Brian E. O’Rourke

In-situ characterization of free-volume holes in polymer thin films under controlled humidity conditions with an atmospheric positron probe microanalyzer

A pulsed, slow positron beam, with a diameter of 200 μm, was extracted into air through a thin SiN window of an atmospheric positron probe microanalyzer (PPMA), and used to measure the ortho-positronium lifetimes τ in polyvinyl alcohol and polycaprolactam sub-μm-thick films. By measuring the variation of τ as a function of relative humidity, the effect of water molecules on the hole sizes, deduced from τ, was examined for the films with consideration to the chain mobility. The results demonstrate the usefulness of the atmospheric PPMA to the in-situ characterization of nanoscopic holes in thin films under practical conditions.

Simulations of slow positron production using a low-energy electron accelerator.

Monte Carlo simulations of slow positron production via energetic electron interaction with a solid target have been performed. The aim of the simulations was to determine the expected slow positron beam intensity from a low-energy, high-current electron accelerator. By simulating (a) the fast positron production from a tantalum electron-positron converter and (b) the positron depth deposition profile in a tungsten moderator, the slow positron production probability per incident electron was estimated. Normalizing the calculated result to the measured slow positron yield at the present AIST linear accelerator, the expected slow positron yield as a function of energy was determined. For an electron beam energy of 5 MeV (10 MeV) and current 240 μA (30 μA), production of a slow positron beam of intensity 5 × 10(6) s(-1) is predicted. The simulation also calculates the average energy deposited in the converter per electron, allowing an estimate of the beam heating at a given electron energy and current. For low-energy, high-current operation the maximum obtainable positron beam intensity will be limited by this beam heating.

Evaluation of a positron-beam-pulsing system in KUR reactor-based positron beam facility

The pulsing performance of the positron-beam-pulsing system of the slow positron beamline at Kyoto University research Reactor was tested using electron beams trimmed by 5-mm- or 15-mm-diameter apertures. Electron pulses with full width at half maximum of 143 ps were obtained with a 5-mm-diameter beam which corresponds to the diameter of the brightness-enhanced positron beam. This indicates the pulsing system has a sufficient pulsing performance for positron annihilation lifetime measurements.

Digitized detection of gamma-ray signals concentrated in narrow time windows for transient positron annihilation lifetime spectroscopy.

A pulsed slow-positron beam generated by an electron linear accelerator was directly used for positron annihilation lifetime spectroscopy without any positron storage devices. A waveform digitizer was introduced to simultaneously capture multiple gamma-ray signals originating from positron annihilation events during a single accelerator pulse. The positron pulse was chopped and bunched with the chopper signals also sent to the waveform digitizer. Time differences between the annihilation gamma-ray and chopper peaks were calculated and accumulated as lifetime spectra in a computer. The developed technique indicated that positron annihilation lifetime spectroscopy can be performed in a 20 μs time window at a pulse repetition rate synchronous with the linear accelerator. Lifetime spectra of a Kapton sheet and a thermally grown SiO2 layer on Si were successfully measured. Synchronization of positron lifetime measurements with pulsed ion irradiation was demonstrated by this technique.

Electron Gun using Coniferous Carbon Nano-Structure

We have measured the emission stability of a coniferous carbon nano-structure (CCNS) field emission electron source. Stable emission over the 1274 h measurement was observed at an emission current density of 20 mA·cm−2. The CCNS emitter can generate an emission current of more than 10 mA making it a practical choice for many applications requiring high electron current. For example, we are currently developing a CCNS based electron gun for electron accelerators and a portable X-ray source. Recent progress on both these applications is presented.

Development of a Vertical Slow Positron Beamline at AIST

A new slow positron beamline with two measurement ports has been installed at AIST. Positrons are generated using the 70 MeV AIST LINAC and are guided to the measurement ports using a solenoid magnetic field. Both beam ports are arranged vertically with the positron beam incident on the sample from above and samples loaded horizontally. Port No. 1 is designed for positron annihilation lifetime spectroscopy (PALS) and Doppler broadening of annihilation radiation (DBAR) measurements with a ∼10 mm diameter positron beam. The beamline on port No. 2 contains a transmission type remoderator for a brightness enhanced positron microbeam, similar to the existing positron probe microanalyzer (PPMA) at AIST.

Detection of re-emission positrons on metal surfaces during slow positron measurements

Detection methods for re-emitted positrons from Ni samples were investigated for in-situ positron annihilation lifetime spectroscopy during ion beam irradiation. A conventional mesh electrode and a ring electrode were examined to detect re-emitted positrons. Results of positron trajectory calculation were in good agreement with the experimental results on the mesh electrode. Simulation and experiments indicated that the ring electrode can form almost uniform electric fields in front of the sample and thus it can be used instead of the mesh electrode.

Design Study of Linear Accelerator-Based Positron Re-Emission Microscopy

The positron re-emission microscope (PRM) [1,2] images positrons that are re-emitted from surface after thermalization and diffusion of positrons implanted into material. By using PRM, it is able to nondestructively observe spatial distribution of vacancy-type defects at atomic levels below the detection limits of electron microscopy. Compared with scanning positron microscopy, PRM potentially has better lateral resolution with faster imaging time by more than one order of magnitude. However, in the previous studies of PRM [1-3], a long imaging time (more than 8 hours) was needed to acquire one image because radioisotope (RI) was used as a positron source. For practical use, a substantial reduction of the imaging time is required.

Annealing of a Pre-Assembled Tungsten Positron Moderator by Direct Electron Bombardment

We have developed a positron moderator annealing system with a handmade electron gun which allows the pre-assembled moderator temperature to be monitored directly during heating. The moderator, composed of strips of tungsten arranged in a regular lattice, was heated by bombarding electrons emitted from a hot tungsten filament. Moderator annealing was successfully achieved up to a temperature of 2600 ◦C in the central region of the moderator with a thermionic power of about 1200 W. Positron annihilation lifetime measurements of as-received and annealed tungsten foils were performed using a pulsed slow positron beam. As a result, it was confirmed that residual defects contained in the as-received tungsten foil were eliminated by conducting the annealing process.