Status of the pulsed low energy positron beam system (PLEPS) at the Munich Research Reactor FRM-II

The Munich pulsed low energy positron beam system (PLEPS) is now installed at the high intensity positron source (NEPOMUC) at the Munich Research Reactor FRM-II. In order to enhance the performance of the system several improvements have been implemented: two additional collinear detector ports have been installed. Therefore in addition to the normal lifetime measurements it is now possible to simultaneously perform Doppler-broadening, coincident Doppler-broadening and age momentum correlation experiments. An additional chopper was included to periodically suppress pulses and therefore to extend the standard time window of 20 ns for precise measurements of longer lifetimes. First test-experiments have been performed in May and July 2007. With all pulsing components in operation we achieved a count-rate of 1.4 × 10⁴ counts per second. The total time resolution (pulsing and detector) was about 240 ps (FWHM) with a peak to background ratio up to 6 × 10³:1.     

A positron remoderator for the high intensity positron source NEPOMUC

A remoderator for the high intensity positron source NEPOMUC was developed and installed at the beam facility. A beam of remoderated positrons could be produced with different energies and a diameter of less than 2 mm was obtained. The efficiency of the remoderation setup was determined to be 5%. Due to the brilliance of the remoderated beam, the measurements at the coincidence Doppler broadening spectrometer (CDBS) and at the positron annihilation induced Auger electron spectrometer (PAES) could be improved. The setup and functionality of the remoderation device is presented as well as the first measurements at the remoderator, CDBS and PAES.     

Surface investigation of Si(1 0 0), Cu, Cu on Si(1 0 0), and Au on Cu with positron annihilation induced Auger-electron spectroscopy

The surfaces of polycrystalline Cu, Au-coated Cu, Si(1 0 0) and of Si(1 0 0) coated with 1.5 monolayer Cu were investigated with positron annihilation induced Auger-electron spectroscopy (PAES). Since the electron background has been reduced considerably we observed the Cu M_2,3VV-Auger transition on a copper surface within only three hours which is the shortest acquisition time reported so far for PAES. In order to demonstrate PAES’ high potential the Auger-yield, the signal-to-background ratio as well as the surface selectivity were compared with accompanying EAES-measurements quantitatively. A more efficient electron energy analyzer for the present PAES setup would lead to an additional efficiency gain of more than two orders of magnitude. The presented measurements were performed at the low-energy positron beam of high intensity NEPOMUC at the research reactor FRM II.     

First positron experiments at NEPOMUC

The in-pile positron source NEutron induced POsitron source MUniCh (NEPOMUC) of the new Munich research reactor FRM-II is now operated at the nominal reactor power of 20 MW. Recently, intensity and positron beam profile measurements were performed at 30 eV and 1 keV, respectively. For this purpose, NaI-scintillators detect the 511 keV γ-radiation of positrons that annihilate at a removable target in the beam line. The beam profile is determined with a micro-channel plate detector and a CCD-camera. In the present arrangement of NEPOMUC's instrumentation the positron beam is connected to a coincident Doppler broadening (CDB) facility and to a positron induced Auger electron spectroscopy (PAES) analysis chamber. First experiments were carried out in order to show the performance of these new spectrometers. An overview of the positron beam facility is given and first experimental results of PAES are presented.     

Thermal evolution of vacancy defects induced in sintered UO2 disks by helium implantation

A slow positron beam coupled with Doppler broadening (DB) spectrometer was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in UO₂ disks implanted with different 1 MeV ³He fluences and annealed in ArH₂ or in vacuum. The S(E) and W(E) behaviors indicate that for fluences in the range from 2 × 10¹⁴ to 2 × 10¹⁶³He cm⁻², the vacancy defects distribution evolves with the annealing temperature in the range from 264 to 700 °C under ArH₂. This evolution is found to be dependent on the ³He fluence implanted in the sintered UO₂ disks. For the lowest fluence of 2 × 10¹⁴³He cm⁻², the S(W) plot with positron energy as the running parameter suggests that only the concentration of vacancy defects decreases when annealing temperature increases. For the highest implantation fluences (from 5 × 10¹⁵ to 2 × 10¹⁶³He cm⁻²) the S(W) plot suggests that the nature of the vacancy defects changes in the annealing temperature range from 260 to 400 °C. Measurements performed in implanted UO₂ disks annealed in vacuum have revealed a partial recovery of the vacancy defects possibly due to their recombination with mobile oxygen interstitials. The role of the hydrogen infusion into the disk is also discussed.     

Intense slow positron production at the 15 MeV LINAC at Argonne National Laboratory

An intense slow positron beam using a 15 MeV LINAC (average current 1.25 × 10¹⁵ e⁻/s) at the Radiation and Photochemistry Group, Chemistry Division of Argonne National Laboratory (ANL) has been proposed and studied. Computer simulated results optimizing the positron yield and distribution of energy and angle show that a slow positron production at 10¹⁰ e⁺/s is possible. A proposed design of an intense slow positron beam with optimal conditions of incident electron, converter/moderator configurations, and extraction/transportation is presented.     

Study of the surface contamination of copper with the improved positron annihilation-induced Auger electron spectrometer at NEPOMUC

The high intensity positron source NEPOMUC at the FRM-II in Munich enables measurement times for positron annihilation-induced Auger electron spectroscopy (PAES) of only 2.4 h/spectrum, in contrast to usual lab beams with measurement times up to several days. The high electron background due to surrounding experiments in the experimental hall of the FRM-II has been eliminated and hence background free experiments have become possible. Due to this, the signal to noise ratio has been enhanced to 4.5:1, compared to 1:3 with EAES. In addition, a long-term measurement has been performed in order to observe the contamination of a polycrystalline copper foil at 150 °C.     

An ab initio-based approach to phase diagram calculations for GaN(0 0 0 1) surfaces

Surface phase diagrams of GaN(0 0 0 1)-(2 × 2) and pseudo-(1 × 1) surfaces are systematically investigated by using our ab initio-based approach. The phase diagrams are obtained as functions of temperature T and Ga beam equivalent pressure p_Ga by comparing chemical potentials of Ga atom in the vapor phase with that on the surface. The calculated results imply that the (2 × 2) surface is stable in the temperature range of 700-1000 K at 10⁻⁸ Torr and 900-1400 K at 10⁻² Torr. This is consistent with experimental stable temperature range for the (2 × 2). On the other hand, the pseudo-(1 × 1) phase is stable in the temperature range less than 700 K at 10⁻⁸ Torr and less than 1000 K at 10⁻² Torr. Furthermore, the stable region of the pseudo-(1 × 1) phase almost coincides with that of the (2 × 2) with excess Ga adatom. This suggests that Ga adsorption or desorption during GaN MBE growth can easily change the pseudo-(1 × 1) to the (2 × 2) with Ga adatom and vice versa.     

Fabrication of Al nanoparticles and their electrical properties studied by capacitance-voltage measurements

Nanometer-scale Al particles are fabricated and are embedded in a GaAs matrix using molecular beam epitaxial technique. The Al particle is self-assembled on GaAs by supplying an Al molecular beam. The average particle size is found to be 25 nm. The density is 7 × 10¹⁰ cm⁻² when Al of 6.2 × 10¹⁵ atoms/cm² is supplied on (1 0 0)GaAs at a substrate temperature of 300 °C. Clear hysteresis and plateaus in capacitance-voltage (C-V) curves are found in an Al-embedded sample, whereas monotonic increase of capacitance is obtained in a reference sample having an AlAs layer instead of Al. This difference results from trapping of electrons by the Al particles, suggesting that the particles have metallic character.     

Investigations of He implantation and subsequent annealing effects in InP

The influence of 70 keV He⁺ ion implantation and subsequent annealing of Cz-indium phosphide (InP) samples has been investigated using a slow positron beam-based Doppler broadening spectrometer. Three samples with ion fluences of 1 × 10¹⁶, 5 × 10¹⁶ and 1 × 10¹⁷ cm⁻² were studied in the as-implanted condition as well as after annealing at 640 °C for times between 5 and 40 min. It was found that the line-shape parameter of the positron-electron annihilation peak in the implanted layer increases after 5 min annealing, then after longer annealing times it starts to decline gradually until it reaches a value close to the value of the as-grown sample. This implies that vacancy-like defects can be created in InP by He implantation followed by short-thermal annealing at T > 600 °C. Comparison of the results with a study where cavities were observed in He-implanted InP has been carried out.     

Propagation regimes of intense circularly polarized laser beam in magnetoactive plasma

This paper presents an analytical and numerical investigation of an intense circularly polarized wave propagating along the static magnetic field parallel to oscillating magnetic field in magnetoactive plasma. In the relativistic regime such a magnetic field is created by pulse itself. The authors have studied different regimes of propagation with relativistic electron mass effect for magnetized plasma. An appropriate expression for dielectric tensor in relativistic magnetoactive plasma has been evaluated under paraxial theory. Two modes of propagation as extraordinary and ordinary exist; because of the relativistic effect, ultra-strong magnetic fields are generated which significantly influence the propagation of laser beam in plasma. The nature of propagation is characterized through the critical-divider curves in the normalized beam width with power plane For given values of normalized density (ω_p/ω) and magnetic field (ω_c/ω) the regions are namely steady divergence (SD), oscillatory divergence (OD) and self-focusing (SF). Numerical computations are performed for typical parameters of relativistic laser-plasma interaction: magnetic field B = 10-100 MG; intensity I = 10¹⁶ to 10²⁰ W/cm²; laser frequency ω = 1.1 × 10¹⁵ s⁻¹; cyclotron frequency ω_c = 1.7 × 10¹³ s⁻¹; electron density n_e = 2.18 × 10²⁰ cm⁻³. From the calculations, we confirm that a circularly polarized wave can propagate in different regimes for both the modes, and explicitly indicating enhancement in wave propagation, beam focusing/self-guiding and penetration of E-mode in presence of magnetic field.     

Slow positron beam study of nitrogen implanted CZ-Si subjected to rapid thermal processing

In this experiment, nitrogen ions were implanted into CZ-silicon wafer at 100 keV at room temperature with the fluence of 5 × 10¹⁵ N₂⁺/cm², followed by rapid thermal processing (RTP) at different temperatures. The single detector Doppler broadening and coincidence Doppler broadening measurements on slow positron beam were carried out to characterize the defects in the as-implanted silicon and RTP-treated samples. It is found that both nitrogen-vacancy complexes (N-Vsi) and oxygen-vacancy complexes (O-Vsi) produced by nitrogen implantation diffuse back to the sample surface upon annealing. But the N-Vsi and the O-Vsi complete with each other and give a summed effect on positron annihilation characteristics. It is shown that the N-Vsi win out the O-Vsi in as-implanted sample and by RTP at 650 °C, 750 °C, which make the S-parameter increase; O-Vsi plays a dominant role after annealing above 850 °C, which makes the S parameter decrease.     

Preparation and characterization of indium tin oxide films formed by oxygen ion beam assisted deposition

Indium tin oxide (ITO) films were produced by low-energy oxygen ion beam assisted electron-beam evaporation. The dependence of surface morphology, electrical and optical properties on evaporation rate, oxygen ion beam energy and density, as well as substrate temperatures was characterized by atomic force microscopy, X-ray photoelectron spectroscopy, Hall-effect and optical transmittance measurements. The results show that high-quality ITO films (resistivity of 7.0×10⁻⁴ Ω cm, optical transmittance above 85% at wavelength 550 nm, surface roughness of 0.6 nm in root mean square) can be obtained at room temperature.     

Structural and electrical analysis of S ion bombarded p-InP(1 0 0)

The chemical state of sulfur and surface structure on low-energy S⁺ ion-treated p-InP(1 0 0) surface have been investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). S⁺ ion energy over the range of 10-100 eV was used to study the effect of ion energy on surface damage and the process of sulfur passivation on p-InP(1 0 0) by S⁺ ion beam bombardment. It was found that sulfur species formed on the S⁺ ion-treated surface. The S⁺ ions with energy above 50 eV were more effective in formation of In-S species, which assisted the InP surface in reconstruction into an ordered (1 × 1) structure upon annealing. After taking into account physical damage due to the process of ion bombardment, we found that 50 eV was the optimal ion energy to form In-S species in the sulfur passivation of p-InP(1 0 0). The subsequent annealing process removed donor states that were introduced during the ion bombardment of p-InP(1 0 0). Results of theoretical simulations by Transport of Ions in Materials (TRIM) are in accordance with those of experiments.     

Vacancy profiles and clustering in light-ion-implanted GaN and ZnO

We present experimental results obtained in H-implanted GaN and He- and Li-implanted ZnO. The ion energies were varied in the range 100-850 keV, and the implantation fluences in the range 5 × 10¹³ to 1 × 10¹⁸ cm⁻². In addition, conventional and flash anneals at temperatures 500-1400 °C were performed on the ZnO samples. The data obtained with a slow positron beam show that vacancy clusters are formed in as-implanted samples with fluences above 1 × 10¹⁷ cm⁻². Below this value only single vacancies are detected after implantation, but vacancy clusters can be formed and subsequently dissociated by thermal annealings.     

Observation of ferromagnetism at room temperature for Cr ions implanted ZnO thin films

Single crystalline ZnO films were grown on c-plane GaN/sapphire (0 0 0 1) substrates by molecular beam epitaxy. Cr⁺ ions were implanted into the ZnO films with three different doses, i.e., 1 × 10¹⁴, 5 × 10¹⁵, and 3 × 10¹⁶ cm⁻². The implantation energy was 150 keV. Thermal treatment was carried out at 800 °C for 30 s in a rapid thermal annealing oven in flowing nitrogen. X-ray diffraction (XRD), atomic force microscopy, Raman measurements, transmission electron microscopy and superconducting quantum interference device were used to characterize the ZnO films. The results showed that thermal annealing relaxed the stress in the Cr⁺ ions implanted samples and the implantation-induced damage was partly recovered by means of the proper annealing treatment. Transmission electron microscopy measurements indicated that the first five monolayers of ZnO rotated an angle off the [0 0 0 1]-axis of the GaN in the interfacial layer. The magnetic-field dependence of magnetization of annealed ZnO:Cr showed ferromagnetic behavior at room temperature.     

Ionoluminescence and photoluminescence studies of Ag ion irradiated kyanite

Ionoluminescence (IL) of kyanite single crystals bombarded with 100 MeV swift Ag⁸⁺ ions with fluences in the range 1.87-7.5×10¹¹ ions/cm² has been studied. A pair of sharp IL peaks at ∼689 and 706 nm along with broad emission in the region 710-800 nm are recorded in both crystalline and pelletized samples. Similar results are recorded in Photoluminescence (PL) of pelletized kyanite bombarded with same ions and energy with fluences in the range 1×10¹¹-5×10¹³ ions/cm² with an excitation of 442 nm laser beam. The characteristic pair of sharp emission peaks at 689 and 706 nm in both IL and PL is attributed to luminescence centers activated by Fe²⁺ and Fe³⁺ ions. The reduction in IL and PL bands intensity with increase of ion fluence might be attributed to degradation of Si-O (2ν₃) bonds, present on the surface of the sample.     

Composition dependent structural modification in relaxed Si1−xGex films by 100 MeV Au beam

We report the modification of molecular beam epitaxy grown strain-relaxed single crystalline Si_1−xGe_x layers for x=0.5 and 0.7 as a result of irradiation with 100 MeV Au ions at 80 K. The samples were structurally characterized by Rutherford backscattering spectrometry/channeling, transmission electron microscopy (TEM) and high-resolution X-ray diffraction before and after irradiation with fluences of 5×10¹⁰, 1×10¹¹ and 1×10¹² ions/cm², respectively. No track formation was detected in both the samples from TEM studies and finally, the crystalline to amorphous phase transformation at 1×10¹² ions/cm² was examined to be higher for Si_0.3Ge_0.7 layers compared to Si_0.5Ge_0.5 layers.     

Construction of a positron microbeam in JAEA

We have developed a positron microbeam using magnetic lenses based on the commercial scanning electron microscope (SEM). A slow positron beam was generated using a handmade source with ²²Na and a solid neon moderator. The beam diameter was 3.9 μm on a target. Two-dimensional image of S parameter was successfully obtained. By introducing a beam pulsing section, positron lifetime measurement beam is also available.     

Removal of chloridazon by natural and ammonium kerolite samples

The adsorption of chloridazon (5-amino-4-chloro-2-phenylpyridazin-3(2H)-one) on natural and ammonium kerolite samples from aqueous solution at 10, 25 and 40 °C has been studied by using batch experiments. The experimental data points were fitted to the Langmuir equation in order to calculate the adsorption capacities (X_m) of the samples; two straight lines were obtained, which indicates that the adsorption process takes place in two different stages. Values for X_m1 (first stage) ranged from 1.1 × 10⁻² mol kg⁻¹ for natural kerolite at 40 °C up to 5.1 × 10⁻² mol kg⁻¹ for ammonium kerolite at 10 °C and the values for X_m2 (second stage) ranged from 9.1 × 10⁻² mol kg⁻¹ for natural kerolite at 40 °C up to 14 × 10⁻² mol kg⁻¹ for natural kerolite at 10 °C. The adsorption experiments showed on the one hand, that the ammonium kerolite is more effective than natural kerolite to adsorb chloridazon in the range of temperature studied and on the other hand, that the lower temperature, the more effective the adsorption of chloridazon on the adsorbents studied.