Doppler Broadened Positron Annihilation Radiation Research Articles

A non-beam-based Doppler broadening of positron annihilation radiation (DBAR) spectrometer for a single piece of micron-thickness film

Doppler broadening of positron annihilation radiation (DBAR) spectroscopy is a critical technique to provide electronic momentum information of materials. For conventional non-beam-based DBAR experiments, due to the continuous distribution of initial kinetic energy (0-545 keV) of positrons from 22Na source, two pieces of identical samples with sufficient thickness (> 1 mm for each sample with a density of 1 g/cm3) are required to stop almost all positrons in the samples. However, in many film studies, it is difficult or even impossible to meet this sample thickness requirement. In this work, judging by the output signals of a specially designed anticoincidence detector, the annihilation of positrons in a film sample and positron source are selectively collected. Furthermore, a new method is proposed to correct high-proportioned source components. Based on this, for the first time, a non-beam-based DBAR spectrometer was developed for a single piece of micron-thickness film. This approach will drastically broaden the application of DBAR spectroscopy in film characterization in future.

Nitrogen Desorption and Positron Sensitive Defect of CVD Diamond

The chemical vapor deposition (CVD) process can produce single or poly-crystalline diamond samples of high purity or with controlled doping concentrations. The defect type in the CVD diamonds can be changed by heating the samples. Controlling the defect type can be used to create devices for quantum diamond switches that could be used in radiation sensors and quantum information technology. Eight samples of CVD diamonds were analyzed with Doppler broadening of positron annihilation radiation (DBAR) before and after annealing in high vacuum with an electron gun. Between temperatures of 1700 - 1850 K, nitrogen was liberated from the diamond sample. At these high temperatures, the surface was graphitized and a change in the color and transparency of the diamond was observed. Some of the samples were analyzed with DBAR during periods with and without light. The defect properties were observed to change depending on the time exposure to the positron beam and were then regenerated by exposure to light. The DBAR data is compared to photoluminescence data and a time varying defect state is discussed for detector and optical grade type II CVD diamonds.

Positron annihilation characteristics in mesostructural silica films with various porosities

Porous silica films with various porosities were prepared via a sol-gel method using a nonionic amphiphilic triblock copolymer F127 as the structure-directing agent. Doppler broadening of positron annihilation radiation (DBAR) spectra were collected for the prepared films using a variable energy slow positron beam. Different linear relationships between positron annihilation line shape parameters S and W are found for the as-deposited films and calcined ones, indicative of the decomposition of the copolymer porogen in the as-deposited films upon calcination. This also reveals the variation of positron annihilation sites as a function of F127 loading or porosity. Strong correlations between positronium 3γ annihilation fraction, S parameter and porosity of the mesoporous silica films with isolated pores are obtained, which may provide a complementary method to determine closed porosities of mesoporous silica films by DBAR.

Mesoporosity in silica films studied by a slow positron beam and ellipsometry

Abstract Porous silica films were synthesized via a sol–gel method using a nonionic amphiphilic triblock copolymer F127 as the structural template. Mesoporosities of the prepared silica films were investigated by Doppler broadening of positron annihilation radiation (DBAR) spectroscopy, positron annihilation gamma-ray energy spectroscopy based on a slow positron beam, and ellipsometry. For the mesoporous silica films, the variation of positron annihilation line shape parameter reveals that the porosity of the silica films increases with loading more F127, which is also confirmed by a decrease of refractive index n. Little variation in positron 3γ-annihilation fraction is found for the silica films prepared with F127 loading less than 15 wt%, whereas a remarkable increment is seen for the films with higher loading. This indicates the pore percolation in porous silica films occurs around a loading of F127 with 15 wt%.

Positron/positronium annihilation as a probe for chemical environments of free volume holes in fluoropolymers

The sensitivity of the Doppler-broadening of positron annihilation radiation (DBAR) for the chemical environment of free volume holes was studied for fluorinated polymers. The free volume was characterized by positron annihilation lifetime spectroscopy (PALS). For this study, we prepared three series of copolymers of methacrylates with various semi-fluorinated methacrylates with increasing fractional number of fluorine atoms fF. Moreover some commercial fluoroelastomers and semicrystalline fluoropolymers were investigated. After the subtraction of a narrow para-positronium DBAR component, calculated on a simplified way using the PALS data, from the conventional Doppler broadened annihilation energy line, the remaining broad distribution shows no response to the size of free volume holes or the positronium formation probability but can be related to the nature and density of the chemical species in the vicinity of the holes. Element specific information of the annihilation site can be expressed in terms of an integral parameter, e.g. the normalised curve shape parameter Sbn of the broad DBAR component. We observed that Sbn does not show the expected exponential decrease but goes down linearly with fF in the range fF<0.45 and levels-off for fF≥0.5. This was explained assuming positronium localization and annihilation in two types of holes: holes, which have walls free of fluorine atoms and holes, which contain fluorine in their walls. A steeper, exponential-like decrease of Sbn with fF seem to occur for the fluoroelastomeric and semicrystalline fluoropolymers which contain not less than one F atom per two backbone carbons. In this, the case ortho-positronium localized at a hole will always find at least one fluorine atom in which neighbourhood it can annihilate.

Doppler broadening measurements of the electron-positron annihilation radiation in nanocrystalline ZrO2

ZrO2 powders have been ground by ball mill grinder to achieve the particle size down to 10 nm. Typical defects introduced during ball mill grinding have been studied by positron annihilation lifetime measurement technique and coincidence Doppler broadened positron annihilation radiation spectroscopic technique. Coincidence Doppler broadened positron annihilation spectra for ball mill ground and unground ZrO2 samples have been analyzed by constructing ratio curve with defects free Al single crystal. Results indicate an increase of cation defects in ZrO2 samples due to the reduction of particle size by the ball mill grinding process.

Electron momentum re-distribution due to superconducting transition in Bi-2212 and (Bi, Pb)-2223 oxides

For single-phase Bi-2212, Doppler broadened positron annihilation radiation line shape (DBPARL) parameter S has been measured from 15 to 300 K. S represents the fraction of low momentum electrons involved in the annihilation process. It shows a broad dip in S over 58 to 90 K in contrast to the 90–110 K complex dip in (Bi, Pb)-2223. The shift of the dip to lower temperature for the lower- T c superconductor proves that our observed decrease of the fraction of low momentum electrons in the T c-region is linked to superconducting pairing.

Complex variation of electron momentum distribution in (Bi,Pb)-2223 oxides across the superconducting transition

High T c superconductors like (Bi,Pb)-2223 show an abrupt change, across T c, in the S-parameter, calculated from Doppler broadened positron annihilation radiation lineshape (DBPARL) data. The S-parameter gives the fraction of low momentum electrons. Here we report the new feature of a sharp fall in the S-parameter on cooling the sample to its T c ( R = 0) and a subsequent complex but partial rise on further cooling. The detailed observations and possible implications are discussed.

Anomalous temperature dependence of electron momentum distribution in cuprate superconductors

Doppler-broadened positron annihilation radiation line shape (DBPARL) and electrical resistance for (Bi, Pb)-2223 oxide superconductors, with T c( R = 0) = 104 K, have been measured from 20 to 300 K. The S-parameter, giving the fraction of low-momentum electrons of the solid involved in the annihilation process, is calculated from the DBPARL data. Both measurements indicate anomalous variations possibly due to lattice instabilities between T c and 300 K. The variation of S with temperature T across the superconducting transition region has, in addition to abrupt changes in S and d S/d T, sharp dips in S. This new feature and its possible origin are discussed.

Multilayered Ni/Hf and solid-state amorphization studied using the slow positron beam technique

A monoenergetic slow positron beam was used to profile thin Ni/Hf multilayers. We measured the changes in the S-parameter of the Doppler broadened positron annihilation radiation through the solid-state amorphization. It is shown that a high density of vacancy-type defects in Hf layers strongly induces the fast diffusion of Ni atoms.

Positron annihilation detection of ultra-violet light induced damage in conjugated polymers

The Doppler-broadened positron annihilation radiation (DBAR) lineshape S-parameter, and the optical absorption spectra were measured during Ultra-violet (UV) exposure to a polydiacetylene (p-4BCMU) which degraded, and a polyquinoline which did not degrade, when exposed to UV light. The S-parameter of p-4BCMU increased during photodegradation and correlated closely to the change in optical absorption spectra. The S-parameter of a polyquinoline remained unchanged.

Positronium-Brönsted site interaction in silica-alumina catalysis

The lineshapes of Doppler-broadened positron annihilation radiation were determined at different acidity content of a silica-alumina catalyst. Analysis of the results together with those obtained from temperature programmed desorption and infrared spectra demonstrated that positronium atoms are selectively oxidized by surface protons and do not react with Lewis acid sites. A mechanism for the oxidation reaction is proposed.

Iterative unfolding method for positron annihilation radiation spectra measured with Ge(Li) detectors

An unfolding method for the analysis of the Doppler broadened positron annihilation radiation spectrum has been developed. The iterative procedure is based on the point-Jacobi method. Positron annihilation spectra measured with a high resolution planar and the coaxial Ge(Li) detector were analyzed and the results were compared with those of angular correlation measurements. The instrumental resolution limits the accuracy of the analysis. However, this unfolding method is effective for the study of the electron momentum distribution in various materials up to high momentum.