1267.pdf

ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to the following people; Dr. Crawford-Brown and Dr. Willhoit for being readers on my Technical Report committee; my parents, Lynette and John for their lifetime support; and last but not

least, my advisor. Dr. Jim Watson for the academic guidance, patience, and friendship shown me over the past few years.

I. INTRODUCTION. ...4

Overview...4

Purpose and Obj ective...8

II LITERATURE REVIEW... ...10

Performance Characteristics of Charcoal Adsorption Collectors...10

III MATERIALS... . . . ...19

Radon chamber...19

Alpha scintillation Cells...22

Alpha scintillation cell counting system...24

Pylon AB-5 Monitor...26

Charcoal canisters...26

Charcoal canister counting system...28

IV PROCEDURES...31

Alpha scintillation cell sampling and counting equipment-procedures for use...31

Calibration of Pylon AB-5 monitors...33

Programming the Pylon AB-5 Monitors...33

Test of Pylon 1 and 2 Efficiencies with standard Radium/Radon source...34

Pylon calibration factors using alpha cell count rates...35

Canister Performance Testing-Adsorption...36

Canister Performance Testing-Desorption...39

V. RESULTS...45

Canister Adsorption Testing Results...45

Canister Desorption Testing Results...51

Canister with Desiccant Testing Results-Adsorption...56

canister with Desiccant Testing Results-Desorption...58

VI. CONCLUSIONS AND RECOMMENDATIONS...62

REFERENCES...66

APPENDIX I PYLON EFFICIENCY TESTING DATA...67

APPENDIX II CHARCOAL CANISTER ADSORPTION DATA...68

APPENDIX III CHARCOAL CANISTER DESORPTION DATA...7 0 APPENDIX IV CHARCOAL CANISTER WITH DESICCANT ADSORPTION DATA...7 2 APPENDIX V CHARCOAL CANISTER WITH DESICCANT DESORPTION DATA...74

APPENDIX VI DESICCANT AND CHARCOAL WATER ADSORPTION DATA...76 APPENDIX VII ADSORPTION AND DESORPTION CURVES CORRECTED

LIST OF FIGURES

FIGURE 1 RADIUM DECAY CHAIN... 5

2 ANNUAL EFFECTIVE DOSE EQUIVALENT TO US POP.... 7

3 RADON-222 EXPOSURE CHAMBER...20

4 RADON-222 SOURCE...21

5 ALPHA SCINTILLATION CELL...23

6 ALPHA SCINTILLATION CELL COUNTING SYSTEM...25

7 PYLON AB-5 MONITOR...27

8 CHARCOAL CANISTER COUNTING SYSTEM...30

9 ADSORPTION CURVES...46

10 ADSORPTION CURVES: 1" CC...47

11 ADSORPTION CURVES: 1.5" CC...48

12 DESORPTION CURVES : OF & SS...52

13 DESORPTION CURVES: 1" CC...53

14 DESORPTION CURVES: 1.5"...55

15 DESICCANT ADSORPTION CURVES...57

INTRODUCTION OVERVIEW

Radon is a naturally occurring, radioactive, inert gas that is one of the nuclides emitted in the uranium-238 {238U)

radioactive decay series. Upon its decay, radon produces

additional radioactive elements which are referred to as radon

progeny. Present throughout the environment, radon-222 (222Rn)

accounts for 55% of the total radiation dose to the United States

population. Concern has grown over the effects of 222Rn

exposures because of the increased lung cancer potential from breathing radon contaminated air in enclosed living and working

spaces.

Radon gas emanates from soils containing uranium deposits. If the soil under a house, or surrounding the basement of a house, contains uranium deposits, the escaping radon gas may become trapped in houses whose foundations are set in or on the

soil. The concentration of radon in a building depends on the

presence of uranium in the soil at that location, the ability of

the radon to seep through the soil, and the entry routes of radon

into the house from the soil.

Radon-222 is emitted from the natural decay of radium-226

(226 Ra) which is part of the 238U decay series. The emission of

radon is represented by the decay chain shown in Figure 1

beginning with 226Ra. Radon-222 and its progeny, polonium-218

RADIUM DECAY CHAIN

226Ra---> 222Rn---> 218po---> Zli^^---.

* 4.78 95% 5.49 100% 6.0 100% 0.65 50% 4.60 5% 0.71 40% 0.98 6%

.> 214Bi---> 214po--->210pb

19.7 m 164 us 21 y

1.51 40% 7.69 100%

1.0 23% 3.26 19<?.

Alpha particles have high linear energy transfer, which results

in a large deposit of energy in a small volume. Alpha particles can give a considerable dose to biological tissues. The lungs absorb most of the dose form alpha particles emitted by inhaled radon progeny. Radon concentration is usually expressed as the activity (picocurie) per unit volume, typically a liter (1). One pCl/1 corresponds to 0.037 222Rn atom disintegrations per second

within one liter of air.

The National Council on Radiation Protection and

Measurements estimates the average annual effective dose

equivalent to United States citizens from 222Rn to be 200 mrem of the 360 mrem total annual effective dose equivalent from all

sources as shown in Figure 2 (NCRP 1988). The dose received from radon is the largest percentage of the dose received from

naturally occurring radioactivity, and is approximately three times that of the dose received from man-made sources (NCRP

1987b).

The Environmental Protection Agency (EPA) states that,

second to smoking, the inhalation of radon gas is the leading

cause of cancer in the United States (EPA 1986). The EPA

estimates that between one and five persons out of 100 exposed to

4 pCi/1 of radon for their lifetime will die from 222Rn induced

lung cancer. As the radon concentration increases, the

associated lung cancer risk increases. For this reason, the

radon concentration in an enclosed living or working area should

Figure 2: Annual Effective Dose Equivalent in U.S. Populations (1980-1982) as a

Percentage of 360 mrem

Internal 10%

CD I

Terrestrial 8%

Medical X-ray 11%

•Other <1%

Occupational...0.3%

Faltout...<0.3%

Nuclear Fuel Cycle...0.1%

Cosmic

Radon 55%

Nuclear Medicine 4%

Consumer Products

^ 1%

The determination of the radon concentration in private homes

has become a national concern. Because most individuals

spend at least one third of each day at home, the inhalation of

radon contaminated air in the home can increase lung cancer risk.

To assess the radon concentration of houses in the United States, an inexpensive, durable, fast, efficient, and reliable method of

testing must be available. The charcoal adsorption canister

satisfies these criteria, and several types of charcoal

adsorption canisters are being used to make screening

measurements in houses today. However, because charcoal canister adsorption can be affected by humidity and temperature

fluctuations, diffusion barriers have been added to the canisters

to enhance the performance characteristics, and to extend the

measurement period.

PURPOSE AND OBJECTIVE

The purpose of this thesis was to compare the performance

characteristics of different types of radon diffusion barrier

canisters. The adsorption and desorption characteristics of fourteen different types of diffusion barrier canisters were

compared to each other and to the adsorption and desorption

characteristics of an open face canister. In a separate

experiment, desiccant packs were added to five different

diffusion barrier canisters, and the radon adsorption and

desorption characteristics of the canister types were compared to

each other and to the adsorption and desorption characteristics

tests were conducted in the UNC radon chamber under controlled

conditions.

LITERATURE REVIEW

PERFORMANCE CHARACTERISTICS OF CHARCOAL ADSORPTION COLLECTORS Adsorption of a gas on granular activated carbon is the

process where the individual molecules of the gas adhere to the

carbon grain surface. Atoms of the radioactive gas 222Rn are bound to the granular activated carbon surface by weak Van der

Waals forces (Cohen 1988). The adsorption of gaseous 222Rn on the

activated charcoal (Q) depends on the airborne radon

concentration (C), the adsorption coefficient of the gas (k), and

the mass of the activated charcoal (m), (Cohen 1983). The activity of the gas adsorbed per unit mass on the granular activated charcoal is the parameter that will be used in this study. This parameter is represented by the following equation:

Q (pCi) / m (g) = k (1/g) C (pCi/l). (Cohen 1983) The physical ability of the radioactive gas to adsorb on granular carbon depends on several factors. The carbon grain size, shape, and pore distribution, as well as environmental

temperature and humidity influence the overall gas adsorption on

the surface of the carbon grain (Thomas, 1991). The pores in the

granular activated carbon maximize the surface area presented by

the small size and cubical shape of the carbon granules, which

makes granular carbon an ideal adsorbing medium for radon gas. When activated and placed in a suitable container, granular

activated charcoal is used for measuring radon concentrations in

indoor air.

Radon adsorption decreases as temperature increases (Cohen

1988). As the temperature of a gas rises, the kinetic energy of

the gas molecules increases. The increased kinetic energy of the

radon molecules exceeds the weak binding energy of the Van der Waals forces that bind the radon molecules to the GAC (Cohen

1988), therefore preventing stable radon adsorption on the GAC.

In practice, most laboratories recycle the exposed radon canisters by baking the GAC in an oven at 100 C (Cohen 1986), which takes advantage of the fact that radon desorption from GAC

increases as temperature rises.

Granular activated charcoal is an excellent adsorber for water vapor as well as radon. As the humidity in a testing

location increases, the quantity of radon adsorbed is reduced because the water vapor competes for adsorption sites on the GAC

(Cohen and Nason 1986). Typically, for a given carbon mass at a

constant temperature, twice as much radon can be adsorbed at 25% humidity as at 80% humidity (Cohen 1988). if the water molecules block the normally available binding sites on the GAC, the lower quantity of radon adsorbed will incorrectly indicate a lower

radon concentration in the room.

A radon sampling device used for indoor radon sampling should be durable, inexpensive, simple to operate, easily

analyzed, and mailable to and from the test site (Cohen 1988). The charcoal canister satisfies each of these criteria. The

charcoal is contained in a metal "ointment tin" which can be sealed air tight until the canister is exposed in the sample location, and resealed once the sampling is complete. The

granular activated charcoal is relatively inexpensive, and

recyclable, as is the outside container. As long as an operator follows a few instructions and reseals the canister correctly, the quantity of radon adsorbed during the exposure period can be correctly determined by gamma ray scintillation counting the canister.

The charcoal canister has been used as an adsorption

collector for measuring the radon concentrations in indoor air for more than ten years. First introduced by George in 1981, the radon adsorption canister has undergone relatively few design modifications. The depth of the charcoal bed in the canister as well as the effects of the addition of diffusion barriers and

desiccant packs to the canisters have been studied. When added to the canisters, diffusion barriers and desiccants enhance the performance characteristics of the basic charcoal canister.

The adsorption of radon in a charcoal bed is a function of

charcoal bed depth and exposure time (Cohen 1983). The charcoal canister contains a bed of granular activated charcoal which adsorbs a sample of radon from the air when the canister is

properly exposed at a specific site for a predetermined amount of time. Usually, the diameter of the granular activated charcoal bed (or canister) does not exceed 7 cm because the diameter of most gamma ray detectors does not exceed 7 cm (Cohen 86). The quantity of radon adsorbed on a charcoal bed does not increase as the bed depth is increased beyond 1.5 cm (Cohen 1983).

The diffusion of the radon into the canister is negated by the radioactive decay and diffusion out of the charcoal bed when

the bed depth exceeds 1.5 cm. The diffusion front of the gas is reflected from the container bottom back to the top of the

charcoal bed (Cohen 1983). Deeper charcoal beds also hinder the

performance of the gamma ray counting device by placing the radon

decay products farther from the counting device and absorbing the

liberated gamma rays before they reach the Nal crystal of the

counting device (Cohen 1986).

Open face canisters and diffusion barrier canisters are the two types of charcoal adsorbers that are currently used to

measure radon. The open face canister exposes the granular activated charcoal directly to the testing atmosphere once the canister is opened. The diffusion barrier canister uses a porous material to protect the charcoal from direct contact with the testing space. The radon gas must first diffuse through the

porous material before it can be adsorbed on the surface of the

charcoal in the diffusion barrier canisters.

The open face charcoal adsorber is typically used for radon

testing with two day exposure periods (George and Weber 1990).

The advantage of the open face canister is that it can adsorb large amounts of radon quickly and efficiently. The principle disadvantage of the open face canister is that its radon

adsorption is not indicative of the average concentration when

there are large fluctuations in room radon concentration (Cohen 1988). The radon level in a room is subject to diurnal cycles, in which the radon concentration in the morning is higher than the concentration in the evening (Cohen 1988). Because radon

desorbs from the GAC while it is adsorbing, the open face

canister measurements will represent the radon concentration in

the room during the last few hours of exposure more than that

concentration present during the first few hours of exposure

(Cohen 1988). If the room experiences radon fluctuations

diurnally, the canister result will depend on the time of day the

operator sealed the canister.

Other disadvantages include the succeptability of the GAC in

the open face canister to be influenced by temperature and

humidity effects, as well as decreased effective exposure time in the testing area. Ultimately, the open face canister results represent "grab samples" (Cohen 1988, George and Weber 1990).

This may or may not adequately describe the average radon

concentration profile of the testing location because grab

sampling does not integrate the concentration adsorbed over a

long period of time.

The main objectives of the use of diffusion barrier

canisters are to decrease the amount of water vapor adsorbed by the GAC; reduce the rate of 222Rn adsorption to, and desorption from the GAC inside the canister; reduce the effects of extreme fluctuations in 222Rn concentration; and extend the exposure time of the collector in the testing location (George and Weber 1990). Diffusion barrier canisters are less sensitive to humidity

effects than those of open face canisters (Cohen 1988) because

the diffusion barrier separates the GAC from the actual testing

environment and physically hinders water molecules from rapidly

entering the canister as experienced with open face canisters.

The reduction of water molecules on the surface of the GAC would allow for more radon adsorption on the GAC.

By fitting a diffusion barrier to the opening of a canister, the radon gas flow in and out of the canister is reduced. The

maximum amount of radon that can be adsorbed in diffusion barrier

collectors is about half of that adsorbed in open face canisters

(Cohen 1988). Thus a disadvantage of diffusion barrier canisters

is the amount of radon adsorbed during the exposure period is reduced, but an advantage is the canister is not as succeptable to radon concentration fluctuations that may exist in the testing

area (Cohen 1988). The diffusion barrier slows down the

diffusion of radon in and out of the canister, which allows the GAC to more closely measure the average concentration of radon present in the room (Cohen 1988). The radon concentration in the room may fluctuate, but the diffusion barrier only allows small quantities of radon near the GAC for adsorption and does not

allow desorbing radon to escape to the atmosphere as fast as open face canisters. The physical hindrance of radon adsorption and desorption by the diffusion barrier allows the radon measurement to be averaged over a longer period of time (Cohen 1988,

Nagarajan, et. al. 1990). Typically, diffusion barrier canisters have an integration time constant of 84 hours (Cohen 1988) and are exposed up to seven days (George and Weber 1990).

A humidity correction must be used with open face canister results to determine room radon concentrations. The open face collectors measure radon concentrations accurately up to

approximately 4 days provided the humidity is less than 55-•%

(George and Weber 1990). Empirically determined calibration

factors are used with open face canister calculations. Each open face canister is weighed before and after exposure and the water weight gain is compared with previous experimental data to

determine the correct humidity correction factor that should be

used.

Diffusion barrier canisters, however, are essentially insensitive to humidity effects and determinations of radon

concentrations do not involve humidity calibration factors (Cohen

1988). The physical hindrance of water vapor adsorption by the diffusion barrier allows the omission of the humidity dependent calibration factor in the concentration calculations. As a result, more homes are tested for radon with diffusion barrier canisters than with open face canisters (Cohen 1988.)

An attempt to control the effects of humidity on GAC

adsorption also has been implemented by placing desiccant packs in close proximity to the GAC. In experiments by Cohen and Nason

(1986), desiccant packs were added in different configurations on the charcoal adsorbers. Insertion of the desiccant did not hinder the diffusion of radon into or out of the canister, yet, did not effectively reduce the moisture uptake on the GAC as expected. The most effective method of attaching the desiccant pack to the canister was securing the pack over the canister opening (Cohen and Nason 1986). The desiccant wrapper acted as a diffusion barrier as well (Cohen and Nason 1986).

A recent study of diffusion barrier canisters (George and Weber 1990) concluded that porous stainless steel diffusion

barriers were the most practical diffusion barrier canisters for household radon sampling. Stainless steel diffusion barriers provided an integration time constant of approximately ten days,

and results were accurate within 10% when tested in known radon concentrations (George and Weber 1990). When compared to other popular diffusion barrier materials, the stainless steel

diffusion barriers used in George and Weber's (1990) study did not reduce water uptake by the GAC contained in the stainless steel diffusion barrier canisters any more than the other diffusion barrier materials. The major advantages in using stainless steel diffusion barriers are an extended integration time period and the durability of the stainless steel frits. The other barrier materials tested were usually delicate and required replacement after every testing series because of handling

(George and Weber 1990).

Recent adsorption canister research by Thomas (1991) at UNC supported the study that diffusion barriers extend the

integration time constant of diffusion barrier canisters.

Consistent with George and Weber (1990), Thomas (1991) concluded that the stainless steel diffusion barrier canisters had a longer integration time constant than the other diffusion barrier

canisters tested, but the stainless steel barriers were difficult to assemble and were more expensive than other types of diffusion barrier materials (Thomas 1991). Of the diffusion barrier

materials studied, Thomas concluded that the Whatman #1 diffusion

barrier material was the best material for use in short term

indoor radon screening measurements when sensitivity,

maintenance, assembly and cost were considered (Thomas 1991)

MATERIALS

RADON CHAMBER

The Radiological Hygiene Laboratory contains a radon

chamber, shown in Figure 3, that allows the establishment of a constant and isolated radon environment for charcoal canister

experiments and equipment calibration. The radon chamber has

dimensions of 48 x 24 x 62 inches which encompasses a 1000 liter volume. The chamber is constructed of lucite and virtually

eliminates uncontrolled leakage of gases in an out of the

chamber.

A radon source, manufactured by the Pylon Electronic Development Company, produced radon by forcing air with an

aquarium pump, across a radium source and is shown in Figure 4. A small fan inside the chamber circulated the air in order to

create a homogeneous 222Rn concentration.

The radon concentration inside the chamber was controlled three ways. First, a "tee" joint was added to the top of the source. One side of the "tee" was clamped almost completely shut in order to limit the amount of radon entering the exposure

chamber. Second, a tube was run from the other side of the "tee" to the exhaust which vented the excess radon from the source

directly to the exhaust hood. Finally, the air flow into the chamber was regulated to 40-45 cubic feet per minute. Using

these three equipment adjustments, the radon concentration in the chamber was regulated between 40 and 50 pCi/1.

One end of the radon chamber could be removed and replaced

Figure 3: Radon-222 Exposure Chamber.

Removable End _Exhaust

Entry Box

Gloves

Sampling Port

Power Supply

ai]- inlet

Figure 4: Radon-222 Source.

glass fiber filter

valve

Ra-226 Source

valve

glass fiber filter

from air pump

Tygon tube

drierite

Tygon tube

Pylon Model Rn-1025

flow through Rn-222 standard

Ra-226 Activity: 488.4 kBq Date: Sept. 20,1987

Rn-222 gas continuously availible: 61,441 mBq/min.

^pr^P^s^^^ipJP^j'

entry-exit hatch had been built into that same panel which

allowed for the insertion of small equipment and charcoal

canisters without removing the entire panel. Sampling ports were located on the side panel which allowed for grab sampling with alpha scintillation cells. Heavy rubber gloves built into the side of the chamber allowed for equipment manipulation and canister loading and unloading. Inside the chamber, the

temperature and humidity were measured with a thermometer and hygrometer. The temperature and humidity of the chamber

atmosphere remained constant at 25 C and 10 mm Hg, respectively.

ALPHA SCINTILLATION CELLS

Alpha scintillation cells were used to take "grab samples" in order to determine the radon concentration in the radon

chamber. The Randam Electronics Model ASC-125 alpha

scintillation cell shown in Figure 6, was used to determine the radon concentration in the chamber and to determine the Pylon

monitor efficiency.

The ASC-125 alpha scintillation cell has a 0.125 1 volume and has an inside coating of ZnS(Ag), which scintillates with alpha particle interactions. Samples are drawn into the

evacuated cell through the stopcock and nozzle on one end of the cell. The transparent window at the opposite end of the cell permits alpha scintillations to pass through it to a

photomultiplier tube, which is part of the alpha scintillation

Rgure 6: AJpha Scintillation Cell

(Thomas 19911

Stopcock

Quartz Window

Air Inlet

ALPHA SCINTILLATION CELL COUNTING SYSTEM

The alpha scintillation cell counting equipment consists of a light proof black box, photomultiplier tube, power supply,

preamplifier, and timer/counter (Figure 7). The photomultiplier

tube was housed inside the black box in order to keep any external background (outside light) from interacting with the photomultiplier tube. The alpha scintillation counting system was checked and calibrated with a known alpha source before each alpha cell was analyzed. A list of the equipment, manufacturer and settings, as well as a schematic diagram of the counting

system follows. INSTRUMENT Black Box Photomultiplier Tube Preamplifier Counter/Timer Counting Times scaler 7 Power Supply Voltage Amplifier coarse gain fine gain BLR input mode

unipolar output range shaping time constant

differentiate

integrate signal input

signal output

=1 - 60 minutes =In pos =1000 V MANUFACTURER UNC-CH Canberra Industries Model 1405

Tennelec TC 545A

Tennelec TC 910

Ortec Model 450

= 50

= 1 :OUt

=negative/differential

=10 V negative = 2

:OUt

=differential

=unipolar

Rgure 6: Alpha Scintillation Cell Counting System

(ThomMlSSI)

"Block Bo»^'

Power Supply

J"

PH-tube

|--J

J

Counter/Timer

PYLON AB-5 MONITOR

A Pylon AB-5 Portable Radiation Monitor was placed inside the radon chamber to continuously monitor the radon concentration while the canister experiments were run. The Pylon monitor has a built in photomultiplier tube and was used with a PRD-1 Passive

Radon Detector. The PRD-1 detector contains an inner coating of zinc sulfide scintillating material. By programming the internal microprocessor, the Pylon monitor is capable of counting for

specific time intervals as well as storing the collected data and allowing it to be recalled. A diagram of the Pylon monitor is

shown in Figure 5.

CHARCOAL CANISTERS

Two different sized charcoal canisters were used in each of

the experiments. The first canister was 4"in diameter and 1 1/8" deep. Each of these canisters contained 70 grams of granular

activated charcoal (GAC) in the adsorption and desorption

experiments and 68 grams of GAC in the desiccant adsorption and desorption experiments (to allow room for the 2 gram desiccant

packs.) The GAC was held in place by a circular brass screen and

a rubber retaining ring. Each canister was sealed airtight with a strip of electrical tape.

Diffusion barrier canisters were constructed from smaller

canisters available in the Radiological Hygiene laboratory.

These canisters were 3 1/8" in diameter and 15/16" deep. Each

diffusion barrier canister contained either 50 grams of GAC or 48

grams of GAC depending on the use of 2 gram desiccant packs in

Rgure 7: P^on AB-6 Portable Radiation Monitor

(Thomas 1991)

PRD-IAMmCoI

Power

Hr^Min/Sec

II (U> II

QuatiA^onL^ate

Statut 1 Pump

part of the experimentation. Two different size holes of 1" and

1 1/2" inches were punched in the canister's lids by the UNC

School of Public Health's Machine Shop. The diffusion barriers

were attached to the underside of the canister lids with rubber

cement or silicone caulking. All canisters were sealed airtight

with a combination of duct tape and aluminum foil across the

diffusion barrier opening, and electrical tape around the

canister seam until ready for use.

CHARCOAL CANISTER COUNTING SYSTEM

The charcoal canister counting system consisted of a low

background chamber, a Nal crystal attached to a photomultiplier

tube, a high voltage power supply, a preamplifier, amplifier, and

multichannel analyzer. The low background chamber contained the

Nal crystal, photomultiplier tube, and preamplifier, and shielded

the crystal from outside sources of background radiation.

Equipment settings and manufacturers, as well as a schematic

diagram (Figure 11) of the canister counting system follows.

INSTRUMENT

Background chamber

Sodium Iodide Crystal Photomultlplier Tube

Preamplifier

input capacitance = 500 pf

MANUFACTURER

Instrument and Development Products Company Amplifier fine gain coarse gain shaping time BLR delay input out

Ortec Model 113

Ortec Model 571

= 0.7 - 1.2 = 100

= 6 microsec

= auto = out

= negative

= uni

High Voltage Power Supply voltage = 1000 V Multichannel Analyzer

Canberra Model 3102

Tractor Northern Model

TN-1705

gain = 0

LLD = 30

ULD = 1200

amp

= _out

offset = ₀

conversion gain = ₁₀₂₄

mode = PHA

preset = live

time base = 100 X 6

= _{100 X 3}

Figure 8: Charcoal Canister Counting System

(Thomas 1991)

Multichannel Analyzer

nnm

Power

Nal Crystal

PM-Tube

Preanplifer Ampiirier

PROCEDURES

ALPHA SCINTILLATION CELL SAMPLING AND ALPHA SCINTILLATION CELL

COUNTING EQUIPMENT-PROCEDURES FOR USE

Alpha scintillation cells and counting equipment were used to determine the radon concentration in the radon chamber and to calibrate the Pylon monitors. Procedures for alpha scintillation cell sampling and counting follow.

1. Turn the MIDIBIN power supply switch to "on" and allow the

equipment to warm up for at least 30 minutes.

2. Press the "reset" button on the counter/timer until the

digital display reads "0."

3. Set the "timer" on the counter/timer for a 10 minute counting period.

4. Turn the voltage on the power supply to 1000 volts.

5. Set the amplifier "coarse gain" on 50 and the "fine gain" on 1.

6. Place the 0.125 1 standard alpha scintillation cell on the

photomultiplier tube inside the black box.

7. Press "count," and the count should begin increasing. The count displayed should be approximately 22,000 at the end of 5 minutes. If not, readjust the fine gain and repeat the ten minute counts until the count is approximately 22,000.

8. Turn the "timer" on the counter/timer to 30 minutes.

9. Take an alpha scintillation cell to the helium source. Turn

the helium source on, and place the helium tube on the nozzle of the alpha cell. Open the stopcock and allow the helium to flow

10. Take the alpha cell to the vacuum. Pull a vacuum on the alpha cell by placing the vacuum hose on the alpha cell nozzle

and opening the stopcock. Allow the vacuum to pull for 30

seconds and close the stopcock.

11. Repeat steps #9 and #10 at least three times.

12. Place the alpha cell on the photomultiplier tube inside the black box.

13. Press the "count" button on the counter/timer. This count

will give you the background counts for the alpha cell.

14. Take the alpha cell to the radon chamber. Place the nozzle

of the alpha scintillation cell inside the tube on the radon chamber sampling port.

15. Open the port valve, and then open the alpha cell stopcock and wait for 30 seconds before closing the stopcock and valve. 16. Pull a vacuum on the alpha cell again as in step #10.

17. Repeat steps #14 and #15 at least three times.

18. Allow the alpha scintillation cells to sit at least four hours before counting on the alpha scintillation cell counting

equipment.

19. Place the alpha scintillation cell on the photomultiplier

tube inside the black box.

20. Press the "count" button on the counter/timer and the counts

should begin increasing.

CALIBRATION OF PYLON AB-5 RADON MONITORS

TWO Pylon AB-5 portable radiation monitors, with the PRD-1 passive diffusion, alpha scintillation cell attachments, were used to measure the radon concentration in the radon testing chamber during the adsorption and desorption canister

experiments. Before testing began, the efficiency of the Pylon

monitors were measured with an alpha emitting standard that

produces alpha disintegrations at a known rate. Then the Pylon monitors were calibrated using alpha scintillation cells with

known efficiencies. Previous Pylon calibrations yielded 1.26 cpm/pci/1 and 1.13 cpm/pCi/1 for Pylons one and two,

respectively.

PROGRAMMING THE PYLON AB-5 MONITORS

Because the Pylon AB-5 monitor is capable of making a variety of radiation measurements, the monitor must be

appropriately programmed to ensure correct radiation measurements integrated over specific increments of time. A sample of Pylon

programming steps follows and should be used for future

reference:

1. Plug the Pylon power cord into an electrical outlet and turn

the "on/off" knob to "on."

2. Turn the "quasi/cont./rate" knob to "cont." and the

"hr/min/sec" knob to "hr."

"Prog./step" button again, to start the pylon monitor's one hour

counting intervals.

TEST OF PYLON 1 AND 2 EFFICIENCIES WITH STANDARD RADIUM/RADON

SOURCE

Each of the Pylon monitors was tested periodically in order to ensure that radiation measurements taken with the monitors

were within certain degrees of error. A protocol for testing the

efficiencies of the Pylon monitors follows:

1. Make sure Pylon is unplugged and turned off so that no digits

or lights are displayed on the panel.

2. Remove the PRD-1 passive radon detector from the face of the Pylon by unscrewing the detector counterclockwise.

3. Attach the sample holster to the face of the Pylon by

screwing the holster clockwise in the threads on the panel until

the holster fits securely.

4. Insert the alpha scintillation cell containing a radium standard in the holster and screw the top on the holster

securely.

5. Plug the Pylon monitor into the electrical outlet and turn

the "On/Off" knob to "On,"

and turn the "hr/min/sec" knob to "min."

6. Press the "prog,/step" button once. Press the "pump" button

until the first of the two digits reads "1." Press the

"start/stop" button until the second of the two digits reads "0."

The program is set for ten-minitue counting intervals.

7. Press the "start/stop" button once and the count should

8. After elapsed time, press "recall" button, and the display should yield "In 01". Press "prog./step" button twice, and the first of the ten minute count results should appear on display. Pressing the "prog./step" button each time thereafter, will

result in the display of successive ten minute count results.

9. Average the results and divide the average count by ten to

obtain counts per minute.

10. Turn the Pylon off and unplug it. Remove the standard from the holster, and seal the holster with the supplied cap.

11. Plug in the Pylon and turn on. Reset the Pylon for ten minute counting intervals and press "start/stop" button. This will measure the background counts.

12. Average the background counts and divide by ten. Subtract this average from the previously obtained count rate.

13. Divide this value by the number of disintegrations per

minute etched on the side of the radium standard. This will give you the efficiency of the pylon.

PYLON CALIBRATION FACTORS USING ALPHA CELL COUNT RATES

Both Pylon AB-5 monitors were calibrated using alpha scintillation cells before the canister exposure experiments began. To calibrate the Pylon monitors the concentration in the radon chamber was monitored with the Pylon monitors during

sampling with the alpha scintillation cells. The parallel

sampling allowed direct comparison between the Pylon monitors and

the alpha cells.

The radon concentration ( pCi/1 ) measured with each alpha cell was determined by dividing the net count rate (cpm) of each alpha cell by the cell's volume (0.125 1) a calibration factor

(4.95 cpm/pCi )(LaVake 1989) and the decay correction factor

{ e'-^t ) .

Eight, one minute counts were made with each Pylon in the

radon chamber. The Pylon count rates were then averaged to give

an average count rate for each of the Pylon monitors. The average Pylon count rates were then divided by the alpha scintillation cell concentrations to produce the calibration

factors (cpm/pCi/1) for each of the Pylon monitors. These values

were then compared to previous calibration factor data.

CANISTER PERFORMANCE TESTING-ADSORPTION

Fourteen different types of diffusion barrier canisters,

along with an open face canister, were chosen to study how 222Rn adsorption varies with time according to the diffusion barrier material and the size of the canister's "opening." Two sizes of canisters were used: The four inch open face canister contained 70 grams of granular activated charcoal as its adsorbent, and the

3 1/2 inch diffusion barrier canisters contained 50 grams of

granular activated charcoal as the adsorbent. The fifteen types of canisters, designated by the type of diffusion barrier, were

as follows:

PORE SIZE

CANISTER OPENING

Open face

Stainless steel 1

Fiberglass Fiberglass Fiberglass Fiberglass Whatman Whatman Whatman Whatman Whatman Whatman Whatman Whatman 1 1 1/2 1 1 1/2 1 1 1/2 1 1 1/2 1 1 1/2 1 1 1/2 0, 0, 1, 1, 2, 2, 6 6 11 11 20-25 20-25 micron micron

At time zero, seven canisters of each type were placed in the radon chamber at 25 C, 10% relative humidity, and a radon concentration of approximately 45 pCi/1. Each day, for seven days, one canister of each type was removed from the chamber and sealed immediately. After allowing the radon progeny to come into equilibrium with the adsorbed radon (at least four hours), each of the canisters was analyzed using the charcoal canister

counting system.

Ten minute background counts were taken on 4 inch and 3 1/2 inch unexposed canisters each day before exposed canisters were counted. These background counts were subtracted from the gross counts obtained from the exposed canisters to give the total net

counts for each of the canisters.

Radon chamber concentrations were continuously measured with the Pylon AB-5 monitor while the canisters were being exposed. The radon concentration in the chamber was integrated over six hour periods and these data were used to obtain the average radon concentration over the exposure period for each set of canisters removed each day. The average concentration measured by the

Pylon was used in calculations to determine the amount of

adsorbed radon on the granular activated charcoal in the canisters per pCi/1 of radon in the chamber.

After counting each of the canisters, the count rate was corrected for radon decay from the time the canisters were

removed from the chamber to the midpoint time of counting. These decay corrected count rates were then used along with the average

radon concentrations in the chamber to determine the amount of radon adsorbed on the granular activated charcoal in the

canisters per pCi/1 of radon in the chamber. After the adsorbed radon per gram of charcoal per pCi/1 of radon in the chamber values were calculated in units of cpm/g/pci/1 , plots of adsorption vs. exposure time were generated for each of the

different types of canisters exposed.

The adsorption plots generated for each canister exposed in

the chamber were used to determine which canister was the most efficient in adsorbing radon over seven days. Sample

calculations used to generate charcoal canister adsorption data

follow.

CANISTER/DAY PREWEIGHT POSTWEIGHT WEIGHT GAIN

CHTy^NEL COUNT RATE 110-201 280-335 TOTAL COUNT RATE

BACKGROUND COUNT RATE NET COUNT RATE

DECAY CORRECTION FACTOR e-'^t

DECAY CORRECTED COUNT RATE RADON CONCENTRATION

ADSORBED RADON

OPEN FACE/1

166 43 g 167 61

1 18 g

1698 4 cpm

+ 504 6

2203 0

- 131 _§.

2071 5 cpm 0 959

2160 1 cpm 42 3 pCi/1

0 730 cpm/g/pCi/1

CANISTER PERFORMANCE TESTING-DESORPTION

Charcoal canister desorption experiments involved placing each of fifteen different types of canisters in the radon chamber

at approximately 45 pCi/1 and 10% relative humidity for seven

days. The fifteen different canister types tested were the same canister types used in the adsorption experiment. All of the canisters were removed from the chamber at the end of seven days. Immediately, one canister of each t3,^e was sealed, and marked as time zero. The remaining canisters were placed in a steel

cabinet and allowed to desorb for specific intervals. The doors of the cabinet were then closed in order to eliminate drafts and circulation that would affect canister desorption rates.

The first set of canisters marked "time zero" was allowed to

sit for at least four hours in order for the radon progeny to

reach equilibrium. Ten minute background counts were taken on blank charcoal canisters before the first set of canisters were

counted on the charcoal canister counting system. The ten minute background counts were subtracted from the gross counts obtained by counting each of the canisters in the first set.

Each day for six days, one canister of each type was removed from the cabinet, sealed, allowed to sit for at least four hours in order for the radon progeny to come into equilibrium with the

radon, and counted on the charcoal canister counting system after

background counts were made.

The time each canister was sealed was recorded and the decay time was determined from the time the canister was removed from the radon chamber to the midpoint of the canister's counting

period. Data were recorded as the adsorbed radon remaining upon

removal from the radon chamber (George and Weber 1990.)

A plot of the desorption rate over seven days was generated for each canister type. A calculation example of the charcoal

canister desorption data follows this section.

CANISTER/DAY

CHANNEL COUNT RATE 110-201 280-335

TOTAL COUNT RATE

BACKGROUND COUNT RATE

NET COUNT RATE

DECAY CORRECTION FACTOR e-'"t

DECAY CORRECTED COUNT RATE RADON CONCENTRATION

ADSORBED RADON REMAINING

OPEN FACE/4 575.3 cpm

+ 175.6 750.9 - 111.9

639.0

0.519

3517.8 cpm 45.5 pCi/1

1.103 cpm/g/pCl/1

CANISTER WITH DESICCANT PERFORMANCE TESTING-ADSORPTION

In a separate experiment, one silica gel desiccant pack was added to each charcoal canister before the canisters were exposed

in order to determine the effects of the desiccant on the overall performance of the charcoal canisters. Theoretically, the

desiccant would remove some of the water vapor from the

atmosphere before it could adsorb on the granular activated charcoal inside the canister. In this experiment, five canister types were evaluated. Each canister contained either 68 grams or 48 grams of granular activated charcoal (depending on the canister size) and a 2 ounce desiccant pack. In addition, one

open face canister without a desiccant pack was used for comparison. The canister types used were as follows:

CANISTER

Open face 4 «

Open face + desiccant 4

No barrier + desiccant 3.5 1.5" _opening

Stainless steel + desiccant 3.5 1.5 1.5 Glass + desiccant 3.5 1.5

11 Whatman + desiccant 3.5" 1.5" _opening

Before exposure, each desiccant pack was weighed and

assigned to a specific canister. Each canister was then sealed and weighed. Next, all canisters were placed in the radon

chamber that contained a radon concentration of about 45 pCi/1. Each day for seven days, one canister of each type was removed from the chamber. Immediately upon removal from the chamber, the desiccant pack was removed from the canister, weighed and placed back in the canister. The canisters were then weighed and both

weights recorded.

Each canister was allowed to sit at least four hours after

sealing in order for the radon progeny to come into equilibrium

with the adsorbed radon. The canisters were then counted on the

radon canister counting system for ten minutes. The canister data

were recorded and the net counts obtained were corrected for

decay. The amount of radon adsorbed in each canister

(cpm/g/pCi/1) was calculated and a plot of the adsorbed radon versus days of exposure was generated. A sample calculation follows:

CANISTER/DAY OPEN FACE+ DESICCANT/1

DESICCANT PREWEIGHT 2.17 g DESICCANT POSTWEIGHT 2.35 DESICCANT WEIGHT GAIN 0.18 g CANISTER PREWEIGHT 158.98 g CANISTER POSTWEIGHT 169.67 CANISTER WEIGHT GAIN 0.69 g CHANNEL COUNT RATE 110-201 1229.5 cpm

280-335 363.5

TOTAL COUNT RATE 1593.0 cpm

BACKGROUND COUNT RATE - 99.4

NET COUNT RATE 1493.6 cpm

DECAY CORRECTION FACTOR e-'^t 0.769

DECAY CORRECT COUNT RATE 1942.3 cpm

RADON CONCENTRATION 43.6 pCi/1 ADSORBED RADON cpm/g/pCi/1 0.655

CANISTER WITH DESICCANT PERFORMANCE TESTING-DESORPTION

In a separate experiment, one silica gel desiccant pack was added to each charcoal canister before the canisters were exposed

seven days in order to determine the effects of the desiccant on

the overall desorption rates of the charcoal canisters. The canister types tested in this experiment were the same types of canisters tested in the canister with desiccant, adsorption

experiment.

Before exposure, each desiccant pack was weighed and

assigned to a specific canister. Each canister was then sealed

and weighed. All canisters were placed in the radon chamber for

seven days at an average radon concentration of 44.5 pCi/1, and 10% relative humidity. At the end of seven days, all canisters

were removed and one canister of each type was sealed

immediately, marked "time zero" and allowed to sit for at least four hours before counting on the charcoal canister counting equipment. The remaining canisters were placed in a steel

cabinet and allowed to desorb at specific intervals. The doors of the cabinet were kept closed in order to eliminate drafts and circulation that would affect canister desorption rates.

Each day for six days, one canister of each type was removed from the cabinet, sealed, and allowed to sit for at least four

hours before being counted on the charcoal canister counting equipment. Before each canister set was counted, a ten minute background count was taken on an unexposed charcoal canister and the result was subtracted later from the gross counts obtained from counting each of the canisters in each set. The decay time was determined from the time each canister was removed from the chamber to the midpoint of the canister's counting period. The data were recorded as the amount of adsorbed radon remaining upon removal from the radon chamber (George and Weber 1990.) A sample calculation for the determination of the amount of radon

remaining in a charcoal canister follows.

CANISTER/DAY OPEN FACE + DESICCANT/3

CHANNEL COUNT RATE 110-201 847.1 cpm 280-335 + 243.7

TOTAL COUNT RATE 1090.8 BACKGROUND COUNT RATE - 109.2 NET COUNT RATE 981.6

DECAY CORRECTION FACTOR e-"t 0.646

DECAY CORRECTED COUNT RATE 1519.5 cpm

RADON CONCENTRATION 44.5 pCi/1

ADSORBED RADON REMAINING 0.502 cpm/g/pCi/1

CANISTER RECYCLING

The granular activated charcoal used in the canisters was reactivated and stripped of radon and moisture by placing the

charcoal in the canisters in a 200 c oven for at least six hours.

The charcoal was allowed to cool and then placed in airtight

containers (filled to the maximum) until ready for weighing or

use in the laboratory.

Routine diffusion barrier replacement was necessary for the glass filters. This was easily achieved by rubbing the rubber cement from the lid of the canister and replacing the torn barrier with a fresh one.

RESULTS AND DISCUSSION CANISTER ADSORPTION TESTING RESULTS

Adsorption plots of cpm/pCi/1 vs. days of exposure were constructed for the fifteen canister types tested in this

experiment. All canisters were tested at 10% relative humidity and 25 C. Canister exposure began at a radon concentration of 42.3 pCi/1 and ended on day seven at a concentration of 49.4

pCi/1. The radon concentration in the chamber seemed to increase

over the exposure period because of gradual decreases in the airflow into the chamber. Over the seven day exposure period, the airflow dropped about 5 cfm. Also, the granular activated

charcoal in the canisters adsorbed more of the available radon in

the chamber at the beginning of the exposure period than at the

end of the exposure period because of charcoal saturation with

radon and water vapor. Integrated radon concentrations were measured with the Pylon monitors from the beginning of the exposure period to the removal of each canister group. The

adsorbed radon values for each exposure group are printed in the Appendices (II-V). Because of computer limitations and for

simplicity, three separate plots were constructed for the fifteen canister types. Figure 9 contains results for canisters with 1" and 1.5" openings and stainless steel diffusion barriers. Figure 10 contains results for 1" opening diffusion barrier canisters, and Figure 11 contains results for 1.5" diffusion barrier

canisters. Each plot contains an identical open face canister adsorption curve for comparison. A chart of the canister types

used follows:

FIGURE 9: ADSORPTION CURVES 10% RH 26 C

1.400T

I

1.200-

1.000-- 0.800

0.600-0.400

0.200-DAYS O.OOO

T 1.400

-1.200

ͣ

1.000

0.800

-O

Q.

E

a

- 0.600 "

ͣ

-0.400

ͣ

-0.200

0.000

FIGURE 10: ADSORPTION CURVES: 1" CC 10% RH 26 C

I

1.400 T

1.200-1.000

:r 0.800

" 0.600

0.400

0.200

0.000'

-•- 0.3 glass 1 •*- 6 Whatman 1

DAYS

Hi- 1.6 glass 1

-O- 11 Whatman 1

T 1.400

••1.200

ͣ

•1.000

0.800 c; ()

a. o>

g

Q.

0.600 "

• -0.400

ͣ

0.200

0.000

2.6 Whatman 1

FIGURE 11: ADSORPTION CURVES: 1.6" C 10% RH 26 C

1.400 T

1.200 ͣͣ

1.000-ͣ

1.400

!5'

^ ::^ 0.800

w o

a. 1 ^-~ ai

s E

a. o

0.600

0.400 ͣͣ

0.200-0.000

ͣ

1.200

ͣ

1.000

ͣ

0.800 ^

ͣͣ0.600 "

• ^0.400

ͣ

0.200

-•- 0.3 glass 1.6

-*ͣ 11 Whatman 1.5

3 4 DAYS

•1.6 glass 1.6

-©- open face

0.000

CANISTER PORE SIZE

Open face

Stainless s teel

stainless £ teel

Fiberglass 0.3 micron

Fiberglass 0.3 Fiberglass 1.5 Fiberglass 1.5 Whatman 2.5 Whatman 2.5 Whatman 6 Whatman 6 Whatman 11 Whatman 11 Whatman 20-25 Whatman 20-25 OPENING 1 1.5 1 1.5 1 1.5 1 1.5 1 1.5 1 1.5 1 1.5

Figure 9 shows that the 1.5" opening on the stainless steel canisters allows more radon to be adsorbed on the granular

activated charcoal (GAG) than does the 1 inch opening. The open

face canister allows more radon to be adsorbed on the GAG than

either stainless steel diffusion barrier canisters.

The adsorption curves for 1 inch opening diffusion barrier canisters show that the fiberglass diffusion barrier canisters consistently adsorbed more radon than any of the Whatman paper diffusion barrier canisters. On the second day of exposure, the

11 micron Whatman paper canister (1 inch) showed a slight decline

in its adsorption value. The weight gain of this particular canister was similar to the other canisters removed that day.

The occurrence of this low value, therefore, cannot be explained.

Adsorption curves for 1.5 inch diameter opening diffusion barrier canisters (Figure 11) also show that the fiberglass

diffusion barrier canisters consistently adsorbed more radon than

the Whatman paper diffusion barrier canisters. The most striking

result of this plot occurred on the first day of exposure with

the 0.3 micron fiberglass diffusion barrier canister. This spike

in radon adsorption occurred because the diffusion barrier became

separated from the top of the canister, which allowed the radon

to pass directly to the GAC without diffusing through the

barrier. The weight gain of this canister was double the average

of the other diffusion barrier canisters as a result of the

separation.

The plot of the open face canister data shows an adsorption spike at day 5. This occurrence cannot be explained because there is no barrier to separate in this canister, and the other canisters did not show a similar spike in the plot of their data. The open face canister used for reference on the fifth day of

adsorption showed no abnormal weight gain.

The adsorption plots in Figures 10 and 11 show that the

canisters with the 1.5 inch diameter openings adsorbed more radon than the canisters with the 1 inch diameter openings. The open

face canister data consistently showed more radon adsorption than any of the diffusion barrier canisters. All canisters tested

show rapidly increasing adsorption rates followed by gradual

plateaus representing GAC saturation. Between days six and seven of exposure, most canisters showed a slight drop in radon

adsorption.

CANISTER DESORPTION TESTING RESULTS

Desorption plots were created for the fifteen canister types tested in this portion of the experiment. All canisters were tested at 10% relative humidity and 25 C. The canisters were exposed over a seven day period to an average radon concentration of 45.6 pCi/1. Because of computer limitations, and simplicity, the three plots are separated into three categories: (1)

stainless steel 1" and 1.5" diameter opening diffusion barrier

desorption curves; (2) 1" diameter opening diffusion barrier canisters; and (3) 1.5" diameter opening diffusion barrier canisters. Each plot contains an open face canister desorption

curve for comparison.

The stainless steel 1" and 1.5" canister desorption curves (Figure 12) show gradual radon desorption compared to the curve for the open face canister. The slope of the 1.5" stainless

steel canister exceeded the slope of the 1" stainless steel

canister. The "time zero" adsorbed radon remaining value for the

1.5" stainless steel canister, when compared with the 1" canister, showed that the 1.5" canister allowed more radon adsorption than the 1" canister. This is consistent with the

adsorption data previously discussed.

The desorption plots for the 1" diffusion barrier canisters

(Figure 13) illustrate near-linear slopes for each of the canister types. A small spike in the 0.3 micron fiberglass diffusion barrier data for day 2 is the result of a separated

diffusion barrier in that canister. During exposure, the radon would have passed around the barrier directly to the GAC instead

FIGURE 12: DESORPTION CURVES; OF & SS 10*/. RH 25 C

1.200 T

(D I

V5

1.000

0.800

n 0.600

0.400

0.200

T 1.200

0.000

OPEN FACE

3

DAYS

STAINLESS STEEL 1"

ͣ

1.000

ͣ

0.800

a.

ͣ

0.600 o)

E

Q. O

ͣ

0.400

ͣ

0.200

0.000

FIGURE 13: DESORPTION CURVES: 1" CC

10% RH 26 C

(D I

1.200 T

1.000 ͣͣ

0.800ͣ•

o

Q.

oi 0.600

E

a o

0.400 ͣ

0.200ͣ•

o.ooo

1 2 3 DAYS

-•- 0.3 glass 1" ͣm- 1.6 GLASS 1"

ͣ

«- 6 WHATMAN 1" _{ͣ©- open face}

1.200

ͣ

1.000

•0.800

o

Q.

• 0.600 oi

E

n. o

0.400

ͣ

-0.200

0.000

-- 2.6 WHATMAN 1"

of diffusing through the barrier as planned, resulting in a

higher adsorbed radon remaining value.

The desorption curves of the 1.5" diffusion barrier

canisters (Figure 14) show a steeper slope than the 1" canister curves. Three noticeable spikes in the curves occur in the 2.5 and 6 micron whatman canisters at days 1 and 5, respectively, and

a trough in the curve for the 0.3 micron fiberglass canister

occurs at day 3. The trough occurring on day three is a result

of the diffusion barrier becoming separated after the canister's

exposure and removal from the chamber. Barrier separation after exposure during desorption would allow faster radon desorption directly to the atmosphere instead of being inhibited by the diffusion barrier. The spikes would occur if the diffusion barriers became separated from the canister tops before the

canisters were exposed.

The open face canister curve showed a rapid decline in

remaining adsorbed radon followed by a more gradual decline. The 1.5" canister curves illustrated lower radon adsorption than open face adsorption at time zero, which is consistent with earlier adsorption data. The adsorbed radon remaining values for 1.5"

canisters exceeded the values for the open face canisters by the end of the first day of desorption. The slope of the open face canister curve is nearly identical to the slope of the 1.5" canisters after the second day of desorption.

The 1" canisters' radon adsorption at time zero is less than

that of the 1.5" canisters, which is consistent with previous adsorption data. The adsorbed radon remaining values for the 1"

FIGURE 14: DESORPTION CURVES: 1.5" 107. RH 25 C

I

a.

1.200 T

1.000 •

C.800ͣͣ

ra 0.600 ͣ •

E

a. o

0.400 ͣ•

0.200 ͣ

0.000

-•- 0.3 glass 1.6"

-*- 6 WHATMAN

3

DAYS

-m- 1.5 GLASS

-G- open face

T 1.200

•1.000

•0.800

0.400

•0.200

0.000 o

o.

• -0.600 ra

E

a. o

-- 2.5 WHATMAN

canisters exceeded the open face canister values by the second day of desorption. The 1" and 1.5" canister curves intersect at the end of the fifth day of desorption at approximately 0.500

cpm/g/pCi/1, which suggests that the radon desorption data for

all diffusion barrier canisters tested become identical near the

end of the fifth day.

CANISTER WITH DESICCANT TESTING RESULTS-ADSORPTION

Adsorption curves (Figure 15) were constructed for six

different canister types including three 1.5" diameter opening diffusion barrier canisters with desiccants, two open face

canister types (one 4" open face canister and one 3.5" canister with 1.5" diameter openings) with desiccants, and one open face canister without a desiccant pack that was used for comparison.

CANISTER OPENING Open face 4" Open face with desiccant 4

3 1/2" with desiccant 1.5

Stainless Steel with desiccant 1.5 1.5 micron Fiberglass with desiccant 1.5 11 micron Whatman with desiccant 1.5"

All canisters were exposed at 10 % relative humidity and 25 C. The integrated radon concentrations during canister exposure

ranged between 43.6 pCi/1 on day one and 47.8 pCi/1 on day seven.

The effects of the desiccant packs are illustrated in the adsorbed radon curves of the 4" and 3 1/2" desiccant canisters.

The desiccant pack seemed to act as a diffusion barrier for the 4" with desiccant canister by hindering the ability of the radon

FIGURE 15: DESSICANT ADSORPTION CURVES 10% RH 26 C

<^

CD i

o

a.

D) 0.6-•

E

o. o

-•- OPEN FACE -»- SS 1.6 + DESS

3 DAYS -ͣ- 4" + dess

-e- 1.6 GLASS 1.6" + DE

t1.2

•ͣ1.0

ͣ

0.8

o

a

ͣ

•0.6 u)

E

Q. O

•ͣ0.4

ͣ

0.2

0.0

-<- 3 1/2" + dess

to adsorb on the GAC as quickly as on the open face canister. At

the fifth day of exposure, the open face and 4" with desiccant

canister curves intersect which suggests that the GAC in the 4"

with desiccant canister does not become saturated with radon as

quickly as the GAC in the open face canister.

The adsorption curves for the 1.5" diameter opening

diffusion barrier canisters with desiccant when compared to the

3 1/2" with desiccant canisters illustrate that the desiccant packing material in the open face canister acts as a diffusion

barrier for the canisters in this series (Figure 15.) Two spikes

occurred in the 1.5" open face canister curves which resulted

from inefficient sealing of the desiccant packs to the lids of those canisters. If those spikes are overlooked, the 1.5" open

face plus desiccant curve follows the slope of the other 1.5" diffusion barrier plus desiccant canisters.

As seen in earlier adsorption experiments, the 1.5 micron

fiberglass diffusion barrier canisters yield higher adsorbed

radon values than the other 1.5" diffusion barrier canister types tested. All canister types tested in this experiment show rapid

radon adsorption initially, followed by slow but steadily

increasing radon adsorption. The 4" open face canisters adsorbed

more radon than the 3.5" canisters which supports earlier experimental results.

CANISTER WITH DESICCANT TESTING RESULTS-DESORPTION

Desorption curves were created for six canister types tested (Figure 16.) Three 1.5" diameter opening diffusion barrier plus

FIGURE 16: DESICCANT DESORPTION CURVES 10% RH 26 C

V

o

1 a.

0\

D)

NO E

Q. O

1.2001

1.000 ͣͣ

0.800•

ra 0.60O

0.400 •ͣ

0.200

0.000

-•- open face

-*- stainless steel + d

3

DAYS -»ͣ 4" with dess

•©- 1.6 glass + dess

•• 3 1/2" + dess

-H- 11 Whatman + dess

T 1.200

•1.000

ͣ

0.800

o

Ol

ͣ

-0.600 rii

E

a. o

ͣ

-0.400

0.200

desiccant canisters were tested and compared to three open face

canister types.

The desiccant packs contained in the open face plus

desiccant canisters did not seem to have a major effect on the

initial radon adsorption at time zero, or desorption of the radon

from the GAC. The open face canister data were very similar to

the data obtained with the 4" canister with desiccant. The open

face canister data were equal to or exceeded the 4" with

desiccant canister data except on two occasions.

The 3 1/2" with desiccant canister curve exceeded all other

canister adsorption curves at time zero, day one, three, four

five and six. This effect may be a result of failed seals in the

canisters used in this portion of the experiment. A failed seal

would allow the radon to freely adsorb on the GAC without

diffusing through the packing material. The curve for the 3 1/2"

with desiccant canisters seems to approach the same desorption

rates as the other 1.5" canisters at the fifth day of desorption.

The 1.5" opening diffusion barrier with desiccant canisters

seem to desorb at similar rates. The slopes of the desorption

curves for each of the canisters appear to be identical with the

exception of one stainless steel canister desorption value.

The 1.5 micron fiberglass diffusion barrier and the 11 micron

Whatman paper diffusion barrier curves were nearly identical

except at time zero, which supports earlier desorption and

adsorption data. The 1.5" opening canisters tested in this

series seem to become identical to each other between the third

and fourth days of desorption. The initial slope of the open

face canister curve rapidly declines and later falls below, yet

CONCLUSIONS AND RECOMMENDATIONS

The performance characteristics of the different types of

diffusion barrier canisters will be discussed in four sections: 1. Adsorption. 2. Desorption. 3. Adsorption with Desiccant. and 4. Desorption with desiccant.

Diffusion barrier canister adsorption curves (Figures 9-11) show that using 1 1/2" diameter canister openings allows more radon to adsorb on the GAG than with 1" diameter openings.

Therefore, using the 1 1/2" opening diffusion barrier canisters might appear to be better than using the 1" canisters because more radon would be adsorbed on the GAC from the testing site. The fiberglass filter, diffusion barrier canisters consistently had higher count rates per gram of charcoal than the porous

stainless steel or Whatman paper diffusion barrier canisters when exposed to the same radon concentrations. The adsorption of all diffusion barrier canisters, however, was well below the

adsorption of the open face canister.

Diffusion barrier desorption curves (Figures 12-14) show

that desorption occurs faster in 1 1/2" diffusion barrier

canisters than in 1" diffusion barrier canisters, even though the diffusion for each set of diffusion barrier canisters became

nearly identical after five days of desorption. The stainless steel diffusion barrier canisters' desorption is nearly identical to the other diffusion barrier canisters' desorption. The open face canister begins with more radon adsorbed than all the

diffusion barrier canisters, desorbs rapidly, and falls below the

The desiccant wrapper may be used as an effective diffusion barrier when properly sealed across the opening of the lid of the

canister, as shown by the curve in Figure 15. When the desiccant

pack was improperly sealed to the lid of the canister, the 3 1/2" canister with desiccant mimics the open face canister adsorption.

The 4" canister with desiccant did not adsorb as much radon as

the open face canister, but did exceed the radon adsorption of

the diffusion barrier canisters with desiccant that were tested

in this portion of the experiment. The radon adsorption of the

1 1/2" fiberglass diffusion barrier canister with desiccant exceeded the radon adsorption of the stainless steel canister with desiccant and the 11 micron whatman filter canister with

desiccant.

The desorption curves in Figure 16 show that the desiccant

packs may be used as an effective diffusion barrier if properly sealed to the opening of the lid on the canisters. The slope of the 3 1/2" with desiccant canister curve closely resembles the other diffusion barrier curves in the figure, and did not

resemble the slope of the open face canister curve. The 4" with

desiccant canister curve resembles the open face canister curve,

and both undergo rapid desorption during the first three days of

removal from the chamber.

Placing a desiccant in the diffusion barrier canister had some effect on the overall water adsorption experienced by the charcoal in the canister. The desiccant typically adsorbed