The influence of geometry on the resolution of a positron annihilation lifetime spectrometer

A detailed study of the prompt resolution curve of a positron annihilation lifetime spectrometer shows that the geometry of the source-detector set-up has great influence on its shape.     

The differential method for the study of small variations of positron lifetime spectra

A new method for the study of small perturbations of positron annihilation characteristics is proposed. The method is based on the direct comparison of two lifetime spectra, leading to the determination of the mean life variation and of the difference spectrum. The combined knowledge of and of a shape parameter of the difference spectrum is often sufficient for the complete quantitative and qualitative characterization of the observed spectral modifications. Specific examples are discussed.Work jointly supported by the Gruppo Nazionale di Struttura della Materia del Consiglio Nazionale delle Ricerche, and by the Centre di Ricerche FIAT, Orbassano (Torino)In partial fulfilment of the requirements for the degree of Dottore in Ingegneria Nucleare at the Politecnico di Milano     

A compact positron annihilation lifetime spectrometer

Using LYSO scintillator coupled on HAMAMATSU R9800 (a fast photomultiplier) to form the small size γ-ray detectors, a compact lifetime spectrometer has been built for the positron annihilation experiments. The system time resolution FWHM=193 ps and the coincidence counting rate ～8 cps/μCi were achieved. A lifetime value of 219±1 ps of positron annihilation in well annealed Si was tested, which is in agreement with the typical values published in the previous lectures.     

A compact positron annihilation lifetime spectrometer

Using LYSO scintillator coupled on HAMAMATSU R9800(a fast photomultiplier)to form the small size γ-ray detectors,a compact lifetime spectrometer has been built for the positron annihilation experiments.The system time resolution FWHM=193 ps and the coincidence counting rate -8 cps/μCi were achieved.A lifetime value of 219±1 ps of positron annihilation in well annealed Si was tested,which is in agreement with the typical values published in the previous lectures.     

The accurate measurement of weak three-photon annihilation by a simple triple coincidence technique

The method of triple coincidence is optimized for the detection of the orthopositronium decay. Two positron sources 86-Ge and 22-Na, are compared. The influence of sandwich thickness is described and a method for the correction for those effects is given. This work is supported by the International Atomic Energy Agency, Vienna, through research contract 2516/R.B.     

The influence of several parameters on the lifetimes and intensities of positron lifetime spectra of metals

The influence of the instrumental time-resolution function and the source-surface spectrum on the lifetimes and intensities of positron lifetime spectra is studied by a computer simulation of such spectra. The prompt curve is described not by a sum of Gaussian curves but by the convolution of a Gaussian curve with a double-sided exponential. It is shown that the fitted lifetimes and intensities considerably depend on these parameters.     

A new positron source for positron annihilation lifetime experiments

A method using ion-implantation of Na²² to produce source-sample systems applicable to positron annihilation lifetime experiments at elevated temperatures in metals is presented.     

Source effects on positron annihilation in copper

It is shown that in sample-source sandwich arrangements the fraction of positrons annihilating in the source material itself contributes mainly to the source term which is also sensitive to the thermal treatment. The diffusion of the direct deposited source produces some type of surface defects which act as positron trapping sites. Work supported by the National Research Council of Canada, the Department of Energy, Mines and Resources and le Conseil de Recherche, Université de Moncton.     

Temperature dependence of positron annihilation in iron and vanadium

The Doppler broadened lineshape is measured as a function of temperature for iron and vanadium; vacancies produce a strong trapping effect in γ-Fe, but a weak effect in V. Threshold temperaturesT _t for trapping are 1280±25 and 1370±30 K. Empirical linear relationships betweenT _t , self-diffusion energyQ ^SD and monovacancy formation energyH _1v ^F are discussed and used to determineH _1v ^F values for γ-Fe and V.     

Temperature dependence of positron annihilation parameters in solid gallium

Positron annihilation measurements between 80 and 300 K indicate that in gallium positrons are not trapped in thermally induced vacancies. Also no clear evidence is obtained for self-trapping of positrons in gallium. The slight temperature dependence of the lineshape measurements is probably due to thermal expansion.     

A two-parameter measurement of the correlation of positron age with the momentum of the annihilating positron-electron pair

The momentum spectrum of annihilating positron-electron pairs is normally analysed in terms of two or more momentum groups. If the medium is heterogeneous, or if it permits the formation of positronium, the annihilation time spectrum also becomes complex with as many as four resolvable exponential components. The interrelationship of these groups is difficult to establish because the usual measurements of time and momentum spectra are done with instruments of low absolute efficiency. We have made simultaneous measurements of time and energy spectra using a Ge(Li) spectrometer as a momentum analyser, obtaining an approximate hundredfold increase in data output at a cost of a factor of 3 in momentum resolution. The momentum spectrum of the intermediate lifetime component in a plastic scintillator is shown to be very similar to that of the long-lived orthopositronium. Recent developments in fast timing and in high purity Ge detectors should improve both the quality and quantity of data from these techniques, making them feasible for routine studies of polymers, chemical solutions and metallic voids.     

Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders

Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 25° and 600 °C. Three positron lifetimes were found in these systems: a short-lived component (0.2 ns) due to positrons in the bulk; a component (0.45 ns) due to surface-trapped positrons; and a long-lived component (2 ns) ofo-Ps in the voids or the interfacial spaces of powders. Both bulk and surface positron lifetimes increase as a function of temperature. Correlations between the intensity of surface-trapped positrons and the surface area and between Ps formation and the surface area of graphite are found. The Ps formation probability increases as a function of temperature. A thermal desorption model interprets the emission process of Ps atoms from the surface of graphite to the vacuum and gives an activation energy of 0.23±0.02 eV.Preliminary results of this paper were presented at the March Meeting of American Physical Society, Los Angeles. Bull. Am. Phys. Soc.28, 347 (1983)     

A positron annihilation study of carbon black and carbon-black-filled polybutadiene

Studies have been made on positron lifetimes in carbon blacks and carbon black filled polybutadiene. The results for the carbon blacks can be interpreted with the aid of the theoretical results of Brandt and Paulin: Phys. Rev. B5, 2430 (1972) and show that while positron annihilation occurs principally from a bound state, diffusion in the carbon black is extremely small. A numerical upper limit on the diffusion coefficient has been obtained. The data from the rubber is discussed with the aid of a specially developed extension of the previous theory. Here one finds that whereas the variations in the long-component intensity with carbon black loading can be very satisfactorily explained, there is again no evidence for any diffusion effects in the rubber, and an upper limit can be placed on the diffusion coefficient here also. Indeed the variation in the long-component intensity with particle size goes the opposite way to what one would expect if diffusion were dominant. This effect is attributed to the increase in the amount of bound rubber as the particle size is reduced, and the consequent increase in the number of annihilation sites.     

Studies of martensitic transformation in Cu-Al alloys by positron annihilation

The reverse martensitic transformations in Cu-23.5at-%Al, and Cu-25.3at-%Al have been studied by means of positron annihilation. The coincidence counting rates of angular correlation were measured as a function of the specimen temperature. The change of counting rates in heating run was rather different from that in cooling run due to the influence of tempering of martensitic structure. The results were interpreted by the change of the formation energy of a vacancy with phase transition. Influence of heating rate is also discussed.     

A new positron annihilation lifetime spectrometer based on DRS4 waveform digitizing board

Abstract： A new simple digital positron lifetime spectrometer has been developed. It includes a DRS4 waveform digitizing board and two scintillation detectors based on the XP2020Q photomultiplier tubes and LaBr3 scintillators. The DRS4 waveform digitizing can handle small pulses, down to few tens of millivolts, and its time scale linearity and stability are very good. The new system has reached a 206 ps time resolution, which is better than the conventional analog apparatus using the same detectors. These improvements make this spectrometer more simple and convenient in comparison with other spectrometers, and it can be applied to the other scintillation timing measurements with picosecond accuracy.     

A new positron annihilation lifetime spectrometer based on DRS4 waveform digitizing board

A new simple digital positron lifetime spectrometer has been developed. It includes a DRS4 waveform digitizing board and two scintillation detectors based on the XP2020Q photomultiplier tubes and LaBr₃ scintillators. The DRS4 waveform digitizing can handle small pulses, down to few tens of millivolts, and its time scale linearity and stability are very good. The new system has reached a 206 ps time resolution, which is better than the conventional analog apparatus using the same detectors. These improvements make this spectrometer more simple and convenient in comparison with other spectrometers, and it can be applied to the other scintillation timing measurements with picosecond accuracy.     

The use of running integrated difference curves for analysing momentum distributions from positron annihilation experiments

In positron annihilation measurements it is often necessary to compare annihilation line shapes or angular distributions obtained from a single sample subjected to different experimental conditions, or from a range of samples subjected to different mechanical or metallurgical treatments. The difference of the normalised, distributions provides a convenient, easily visualised, and comprehensive summary of the experimental results. Changes in the shape of these difference curves can be shown to arise only when within the material under examination competition occurs between different types of positron trap. Some of the factors affecting the shape of these difference curves are discussed in terms of trapping model concepts. By using the running integrals of the difference curves (RID curves) a representation is obtained which is very little affected by the number of channels spanning the line, provided that there are enough, leads naturally to an optimised determination of the line shape parameterS, leads to the definition of a number of parameters related to the shape of the difference curve and has well defined statistical properties. Examples of RID curves show the range of shapes encountered, shape changes associated with competition between different types of positron trap, and artefact shapes associated with rate dependent effects in the detector and electronics.     

Vacancies and impurities in InP studied using positron lifetime and an improved Doppler-broadening spectrometer

In ^- in Zn-doped p-InP. The line-shape parameter measured by an improved Doppler-broadening spectrometer shows a significant difference between n-type and Fe-doped SI-InP materials: the S-parameter varies from 0.5203 in n-InP to 0.5184 in Fe-doped SI-InP by four times the standard deviation. A possible reason for this difference is discussed. Received: 7 January 1997/Accepted: 8 September 1997     

Temperature dependence of positron annihilation parameters in neutron irradiated molybdenum

Positron lifetime measurements have been performed for molybdenum samples containing different densities of voids and dislocation loops. The samples consisted of single crystal molybdenum exposed to 2.7×10¹⁸ fast neutrons/cm₂ at 60°C, and subsequently annealed at 650°, 725°, 800°, and 875°C in vacuum (p<10^–7 Torr). After each annealing, where the densities of voids and loops were changed, positron lifetime measurements were performed in the temperature interval [–194°, 285°C]. In two-term fits of the measured spectra the longer lifetime, _e2-460 ps corresponds to an intensityI _e2 increasing with sample temperature. The shorter lifetime _e1 decreases with increasing temperature. A three-state trapping model with and without detrapping is discussed, and appears to be incapable of explaining the observed temperature dependences. A four-state positron trapping model including detrapping is necessary and satisfactory. It describes positron trapping to voids and trapping to dislocation loops, which is followed by a competition between detrapping and positron transition to jogs or other dislocation-bound defects. Mathematical expressions of the four-state trapping model including detrapping are worked out and calculations of the intensityI _e2 are compared with the experimental values ofI _e2. By use of special models for the temperature dependence of trapping rates, numerical values can be determined for the positron-dislocation-binding energy and for specific positron trapping rates.