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To investigate inhomogeneous defect distributions, e.g. close to fatigue cracks or dispersive alloys with positron annihilation lifetime spectroscopy a pulsed positron beam with a diameter in the range of 1 µm and with a time resolution in the order of 250 ps is needed. The Scanning Positron Microscope (SPM) fulfills the requirements for position resolved positron lifetime measurements and was operated by the Universität der Bundeswehr München. To increase the countrate, the SPM is currently transferred to the intense positron source NEPOMUC. The SPM Interface connects the SPM to the NEPOMUC source and en-hances the beam quality to the needs of the SPM. In this contribution we will give an overview of the SPM Interface and its performance. The finale stage of the SPM Interface is the positron elevator which has been further developed. We will discuss the apparatus in detail and show measurements of the beam quality with a beam elevation of 8 keV. From our measurements we are able to predict the performance of the SPM at NEPOMUC. In the future position resolved measurements will be possible with an improved spatial resolution of about 0.3 µm and an event rate of about 3.7 kHz. [ABSTRACT FROM AUTHOR]

In the field of positron annihilation spectroscopy, the investigation of a thin near-surface region, modified with ion implantation, has been usually performed by slow positron beams. Experiments carried out to simulate radiation effects in Fe-based alloys, were reported widely in the past [1,2]. Radioisotope positron sources, such as 22Na, provide only a little information from the near-surface area, probing the bulk region particularly. With more and more accessible high-energy particle accelerators in research institutes, modified layers can reach up to several tens of micrometres into the bulk, enabling to use conventional 22Na positron source for their investigation. This paper addresses some practical issues of the application of unmoderated radioisotope sources for the investigation of radiation effects in solids, simulated by ion implantation. [ABSTRACT FROM AUTHOR]

A brief description is given of the most recent version of the PALSfit3 program. Based on previous programs (PALSfit and PATFIT) which have been used by a number of positron annihilation groups, PALSfit3 has been extended to include the option of log-normal broadening of the lifetime components. In addition more detailed graphics output has been added, and the possibility to select output in spread sheet format included. A special feature is the possibility to exclude certain channel ranges in a lifetime spectrum from the analysis, which may be valuable if spectra contain ”bumps”, for example due to scattered positrons in beam experiments. Details of PALSfit3 can be found on the PALSfit web page (www.palsfit.dk). [ABSTRACT FROM AUTHOR]

Vacancy-type defects in Mg-implanted GaN were probed using monoenergetic positron beams. For the samples after annealing at 1300°C, the line shape parameter S and the positron lifetime increased under illumination with a 325-nm He-Cd laser. The observed increase in the trapping rate of positrons by vacancies was associated with the trapping of the excited electrons by the defects. Native defects in GaN grown on Si substrate were also studied. A similar illumination effect on the positron annihilation parameters was observed for GaN with the carbon concentration of 2×1016 cm−3. From the relationship between S and the photon energy, it was found that the transition of an electron from a carbon atom to the vacancies plays an important role in the charge shift of the vacancies under illumination. [ABSTRACT FROM AUTHOR]

Jefferson Lab is interested in the production of positrons for nuclear physics applications but also has the capability of producing low energy positrons that could be used for positron annihilation studies. Using the LERF accelerator at the Thomas Jefferson National Accelerator Facility, it is possible to produce a high brightness source of very low-energy positrons. The accelerator requirements are well within the capabilities of the installed hardware. For these experiments, we only need run at up to 120 MeV. A challenge is the production of the positrons. The gamma converter must be able to absorb the 50 kW of beam power that the linac delivers. At this low an energy, the converter, though challenging, is feasible. The transport of the low energy positrons from the production target to the next stage, where the energy is reduced even further, must have a very large acceptance to be able to efficiently transport the flux of positrons from the positron production target to the moderator. We propose to accomplish such a transport by means of a guiding solenoidal field with a novel endcap design. Finally, it is in principle possible to produce a spin polarized beam by filtering the positrons at low energy. The layout of the proposed device will be described in this presentation. [ABSTRACT FROM AUTHOR]

A slow positron facility is being built in Israel, at the Hebrew University, for basic and applied research. It consists of a slow positron beam and a compact Positrons Annihilation Lifetime (PAL) spectrometer. The slow positron beam follows a traditional design, using a 22Na source, of about 40mCi, a Tungsten moderator and a unique grounded target cell, with positrons energy that can vary between 0.03 keV and 30 keV. The detection system will be comprised of High Purity Germanium and BaF2 detectors, facing each other, for low background Doppler Broadening (DB) measurements. The target cell is designed to allow a combined measurement of sample conductivity and DB, with the flexibility to add more detection options in the future. The compact PAL spectrometer includes two fast scintillation detectors read by a fast digitizer (DRS4), with a sampling rate of 5.12 GS/s. A dedicated software package was developed to emulate analogue data acquisition. Lifetime measurements were performed using a ∼25 µCi 22Na source. The time resolution was defined using a 60Co source, to be 180-200ps. First positron lifetime validation measurements of Ti resulted in positrons lifetime of 157 ± 4 ps, consistent with previously published values. [ABSTRACT FROM AUTHOR]

The intense positron beam user facility at the PULSTAR reactor of the North Carolina State University has undergone recent upgrades to address issues caused by radiation damage over the years to the Tungsten moderator and the main beam structure. The two-bank Tungsten moderator is replaced with a four-bank Platinum moderator. The effect of moderator geometry has been systematically studied using SIMION and COMSOL multiphysics and the positron transport efficiency is optimized. Initial tests showed a positron rate of nearly 6×108 e+/s. The two associated spectrometers were also modified to improve their performance. The magnetically guided spectrometer with an RF buncher (e+-PAS) has been upgraded to include a new pre-buncher and a modified chopper to significantly improve the signal to noise ratio for PALS spectroscopy. In addition, a sample changer with heating and cooling capabilities is under development to address the sample turn-around efficiency and evacuation time. Meanwhile, the electrostatically focused Ps-PAS spectrometer is under an upgrade to further shrink the beam spot size and improve the beam brightness. Magnetic shielding is being implemented between the two remoderator stages and the electrostatic lenses have been redesigned to achieve better beam focusing. After the first-phase of modifications, the on-sample spot size is expected to be improved from ~2 mm to ~0.5 mm. The second-phase of modifications involves the implementation of a third stage of brightness enhancement and a final magnetic focusing lens. The new features will allow the beam to be focused down to less than 100 μm and enable measurement with a spatial resolution of several tens of micrometers. [ABSTRACT FROM AUTHOR]

Preliminary results are presented from a new positron beam system currently under development at the University of Texas at Arlington for the coincident energy measurement of Doppler-shifted annihilation quanta and positron-induced Auger electrons. We report data based on an analysis of the pulses resulting from the detection of positron induced secondary electrons by a micro-channel plate detector in coincidence with the pulses resulting from the detection of associated annihilation gamma rays in a NaI(Tl) gamma detector. [ABSTRACT FROM AUTHOR]

We report the development of a new positron diffractometer that will be coupled to the high-intensity positron source NEPOMUC located at the research neutron source FRM II. An optional transmission-type remoderator stage utilizing a Ni (100) foil will be installed to further enhance the brightness of the positron beam. The acceleration and focus onto the remoderator foil are realized by an electrostatic lens system. At the sample position we aim to obtain an almost parallel, slightly converging beam with a diameter of less than 1mm. Simulations were run to optimize the focus onto the remoderator foil and the electrostatic acceleration up to energies of 25 keV after remoderation. [ABSTRACT FROM AUTHOR]

We developed a new scanning positron microbeam for spatially resolved (coincident) Doppler broadening spectroscopy ((C)DBS) at the neutron induced positron source Munich (NEPOMUC). For this purpose, positrons are moderated three times: First, positrons generated by pair production are self-moderated in annealed polycrystalline Pt foils inside the beam tube housing the positron source NEPOMUC. Then, the brightness of the primary 1 keV positron beam with 109 moderated positrons per second is enhanced by a W(100) single crystal remoderator operated in back reflection geometry. Finally, in the upgraded CDB spectrometer we apply a 100 nm thin Ni(100) foil acting as transmission remoderator to generate a positron microbeam with a minimum beam diameter of 33 µm. We demonstrate the potential of laterally resolved DBS by imaging the defect distribution around a laser beam weld of the high-strength age-hardenable Al alloy AlCu6Mn. CDBS was applied with unprecedented spatial resolution to reveal the dissolution of Cu rich precipitates and the formation of Al-vacancy-Cu-complexes. [ABSTRACT FROM AUTHOR]