A novel design for a variable energy positron lifetime spectrometer

We present computer simulations of a new design of a variable energy positron lifetime beam that uses for a start signal the secondary electron emission from a 25-nm thick carbon foil (C-foil) located in front of the sample. A needle of ∼30 μm diameter is positioned on-axis right behind the foil, creating a radial electric field that deflects the secondary electrons radially outward so as to miss the sample and to hit the micro-channel plate (MCP) detector placed down beam. The MCP signal provides the start signal for the positron lifetime spectrometer. A grid can be further introduced between the sample holder and the MCP to yield a cleaner signal by preventing the positrons with large transmitted scattering angle from hitting the MCP. The cylindrical symmetry of this design reduces the experimental complexity and offers good timing resolution. We show that the design is robust against the transmitted energy and angle of the secondary electrons and positrons.     

Performance of the Beijing pulsed variable-energy positron beam

In order to study the depth-dependent characteristics of open-volume defects in thin surface layers, the variable-energy positron lifetime spectroscopy (VEPLS) has been enabled by pulsing a continuous positron beam. The buncher is a quarter-wave coaxial resonator and the RF-signal is fed in by a coupling loop with a frequency of 149.89 MHz and the reflection factor of 0.05 measured by a Network Analyzer. Three synchronic signals with their phases and amplitudes adjusted independently are supplied for start signal of the positron lifetime measurement and the power signal by an electronic system. The stop signal is derived from a detector, a BaF₂ scintillator coupled to a photomultiplier-tube (Hamamatsu). The time resolution of 295 ps (FWHM) was achieved for a Kapton film and a Ti sample at positron energies in the range between 1 keV and 30 keV.     

Using a LaBr3:Ce scintillator for positron annihilation lifetime spectroscopy

A LaBr₃:Ce scintillator has a high light output (~60000 p.e/MeV) and a short decay constant (<25 ns), which makes it good for time spectrometry. Compared with a BaF₂ scintillator, it can bear a much higher count rate, and can be coupled to photomultipliers without using a quartz window. In this work, a positron annihilation lifetime spectrometer (PALS) consisting of two bulks of φ25 mm×25 mm LaBr₃:Ce scintillator coupled to two XP20D0 photomultipliers, respectively, was built. A time resolution of FWHM=206 ps was measured for the PALS with a ⁶⁰Co source at the energy window for ²²Na. With this spectrometer, a reasonable lifetime value τ=221±4 ps in a pure Si sample is obtained, which means that the utilization of LaBr₃:Ce as the detector for a PALS is feasible.     

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ₂ = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ₁, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ₁ and the intensity I₂, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.     

Ion implantation induced by Cu ablation at high laser fluence

High energy laser plasma-produced Cu ions have been implanted in silicon substrates placed at different distances and angles with respect to the normal to the surface of the ablated target. The implanted samples have been produced using the iodine high power Prague Asterix Laser System (PALS) using 438 nm wavelength irradiating in vacuum a Cu target. The high laser pulse energy (up to 230 J) and the short pulse duration (400 ps) produced a non-equilibrium plasma expanding mainly along the normal to the Cu target surface. Time-of-flight (TOF) technique was employed, through an electrostatic ion energy analyzer (IEA) placed along the target normal, in order to measure the ion energy, the ion charge state, the energy distribution and the charge state distribution. Ions had a Boltzmann energy distributions with an energy increasing with the charge state. At a laser fluence of the order of 6 × 10⁶ J/cm², the maximum ion energy was about 600 keV and the maximum charge state was about 27+.In order to investigate the implantation processes, Cu depth profiles have been performed with Rutherford backscattering spectrometry (RBS) of 1.5 MeV helium ions, Auger electron spectroscopy (AES) with 3 keV electron beam and 1 keV Ar sputtering ions in combination with scanning electron microscopy (SEM). Surface analysis results indicate that Cu ions are implanted within the first surface layers and that the ion penetration ranges are in agreement with the ion energy measured with IEA analysis.     

Secondary electron spectra of gold under bombardment by very low-energy positrons

Measurements of the secondary electron energy spectra resulting from very low-energy positron bombardment of a polycrystalline Au surface are presented. The low-energy part of the secondary spectra contain significant contributions from two processes: (1) annihilation-induced Auger electrons that have lost energy before leaving the surface and (2) secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions (∼3 eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. These measurements were also used to obtain the first experimentally determined upper limit on the intensity of the spectrum of Auger-induced secondary electrons.     

Development of a nearly transform-limited, low-repetition-rate, picosecond optical parametric generator

A low-repetition-rate (10-Hz), picosecond (ps) optical parametric generator (OPG) seeded at the idler wavelength with a high-power diode laser is demonstrated. The output of the OPG at ∼566 nm is amplified in dye cells, resulting in signal enhancement by more than three orders of magnitude. The nearly transform-limited beam at ∼566 nm has a pulsewidth of ∼170 ps, with an overall output of ∼2.3 mJ/pulse. The laser is tuned either by tuning the nonlinear crystal or the seed-laser current. The applications of such a simple, compact, high-performance, tunable ps laser system for linear and nonlinear spectroscopies are outlined.     

Determination of the timing properties of a pulsed positron lifetime beam by the application of an electron gun and a fast microchannel plate

We show that the timing properties of a pulsed low-energy positron lifetime beam can be conveniently tested by an electron beam. We apply this method to study the time resolution of the beam and electron scattering in flat and ‘sawtooth’ shaped choppers. The results show that (i) time resolution of 160 ps is obtained, (ii) the scattering of the electrons and the secondary electron yield of the flat chopper make the time resolution worse and background poor, and (iii) both these problems can be solved by using a ‘sawtooth’ shaped chopper. We also compare these results to beam simulations.     

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.     

Fast photovoltaic characteristic of silver nano-cluster doped ZnO thin films induced by 1.064 μm pulsed laser

Fast photovoltaic characteristic was found from silver nano-cluster doped ZnO thin films when it was irradiated by 1.064 μm infrared laser. Its arising time is about 1 ns with an open-circuit photovoltage of ∼2 ns full width at half-maximum. A transient photovoltaic signal of ∼80 mV occurred with a full width at half-maximum of about 2 ns was observed under the illumination of a 4 mJ pulsed laser in duration of 25 ps. This fast photovoltaic response perhaps relate to the non-uniform distribution of the silver clusters along direction perpendicular to ZnO surface.     

Application of positron beams to the study of positronium-forming solids

Doppler broadening of annihilation radiation (DBAR) and positron annihilation lifetime spectroscopy (PALS) have been successfully applied to the study of positronium (Ps)—forming amorphous solids such as polymers and silicon oxide in the bulk. Implementing depth selectivity to DBAR and PALS by combining them with variable-energy positron beams considerably broadens their applicability. Variation of incident positron energy over a wide range enables depth-profiling, whereas tuning of the beam energy enables the studies of surfaces, interfaces and thin films. In this paper, we discuss fundamentals and applications of energy variable DBAR and PALS for Ps—forming polymers and silicon oxide.     

A novel multilevel coding technique for high speed optical fiber communication systems

This paper delineates a novel coding technique, a combination of coding and multiplexing technique, known as mapping multiplexing technique (MMT). A mapping algorithm is used to multiplex four channels, each of which running at 10 Gbit/s. Up to four bits per symbol at a symbol rate of 10 Gboud was successfully transmitted over a single wavelength. Compare to the conventional return to zero (RZ) and none-return to zero (NRZ)-time division multiplexing (TDM) more than 75% and 50% reduction in the spectral width are achieved, respectively. Calculated chromatic dispersion tolerance of ±130 ps/nm is achieved for the 40 Gbit/s MMT transmission system at the expense of ∼2 dB degradation in the receiver sensitivity (compared to NRZ) due to an increase in the number of levels and in the signal dependency of signal-spontaneous beat noise. In comparison with conventional 4-ary system, the proposed system depicts ∼6.5 dB improvement in terms of receiver sensitivity.     

Gain-saturated Ni-like antimony laser at 11.4 nm in grazing-incidence pumping geometry

We report on gain-saturated operation of the 4d → 4p, J = 0-1, 11.4 nm soft-X-ray laser line in Ni-like antimony (Sb) at a pump energy of only 2.5 J. The driving laser used was a 1054 nm Nd:glass CPA laser system with a pulse duration of 7 ps (FWHM). The pump beam was focused with a tilted on-axis parabolic mirror in a grazing-incidence (GRIP) pumping configuration at an incidence angle of 45°. A fraction of 2.8% of the pump energy (∼70 mJ) was used for the prepulse, which was propagated along the same beam line as the main pulse and arrived at the target 4.4 ns before the main pulse.     

Characterization of ultra-weak fluorescence using picosecond non-collinear optical parametric amplifier

We report a technique for characterization of ultra-weak fluorescence based on a 355-nm pumped picosecond non-collinear optical parametric amplifier (OPA). In the experiment, we effectively reduced the strong super-fluorescence background by using a series of methods. With the picosecond OPA as the pre-amplifier and the gating pulse, the decay of the fluorescence of Rhodamine 6G dye in ethanol was measured and the fluorescence lifetime was found to be about 941 ps. The gain factor of this parametric fluorescence amplifier was measured to be ∼4.2 × 10⁶, while the energy detection limit was ∼160 aJ per pulse within a 15-ps gating pulse.     

Study of the effect of annealing on defects in Fe-Mn-Si-Cr-Ni-C alloy by slow positron beam

FeMnSi shape memory alloys (SMAs) have received much attention as one-way SMAs due to their cost-effectiveness. Variable-energy (0-30 keV) positron beam studies have been carried out on a Fe-Mn-Si-Cr-Ni-C alloy with different degrees of deformation. Doppler broadening profiles of the positron annihilation as a function of incident positron energy were shown to be quite sensitive to defects introduced by deformation. The variation of the nature and the concentration of defects are studied as a function of isochronal annealing temperature. These results are correlated with the data measured with the positron annihilation lifetime spectroscopy (PALS). The positron annihilation results are compared to XRD and optical microscopy (OM).     

Application of positron annihilation spectroscopy on the ion implantation damaged Fe-Cr alloys

Four different Fe-Cr binary alloys with Cr content 2.5-11 wt% were studied in details using various methods. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were applied to obtain basic information, required for standard positron annihilation lifetime spectroscopy (PALS) spectra analysis. Additionally, PALS measurements were performed on as-received state as well as on helium implanted specimens. The He implantation was proposed for simulation of radiation damage and obtain high doses even in near surface areas (up to 1 μm). The implantation was based on the SRIM code simulation and next DPA calculations. Final concentration of vacancy type defects were calculated for 250 keV He²⁺ beam and the maximum was determined in 600 μm depth. Such specimens are very suitable for positron beam study of vacancy type defect mobility as a result of thermal treatment, which will be performed simultaneously in the future.     

Modifications of AISI 1045 steel by picosecond Nd:YAG laser at 266 nm; comparison with 532 and 1064 nm pulses

Interaction of Nd:YAG laser, operating at 266 nm wavelength and a pulse duration of 40 ps, with AISI 1045 steel was studied. Surface damage threshold was estimated to be 0.14 J/cm². The steel surface modification was studied at the laser fluence of ∼1.0 J/cm². The energy absorbed from Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following AISI 1045 steel surface morphological changes and processes were observed: (i) intensive damage of the target in the central zone of irradiated area; (ii) appearance of periodic surface structures at nano-level, with periodicity in agreement with the used wavelength; (iii) reduction of oxygen concentration in irradiated area; and (iv) development of plasma in front of the target. Generally, interaction of laser beam with AISI 1045 steel (at 266 nm) results in a near-instantaneous creation of damage, meaning that large steel surfaces can be modified in short times.     

Excitation relaxation and structure of TPPS4 J-aggregates

The energy relaxation kinetics and the structure of the J-aggregates of water-soluble porphyrin 5,10,15,20-tetrasulphonatophenyl porphine (TPPS₄) were investigated in aqueous medium by means of time-resolved fluorescence spectroscopy and confocal laser-scanning fluorescence microscopy. The excitation of the J-aggregates, at excitation intensities higher than ∼10¹⁵ photons/cm² per pulse, results in a remarkable decrease of the fluorescence quantum yield and in the appearance of an additional, non-exponential energy relaxation channel with a decay constant that depends on the excitation intensity. This relaxation mechanism was attributed to the exciton single-singlet annihilation. The exciton lifetime in the absence of the annihilation was calculated to be ∼150 ps. Using exciton annihilation theory, the exciton migration within the J-aggregates could be characterized by determining the exciton diffusion constant (1.8±0.9)  10⁻³ cm²/s and the hopping time (1.2±0.6) ps. Using the experimental data, the size of the J-aggregate could be evaluated and was seen to yield at least 20 TPPS₄ molecules per aggregate. It was shown by means of confocal fluorescence laser scanning microscopy that TPPS₄ does self-associate in polyvinyl alcohol (PVA) at acidic pH forming molecular macro-assemblies on a scale of ∼1 μm in PVA matrices.     

Tunable slow and fast light with large bandwidth in the band-edge of gain spectrum of the wide band fiber optical parametric amplifier

We propose a novel method theoretically to generate the slow and fast light with large bandwidth and low gains, which is based on the parametric process in fiber. In our scheme, the wide band fiber optical parametric amplifier is employed and the whole signal bandwidth should be located at a certain frequency range of the band-edge of gain spectrum, and then signal waves will be delayed or advanced with low signal gains because of the peculiar feature of signal gain and phase shift. By changing the pump power, the delay time is continuously-tunable optically. The ultimate delay bandwidth and the delay bandwidth product are constrained by the shape of time delay spectrum. Our simulation verifies that 22.4 ps delay or advanced time for the bandwidth of 10 GHz with little distortion can be obtained at certain wavelengths in the optical communication waveband, and their gains are nearly zero. The tunable range is from 0 ps to 22.4 ps for the signal bandwidth of 10 GHz, and it is from 0 ps to 15.6 ps for the bandwidth of 15 GHz. This type of slow and fast light in wide band FOPA has the potential capability to produce the tunable slow and fast light for large bandwidth with low signal gains in future.     

Positron trapping at quantum-dot-like particles on metal surfaces

Measurements of the positron annihilation-induced Auger electron (PAES) spectra from the Fe-Cu alloy surfaces with quantum-dot-like Cu nanoparticles embedded in Fe reveal a decrease of the Fe M_2,3VV positron annihilation-induced Auger signal intensity and an enhancement of the Cu one for surfaces created by enriching the Cu content of the Fe-Cu alloy. These experimental results are analyzed by performing calculations of positron surface states and annihilation characteristics at the Fe(1 0 0) surface with quantum-dot-like Cu nanoparticles embedded in the top atomic layers of the host substrate. Estimates of the positron binding energy and annihilation characteristics reveal their strong sensitivity to the nanoparticle coverage. Theoretical core annihilation probabilities are compared with experimental ones estimated from the measured Auger peak intensities. The observed behavior of the Fe and Cu PAES signal intensities is explained by theoretical calculations as being due to trapping of positrons in the regions of Cu nanoparticles embedded in the top atomic layers of Fe.