United statM
EnvlronrrwnUI Protection
Ag«ncy
Offteaof
Radiation Program*
WMiilngton, D.C. 20480
August 1984
EPAS20/ 1-85-012
(PM.-5235)
Radiation
METHODS FOR MEASUREMENT OF 222Sn EMISSIONS
FEOM DHDERGSOOND CRANIUM MINIS
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BIBLIOGRAPHIC INFORMATION
PB85-187953
Methods for Measurement of (222)Rn
Emissions from Underground Uranium Mines
Aug 84
by P. 0. Jackson.
PERFORMER: Office of Radiation Programs, Washington, DC.
EPA/520/1-85/012
SPONSOR: Department of Energy, Washington, DC.
Sponsored by Department of Energy, Washington, DC.
Three methods are described for use in monitoring the
emission rate (activity per unit time) of 222Rn in
ventilation air exhausted from active underground uranium
mines. The methods include procedures for making (1)
concentration measurements, (2) flow-rate measurements, and
(3) emission rate calculations.
KEYWORDS: *Radioactivity, *Mine gases, *Uranium mines,
*Radon 222.
Available from the National Technical Information Service,
SPRINGFIELD, VA. 22161
PRICE CODE: PC A03/MF A01
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing;
1, REPORT NO. 2.
520/1-85-012
4. TITLE AND SUBTITLE
Methods for Measurement of 222Rn Emissions from
Underground Uranium Mines
7. AUTHOR(S)
P.O. Jackson
9. PERFORMiNG ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
3, RECIPIENT'S ACCESSION NO.
PB85 1879537AS
5. REPORT DATE
August 1984
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Three methods are described for use in monitoring the emission rate (activity
per unit time) of 222Rn in ventilation air exhausted from active undergroung
uranium mines. The methods include procedures for making (]) concentration
measurements, (2) flow-rate measurements, and (3) emission rate calculations.
17. KEY WORDS AND DOCUMENT
a. DESCRIPTORS b.lDENTI
18. DISTRIBUTION STATEMENT 19. SECU
_-tJm4.-_j unc
unlimited 2Q SEC(J
unc
ANALYSIS
FIERS/OPEN ENDED TERMS C. COSATI Field/Group
RITY CLASS (This Report} 21, NO. OT PAGES
lassified 32
RITY CLASS (Tills page) 22. PRICE
lassified
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
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EPA 520/1-85-012
(PNL-5235)
METHODS FOR MEASUREMENT OF 222Rn EMISSIONS
FROM UNDERGROUND URANIUM MINES
P. 0. Jackson
August 1984
Prepared for the U.S. Environmental
Protection Agency under a Related Services
Agreement with the Department of Energy,
Contact No. DE-AC06-76-RLO 1830
Office of Radiation Programs
U.S. Environmental Protection Agency
Washington. D.C. 20460
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DISCLAIMER
This report has been reviewed by the Office of Radiation Programs,
U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views
and policies of the U.S. Environmental Protection Agency, nor does mention
of trade names or commercial products constitute endorsement or
recommendation, for use.
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METHODS FOR MEASUREMENT OF 222Rn EMISSIONS
FROM UNDERGROUND URANIUM MINES
Forward:
The following three methods are intended for use in monitoring the emission
rate (activity per unit time) of 222Rn in ventilation air exhausted from active
underground uranium mines. The methods include procedures for making 1) concen-
tration measurements, 2) flow-rate measurements, and 3) emission rate calcula-
tions. The general format follows the usage found in the Code of Federal
Regulations -Title 40, Part 60, Appendix A.
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METHOD 1—DETERMINATION OF RADON-222 EMISSION RATES FROM UNDERGROUND
URANIUM MINES; GRAB SAMPLING METHOD FOR MEASURING RADON-222 CONCENTRATIONS
i.N MINE EXHAUST AIR
Introduction
A method Is described that uses scintillation flasks for grab sampling and
measuring the concentration of 222Rri in air. There is a variety of commercial
and shop made flasks and counting equipment that can be used with them, and
any system which uses a scintillation flask which can satisfy the criteria
below may be used as a measurement system. Since this technique is to be used
to evaluate the average 222Rn emission rate from air streams that often are
characterized by significant temporal variations of radon concentrations, it
will be necessary to average samples collected at several times tj achieve the
necessary precision and accuracy, as outlined below. It will also be-necessary
to measure the flow-rates of exhaust air at approximately the same times that
samples are collected.
1. Principle and Applicability
1.1 Principle. Air containing gaseous radon is withdrawn from each mine
vent using either an in-line probe or a vent-outlet probe which uses the velo-
city of the air stream to force a sample through the collection apparatus. A
fraction of the air is filtered and drawn into a scintillation flask. After
allowing the ingrowth of 218Po, 214Bi, 2i4Pb and 214Po, which are the short
lived daughters of radon, the radon is determined by counting the scintilla-
tions produced in the zinc sulfide coat-ing on the flask walls by alpha-particles
within the flask.
1.2 Applicability. This method applies to radon-222 (radon) emissions. It
does not measure radon-220 (thoron) emissions. Both long-term and short-term
variations in emissions occur at uranium mines. This procedure will yield
estimates of the average emission rate for a calendar quarter when several
samples are collected over an interval of several days within one quarter. To
estimate an annual average, the measurements must be repeated quarterly and
the results for each quarter must be averaged. Since existing uranium mine
vents have several configurations, two sampling techniques and equipment are
specified - one for in-stack sampling where accessible sampling ports are pre-
sent and the other for sampling at the outlet of the vent.
2. Sensitivity and Range
2.1 Sensitivity. The sensitivity of this method depends on the size of the
scintillation flask and its background scintillation rate, which normally in-
creases with use because of the accumulation of 210pjj ancj its daughter 210p0
from the decay of radon in air samples stored in the flasks. When new, back-
ground counting rates from about 0.1 counts per minute (c/m) up to 1 c/m have
been reported for commercially available flasks, depending on make and model.
Typically, the backgrounds of 0.1 c/m are associated with small flasks which
have a capacity of about 100 ml and response factors of about 0.5 c/m per pCi/1.
Backgrounds of 1 c/m are associated with larger flasks with capacities up to
1.4 liters and response factors of about 6 c/m per pCi/1.
The limiting sensitivity is defined here as that concentration which is needed
to achieve the overall precision standard of 22% total error at the 95% confi-
dence level. As will be discussed under Paragraph 4, sensitivity is limited
by the statistical variations inherent in counting radioactive decay events as
well as three other sources of variation. Using reported estimates for the
magnitude of these terms, this method has a minimum sensitivity of about 10
pCi/1 for a 30-minute counting time for 100 ml scintillation flasks with back-
grounds of 1 c/m and efficiencies of 0.5 c/m per pCi/l«-
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2.2 Range. The maximum concentration that can be determined is limited by
the maximum counting rate that can be measured without unacceptable losses
caused, by the pulse pair resolution of the instrument. The pulse pair resolu-
tion time is the time required for the instrument to process and record a count
before a second count can be detected and processed. As counting rates get
progressively higher, the time between individual pulses gets shorter and shorter.
Eventually a significant fraction of the pulses are lost because the instrument
can't detect them until it has completed the processing of the preceding pulses.
Corrections can be made for such losses, but it is best to limit the counting
rate so that the maximum correction will be less than 10% because such correc-
tions are normally only approximate.
Typical laboratory scintillation counters have pulse pair resolution times
ranging from about 2 microseconds up to 20 microseconds. However, some commer-
cial counting systems may have longer resolving times because they are designed
for portability or for environmental samples with low activity. For resolving
times less than 20 microseconds, count-rates up to 3 X 10$ counts per minute
can be tolerated. This is equivalent to a radon concentration of about 1.5 X
105 pCi/i for a 100 ml flask.
3. Interferences
3.1 Moisture Entrapment. Air from some uranium mine vents contains signi-
ficant quantities of entrained water droplets that will be collected by the
sample delivery system. Normally, if a small quantity of entrained water is
present, it will be stopped by the inlet filter. However, when a visible spray
of water is present, the droplets can plug the inlet filter or pass through it
.into the scintillation flask where the water will affect the detection effi-
ciency of' the scintillator for alpha particles. Therefore, filters should be
checked to be sure that they have not become plugged after the sample has been
collected, and the interior of the sampling flask should also be checked for
liquid water. Visible droplets on the window of the flask indicate the presence
of excess moisture. In such cases, a water spray removal system must be employed
in the sampling apparatus.
3,2 Particulate Matter. Occasionally the quantity of particulate matter
present in mine ventilation air may be sufficient to block the inlet filter.
For this reason, high efficiency fiberglass filters have been specified for
the sample collection system because they plug more slowly than other high
efficiency filters. If air is no longer passing through the filter after the
scintillation flask has been filled and removed from the sampling line, the
filter is plugged. Another sample should be collected when dust levels are
lower, or a larger area in-line filter should be used.
3.3 Static Charge. Scintillation flasks often acquire static charges on
their surfaces during handling. Such charges can drastically change the count-
ing efficiency of the flask by attracting the charged ions of radon progeny
born in the flask to areas of the flask where the detection efficiency for the
alpha-particles emitted in their decay is significantly different from normal.
This is especially true for scintillation flasks constructed of plastic materials
which are inherently non-conductors of electrical charge. Static charge can
also occur on the glass viewing window of metallic flasks and on inlet tube
connections. For this reason, flasks which have non-conducting parts must be
treated to make them at least temporarily conductive. Some manufacturers apply
coatings to the interior of such surfaces to make them conductive. All noncon-
ducting exterior surfaces must be sprayed with an antistatic spray before using
the flasks. This must be done at least three hours before counting to permit
the decay of any daughter ions attracted to charged flask areas prior to spray-
ing. The flasks should not be rubbed or wiped between spraying and counting.
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3.4 Daughters of 220Rn. In some cases, there is a possibility that signifi-
cant quantities of 220Rn will be present in uranium mine air from deposits of
thorium at the mine site. Although 220Rn w-m decay to insignificance before
counting because it has a half life of only 56 seconds, its longer lived daugh-
ters, 2l2pb and 2081*1, will form within the scintillation flask between sample
collection and counting. This activity will be an insignificant fraction of
the total activity unless the thorium content of the ore exceeds 5% of its
uranium content by weight. For mines that have thorium assays which average
below 5 weight % of the uranium content, no corrections for 220Rn daughters
are necessary. For mines that do not have such assay information, or that
have assay information indicating that the thorium content of the ore exceeds
5% of its uranium content, a test must be performed to determine if a correc-
tion for the 220Rn daughters will be necessary. The correction must be deter-
mined by re-counting one of the scintillation flasks, collected at each vent
of the mine, three times at four-hour intervals after the initial count (one
of the intervals may be as long as 20 hours if required by working shifts).
The resulting count-rates should be plotted on semi-logarithmic graph paper.
The final count should be decay corrected to the time of the initial count
using the half-life of 222Rn. if 220Rn daughters are present, the count-rate
of the initial count will be higher than predicted by the extrapolation of the
later counts. The difference between the measured activities can give an in-
dication of the amount of 220Rn daughters present. Because this technique is
sensitive to changes in count-rate, it is essential that the operator verify
that the flasks being used do not leak significantly during the period of the
.evaluation and that procedures to minimize static charge have been used.
If the'decay study of one set of samples indicates that the excess activity
from the first count was less than 10X of the decay corrected activity from
the later count, then the normal counting and delay intervals can be used for
that vent. If the excess exceeds 10%, the recommended procedure must be modi-
fied on all subsequent samplings to permit a 2-3 day holding period before
counting. After that time the daughters of 220Rn will have decayed to less
than 4% of their initial activity and can be assumed to be an insignificant
fraction of the 222Rn activity, except for the highly unlikely event that the
thorium content of the ore exceeds its uranium content, (in which case, the
delay interval must be extended until counts taken at successive 24 hour inter-
vals decrease with the characteristic half-life of 222Rn).
4. Precision and Accuracy
4.1 Accuracy. At the present time (Aug., 1984) there are no recognized
primary national or international standard radon sources. Laboratories per-
forming radon calibrations normally use the equipment and method described in
the ASTM standards (12.1). In that method, radon ingrown in a sealed flask
containing a standard solution of 226Ra> traceable to the National Bureau of
Standards (NBS), is purged from the solution and transferred to the scintilla-
tion flask using a prescribed transfer apparatus. The radon ingrown in the
sealed flask between purges can be computed from the 226Ra concentration and
the time interval.
Several governmental laboratories that routinely perform radon measurements
using the flask method have been participating in an ongoing intercomparison
study to evaluate the accuracy and precision of their measurements (12.2, 12.3).
These laboratories send four scintillation flasks to the Environmental Measure-
ments Laboratory (EML) of the U. S. Department of Energy. There, the flasks
are tested for leaks and filled from a large chamber containing air mixed with
radon. The scintillation flasks are returned to the laboratories where the
radon concentrations are measured. Fisenne (12.2, 12.3), reported the results
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of the first four intercomparisons. Those results indicated that there were
difficulties with leaks and/or the calibrations of the radon flasks. The range
of average results for eight laboratories was 22% in the first intercomparison,
and 26% in the second. This scatter 1n calibrations is greater than would be
predicted on the basis of the precision of replicate measurements and is also
much greater than the errors normally estimated for NBS radium standards. For
this reason, the calibration of scintillation flasks by the ASTM method is not
recommended at this time.
Currently, two laboratories have tentatively been designated to provide cali-
brated mixtures of radon in air based on their in-house calibration techniques.
EML can provide radon concentrations appropriate for environmental measurements.
The Bureau of Mines maintains a laboratory in Denver, Colorado which can provide
radon at concentrations typical of those encountered in uranium mine air. For
calibrations, scintillation flasks must be shipped to that laboratory where
they will be filled with the standardized mixture.
4.2 Precision. Analysis of the errors contributed by various sources inherent
In this method indicates that it is practical to estimate the quarterly average
radon concentration in air from a uranium mine vent with a total error of 22%
at the 95% confidence level. Therefore, an arbitrary limit of 22% has been
set for this total error. The total % error, (1.96 ST), can be derived from
the following expression:
ST =
where, .. C
ST = the total % standard deviation of the method
sc = the % standard deviation from counting statistics
sm = the inherent % standard deviation from replicate measurements
sa = the stated % standard deviation of the calibration standard
and,
ss = the standard deviation of the measured radon concentrations re-
sulting from the temporal variations of concentrations present in air
from a mine vent.
This formula must be adjusted for the numbers of repeated sampling and repli-
cate flasks that must be collected from a mine vent to achieve the required
precision, as will be described later in this section.
The % standard deviation from counting statistics, sc.
The formula for sc is as follows:
. . 100 (EC + 2B)1/2
c " ECt1/2
where,
E = the counting efficiency, counts/minute per pCi/1,
C = the measured radon concentration, pCi/1,
B = the background count-rate, counts per minutes,
and t = the length of sample and background counts, minutes
Example:
For a scintillation flask with a background of 1 c/m, an efficiency of 0.5 c/m
per pCi/liter and counting intervals of 30 minutes for both sample and back-
ground measurements,
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100 (0.5C + 2)1/2
c " 0.5COO)"172
(667C * 2670)1/2
sc " C
For C = 10 pCi/1, sc = 9.7%.
The inherent error of the flask method, sm.
This error must be determined by filling several scintillation flasks with a
constant known source of radon and calculating the standard deviation of the
measured radon concentrations using the ordinary statistical formula, and not
counting statistics (9.7). Results reported by Jackson (12.4) indicate that
the maximum inherent error should be about 7% for this method. The measured
value of sm must not exceed 7%.
Precision of calibrations, sa. This term should be obtained from precision
estimates supplied by the calibration laboratory of'the Bureau of Mines. If %
standard deviations are not supplied, the reported standard deviations and
average concentrations should be used to compute sa (using the same formula
used for calculating s^ from the measured standard deviation (9.7).
Variability of source term concentrations. ss. The variability of concen-
trations at individual mine vents cannot be determined in advance of measure-
ments.- For the purpose of this method-the average value of ss reported by
Jackson (12.4) for short term temporal variations of radon concentration
measured at 150 vents of 27 operating mines will be assumed to apply.
Ss * 24X.
Since the error associated with temporal variations of the source term alone
exceeds the prescribed limit for total error, this method requires that samples
be collected from each vent on more than one occasion. The collection times
should be spaced at intervals. It is also necessary to collect a duplicate
set of flasks at each vent at each sampling time to assure that the scintilla-
tion flasks have been properly filled and have not leaked. Pairs of measure-
ments differing by more than the maximum allowable error (11.1) should be re-
peated at about the same collection time on another day. The replicate measure-
ments will affect sc, sm and ss, but sa will not be affected. A total of six
acceptable pairs of samples must be collected.
To account for the total number of samples which will be collected, the formula
for the total error must be modified. The standard deviation of the average
of several measurements is equal to the standard deviation of the individual
measurements divided by the square-root of the number of measurements. Since
sc and sm are affected by the total number of samples measured, but ss is only
affected by the number of repeated samplings, the total % error becomes:
Total % Error « 1.96 sy = 1.96
where
12 = the total number of samples (6 times 2) and
6 = the number of repeated samplings.
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Using the previous estimates of sc, sm» sa, and Ss» the maximum total error is
as follows:
Tot.1 Error - 1.96
= 22%
5. Apparatus
5.1 Sampling Apparatus (See Figures 1 through 4)
5.1.1» In-Stack Grab Sampling Probe. This unit must comply with specifica-
tions given in 10CFR60 Appendix A, Method 3, (2.1.1 and 2.1.2). (See Figure 1).
5.1.2 Vent-Outlet Sampling Probe* The probe must be made of metal or plastic
components joined with flexible tubing. Joints must be clamped. It must incor-
porate a funnel at the inlet to collect sufficient air to maintain a positive
air flow. The funnel must have a diameter of at least 10 cm at the larger end
and no less than 1 cm at the smaller end. The funnel should be connected to a
stainless steel tube of 1.3 cm (1/2 inch) diameter and should be formed with a
curve at one end with a radius of approximately 23 cm (9 inches) so that the
funnel can be inserted into the outlet of a vent. The overall length should be
about 1.8 meters (6 feet) and the short end should be about 38 cm (15 inches)
from the top (see Figure 2).
5.1.3 Water Separator. When water is entrained in the ventilation air, a
separator for removing the water must be attached to the sampling probe at the
.bottom of the long end. Flexible tubing-should be used for attachment. Joints
should be" leak tight. Water separators may be constructed of metal, glass, or
plastic. However, glass impingement separators are not recommended for field
work unless properly housed, because of the possibility of breakage. Water
separators can be obtained from commercial sources or can be shop-made.
5.1.4 In-line Filter. The in-line filter apparatus must use high efficiency
fiberglass filters with an active filter area of at least 7.5 cm2. Filters
must be at least 99.9% efficient for standard 0.3 |im OOP particles in air.
Filter papers must pass at least 20 liters/minute at 2.75 psi head pressure.
Filter housings must be leak tight and suitable for fiberglass filters. To
prevent tearing when using units with replaceable filter elements, the com-
pressive connections with the edge of the filter paper must not rotate as the
assembly is tightened. Filter housings which prevent the filter paper from
tearing with flow in either direction are recommended but not essential. Plastic
or metal tubing connectors and adaptors with leak tight ribbed or serrated
tapers may be used. At least two filter units will be needed. Ready-to-use
one-piece filtration devices with suitable characteristics may be used if integ-
rity of filter and seals are verified.
5.1.5 Flowmeter. The flowmeter must be a rotameter that is direct reading
for air and has a minimum scale reading 1 liter/minute or less and a maximum
reading between 9 and 15 liters/minute. The scale accuracy must be certified
by the manufacturer or a test laboratory to ± 10% or less of full scale.
5.2 Scintillation Flasks. At least four scintillation flasks of the same
type must be available for measurements. Scintillation flasks may be con-
structed of metal, glass, or plastic. Flasks must be equipped with a flat
transparent viewing window. Viewing windows must not be larger in diameter
than the diameter of the face of the photomultiplier tube used in the counting
system. Flasks must be coated with silver-activated zinc sulfide phosphor on
all interior surfaces except the viewing window. They must be equipped with
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PROBE
FLEXIBLE TUBING
FILTER (GLASS WOOL)
TO ANALYZER
SQUEEZE BULB
FIGURE 1. In-stack grab sampling probe
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18" —
FIGURE 2. Vent-outlet sampling probe
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Mine Vent (not drawn to scale)
\
• Vent Outlet Sample Probe
Rotameter
Water Separator
Flexible
Tubing
In-Line Filter
Scintillation
Flask
FIGURE 3. Vent-outlet radon sampling apparatus
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Gauge
Valve
Gauge «—i '
Tube r—*
Pump
Valve
A* V -A I Vacuum
fc— I Gauge
OO O
/"/Vacuum Hose
Filter Assembly
Filter
Valve
Flask
Valve
Scintillation
Flask
FIGURE 4. Flask evacuation assembly
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Clear Silica or Pyrex
Glass Window Coated
with Stannic Oxide
Stopcock
Zinc Sulf ide
Scintillator
Coated .
Metal Walls
FIGURE 5. Scintillation flask
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one or two valves or stopcocks suitable for high vacuum and positive pressures
up to 14.7 psig. Flasks may have any combination of efficiency, volume, and
background counting rate that produces a relative counting error sc (4.2) no
greater than 10% with air containing 10 pCi 222Rn/liter using a 30 minute count-
ing interval on the counting system in use. A typical small scintillation
flask is shown in Figure 5.
5.3 Counting System. The counting system must have at least the following
components.
5.3.1 Light Tight Counting Chamber to house photomultiplier tube and scin-
tillation flask. The counting chamber must be equipped with a photomultiplier
tube base and a resister string capable of supplying appropriate bias voltages
to the photomultiplier (P.M.) tube dynode stages and anode. Radio frequency
and magnetic shielding for the P.M. tube must be provided. Suitable con-
nectors for applying high voltage to the P.M. tube must be provided. The
chamber must be equipped to supply a pulse signal from the P.M. tube anode
that is decoupled from high voltage. A preamplifier may be included in the
electrical circuit.
5.3.2 Photomultiplier (P.M.) Tube. The P.M. tube-must be capable of opera-
tion at overall bias voltages of at least 1400 volts, and must have a peak
spectral response for emission wavelengths of 440-450 urn. P.M. tubes must
have flat end viewing windows.
5.3.3 High Voltage Power Supply. The power supply should be capable of
delivering the D.C. voltage requirements of the photomultiplier tube and the
current requirements of the resister string of the tube base. High voltage
must be filtered, to give <10 mV ripple peak to peak 5 hz to 50 mHz and regu-
lated to provide ? 0.0025% variation in" the output voltage-for any combination
of line and load variation within the operating voltage and current requirements
of the photomultiplier tube and tube base in normal counting operations. Voltage
settings must be indicated and adjustable in steps of "<25 volts; any setting
must be repeatable to within ± 0.5%. Temperature stability must be T ± 50 ppm/°C
from 0 to 50°C. Voltage and current requirements of the power supply must be
compatible with either the power supplied by the low voltage power supply to
the counting system, or by 110V AC lines.
5.3.4 Amplifier. The counting system amplifier must be compatible with the
pulses from the output of the photomultiplier tube base (voltage, time con-
stant and electrical impedence). Amplifier overall gain must be sufficient to
produce signal pulses larger than the noise discrimination level used in either
the amplifier output or the sealer input when the photomultiplier tube is operat-
ing in its normal voltage range. Gain must be externally adjustable or preset
for optimum system performance. If preamplification is not provided by the
tube base (5.3.1), a suitable preamplification stage must be provided in the
amplifier with an externally accessable signal test point at its output. Gain
drift must be ? 0.005%/°C. The amplifier must provide pulse shaping networks,
and baseline restoration suitable for zinc sulfide scintillator count-rates
expected from radon concentrations up to 1.5 X 10^ pCi/liter. It must have an
operating temperature range from 0 to 500C.
5.3.5 Scaler/Discriminator/Timer. Sealer input requirements must be com-
patible with amplifier output pulses and impedence. The sealer must have a
preadjusted input discriminator, or if a discriminating stage with an indicating
external adjustment to eliminate spurious counting from noise or other erroneous
pulses. If the amplifier contains an indicating output discriminator, or if a
separate discriminator stage 1s provided, the sealer discriminator need not be
indicating. Sealer circuits must have at least 20 MHz count-rate capability.
The count readout must be at least six decades with either LED or LCD display.
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An overflow indicator must be provided if less than seven decades are displayed.
Leading zeros must be suppressed. If a separate timing unit is used, the sealer
must be provided with a gate to allow the timer to turn it on and off. A test
feature which illuminates all segments must be provided for readouts. The
timer must be crystal timebase controlled, with an inherent accuracy of at
least ± 0.0025%, and setable in minutes, with increments no larger than I minute
and a maximum interval that is not less than 60 minutes. The timer must be
accurate to within ± 0.01% over an operating temperature range of 0 to 50°C.
The timer must provide gating signals compatible with the sealer.
5.3.6 Cabinet/Power Supply. The amplifier, sealer, (discriminator), and
timer must be housed in a suitable cabinet that has provisions for housing the
power supply where necessary. The cabinet must provide all power needs of the
equipment it houses with suitable regulation of the needed power. Ventilation
must be adequate to prevent internal temperatures greater than 50°C with all
housed equipment operating, in an environment which is less than 35°C.
5.4 Vacuum Pump. A small one or two stage rotary vane vacuum pump must be
used. The ultimate blank-off vacuum must be 20 microns or less and free air
pumping rates must be at least 20 1/min. The exhaust outlet must be connectable
to tubing or hose no larger than 3/4" I.D. to permit exhausting to outdoors.
5.5 Vacuum Sauge. The vacuum-gauge must be a thermocouple type with a range
of 0-1000 microns of mercury (0-5000 microns optional) and a direct meter readout.
It must be equipped with a metal gauge tube with a male npt thread connector,
threaded to metal or plastic tee adapter. The metal gauge tube and tee adapter
are to be coupled to vacuum pump shut-off valve (5.8).
5.6 Pressure Gauge, Air. The range, ofcthe pressure gauge must be 0-15 psig.
It must b'e accurate to 3% of its scale range.
5.7 Compressed Air Regulator. The regulator must have single or double stage
control and a flow indicator. The regulator must conform to the following
specifications: maximum inlet pressure: 3000 ps1; outlet pressure range: 4-
80 psi; flow range: 0.2-2 slpm. The Inlet connector must be CGA 346 or other
as used for air cylinders specified In Paragraph 6.2. Regulator must be equipped
with shut-off and flow control valves.
5.8 Vacuum/Pressure Valves (three required). The valve shut-off, and connec-
tions must be leak tight. The internal flow restriction must provide a maximum
flow of at least 20 SCFM air at a pressure drop of 10 psi. Valves isolate the
vacuum pump, (5.4) vacuum-gauge, (5.5) and the outlet of in-line filter (5.1.4).
5.9 Explosion Shield. The explosion shield must be suitable for scintilla-
tion flask pressurization or evacuation operations.
5.10 Barometer (optional). The barometer must be calibrated for the alti-
tudes at the sampling site against a mercury barometer.
5.11 Oscilloscope (optional). The oscilloscope must provide a bandwidth 100
M Hz or more; a minimum sensitivity of 5 mV/div; fastest sweep of no more than
20 ns/div; a triggered sweep. It must provide sweep speeds from 20 ns/div to
at least 50 ms/div and calibrated verticle scale from 5 mV/div to at least 5
V/div.
6. Reagents
6.1 Anti-Static Spray. Any commercial anti-static spray in an aerosol can .
may be used. Test a small area to be certain that spray constituents do not
attack plastic components of scintillation flasks.
6.2 Air, Dry Compressed . Compressed air must comply with the following
specifications: maximum dew point -75°F, or maximum moisture 3 ppm; maximum
pressure 2200 psi; outlet connector compatible with regulator (5.8). This air
must be aged at least 30 days befora use, to allow 222Rn to decay.
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7. Standards
7.1 Calibration Sources. Caution. Radioisotope standards are available
from commercial and governmental suppliers. However, the possession, trans-
portation, and use of radioisotopes are regulated by federal, state and local
statutes. For possession of many radioisotopes, licenses are required. Per-
sons not specifically trained in the safe handling of radioactive materials
'should not perform analyses involving their use. Compliance with institu-
tional, local, state, and federal regulations concerning the procurement and
use of radioactive materials is the responsibility of the laboratory using
this method.
7.1.1 Radon-222. Institutionally calibrated 222Rn jn air is available from
the U. S. Bureau of Mines, Denver Research Center. Flasks to be calibrated
must be appropriately packaged to prevent damage and shipped to the center
where they will be filled with air containing radon. A range of concentrations
is available. For the purpose of this method, a concentration of 1000 ± 200
pCi/liter should be requested. The shipping address is:
U. S. Bureau of Mines
Denver Research Center
Bldg. 20, Denver Federal Center
Denver, CO 30255
Attention: Radiation Hazards Group
Prior to sending flasks, a written request for calibration should be sent to
the Research Center. The type of flask to be filled, its approximate volume,
the desired concentration, and the approximate date when the calibration will
.be needed, should be specified. Details can then be worked out between the
laboratories. The radon concentration, internal pressure, relative humidity
of fill air, calibration date and time, and the estimated accuracy of the cali-
bration at one standard deviation will be needed from the research center.
A convenient regenerative source of radon in air using 22bRa •}„ solution can
be constructed (12.1) or a radon generator using 226Ra in a solid substrate
can be purchased commercially. These sources can be used to make periodic
checks of counter characteristics but must not be used for flask calibrations.
7.1.2 Control Source Scintillation Standard. Point source reference stan-
dards of alpha-particle emitting radionuclides are available from the National
Bureau of Standards or commercial sources. These standards are prepared from
radioisotopes which require a federal or state license for repossession. In
Table 7.1 are listed two appropriate alpha-particle emitting point source stan-
dards and potential suppliers (Other commercial suppliers may be available.
The listing of a commercial supplier does not constitute endorsement by the
U.S. Environmental Protection Agency or its contractors).
TABLE 7.1
Point Source Alpha-Particle Emitting Standard Sources
Radionuclide Nominal Activity Range Supplier
Plutom'um-238 11 - 540 nCi (specify) U.S. Bureau of Standards
Americium-241 50nCi New England Nuclear Corp.
Standards purchased for this method must have a nominal activity in the
range from 10-50 nCi.
-------
After receipt, the alpha-particle-emitting control source should be mounted
on a disc of transparent plastic coated with silver activated zinc sulfide
scintillator. Material of this type is available from William B. Johnson
and Associates, Research Park, Montville, N.J. 07045. Other suppliers may
be available. See endorsement disclaimer above.
These scintillation control sources should be prepared by mounting the
alpha-particle emitting source(s) with emitting surface facing the scintillator
surface. (Note: if the two-source technique will be used to determine the
instrument dead time (9.3), the alpha particle emitting sources should not
be permanently attached to scintillator discs until after measuring the dead
time. With that technique, both sources are mounted precisely on a scintilla-
tor disc and the counting rate measured. Then each must be mounted separately
in the exact original positions and each counted.) After dead time measure-
ments are taken, each source must be permanently attached to a scintillator
disc. A diagram of one possible method for construction of this control
source is shown in Figure 6. Specific dimensions of the scintillator disk
are not critical as long as it is larger in diameter than the mount of the
alpha-particle emitting point source, and equal to or. less than the diameter
of the face of the photomultiplier (P.M.) tube used for counting scintillation
flasks. If the outside diameter of the scintillator disk is the same as the
face of the P.M. tube, it will be easier to reproducably mount the source at
one location on the P.M. tube. An index line should be marked on the edge
of the source so that its orientation can be reproduced in counting.
8. Safety Precautions. The laboratory is responsible for compliance with
all applicable federal, state, local, and institutional health and safety
regulations in performing this method. "Appropriate eye, ear and hand protec-
tion and protective clothing should be worn. Sorcie of the safety considera-
tions when sampling uranium mine vents are as follows:
8.1. Operations involving handling pressurized or evacuated scintillation
flasks involve risk of explosion or implosion. Care should be used to prevent
dropping or damage. Damaged scintillation flasks should not be used until
repaired. Appropriate explosion shields should be used when pressurizing or
evacuating.
8.2. The sound pressure level near some operating mine vents can be suffi-
cient to cause hearing imparement, so effective ear protection should be
worn in these cases.
8.3. Concentrations of 222Rn in air emitted from some mine vents may be
orders of magnitude greater than the concentrations specified for occupational
exposures. Although potential exposure intervals are very brief, for those
occasions where there is risk of excessive inhalation exposure, appropriate
respiratory protection should be worn.
8.4. When sampling or measuring flow rates from operating vents, it is
necessary to be aware of the location of air moving components, so that acci-
dental contact with moving parts will be avoided.
8.5. Since the air velocity from mine vents can be of the order of several
hundred meters per second, entrained sand, dust and gravel can be thrown
from the vent outlet with considerable force. Appropriate eye protection
should be worn.
8.6. The operating voltage for some fan motors is as high as 440V. Care
should be taken to avoid sources of electric shock.
?. Pretest Procedures
9.1 Leak-testing Scintillation Flasks
Scintillation flasks must be tested for leaks before and after calibrating
and at weekly intervals when samples are being collected at daily intervals
-------
Point Source Attached
with Pressure Sensitive
Plastic Tape
Alpha Particle Emitting
Point Source (Activity
Facing Scintillator)
Top Surface Coated
with Silver Activated
Zinc Sulfide Phosphor
Transparent
Plastic Film
Reference Line for Positioning Source
Drawn on Edge
Alpha-Particles
Pressure Sensitive
Plastic Tape
Point Source
Zinc Sulfide
Phosphor on
Plastic Film
FIGURE 6. Suggested scintillating control source assembly details
-------
or oftener. Both tests (9.1.1) and (9.1.2) should be performed initially,
then (9.1.1) can be used for routine testing. Before vacuum leak testing,
the vacuum system between the pump shut-off valve and the gauge, and filter
assembly must be sufficiently leak tight that pressures do not drift more
than 5% in a one-hour period at total pressures below 800 microns of Hg.
Before pressure testing, verify that scintillation flasks are designed to
withstand high vacuum and internal pressures exceeding 14.7 psig because of
the explosion hazard. During vacuum and pressure testing, scintillation
flasks should be behind a suitable explosion shield.
9.1.1 Vacuum Testing. Assemble the vacuum pump, gauge, and one of the
on-line filters. Attach a vacuum valve (5.8) to the pump (5.4) inlet, with
the flow direction into the pump. Attach the vacuum-gauge (5.5) to the valve
inlet using a tee fitting. Attach a second valve to the third leg of the
tee fitting. Connect a flexible tube to the inlet of the second valve. Attach
a tee fitting to the flexible tubing . To one leg of the tee, attach the
third vacuum valve (5.8). To the vacuum valve attach the outlet of an in-
line filter (5.1.4). To the other leg of the tee, attach a length of flexible
tubing which can be attached to the scintillation flask Inlets (see Figure 5.)
These connections may be made with flexible tubing and ribbed connectors or
with threaded connectors. All connections must be vacuum tight with the
scintillation flask inlet plugged (see above). Attach a scintillation flask
to the vacuum pump, gauge and filter assembly. Evacuate the flask to give a
measureable vacuum less than 800 microns. Record the pressure, close the
scintillation flask valve and store for at least 16 hours. Reattach the
flask to the pump assembly. Evacuate all lines to the initial internal pres-
sure of the flask. Close valve between-the vacuum pump and gauge and open
the scintillation flask valve. Residual pressure in the scintillation flask
must not have increased to more than 1000 microns. If a greater pressure is
measured, determine source of the leak and repair it.
9.1.2 Pressure Testing. Attach the output of the compressed air regulator
(5.7) to the inlet of filter assembly. Close the gauge valve. Insert a tee
fitting between a scintillation flask and the pump assembly inlet hose and
install the pressure gauge (5.6). With regulator shut-off valve closed, set
the pressure of the regulator output to 4 psi. Open the regulator shut-off
valve and flask valve. Slowly open the flow control valve and pressurize
the flask to 4 psi on the pressure gauge attached to the tee. Close the
flask valve and regulator shut-off valve. Remove the flask and store it for
at least 16 hours. Re-attach the flask to the tee. Open the regulator shut-
off valve and again adjust line pressure to 4 psi on gauge attached to tee.
Close the regulator shut-off valve and open the flask valve. The pressure
should be at least 3.9 psi. If a pressure leak is observed, the leak point
may be determined using a leak-testing bubble solution or by emersion of the
flask in water. Repair pressure leaks before using the flask.
9.2 Optimizing Electronic Set-points. Unless the optimum settings for
counting scintillation flasks have been previously determined, the operating
high voltage (H.V.) and set-points for the counting system must be established.
The H.V. applied to the P.M. tube must be within the operating range for the
tube specified by the manufacturer. In no case should the H.V. setting be
above the maximum specified for the tube.
Place a scintillation flask filled with air containing at least 800 pCi
222Rn/liter on the P.M. tube in the light tight chamber. Close the chamber
and wait 5 minutes. Attach a suitable oscilloscope to the test point between
the preamplifier output and amplifier input. Turn on the H.V. Increase the
H.V. while observing pulses from preamplifier. Pulses will increase in size
-------
as voltage increases. Increase the H.V. until the largest pulses from pre-
amplifier begin to clip (flatten at their top). Then decrease the voltage
slightly until no clipping is observed. Record this initial H.V. setting.
If the amplifier has adjustable gain, set the control(s) to the approximate
center of the gain span. Adjust the discriminator between amplifier and
sealer to minimum discrimination. Take a count and record the count-rate.
Increase the discriminator setting by 10% of full scale. Take a count and
record the count-rate. Repeat over the full range of the discriminator. At
minimum discrimination the highest count-rates should be observed. The count-
rate should decrease with increasing settings.
In the center of the span there should be a range of settings where count-
rates change by about 5% between settings. (This is the discriminator plateau.)
The set-point for the discriminator is the setting at about the center of
the plateau. If count-rate does not drop by at least a factor of two between
the discriminator plateau counts and those at the maximum setting, the amplifier
gain setting should be reduced by about 10%. Re-set the discriminator to
maximum and measure the count-rate. Continue decreasing amplifier gain by
10% steps and determine the point where the count-rate is about a factor of
two lower than it is on the discriminator plateau region. This is the ampli-
fier set-point. Then take a new series of measurements with 10% increments
of the discriminator. Set the discriminator at the set-point where the count-
rate is in the center of the discriminator plateau region as defined above.
If there was no discriminator plateau on the original set of measurements,
and count-rates fell by a factor of two or more at lower discriminator settings,
the amplifier gain should be increased i.;n 10% steps until the count-rate at
a discriminator setting of 30% of full scale is less than 10% lower than the
count-rate at a setting of 20% of full scale. This is the approximate ampli-
fier set-point. Then the count rate should be remeasured at each discriminator
satting to establish the discriminator plateau and set-point. The amplifier
set-point can be readjusted slightly if necessary to optimize the upper end
of the plateau curve.
After the set-points for the discriminator and amplifier gain have been
established, the H.V. plateau should be determined. Set the amplifier and
discriminator to their set-points. Lower the H.V. by 300 volts from the
initial set-point, and measure the count-rate. Increase H.V. in 100 volt
steps, measuring the count-rate at each step. The count-rate will increase
rapidly at first, then increase more slowly. When the count-rate again in-
creases rapidly, discontinue increasing the H.V. Reduce the H.V. by 200 V.
Make a linear plot with the count-rate as the ordinate and the H.V. as the
abscissa. The region of the curve through the data which has the minimum
slope is the H.V. plateau. The slope of the H.V. plateau should be less
than 5% increase of count-rate per 100 V increase of H.V. The length of the
H.V. plateau should be at least 100 V. If necessary, intermediate points on
the plateau can be measured at 50 V intervals. When measurements are com-
pleted, turn off the H.V. and remove the flask. The useful settings for
H.V. are approximately in the center of the H.V. plateau but should not ex-
ceed the initial H.V. setting determined by oscilloscope testing, by more
than 50V. A voltage plateau for the instrument background should also be
measured using a new flask filled with aged air (9.5). A series of overnight
background counts should be taken, one at each of the previously determined
H.V. plateau set points. The optimum H.V. setting will have the minimum
background count for a voltage setting that is on the original plateau. If
necessary, the H.V. set-point can be at any point that is more than 50V from
the extremities of the H.V. plateau. The optimum settings and all sample,
-------
background, and calibration counts made at those settings must be recorded.
For nominally identical scintillation flasks, these settings may be assumed
to be the same from flask to flask. Otherwise, optimum settings for H.V.
should be determined for each kind of flask, but amplifier and discriminator
settings should not be readjusted.
9.3 Instrument Dead Time. Unless the instrument dead time for the scintilla-
tion flask counting system has been provided by the manufacturer, it must be
determined by. the operator. The dead time should be determined using one of
the methods given by ':he National Council on Radiation Protection and Measure-
ments (12.9). Zinc-sulfide scintillation counters may be assumed to have
non-extendable dead time effects, over the range of count-rates expected for
mine-vent monitoring. One or two alpha-particle emitting source(s) mounted
on zinc sulfide phosphor disc(s) (7.1.2) will be needed for this calibration.
The source-pulser method is recommended for its relative accuracy and sim-
plicity; otherwise, the two-source method is suggested.
Once the dead time has been established, the expression relating the true
count-rate to the observed count-rate is:
N = n/U-nr)
where,
N = the corrected count-rate,
n = the observed count-rate,
and, T = the dead time in the same time units as count-rates.
When T has been measured, this equation can be used to determine the corrected
count-rate for any given measured rate.
Using the expression: ;.
_ 0.01
the count-rate corresponding to a 1% correction (N/n * 1.01) can be determined.
For count-rates below that quantity, the dead time correction may be neglected.
9.4 Control Source Checks. The control source (7.1.2) is used to determine
that the counting efficiency of the counting system remains constant after
calibration. The count-rate for the control source is established during
calibrations. Daily counts of the control source are made when samples are
being analyzed to establish that the efficiency has not changed significantly.
After the H.V. operating point has been established for scintillation flasks,
the control source must be counted at that H.V. Before flask calibration
counts are taken, position the control source on the P.M. tube, close the
light tight counting chamber, wait five minutes, adjust H.V. to the set-
point and take a count of sufficient duration to accumulate at least 20,000
counts. Record the count. Turn off the H.V. Remove the source and then
replace it and repeat the counting process. Repeat removing, replacing and
counting the control source ten times. Remove the source, and measure the
background counting rate for the empty counting chamber using the same count-
ing interval. Compute the count-rate for all counts. Subtract the empty
chamber background count-rate from all control source count-rates. Then
compute the average count-rate for the control source and compute the rela-
tive standard deviation of the net count-rate. If the relative standard
deviation exceeds 1% of the average, the variability of control source counts
is excessive. In that case, it is necessary to determine whether the source
positioning is inconsistent or the counting system is malfunctioning. Once
the source of excess variation has been corrected and a set of 10 control
-------
source measurements with acceptable variability has been accumulated, pre-
pare a control chart with the average count-rate for that counting system
and limits of ±2% of the average count-rate shown on the verticle scale and
the dates of measurements shown on the horizontal scale.
Each day before samples are counted, make one measurement of the control
source and the empty-chamber background using the same time interval as pre-
viously. Plot the net count-rate on the control chart. If the net count-
rate is within ±2% of the previous average, the system is in control and may
be used for samples. If a control source count-rate falls outside the limits,
repeat the count one time. If it falls within limits, the instrument may be
used. If it does not, a counter malfunction is likely, and the cause of the
changed counting efficiency must be determined and corrected. If the first
count falls outside of limits on two consecutive days the cause must also be
determined and corrected.
9.5 Backgrounds. Prior to calibration or sampling, the background count-
rate for each flask must be determined using scintillation flasks filled
with compressed air that has been aged for at least 30 days to permit the
decay of any radon initially present.
9.5.1 Evacuation and Filling. For flasks with two" inlet valves, evacuation
is unnecessary, and flushing may be accomplished with the vacuum-pump and
gauge isolated from the system, using aged compressed air, through the filter
inlet. For flasks with a single inlet valve, the following procedure must
be followed. Attach the vacuum pump, gauge and filter assembly (Figure 4)
to the flask using flexible tubing. With the flask valve closed, evacuate
the system. Turn on the thermocouple gauge. While pumping, open the valve
on the scintillation flask and evacuate-to less than 1000 microns pressure.
Close the valve on scintillation flask. Close the vacuum-guage valve and
open the filter-assembly valve. Open the valve on scintillation flask and
allow it to fill with filtered ambient air. Close the filter assembly valve;
then open the vacuum-gauge valve and evacuate the flask to less than 1000
microns pressure. Repeat filling and flushing two more times. Evacuate the
flask to less than 1000 microns pressure and close the flask and vacuum-
guage valves. Attach the outlet of the compressed air regulator to the filter
inlet. Adjust the regulator pressure to 4 psi with regulator shut-off valve
closed. Open the niter assembly and vacuum-gauge valves and pump the filter
assembly to less than 1000 microns. Shut off the gauge valve. Open the
regulator shut-off valve and flask-valve and slowly open the flow control
valve of the regulator. Allow the flask to fill until flow rate drops. Close
the flask valve and regulator shut-off valve. Remove the flask from the
assembly and open the flask valve briefly to allow pressure to equilibrate
with ambient atmospheric pressure.
9.5.2 Background Count Measurement. Spray the exterior of the evacuated
flask with anti-static spray and store overnight. Place flask in the counting
chamber taking care to center the window of the flask on the photomultiplier
tube. Select one orientation for all flasks (e.g. valve handle facing forward)
so that the geometry is consistent. Close the chamber. Allow five minutes
for the phosphorescence of the scintillator to decay to insignificance and
adjust H.V. to the set-point and initiate a 30-minute background count. Record
the counts registered, the count interval, flask number, and counting date
and time. Compute background counts per minute and record. The background
count-rate for each flask may be written on the flask or a label attached to
the top of the flask. However, static charges may be induced when removing
gummed labels, so they should not be removed until after the sample has been
collected and counted.
20
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9.6 Calibration
Prior to any sampling run, calibrate the system using the following proce-
dures. A complete calibration should be performed before making field measure-
ments and at six month intervals. Send all scintillation flasks used for
field measurements (at least four flasks) to the Bureau of Mines to be filled
with a known quantity of 222Rn in air (1000 ± 200 pCi/1), (7.1.1.) When flasks
are received, examine them for damage in shipping, spray all exterior surfaces
with anti-static spray. Delay counting for at least three hours after spraying.
Place a scintillation flask on the P.M. tube in the counting chamber and
close the lid. After five minutes, adjust the H.V. to the set-point. Preset
the time to at least 30 minutes, and initiate the count. Record the time of
the count, flask identification code and length of the count. After completion
of the count, record the count registered on the sealer. Turn off the high
voltage, remove the flask and replace it with the next flask to be calibrated,
and repeat the process. After all flasks have been counted one time, repeat
the process a second time for all flasks. After 1-2 days, recount all flasks.
If the elevation at the measurement site is more than 1000 feet higher or
lower than the elevation in Denver, Colorado, the pressure within the scintilla-
tion flask will be significantly different from the pressure normally present
when flasks are filled during sampling. This can make a small difference in
the counting efficiency of the flasks. If the elevation is lower, filtered
ambient air may be allowed to bleed into each flask through a length of tubing.
(The radon concentration in the ambient air should be negligible compared to
that inside the flask.) Close flasks as soon as possible after filling to
minimize outward diffusion of the fl as fcv contents.
If the- altitude at the location of sampling is more than 1000 feet higher
than at Denver, CO., it will be necessary to bleed a portion of the air from
each flask to achieve pressure equilibrium. Bleed air from the flask to the
outdoors or a suitable fume hood through a 4-5 foot length of tubing. Close
the flask as soon as possible. The dilution of the flask contents will be
negligible and the net effect will be the removal of a fraction of the radon-
air mixture. The fraction removed can be determined from the barometric
pressures and temperatures at the time of filling and at the sampling site.
If a local weather station cannot provide the barometric pressure, it must
be measured with a barometer (5.11).
After bleeding or filling flasks, repeat the original counting procedure
again, taking three counts spaced by at least 1 to 2 days. After counts
have been completed, efficiency factors should be computed before evacuating
flasks.
9.6.1 Correction for loss of Flask Contents after Bleeding. The correction
factor for computing the fraction of the initial radon-air mixture remaining
in the flask is as follows:
. P2Ti
- --
where:
F = the fraction of initial air containing 222Rn remaining in the flask
P! = the barometric pressure of the fill air at the time of filling, mm
Hg,
?2 - the barometric pressure at the sampling site,. mm Hg,
TI = the absolute temperature of the fill air at the time of filling, °K,
and,
T2 = the absolute temperature at the location when bleeding air, °K.
21
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9.6.2 Efficiency Calculations. The following formula must be used to
compute flask efficiencies:
CCs-CB(ts/tB)](X) exp (XT)
C. """ ""Pi
where,
E = the flask efficiency, counts/min per pCi Z22Rn/iiter,
Cs = the observed standard count during the counting interval
CB = the observed background count prior to filling,
exp (X) is the XHL power of the base of natural logorithm
X = the radioactive decay constant for 222Rn (1.259 x 10-4 min-1),
T » the interval between the start of the count and the date/time of the
fill at calibration, minutes,
ts = the length of the standard count interval, in minutes,
tg ~ the length of the background count, in minutes,
F = the fractional loss of air for flasks bled at the sampling site, (F=1.0
unless flask pressure was reduced by bleeding before the count)
P = the calibration concentration of the 222Rn in the flask at the time of
fill calibration, pCi/liter,
and,
T = the dead time determined for flasks, minutes.
Tabulate counting efficiencies and respective counting dates and times.
For each flask compute the average and relative standard error of the average
for the original set of three counts and also the second set if flask air
pressure was increased or decreased. Use ordinary statistical formulas and
not errors based on counting statistics. The statistical formula to use is
as follows:
^ fxAc rv *2 1/2
1. = +(100) 1=1 1=1
x ~
where,
Eg - the relative standard error of the average efficiency, *,
n = the number of replicate counting measurements used to determine the
average efficiency, (3 in this case),
and,
Xi = the efficiency determined from the itn count, c/m per pCi/1.
The relative standard errors of each average must be less than ±3%. If
errors exceed 3%, determine if there is a decrease in efficiency with time
for each set of counts. Such decrease indicates that a flask is probably
leaking. Scattered efficiencies indicates that there has probably been a
problem with static charge buildup on flasks. In the latter case, an addi-
tional set of counts may be taken after again spraying with anti-static spray
and waiting at least three hours. If flask internal pressures have been
adjusted, this additional set should correspond to the set taken after adjust-
ment. A new overall average and relative standard error of the average may
be taken after discarding the efficiency of any single count which represents
an extreme value. This relative standard error must not exceed ±3.
99
-------
If the relative error of the efficiency for any flask exceeds 3%, the flask
should be evacuated, checked for leaks, and leaks repaired, and returned to
the Bureau of Mines for recalibration. The efficiency of a flask should be
constant under constant conditions of internal pressure. A variation of
measured efficiencies can be caused by changes in internal pressure, the
distribution of radon progeny within the flask or the presence of a barrier
such as liquid water on the scintillation surface. The average efficiency
for the set taken after any required pressure adjustment will normally differ
from the original average by less than 15%. If pressure was increased, the
efficiency should decrease, and if pressure was decreased, the efficiency
should increase. The ratio of the efficiency after pressure adjustment to
the original efficiency for flasks of the same nominal construction must be
consistent. Any flask with a ratio differing from the average ratio for
other flasks of the same construction by more than 5% must also be re-cali-
brated.
Average efficiency factors with less than 3% relative standard deviation
may be used in sample computations for the corresponding flask. If adjustment
of the internal pressure of flasks was necessary, the average efficiency for
each flask after the pressure adjustment must be used in sample computations.
After calibration counts are complete, evacuate and flush the flasks with
ambient air, exhausting their contents outdoors or to a fume hood.
Test all flasks for leaks (9.1).
9.7 Replication Errors (sm)
Replication error is the inherent error of a set replicate measurements
when a constant concentration of radon- is measured with several flasks under
conditions such that the inherent counting errors are negligible. Unless a
source of radon, known to have a fixed concentration, is available at the
laboratory, flasks should be returned to the calibration facility of the
Bureau of Mines after the original .calibration. At least four flasks must
be used in this test. When flasks have been filled and returned, the radon
concentrations should be measured and each corrected to a single reference
time using the original calibrations. Counting intervals must be long enough
to accumulate at least 20,000 counts for each flask so that the error from
counting statistics is negligible. Then the standard deviation of the repli-
cate concentration measurements should be calculated using the usual statisti-
cal formula (9.6.2). sm must be equal to or less than 7X, which is the
assumed maximum inherent error for this method.
10. Sampling and Analysis Procedure.
10.1 General Considerations. This section describes the use of apparatus
and protocol for sampling and measuring 222Rn concentrations. To establish
the average characteristic emission rate during a particular quarterly interval,
it is necessary to measure both the 222Rn concentrations and vent flows from
all vents of a mine. Because of the short-term variability of the emission
rate, grab-sampling measurements must be repeated on six occasions and averaged.
Duplicate grab samples must be taken on each occasion. The sampling schedule
must be adjusted to include emissions under typical operating conditions and
to exclude occasions when non-typical short-term variations are probable.
All six measurements must be completed in a calendar quarter of a year. Samples
must not be collected at the following times:
1. Within one hour of shift change or blasting.
2. During a period when mine ventilation is other than that typical during
active mining, (e.g. a shut-down or flow-reversal).
3. Within two hours after shut-down or reversal of vent fans.
4. During or within 24 hours after passage of a local weather disturbance
(storm) that produces an atypical barometric pressure. '
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Sampling times must be spaced so as to obtain a representative sampling of
the air concentrations present during normal operating conditions. One of
the six samples must be collected between 1:00 and 3:00 P.M. to include the
normal daily concentration maximum reported by Jackson (12.4). The remaining
samples should be collected at uniform intervals during working shifts. At
mines which have more than one exhaust vent, samples should be collected
from all exhaust vents before resampling any vent.
Since some existing mine vents are not equipped with in-line sampling ports,
two alternate sampling methods are given. For sampling from vents with exist-
ing sampling ports, the in-line sampling technique should be used.
10.2 Sample Collection. Use scintillation flasks that have been sprayed
with anti-static spray. Measure flask background count-rates before using.
For flasks with a single valve, evacuate before sampling. Flasks with two
valves need not be evacuated.
10.2.1 Sampling vent stacks with access sampling ports. A modification of
the in-stack grab sampling procedure specified in Method 3 of Appendix A,
40CFR Part 60 (12.8) must be used. When entrained water is present, fit the
water separator (5.1.3) to the outlet of the squeeze iulb (5.1.1). Then
attach the inlet of the in-line filter assembly (5.1.4) either directly to
the squeeze bulb outlet or to the outlet of the water separator. Insert the
probe into the access port and position as instructed in Method 3 of (12.8).
Squeeze the bulb several times to completely flush the sampling system. (At
least enough times to pump 10 times the normal volume of the system). Attach
a scintillation flask (5.7) to the outlet of the in-line filter using flexible
tubing. Open the inlet valve of an evacuated flask or both valves of dual
inlet flasks. Squeeze the bulb a sufficient number of times to pressurize
or flush the scintillation flask. Flushing should be continued uoiil at
least 10 flask volumes have been pumped through. Close the flask valve(s).
Remove the flask and attach a second flask. Repeat filling or flushing the
flask. After both samples have beetr collected, partially open one valve of
each for 1-2 seconds, then close, to allow the internal pressure to equilibrate
with ambient atmospheric pressure. Record the flask identification codes,
sampling site, and the date and time of sampling in a field notebook. The
vent flow must be measured immediately preceding or following sample collection.
Return flasks to the laboratory for counting. Avoid rubbing flask exterior
surfaces to minimize the possibility of inducing static charges.
10.2.2 Sampling Vent Stacks without Access Ports. Assemble vent outlet
sampling apparatus (5.2). When entrained water is present, fit the water
separator (5.1.4) to the outlet of the probe (5.1.2) with flexible tubing.
Then attach the inlet of the in-line filter assembly (5.1.4) either directly
to the probe outlet of the outlet of the water separator. Connect the flow-
meter (5.1.5) to the outlet of the in-line filter. Flexible tubing should
be attached to the outlet of the flow-meter to permit attaching the scintilla-
tion flasks. See Figure 3.
Before inserting the funnel end of the probe into the outlet of a vent,
observe the interior of the vent to determine the locations of moving parts
and obstacles. A suitable ladder may be needed. Cautiously insert the funnel
end of the probe into the air stream and into the vent. With the outlet of
the probe parallel to the air stream, the plane of the face of the probe
funnel inlet will be facing to the stream. The bend of the metal probe tube
will limit the depth that the probe ran be inserted into the vent. It is
necessary to insert the proba as deeply as possible without touching moving
parts. Do not insert deeper than tlie limit created by the bend in the metal
tubing. Air will be forced through the probe and attached equipment. Adjust
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the position of the probe to obtain the maximum possible flow, indicated by
the flow-meter. Any sampling flow-rate greater than 2 1/min will supply a
sample in a reasonably short time. Attach a scintillation flask inlet valve
to the tubing on the outlet of the flow-meter. Slowly open the inlet valve
of the scintillation flask. For evacuated flasks, allow to fill slowly so
that the fill rate does not exceed the rate of supply. For flasks with dual
inlet valves, open the second valve and allow the flask to flush for the
interval necessary to pass at least 10 flask volumes. When evacuated flasks
are filled, the flow indication will drop to zero. Close flask valve(s) and
remove from the hose. Attach a second scintillation flask and repeat the
sampling. After both samples have been collected, partially open the inlet
valve for 1-2 seconds, then close, to equilibrate the interior pressure with
the ambient atmospheric pressure. Record the flask identification codes,
sampling site and the date and time of sampling in a field notebook. The
vent flow must be measured immediately preceding or following sample collec-
tion. Return the flasks to the laboratory for counting. Avoid rubbing the
flask exterior surfaces to minimize the possibility of inducing static charges.
10.3 Counting Scintillation Flask. Wait at least three hours after filling
scintillation flasks to permit the ingrowth of the short-lived daughters of
222Rn. The counting system must be in control (9.3) before using. Place a
scintillation flask in the light tight chamber in the same orientation to
the P.M. tube that was used during calibration. Close the light tight chamber,
turn on H.V. and adjust to the set-point. Wait 5 minutes. Adjust the timer
for a 30-minute count interval. Initiate the count. Record the time that
the count started, the flask identification code, sampling site, and the
date and -time of sampling. When the count is completed, record the number
of counts. Turn-off the H.V. and, change samples. Repeat the counting-proce-
dure for all samples. Calculate the radon concentrations before pumping
samples from scintillation flasks. If a pair of duplicate measurements differs
by more than the maximum allowable error (e^x) Siven below, recount both
flasks and recalculate radon concentrations and allowable errors. Record
all results. Any pair of the measurements from two flasks not differing by
more than eraax can be averaged and the average used as the acceptable measure-
ment for a given sampling. If no pair of measurements are acceptable, the
sampling must be repeated. If there is a possibility that 220Rn -js present
in mine air recount flasks as indicated in Paragraph 3.4 and use the results
to determine an appropriate delay time between sampling and counting.
11. calculations. Carry out all calculations retaining one extra decimal
place beyond that present in the acquired data. Round off the numbers follow-
ing calculations, retaining three significant figures.
11.1 Allowable Error. The formula for the allowable error (e) between
replicate samples is as follows:
e = 200
1/2
100,
where,
e = the allowable error between duplicates (%)
Cs - the gross sample count including background,
ts = the sample count interval, minutes,
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CB = the background count interval, minutes
tB - the background count interval, minutes
and
sm = the measured inherent limiting precision (9.7) of scintillation flask
measurements, %.
The allowable error or each of the pair of flasks must be calculated. The
maximum allowable error (emax) 1s the larger of the two values of e.
11.2 Radon.Concentration. The formula for calculating the concentrations
of radon is as follows:
[exp(XT)]
(l-exp(-Ats}] E [l-Cs(T/ts)]
where,
P = the z"Rn concentration in pCi/liter,
X = the decay constant for 222Rn (=1.259 X 10 "4 min'1),
exp (X) = the X power of the base of the natural logarithm
T = the interval between sample collection and the start of the sample
count, minutes,
E = the efficiency factor for the scintillation flask, c/m per pCi/1,
and
T = the dead time for the counting system in minutes. (T may be assumed
to be zero for sample count-rates which give a dead time correction less
than 1% (9.3).
After calculating the average concentrations for pairs of sampling flasks
with acceptable allowable error, for each of six samplings, record all results
on a sample record sheet. (See Figure 7 for an example.)
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FIGURE 7. Example of Radon Count/Calculation Record'Table.
Record of Radon Concentration Measurements
Company_
Mine
Vent Identification
Sample 1.
Sampling Date/T1me_
Background Counts
Flask
Date/Time
Length of Count
Total Counts
Flask
Instrument
Date/Time Length of Count Total Count Radon Cone.
Allowable Error
Test A. Minimum % Range Between Duplicate Flasks
B. Maximum % Allowable Error
*. Test
_
concentration from Test "A" Flask Measurements
pd/llter. IF "A" > "BB". Resamplc.
_
£ If "B" > "A". Average
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12. References
12.1 Procedure 03454. 1984, Annual Book of ASTM Standards. Section 11,
Water and Environmental Technology. Volume 11.02-Water (II). American Society
for Testing and Materials, Philadelphia, PA 15103.
12.2 Fisenne, I. M., A. C. George, and H. W. Keller. Radon Measurement
Intercomparisons at EML. Environmental Measurements Laboratory, Department
of Energy, New York, NY. DOE/EML-397, October 1981.
12.3 Fisenne, I. M., A. C. Seorge, and H. W. Keller. The 1982 Radon Inter-
comparison Exercises at EML. Environmental Measurements Laboratory, Department
of Energy, New York, NY. EML-413, March 1983.
12.4 Jackson, P. 0., et al. An Investigation of Radon-222 Emissions from
Underground Uranium Mines. Pacific Northwest Laboratory, Department of Energy,
Rlchland, WA. NUREG/CR-1273, PNL-3262, RU. February 1980.
12.5 Beckman, R. T. Calibration Procedures for Radon and Radon-Daughter
Measurement Equipment. Denver Technical Support Center, Mining Enforcements
and Safety Administration, Department of Interior, Denver, Colorado. Informa-
tional Report MESA-IR-1005.
12.6 Lucas, H. F. Improved Low-Level Alpha Scintillation Counter for
Radon. Review Sci. Instrum., Volume 28, pg. 680, 1957.
12.7 George, A. C. Scintillation Flasks for the Determination of Low-
Level Concentrations of Radon. Proceedings of the Ninth Midyear Health Physics
Symposium, Denver, Colorado. February, 1976.
12.8 Part 60, Appendix A-Reference Methods. Code of Federal Regulations-
Title 40, Protection of Environment. Parts 53 to 80. Revised as of July 1,
-1982. Available from Superintendent offDocuments, U.S. Government Printing
Office, Washington, D.C. 20402.
12.9 Dead-Time Corrections. Handbook of Radioactivity Measurement Procedures.
Recommendations of the National Council on Radiation Protection and Measurements,
7910 Woodmont Avenue, Washington, D.C. 20014, pp. 54-66, November, 1978.
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METHOD 2 - VENT FLOW MEASUREMENTS
1. Principle and Applicability.
1.1 Principle. To determine the rate of 222Rn emissions from underground
uranium mines, both the average radon concentration in the air and the volu-
metric flow rate must be measured. The volumetric flow rate measurements
must be taken at the above ground portion of the mine exhaust vent using the
reference technique found in Method 1 of Appendix A, 4QCFR Part 60, (See
Method 1, 12.8 for reference). A traverse of air velocity measurements
must be taken in the accessible portion of the vent which best conforms to
the requirements of that method.
1.2 Applicability. Method 1 (12.8) specifies that it can not be used
when (1) flow is cyclonic or swirling. (2) a stack is smaller than about
0.3 m (12 in.) in diameter or 0.071 m2 (113 in.2) in cross-sectional area or
(3) the measurement site is less than two stack or duct diameter downstream
or less than a half diameter upstream of a flow disturbance. However, measure-
ment points at existing mine vents rarely conform to item (3) and occasionally
fail to conform to items (1) and (2). At some vents-no in-line access ports
have been provided for sampling and velocity measuring traverses. At some
mines, air is exhausted from large entry portals where no stack is present.
Procedures for a detailed assessment of the applicability of this method are
given in the EPA Quality Assurance Handbook for Air Pollution Measurement
Systems-Volume 3. Stationary Source Specific Methods, EPA-600/4-77~027b
August, 1977. This volume also gives more detail about measurement and sampl-
. ing considerations than is present in ('|2.8). If it is determined that the
reference method is not applicable to a'measurement site, suitable modifica-
tions to the sampling site must be made or the administrator must be consulted
about alternative measurement methods, such as the anemometer technique given
in Method 14 of (12.8).
2. Computations. For flow measurements from mine vents, the computation
of average stack gas velocity conforms to Paragraph 5.2 of Method 1 (See
Method 1, 12.8 for reference). However, 2"Rn concentration measurements
are made after equilibrating the internal pressure of the sampled air in
scintillation flasks with the local barometric pressure. The normal computation
for stack gas volumetric flow given in Paragraph 5.3 of the reference method
converts the velocity to the equivalent volume of dry air at standard barometric
pressure and temperature. For the purpose of this method, it will not be
necessary to make this conversion. The applicable formula is as follows:
3.6X106 VCAPC
Qe = p —
pbar
where,
Qe - volumetric ambient stack gas flow rate corrected to local barometric
pressure, liter/hour),
3.6 X 10° = conversion factor, (sec/hr)(liters/m3),
vs s Average stack gas velocity, m/sec,
A = Cross sectional area of stack, m2,
Ps * Absolute stack gas pressure, mm Hg,
(Ps = Pbar + Pg)
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Pbar = barometric pressure at measurement site, mm Hg,
and
Pg = stack static pressure, mm Hg.
2.1 Summary Table. A summary table must be prepared listing the mine,
vent number, and flow rates calculated for each of the six sampling intervals.
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METHOD 3—COMPUTATION OF RADON EMISSION RATES
1. Principles and Limitations
1.1 Principals. Radon emission rates must be computed for each mine vent
from the average radon concentration measured in Method 1 and the flow rate
measured in Method 2. An emission rate must be computed for each of six
sampling occasions. Then the six emission rates must be average to give the
average rate for that vent during one quarter of a year. The annual average
emission rate is the average of four quarterly averages.
1.2 Limitations. Limitations of the measurement processes are discussed
for methods 1 and 2. The computational process produces average emission
rates relative to the barometric pressure at the sampling site. Thus, the
emission rates are useful for predicting local concentrations of radon as a
result of atmospheric dilution. (If exposure rate estimates at distant sites
are required, radon concentrations and stack flow rate determinations would
have to be corrected to the equivalent dry air at standard temperatures and
pressures.)
2. Calculations. Carry out calculations retaining one extra decimal figure
beyond that present in the required data or previously calculated values.
Round-off results to three significant figures.
2.1 Summary Table
Prepare a summary table listing the radon concentration and flow rate for
each of the six sampling occasions at each vent of the mine. An example of
such a table is shown in Figure 8. Reserve the final column for the results
of emission rate calculations.
Operator:
Figure 8. Sample Summary Table
Summary Table
Mine Identification:
Quarterly Interval; 4/84 - 7/84
Vent
1
1
1
1
1
1
2
2
Samp! ing
Date/Time
6-3-84/08:40
6-4-84/11:25
6-3-84/14:25
6-4-84/16:15
6-3-84/18:30
6-3-84/21:10
6-4-84/09:20
6-3-84/11:35
Vent/Flow fl/hr)
8.49 X 107
8.62 X 107
8.52 X 107
8.47 X 107
8.53 X 107
8.41 X 107
5.13 X 107
5.21 X 107
222f,oncentration
fpCi/1)
528
541
823
613
548
534
1560
1610
Emission
rate fl/hr)
2.2 Emission Rate Calculation. The emission rate for any sampling of a
mine vent should be calculated using the following equation:
Evt • 10~12
where:
Evt = the hourly emission rate of 222Rri from vent v at sampling time t,
Ci/hr
Cyt = the measured average 222Rn concentration in air from vent v at
sampling time t, pCi/liter
-------
Fvt = the measured average volumetric stack gas/low rate for vent v at
sampling time t, liter/hr.
After calculation, emission rates should be placed in the summary table.
2.3 Calculation of quarterly average vent emission rate. The quarterly
average emission rate for each mine vent is the arithmetic mean of the six
measurements taken at that vent (all taken within a 30 day interval).
Ey » 1/6 I Evt
WnfilTS* *.— 1
Fv = the quarterly average emission rate of vent V, li/hr,
*vt = the emission rate of vent V at the ttn sampling, Ci/hr.
2.4 Calculation of quarterly average mine emission rate. The total emission
rate for a mine is the sum of the average emission rates determined for all
exhaust vents for that quarter. The emission rate is computed as follows:
J E
Ev
where: v=l ««_
!ma = tne quarterly average *"Rn emission rate for mine m, with n vents,
Ci/hr.
2.4 Calculation of annual average mine emission rate. The annual average
emission rate for a mine is the arithmetic mean of four quarterly average
emission rate measurements taken at intervals of about three calendar months
The formula for calculating the annual average is as follows:
Ema = 1/4 I Emq
q=l
where: ,,.,,
Ema * the annual average "^Rn emission rate for mine m, C1/hr.
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