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GAMMA-RAY CHARACTERIZATION OF THE U-SERIES INTERMEDIATE

DAUGHTERS FROM SOIL SAMPLES AT THE PEÑA BLANCA

NATURAL ANALOG, CHIHUAHUA, MEXICO.

DIANA CAROLINE FRENCH

Department of Geological Sciences

APPROVED:

________________________________Elizabeth Y. Anthony, Ph.D., Co-Chair

________________________________Philip C. Goodell, Ph.D., Co-Chair

________________________________John Walton, Ph.D.

_______________________Pablo Arenaz, Ph.D.Dean of the Graduate School


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GAMMA-RAY CHARACTERIZATION OF THE U-SERIES INTERMEDIATE

DAUGHTERS FROM SOIL SAMPLES AT THE PEÑA BLANCA

NATURAL ANALOG, CHIHUAHUA, MEXICO.

by

DIANA CAROLINE FRENCH, B.S.

THESIS

Presented to the Faculty of the Graduate School of

The University of Texas at El Paso

in Partial Fulfillment

of the Requirements

for the Degree of

MASTER OF SCIENCE

Department of Geological Sciences

THE UNIVERSITY OF TEXAS AT EL PASO

May 2006


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UMI Number: 1434288

1434288

2006

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by ProQuest Information and Learning Company.


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ACKNOWLEDGEMENTS

I would first like to thank my parents for always supporting my interest in geology and

helping me through school. Thanks to my brothers and sisters for emotional support through out

my whole life. I would also like to thank my advisor Dr. Elizabeth Anthony; without her I would

not have been able to stay on track. Dr. Philip Goodell and Dr. John Walton for all their help

with research and interpretation. Dr. Minghua Ren for his assistance in the gamma-ray lab.

Thanks to Charles Beshears for his help in field sample collection and with the surface gamma

survey calculations. A special thanks to Aaron Kelts for his help in the field and in keeping me

sane for the last year and a half. Lastly, would like to thank also the U.S. DOE (TCO-TWA-0313, r.00), Office of Civilian Radioactive Waste Management, Office of Science and

Technology and International Program, and the NSF ADVANCE program (SBE 0245071) for

field and analytical expenses and summer salary support.


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ABSTRACT

The Nopal I uranium deposit is located in the Sierra Peña Blanca, Mexico. The deposit

was mined in the early 1980s, and ore was stockpiled close by. This stockpile area was cleared

and is now referred to as the Prior High Grade Stockpile (PHGS). Some of the high-grade

boulders from the site rolled downhill during stockpiling; for this study soil samples were

collected from the alluvium surrounding and underlying one of these boulders. A bulk sample of

the boulder was also collected. Because the Prior High Grade Stockpile had no ore prior to the

1980s, a maximum residence time for the boulder is about 25 years; this means that the soil was

at background as well. The purpose of this study is to characterize the transport of uranium seriesradionuclides from ore to the soil.

Transport is characterized by determining the relative activities of individual

radionuclides and daughter to parent ratios. Isotopes of the uranium series decay chain detected

include 210Pb, 234U, 230Th, 226Ra, 214Pb, and 214Bi. Peak areas for each isotope are determined

using gamma-ray spectroscopy with a Canberra Ge (Li) detector and GENIE 2000 software.

The boulder sample is close to secular equilibrium when compared to the standard BL-5

(Beaver Lodge Uraninite from Canada). Results for the soils, however, indicate that some

daughter/parent pairs are in secular disequilibrium. These daughter/parent (D/P) ratios include

230Th/234U, 226Ra/230Th, and 210Pb/214Bi. The gamma-ray spectrum for organic material lacks

230Th peaks, but contains 234U and 226Ra. The results, combined with previous studies require

multistage history of mobilization of the uranium series radionuclides. Earlier studies at the ore

zone could only limit the time span for mobilization to a few thousand years. The contribution of

this study is that the short residence time of the ore at the Prior High Grade Stockpile requires a

time span for mobilization of 20-30 years.


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TABLE OF CONTENTS

ACKNOWLEDGEMENTS...........................................................................................................iii

ABSTRACT................................................................................................................................... iv

TABLE OF CONTENTS................................................................................................................ v

LIST OF TABLES......................................................................................................................... vi

Chapters

1. INTRODUCTION ............................................................................................................. 1

2. GEOLOGIC SETTING ..................................................................................................... 3

3. DESCRIPTION OF THE SOILS....................................................................................... 54. PREVIOUS STUDIES IN RADIONUCLIDE MOBILITY AT NOPAL I....................... 6

5. FIELD METHODOLOGY................................................................................................ 8

6. ANALYTICAL PROCEDURES..................................................................................... 10

7. URANIUM SERIES DECAY ......................................................................................... 12

7.1 Types of Decay ..................................................................................................... 12

7.2 Secular Equilibrium and Disequilibrium .............................................................. 13

8. RESULTS ........................................................................................................................ 14

9. DISCUSSION.................................................................................................................. 17

10. CONCLUSIONS AND IMPLICATIONS..................................................................... 20

REFERENCES ............................................................................................................................. 21

APPENDIX A: Data tables and other figures……………………………………………………40

CURRICULUM VITA…………………………………………………………………………..64


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LIST OF TABLES

Table 1: Table with all sample D/P ratios with attenuation correction…………………………..24

Table 2: Table with percents of daughter ingrowth due to parent decay………………………...25


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LIST OF FIGURES

Figure 1: Map of the Nopal 1 Mine and Prior High Grade Stockpile......................................... 266

Figure 2: Surface radiometric survey............................................................................................ 27

Figure 3: Plan view and cross-section of sample area…………………………………………...28

Figure 4: Soil samples in the lab................................................................................................. 299

Figure 5: Self-attenuation curve for bulk samples........................................................................ 30

Figure 6: Self-attenuation curve for BL-5……………………………………………………….31

Figure 7: Self-attenuation curve for boulder sample…………………………………………….32

Figure 8: U-238 decay series…………………………………………………………………….33

Figure 9: Raw counts for B1 vs. Depth graph…………………………………………………...34

Figure 10: Raw counts for B3 vs. Depth graph………………………………………………….34

Figure 11: All bulk samples normalized to BL-5 ....................................................................... 355

Figure 12: Th-230/U-234 pair for all samples ............................................................................ 355

Figure 13: Ra-226/Th-230 pair for all samples........................................................................... 366

Figure 14: Pb-210/Bi-214 pair for all samples ........................................................................... 366

Figure 15: Spectra for BL-5 and organic sample........................................................................ 377

Figure 16: Cartoon for 230Th/ 234U.................................................................................................38

Figure 17: Cartoon for 226Ra/ 230Th...............................................................................................38

Figure 18: Cartoon for 210Pb/ 214Bi................................................................................................39


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1. INTRODUCTION

The Peña Blanca natural analog is located in the Sierra Peña Blanca, approximately 50

miles north of Chihuahua City, Mexico. The Peña Blanca site is considered a natural analog to

the proposed Yucca Mountain nuclear waste repository because they share similar characteristics

of structure, volcanic lithology, tectonic activity, and hydrologic regime.

One of the uranium-mineralized zones at Peña Blanca is the Nopal I mine. This deposit

lies mostly in the Tertiary Nopal formation, which is silicic ash flow tuff. The uranium bearing

deposit is part of a large breccia pipe in ignimbrites dated to be 44 Ma (Alba and Chavez 1974).

Previous studies focused on the brecciated zone and the surrounding area (Pearcy et al. 1994,Pearcy et al. 1995, Prikryl et al. 1997, Leslie et al. 1999, Murrell et al. 1999, and Wong et al.

1999). Most samples that were analyzed for these studies were infillings from several fractures

that make up an E-W fracture zone that runs from the middle of the breccia zone to the non-

brecciated country rock (Fig.1).

The focus of this study is the mobility of radionuclides from high-grade ore boulders of

the Prior High Grade Stockpile into local soils. The Prior High Grade Stockpile was located

approximately 250 meters west of the Nopal I mine. High-grade ore from the mine was

stockpiled here in the 1981 and boulders rolled down hill onto uncontaminated ground. The

stockpile was later moved to another location in the 1993. The advantage of this is a short, well

constrained residence period. One of the boulders that rolled down hill is the focus of this study.

This situation presents a unique opportunity to analyze the mobility of the radionuclides from

this boulder into the soil, to accomplish this soil samples were collected under, around, and into

the subsurface near the boulder.


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The soil samples were then analyzed using gamma ray spectroscopy. This method was

used because individual radionuclide peaks from the uranium decay series could be produced

and relative activities determined. Then, these peaks could be used to form ratios using

daughter/parent pairs to determine equilibrium or disequilibrium for that pair. By knowing the

characteristics of each radionuclide and this ratio, a deficiency or excess could be determined for

either the daughter or parent.

Substantial portions of this study are published in a CD volume from the International

High Level Radioactive Waste Management Conference in Las Vegas, NV in May 2006 (French

et al. 2006). The publication was prepared and edited by French.


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2. GEOLOGIC SETTING

The Sierra Peña Blanca is located about 50 miles north of Chihuahua City, Chihuahua,

Mexico. The Sierra Peña Blanca is a horst block in a basin and range province known as the

Chihuahua tectonic belt that contains many northwest striking faults (Goodell 1981). Before this

region experienced extension, it was part of an inland sea that covered parts of Mexico and the

southwestern United States. Therefore, many carbonate sequences can be found at the Sierra

Peña Blanca. Following the retreat of the inland sea, Laramide compression initiated volcanism

creating the volcanic stratigraphy of tuff sequences that cover a majority of Sierra Peña Blanca

(Reyes-Cortez 1997).Uranium at the Nopal I mine is located in a brecciated zone about 40 meters in diameter

and at least 100 meter thick that occurs at the intersection of two small step faults (Altamirano

1992; Pearcy et al. 1994; Reyes-Cortes 1997; Goodell 1981). This was thought to be a magmatic

breccia pipe or subsurface conduit that lies in the Nopal and Coloradas Formation tuffs. These

two formations lie unconformably above a limestone conglomerate called the Pozos Formation.

Whatever the ultimate origin of the brecciated zone, the breccia allowed hydrothermal waters to

travel through conduits and precipitate uranium in fractures or voids (Wong 1994). In the early

history of the deposit, reducing conditions caused U+4 to be precipitated producing uraninite, but

more recently water and other hydrothermal processes have caused the uraninite to oxidize

forming minerals like weeksite and uranophane.

There are many similarities that make Peña Blanca an excellent natural analogue to

Yucca Mountain. Yucca Mountain is a horst block in a region of basin and range dominated by

strike-slip and intrablock faulting (Day et al. 1998). Yucca Mountain is also composed of

sequences of welded tuffs and non-welded tuffs from calderas that lie to the north, which are


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rhyolitic in composition (Day et al. 1998). The unit selected to hold the proposed high-level

nuclear waste repository is the Topopah Spring member of the Paintbrush Tuff, which is a

welded tuff in the unsaturated zone (www.ocrwm.doe.gov/documents).

Both sites are semi-arid much like the rest of northern Mexico and the southwestern U.S.

The semi-arid conditions cause the water table to be deep below the surface, implying that

ground water does not play a large role in spreading contaminants from the surface, though

meteoric waters possibly could. At the Nopal I site the water table lies about 100 meters below

the uranium-mineralized zone (Wong 1994). This hydrologic regime is also similar to that of

Yucca Mountain, Nevada where the water table lies 167 meters below the level at which thewaste is to be buried (Wong 1994; www.ocrwm.doe.gov/documents).


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3. DESCRIPTION OF THE SOILS

The main focus of this study has been the soils surrounding boulders of high-grade ore

from the Prior High Grade Stockpile. The site where these samples were collected lays on a

dipping slope away from the Nopal I mine. The slope is estimated to be about 11º. Even though a

systematic description has not been done, a simplified one will be done here. Samples were taken

from several different levels into the subsurface. All samples had pebble to clay-sized grains and

were dark chocolate in color. Most of the large grains tend to be angular suggesting that they

have not traveled far and are composed of the local rocks. The pebble-sized grains tended to be

weathered pieces of tuff, while smaller factions may be tuffaceous or have an alternate origin.This area experiences high winds that could bring in other material. All samples also contained

some organic fraction. This can be roots, small leaves, or twigs. Some body casts from insects

have also been found.


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4. PREVIOUS STUDIES IN RADIONUCLIDE MOBILITY AT NOPAL I

Radionuclide mobility at the Nopal 1 deposit has been studied in the past. Southwest

Research Institute (SWRI) did some of the first work in the 1990s (Pearcy et al. 1995; Prikryl et

al. 1997). Their main goal was to describe the petrology of the brecciated zone and uranium

source and, to monitor migration of uranium radionuclides in the E-W fracture zone. In these

studies, gamma-ray spectroscopy and other techniques were employed. They discovered secular

disequilibrium in the fracture infillings. Ratios greater than unity were found when comparing

234U/ 238U, 230Th/ 234U, and 226Ra/230Th inside and outside the brecciated zone of the Nopal 1

deposit. They determined that there were excesses of

234

U,

230

Th, and

226

Ra in the fractureinfillings outside the brecciated zone. Their interpretation was multistage mobilization that

requires 230Th>234U>238U. Based on the half-life of 234U they placed mobilization of the

radionuclides within the last million years.

Wong and others conducted a similar study in 1999 (Wong et al. 1999). Again,

gamma-ray spectroscopy was employed, and the fracture infillings and ore were analyzed. They

found secular disequilibrium between 230Th and 234U and also between 226Ra and 230Th similar to

SWRI. Inside the brecciated zone, daughter/parent (D/P) ratios of 230Th/ 234U and 226Ra/ 230Th

were greater than unity. They also believed that the greater than unity ratio for 226Ra/ 230Th

outside the breccia zone means precipitation rather than leaching. The most pronounced mobility

was in veins and fractures with oxidized alteration minerals, e.g. hematite. Contrary to the

studies at SWRI, they thought that mobility had been within the last 8 ka, based on the mobility

of 226Ra.

A recent study of the Nopal 1 deposit was conducted by Murrell and others in 2002

(Murrell et al. 2002). Thermal Ion Mass Spectrometry (TIMS) was used to analyze samples from


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the E-W fracture zone of the Nopal 1 deposit. Disequilibrium between 226Ra / 230Th was seen.

This ratio was found to be greater than unity, similar to earlier studies, and thought to be due

mostly to surface water infiltration. They did not observe disequilibrium within 230Th/234U. The

time line proposed for radionuclide mobility for this study was set at 50,000 years based on the

half-lives of the radionuclides analyzed.

Leslie and others (Leslie et al. 1999) conducted a study of radium activity of the Phacelia

robusta, growing among the ore on the Prior High Grade Stockpile. Using photonegative paper

they were able to see the tracks of alpha particles produced as the radionuclides in the plants

cells decayed. The plant was powdered and analyzed. Samples contained large excesses of

226

Ra,

indicating that the plants were fixing radium into their cellular structure. They also determined

that the amount of calcium in the soil was inversely related to the plants uptake of 226Ra and that

a low concentration of calcium in the ore rocks supports a higher concentration of 226Ra. They

also hypothesized that once radium is released in to solution that the plants can scavenge it and

this leads them to believe that radium is deficient to its parent in the ore.


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5. FIELD METHODOLOGY

Soil samples were collected from the surface and subsurface, under and around a point

source. The point source is a boulder that rolled downhill from where it was originally deposited

during the placement of the Prior High Grade Stockpile in the 1981. The removal of the uranium

ore stockpile took place in 1993 (Reyes-Cortes 1997).

The point source was identified in an initial field trip to the site and designated Potential

Scientific Target (PST) #110. Field scintillometer readings from the boulder yielded

approximately 100,000 counts per minute.

After the site around the boulder was roped off, an estimated drip line was outlined using

string and anchored to the surface. The drip line is approximately where water fell from the

boulder onto the soil surface during rain events. After the drip line was marked, the boulder was

rolled about three meters up slope from the sample site. Three reference points were placed

around the site with rebar and cement. These reference points made it possible for ‘x’ and ‘y’

coordinates to be measured once a grid was set up.

Prior to sampling, a surface gamma survey was conducted using a scintillometer. Fifty

readings were taken around the sample site, with the head of the scintillometer facing down, to

obtain an idea of how the boulder had affected the soil surface around it. Special care was taken

to avoid readings close to where the boulder sat after being rolled away. The gamma survey is

shown in Figure 2 and is described in the results section below. The gamma survey map was

created using Surfer7 software.

After the survey, seven 30 cm by 30 cm squares of rigid plastic cardboard were set on

the surface to mark where samples would be taken (Figure 3). Each square was labeled with a

“B” for “block” and a number. This designator was put on each sample to indicate its location.


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Three squares were placed in a line down slope from the boulder and one where the boulder sat

prior to being moved. The other three were placed to the sides of the boulder position.

The samples were taken in horizontal layers into the subsurface. A metal frame was used

to keep the soil in place while the samples were collected. The cemented rebar reference points

were used to assign ‘x’ and ‘y’ coordinates for each sample. A laser level was used to create a

datum line so that a ‘z’ coordinates for each sample could be measured. The samples were put

into 700ml plastic containers with a lid and given a DOE designator and a name with PST

number, square number, and layer number (e.g. 110-B3-2). Each sample weighs approximately

1kg. This large amount of sample was needed to insure that each sample had a sufficientradionuclide content to be detectable (Figure 4). Altogether, 31 samples were collected. This

includes eight layers from blocks B1 and B3, and three layers from blocks B4, B5, B6, B7, and

B8.


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6. ANALYTICAL PROCEDURES

The samples were analyzed in the laboratory at the University of Texas at El Paso using gamma-

ray spectroscopy. The instrument used was a Canberra Lithium-drifted Germanium (Ge (Li))

detector. Gamma-ray spectroscopy is an efficient method to analyze a large number of large

samples in a short amount of time with minimum sample preparation. Each sample was counted

in its original container for between three days to a week and a half depending on its activity.

Three types of samples were analyzed for the study: bulk soil samples, crushed boulder,

and an organic fraction. Twelve of the 31 bulk soil samples have been counted: the first three

layers from B1, B3, B7, and B4 (Figure 3). A boulder sample was also counted for comparison.The boulder sample (PointE) was chipped off PST #110, crushed in the lab, and put into a

container like that of the bulk soil samples to keep a consistent geometry. The organic fraction

consists of roots handpicked from sample 110-B3-3 and put into a similar container.

The spectra were produced and analyzed with GENIE 2000 software. The software was

used to generate peak areas or relative activities for each isotope in the sample and calculate the

error for each peak. These isotope peaks then were turned into daughter/ parent (D/P) ratios of

the uranium decay chain.

In order to determine if the D/P pairs for the samples were in equilibrium or

disequilibrium, a standard, BL-5, was used for comparison. BL-5 is uraninite from the Beaver

Lodge deposit, obtained from Canada Centre for Mines and Technology and certified to be in

secular equilibrium. BL-5 was cast in a resin disk by Wong in 1995 and used in that form for this

study. BL-5 was counted in the gamma-ray detector in the same fashion as the soil and boulder

samples and produced the same peaks. D/P ratios were calculated for BL-5 and all samples were

normalized to BL-5.


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Before the data were normalized, a self-attenuation correction was completed. The

National Institute of Standards and Technology (NIST) SRN 4275c standard was used to

perform the attenuation correction. The standard was placed on top of three bulk samples: 110-

B3-1, 110-B1-3, and 110-B7-3, and on top of an empty container. 4275c was also counted on top

of BL-5, an empty resin disk, and the boulder sample as well. Each was counted for five days,

and twelve peaks were used for analysis. The following algorithm was used to produce the

attenuation curve and for interpolation of all unknowns (Cutshall et al. 1983).

A/O=Ln (T/I)/(T/I)-1

I= unattenuated counts per second (empty container)T=attenuation counts per second (full sample container)

A/O=attenuation correction as dependent on energy (keV)

A graph was generated with A/O on the y-axis and energy (keV) on the x-axis. The

coordinates were exported into SigmaPlot 2000, and a polynomial regression was done to fit a

line to the data. The three attenuation curves used can be seen in figure 5-7. The program

produces the equation, and interpolation can be completed for all unknowns. The A/O for each of

the unknowns’ energies was divided through the peaks areas for every bulk sample, boulder

sample, and BL-5.


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7. URANIUM SERIES DECAY

7.1 Types of Decay

For this study the 238U decay chain was employed (figure 8). Every radioactive decay

chain starts out with a parent radionuclide, in this case 238U, and it decays to its daughter

radionuclide, 234Th. Then that decays to the next daughter, which again decays to its daughter,

and this keeps happening until a stable radionuclide is reached, 206Pb for this decay chain. During

the process two types of decay occur, alpha and beta. Alphas are a divalent cations (Murray

2003). In this type of decay a radionuclide decays to another radionuclide and the atomic number

decreases by two (Z-2) and the atomic mass decreases by four (A-4), i.e.

238

U→234

Th +α. Alpha

decay can be very destructive because alphas are large. Alpha recoil can damage the crystal

lattice causing the crystal to break down move readily by leaching. An alpha is relatively small

and a sheet of paper can stop it, so this type of decay is the least dangerous (.1mm).

Betas are similar to an electron; they are anti-matter. Beta decay occurs when a

radionuclide decays to another radionuclide and the atomic number increases by 1 (Z+1); this

also considered negatron decay, i.e. 234Th→234Pa +β. The betas radiation can be stopped by a

sheet of aluminum (1mm).

Another type of particle that is emitted during radioactive decay is the gamma. During

branched decay a radionuclide may decay to different energies or excited states, instead of going

right to ground state. In order to get to the ground state a gamma ray (γ) is emitted at different

energies (keV). The radiation emitted by a gamma ray can only be stopped by a thick lead block,

making this type of radiation the most dangerous (Murray 2003).

Some radionuclides give off more gamma rays than others. For example U-238 only

emitted two gammas while Bi-214 can emit over 100 (Erdtmann and Soyka 1979). Since many


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of the radionuclides in the U-series decay chain emit gamma rays, gamma-ray spectroscopy was

chosen to analyze the samples for this study.

7.2 Secular Equilibrium and Disequilibrium

There are different states of equilibrium and disequilibrium. Ideal equilibrium is when the

activities of both the daughter and parent are in equal, i.e. closed system (Ivanovich 1992).

Secular equilibrium is when “ the parent activity does not decrease measurably during many

daughter half-lives.” This condition can exist as long as there are no breaks in the decay chain,

meaning that the parent is not leaving the system through increasing mobility when compared to

the daughter

Secular disequilibrium is just the opposite; here the system is open and radionuclides are

moving in and out at different rates of mobility. When a daughter/parent pair is in secular

disequilibrium that means there is a break in the chain, indicating that there is either excess or

deficiency of the daughter or the parent usually caused by fractionation of the daughter from the

parent (Murphy and Pickett 2002). Deficiencies of the parent are caused by increased mobility ofthat radionuclide. Excesses of a parent usually result from increased mobility of the daughter. All

disequilibrium situations go back to being in secular equilibrium. In some cases a permanent

state of disequilibrium can exist “ If the half-life of the parent nuclide is smaller than that of the

daughter then the activity ratio increases continuously as t increases” (Ivanovich 1992).


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8. RESULTS

Figure 2 shows the result of the field gamma survey. This map shows an area of high

counts where a large concentration of radionuclides resides in the soil, coincident with the drip

line of the boulder. Dripping moisture, therefore, is probably the main cause of mobilization of

the radionuclides via mechanical and chemical weathering. A few lobes of intensity surround this

hot spot, indicating other surface transport processes, such as wind, but it is noteworthy that the

radioactivity drops off over a very short distance away from the boulder. Radionuclide transport

occurs only a limited distance from the place of origin, e.g. the boulder.

Back in the lab, raw counts were also taken for all eight layers of B1 and B3 using the

same scintillometer as used in the field. This was done so that an approximate depth to

background could be calculated without the affect from the boulder that was observed in the

field. The results can be seen in figures 9 and 10. In figure 9 the depth at which the samples fall

off to background is about 5 cm or the 5th layer of B1. In figure 10 the depth at which the

samples fall off to background again is about 5 cm or layer 5 of B3. This is why when samples

from the bottom most layers of B1 and B3 were analyzed they didn’t yield significant enough

activity to be reported here. The field gamma survey and in lab raw counts record only total

intensities of radionuclides, while the gamma ray spectroscopy analysis done in the lab yields

relative activities of the individual radionuclides (Figs. 11-14). For these figures, the samples are

normalized to BL-5 (data from Table 1), as discussed above. Non-unity ratios imply secular

disequilibrium.

Figure 11 shows that 214Pb/ 226Ra and 214Bi/214Pb are in secular equilibrium. 214Bi and

214Pb have half-lives of 27 min. and 20 min., respectively, and therefore these ratios should be in

secular equilibrium given the counting protocols (detailed graphs of these pairs can be seen in


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appendix A). The ratios for these isotopes were used as a control to monitor the precisions and

accuracy of the isotope pairs that did exhibit disequilibrium. Figures 12-14 through show the

disequilibrium daughter/parent pairs. Values greater than unity were observed for most samples

for 230Th/ 234U and 226Ra/230Th. The disequilibrium for the pair 230Th/ 234U indicates a deficiency

of 234U. The disequilibrium between 226Ra/230Th indicates an excess of 226Ra. The last pair that

was found to be in disequilibrium was the 210Pb/ 214Bi, which exhibits a 210Pb deficiency.

Figures 12 through 14 show both lateral and vertical trends for the soil samples. In the

disequilibrium pair 230Th/ 234U (Figure 12) there is a similar trend for samples in B1, B3, and B4:

the deepest layers analyzed thus far (i.e. 110-B1-3, 110-B3-3, and 110-B4-3) are the furthestfrom equilibrium. The first two layers of B1 are close to equilibrium (110-B1-1, 2), along with

the boulder sample (PointE), while the first two layers on B3 and B4 show some disequilibrium

(110-B3-1, 2; 110-B4-1, 2). For B7, the samples in the vertical profile (110-B7-1, 2, 3) show

similar disequilibrium within error. These trends may be caused by the proximity of B1, B3, and

B4 to the drip line, whereas B7 is farther a field (Figure 3).

The graph of 226Ra/230Th exhibits another pattern (Figure 13). The trend is almost the

opposite of Figure 12. The boulder is very close to equilibrium (PointE). The sample that is

farthest from equilibrium is the topmost layer from B1 (110-B1-1), which lies directly

underneath the boulder. The two layers below it (110-B1-2, 3) begin to approach the same

degree of disequilibrium as the remainder of the samples. Samples from B3, B7, and B4 have

similar patterns of disequilibrium. The ratio of 210Pb/ 214Bi (Figure 14) does not exhibit trends

with respect to either depth or distance from the boulder.

The 226Ra excesses were investigated in the organic material from one of the soil

samples. The organics were counted for two weeks to obtain the best resolution. A large 226Ra


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peak was observed, whereas a 230Th peak was not observed at all (Figure 15). This is similar to

the results obtained by Leslie et al. 1999 from plants growing on the Prior High Grade Stockpile.


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9. DISCUSSION

These results indicate that non-aqueous transport (mechanical weathering) is not the only

mechanism by which radionuclides are being transported on the surface and subsurface in this

region. The relative activities and disequilibrium shown in Figures 11 through 14 indicate that

the radionuclides have traveled vertically as much as 5 to 6 cm into the soil. The hypothesis is

that those layers are deeper than mechanical transport would penetrate, thus requiring aqueous

transport (chemical weathering), i.e. leaching. As water fell on the boulder, the radionuclides

were leached and percolated into the soil. The maximum residence time of 25 years for this

boulder indicates that mobilization was quick, but the results also show that the radionuclideshave not traveled far from the original location of the boulder. To see the percent of each

daughter that could be produced by a decaying parent in this 25 year time span refer to table 2.

From this study it can be concluded that mobilization of the uranium radionuclides has

happened within the last 20-30 years. This differs from previous studies by SWRI (Pearcy et al.

1995; Prikryl et al. 1997) that places mobilization sometime within the last 1 million years.

Murrell et al. 2002 placed mobilization within the last 50,000 years, and Wong et al. 1999 that

placed mobilization within the last 8,000 years. All of these studies used the half-lives of

radionuclides to develop their time constraints. The importance of this study is that patterns of

mobility similar to those of previous studies were found, but given the residence time of the

boulder; the time scale could be narrowed down considerably.

The relative abundance of radionuclides in the soil indicates that multistage mobilization

has occurred. The deficiencies of 234U relative to 230Th imply that via non aqueous transport a

piece of the boulder fell onto the soil and was then leached in situ (figure 16). This indicates that


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230Th is being sorbed and retained in the soils, while 234U is moving through simply because 234U

is more soluble.

The excesses of 226Ra can be accounted for by the fact that 226Ra is more soluble than

230Th and can be put into solution in the soils by meteoric waters; it then can be taken up by

plants providing the excess in the soils (figure 17).

These observations have been documented in several other studies in the area (Pickett

and Murphy 1999). The studies by Wong et al. (1999) and SWRI (Pearcy et al. 1995; Prikryl et

al. 1997) in which gamma ray counting was also used found similar results concerning the

disequilibrium between

230

Th/

234

U

and

226

Ra/

230

Th. These studies both found deficiencies of

234

Uand large excesses of 226Ra in the large E-W fracture system. These results are very similar to

what has been found in the soils of Prior High Grade Stockpile. Our results are comparable to

previous studies because the boulders from the Prior High Grade Stockpile were once part of the

in-situ uranium deposit, so it appears that the radionuclides are behaving the same, only in a

different location. Water transport has played a large role in mobilization in both systems.

The study done by Murrell and others (Murrell et al. 2002) showed different results using

Thermal Ion Mass Spectrometry (TIMS). Instead of finding excesses of 226Ra, they found

deficiencies and they also reported equilibrium between U and Th pairs. The discrepancies may

be because they used a different technique to analyze their samples, or because of their sample

preparation method.

The other D/P pair that was greater than unity in our study and has not been documented

by previous studies is 210Pb/ 214Bi; with a deficiency of 210Pb. 210Pb was depleted in the soils,

presumably because 222Rn loss to the atmosphere from the time of stockpiling of the ore to

encapsulation of soil samples a year ago and, insufficient rain water to sequester 210Pb and bring


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it back down to the surface (figure 18). This decay to 210Pb in the atmosphere is a well known

phenomenon because it has been used to date glacial ice packs and snow (Faure 1977; El-

Daoushy 1986) This deficiency of 210Pb has been seen in waters collected from local water wells

(unpublished data) and in the Leslie et al. 1999 study. Since 214Pb is in equilibrium in the

encapsulated samples shows that radon is now being retained in the sample containers. Evidence

for this is that 214Pb was found in the samples and is now come into equilibrium with its daughter

214Bi and its parent 226Ra over an encapsulation period of a few days.


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10. CONCLUSIONS AND IMPLICATIONS

This study at Peña Blanca shows that environment plays a large role in how radionuclides

are mobilized. The disequilibrium that exists between some of the D/P pairs shows that the soil is

an open system in which radionuclides move freely. Multistage mobilization is prominent since

radionuclides were found in soil and organic samples, and disequilibrium was found between

D/P pairs. These conclusions agree with several previous studies. Water and spalling of the

boulder are dominant factors in radionuclide mobilization because the most radioactive samples

were found closest to the drip line, while radioactivity decreased in samples that were farther

from the drip line. The idea of water being the dominant mode for movement is supported by thesurface gamma survey.

The 226Ra excesses found in all the soil samples and organic material are indicative of

226Ra fixation by plants as described by Leslie et al. (Leslie et al. 1999). The 234U deficiencies

described in this study and in others can be attributed to uranium solubility and the relatively

quick movement of uranium in soils. The 210Pb deficiency can simply be explained by 222Rn loss

to the atmosphere.

Implications of this study are that mobilization was quick, with a time span of 25 years,

but the radionuclides have only traveled a matter of centimeters from the original source boulder.

In reference to Yucca Mountain, the Nopal I deposit would be analogous to the spent nuclear

fuel that is proposed to be stored. Though this study does not focus on the deposit itself, the

results found here further support the results from the studies at the deposit, and actually shorten

the time frame for mobility. Future work on this project will mean working toward total

concentrations of the radionuclides studied here in order to model the system and find rates of

mobility.


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21

REFERENCES

Alba, L. A. and R. Chavez (1974). "K-Ar Ages of Volcanic Rocks From the Sierra Peña Blanca,

Chihuahua, Mexico." Isochron West 10: 21-22.

Altamirano, J. R. (1992). Recent Explorations for Uranium and Molybdenum in the Cuerva

Amarilla, Sierra de Peña Blanca, Chihuahua, Mexico: Energy Resources of the

Chihuahua Desert Region, The El Paso Geological Society: 192-220.

Cutshall, N. H., I. L. Larsen, and C.R. Olsen (1983). "Direct Analysis of 210 Pb in Sediment

Samples: Self-Absorption Corrections." Nuclear Instruments & Methods In Physics

Research 206: 309-312.

Day, W. C., R. P. Dickerson, C.J. Potter, D.S. Sweetkind, C.A. San Juan, R.M. Drake, and C.J.

Fridich (1998). "Bedrock Geologic Map of the Yucca Mountain Area, Nye County,

Nevada." U.S. Geological Society Geological Investigation Series I-2627.

El-Daoushy, F. (1986). “The Value of 210Pb in Dating Scandinavian Aquatic and Peat Deposits”,

Radiocarbon 28: 1031-1040.

Erdtmann, G. and W. Soyka (1979). The Gamma Rays of the Radionuclides-Tables for Applied

Gamma Ray Spectroscopy, Verlag Chemie, 7: 235-236, 258.

Faure, G. (1977). Principles of Isotope Geology, 1st edition, John Wiley & Sons, Inc: 294-296

French, D. C., E. Y. Anthony, and P.C. Goodell (2006). U-Series Disequilibrium in Soils, Peña

Blanca Natural Analog, Chihuahua, Mexico. International High Level Radioactive Waste

Management Conference, Texas Station Hotel Las Vegas, NV.


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Goodell, P. C. (1981). Geology of the Peña Blanca uranium deposit, Chihuahua, Mexico:

Uranium Deposits in Volcanic and Volcaniclastic Rocks, P.C. Goodell and A.C. Waters,

American Association of Petroleum Geologists Studies in Geology, 13: 275-291.

Ivanovich, M. (1992). The Phenomenon of Radioactivity. Uranium-Series Disequilibrium :

Applications to Earth, Marine, and Environmental Sciences. M. Ivanovich and R. S.

Harmon, Clarendon Press: 14-33.

Leslie, B. W., D. A. Pickett, and E.C. Pearcy (1999). "Vegetation-Derived Insights on the

Mobilization and Potential Transport of Radionuclides From the Nopal I Natural Analog

Site, Mexico." Material Research Society Symposium Proceedings 556: 833-842.

Murphy, W. M. and D. A. Pickett (2002). "Radioisotope Fractionation and Secular

Disequilibrium in Performance Assessment for Geological Disposal of Nuclear Waste."

Material Research Society Symposium Proceedings 713: 867-874.

Murray, R. L. (2003). Understanding Radioactive Waste, 5th edition, K.R. Manke, Battelle Press:

10, 22.

Murrell, M. T., S. J. Goldstein, and P.R. Dixon (2002). "Uranium Decay Series Mobility at Peña

Blanca, Mexico: Implications for Nuclear Repository Stability." Proceedings of the 8th

European Commission Natural Analogue Working Group Meeting: 339-343.

Pearcy, E. C., J. D. Prikryl, and B.W. Leslie (1995). "Uranium Transport Through Fractured

Silicic Tuff and Relative Retention in Areas With Distinct Fracture Characteristics."

Applied Geochemistry 10: 685-704.

Pearcy, E. C., J. D. Prikryl, W.M. Murphy, and B.W. Leslie (1994). "Alteration of Uraninite


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From the Nopal 1 Deposit, Pena Blanca District, Chihuahua, Mexico, Compared to

Degradation of Spent Nuclear Fuel in the Proposed U.S. High-Level Nuclear Waste

Repository at Yucca Mountain, Nevada," Applied Geochemistry 9: 713-732.

Pickett, D. A. and W. M. Murphy (1999). "Unsaturated Zone Waters From The Nopal I Natural

Analog, Chihuahua, Mexico-Implications For Radionuclide Mobility at Yucca

Mountain." Material Research Society Symposium Proceedings 556: 809-816.

Prikryl, J. D., D. A. Pickett, W.M. Murphy, and B.W. Leslie (1997). "Migration Behaviour of

Naturally occurring Radionulcides at the Nopal I Uranium Deposit, Chihuahua, Mexico."

Journal of Contaminant Hydrology 26: 61-69.

Reyes-Cortes, I. A. (1997). Geologic Studies in the Sierra de Peña Blanca Chihuahua, Mexico.

Geological Sciences, University of Texas at El Paso. PhD: 342.

Wong, V. (1994). Nopal 1 Uranium Deposit, Peña Blanca District, Chihuahua, Mexico: A Study

of Radionuclide Migration in a Natural Analogue to Yucca Mountain, Nevada. .

Geological Sciences, Unversity of Texas at El Paso. MS: 62.

Wong, V., P. C. Goodell, and E.Y Anthony (1999). "Characterization of U-Series Disequilibria

at the Peña Blanca Natural Analogue Site, Chihuahua, Mexico." Material Research

Society Symposium Proceedings 556: 801-809.


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T a b l e 1 : C h a r t w i t

h a l l s a m p l e s n o r m a l i z e d t o B L - 5 a f t e r s e l f - a t t e n u a t i o n c o r r e c t i o

n , u s e d t o m a k e f i g u r e s

1 1 , 1 2 , 1 3 , a n d 1 4 .

I n f o r m a t i o n f o r t h i s t a b l e w a s c

o m p i l e d f r o m

t a b l e s 1 - 1 6 i n a p p

e n d i x .

24


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Table 2: This figure shows the percent of each daughter that has ingrown as a result of parentdecay for 25 years. D* is the daughter product, λ is the decay constant, and t1/2 is the half-lifefor that isotope.

25


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Figure 1: Site map of the Nopal 1 Mine (brecciated zone) and Prior High Grade Stockpile.

Contour interval is 20 m. E-W fracture represented by red line.

26


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Figure 2: Surface Radiometric survey of PST 110. The red dots are reference points in which xand y coordinates are based. Readings are not shown above a certain point because the boulderwas upslope and was most likely giving false readings.

27


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28

F i g u r e 3 : P l a n v i e w a n d

c r o s s - s e c t i o n o f s a m p l e a r e a .


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Figure 4: Soil samples in the lab.

29


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Self-Attenuation Curve for Bulk Samples

keV

0 200 400 600 800 1000 1200 1400 1600 1800

A / O

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

Figure 5: This is the attenuation curve used to do attenuation correction for all bulk samples.The line is fit to the curve and then the equation is extracted. The attenuation correction factor

(A/O) is then found for all unknowns at a given energy (keV).

30


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Self-attenuation curve for BL-5

energy (keV)

0 200 400 600 800 1000 1200 1400 1600 1800

A / O

1.05

1.10

1.15

1.20

1.25

1.30

Figure 6: This is the attenuation curve used to do attenuation correction for BL-5. The line is fitto the curve and then the equation is extracted. The attenuation correction factor (A/O) is then

found for all unknowns at a given energy (keV). A different correction had to be done becauseBL-5 had a different geometry than bulk soils samples.

31


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Self attenuation curve for Boulder sample

energy (keV)

0 200 400 600 800 1000 1200 1400

A / O

1.82

1.84

1.86

1.88

1.90

1.92

1.94

1.96

1.98

2.00

Figure 7: This is the attenuation curve used to do attenuation correction for PointE, the bouldersample. The line is fit to the curve and then the equation is extracted. The attenuation correction

factor (A/O) is then found for all unknowns at a given energy (keV). A different correction hadto be done because PointE had a different volume of material than bulk soils samples.

32


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Figure 8: Uranium-238 decay series. Boxes in yellow represent isotopes detected in this study.

represents alpha decay. represents beta decay.

33


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Raw counts for B1 vs. depth

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10

depth (cm)

r a w c

o u n t s ( c p m )

B1

Figure 9: Raw scintillometer counts done in the lab for all 8 layers of B1. Background is reachedat around 5 cm or the 5th layer.

Raw counts for B3 vs. depth

0

200

400

600

800

1000

1200

0 2 4 6 8 10

depth (cm)

r a w c

o u n t s ( c p m )

B3

Figure 10: Raw scintillometer counts done in the lab for all 8 layers of B3. Background isreached at around 5 cm or the 5th layer.

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Figure 11: All bulk samples normalized to BL-5 to identify secular equilibrium/disequilibrium.Color-coding is as follows: B1 samples are in oranges, B3 samples are in greens, B7 samples arein blues, and B4 samples are in purples. Samples for each block are listed in order of increasingdepth.

Figure 12: 230Th/234U for samples from the Prior High Grade Stockpile. Color-coding as inFigure 11.

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Figure 13: 226Ra/230Th for samples from the Prior High Grade Stockpile. Color-coding as inFigure 11.

Figure 14: 210Pb/214Bi for samples from the Prior High Grade Stockpile. Color-coding as inFigure 11.

36


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226Ra

Th

234Th234Th

226Ra

230Th

234Th

Th

Figure 15: Spectra produced with GENIE 2000 software – BL-5 (above) and organic sample(below). In the organic fraction a 230Th peak was not resolved, indicating a very low activity of230Th in the organic fraction relative to BL-5.

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Figure 16: Cartoon explaining the deficiency of 234U relative to 230Th in bulk samples.

Figure 17: Cartoon explaining the excess of 226Ra relative to 230Th in bulk samples.

38


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Figure 18: Cartoon explaining the deficiency of210

Pb relative to214

Bi in bulk samples.

39


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APPENDIX A: Data tables and other figures

T a b l e 1 : T h e s e c a

l c u l a t i o n s w e r e d o n e 1 1 0 - B 1 - 1 .

T h e t o p t w o s h o w t h e c a l c u l a t i o

n s d o n e t o g e t D / P p a i r s b e f o r e

a s e l f -

a t t e n u a t i o n c o r r e c

t i o n w a s d o n e a n d t h e b o t t o m t w

o s h o w c a l c u l a t i o n s d o n e t o g e

t D / P p a i r s a f t e r s e l f - a t t e n u a t i o n

c o r r e c t i o n s . T h e d

e a d t i m e

e r c e n t f o r t h i s a n a l s

i s w a s 0 . 2 1 %

a n d t h e t o t a l t i m e

t h e s a m

l e w a s a n a l z e d w a s 6 0 8 4 0 0 s e c .

40


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41 T

a b l e 2 : T h e s e c a l c u l a t i o n s w e r e d o n e f o r 1 1 0 - B 1 - 2 . T h e t o p t w o s h o w t h e c a l c u l a

t i o n s d o n e t o g e t D / P p a i r s b e f o

r e a s e l f -

a t t e n u a t i o n c o r r e c t

i o n w a s d o n e a n d t h e b o t t o m t w

o s h o w c a l c u l a t i o n s d o n e t o g e t

D / P p a i r s a f t e r s e l f - a t t e n u a t i o n c o r r e c t i o n s .

T h e d e a d t i m e

e r c

e n t f o r t h i s a n a l s i s w a s 0 . 1 9 %

a n d t h e t o t a l t i m e t h e s a m



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T a b l e 3 : T h e s e c a l c u l a t i o n s w e r e d o n e f o r 1 1 0 - B 1 - 3 . T h e t o p t w o s h o w t h e c a l c u l a


r e a s e l f -




D / P p a i r s a f t e r s e l f - a t t e n u a t i o n

c o r r e c t i o n s . T h e d e

a d t i m e

e r c e n t f o r t h i s a n a l s i s w a s 0 . 1 7 %



42


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43 T

a b l e 4 : T h e s e c a l c u l a t i o n s w e r e d o n e f o r 1 1 0 - B 3 -

1 . T h e t o p t w o s h o w t h e c a l c u l a


r e a s e l f -

a t t e n u a t i o n c o r r e c t i o n w a s d o n e a n d t h e b o t t o m

t w




e r c





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44 T



r e a s e l f -






e r c





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45 T



r e a s e l f -






e r c





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46 T

a b l e 7 : T h e s e c a l c u l a t i o n s w e r e d o n e f o r 1 1 0 - B 7 - 1 . T h e t o p t w o s h o w t h e c a l c u l a t i o n s d o n e t o g e t D / P p a i r s b e f o r

e a s e l f -


t w o

s h o w c a l c u l a t i o n s d o n e t o g e t D / P p a i r s a f t e r s e l f - a t t e n u a t i o n c

o r r e c t i o n s .




l e w a

s a n a l z e d w a s 6 0 8 4 0 0 s e c .


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47 T



r e a s e l f -


t w


D / P p a i r s a f t e r s e l f - a t t e n u a t i o n c o r r e c t i o n s . T h e

d e a d t i m e


a n d

t h e t o t a l t i m e t h e s a m

l e w a s a n

a l z e d w a s 6 0 4 8 0 0 s e c .


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48 T



r e a s e l f -






e r c





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T a b l e 1 0 : T h e s e c a

l c u l a t i o n s w e r e d o n e f o r 1 1 0 - B 4 - 1 . T h e t o p t w o s h o w t h e c a l c u l a t i o n s d o n e t o g e t D / P p a i r s b e f

o r e a s e l f -






e r c




49


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50 T

a b l e 1 1 : T h e s e c a


o r e a s e l f -






e r c





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51 T

a b l e 1 2 : T h e s e c a


o r e a s e l f -





T h e d e a d t i m e p e r c

e n t f o r t h i s a n a l y s i s w a s 0 . 1 4 % a n d t h e t o t a l t i m e t h e s a m p l e w a s a n a l y z e d w a s 6 0 4 , 8 0 0 s e c .


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52 T

a b l e 1 3 : T h e s e c a l

c u l a t i o n s w e r e d o n e f o r B L - 5 ’ s

f i r s t a n a l y s i s . T h e t o p t w o s h o w

t h e c a l c u l a t i o n s d o n e t o g e t D / P p a i r s b e f o r e

a s e l f - a t t e n u a t i o n c o r r e c t i o n w a s d o n e a n d t h e b o t t o

m

t w o s h o w c a l c u l a t i o n s d o n e t

o g e t D / P p a i r s a f t e r s e l f - a t t e n u a t i o n

c o r r e c t i o n s . T h e d e a d t i m e p e r c e n t f o r t h i s a n a l y s i s

w a s 1 . 6 8 % a n d t h e t o t a l t i m e t h

e s a m p l e w a s a n a l y z e d w a s 2 5 0

, 0 0 0 s e c .


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T a b l e 1 4 : T h e s e c

a l c u l a t i o n s w e r e d o n e f o r B L - 5 ’ s s e c o n d a n a l y s i s . T h e t o p t w o s h o w t h e c a l c u l a t i o n s d o n e t o g e

t D / P p a i r s

b e f o r e a s e l f - a t t e n

u a t i o n c o r r e c t i o n w a s d o n e a n d

t h e b o t t o m

t w o s h o w c a l c u l a t i o n s d o n e t o g e t D / P p a i r s a f t e r s e

l f - a t t e n u a t i o n

c o r r e c t i o n s . T h e l a s t c h a r t g i v e s t h e a v e r a g e D / P f o r b o t h a n a l y s e s w i t h s e l f - a t t e n u a t i o n c o r r e c t i o n s . T h e d e a d t i m

e p e r c e n t f o r

t h i s a n a l y s i s w a s 1 . 6 4 % a n d t h e t o t a l t i m e t h e s a m

p l e w a s a n a l y z e d w a s 2 5 9 , 2 0 0 s e c .

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T a b l e 1 5 : T h e s e c a l c

u l a t i o n s w e r e d o n e f o r P o i n t E ’ s

f i r s t a n a l y s i s . T h e t o p t w o s h o w

t h e c a l c u l a t i o n s d o n e t o g e t D /

P p a i r s b e f o r e

a s e l f - a t t e n u a t i o n c o r r e c t i o n w a s d o n e a n d t h e b o t t o m

t w o s h o w c a l c u l a t i o n s d o n e t o

g e t D / P p a i r s a f t e r s e l f - a t t e n u a t i o n

c o r r e c t i o n s . T h e d e a d t i m e p e r c e n t f o r t h i s a n a l y s i s w

a s 2 . 3 7 % a n d t h e t o t a l t i m e t h e

s a m p l e w a s a n a l y z e d w a s 2 5 9 , 2 0 0 s e c .

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T a b l e 1 6 : T h e s e c

a l c u l a t i o n s w e r e d o n e f o r P o i n t E ’ s s e c o n d a n a l y s i s . T h e t o p t w o

s h o w t h e c a l c u l a t i o n s d o n e t o g e t D / P p a i r s

b e f o r e a s e l f - a t t e n

u a t i o n c o r r e c t i o n w a s d o n e a n d

t h e b o t t o m

t w o s h o w c a l c u l a t i o n s d o n e t o g e t D / P p a i r s a f t e r s e

l f - a t t e n u a t i o n

c o r r e c t i o n s . T h e l a s t c h a r t g i v e s t h e a v e r a g e D / P f o r b o t h a n a l y s e s w i t h s e l f - a t t e n u a t i o n c o r r e c t i o n s . T h e d e a d t i m

e p e r c e n t f o r

t h i s a n a l y s i s w a s 2 . 3 8 % a n d t h e t o t a l t i m e t h e s a m

p l e w a s a n a l y z e d w a s 2 5 9 , 2 0 0 s e c .

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Table 17: Counts per second are calculated for the set-up with standard 4275c on an emptysample container. The counts per second represent I in the equation:A/O=Ln (T/I)/(T/I)-1 used to calculate self-attenuation. This setup was run for 432,000 sec.

Table 18: Counts per second are calculated for the set-up with standard 4275c on sample 110-B7-3. The counts per second represent T in the equation: A/O=Ln (T/I)/(T/I)-1 used to calculateself-attenuation. This setup was run for 432,000 sec.

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Table 19: Counts per second are calculated for the set-up with standard 4275c on sample bouldersample 110-PointE. The counts per second represent T in the equation: A/O=Ln (T/I)/(T/I)-1used to calculate self-attenuation. This setup was run for 432,000 sec.

Table 20: Counts per second are calculated for the set-up with standard 4275c on an empty resindisk. The counts per second represent I in the equation:A/O=Ln (T/I)/(T/I)-1 used to calculate self-attenuation. This setup was run for 432,000 sec.

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Table 21: Counts per second are calculated for the set-up with standard 4275c on BL-5. Thecounts per second represent T in the equation: A/O=Ln (T/I)/(T/I)-1 used to calculate self-attenuation. This setup was run for 432,000 sec.

Table 22: These calculations were done to create the curve in figure 5, using the last column,which equals A/O, and the average keV. I is the counts per second for 4275c on an empty samplecontainer and T is the counts per second for 110-B7-3.

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Table 23: These calculations were done to create the curve in figure 6, using the last column,which equals A/O, and the average keV. I is the counts per second for 4275c on an empty resindisk and T is the counts per second for BL-5.

Table 24: These calculations were done to create the curve in figure 7, using the last column,which equals A/O, and the average keV. I is the counts per second for 4275c on an empty samplecontainer and T is the counts per second for boulder sample 110-PointE.

Table 25: This is the formula of the curve from figure 5 that was extracted using Si

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