radium-226 sources are used to provide a continuous
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FIGURE 2
Four detectors in counting shield
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,
FIGURE 3
Two shields with electronics
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flow of radon through the chambers. In the chamber, controlled
conditions include relative humidity (10 to 90 percent),
temperature (0° to 40°C) , condensation nuclei (103 to 106/ml) ,
radon concentration (10 to 1,000 picocuries per liter
(pCi/liter)), and radon decay product concentration or working
level (0.01 to 5 WL).
The temperature, humidity, radon concentration, radon decay
product concentration, and condensation nuclei concentration are
monitored continuously and printed out each hour. Radon
concentrations are monitored using a 1.2 liter flow-through
scintillation cell and decay product concentrations are monitored
by counting, with a surface barrier detector, the alpha activity
from decay products collected on a filter. Grab samples, which
are done in conformance with recently published EPA procedures
(EPA89), are used to calibrate the hourly printouts.
D. Standard and Background Canisters
Counting system performance is determined daily by counting
standard canisters containing known radioactivity and background
canisters. The standard canisters were made by the quality
assurance group at NAREL in the following manner: a known
activity of about 20.5 nCi, liquid radium-226 was slurried into
the 70 grams of charcoal in a canister with the retainer screen
removed; this was heated to dry all liquid from the charcoal; the
canister was sealed by soldering a galvanized metal disc in the
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can where the screen normally is located; the lid was placed on
the canister and secured with tape; and the standard was stored
for thirty days to allow secular equilibrium to be reached
between the radium, the radon, and the progeny. Standard and
background canisters were prepared for each crystal and are
counted every morning to establish a daily counting efficiency
for each detector system. The counting efficiency is determined
by counting each standard and background canister for 10 minutes.
The background count is subtracted from the standard count to get
the net count. This is divided by the time, 10 minutes, to get
net counts per minute (CPM). By dividing the net CPM by the
known activity (pCi) of the standard the efficiency (CPM/pCi) of
the detector for that day is determined.
8
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III. Laboratory Methods
A. Canister Preparation
The gamma emitter content of the charcoal is determined for
each new batch of canisters from a supplier to make sure no
contaminated charcoal is being used. Representative samples of
canisters from each new batch are also checked for moisture
content of the charcoal.
Each canister is given an individual serial number which is
printed on the data label attached to the top of the canister and
automatically entered in the NAREL computer. The canister is now
ready to use for calibration runs or for a radon measurement.
B. Calibration
The diffusion barrier canisters are calibrated over exposure
periods of one to ten days. The radon concentration is kept as
constant as possible during all calibration work. The procedure
is described in the following steps:
1. Canister Exposure Sequence
When the chamber is stabilized, a set of 50 canisters is put
into the chamber and exposed in the following manner: a group of
five each for 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 days,
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respectively. The canisters are opened inside the chamber and
the exact start times are recorded.
2. Removal and Counting
At the end of each 1 day period specified above, a group of
five canisters is removed for counting. The lids are placed on
the canisters while they are still inside the chamber and the
canisters are then removed, taped, and the exact time is
recorded. The canisters are allowed to sit for at least three
hours to allow progeny to equilibrate with the radon before
counting.
Each canister is counted for ten minutes on a sodium-iodide
detector. The count time and detector number are recorded so
that the efficiency of that detector, from previous counting of
standard and background, can be incorporated in the calculation
of calibration factors.
3. Calculation of Calibration Factors
A calibration factor is calculated for each canister using
the following equation:
CF = NET CPM (Equation 1) (Ge84)
(Ts) (E) (RN) (DF) '
where
CF = Calibration factor, radon adsorption rate (1/min),
10
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NET CPM = Gross CPM for that canister - background CPM for
that detector for that day,
T = Exposure time of the canister (minutes),
S
E = Detector efficiency for the appropriate detector
(CPM/pCi),
RN = Radon concentration in the chamber for the
exposure period (pCi/liter),
DF = Decay factor from the midpoint of exposure to the
time of counting, which is calculated from
.693t _ .693t
DF = e T% Rn (min) °r e 5501 min
where t = time in minutes from midpoint of exposure to the
start of counting.
4. Generation of Calibration Equations
The calibration factors in Table 1, derived from the
equation above, relate exposure time of the canister as a
function of 20, 50, and 80 percent humidity. Data in this table
are plotted to generate Figure 4. Due to the uniformity in the
sampling rate (CF) of the diffusion barrier canister over the
entire humidity range, a single equation that describes the
response of the canister as a function of exposure time was
generated. Using a linear regression analysis of the data in
Table 1, this best fit equation is plotted in Figure 5.
11
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Table 1
Exposure Time Versus Calibration Factors
for Low, Medium, and High Humidity
Time of Exposure
(Hours)
24
24
24
24
24
48
48
48
48
48
72
72
72
72
72
96
96
96
96
96
120
120
120
120
120
144
144
144
144
144
168
168
168
168
168
192
192
192
192
Calibration
Factors (
20 Percent 50 Percent
0.03301
0.03428
0.03564
0.03180
0.03393
0.02900
0.03020
0.02997
0.03109
0.03167
0.03249
0.02946
0.03156
0.03230
0.03112
0.03154
0.03204
0.02852
0.02965
0.03130
0.03124
0.02874
0.03179
0.02978
0.02955
0.02922
0.03011
0.03069
0.02895
0.02817
0.03018
0.02810
0.02890
0.02970
0.02965
0.02810
0.02850
0.02908
0.02673
0.02909
0.02957
0.03078
0.03128
0.03173
0.02923
0.03139
0.03233
0.03092
0.02864
0.02802
0.02817
0.03022
0.02939
0.02982
0.02638
0.02816
0.02902
0.02598
0.03055
0.02787
0.02970
0.02630
0.03017
0.02949
0.02739
0.02704
0.02689
0.02969
0.02574
0.02639
0.02663
0.02971
0.02917
0.02857
0.02662
0.02518
0.02616
0.02659
liters/min)
80 Percent
0.03329
0.02912
0.03055
0.02948
0.02982
0.03195
0.02780
0.03099
0.02827
0.02961
0.03099
0.03126
0.03151
0.02773
0.02939
0.02751
0.02775
0.02722
0.02792
0.02747
0.02912
0.02885
0.02738
0.02521
0.02711
0.02617
0.02679
0.02734
0.02550
0.02724
0.02611
0.02504
0.02644
0.02661
0.02629
0.02493
0.02577
0.02443
0.02417
12
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Table 1 - Continued
Exposure Time Versus Calibration Factors
for Low, Medium, and High Humidity
Time of Exposure
(Hours)
Calibration Factors fliters/min)
20 Percent
50 Percent
80 Percent
192
216
216
216
216
216
240
240
240
240
240
0.02718
0.02977
0.02756
0.02822
0.02878
0.02670
0.02897
0.02802
0.02795
0.02658
0.02763
0.02693
0.02470
0.02473
0.02399
0.02721
0.02684
0.02737
0.02849
0.02660
0.02714
0.02706
0.02484
0.02440
0.02297
0.02439
0.02408
0.02420
0.02367
0.02215
0.02380
0.02338
0.02367
13
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-------
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.03
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o
CO
LL
C
g
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CB
k_
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"co
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.02 _
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.01
J I I I I L
10 20 30 40 50 60
I I I I l_l I I I I L
I I I L
70 ao
90
100 110 120 130 140 150 160 17O 180 190 200 210 220 230 240
EXPOSURE TIME (HOURS)
Figure 5. Best fit regression analysis of calibration factors versus
exposure time
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Since the diffusion barrier significantly reduces the
amount of water vapor adsorbed by the charcoal, the calibration
procedure has been simplified. Instead of calibrating diffusion
barrier canisters over a range of humidities, each new batch of
canisters from a supplier is re-calibrated only at medium
humidity for a period of one to seven days, and periodic two day
spot checks are performed on the canisters at low and high
humidities.
C.
Calculation of Radon in an Exposed Canister
Equation 1 above may be rearranged to solve for the radon
concentration if the calibration factor is known. This equation
will be
RN
where
NET CPM
(TJ (E) (CF) (DF)
(Equation 2)
RN
NET CPM
E
CF
DF
Radon concentration in pCi/liter,
Gross CPM for the canister - background CPM for
that detector for that day,
Canister exposure time (minutes),
Detector efficiency (CPM/pCi),
Calibration factor (1/min), and
Decay factor from the midpoint of exposure to the
start of counting.
The following steps are taken to analyze the canister and
calculate the radon concentration to which a canister has been
16
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exposed:
1. Count the standard canister on the detector to be used.
Calculate the efficiency (E) for that detector in CPM/pCi as
in section II-D. This efficiency is used for all counting
done on that detector for that day.
2. Count the background canister on the detector. This
background count is used for all counting done on that
detector for that day.
3. Determine the exposure period (Ts) in minutes for the
canister and divide by 60 to determine the amount of hours
the canister was exposed.
4. Use the amount of hours of exposure on the X-axis on
Figure 5 to obtain the CF for the canister from the Y-axis.
5. The decay factor (DF) is used to correct for
radiological decay between exposure and counting. It is
calculated from the midpoint of exposure to the start of
counting using the formula
.693t
DF = e"
T% for Radon
where t is the time from midpoint of exposure to the start
of counting and T,A is the half-life of radon.
17
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EXAMPLE CALCULATION
To calculate the concentration of radon in a canister we use
Equation 2,
RN = NET CPM
(TJ (E) (CF) (DF)
The data given for this example calculation are the following:
Exposure Period = 12-29-86 at 1052 CST to
01-01-87 at 0950 CST
Start Count = 01-04-87 at 0950 CST
Gross Counts = 2,942 in 10 minutes
Background Counts = 721 in 10 minutes
Standard Counts = 58,423 in 10 minutes for a 20,549 pCi
source
From these data, we first determine that
Net CPM = 2.942 - 721 = 222 CPM
10 min.
T = 4258 minutes (70.97 hours), and
(58.423 - 721)710 minutes = 0.281 CPM/pCi,
20,549 pCi
From Figure 5, we find a CF of 0.03050 on the Y-axis
18
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corresponding to an exposure period of 70.97 hours on the X-axis,
The decay factor for Equation 2 is calculated by
.693t
DF e TK for radon
where t = midpoint of exposure time to start of counting
time,
4,258 + 4,320 ,
2
6,449 minutes,
and T% of radon =3.82 days or 5,501 minutes. The decay factor
then is calculated by
(.693) (6,449)
DF = e~
5,501
= 0.444.
The final radon concentration is calculated by substituting
the above values into Equation 2 as follows:
RN =
(4258) (0.281) (0.03050) (0.444)
13.7 pCi/liter.
19
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The 2-sigma counting error (Kn79) is calculated as follows:
2
-------
BKG Count = total background counts, and
BKG Count Time = counting time of background.
For the NAREL counting system, this calculates as
7 750
MDA (CPM) = 3 X ,
10
= 8.2 counts per minute.
This may be converted to activity of radon in pCi/liter by
assuming exposure conditions. For the exposure range of the
diffusion barrier canister this calculates as follows:
Assumed conditions:
Exposure time = 2 days (48 hours) (This is the minimum
exposure time recommended for this canister.)
Time in shipment = 3 days (4,320 minutes)
Counter efficiency =0.28 CPM/pCi
MDA (CPM) =8.2 (from above)
_ MDA
MDA (pCi/liter) = - ~~~. T 7TT ,„,.
v* ' (Exposure Time) (E) (CF)
8^,2
(2,880) (0.28) (0.031) (.48)
= 0.68 pCi/liter
21
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Exposure time = 7 days (168 hours) (This is the recommended
exposure time for this canister.)
Time in shipment = 3 days (4,320 minutes)
Counter efficiency = 0.28 CPM/pCi
MDA (CPM) =8.2 (from above)
MDA (pCi/liter) =
(10,080) (0.28) (0.028) (.31)
= 0.34 pCi/liter
Since assumed conditions are not always met and the MDA will
change as a function of the length of the exposure period, NAREL
reports an MDA of less than 1.0 pCi/liter for an exposure period
less than 5 days and an MDA of less than 0.5 pCi/liter for an
exposure period of 5 days or longer.
E. Quality Assurance Procedures
The NAREL radon calibration chambers are the principal source
of quality assurance exposures for charcoal canisters. After the
initial calibration run on each new batch of canisters from a
supplier, periodic checks are made on randomly selected canisters
to assure no deviation.
Reliability of the NAREL calibration chambers is assured
through several programs. As described previously, the chamber
is monitored continuously while in operation. Also, daily checks
22
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on the radon and radon decay product concentrations are made
using grab sampling techniques. These same techniques are used
by NAREL in national and international measurement cross-check
programs. NAREL participates in these programs every time they
are offered, usually several times per year, and also sends
groups of its canisters to operators of other radon calibration
chambers for blind exposures as verification checks.
As part of NAREL's participation in the state radon survey
program, participating states are requested to expose duplicate
canisters and return them to NAREL for routine processing. This
is usually done in five percent of the homes surveyed. NAREL
also requests that the states submit blank canisters for
analysis. These are sent in by each state at a rate of
approximately two canisters per week.
The sodium iodide detectors and counting systems used for the
analysis of canisters are calibrated daily using standard sources
described in section II-D. The standard count for each detector
system each day is entered into a computer-maintained control
chart (Ro65). If the daily standard count exceeds the 2-sigma
variation of the average of all previous counts for that system,
the system is not used for counting until the cause of the
excessive variation is determined. The same type analysis is
also performed daily for a background canister on each system.
23
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In actual field use, the coefficient of variation of a
canister should not exceed 10 percent at a radon concentration of
4 pCi/liter or greater. This precision is monitored using
duplicate canisters (EPA86).
24
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IV. Survey Methods
A. Shipping of Canisters
Canisters are prepared for use as described in section III-A.
After preparation, they are individually boxed and the individual
boxes are packed into a shipping case, usually 150 canisters per
case. These are shipped to the users through normal commercial
carriers. An individual canister in its box may be mailed to a
user. Prepaid labels are used to ship the individual boxes to
the user and a prepaid label is provided to return the canister
from the user to NAREL. Each individual box has a gummed flap
for sealing.
B. Deployment
The charcoal canisters should be used as directed in the Indoor
Radon and Radon Decay Product Measurement Protocols, EPA 520/1-89-009,
April 1986. The protocols specify house conditions at the time
of measurement, provide instructions on selecting the location
in the house at which to make the measurement, and suggest how to
interpret results. Copies of the Indoor Radon and Radon Decay Product
Measurement Protocol may be obtained by written request to either of
the following addresses:
25
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U.S. Environmental Protection National Air and
Agency, Office of Radiation Radiation Environmental
Programs (ANR-464) Laboratory
401 M Street, S.W. 1504 Avenue A
Washington, DC 20460 Montgomery, AL 36115-2601
C. Forms
A combination data-information form is shipped with each
canister. This form is printed on yellow paper to easily tell it
from the form for the open-faced canister printed on white paper.
The information side of the form contains detailed instructions
for use and return of the canister. This form is shown in
Figure 6. The other side of the form consists of blanks to be
completed by the homeowner and the government agencies conducting
the study. This data form is shown in Figure 7. Accurate and
complete answers in the blanks are very important.
D. Time Restrictions
The following time restrictions are used for the NAREL
diffusion barrier canister. The exposure time for the diffusion
barrier canister must not be less than 47 hours nor longer than
240 hours (10 days). The time delay in returning the canister to
NAREL for processing should be as short as possible to avoid
26
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Where to Place the Charcoal Canister
please make sure that for 12 hours before and during the 7-day measurement period:
- Windows and external doors are kept closed, except for normal entry and exit,
- Fans or ventilation systems that use outside air, such as attic fans, are not operated.
If your house has a basement, place the canister anywhere in the basement, except in a
garage, root cellar, or crawl space. If your house does not have a basement, place the canister
in any room on the lowest floor of the house, except in a bathroom, kitchen, or porch.
Within the selected room, the canister should not be 1n a location frequently exposed to
noticeable drafts of an open door, window, fireplace, etc. The canister should be exposed to air
people breathe. It should be placed on a table or shelf at least 2 feet above the floor and
should be in open air, not in a closet, drawer, cupboard, etc.
The canister should be opened and exposed to the air for 7 full days (168 hours) and then
returned Immediately for processing. Follow the PROCEDURE below for opening and reseal1ng the
charcoal canister.bo not open the canister to begin the measurement if you cannot end the
measurement In 7 days.
PROCEDURE
1. Remove the tape from around the canister. Save the tape to reseal the canister at the end of
the measurement.
2. Remove the lid from the canister. Place the lower half of the canister, with the screen side
up toward the open air, on a table or shelf 1n the room chosen according to the above
instructions.
3. Fill In the start date and time on the label on the canister Hd.
4. After 7 full days (168 hours), replace the lid on the canister and reseal 1t with the tape.
5. Fill In the stop date and time on the label. This is very Important. Also, please fill 1n
the Information requested (To Be Completed By Occupant) on the other side of this sheet.
6. Place the canister and this sheet in the box provided. Seal the box as shown below and place
the addressed, postage-paid label on the box.
7. Mail the box within 1 day of resealing the canister. No postage Is required.
1. P««l th. p.p.r ott UK
TO SEAL THE BOX FOR MAILING:
2, Clo»t th« boi with eanlstir
•no thta ahegt Insidt box.
X Pr>» t>» front ol lh« taoi to *uL
FIGURE 6
Instruction sheet
27
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TO BE COMPLETED BT OCCUPANT
Date and tine you started and stopped the wasurenent:
Start date / / Time [J PM stop date / / Tine H
1. Postal zip code at location of measurement:
2. Type of building In which measurement was nade: Q single family home
Q nultl-family building Q business Q school Q other
3. Floor of building where measurement was made: H basement H first floor
(_J second floor or above
4. Room In which measurement was made: H bedroom [~| family room Q living room
[J unfinished basement £] office M classroom |_j other
5. Mere exterior doors and windows mostly open or closed during measurement?
[] Mostly open Q Mostly closed
6. Comments:
FOR GOVERNMENT USE
House l.D. Code Canister Number
(On canister label)
Geologic Region Code
EPA USE
Date Received / / Date Counted / / Time H PM
m d yr m d yr
Detector Number __ _ Gross Counts
"t _____ _ weight In
Counted by Checked by
Entered by Verified by
Comments:
FIGURE 7
Data form
28
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excessive decay. All canisters received at NAREL are counted,
regardless of the delay time. If the 3-sigma counting error is
less than 100 percent, the results are calculated. If the 3-
sigma counting error is greater than 100 percent, the canister is
not calculated.
E. Reporting
Usually, all canisters received at NAREL are counted on the
day they are received. NAREL reports results from canisters
processed each two weeks. The report contains data for the
canisters received during the previous two week period. Summary
reports may be provided on request. The reports contain all the
responses to questions on the data forms. If a form was not
fully completed, this section will be blank on the report.
Certain omissions, such as a start or stop date/time, make it
impossible to calculate a radon value. The canister, however, is
counted and the count data filed by canister serial number. If
the missing data are supplied at a later date, the results will
be calculated and reported on the next report. If any
irregularities are noted in a canister, such as the top not taped
properly, this will be noted in the comment section of the
report. The responsible agency conducting the survey will be
notified by telephone if any canister result is greater than
100 pCi/liter.
29
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Agencies conducting surveys with NAREL canisters, if they
have the appropriate computer equipment, may access their data
directly from the NAREL computer. They will be given access only
to their own data. NAREL also can arrange to send data directly
to agencies which have access to electronic mail. Interested
persons should contact NAREL to arrange to get computer access or
electronic mail.
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REFERENCES
A163
Altshuler, B. and Pasternack, B., Statistical Measures
of the Lower Limit of Detection of a Radioactivity
Counter, Health Physics, Vol. 9, pp. 293-298, 1963.
EPA89 Environmental Protection Agency, Indoor Radon and Radon
Decay Product Measurement Protocols, EPA 520/1-89-009,
Office of Radiation Programs: Washington, D.C., 1989.
Ge84
George, A.C., Passive, Integrated Measurement of Indoor
Air Using Activated Carbon, Health Physics, Vol. 46,
No. 4, 1984.
Kn79
Knoll, G.F., Radiation Detection and Measurement,
New York: John Wiley & Sons, 1979.
Ro65
Rosenstein, M. and Goldin, A.S., Statistical Techniques
for Quality Control of Environmental Radioassay, Health
Laboratory Science, Vol. 2, pp. 93-102, 1965.
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