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The motivation for our research was to determine whether positron annihilation
(PA) spectroscopy methods can be utilized for the characterization of rocks.
We studied rocks of geophysical interest both at room temperature and as
a function of temperature to approach the conditions in a natural rock formation.
The ultimate objective was to provide laboratory data from which it would
be possible to develop a positron-based well-log probe that would detect
the hydrocarbon content in the rock formation or provide additional information
on the characterization of the rock formation which would supplement the
conventional nuclear and other well-logs.
In order to characterize the rocks we used Doppler Broadening Spectroscopy
(DBS) and Lifetime Spectroscopy (LT). Initially we concentrated on sandstone
and carbonate rocks of different porosity. We found that the PA signal depends
on the rock state. But, this dependence was more complex than we originally
expected. In order to understand the positron interactions in rocks, we
found it necessary to consider the composition and structure of the rocks,
the dependence on sample history, and such factors as temperature and grain
size. We broadened the range of materials studied. We analyzed materials
similar in composition but different in structure (such as sandstones versus
opals) or similar in structure but different in composition (such as sandstones
versus anhydrite). We also realized that many measurements on a wide variety
of samples would be necessary if we were to understand the physical interaction
of positrons with rocks.
No one to our knowledge has previously investigated rocks by use of positron
techniques. One reason may be the complexity and inhomogeneity of the rock
structure and composition which makes a straightforward interpretation of
the positron annihilation results complex.
The dissertation is divided into three parts:
The first part (Chapters II- V) contains the description of PA techniques,
rock samples and a summary of general results. In Chapter II, the basic
concepts and methods of positron annihilation are discussed. Chapter III,
and Appendix A, contains the broad characterization of the samples with
information about their origin and properties measured by use of the XRF,
SEM and porosity methods. Chapters VI and V contain the results of the DBS,
LT and the Residual Gas Analysis Spectroscopy (RGAS) measurements at room
temperature and during the heating/cooling cycle.
The second part (Chapters VI-VII) includes a detailed discussion of the
PA results for specific rocks according to their composition, structure
and state. In Chapter VI, a detailed discussion of the PA results with a
qualitative physical explanation of positron interactions with the sandstone
and carbonate samples is presented. In Chapter VII the discussion of the
results on opals, synthetic sandstones and anhydrite is presented with the
emphasis on how the differences in structure or in composition affect the
PA signal.
Finally, the third part (Chapters VIII-IX) contains the plans for future
development. Novel approaches to DBS analysis are discussed in Chapter VIII.
Chapter IX contains a summary of this research and suggestions for future
research on rocks.
This work is a comprehensive study of rocks by use of positron annihilation
spectroscopy methods. A variety of rock samples, mainly sandstones, carbonates
and some other rocks with known composition and structure investigated by
XRF, SEM, EDX and BET methods have been studied. According to our investigations,
the positron annihilation signal depends on the rock composition, structure
and state (soaked in fluids or dry) but it does not depend on the history
of the rock (whether the rock went through a process of heating/cooling
or soaking/drying/cleaning ).
This work represents the first application of positron annihilation spectroscopy
to the study of rocks. There was some initial skepticism in the positron
annihilation community toward our research based on the fact that rocks
were perceived as very heterogeneous in structure and composition. The
results of our measurements, however, indicate that rocks can be described
and characterized or classified by their PA-parameters.
An important part of my dissertation is the development of the physical
models that explain how the positron annihilation response depends on
the structure, composition and state of the rocks.
One model is based on the understanding of the differences between the sandstones
and carbonates but can be applied to other, low porosity rocks such as
anhydrites. This model explains the observed differences in the PA-parameters
mainly on the basis of the contributions from the major constituents of
the rock composition. In addition, the presence of minor elements (such
as Mg in carbonates) influences the PA-parameters so that there is a clear
division between the carbonates without the magnesium (limestones) and with
magnesium content > 5%( dolomites).
Another model explains the PA-parameters on the basis of structural differences,
such as microporosity, powderization or compactness. The higher S-parameter
and lifetime values in opals (SiO2 *nH2O) in comparison to sandstones (SiO2),
although both have similar composition, have been explained by the high
microporosity in opals. On the base of the LT measurements the size of
the micropores in opals has been determined . In addition, the different
stages of maturation in opals have been characterized by different values
of the PA-parameters.
The differences in the PA-parameters in powdered versus non-powdered samples
have also been explained. We discussed three groups of powders (1. fine
powders in low-porous rocks, 2. rough powders in low-porous rocks, 3. rough
powders in high-porous rocks). The mechanism of the annihilation process
in these three groups was different. We discussed also how compactness
and cementation influences the S-parameter values in synthetic sandstones.
Several novel approaches to the analysis of the DBS spectra that have
been made possible with the introduction of improved background reduction
methods have been discussed. The examples of application of these techniques
which we presented are promising, although we are still in the stage of
testing and understanding some of the physics behind them.
The explanations of positron interactions with rocks presented in the dissertation
provide an insight into the rock microstructure on the atomic and molecular
level. In the future this research could lead to the development of a
new nuclear well-logging tool using the positron annihilation technique,
especially measurement of the Doppler broadening parameter.
In order to understand the PA processes in rock better, we propose the
continuation of the research in four areas:
1. Study of a wider variety of anhydrite and gypsum rocks during heating/cooling
cycle. Observation of the possible phase changes.
2. Analysis of the powdered clays with different density and different crystalline
structure.
3. Study of meteorite rocks.
4. Application of new measuring techniques, like 2-dimentional DBS or the
LT-DBS correlation measurements (for opals). Application of the improved
novel analysis methods to broader range of samples.
In conclusion this work has demonstrated the usefulness of the application
of positron annihilation studies to rocks and may lead to a better understanding
of the physics of rocks.
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