Pulsed Positron Beam Research Articles

Vacancy defects in epitaxial InN: identification and electrical properties

We have used a low-energy positron beam to identify and quantify the dominant vacancy defects in InN layers grown on Al 2O 3 by molecular beam epitaxy. By applying both continuous and pulsed positron beams, we can show that In vacancies are formed during the crystal growth. Their concentration decreases from ∼5×10 18 to below 10 16 cm −3 with increasing layer thickness (120–800 nm). The In vacancy concentration correlates with the free electron concentration and decreases with increasing electron Hall mobility. These results suggest that In vacancies act as both compensating defects and electron scattering centers in InN films.

Positron and Positronium Annihilation in Low-Dielectric-Constant Films Studied by a Pulsed Positron Beam

Positron and Positronium Annihilation in Low-Dielectric-Constant Films Studied by a Pulsed Positron Beam

Thermal Isomerization of Azo-Benzene in PMMA Probed by a Pulsed Low-Energy Positron Beam

Polymers containing the Azo-benzene group have attracted attention due to anticipated uses such as materials for data storage, membranes with controllable permeability and materials with changeable solubility [1,2]. These expectations are due to the change in shape and dipole moment of the Azo-compound when excited by photo isomerization from the trans-state to the cis-state by UV light of ca 360 nm. Films of copolymers of poly-methyl methacrylate (PMMA) containing 8 wt-% Azo-benzene were photo isomerized with a UV lamp and then measured as a function of time in a lowenergy positron beam. The observed changes in the lifetime parameters as a function of time were in agreement with the thermal isomerization (decay) of the cis-isomer as measured by conventional UV spectroscopy. The results show that positron lifetime spectra at low energy are sensitive for the cis-trans isomerization of the Azo-compound, suggesting trapping and annihilation of positrons at free volume sites around the Azo-benzene groups. (Less)

Application of positron annihilation to polymer and development of a radioisotopes-based pulsed slow positron beam apparatus

Application of positron annihilation to polymer and development of a radioisotopes-based pulsed slow positron beam apparatus

Positron and positronium annihilation in silica-based thin films studied by a pulsed positron beam

Positron and positronium annihilation in silica-based thin films has been investigated by means of measurement techniques with a monoenergetic pulsed positron beam. The age–momentum correlation study revealed that positron annihilation in thermally grown SiO 2 is basically the same as that in bulk amorphous SiO 2 while o-Ps in the PECVD grown SiCOH film predominantly annihilate with electrons of C and H at the microvoid surfaces. We also discuss time-dependent three-gamma annihilation in porous low- k films by two-dimensional positron annihilation lifetime spectroscopy.

Characterization of polymer sub-surface using slow positron beam

A new pulsed mono-energetic slow positron beam as well as the conventional positron annihilation lifetime spectroscopy (PALS) have been applied to study the sub-surface and the bulk of epoxy polymer. Significant changes of o-Ps parameters were found at a short distance from the surface. The lifetime of o-Ps was observed to decrease with increasing the positron implantation depth, while its intensity increased. The temperature effect on o-Ps parameters at sub-surface was also investigated. The glass transition temperature for the sub-surface was lower than that for the bulk. Furthermore, the thermal expansion coefficient of the sub-surface was found smaller than that of the bulk.

Study on laser cooling of ortho-positronium

We have been theoretically and experimentally studying details of laser cooling of ortho-positronium. Experimental apparatus consists of a pulsed positron beam generator, a long pulse wide bandwidth ultraviolet laser and a time-of-flight system to measure kinetic energy of positronium atoms is constructed and tested. Using this apparatus, production of thermally activated positronium is confirmed. Its production rate is larger as the target temperature increases and ratio of thermally activated ortho-positronium to all γ-ray annihilation events is estimated to be 32.3±6.7% for the target temperature 1000 K.

Positron annihilation in SiO 2–Si studied by a pulsed slow positron beam

Positron and positronium (Ps) behavior in SiO 2–Si have been studied by means of positron annihilation lifetime spectroscopy (PALS) and age-momentum correlation (AMOC) spectroscopy with a pulsed slow positron beam. The PALS study of SiO 2–Si samples, which were prepared by a dry-oxygen thermal process, revealed that the positrons implanted in the Si substrate and diffused back to the interface do not contribute to the ortho-Ps long-lived component, and the lifetime spectrum of the interface has at least two components. From the AMOC study, the momentum distribution of the ortho-Ps pick-off annihilation in SiO 2, which shows broader momentum distribution than that of crystalline Si, was found to be almost the same as that of free positron annihilation in SiO 2. A varied interface model was proposed to interpret the results of the metal-oxide-semiconductor (MOS) experiments. The narrow momentum distribution found in the n-type MOS with a negative gate bias voltage could be attributed to Ps formation and rapid spin exchange in the SiO 2–Si interface. We have developed a two-dimensional positron lifetime technique, which measures annihilation time and pulse height of the scintillation gamma-ray detector for each event. Using this technique, the positronium behavior in a porous SiO 2 film, grown by a sputtering method, has been studied.

Development and application of the Ghent pulsed positron beam

We present the development and application of a compact pulsed positron beam at the Ghent University. The pulsing system of this source-based beam consists of a chopper–prebuncher and a resonator buncher both running at a repetition rate of 50 MHz (20 ns time window). A description of the timing components is given. The energy of the positrons can be adjusted between 0.5 and 30 keV by changing the potential of the sample. Finally, we propose a refined method to analyse lifetime spectra from pulsed positron beams using a resolution function with the shape of the time structure of the beam.

Lifetime measurements with a scanning positron microscope.

First lifetime results obtained with a scanning positron microscope will be presented. A pulsed positron beam with a variable energy from 0.5 to 20 keV, with a spot diameter of 2 microm, can be electronically scanned over an area of 0.6x0.6 mm(2). This beam is formed after a double-stage stochastic cooling (moderation) of positrons emitted from a radioactive isotope. Included in the system is a conventional scanning electron microprobe for surface analysis. Three-dimensional positron lifetime spectra of a GaAs sample with a small surface scratch reveal the range due to the mechanical damage.

Identification of open-volume defects in disordered and amorphized Si: A depth-resolved positron annihilation study

Depth-resolved positron beam studies have been carried out on Ar-irradiated Si using Doppler S parameter and lifetime measurements. Si samples have been irradiated with 140-keV Ar ions to a dose of $2\ifmmode\times\else\texttimes\fi{}{10}^{13}$ and $5\ifmmode\times\else\texttimes\fi{}{10}^{16} \mathrm{Ar}/{\mathrm{cm}}^{2},$ respectively, so as to produce disordered and amorphous states in near-surface regions. The observed features of the defect sensitive line shape S parameter indicate the presence of small vacancylike defects in the disordered sample and higher-order vacancy clusters in an amorphous sample. Pulsed positron beam lifetime results indicate that the disordered Si sample exhibits lifetime distribution ascribable to mostly divacancies. In the case of an amorphous sample, the lifetime distribution is broad with larger lifetime values indicating the presence of a distribution of large vacancy clusters or nanovoids. By using theoretical lifetime values for Si reported in the literature, an empirical fit to the lifetime variation as a function of vacancy cluster size is obtained. By comparing the experimental lifetime distribution with this data, the vacancy cluster size distribution in disordered and amorphous Si is deduced. In disordered Si, divacancies are found to be the dominant defects species followed by small concentration of ${\mathrm{V}}_{3}.$ In amorphous Si, nanovoids in the size range of four to seven vacancy clusters are present with ${\mathrm{V}}_{5}$ and ${\mathrm{V}}_{6}$ clusters being the dominant defect species. The implication of these results is discussed in light of recent computer-simulation studies.

'Smart Cut' Silicon by Proton Implantation: Lifetime Studies with a Pulsed Positron Beam

'Smart Cut' Silicon by Proton Implantation: Lifetime Studies with a Pulsed Positron Beam

Research and Development of High Energy Pulsed Positron Beam

Research and Development of High Energy Pulsed Positron Beam

Development of pulsed MeV positron beam line

A high-energy (1 MeV) pulsed positron beam line has been developed to aim at the application of this system for positron annihilation lifetime spectroscopy (PALS). The performance tests of this beam line were made using an electron beam, and as a result, we have obtained the beam energy, size and emittance expected from the machine design. From the observed time structure of pulsed beams, further improvement of a chopper system was shown to be required to accomplish single fine pulses necessary for PALS.

Development of a short-pulsed slow positron beam for application to polymer films

We have recently developed a pulsed slow positron beam to be used for the characterization of polymer thin films. It consists of the sections of stretching and pulsing of slow positrons from an electron linac. The positron lifetime spectra measured using this system had a time resolution of 1.4 ns. The details of the system are described.

Intense positron beam at KEK

A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e +/s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2. We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

Positron annihilation in near-surface regions and layered structures

Positrons are very sensitive probes for vacancy-type defects of atomic dimensions, e.g. vacancies, vacancy agglomerates, dislocations or inner surfaces. It is well established that positrons can be trapped at these defects and, because of the locally reduced electron density, the lifetime of the positron localized at the defect increases. This lifetime has characteristic values for each defect type and therefore it is possible to separate out various atomic defect configurations and their relative abundance with very high sensitivity (∼1 ppm) and in a nondestructive way. With a pulsed positron beam (∼3 mm diameter) of variable energy, lifetime studies can be performed as a function of the positron energy and hence the penetrations depth. Results on silicon and silicon carbide subjected to different treatments will be discussed. For many applications in materials science a positron beam in the micrometer range is desirable. This leads to a scanning positron microscope consisting of a pulsed positron beam of micrometer dimension with a scanning facility. The design and the performance of this first system of its kind will be presented.

A positron lifetime spectroscopy apparatus for surface and near-surface positronium experiments

A positron lifetime spectroscopy apparatus, which utilizes an intense pulsed positron beam generated by an electron linear accelerator, is presented. Using this apparatus we can measure positron lifetime spectra with variable energy, high count rate, wide measurable time range, and high peak-to-background ratio. These features are suitable for surface and near-surface positronium experiments.

Construction of the Helsinki University of Technology (HUT) pulsed positron beam

We are constructing a pulsed positron beam facility for lifetime measurements in thin surface layers. Our beam system comprises a 22 Na positron source and a tungsten foil moderator followed by a prebuncher and a chopper. A double-drift buncher will compress the beam into 120-ps pulses at the target. The end energy of the positron beam can be adjusted between 3 keV and 30 keV by changing the potential of the source end of the beam. The bunching electronics and most of the beam guiding magnets are also floating at the high voltage. The sample is at ground potential to facilitate variable temperature measurements. With a test source of 6 mCi 22 Na we get a prebunched beam intensity of 4×10 3 positrons per second in 1.5-ns wide pulses (the bunching frequency is 33.33 MHz). We are currently testing the chopper and the following buncher stages and building the final accelerator/decelerator system.

Positron re-emission from tungsten surfaces

Positron re-emission properties of 50 μm thick polycrystalline tungsten foil and single crystalline tungsten specimen with (110) surface were measured as a function of successive heat treatment with and without oxygen by a pulsed monoenergetic positron beam. The tungsten specimens after in situ annealing with oxygen have significantly higher positron re-emission fraction and lower positronium fraction than those after in-situ annealing without oxygen. Furthermore, re-emission fraction at high temperatures of 800–1000°C with an oxygen partial pressure of 2×10 −7 Torr is significantly higher than that without oxygen, suggesting that surface oxygen inhibits positronium formation. This feature is good for electron-linac based slow positron beamlines.