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2.6. Some Final Remarks
2.6.
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SOME FINAL REMARKS
done by calculating the centroid for each individual spectrum, then aligning or shifting the spectra according to their centroids, and finally adding all the aligned spectra together. Such a procedure becomes especially simple when only integer displacements of the microscopic data are carried out. In this case the spectra are being stabilized to f 0.5 channels, which means that the conversion gain (time per unit channel) should be 20 ps per channel or less. The average broadening of a lifetime spectrum due to this roundoff error will be 0.4 ps, a value that is small enough for most realistic applications. Any stabilization procedure, whether it takes place in real time or not, is based upon information regarding the time-dependent status of the system (e.g., resolution function, dc levels, and gain), and the procedure represents a hypothesis about how changes in system status affect the data. Therefore a statistical test of a physical model against the observations is a test not only of the model but of the stabilization hypothesis as well. From this point of view, postacquisition procedures offer clear advantages over real-time procedures since the stabilization hypothesis and the physical model can be viewed correctly as a single entity describing the observed (unstabilized) data. This continuous and direct test of the instability correction procedure cannot be achieved by real-time stabilization since the corrections are applied prior to data storage. The disadvantage of the superior postacquisition method is the observational technique (e.g., microscopic experiments) along with the large amounts of concomitant mass data storage that are required. These disadvantages can be anticipated to diminish rapidly in the future with the appearance of more advanced and less costly computer hardware. For a further discussion of the mathematical basis of various stabilization procedures see Smedskjaer et
-
-
2.6. Some Final Remarks It is appropriate for us to end this part by outlining some of the areas where the positron technique requires extra care while remembering that the unique nature of the technique and its general prowess in the defect area make it a useful tool in metallurgical studies. Care must always be taken in positron experiments to ensure that the defect population being studied is indeed the dominant defect in the system with respect to positron trapping. At a low defect concentration, lo4, which is the sensitivity limit of the positron technique for vacancy point defects, extreme care is required. It is under conditions such as this that low concentrations of additional defects (dislocations for example) have been shown to have significant or even dominant effects on the resulting data.
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144
2.
POSITRON ANNIHILATION
When one is dealing with a complex population of defects, vacancies, vacancy clusters, voids, dislocations, etc., questions will necessarily arise concerning the way in which the positron is partitioned among such a population. The partitioning of the positron among defect types present simultaneously in a system is not fully understood, and the interpretation of such data usually has a very qualitative character. Clear improvements are called for in the identification of parameters specific to different defects that can be derived from the experimental data. The number of positron states that can now be resolved is no more than 2 or 3, depending on how different the signals from these states are. The best results in separatingsuch states are usually obtained in lifetime experiments. To some degree one is always faced with contributions from positron annihilation events that do not take place in the part of the sample one desires to study. In experiments where bulk properties of the sample are to be investigated, the unwanted contribution to the observed data comes from annihilations in either the source or its supporting structures and/or those positrons that reach and eventually annihilate at or near the surface, or for that matter any place that can be viewed by the detection system. Currently, the best way to minimize such effects is the use of internal sources, produced by either ion implantation or activation techniques. The positron technique has now gone through an initial period of growth with respect to its application to the study of defects in metals. This period has been successfulin that increasing use is being made of the technique, not just by specialists, but by a broad group of material scientists. The technique is now entering a new stage of development where new methodologies are being explored that will uncover those properties of the annihilation signal that might be considered as defect specific. At the same time new information is being uncovered with respect to the trapping mechanism itself, particularly in traps where the binding energjes of the positron are small. The use of positron annihilation to measure vacancy formation enthalpies is well developed and is currently believed to be limited only by the variations in sample properties themselves: impurities and nonequilibrium defect concentrations arising from them. On the instrumentation side, one can identify three general areas which hold special promise of yielding improvements in the amount and details of the data: 1. The use of large-area position-sensitive detectors operating in coincidence in ACAR experiments clearly increases data rates as well as providing considerably more flexibility in the analysis of momentum data. It is hoped that combining Doppler-broadening experiments with detailed ACAR work will lead to an efficient identification of interesting metal defects studies.
2.6.
SOME FINAL REMARKS
145
2. Continued improvements are being made in lifetime techniques. The use of large-volume plastic scintillators is particularly noteworthy. Combined simultaneous lifetime and momentum (Doppler broadening) measurements are much more tractable, and these combined experiments yield an extra dimension of sensitivity and possible specificity. Earlier work has demonstrated that these two types of experiments are often invaluable together and provide additional insight into the underlying physical principles themselves. 3. The use of slow beams for positron sources opens the door to a whole new class of positron experiments, not only on surfaces and interfaces but also in thin foils; the latter are amenable to many other types of materials studies, from electron microscopy to resistivity, and are particularly valuable in nonequilibrium experiments such as radiation damage and quenching- annealing studies. The techniques for analyzing experimental data are improving continually. Much more care is being taken in dealing with the statistical properties of the data and in applying stabilization methods. The relation between the physical models and the experimental uncertainties is appreciated, and more accurate results are being obtained by understanding this relation and recognizing the various limitations of the instrumental techniques.
143
SOME FINAL REMARKS
done by calculating the centroid for each individual spectrum, then aligning or shifting the spectra according to their centroids, and finally adding all the aligned spectra together. Such a procedure becomes especially simple when only integer displacements of the microscopic data are carried out. In this case the spectra are being stabilized to f 0.5 channels, which means that the conversion gain (time per unit channel) should be 20 ps per channel or less. The average broadening of a lifetime spectrum due to this roundoff error will be 0.4 ps, a value that is small enough for most realistic applications. Any stabilization procedure, whether it takes place in real time or not, is based upon information regarding the time-dependent status of the system (e.g., resolution function, dc levels, and gain), and the procedure represents a hypothesis about how changes in system status affect the data. Therefore a statistical test of a physical model against the observations is a test not only of the model but of the stabilization hypothesis as well. From this point of view, postacquisition procedures offer clear advantages over real-time procedures since the stabilization hypothesis and the physical model can be viewed correctly as a single entity describing the observed (unstabilized) data. This continuous and direct test of the instability correction procedure cannot be achieved by real-time stabilization since the corrections are applied prior to data storage. The disadvantage of the superior postacquisition method is the observational technique (e.g., microscopic experiments) along with the large amounts of concomitant mass data storage that are required. These disadvantages can be anticipated to diminish rapidly in the future with the appearance of more advanced and less costly computer hardware. For a further discussion of the mathematical basis of various stabilization procedures see Smedskjaer et
-
-
2.6. Some Final Remarks It is appropriate for us to end this part by outlining some of the areas where the positron technique requires extra care while remembering that the unique nature of the technique and its general prowess in the defect area make it a useful tool in metallurgical studies. Care must always be taken in positron experiments to ensure that the defect population being studied is indeed the dominant defect in the system with respect to positron trapping. At a low defect concentration, lo4, which is the sensitivity limit of the positron technique for vacancy point defects, extreme care is required. It is under conditions such as this that low concentrations of additional defects (dislocations for example) have been shown to have significant or even dominant effects on the resulting data.
-
144
2.
POSITRON ANNIHILATION
When one is dealing with a complex population of defects, vacancies, vacancy clusters, voids, dislocations, etc., questions will necessarily arise concerning the way in which the positron is partitioned among such a population. The partitioning of the positron among defect types present simultaneously in a system is not fully understood, and the interpretation of such data usually has a very qualitative character. Clear improvements are called for in the identification of parameters specific to different defects that can be derived from the experimental data. The number of positron states that can now be resolved is no more than 2 or 3, depending on how different the signals from these states are. The best results in separatingsuch states are usually obtained in lifetime experiments. To some degree one is always faced with contributions from positron annihilation events that do not take place in the part of the sample one desires to study. In experiments where bulk properties of the sample are to be investigated, the unwanted contribution to the observed data comes from annihilations in either the source or its supporting structures and/or those positrons that reach and eventually annihilate at or near the surface, or for that matter any place that can be viewed by the detection system. Currently, the best way to minimize such effects is the use of internal sources, produced by either ion implantation or activation techniques. The positron technique has now gone through an initial period of growth with respect to its application to the study of defects in metals. This period has been successfulin that increasing use is being made of the technique, not just by specialists, but by a broad group of material scientists. The technique is now entering a new stage of development where new methodologies are being explored that will uncover those properties of the annihilation signal that might be considered as defect specific. At the same time new information is being uncovered with respect to the trapping mechanism itself, particularly in traps where the binding energjes of the positron are small. The use of positron annihilation to measure vacancy formation enthalpies is well developed and is currently believed to be limited only by the variations in sample properties themselves: impurities and nonequilibrium defect concentrations arising from them. On the instrumentation side, one can identify three general areas which hold special promise of yielding improvements in the amount and details of the data: 1. The use of large-area position-sensitive detectors operating in coincidence in ACAR experiments clearly increases data rates as well as providing considerably more flexibility in the analysis of momentum data. It is hoped that combining Doppler-broadening experiments with detailed ACAR work will lead to an efficient identification of interesting metal defects studies.
2.6.
SOME FINAL REMARKS
145
2. Continued improvements are being made in lifetime techniques. The use of large-volume plastic scintillators is particularly noteworthy. Combined simultaneous lifetime and momentum (Doppler broadening) measurements are much more tractable, and these combined experiments yield an extra dimension of sensitivity and possible specificity. Earlier work has demonstrated that these two types of experiments are often invaluable together and provide additional insight into the underlying physical principles themselves. 3. The use of slow beams for positron sources opens the door to a whole new class of positron experiments, not only on surfaces and interfaces but also in thin foils; the latter are amenable to many other types of materials studies, from electron microscopy to resistivity, and are particularly valuable in nonequilibrium experiments such as radiation damage and quenching- annealing studies. The techniques for analyzing experimental data are improving continually. Much more care is being taken in dealing with the statistical properties of the data and in applying stabilization methods. The relation between the physical models and the experimental uncertainties is appreciated, and more accurate results are being obtained by understanding this relation and recognizing the various limitations of the instrumental techniques.