A review of positron studies of the annealing of the cold worked state
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measurement of positron lifetime in a solid permits one t o deduce something about the s t a t e of imperfection of the solid#-4 Crystalline defects s e r v e as trapping sites for positrons and such trapped positrons survive on the a v e r a g e longer than an untrapped positron which annihilates with an electron in a more perfect r e g i o n of the lattice. By measuring the a v e r age positron lifetime, then, one may follow quite s e n s i tively increases or d e c r e a s e s in the s t a t e of perfection of a material. Similarly, by measuring the Doppler broadening of the e n e r g i e s of g a m m a r a y s emitted with the annihilation events, one can ascertain the s t a t e of crystalline perfection.5 This is possible bec a u s e defect trap sites are depleted of higher energy core electrons therefore a trapped positron is more likely to annihilate with a lower energy conduction electron. This in turn causes a narrowing of the energy distribution about the v a l u e of 511 keV, the g a m m a ray e n e r g y if the electron-positron center of mass were stationary. Since the positron is essentially thermalized by the time annihilation occurs, one can consider the Doppler energy shift from the 511 keV value t o be only due t o the e n e r g y of the electron involved in the annihilation. EXPERIMENTAL PROCEDURES The Doppler broadening measurements were perf o r m e d with a Ge (Li) detector, multichannel analyzer and digital stabilizer. The system was adjusted t o have a resolution of 1.24 keV full width half m a x i mum at the total count rate of 14 kHz which was used. Complete details of the system are presented e l s e where.6 Changes in the spectrum of the annihilation photon energies were described by a shape factor (P/W) which represents the sum of counts in a peak J. G. BYRNE is Professor ofMaterials Science and Engineering, Department of Materials Science and Engineering, University ofUtah, Salt Lake City, UT 84112. This paper is based on a presentation made at a symposium on "Recovery Recrystallization and Grain Growth in Materials" held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee. METALLURGICAL TRANSACTIONSA
region divided by the total counts in two wing regions. The exact, locations used in the spectrum to delineate the peak and wings regions were determined for each material by an analysis which maximized differences with a standard sample.6 Both Ge68 and Na22CI positron sources were utilized. The positron lifetime measurements were at first performed on a slow-slow coincidence system7 and more recently on a fast-fast coincidence system designed by Halls and reproduced at the University of Utah by Lynn9 and Hadnagy. I° Complete details may be found in the latter reference. Nae2C1 served as the positron source for all lifetime measurements. The data were analyzed using a modifiedIx version of a program called Positronfit whichis capable of fitting as many as three positron lifetimes and intensities. T
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