SWRHL-91
.GAS ANALYSIS CAPABILITIES OF THE
SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY
by
Frederick B. Johns and Richard E. Jaquish
Technical Services
Southwestern Radiological Health Laboratory
U. S. Department of Health, Education, and Welfare
Public Health Service
Environmental Health Service
April 1970
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SWRHL-91
GAS ANALYSIS CAPABILITIES OF THE
SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY
by
Frederick B. Johns and Richard E. Jaquish
Technical Services
Southwestern Radiological Health Laboratory
U. S. Department of Health, Education, and Welfare
Public Health Service
Environmental Health Service
Environmental Control Administration
Bureau of Radiological Health
April 1970
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ABSTRACT
The Southwestern Radiological Health Laboratory
collects and analyzes radionuclides which occur
in the gaseous state in the environment. A variety
of samplers are used depending upon the type of gas
being sampled. Most of the sampling is from the
atmosphere, but natural gas samples are also collected
and analyzed. The mixtures of gases are separated
in the laboratory using low temperature adsorption and
gas chromatograph techniques. After separation, the
quantity of radioactivity is determined by counting.
The gases routinely sampled and analyzed are xenon,
krypton, water vapor, carbon dioxide and radon.
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TABLE OF CONTENTS
Page
ABSTRACT i
LIST OF FIGURES iii
LIST OF TABLES iv
INTRODUCTION 1
SAMPLING EQUIPMENT 1
A. Freezing Traps 1
B. Molecular Sieve Sampler 3
C. Aerial Grab Sampler 5
D. Cryogenic Sampler 5
E. Radon Sampler 6
F. Natural Gas Sampler 7
SEPARATIONS 8
A. Water and Carbon Dioxide 8
B. Xenon and Krypton 9
C. Radon 11
COUNTING 12
REFERENCE 16
BIBLIOGRAPHY 16
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LIST OF FIGURES
Figure Page
1. Freezing Trap in Field Installation 3
2. Small Molecular Sieve as Currently Used 4
3. Large Molecular Sieve Configured for Aerial Sampling 4
4. Large Molecular Sieve Configured for Ground Sampling 4
5. Disassembled Cryogenic Sampler 5
6. Cryogenic Sampler Installed in Aircraft 6
7. Pump Used for Integrated Radon Sampling 7
8. Combustion and Collection Apparatus for Natural 8
Gas Analysis
9. Schematic, Natural Gas Separation Apparatus 10
10. Gas Envelope Geiger-Mueller Tube 14
11. Radon Scintillation Cell 15
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LIST OF TABLES
Table Page
I. Radiological Properties of Gases 2
II. Mirimum Detectable Concentrations 13
IV
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INTRODUCTION
The Southwestern Radiological Health Laboratory has the capability of
collecting and analyzing samples for radionuclides which occur in the
gaseous state in the environment. Radionuclides which are found in
the environment and are of significant health importance include
tritium (3H), carbon-14 (^C), radon-222 (222Rn), and the isotopes of
xenon, krypton and argon. A list of the nuclides of interest and
their properties is found in Table 1.
Sampling for radioactive gases is usually from the atmosphere but with
recent Plowshare projects to stimulate natural gas fields with nuclear
explosives, the analysis of radioactive gases in natural gas is of
interest.
SAMPLING EQUIPMENT
Air sampling and analysis procedures are very much interrelated; the
purpose of sampling is to collect only the gases of interest, thereby
increasing the sensitivity of the method. Based on this criteria,
several sampling devices have evolved.
A. Freezing Traps - water for tritium analysis
This device consists of two glass traps immersed in dry-ice acetone
(DIA) slush. Air is passed through at a flow rate of three to six
liters per minute. To check on collection efficiency,data are
collected at the same time on the humidity in the ambient air.
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TABLE I
RADIOLOGICAL PROPERTIES OF GASES
NUCLIDE
Tritium 3H
Carbon 14C
Argon 37Ar
39Ar
Krypton 85Kr
87Kr
88Kr
Xenon 127Xe
BOILING
POINT °C HALF-LIFE
0 12.262a
-78.6 5730a
(as carbon dioxide)
-185.7 35. Id
269a
1.83h
-152 10.76a
4.4h
76m
2.80h
-107 36. 4d
ENERGY (MeV)
TYPE OF DECAY AND ABUNDANCE
3- 0.0186 MAX
6- 0.156 MAX
EC X-rays
3- 0.565 MAX
B- 2.49 MAX
Y 1.293 (99%)
3- 0.67 MAX
Y 0.514(0/41%)
3- (77%) 0.82 MAX
Y 0.150 (74%)
0.305 (13%)
3 3.8 MAX
Y 0.403 (84%)
0.85 (16%)
2.57 (35%)
3- 2.8 MAX
Y 0.166 (7%)
0.191 (35%)
0.56 (5%)
0.85 (23%)
1.55 (14%)
2.19 (18%)
2.4 (35%)
EC, Y X-rays, 0.058 (1.4%)
0.145 (4.2%)
0.172 (22%)
0.203 (65%)
133X£
135Xe
Radon 222Rn
-61.8
5.27d
9.14h
3.8229d
2
0.375 (20%)
0.346 MAX
0.081 (37%)
3-
Y
0.92
0.250
0.61
5.49
MAX
(91%)
(3%)
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>^*-
Figure 1 -
Freezing trap in
field installation
B. Molecular Sieve Sampler - water for 3H and carbon dioxide
for 14C analysis
Molecular sieves have the unique property of selectively absorb-
ing atoms or molecules based on their diameter. This feature is
used in the molecular sieve samplers. Two sampling devices are
used. A large sampler containing 2000 grams of 13x molecular
sieve operating at a flow rate of 120 liters is used in an air-
craft or in semi-permanent installations. A small portable
sampler is also used which contains 200 grams of molecular sieve
and operates at a flow rate of 15 liters per minute.
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Figure 2 -
Small molecular sieve
sampler as currently
used.
Figure 4 -
Large molecular sieve con-
figured for ground sampling.
Figure 3 -
Large molecular sieve sampler
configured for aerial sampling
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C. Aerial Grab Sampler - xenon and krypton analysis
This sampling device is installed in the sampling aircraft. Air
is pumped for thirty seconds from outside the airplane into a
30-liter plastic bag. The air in the bag is then pumped by means
of a compressor into a steel gas bottle that has been previously
evacuated. The volume of air is determined by weighing the evacu-
ated bottle before and after it has been filled. Approximately
0.3m3 of air is collected as a sample for analysis for xenon and
krypton.
D. Cryogenic Sampler - xenon and krypton analysis
This device utilizes a cannister of molecular sieve 5A immersed
in liquid nitrogen at 12 psig (giving a temperature of -182°C) to
Figure 5 -
Disassembled cryogenic sampler
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absorb xenon and krypton, although water vapor and carbon dioxide
are also quantitatively collected. Air is pumped through the
sampler and a dry gas meter measures the sample volume. The initial
flow rate of 1250 liters per minute gradually decreases due to the
freezing out of the water and carbon dioxide to a flow of 70 liters
per minute. Average collection is 1.5 to 4m3.
t *
Figure 6 -
Cryogenic sampler
installed in aircraft.
E. Radon Sampler - radon-222 analysis
Two types of sampler are used to collect radon gas in air:
1. An integrated sampler consists of a small pump, Figure 7,
which fills a plastic bag with a volume of 30 liters. The
sample, collected over a 24-hour period, includes all gases
in air although radon is the nuclide of interest.
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2. A grab sample for radon consists of a 2-liter gas bottle
through which air is pumped until seven or eight volume
changes of air in the bottle occur. This collection is made
at atmospheric pressure for five to ten minutes.
Figure 7 -
Pump used for
integrated radon
sampling.
F- Natural Gas
Natural gas is collected in 8-liter steel gas bottles which are
evacuated to a pressure of lO'^mm of mercury prior to use. In
the field, the bottle is attached to the high pressure gas line.
The valve is opened to allow gas to enter the bottle. The
pressure is not allowed to exceed 1000 psi. The bottle is emptied
and refilled eight times, then, under pressure, is returned to the
laboratory. Before analysis, a measured volume of the natural gas
sample is ignited with excess oxygen, Figure 8. Water vapor is
collected and the remaining products of combustion and noble gases
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are introduced into the gas analysis apparatus.
Figure 8 - Combustion and collection apparatus for natural gas analysis
i
SEPARATIONS
A. Water and Carbon Dioxide
The collected gases from the molecular sieve, cryogenic and aerial
grab samplers and products of combustion from natural gas are
introduced into the separation apparatus, Figure 9, by a combination
of heat and helium purge. In all cases, the water vapor is collected
in the steel ball trap (Tj at liquid nitrogen (LN) temperature. A
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portion qf the carbon dioxide, xenon and krypton is also collected
in the stjeel ball trap and the remainder on the charcoal in Cj at LN
tempera ti^re. When only tritium and carbon-14 analyses are to be
conducted!, T: is heated to approximately 30°C to melt the water and
vaporize the carbon dioxide. The water is refrozen on the steel
balls at -80°C with DIA and the carbon dioxide is transferred to C2.
This step is repeated until all of the carbon dioxide is transferred.
The water is distilled into a receiver at MI. Oxygen and nitrogen
are removed from GI by purging with helium at DIA temperature. The
carbon dioxide is desorbed from Ci at 350°C and transferred to T2 of
LN temperature with a helium carrier gas. After removing the helium
in T2 (by vacuum), the trap is allowed to come to room temperature and
the carbon dioxide transferred to Section I for volume measurement and
solution in Hydroxide of Hyamine (10 ) (a product of Rohm & Haas
* A
Chemical Corporation). The water, after distillation, and the carbon
dioxide are prepared for liquid scintillation counting as described in
Part IV below.
B. Xenon and Krypton
When analysis for xenon and krypton is to be performed, known volumes
of xenon and krypton carrier are added to Tj. and C^ TI is then heated
to 30°C and refrozen with DIA and the xenon, krypton and carbon dioxide
transferred to Ci. The water is distilled to W^ After the oxygen and
nitrogen on the charcoal in Ci are removed by purging with helium at
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Figure 9 - Schematic, Natural Gas Separation Apparatus
.^lomzotion Chamber & Thermal Conductivity
yRough Vacuum
C, M, C2
Wj - Water Trap
M.
T! - Steel Ball Trap
Ci - Charcoal
M! - Molecular Sieve (5A)
T2 - Steel Ball Trap
C2 - Charcoal Trap
T3 - Empty Trap
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DIA temperature, heat is applied to Ci and the gases of interest are
transferred to the molecular sieve column (l^) which is at LN tempera-
ture. The temperature of the molecular sieve column is raised to wet
ice temperature (0°C) which will elute the krypton more rapidly than
the xenon. The thermal conductivity cell and the ionization chamber
indicate the elution of the various gases. After krypton has passed
through the column to C3, the temperature of the column is raised to
boiling water temperature (100°C) to speed the removal of the xenon.
The xenon is transferred to T3 and the temperature is raised to 350°C
to remove the carbon dioxide to T2. The xenon is then transferred to
Section I by heating T3. The recovered volume is determined for yield
determination and a portion of the xenon is then transferred to a gas
envelope Geiger-Mueller tube for counting.
The krypton is transferred to Section I in the same manner prescribed
for the xenon. The volume is determined and the gas transferred to a
counting cell.
The carbon dioxide is transferred to Section I, the volume determined,
and a portion dissolved in Hydroxide of Hyamine (10X) for liquid
scintillation counting.
C. Radon
When radon-222 analysis is required from the cryogenic sample, a portion
of the xenon is transferred to an alpha scintillation counting cell for
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radon-222 determination. The radon remains with the xenon fraction.
Radon in natural gas is determined by direct transfer of the natural
gas to an alpha scintillation counting cell. The cell is then alpha
counted. Radon in air is determined by adsorbing the radon in a
known volume of air on charcoal and then desorbing with helium into an
alpha scintillation counting cell.
COUNTING
Tritium is determined by liquid scintillation counting. A measured
volume of sample water is transferred to a liquid scintillation count-
ing cell containing the scintillator cocktail. (1) Since gamma-emitting
nuclides interfere with the tritium counting, all water samples are
gamma scanned and, when gamma emitters are present, the water is re-
distilled with holdback carriers. All samples are counted on the
liquid scintillation spectrometer for a minimum of 100 minutes (See
Table II). Carbon-14 is also determined by liquid scintillation count-
ing. The carbon dioxide is dissolved in 5 ml of Hydroxide of Hyamine
(10X) which will dissolve 4.5 millimoles of gas. This solution is then
added to a liquid scintillation cocktail and counted in the liquid
scintillation spectrometer.
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TABLE II
MINIMUM DETECTABLE CONCENTRATION (MDC)
Tritium
Carbon-14
Xenon and
Krypton
Radon
AIR
0.4 pCi/ml H20 collected3
0.02 pCi/ml C02 collected
100 pCi/total sample
0.04 pCi/liter
NATURAL GAS
1 pCi/liter
20 pCi/ liter
100 pCi/total
sample
0.04 pCi/liter
a = based on 5 ml sample counted for 100 minutes
The radioxenon and -krypton are presently counted in a gas envelope
Geiger-Mueller tube, Figure 10, to determine the total beta activity.
A portion of the sample is also gamma scanned for isotopic identification
and relative concentration.
Radon-222 is determined by alpha counting the radon gas in a scintilla-
tion counting cell, Figure 11, the interior surface of which is coated
with ZnS. The counting cell containing 125 ml of gas is placed on a
photomultiplier tube for scintillation counting. The counting time is
usually 60 minutes.
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G. M. TUBE
GAS ENVELOPE
Figure 10. Gas Envelope Geiger-Mueller Tube
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67mm
90mm
Corning No. 2
or Equivalent
Brass Collar
Kovar Metal
Clear Silica
Window
50 mm
Figure 11. Radon Scintillation Cell
15
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REFERENCE
A. A. Moghissi, H. L. Kelley, J. E. Regnier, M. W. Carter.
Low-Level Counting by Liquid Scintillation - I. Tritium
Measurement in Homogeneous Systems. International Journal of
Applied Radiation and Isotopes, Vol. 20, pp 145-156. Pergamon
Press. (1969)
BIBLIOGRAPHY
Breck, D. W. and Smith, J. V. Molecular Sieves. Scientific
American. (January 1959)
Linde Molecular Sieves Non-hydrocarbon Materials Data Sheets.
Union Carbide Corporation.
Momyer, F. F. The Radiochemistry of the Rare Gases. National
Academy of Sciences, Nuclear Science Series.
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