Positron Lifetime Measurements Research Articles

Digital positron lifetime spectroscopy: present status and outlook

AbstractThis contribution summarizes achievements in instrumentation for positron lifetime measurements with emphasis on digital spectrometric systems. A significant part of the data presented are based on a long‐term exploitation of a conventional positron lifetime spectrometer developed at Charles University in early 90s, on bench‐mark testing measurements with this spectrometer working temporarily in conjunction with a pair of 8‐bit, 4 GS/s digitizers and on analogous measurements with a recently assembled digital positron lifetime spectrometer. In addition, results from testing experiments with microchannel plate photomultipliers are briefly reported. Further development of positron lifetime technique is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Digital positron lifetime spectrometer for measurements of radioactive materials

AbstractA digital positron lifetime spectrometer for the measurements of 2 GBq level radioactive materials has been constructed and successfully tested. The equipment consists of a state‐of‐the‐art digital oscilloscope and three movable scintillation detectors operating in triple‐coincidence mode. The time resolution of 188 ps full‐width at half‐maximum (FWHM) is achieved for the positron lifetime measurements of radioactive specimens. Also by comparison of measurements of well‐known material with and without high 60Co background radiation the ability to measure radioactive specimens is proved. In this paper the performance, construction, installation, optimization and calibration of the setup is reported. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Implementation of the Munich scanning positron microscope at the positron source NEPOMUC

AbstractThe Munich scanning positron microscope (Spm) permits positron lifetime measurements with a lateral resolution down to 2 µm within an energy range of 1‐20 keV. One practical limitation of the Spm is set by the long measurement times of several days per a 2D‐scan due to the low intensity positron beam produced by standard 22Na sources. This disadvantage will be overcome by installing the Spm at the high intense positron beam facility Nepomuc at the research reactor FRM II in Garching. Thus the time for one measurement will be shortened by a factor of 60. In addition it is expected to reduce the lateral resolution to about 100 nm. Due to the beam characteristics of the Nepomuc facility an interface is needed, which enhances the phase space density of the beam. The requirements, which have to be fulfilled by the interface, will be described and an overview of the different components such as bunching units, remoderation stages and rf‐elevator will be given. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation spectroscopy in the study of quasicrystals and their related materials

AbstractRecent advances in the study of quasicrystals (QC's) and their related materials (crystalline approximants) by using positron annihilation spectroscopy are reviewed. We describe how a series of positron annihilation spectroscopy has been applied for the studies of aperiodic QC's. Positron lifetime measurements revealed the presence of high concentration of structural vacancies giving rise to saturation trapping for QC's and approximants studied. Chemical environments around the structural vacancies were successfully clarified by coincident Doppler broadening measurements. In addition, the concentrations of structural vacancies were estimated by analyzing the positron diffusion data obtained by a slow positron beam under the saturation trapping condition. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation in three zirconia polymorphs

AbstractPositron lifetimes and high momentum profiles both for the perfect lattice and selected defects are calculated in three (cubic, tetragonal and monoclinic) zirconia polymorphs using the atomic superposition method. Theoretical data are compared with the measured positron lifetime for cubic and tetragonal monocrystals of yttria‐stabilized zirconia (YSZ) and coincidence Doppler broadening measurements on tetragonal monocrystals of YSZ. Positron lifetime spectra of YSZ monocrystals exhibit a single component spectrum with lifetimes 178 ps and 174 ps for cubic and tetragonal phases, respectively. Possible interpretations of measured lifetime and Doppler data are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High moderation efficiency positron beamline

AbstractA new positron beam system, which has high moderation efficiency and can measure positron lifetime spectra, has been constructed at Wuhan university. A solid neon moderator with about 1% moderation effeciency is used. Secondary electron time‐tagging can be used to measure the positron annihilation lifetime. A target chamber has been designed for positron annihilation lifetime spectroscopy (PALS), Doppler broadening (DB) and coincidence Doppler broadening (CDB). The preliminary beam parameters are: a slow positron flux of 5 million per second for 40 mCi 22Na radioactive isotope, a positron energy spread as narrow as 2.0eV, and a moderator decay rate of 4% per day. Variable energy Doppler broadening annihilation radiation data for stainless steel samples are shown, which demonstrate the capabilities of the system. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation spectroscopic studies of fluorinated ethylene propylene copolymer-g-polystyrene

Polystyrene was grafted onto FEP by simultaneous irradiation using a 60Co γ-irradiator. Graft copolymers of different degree of grafting were obtained by varying the irradiation dose. Positron lifetime measurements were carried out on FEP and the grafted samples. A systematic decrease in o-Ps lifetime and intensity with grafting is seen indicating a decrease in pore size as well as total free volume fraction. Temperature dependent (95–250K) S-parameter measurements in FEP showed saturation of free volume fraction at temperatures corresponding to the β-relaxation, the onset of which is seen at 150K. Coincidence Doppler broadening results indicate increase in specific volume in the crystalline region of the polymer. Dynamic mechanical analysis (DMA) showed that tanδ value for β-relaxation of the samples decreased with increasing graft percentage. The results are discussed.

Investigation of vacancy defect in InP crystal by positron lifetime measurement

Positron lifetime measurements have been carried out on liquid-encapsulated Czochralski-grown undoped InP samples sliced from the middle part of ingots over the temperature range 10–300 K. And at 70 K, the spectra have been measured in darkness, under illumination of infrared LED, and with illumination off is one sample. The measurements at low temperature reveal different concentration of hydrogen indium vacancy complex V InH 4 in these samples. A relatively higher concentration of V InH 4 in samples grown from P-rich undoped InP melts can be shown. The increase of resistivity of these samples can be speculated when temperature is low enough.

Identification of ultra-fine Ti-rich precipitates in V–Cr–Ti alloys irradiated below 300 °C by using positron CDB technique

Irradiation-induced Ti-rich precipitates in V–Ti and V–4Cr–4Ti alloys are studied by TEM and positron annihilation methods (positron lifetime, and coincidence Doppler broadening (CDB)). The characteristics of small defect clusters formed in V alloys containing Ti at irradiation temperatures below 300 °C have not been identified by TEM techniques. Strong interaction between vacancy and Ti solute atoms for irradiated V alloys containing Ti at irradiation temperatures from 220 to 350 °C are observed by positron lifetime measurement. The vacancy-multi Ti solute complexes in V-alloys containing Ti are definitely identified by using CDB measurement. It is suggested that ultra-fine Ti-rich precipitates or Ti segregation at periphery of dislocation loops are formed in V alloys containing Ti at irradiation temperatures below 300 °C.

Thermal evolution of boron irradiation induced defects in predoped Si revealed by positron annihilation experiments

The isochronal annealing behavior of high energy (25–72 MeV) boron ion irradiation induced defects in boron-doped silicon is monitored through measurements of positron lifetimes and three distinct defect-evolution stages are identified. The initial boron doping created a defect environment where positrons could sensitively annihilate with the boron electrons, suggesting boron-decorated Si monovacancies as potential trapping sites. The irradiation results in the dissolution of boron from these sites and positrons are then trapped by the empty divacancies of Si. Charge neutralization of divacancies through interaction with boron atoms leads to enhanced positron trapping in the initial stages of isochronal annealing. The divacancies start annealing above 673 K. However, a remarkable defect evolution stage due to the diffusion of the boron atoms beyond their initial depths of penetration is seen above 873 K and it leaves the sample with defects still present even at the highest annealing temperature 1273 K used in this work.

Investigation of Vacancy Defect in InP Crystal by Positron Lifetime

Positron lifetime measurements were carried out on liquid-encapsulated Czochralski-grown undoped InP samples sliced from middle of ingots over the temperature range of 10 ∼ 300 K. At 70 K, the spectra were measured in darkness, under illumination of infrared LED, and while illumination off respectively on one of samples. The measurements at low temperature revealed different concentration of hydrogen indium vacancy complex V In H 4 in these samples. A relatively higher concentration of V InH 4 existing in that grown from P-rich undoped InP melts could be shown. The increase of resistivity of these samples could be speculated when temperature was low enough.

Positron lifetime and residual gas analysis studies of [formula omitted] hydride system

The positron lifetime measurements were carried out to confirm the hydrogen interaction process with compound. It was found that positron lifetime of compound hydride increased in comparison to the parent compound and it is suggested that the interaction of hydrogen with compound obeyed the anionic model. The rate of hydrogen desorption for material MmNi 4.5 Al 0.5 increases with number of cycles. The residual gas analysis shows that hydrogen releases with all isotopes or satellites ( m 3 , m 4 and m 5 ) in third cycle of desorption at pressure 10 - 7 torr . The X-ray diffraction pattern shows that most of the peaks of a structure were found to be rhombohedral and resembled a bunsenite mineral structure. However, few peaks of material were satisfied by mixed phase of hexagonal and cubic with rhombohedral structure. The rhombohedral phase was the most suitable structure for hydrogen absorption and desorption process. The X-ray fluorescence of material shows the different peaks of misch metal ( Mm = Ce , Nd, Pr with background peaks) and Ni peaks but Al and La peaks do not appear. These peaks could not be detected by the detector as these peaks were found in the lower energy region of the spectrum.

Positron lifetime studies and coincidence Doppler broadening spectroscopy of Al–6Mg–xSc (x = 0 to 0.6 wt.%) alloy

Positron annihilation spectroscopy (PAS), comprising of both positron lifetime and coincidence Doppler broadening measurements, has been employed for studying the phase decomposition behaviour of scandium doped Al–6Mg alloys. Micro structural and age hardening studies have also been conducted to substantiate the explanation of the results of PAS. Samples with scandium concentration ranging from 0 to 0.6 wt.% have been studied. The measured positron lifetimes of undoped alloy reveal that GP zones are absent in the as-prepared Al–6Mg alloy. The observed positron lifetimes and the results of coincidence Doppler broadening measurements largely stem from the entrap of positrons at the interface between aluminium rich primary dendrites and the magnesium enriched interdendritic eutectic mixture of Mg5Al8 (β) and the primary solid solution of aluminium (α). The study also provides evidence of the formation of scandium vacancy complexes in Al–6Mg alloys doped with scandium upto a concentration of 0.2 wt.%. However such complex formation ceases to continue beyond 0.2 wt.% Sc; instead, the formation of fine coherent precipitates of Al3Sc is recorded in the as prepared alloy containing 0.6 wt.% scandium. The positron annihilation studies coupled with CDBS have also corroborated with the fact that the fine coherent precipitates of Al3Sc are formed upon annealing the Al–6Mg alloys doped with scandium of concentration 0.2 wt.% and above. Transmission electron microscopic studies have provided good evidence of precipitate formation in annealed Al–6Mg–Sc alloys. Elevated temperature annealing leads to dissociation of the scandium-vacancy complexes, thereby leading to the enhancement of the mobility of magnesium atoms. This has facilitated fresh nucleation and growth of Mg5Al8 precipitates in the above alloys at 673 K.

Methodology of positron lifetime spectroscopy: Present status and perspectives

Achievements in instrumentation for positron lifetime measurements are summarized with emphasis on digital spectrometric systems. The main part of the data presented are based on the long-term exploitation of a conventional positron lifetime spectrometer developed at Charles University in early 90s, on bench-mark testing measurements with this spectrometer working in conjunction with a pair of 8-bit ultra-fast digitizers and on analogous measurements with a new, recently assembled digital positron lifetime spectrometer. Perspectives of further improvements of positron lifetime instrumentation are discussed.

Identification of lattice vacancies in the B2-phase region of Ni–Al system by positron annihilation

The point defects of B2-type intermetallic compound Ni–Al have been investigated as a function of different Ni compositions ranging from 47 to 55 at.% by means of positron lifetime and coincidence Doppler-broadening (CDB) measurements. The position lifetime of the annealed samples, expressed by single-component lifetime of τ = 170–180 ps, indicates positron saturation trapping at single vacancies in Ni–Al samples. The CDB measurements have been applied to identify these vacancy-type defects formed on the Ni sublattice in the B2-phase Ni–Al alloys.

Vacancies on the Ti sublattice in titanium monoxideTiOystudied using positron annihilation techniques

The local atomic environment of lattice vacancies in titanium monoxide has been specifically studied by positron annihilation techniques in the composition range from ${\mathrm{TiO}}_{0.74}$ to ${\mathrm{TiO}}_{1.26}$. For nonstoichiometric titanium monoxide, a high concentration of vacancies has been suggested on both the titanium and the oxygen sublattice. From the analysis of the core electron momentum distribution of the atoms surrounding the vacancies in disordered as well as in ordered titanium monoxide, oxygen atoms are identified to form the local vacancy environment, indicating that positrons are trapped by titanium and not by oxygen vacancies. Positron lifetime measurements have shown that the decrease of the oxygen content, $y$, and the process of disordering in nonstoichiometric titanium monoxide ${\mathrm{TiO}}_{y}$, are accompanied by an increase of the valence electron density on the titanium vacancy.

Implantation profile of Na22 continuous energy spectrum positrons in silicon

The implantation profile of positrons emitted from a continuous energy spectrum source of Na22 in close proximity to a silicon target is modeled. The primary motivation is the use of positron lifetime spectroscopy to characterize layers of defects such as those created by ion irradiation, usually deemed accessible only to techniques which utilize slow positrons. The model combines the Makhov profile, used with considerable success to describe the profile of low energy (<30keV) monoenergetic positrons, with the well-established, universal β-decay energy spectrum. The success of this approach is verified by measuring the fractions of positrons absorbed in thinned silicon samples. This verification utilizes lifetime measurements performed on silicon in a bilayer sandwich configuration with copper as a backing. The model accounts for the uncertainty in the positron backscattering at the silicon∕copper interface. An optimal fit of the model to the experimental data requires that the parameter defining the mean depth of the Makhov profile (usually denoted r) is energy dependent. An example of application is provided in the form of a positron lifetime measurement of defects in silicon introduced by 1.5MeV proton irradiation. Excellent agreement is found between the lifetime data and those obtained using a slow positron technique.

Positron trapping kinetics in thermally generated vacancy donor complexes in highly As-doped silicon

We have measured positron lifetime and Doppler broadening in highly As-doped silicon containing thermally generated $V\text{\ensuremath{-}}{\mathrm{As}}_{3}$ defect complexes (vacancy is surrounded by three arsenic atoms). We observe positron detrapping from the $V\text{\ensuremath{-}}{\mathrm{As}}_{3}$ defect complex and determine the binding energy of $0.27\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ of a positron to the complex. The results explain why 85% of the thermal vacancies formed in highly As-doped Si at temperatures over $700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are invisible to positron measurements at elevated temperatures.

Investigation of molecular modes in K18 employing positron lifetime spectroscopy

Molecular modes executed by the liquid crystal forming compound n-p-cyano-p-hexylbiphenyl (K18) have been studied using positron lifetime spectroscopy (PLS). Positron lifetime measurements have been carried out during the heating cycle of samples prepared by (i) slow cooling from the isotropic phase and (ii) quenching from either the liquid crystalline phase or the isotropic phase of the compound. The material in the quenched samples, unlike the slow-cooled sample, exhibits strong temperature dependence before it undergoes a glass transition. The temperature dependence of o-Ps pick-off lifetime in the quenched samples shows broad peaks at various characteristic temperatures. These peaks have been attributed to various intra- and inter-molecular motions associated with these compounds. The characteristic frequencies of the modes observed in the present work agree well with the literature reported values obtained from FIR studies. The two peaks corresponding to the collective modes observed in the sample quenched from the liquid crystalline phase have been found to be missing in the sample quenched from the isotropic phase. The present work demonstrates the usefulness of PLS in the study of molecular motions.

Microdefects and electron densities in NiTi shape memory alloys studied by positron annihilation

The microdefects and free electron densities in B2, R and B19′ phases of Ni 50.78Ti 49.22 alloy were studied by positron lifetime measurements. Comparing the lifetime parameters of the Ni 50.78Ti 49.22 alloy measured at 295 K and 225 K, it is found that the free electron density of the R phase is lower than that of the B2 phase; the open volume of the defects of the R phase is larger, while the concentration of these defects is lower than that of the B2 phase. The Ni 50.78Ti 49.22 alloy exhibits B19′ phase at 115 K. In comparison with the R phase, the free electron density of the B19′ phase increases, the open volume of the defects of the B19′ phase reduces, and the concentration of these defects increases. The microdefects and the free electron density play an important role during the multi-step transformations ( B2→ R→ B19′ phase transformations) in Ni 50.78Ti 49.22 alloy with the decrease of temperature.