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First Positron Annihilation Lifetime Measurement Of Pu
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Book Synopsis First Positron Annihilation Lifetime Measurement of Pu by :
Download or read book First Positron Annihilation Lifetime Measurement of Pu written by and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: We have made the first measurement of defects in an aged sample of[delta] phase, Ga stabilized Pu, using positron annihilation lifetime spectroscopy. This measurement validates the procedure necessary to perform measurements on this highly toxic material and obtain data representative of sample conditions. Comparison of the positron annihilation lifetime analysis of the data with calculated values suggests that He filled vacancies or vacancy clusters dominate the defect population. Such defects are the necessary precursor to void growth and swelling. The evolution of defects resulting from the radioactive decay of Pu during its life in the stockpile is one of the unknown quantities affecting our confidence in predictions of the limit on stockpile components. Radiation damage leads to changes in the size and strength of metals studied for reactor and accelerator use and similar effects may be expected in Pu. The evolution of radiation produced vacancies into larger void structures and accompanying macroscopic swelling may occur in Pu at some age. A detailed understanding of the defects in self irradiated Pu is required to predict the time scale of void swelling and related radiation effects. 1 fig.
Book Synopsis Construction and Testing of a Positron Annihilation Lifetime Spectrometer by : Brian M. Wieland
Download or read book Construction and Testing of a Positron Annihilation Lifetime Spectrometer written by Brian M. Wieland and published by . This book was released on 2012 with total page 180 pages. Available in PDF, EPUB and Kindle. Book excerpt: Positron Annihilation Lifetime Spectroscopy (PALS) is a nuclear technique that was developed in the 1970s as a method to assess defects in materials. This technique has tremendous potential as a powerful tool for non-destructive quantification of the types and densities of defects within solids. Following the injection of positrons into a solid material, the positrons pair-annihilate at a rate dependent upon the density of electrons near the injection site. If there are lattice vacancies or dislocation defects near the injection site, the positrons are attracted to these areas, which have lower electron densities and thus give rise to longer positron lifetimes. Typical positron lifetimes in defect-free conductor-type metals are -'-'1OO ps, while lifetimes within defect regions are 2OO ps. In order to discern between these minute lifetime differences, a spectrometer with very good timing resolution has been constructed and optimized. The capabilities of the spectrometer have been tested using a 22Na positron emitter by performing source-based measurements of positron lifetimes in high-purity samples of aluminum, nickel, copper, and lead. Test results are in excellent agreement with currently accepted literature values, thus validating the apparatus and data analysis technique. This study is an important first step towards the development of a modern positron annihilation lifetime spectrometer for use in accelerator-based, gamma-induced positron annihilation spectroscopy (AG-PAS).
Book Synopsis Ageing Studies and Lifetime Extension of Materials by : Les Mallinson
Download or read book Ageing Studies and Lifetime Extension of Materials written by Les Mallinson and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 659 pages. Available in PDF, EPUB and Kindle. Book excerpt: The first International Conference on Ageing Studies and Lifetime Extension of Materials was held on th July 12-14 , 1999 at St. Catherine's College, Oxford, United Kingdom. Over 230 delegates attended during the three days and heard nearly ninety papers, together with over thirty poster presentations. Sixteen of these papers were keynotes from invited speakers eminent in their field of research. The proceedings were organised into six separate sessions: observation and understanding of real-time and accelerated ageing; experimental techniques; modelling and theoretical studies; lifetime prediction and validation; lifetime extension; and material design for ageing. In doing this, it was hoped to cover most issues of scientific concern inthefield ofmaterials ageing. One important aspect was that the conference did not concentrateon any particular group or type ofmaterial; rather the aim was to attract contributions from workers engaged in ageing studies with as wide a range of materials as possible. In this way, it was hoped that delegates could interactwith and learnfrom those whom they perhapswould not normally come across and that metallurgists could learn from polymer scientists, ceramicists could talk to modellers, and so on, in this important field. A read through the diverse papers contained within these proceedings will confirm that this aim was happily satisfied. Why hold such a meeting? In the modem world, engineered systems are expected to last longer.
Book Synopsis Theory of Positron Annihilation in Helium-Filled Bubbles in Plutonium by : J. E. Pask
Download or read book Theory of Positron Annihilation in Helium-Filled Bubbles in Plutonium written by J. E. Pask and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Positron annihilation lifetime spectroscopy is a sensitive probe of vacancies and voids in materials. This non-destructive measurement technique can identify the presence of specific defects in materials at the part-per-million level. Recent experiments by Asoka-Kumar et al. have identified two lifetime components in aged plutonium samples--a dominant lifetime component of around 182 ps and a longer lifetime component of around 350-400ps. This second component appears to increase with the age of the sample, and accounts for only about 5 percent of the total intensity in 35 year-old plutonium samples. First-principles calculations of positron lifetimes are now used extensively to guide the interpretation of positron lifetime data. At Livermore, we have developed a first-principles finite-element-based method for calculating positron lifetimes for defects in metals. This method is capable of treating system cell sizes of several thousand atoms, allowing us to model defects in plutonium ranging in size from a mono-vacancy to helium-filled bubbles of over 1 nm in diameter. In order to identify the defects that account for the observed lifetime values, we have performed positron lifetime calculations for a set of vacancies, vacancy clusters, and helium-filled vacancy clusters in delta-plutonium. The calculations produced values of 143ps for defect-free delta-Pu and 255ps for a mono-vacancy in Pu, both of which are inconsistent with the dominant experimental lifetime component of 182ps. Larger vacancy clusters have even longer lifetimes. The observed positron lifetime is significantly shorter than the calculated lifetimes for mono-vacancies and larger vacancy clusters, indicating that open vacancy clusters are not the dominant defect in the aged plutonium samples. When helium atoms are introduced into the vacancy cluster, the positron lifetime is reduced due to the increased density of electrons available for annihilation. For a mono-vacancy in Pu containing one helium atom, the calculated lifetime is 190 ps, while a di-vacancy containing two helium atoms has a positron lifetime of 205 ps. In general, increasing the helium density in a vacancy cluster or He-filled bubble reduces the positron lifetime, so that the same lifetime value can arise fi-om a range of vacancy cluster sizes with different helium densities. In order to understand the variation of positron lifetime with vacancy cluster size and helium density in the defect, we have performed over 60 positron lifetime calculations with vacancy cluster sizes ranging from 1 to 55 vacancies and helium densities ranging fi-om zero to five helium atoms per vacancy. The results indicate that the experimental lifetime of 182 ps is consistent with the theoretical value of 190 ps for a mono-vacancy with a single helium atom, but that slightly better agreement is obtained for larger clusters of 6 or more vacancies containing 2-3 helium atoms per vacancy. For larger vacancy clusters with diameters of about 3-5 nm or more, the annihilation with helium electrons dominates the positron annihilation rate; the observed lifetime of 180ps is then consistent with a helium concentration in the range of 3 to 3.5 Hehacancy, setting an upper bound on the helium concentration in the vacancy clusters. In practice, the single lifetime component is most probably associated with a family of helium-filled bubbles rather than with a specific unique defect size. The longer 350-400ps lifetime component is consistent with a relatively narrow range of defect sizes and He concentration. At zero He concentration, the lifetime values are matched by small vacancy clusters containing 6-12 vacancies. With increasing vacancy cluster size, a small amount of He is required to keep the lifetime in the 350-400 ps range, until the value saturates for larger helium bubbles of more than 50 vacancies (bubble diameter> 1.3 nm) at a helium concentration close to 1 He/vacancy. These results, taken together with the experimental data, indicate that the features observed in TEM data by Schwartz et al are not voids, but are in fact helium-filled bubbles with a helium pressure of around 2-3 helium atoms per vacancy, depending on the bubble size. This is consistent with the conclusions of recently developed models of He-bubble growth in aged plutonium.
Book Synopsis Positron Annihilation by : A.T. Steward
Download or read book Positron Annihilation written by A.T. Steward and published by Elsevier. This book was released on 2012-12-02 with total page 455 pages. Available in PDF, EPUB and Kindle. Book excerpt: Positron Annihilation focuses on the process of positron annihilation in different environments. Partitioned into two parts with 42 chapters, the book contains the contributions of authors who have done research on the annihilation of positrons, which brought about valuable information on the properties of matter. The first part of the book deals with lengthy review articles, including a survey of the physics of positron annihilation; positron annihilation in metals and the theory involved in the process; and positron annihilation in alkali halides and ionic crystals. Positronium formation and interaction in gases, molecular substances, and ionic crystals are also given attention. Gaseous positronics and positron annihilation in condensed gases and liquids are also discussed. The second part of the book focuses on developments on positron annihilation and the direction of research on this field. The studies concentrate on positron annihilation in various crystals, metals, mercury, liquefied gases, helium, and metal oxides. Numerical representations and analyses are presented to support the processes involved. The book can best serve the interest of those who want to explore further the annihilation of positrons.
Book Synopsis Design and Implementation of a Digital Positron Annihilation Lifetime Spectrometer for Measurements in Graphite by : Ethan Wesley Bodnaruk
Download or read book Design and Implementation of a Digital Positron Annihilation Lifetime Spectrometer for Measurements in Graphite written by Ethan Wesley Bodnaruk and published by . This book was released on 2008 with total page 134 pages. Available in PDF, EPUB and Kindle. Book excerpt: Keywords: positron annihilation, coincidence timing, digital spectrometer.
Book Synopsis Positron Lifetime Measurement in Solids by : Xuejun Zhang
Download or read book Positron Lifetime Measurement in Solids written by Xuejun Zhang and published by . This book was released on 1994 with total page 100 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Nuclear Science Abstracts written by and published by . This book was released on 1976 with total page 1026 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Positron Annihilation by : Alec Thompson Stewart
Download or read book Positron Annihilation written by Alec Thompson Stewart and published by . This book was released on 1967 with total page 464 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Nuclear Science Abstracts written by and published by . This book was released on 1974 with total page 1036 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Positron Annihilation by : L. Dorikens-Vanpraet
Download or read book Positron Annihilation written by L. Dorikens-Vanpraet and published by . This book was released on 1989 with total page 1038 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Evolution of Defects in Pu During Isochronal Annealing and Self Irradiation by : R. Howell
Download or read book Evolution of Defects in Pu During Isochronal Annealing and Self Irradiation written by R. Howell and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: We report on the evolution of defects in Pu during isochronal annealing and self irradiation using positron annihilation spectroscopy. Positron annihilation spectroscopy is a sensitive probe (part per million level) for atomic-scale defects. The spectroscopic tools available at LLNL allow the determination of size, concentration, and chemical surroundings of defects in aged Pu samples. Positron lifetime analysis was performed on eight samples aged 7 months to 42 years. All samples except the 7-month old sample contained a high concentration of positron trapping centers. The dominant component yielded a lifetime value of {approx}182 ps. In aged samples, a second longer lifetime component was observed that appears to increase in strength with the age of the sample. The observed lifetime values and their relative strengths are shown in figure 1. The top panel corresponds to the lifetime values and the bottom panel corresponds to the intensity of the long lifetime component. Positron lifetime values are determined by the bubble size and He content. When He is added to a bubble, the positron lifetime is shortened due to the increased electron density. When the size of the bubble is known from an independent measurement, the observed positron lifetime values and the associated first principle calculations can be used to estimate the He content of the bubble. Our positron results can be combined with the recent transmission electron microscopy measurements of Schwartz et al to estimate the He content of the bubbles. The short lifetime component corresponds to 2-3 He atoms per vacancy. A 35-year old sample was annealed to 440 C in several steps to examine the temperature stability of the two lifetime features. The isochronal anneals were performed for 30 minutes, increasing the temperature with each subsequent step. This measurement complements a previous annealing study on recast, 7-month old material. The primary objective of these studies is to establish the appropriate limits on the storage temperature of the accelerated aging samples. Figure 2 shows the intensity of the long lifetime component as a function of the annealing temperature. Also shown is the corresponding He evolution from the capsule measured using a standard He leak detector. The intensity of the long lifetime component starts to increase at about 150 C and reaches a maximum at around 250 C.A similar behavior is seen in He evolution. The storage temperature of the accelerated aging sample should be below the onset of these annealing effects. We also report results from a new spectroscopic method that examines chemical surroundings of the bubbles observed in aged Pu material. The new spectroscopy determines the momentum distribution of the electrons participating in the annihilation process. At high momentum values these distributions are dominated by electrons bound to atoms and is therefore element specific. No significant variations are observed in aged samples, suggesting that the chemical surroundings of voids and bubbles are not evolving with age.
Book Synopsis On the Effect of Backscattering of [gamma] Quanta and Statistics in Positron-annihilation Lifetime Measurements by : Steen Dannefær
Download or read book On the Effect of Backscattering of [gamma] Quanta and Statistics in Positron-annihilation Lifetime Measurements written by Steen Dannefær and published by . This book was released on 1981 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Positron Annihilation written by and published by . This book was released on 1979 with total page 1008 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Positron Annihilation as a Probe of Polymer Thin Films, Surfaces, and Interfaces by : Gregory Bernard DeMaggio
Download or read book Positron Annihilation as a Probe of Polymer Thin Films, Surfaces, and Interfaces written by Gregory Bernard DeMaggio and published by . This book was released on 1997 with total page 294 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Positron Annihilation in Chemistry by : Ole E. Mogensen
Download or read book Positron Annihilation in Chemistry written by Ole E. Mogensen and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 279 pages. Available in PDF, EPUB and Kindle. Book excerpt: The only critical discussion available on the chemistry of the two "strange" light particles, the positron and positronium, with much space devoted to the excess electron. Positron annihilation allows the investigation of many unusual phenomena in the reaction kinetics of the positron, positronium, and excess electron, and in radiation chemistry and physics, while also providing important information on defects in solids.
Book Synopsis Plutonium Futures -- The Science by : Gordon D. Jarvinen
Download or read book Plutonium Futures -- The Science written by Gordon D. Jarvinen and published by American Institute of Physics. This book was released on 2003-07-09 with total page 448 pages. Available in PDF, EPUB and Kindle. Book excerpt: Over 1,000 tons of plutonium exist throughout the world in the form of nuclear fuel, nuclear weapons, inventories of various types, and legacy materials. Regardless of ones views of how this condition came to be, or what actions are taken in the future, it is a certainty that large quantities of plutonium must be managed for many decades. Thus it is clear that the plutonium challenge is not only scientific, but political and socioeconomic as well. In view of the global nature of plutonium research efforts, the Plutonium Futures - The Science conference was established to increase awareness of the importance of the scientific underpinnings of plutonium research, and facilitate communication among its international practitioners. The 2003 conference is the third in this series; the two previous conferences were held in Santa Fe in 1997 and 2000. Extended abstracts of the presentations cover the latest results in plutonium condensed matter physics, materials science, compounds and complexes, environmental behavior, detection and analysis, separations and purification, nuclear fuel cycles, and waste isolation and disposal.
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