W T RADON EMANATION STANDARD  SRM 4968
THEORETICAL AND PRACTICAL ASPECTS OF IT'S APPLICATIONS
Paul Kotrappa
Rod Bee,Inc.
Frederick, MD
ABSTRACT
Several years of research in developing a radon emanation standard by the National Institute of Standards
and Technology (NIST) has led to NIST Radon Emanation Standard SRM 4968 (Journal of Research of the MST
100:629639; 1996). This new standard is based on a polyethylene encapsulated ""Ra solution that has been
demonstrated to emanate a wellcharacterized and known quantity ^"Rn when employed in "accumulation mode*.
The encapsulated standard is intended to serve as a more convenient, easiertouse, alternative to the conventionally
employed "^Ra solution standards that have been disseminated by MIST for 5 emanation measurements for the
past 40 years. The latter standards were certified only for ^Ra content only, whereas the new standards are
certified in termsof two parameters, both the ""Ra content and the "^Rn emanation fractions. Development of this
standard is considered as a land mark in the standardization of radon measurement. The ease with which the source
can be used lends itself to a wide spread use by any quality conscious radon measurement laboratory. These
emanation standards are already in use in the USA, Europe, Canada, and Mexico. Their use is expected to grow.
Hie purpose of this presentation is to describe (1) the standard and its characteristics, (2) theoretical and practical
aspects, (3) its application for routine calibration of continuous radon monitodintegrating monitors used in indoor
radon measurements, and (4) the precautions needed for its use.

Pennisswnfor reproduction of this paper is being obtained from the publisher: Nuclear Tecfmology Publishing.

1996 International Radon Symposium UP 2.1
TECHNICAL NOTE
~ a d i i~relection
~ i
D<Ãˆimu
Vol. 55. Na 3. pp. 21 1218( 1994)
Nuclear Technology Publishing
APPLICATION OF NIST ^Rn EMANATION STANDARDS FOR
CALIBRATING ^Rn MONITORS
P. Kotrappa and L. R. Stieff
Rad Elec Inc., 5714C. Industry Lane
Frederick, MD 2 1701. USA
Received January 21 1994, Amended May 4 1994. Accepted May 10 1994
AbstractÃ‘Th NIST (National Institute of Standards and Technology) has recently made available '"Rn emanation standards.
The NIST certified parameters include the '*'Ria strength and the emanation coefficient. When such a source is loaded into a
leak tight enclosure of a known volume. '"Rn accumulates over time and it is possible to calculate precisely both the '"Rn
concentration after any accumulation time and also the time integrated average '"Rn concentration after any given accumulation
time. Equations needed for calculations are given. For example, i f a 25 Bq (676 pCi) NIST source is loaded into a jar with an
air volume of 3.72 litres, the time integrated average "'Rn concentration is 973 Bq.m" (26.3 pCi.1"') after an accumulation
time of two days. If the "'Rn detector placed in the jar is nonradonabsorbingand a m e integrator, i t must yield the theoretically
predicted results. If not. on appropriate calibration correction factor can be calculated The paper describes a study involving 34
randomly chosen commercially available EPERMtR)(ElectretPassive Environmental Radon Monitor) and 17 NIST sources in
17 different calibration (accumulator) jars. The study indicated that EPERMt% give results within about 5% of the predicted
results. The study also includes a study of continuous radon monitors. The data obtained show that. i n practice, NIST sources
can be used to calibrate both passive and active continuous '%I
monitors. The availability o f NIST sources with a precisely
known ^^Ru emanation characteristic is a major advance in radon metrology. Practical problems in (he successful use of these
sources are discussed.
INTRODUCTION
The NIST (National Institute of Standards and
Technology) has recently made available ^Rn emanation standards*'). The NIST certified parameters
include the "'Ra strength and the emanation coefficient.
When such a source is loaded into a leak tight jar of a
known volume 2aRn will accumulate over time. It is
possible to calculate precisely the time integrated average radon concentration after any given accumulation
time. If the "'Rn detector present in the jar is nonradonabsorbing and a (me integrator, this radon detector
must yield the theoretically predicted results. If there
is some consistent difference, suitable NIST traceable
calibration corrections can be derived. The current study
involves 34 randomly chosen EPERMqR)S*and 17
NIST sources in 17 different calibration jars. The purpose of the study is to show how a NIST source can be
used in practice. The availability of NIST sources with
precisely known radon emanation characteristics is considered to be a major advance in radon metrology.
MATERIALS AND METHODS
The recently developed ^"Rn emanation standards
that are based on polyethyleneencapsulated ^Ra
solutions(u are used in the present study. These sources
*EPERM1') is a registered trade mark of the electret ion
chamber system patented and manufactured by Rad Elec Inc.,
57144 Industry Lane. Frederick. MD 21701. USA.
are certified by NIST in terms of 226Racontent and the
^Rn emanation fraction. These have been demonsinned to emanate a well characterised and known quantity of "'Rn when employed in 'accumulation mode'('*.
A total of 17 such sources with "'Ra content ranging
from about 4.5 Bq to about 450 Bq with an emanation
fraction of 0.890 were made available by NIST for this
study. Specially prepared glass jars with a capability of
being made leak tight were used as accumulators (see
Figure 1). Such glass jars were previously used for the
measurement of ^Rn in water"'. The %n detectors
used were the standard commercially available EPERM"11 210 ml volume 'S' chambet*'** configured
with either short term (ST) or long term (LT)electrets.
Ã‘^^^^^^Q
Radon leaktlaht
IId
Temperature
strip
â‚¬PE
radon monitor
Figure 1. Experimental arrangement. Detectors used: two SST
or SLT EPERMS.

1996 International Radon Symposium IIP 2.2
P. KOTRAPPA and L R. STIEFF
given by Equation I since the chamber starts with 0
concentration and builds up to a maximum:
THEORETICAL CALCULATIONS OF ^Rn IN AN
ACCUMULATOR
Equation I gives the ^Rn concentration in the
accumulator at any time TA after the start of accumulation. This is the maximum '"Rn concentration seen by
the detector in the accumulator. The ^Rn concentration
varies from zero at the start of the experiment to the
concentration given by Equation 1 at the end of the
accumulation lime of TA days:
ARn =
f AR, ( 1  exp(ARnTA))
vA
(1)
where An, is the '"Ra content of the NIST source. Ann
is the ^Rn concentration after an accumulation time of
TA days. f is the emanation coefficient of NIST source,
ARn is the decay constant of ^=Rn, VA is the air volume
of the accumulator.
The time integrated concentration of ^Rn is obtained
by integrating Equation 1 from time zero to TA. Further,
if the time integrated "'Rn is divided by the accumulation time TÃˆwe arrive at the average concentration
over the time TAPCan (Equation 2 gives the result). This
is always smaller than the maximum concentration
(  1  expib.TA))
ÃˆRnT
f
C" 'VA
(2)
The "*Ra concentration is in Bqunits, f is the emanation coefficient and is determined by NIST as 0.890,
the volume of the jar should be in units of cubic metres
(3720 ml = 0.00372 m3), the time is in units of days and
the decay constant of radon is in units of day"'
(0.1812). then the "'Rn concentration is in units of
Bq.m"'. This is divided by 37 to get the concentration
in pCi.1I.
AIR VOLUME OF THE ACCUMULATOR
The air volume of the accumulator depends upon the
number and the type of EPERMR*used in the accumulator jar. The different air volumes to be used are given
in Table 1. These were determined experimentally by
carefully measuring the volume of the empty jar and the
volume of the jar when one, two or three 'S' chambers
were loaded into the jar. The empty accumulator jars
were filled with distilled water and the water needed to
Table 1.Air volume of the accumulator when different fill the jar weighed. Measurements were then made by
numbers and types of EPERMS are used in the accumu loading the accumulator jar with different numbers of
'S' chambers. The difference in the weight of the water
lator.
between the two measurements gives the air volume of
the jars, when the respective number of detectors were
Number
Air volume
EPERM
loaded.
Experiments were repeated at least 10 times and
of
units
(ml)
type
the values reported in the table have an error of less
than 3%. Similar measurements were made for one, two,
SST or SLT
three and four 'L' chambers. The volume of the empty
SST or SLT
jar is 3996 ml and the air volume occupied by one
SST or SLT
SST or SLT EPERM is 123 ml and one LST or LLT
LST or LLT
LST or LLT
EPERM is 47 ml.
LST or LLT
Depending upon how many EPERMS are inside the
LST or LIT
accumulator, the appropriate air volume (Vi) given by
Table 1 should be used. For example, if 2 SST (or SLT)
are used in the accumulator, the appropriate value for
VA to be used is 3720 ml. See Table 1 for the appropriate value to be used when a different number of detectors are used in the accumulator.
Figure 2 is a graphical representation of Equations 1
and 2. The top curve gives the maximum ^ R n concentration seen by the detector (Equation 1) and the lower
curve gives the time averaged concentration (Equation
2) seen by the detector. If the detector is a true integrator, the response should be in accordance with the
lower curve. If a continuous radon monitor is inside the
accumulator, it should track the upper curve.
Figure 2. "'Rn concentration in a 3.72 litre accumulator.
Detectors used: two SST EPERMS. Source: NIST source with
25 Bq "'Ra and f of 0.890. ( a ) radon concentration. (+)
average radon concentration.
EXPERIMENTS AND RESULTS
Each source was fixed inside each jar using the clip
arrangement. The NIST serial number was marked on
each jar. NIST instructions require $at the source

1996 International Radon Symposium IIP 2.3
M5T STANDARDS FOR CALIBRAT/NG ^Rn MONITORS
should be open to the atmosphere for at least 24 h prior
to the use of the source in the accumulator mode'". This
boundary condition was achieved by keeping the lid of
the jars open for at least 24 h. The jars were also left
in a low radon area (outside environment) so that the
contribution from the residual radon in the jar is minimised before starting the accumulation. A pair of premeasured EPERMS (initial voltage IV) were loaded
into each jar. lids closed, (he collars tightened and the
time noted (please see Figure 1). This is the start of
the experiment After the desired specific accumulation
period. the EPERMS were removed from (he jar and
left in a low radon area in the 'on' position for a period
of 3 h. Only after this delay was the final voltage reading taken (FVJ. In the normal use of EPERMS the
final reading is taken immediately after the termination
of a measurement. The reasons for taking a delayed final
reading in the present case warrant additional discussion. In the accumulator mode the radon concen(ration changes from zero at the beginning to a
maximum concentration at the end of (he experiment.
The ^"Rn and associated progeny signals at the beginning and at the end are very different. The EPERMS
are usually calibrated in a radon calibration chamber
with a steady "'Rn concentration"*. Boundary conditions are not the same in the accumulator mode and
in steady state modes. The end effect in accumulation
mode is compensated by leaving the EPERMS in a low
radon area for about 3 h so that the contribution from
the deposited decay products is taken into account. By
following (his procedure it has been found that the
accumulation periods can be as short as 6 h. The "'Rn
concentration was calculated by the standard
pr~cedure"~~
using the calibrations based on steady
state radon concentration. It is important to note that (he
actual accumulation time is used in the calculation and
this does not include (he3 h delay time.
Results are listed in Tables 2 to 6. In these tables,
column 2 gives the source strength and column 3 gives
the theoretically calculated average concentration in the
Table 2. Experimental results. Detectors used: two SST EPERMS. Accumulation time 3.07 days.
Source
CP5
CP5
CP6
CP6
CP7
CP7
CP27
CP27
CP28
CP28
CP30
CP30
CP31
CP3 1
CP32
CP32
CP33
CP33
CP35
CP35
CP36
CP36
CP37
CP37
CP38
CP38
CP4 1
CP4 1
CP42
CP42
CP44
44
blank
blank
NS
NS
(Ra Bq)
(I3q.m')
El
W
Fv:
EP
EPB
(Bq.m9
(EPB)/
Av.
NS
ratio
0.898
0.897
1.075
0.962
0.984
1.023
1.029
1.152
1.183
0.980
1.096
I .040
1.056
1.014
0.945
0.944
0.88 1
0.995
0.976
0.843
1.038
I .022
0.914
1.001
1.018
1.OX
0.892
0.848
0.881
0.903
0.937
0.997
SK6439
SK47O1
SK4697
SKI 126
S13944
SK3612
SK1301
SK4734
SJ6073
SK4733
SK3574
SK3786
SKI255
SK4722
SK6435
SK6412
SK6446
SKI118
SKI217
SK4696
SK3601
SK3533
SK6405
SK4715
SJ3779
SK3728
SK4699
SK6451
SK3682
95778
S13986
SK3768
S11457
SI14I9
Av.
% STD

1996 International Radon Symposium IIP 2.4
L R. STIEFF
accumulator. Column 7 gives the average concentration of 4 to 5 Bq "'Ra. Because the usable range of electret
as measured by EPERMS, Column 8 gives the net E surface voltage is from 700 to 200 V, electrets of differPERM value after subtracting the blank readingB' and ent surface voltages were used to verify their performcolumn 9 gives the ratio of the concentration measured ance over the entire usable voltage range. Table 3 is a
by EPERM and the theoretically expected concen repeat of the experiment, using the same detectors and
tration. Column 10 gives the average ratio of the two same sources. Table 4 gives the results of experiments
detectors in the same accumulator. The grand average with stronger sources (33 to 460 Bq). Table 5 and Table
ratio and the associated standard deviation is given at 6 give results using SLT EPERMS.
Results indicate that, on average, SST devices give
the end of column 10. To obtain the blank reading, two
EPERMS were introduced into an empty jar similar to results nearly 3% lower than (he NIST predicted values.
the one used for experiments. These EPERMS were Similarly, on average, SLT devices give results nearly
exposed for the same time that was used for the experi 6% lower than the NIST predicted values. This is within
the accuracy expected from EPERMS~'.~'.However,
menial ones.
individual EPERMS can vary by wider margins. For
very accurate measurements, individually calibrated
DISCUSSION OF RESULTS
EPERMS can be used (please see later section). The
Table 2 gives the results for a 3.07 day accumulation. procedure developed in this study is demonstrated to
Two SST EPERMSwere used. NIST sources used were be usable for different accumulation times, for different
P. KOTRAPPA and
Table 3. Experimental results. Detectors used: two SST EPERMS. Accumulation t h e 3.00 days.
Source
CP5
CP5
CP6
CP6
CP7
CP7
CP27
CP27
CP28
CP28
CP30
CP30
CP31
CP31
CP32
CP32
CP33
CP33
CP35
CP35
CP36
CP36
CP37
CP37
CP38
CP38
CP40
CP40
CP4 1
CP4 1
CP42
CP42
CP44
CP44
blank
blank
NS
NS
(RaBq)
(Bq.m")
El
IV
FV,
EP
(Bq.mI)
EPB
(EPB)/
NS
0.934
0.886
1.079
0.98 1
0.965
I .008
1.014
1.103
1.219
1.000
1.139
1.O8O
1.063
0.994
0.947
0.988
0.799
0.968
0.975
0.832
1.O87
1.024
0.910
0.98 1
1.038
1.O37
0.90 1
1.100
0.842
0.898
0.870
0.893
0.960
1.a24
SK6439
SK47O1
SK4697
SKI 126
$13944
SK3612
SKI301
SK4734
SJ6073
SK4733
SK3574
SK3786
SK I255
SK4722
SK6435
SK6412
SK6446
SKI118
SKI217
SK4696
SK3601
SK3533
SK6405
SK4715
SJ3779
SK3728
SK6434
SK6415
SK4692
SK6422
SKI253
SK6448
SI3986
SK3768
SI1457
SI1419
GRD A
% STD
214

1996 International Radon Symposium IIP 2.5
Av.
ratio
N1ST STANDARDS FOR CALIBRATING '"Rn
EPERM configurations
and for sources o f different
strengihs. EPERMS were found to function as truly
integrating "Qn measuring devices, integrating radon
concentrations from near zero to the highest
(17.500 Bq.me3) encountered in the study.
Before using a certain configuration of EPERMwith
a source of certain strength, one should do a theoretical
analysis so as not to discharge the electret beyond the
operating voltages of electrets.
I n order to use NIST sources i n accumulation mode:
(1) It is important to know the exact air volume of the
accumulator, when test devices are inside the
MONITORS
accumulator. This may be more complicated when
complex instruments such as continuous radon
monitors are located inside the accumulator.
( 2 ) The accumulator should be leak tight so that
accumulated radon does not escape.
(3) The test devices themselves should not absorb or
affect the radon concentration i n the accumulator.
(4) Instruction given by NIST on the use of these
sources must be followed'". One imponant requirement is to leave the source in infinite volume for at
least 24 h before using the source in the accumulator.
amends
the sources be stored
in lms R,ative ,,umidiv,
when not in uw.
Table 4. Experimental results. Detectors used. two SST EPERMs. Accumulation time: 1 day for CP4S to CPS9 and 2
days for CP17 to CP26.
Source
NS
NS
(Ra Bq)
(mm')
.
El
IV
Fv2
EP
(6q.m')
EP0
(EPBY
NS
0.951
0.959
I .043
0.992
0.971
0.949
0.975
0.96 1
0.960
1.009
0.986
0.960
0.954
1.035
0.9 17
0.893
1 .on
0.9 19
0.941
0.946
1.058
I .O53
1.038
1 .&to
0.978
I.024
1.030
1.018
1.062
1.064
1.030
1.010
1.057
0.990
0.994
0.924
0.990
1.004
Av.
% STD

1996 International Radon Symposium IIP 2.6
Av.
ratio
P. KOTRAPPA andL R. STIEFF
Table 5. Experimental results. Detectors used: two SLT EPERM& AccumulaUon time: 1 day.

Source
(Ra Bq)
NS
El
IV
W,
(Bq.m')
EP
(Bq.rn')
EPB
(EP By
AV.
NS
ratio
0.901
0.881
0.91 1
1.O53
0.897
0.9 18
0.838
0.903
0.973
0.902
0.900
1.015
0.935
0.948
0.980
0.913
0.912
0.9 16
0.932
0.932
Av.
%STD
Table 6. Experimental results. Detectors used: two SLT EPERMS. Accumulation Ume: 2 days.
Source
NS
NS
(Ra Bq)
(Bq.m')
El
IV
Wa
Ep
(Bq.m')
EPB
(Bq.m')
(EPBY
Av.
NS
ratio
0.950
0.923
0.947
1.016
0.927
0.99 1
0.890
0.945
0.965
0.923
0.910
1.014
0.923
0.9 15
0.972
0.953
0.922
0.938
0.976
0.964
Av.
%ST

1996 International Radon Symposium IIP 2.7
0.936
0.982
0.959
0.918
0.944
0.962
0.9 19
0.962
0.930
0.970
0.948
2.204
NIST STANDARDS FOR CALIBRATING '"Rn MONITORS
CALIBRATING CONTINUOUS RADON
MONITORS
Table 7 gives the comparison between NIST estimated values with the values given by a CRM (Femtotech model 210, manufactured by FemtoTech Inc..
Carisle, OH, USA). The results indicate that the instniment is well calibrated.
The NIST sources have been previously used for
determining the pressure correction factors, also called
elevation correction factors by maintaining different
pressures simulating different elevations inside the
accumulators(".
The use of NIST emanation standards also gives an
elegant method of calibrating a continuous radon monitor (CRM) in a single experiment over a wide range of
radon concentrations. This applicationis based on the
fact that Equation I gives the radon concentration at any
accumulation time and that these theoretical estimates
should match the reading given by CRM. For example,
if one uses a 127 litre accumulator with a source of
880 Bq, the concentration varies from 46.4 Bq.m"' after
I h to I980 Bq.m' after 48 h. If such an instrument
also gives a cumulative average, then this can be compared to the NIST calculated value. Larger accumulation times will yield higher concentrations.
CONCEPT OF INDIVIDUALLY CALIBRATED
EPERM or CRM
If an EPERM shows 5% lower than what is predicted
Table 7. Experimental results. NIST source strength: 879.9 Bq. Detector used: Femtotech 210 continuous radon
monitor. Accumulation time: 17 to 48 hours. R l Is the ratio of radon measured by CRM to radon calculated.
R2 is the ratio of average radon measured by CRM to the average radon calculated. Accumulator volume 126.6
litres.
(hours)
NIST
Rn
(Barn')
NIST
Av. Rn
(Bq.m')
CRM
CRM
Rn
Av. Rn
(Bq.rn')
(f3q.m')
444.00
466.20
484.70
510.60
536.50
555.00
573.50
595.70
617.90
640.10
654.90
669.70
691.90
717.80
736.30
754.80
773.30
791.80
810.30
828.80
847.30
865.80
884.30
906.50
928.70
947.20
965.70
987.90
1006.40
1024.90
1039.70
1058.20
Av.
STD
217
1996 International Radon Symposium IIP  2.8
RI
R2
P. KOTRAPPA and L R. STIEFF
by the NIST source, the results of the measurements
made by using this unit in a later measurement should
be divided by 0.95 to get the correct results. Using a
portion (say from 700 to 650 V) it is possible to calibrate an EPERM to NIST traceability. This still has a
large range of usable voltage. An individually calibrated
EPERM can be used as reference EPERM or as transfer standard for calibrating continuous "^Rn monitors
used in '"Rn Calibration Test Chambers. The same
logic applies to a CRM calibrated using NIST source.
CONCLUSIONS
For the first time. it is possible to calibrate both passive and continuous radon monitors to NIST traceability.
The procedures developed in this work are usable with
simple equipment. Typical passive devices (EPERMS)
were found to give predicted measurement with an accuracy of about 5%. Similarly, commercially available
continuous radon monitors (Femto Tech) also gave satisfactory performances. With the availability of this
technology, ^Rn measurement instruments can all be
made NIST traceable. This is indeed a great step forward in radon metrology.
ACKNOWLEDGEMENTS
The authors are grateful to Dr Ron Colle of NIST for
providing the sources for this study. They also wish to
thank Mr Shiva Veerabhadraiah for technical help and
for data processing.
REFERENCES
1. Colli, R. and Hutchinson, J. M. R. Technical Notes on the Use of Standard Reference Material. Radon222 Emnation
Standard. (RadioactivityGroup. National Institute of Standards and Technology. MD.USA) Technical Report No 4968 (1993).
2. Kotrappa, P. and Jester, W. A. Measurement of Dissolved '"Rn in water using Electret Ion Chamber "'Rn Monitors. Health
Phys. 64. 397405 (1993).
3. Komppa, P.. Dcmpscy. J. C.. Hickey. L. R and Stieff. L. R. An Electret Passive Environineml Rn222 Monitor Based on
Ioniwion Measurement. Health Phys. 542756 (1988).
4. Kotrappa. P.. Dempscy, J. C.. Ramsey. R W. and Stieff. L. R. A Practical EPEKM"' (Passive Environmental '"Rn Monitor)
for Indoor lZzRnMeasurement. Health Phys. 58.461167 (1990).
5. Kouappa. P. and Stieff, L. R. Elevation Correction Factors for EPERM Radon Monitors. Health Phys. 62. 8286 (1992).

1996 International Radon Symposium IIP 2.9