Applied Surface Science (v.252, #9)

Preface by Ilham Al-Qaradawi; Paul Coleman (3093).

In memory of Tom van Veen (1942–2004) by H. Schut; L.V. Jørgensen (3094-3097).

First positron experiments at NEPOMUC by C. Hugenschmidt; K. Schreckenbach; M. Stadlbauer; B. Straßer (3098-3105).
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.
Keywords: NEPOMUC; Positron beam; High intensity; PAES; CDB;

EPOS—An intense positron beam project at the ELBE radiation source in Rossendorf by R. Krause-Rehberg; S. Sachert; G. Brauer; A. Rogov; K. Noack (3106-3110).
EPOS, the acronym of ELBE Positron Source, describes a running project to build an intense pulsed beam of mono-energetic positrons (0.2–40 keV) for materials research. Positrons will be created via pair production at a tungsten target using the pulsed 40 MeV electron beam of the superconducting linac electron linac with high brilliance and low emittance (ELBE) at Forschungszentrum Rossendorf (near Dresden, Germany). The chosen design of the system under construction is described and results of calculations simulating the interaction of the electron beam with the target are presented, and positron beam formation and transportation is also discussed.
Keywords: Positron beam; Intense positron source;

Planned positron experiments at FRM-II by G. Kögel; G. Dollinger (3111-3120).
The new research reactor FRM-II near Munich has a strong positron source, which delivers an intense, nearly monoenergetic positron beam. Our positron systems, the pulsed low energy positron source (PLEPS) and the scanning positron microscope (SPM) will be operated at this beam. Some aspects of matching these systems to the new positron source will be discussed.Considerable improvements are expected, e.g. more than 105  s−1 recorded events at PLEPS and sub-micrometre resolution at SPM. They will enable investigations in so far inaccessible problems like the evaluation of annihilation characteristics and trapping constants of individual defects or studies of fast dynamical processes. In applied materials science complex defect structures will be studied which demand a resolution into many differing lifetimes, e.g. fractured specimens, wear, corrosion, etc. Also large series of measurements at small systematic modifications are planned. There is also the opportunity to analyse in addition the chemical microstructure of the specimens by means of a hydrogen microprobe and other ion beam techniques available close to FRM-II at the Technical University of Munich.
Keywords: Intense pulsed positron beams; Dynamic positron studies; Positron microbeam; Positron microscopy;

Design of a new type positron beam system by Y.C. Wu; B. Wang; S.J. Wang (3121-3125).
A new type positron beam system is being constructed in Wuhan university. The goal of this project is to build a positron beam which can measure positron lifetimes and has high moderation efficiency. The system utilizes a magnetically guided incident positron beam and the sample is biased to a high negative potential to achieve the desired implantation energies. A conventional tungsten moderator is replaced by a solid Ne moderator with high moderation efficiency (about 1%). A multi-functional target chamber for slow beam studies is designed, which can be used for positron annihilation lifetime spectroscopy (PALS), Doppler broadening (DB) and coincidence Doppler broadening (CDB) measurements.
Keywords: Positron; Slow positron; Positron beam; Moderator; Secondary electron; Positron lifetime; CDB;

Transmission positron microscopes by Masao Doyama; Yoshiaki Kogure; Miyoshi Inoue; Toshikazu Kurihara; Toshimasa Yoshiie; Ryuichiro Oshima; Miyuki Matsuya (3126-3131).
Immediate and near-future plans for transmission positron microscopes being built at KEK, Tsukuba, Japan, are described. The characteristic feature of this project is remolding a commercial electron microscope to a positron microscope. A point source of electrons kept at a negative high voltage is changed to a point source of positrons kept at a high positive voltage. Positional resolution of transmission microscopes should be theoretically the same as electron microscopes. Positron microscopes utilizing trapping of positrons have always positional ambiguity due to the diffusion of positrons.
Keywords: Positron microscopes; Positron–electron microscopes; Transmission; Positron microscopes;

Performance of a slow positron beam using a hybrid lens design by C.K. Cheung; P.S. Naik; C.D. Beling; S. Fung; H.M. Weng (3132-3137).
The University of Hong Kong positron beam employs conventional magnetic field transport to the target, but has a special hybrid lens design around the positron moderator that allows the beam to be focused to millimeter spot sizes at the target. The good focusing capabilities of the beam are made possible by extracting work-function positrons from the moderator in a magnetic field free region using a conventional Soa lens thus minimizing beam canonical angular momentum. An Einzel lens is used to focus the positrons into the magnetic funnel at the end of transportation magnetic field while at the same time bringing up the beam energy to the intermediate value of 7.5 keV. The beam is E  ×  B filtered at this intermediate energy. The final beam energy is obtained by floating the Soa–Einzel system, E  ×  B filter and flight tube, and accelerating the positrons just before the target. External beam steering saddle coils fine tune the position, and the magnetic field around the target chamber is adjusted so as to keep one of the beam foci always on the target. The system is fully computer controlled. Variable energy-Doppler broadened annihilation radiation (VEDBAR) data for a GaN sample are shown which demonstrate the performance of the positron beam system.
Keywords: Focusing; Positron beam; Hybrid lens; S-parameter imaging;

Preliminary studies on a variable energy positron annihilation lifetime spectroscopy system by P.Y. Kwan; C.K. Cheung; C.D. Beling; S. Fung (3138-3142).
There are many advantages in being able to perform positron annihilation lifetime spectroscopy (PALS) using a variable energy positron beam, the most obvious being the easy identification of different defect types at different depths. The difficulty in conducting variable energy (VE) PALS studies lies in the fact that a “start” signal is required to signal the entry of the positron into the target. Two methods have been used to overcome this problem, namely the bunching technique, which employs radio frequency (RF) cavities and choppers, and secondly the use of secondary electrons emitted from the target. The latter technique is in terms of experimental complexity much simpler, but has in the past suffered from poor time resolution (typically ∼500 ps). In this work, we present a series of computer simulations of a design based on the secondary electron emission from thin C-foils in transmission mode which shows that significant improvements in time resolution can be made with resolutions ∼200 ps being in principle possible.
Keywords: Lifetime spectroscopy; Secondary electrons; Timing;

The design of the main accelerator for a pulsed positron beam by A. Pelli; A. Laakso; K. Rytsölä; K. Saarinen (3143-3147).
The pulsed positron beam at the Helsinki University of Technology is designed for the end energy of 3–30 keV and grounded target. This is achieved with a constant voltage acceleration followed by an adjustable deceleration. In the design of this accelerator–decelerator the possibility for electrical breakdowns and partial discharges must be eliminated.For designing the electrode and insulator structures for accelerator–decelerator configuration electric field simulations were carried out with the finite element analysis program. In this paper we present the design of the accelerator–decelerator and the results of the electric field simulations. The results of high voltage tests will also be presented and compared with the simulations.
Keywords: Pulsed positron beam; High voltage; Accelerator; Breakdown; Partial discharge;

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.
Keywords: Positron spectroscopy; Pulsed positron beam; Electron scattering;

We have constructed a simple positron beam for measuring both the efficiencies of positron moderator foils and the positron yields of encapsulated positron beam sources. The slow-positron emission rate from the moderator is determined from the positron annihilation radiation. This technique is also applied to measure source yields. In addition to the coincidence technique, the source yield is determined by measuring electric currents generated by the annihilating positrons. The performance of the system is demonstrated with different tungsten moderator foils before and after heat treatments, and several Na 22 sources with primary activities in the range 1–50 mCi.
Keywords: Positron beam; Positron moderator; Positron source;

Intense slow positron production at the 15 MeV LINAC at Argonne National Laboratory by H.M. Chen; Y.C. Jean; C.D. Jonah; S. Chemerisov; A.F. Wagner; D.M. Schrader; A.W. Hunt (3159-3165).
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.
Keywords: EGS4; Computer simulation; Intense slow positron beam; LINAC;

Applications of slow positrons to cancer research: Search for selectivity of positron annihilation to skin cancer by Y.C. Jean; Ying Li; Gaung Liu; Hongmin Chen; Junjie Zhang; Joseph E. Gadzia (3166-3171).
Slow positrons and positron annihilation spectroscopy (PAS) have been applied to medical research in searching for positron annihilation selectivity to cancer cells. We report the results of positron lifetime and Doppler broadening energy spectroscopies in human skin samples with and without cancer as a function of positron incident energy (up to 8 μm depth) and found that the positronium annihilates at a significantly lower rate and forms at a lower probability in the samples having either basal cell carcinoma (BCC) or squamous cell carcinoma (SCC) than in the normal skin. The significant selectivity of positron annihilation to skin cancer may open a new research area of developing positron annihilation spectroscopy as a novel medical tool to detect cancer formation externally and non-invasively at the early stages.
Keywords: Cancer; Positron annihilation; Skin; Medicine; Non-invasive;

Positron beam studies of transients in semiconductors by C.D. Beling; C.C. Ling; C.K. Cheung; P.S. Naik; J.D. Zhang; S. Fung (3172-3182).
Vacancy-sensing positron deep level transient spectroscopy (PDLTS) is a positron beam-based technique that seeks to provide information on the electronic ionization levels of vacancy defects probed by the positron through the monitoring of thermal transients. The experimental discoveries leading to the concept of vacancy-sensing PDLTS are first reviewed. The major problem associated with this technique is discussed, namely the strong electric fields establish in the near surface region of the sample during the thermal transient which tend to sweep positrons into the contact with negligible defect trapping. New simulations are presented which suggest that under certain conditions a sufficient fraction of positrons may be trapped into ionizing defects rendering PDLTS technique workable. Some suggestions are made for techniques that might avoid the problematic electric field problem, such as optical-PDLTS where deep levels are populated using light and the use of high forward bias currents for trap filling.
Keywords: Positron beam; Vacancy sensing; Deep level; Deep level spectroscopy;

Real-time full spectrum fitting of beam-based Doppler broadening data by P.G. Coleman; C.P. Burrows; R.E. Mason (3183-3187).
A full spectrum fitting code, developed and used by the Bath positron group to compute the fractions of positrons annihilated from surface, defect and bulk states as a function of incident positron energy, is described. To date, the most useful application of the code has been in studies of ion-implanted silicon, being particularly useful in the observation of the formation of oxygen-divacancy complexes. The advantages and limitations of the code are discussed.
Keywords: Data analysis; Annihilation spectra; Vacancies; Silicon;

In thin film electronic applications, the limiting factor, in terms of cost and usability, is generally the substrate material. As a consequence, different materials are being investigated as potential lightweight, inexpensive and flexible substrates. In this respect, we have been the first research collaboration to produce silicon-based electronics on paper substrates. Here we present structural characterisation of hydrogenated amorphous silicon (a-Si:H) layers deposited on 80 g m−2 wood-free paper, with and without an intermediate metallic interlayer, using low temperature hot wire chemical vapour deposition (HW-CVD). Both pulsed positron beam profiling and X-ray diffraction studies indicate that the growth rate on the uncoated substrate is slightly higher than with prior metallization. There is no evidence of a crystalline phase or voids in the a-Si:H layers. The internal defect structure is similar, with a dominant dangling bond complex of similar size, which has a slightly longer lifetime than in layers grown at higher temperatures on conventional substrates.
Keywords: Hydrogenated amorphous silicon; Positron lifetime spectroscopy; Cheap paper substrate;

Local structure reconstruction in hydrogenated amorphous silicon from angular correlation and synchrotron diffraction studies by D.T. Britton; E. Minani; D. Knoesen; H. Schut; S.W.H. Eijt; F. Furlan; C. Giles; M. Härting (3194-3200).
Hydrogenated amorphous silicon (a-Si:H) is a widely used thin film semiconductor material which is still incompletely understood. It is generally assumed to form a continuous random network, with a high concentration of coordination defects (dangling bonds), which are hydrogen terminated. Neither the exact nature of these sites nor the degree of medium range order has been fully determined. In this paper, we present the first results for the local structure, from a combined study using angular correlation of positron annihilation radiation (ACAR) and synchrotron radiation diffraction. Reciprocal space information is obtained directly, for the mesoscale structure and the local defect structure, from the orientation dependent diffraction and 2D-ACAR patterns, respectively. Furthermore, inversion of both patterns yields a comparison of real space information through maps of the silicon–silicon pair correlation function and the electron–positron autocorrelation function B 2γ (r). From this information, it is possible to identify the dominant structural defect as a vacancy-size dangling bond cluster, around which the network strain is fully relaxed.
Keywords: Amorphous silicon; ACAR; Synchrotron; Autocorrelation; Pair correlation; Continuous random network;

Electrical properties of MOS structures on nitrogen-doped Czochralski-grown silicon: A positron annihilation study by V. Slugeň; L. Harmatha; M. Ťapajna; P. Ballo; P. Písečný; J. Šik; G. Kögel; V. Kršjak (3201-3208).
Measurements of interface trap density, effective generation lifetime (GL) and effective surface generation velocity have been performed using different methods on selected MOS structures prepared on nitrogen-doped Czochralski-grown (NCz) silicon. The application of the positron annihilation technique using a pulsed low energy positron system (PLEPS) focused on the detection of nitrogen-related defects in NCz silicon in the near surface region. In the case of p-type Cz silicon, all the results could be used for the testing of homogeneity. In n-type Cz silicon, positron annihilation was found insensitive to nitrogen doping.
Keywords: Czochralski-grown silicon; Nitrogen doping; Positron annihilation; Slow positron beam; MOS structure; Generation lifetime;

Self-implantation of Cz-Si: Clustering and annealing of defects by D.A. Abdulmalik; P.G. Coleman; I.Y. Al-Qaradawi (3209-3214).
Observations of vacancy clusters formed in Czochralski (Cz) Si after high energy ion implantation are reported. Vacancy clusters were created by 2 MeV Si ion implantation of 1 × 1015  ions/cm2 and after annealing between 600 and 650 °C. Doppler broadening measurements using a slow positron beam have been performed on the self-implanted Si samples, both as-implanted and after annealing between 200 and 700 °C for time intervals ranging from 15 to 120 min. No change in the S parameter was noted after the thermal treatment up to 500 °C. However, the divacancies (V2) created as a consequence of the implantation were found to start agglomerating at 600 °C, forming vacancy clusters in two distinct layers below the surface; the first layer is up to 0.5 μm and the second layer is up to 2 μm. The SW plots of the data suggest that clusters of the size of hexavacancies (V6) could be formed in both layers after annealing for up to an hour at 600 °C or half an hour at 650 °C. After annealing for longer times, it is expected that vacancies are a mixture of V6 and V2, with V6 most probably dominating in the first layer. Further annealing for longer times or higher temperatures breaks up the vacancy clusters or anneals them away.
Keywords: Ion implantation; Cz-Si; Vacancy clusters; Positron annihilation;

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.
Keywords: InP; Defect evolution; Cavities; Annealing; Doppler broadening;

Positron annihilation studies of mesoporous silica films using a slow positron beam by Chunqing He; Makoto Muramatsu; Toshiyuki Ohdaira; Atsushi Kinomura; Ryoichi Suzuki; Kenji Ito; Yoshinori Kabayashi (3221-3227).
Positron annihilation lifetime spectra were measured for mesoporous silica films, which were synthesized using triblock copolymer (EO106PO70EO106) as a structure-directing agent. Different positron lifetime spectra for the deposited and calcined films indicated the formation of meso-structure after calcination, which was confirmed by Fourier transform infrared (FTIR) spectra and field emission-scanning electron microscopy (FE-SEM) observation. Open porosity or pore interconnectivity of a silica film might be evaluated by a two-dimensional positron annihilation lifetime spectrum of an uncapped film. Pore sizes and their distributions in the silica films were found to be affected by thermal treatments.
Keywords: Positron annihilation; Mesoporous film; Slow positron beam;

A system for positron beam-based Doppler broadening spectroscopy of the formation and evolution of monovacancy defects in silicon is described. The apparatus allows in situ ion implantation at low temperatures (∼50 K) followed by positron beam assay. First measurements for 6 keV He implantation, at post-implant temperatures between 60 and 300 K are presented. Benefits and drawbacks of this system are discussed.
Keywords: Monovacancies; Ion implantation; Positron beams;

Modifications of He implantation induced cavities in silicon by MeV silicon implantation by P. Desgardin; M.-F. Barthe; E. Ntsoenzok; C.-L. Liu (3231-3236).
Doppler broadening spectroscopy (DBS) coupled to a slow positron beam has been used to investigate the formation of He-cavities in the presence of high vacancy concentrations in Cz-Si (1 1 1). Si samples were first implanted with MeV Si ions in order to create a damaged Si layer. DBS measurements show the presence of divacancy ( S V 2 / S Si lattice = 1.052 , W V 2 / W Si lattice = 0.83 ) from the surface up to 4.2 μm depth with a concentration higher than 1018  cm−3. The thickness of this damaged layer was confirmed by spreading resistance measurements. In the second step, samples were implanted with 50 keV 3He with fluence of 1016  cm−2. DBS results show that the apparent divancancy concentration decreases at 3He implantation depth ∼435 nm due to 3He passivation of vacancies that occurs during the implantation process. After 900 °C annealing, large defects are detected at depth up to 2 μm and (S, W) values suggest the detection of cavities at the implantation depth. We also report the possible presence of impurity complexes. The formation of these complexes is attributed to the gettering of metallic impurities present in the Si sample.
Keywords: DBS measurements; Ion implantation; Silicon; He-cavities; XTEM; Impurities; Contamination;

Defect studies of hydrogen-loaded thin Nb films by J. Čížek; I. Procházka; G. Brauer; W. Anwand; A. Mücklich; R. Kirchheim; A. Pundt; C. Bähtz; M. Knapp (3237-3244).
Hydrogen interaction with defects in thin niobium (Nb) films was investigated using slow positron implantation spectroscopy (SPIS) combined with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thin Nb films on Si substrates were prepared using cathode beam sputtering at room temperature. Initially, the microstructure of the virgin (hydrogen-free) films was characterized. Subsequently, the films were step-by-step electrochemically charged with hydrogen and the evolution of the microstructure with increasing hydrogen concentration was monitored. Hydrogen loading leads to a significant lattice expansion which was measured by XRD. Contrary to free-standing bulk metals, thin films are highly anisotropic. The in-plane expansion is prevented because the films are clamped on the elastically hard substrate. On the other hand, the out-of-plane expansion is substantially higher than in the bulk samples. Moreover, an enhanced hydrogen solubility in the α-phase was found in nanocrystalline Nb films. It was found that most of positrons in the films are trapped at open-volume defects at grain boundaries (GBs). These defects represent trapping sites also for hydrogen atoms. Hydrogen trapping at vacancy-like defects like GBs leads to a local increase of the electron density and is reflected by a pronounced decrease of the S parameter in the hydrogen-loaded samples. In addition, it was found that new defects are introduced at higher concentrations of hydrogen due to the formation of NbH (β-phase) particles.
Keywords: Niobium films; Hydrogen; Slow positron implantation spectroscopy; Doppler broadening; X-ray diffraction;

Defects in nanocrystalline Nb films: Effect of sputtering temperature by J. Čížek; O. Melikhova; I. Procházka; G. Brauer; W. Anwand; A. Mücklich; R. Kirchheim; A. Pundt (3245-3251).
Thin niobium (Nb) films (thickness 350–400 nm) were prepared on (1 0 0)Si substrate in a UHV chamber using the cathode beam sputtering. The sputtering temperature T s was varied from 40 up to 500 °C and the influence of the sputtering temperature on the microstructure of thin Nb films was investigated. Defect studies of the thin Nb films sputtered at various temperatures were performed by slow positron implantation spectroscopy (SPIS) with measurement of the Doppler broadening of the annihilation line. SPIS was combined with transmission electron microscopy (TEM) and X-ray diffraction (XRD). We have found that the films sputtered at T s  = 40 °C exhibit elongated, column-like nanocrystalline grains. No significant increase of grain size with T s (up to 500 °C) was observed by TEM. The thin Nb films sputtered at T s  = 40 °C contain a high density of defects. It is demonstrated by shortened positron diffusion length and a high value of the S parameter for Nb layer compared to the well-annealed (defect-free) bulk Nb reference sample. A drastic decrease of defect density was found in the films sputtered at T s  ≥ 300 °C. It is reflected by a significant increase of the positron diffusion length and a decrease of the S parameter for the Nb layer. The defect density in the Nb layer is, however, still substantially higher than in the well-annealed reference bulk Nb sample. Moreover, there is a layer at the interface between the Nb film and the substrate with very high density of defects comparable to that in the films sputtered at T s  < 300 °C. All the Nb films studied exhibit a strong (1 1 0) texture. The films sputtered at T s  < 300 °C are characterized by a compressive macroscopic in-plane stress due to lattice mismatch between the film and the substrate. Relaxation of the in-plane stress was observed in the films sputtered at T s  ≥ 300 °C. The width of the XRD profiles of the films sputtered at T s  ≥ 300 °C is significantly smaller compared to the films sputtered at lower temperatures. This is most probably due to a lower defect density which results in reduced microstrains in the films sputtered at higher temperatures.
Keywords: Niobium films; Cathode beam sputtering; Slow positron implantation spectroscopy; X-ray diffraction;

Positron and deuteron depth profiling in helium-3-implanted electrum-like alloy by R.I. Grynszpan; N. Baclet; A. Darque; J.L. Flament; F. Zielinski; W. Anwand; G. Brauer (3252-3255).
In spite of previous extensive studies, the helium behavior in metals still remains an issue in microelectronics as well as in nuclear technology. A gold–silver solid solution (Au60Ag40: synthetic gold-rich electrum) was chosen as a relevant model to study helium irradiation of heavy metals. After helium-3 ion implantation at an energy ranging from 4.2 to 5.6 MeV, nuclear reaction analysis (NRA) based on the 3He(d,p)4He reaction, was performed in order to study the thermal diffusion of helium atoms. At room temperature, NRA data reveal that a single Gaussian can fit the He-distribution, which remains unchanged after annealing at temperatures below 0.45 of the melting point. Slow positron implantation spectroscopy, used to monitor the fluence dependence of induced defects unveils a positron saturation trapping, which occurs for He contents of the order of 50–100 appm, whereas concentrations larger than 500 appm seem to favor an increase in the S-parameter of Doppler broadening. Moreover, at high temperature, NRA results clearly show that helium long range diffusion occurs, though, without following a simple Fick law.
Keywords: Ion-implantation; Gold; Silver; Helium; Positrons; NRA;

Vacancy defects induced in sintered polished UO2 disks by helium implantation by H. Labrim; M.-F. Barthe; P. Desgardin; T. Sauvage; G. Blondiaux; C. Corbel; J.P. Piron (3256-3261).
Vacancy defects have been investigated in sintered polished and annealed uranium oxide disks. Slow positron beam coupled with Doppler broadening spectrometer was used to probe the track region of 1 MeV 3He ions implanted in uranium dioxide (UO2) disks. The low and high momentum annihilation fractions, S and W, respectively, were measured in the first micrometer near surface region of the disks as a function of positron energy. The S and W values indicate that the 1 MeV He ions induce vacancy defects in the track region of their range. The vacancy defect depth distribution is heterogeneous. The positron trapping at these vacancy defects increases with the depth and with the implantation fluence indicating an increase of the vacancy defect concentration. The nature of the induced vacancy defects does not change with the fluence.
Keywords: Slow positron beam; Uranium dioxide; Vacancy defects; Helium implantation; Doppler broadening;

Thermal evolution of vacancy defects induced in sintered UO2 disks by helium implantation by H. Labrim; M.-F. Barthe; P. Desgardin; T. Sauvage; G. Blondiaux; C. Corbel; J.P. Piron (3262-3268).
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 × 1016 3He 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 × 1014 3He 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 × 1016 3He 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.
Keywords: Slow positron beam; Uranium dioxide; Vacancy defects; Helium implantation; Doppler broadening; Thermal evolution; Hydrogen;

Spatially resolved investigation of thermally treated brass with a coincidence Doppler spectrometer by M. Stadlbauer; C. Hugenschmidt; C. Piochacz; B. Straßer; K. Schreckenbach (3269-3273).
A spatially resolving coincidence Doppler spectrometer has been developed at the intense positron beam NEPOMUC at the nuclear research reactor FRMII in Garching near Munich. The positrons are focused onto the sample which is positioned in a vacuum chamber in order to achieve lateral resolution. The emitted annihilation radiation is detected by two facing germanium detectors in coincidence in order to enable background-free measurements. Since the measured spectra are not degraded by background, the high momentum region of the annihilation line can be precisely analysed. The positron beam structure is described and first measurements are presented in which thermally treated brass was investigated in order to determine the chemical environment of the positron at the annihilation site.
Keywords: NEPOMUC; Coincidence Doppler broadening; Brass;

Slow positron beam study of corrosion-related defects in pure iron by Y.C. Wu; Y.Q. Chen; B. Wang; S.J. Wang; Y.C. Jean; R. Suzuki; T. Ohdaira (3274-3277).
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 τ 2  = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ 1, 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 τ 1 and the intensity I 2, 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.
Keywords: Positron annihilation; Slow positron beam; Corrosion; Iron; Defect;

Hydrogen damage of AISI 304 stainless steel has been systemically investigated by measuring Doppler broadening of positron annihilation. Defect profiles of the S-parameter, the low-momentum annihilation fraction as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) have been analyzed. Experimental results show that hydrogen damage between the surface and the bulk has a significant variation with depth, and strongly depends on the condition of hydrogen-charging, i.e. current density and charging time. It has been suggested that the increase in S-parameter near the surface after hydrogen-charging mainly comes from the formation of voids; however the increase in S-parameter in the bulk after hydrogen-charging mainly comes from the production of structural defects (dislocations). Defect densities induced due to hydrogen-charging in some cases (e.g. dislocation density in the bulk) are estimated based on the simple two-state trapping model.
Keywords: Positron annihilation; Slow positron beam; Hydrogen; Steel; Defects;

Vacancy–solute complexes in aluminum by O. Melikhova; J. Kuriplach; J. Čížek; I. Procházka (3285-3289).
Several vacancy–solute complexes in the Al matrix are examined theoretically. In particular, these are V–Cu, V–Cd, V–In, V–Sn, V–Si and V–Fe. We concentrate on coincidence Doppler broadening (high momentum) profiles and positron lifetimes that bring complementary information about these defects. Positron calculations are carried out utilizing the atomic superposition method employing realistic atomic configurations obtained using an ab initio pseudopotential method. In this study we inspect to what extent such defects are detectable and differentiable using positron annihilation techniques. The influence of lattice relaxations around defects on the positron properties turns out to be important and is also debated. The obtained results are discussed in connection with experimental data published in literature.
Keywords: Aluminum alloys; Vacancy–solute complexes; Electron–positron momentum distribution; Atomic superposition method; Lattice relaxations;

Assessment of the correlation between mechanical testing and positron annihilation outcomes for RPV model alloys by Andrej Zeman; Luigi Debarberis; Vladimír Slugeň; Beatriz Acosta (3290-3296).
The correlation between recent PAS results and the outcomes from mechanical testing of RPV model alloys are presented, here significant changes due to different chemical composition and different irradiation levels are observed. The influence of alloying elements to the microstructure degradation process following irradiation was identified by analysis of the mean-lifetime parameter, since an interesting interdependency of this parameter with hardness was observed.
Keywords: Reactor pressure vessel; Model alloys; Irradiation embrittlement; Alloying elements; Positron annihilation;

Studying the recovery of as-received industrial Al alloys by positron annihilation spectroscopy by E.E. Abdel-Hady; A. Ashry; H. Ismail; S. El-Gamal (3297-3302).
Positron annihilation lifetime (PAL) spectroscopy, Doppler broadening of annihilation radiation (DBAR) spectroscopy and Vickers microhardness (Hv) measurements were performed to study the micro- and macro-structure variations during isochronal annealing from room temperature (RT) to 500 °C of commercial pure Al (1 1 0 0), Al–Mn–Mg (3 0 0 4) and Al–Mg–Si (6 2 0 1) alloys. Three annealing stages of microstructures have been identified as recovery, partial recrystallization and complete recrystallization followed by grain growth. A positive correlation between the macroscopic mechanical properties (Hv) and positron annihilation parameters has been achieved for the three samples under investigation.
Keywords: Al alloys; Positron annihilation lifetime; Doppler broadening; Vickers microhardness;

Vacancy-solute complexes and their clusters in iron by J. Kuriplach; O. Melikhova; C. Domain; C.S. Becquart; D. Kulikov; L. Malerba; M. Hou; A. Almazouzi; C.A. Duque; A.L. Morales (3303-3308).
In this contribution, several vacancy-solute complexes in iron are investigated theoretically from the viewpoint of positron annihilation. In particular, V-Si, V-P, V-Cr, V-Mn, V-Ni, V-Cu and V-Mo complexes are examined. In addition, nano-sized vacancy-Cu clusters in the Fe matrix are also studied. We concentrate on positron lifetimes and coincidence Doppler broadening profiles that bring complementary information about the studied complexes and their clusters. Positron calculations are carried out using the atomic superposition method employing realistic atomic configurations obtained recently using an ab initio pseudopotential method (vacancy-solute complexes) and Monte Carlo/molecular dynamics methods (vacancy-Cu clusters). The main aim of this study is to predict as to what extent such defects are detectable and differentiable using positron annihilation techniques. The results obtained are discussed in the context of experimental data available in the literature.
Keywords: Vacancy-solute complexes; Vacancy-solute clusters; Electron–positron momentum distribution; RPV steels; Atomic superposition method;

Positron study of radiation embrittlement of steels used in water cooled, water moderated energy reactors by Vladimír Slugeň; Andrej Zeman; Martin Petriska; Vladimir Kršjak (3309-3315).
Positron annihilation spectroscopy (PAS) lifetime study was applied in the evaluation of the microstructure parameters and degradation processes of nuclear reactor pressure vessel (RPV) steel surveillance specimens. Study was oriented to the material investigation of Russian WWER-1000 steels (15Kh2MNFAA and 12Kh2N2MAA) with higher Ni content (1.26 wt.% in base metal and 1.7 wt.% in weld). For comparison, the WWER-440 weld metal (Sv10KhMFT) without Ni was measured too. Specimens were studied in as received stage, after irradiation in LVR-15 experimental reactor to the neutron fluence F (E  > 0.5 MeV)  = 4 × 1023  m−2  s−1 and after annealing in vacuum at 475 °C/2 h. Post-irradiation thermal treatment and annealing of defects was well detected by different PAS techniques. It was observed that the sensitivity of PAS parameters to defined irradiation treatment decreases with Ni-content increase. Results confirm the hypothesis that Ni affects size (decrease) and distribution (more homogeneous) of the Cu- and P-rich clusters and M x C x carbides.
Keywords: Positron annihilation; Reactor pressure steels; WWER; Neutron embrittlement;

The effect of gamma irradiation and shelf aging in air on the oxidation of ultra-high molecular weight polyethylene by M.A. Al-Ma’adeed; I.Y. Al-Qaradawi; N. Madi; N.J. Al-Thani (3316-3322).
This study has investigated the effect of shelf aging, for up to one year in air, on the properties of gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE). A variety of techniques were used to characterize the properties of treated samples. Differential scanning calorimetery (DSC) was used to characterize the morphology. The extent of cross-linking in a polymer network was detected by swelling measurements. The durometer hardness test was used to measure the relative hardness of this material, and changes in density were also measured. Results from all these measurements were combined to explain the changes in the microstructure of the aged, irradiated UHMWPE. This study shows that crystallinity is increased with radiation dose and with aging due to chain scission, which leads to a reduction in the molecular weight of the material. This allows the chains to rearrange to form crystalline regions. Positron annihilation lifetime spectroscopy confirms these conclusions. Fractional free volumes have been deduced from lifetime parameters, which correlate with the data obtained by the other techniques.
Keywords: Free volume; Cross-linking; Crystallinity; UHMWPE; Oxidation; Gamma irradiation;

Mn–Zn ferrite substituted with rare earth ions have been investigated by means of positron annihilation lifetime spectroscopy (PALS). The variations of lifetime parameters τ av, I 2, and κ with ionic radius of rare earth ions, grain size and electrical resistivity for all samples have been studied. The intergranular pores increase with increasing the ionic radius of rare earth ions and grain size of the samples.
Keywords: Ferrite; Electrical resistivity; Substituted ferrite; Positron annihilation;

Positron trapping at quantum-dot-like particles on metal surfaces by N.G. Fazleev; J.L. Fry; M.P. Nadesalingam; A.H. Weiss (3327-3332).
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 M2,3VV 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.
Keywords: Quantum dot; Nanoparticle; Positron; Surface; Auger; Annihilation; Binding;

Positron probes of the Si(1 0 0) surface that plays a fundamental role in modern science and technology are capable to non-destructively provide information that is both unique to the probe and complimentary to that extracted using other more standard techniques. This paper presents a theoretical study of positron “image-potential” surface states and annihilation characteristics of surface trapped positrons at the Si(1 0 0) surface. Calculations are performed for the reconstructed Si(1 0 0)-p(2 × 2) surface using the modified superimposed-atom method to account for discrete-lattice effects, and the results are compared with those obtained for the non-reconstructed and reconstructed Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) surfaces. The effect of orientation-dependent variations of the atomic and electron densities on localization and extent of the positron surface state wave function at the semiconductor surface is explored. The positron surface state wave function is found to extend into the Si lattice in the regions where atoms are displaced from their ideal terminated positions due to the p(2 × 2) reconstruction. Estimates of the positron binding energy and positron annihilation characteristics reveal their sensitivity to the specific atomic structure of the topmost layers of Si. The observed sensitivity of annihilation probabilities to crystal face indicates that positron spectroscopy techniques could serve as an important surface diagnostic tool capable of distinguishing different semiconductor surfaces and defining their state of reconstruction.
Keywords: Silicon; Reconstruction; Surface; Positron; Localization; Annihilation;

Characterization of a SiC/SiC composite by X-ray diffraction, atomic force microscopy and positron spectroscopies by G. Brauer; W. Anwand; F. Eichhorn; W. Skorupa; C. Hofer; C. Teichert; J. Kuriplach; J. Cizek; I. Prochazka; P.G. Coleman; T. Nozawa; A. Kohyama (3342-3351).
A SiC/SiC composite is characterized by X-ray diffraction, atomic force microscopy and various positron spectroscopies (slow positron implantation, positron lifetime and re-emission). It is found that besides its main constituent 3C–SiC the composite still must contain some graphite. In order to better interpret the experimental findings of the composite, a pyrolytic graphite sample was also investigated by slow positron implantation and positron lifetime spectroscopies. In addition, theoretical calculations of positron properties of graphite are presented.
Keywords: SiC/SiC composite; Graphite; X-ray diffraction; Atomic force microscopy; Slow positron spectroscopy; Positron lifetime; Positron affinity; Positron re-emission;

The processes undergone by positrons in the interstellar medium (ISM) from the moments of their birth to their annihilation are examined. Both the physics of the positron interactions with gases and solids (dust grains), and the physical conditions and characteristics of the environments where the processes of energy loss, positronium formation, and annihilation taking place, are reviewed. An explanation is given as to how all the relevant physical information are taken into account in order to calculate annihilation rates and spectra of the 511 keV emission for the various phases of the ISM; special attention is paid to positron interactions with dust and with polycyclic aromatic hydrocarbons. An attempt is made to show to what extent the interactions between positrons and interstellar dust grains are similar to laboratory experiments in which beams of slow positrons impinge upon solids and surfaces. Sample results are shown for the effect of dust grains on positron annihilation spectra in some phases of the ISM which, together with high resolution spectra measured by satellites, can be used to infer useful knowledge about the environment where the annihilation is predominantly taking place and ultimately about the birth place and history of positrons in the Galaxy. The important complementarity between work done by the astrophysical and the solid-state positron communities is strongly emphasized and specific experimental work is suggested which could assist the modeling of the interaction and annihilation of positrons in the ISM.
Keywords: Positron processes; Annihilation; Spectra; Interstellar medium; Dust; PAHs;

Backscattering (Bcs) coefficients for low-energy positrons (∼100 eV) from elemental solids have been simulated using an analytic approach. The model is based on the use of the transport cross-sections (TCSs) and the stopping power calculated from partial wave methods and the best-fit stopping power data of Ashley, respectively. The new result is an extension of recent calculations in the medium energy range. Comparisons, when possible, with experimental and Monte-Carlo (MC) simulation data have been made.
Keywords: Backscattering; Simulation;

Results from coincidence Doppler broadening (CDB) measurements on various Si samples and Brazilian quartz having low quartz structure are presented with the aim to give further strong indication of the existence of a low quartz structure, but not of Si divacancies as frequently considered, at the SiO2/Si interface.
Keywords: Slow-positron spectroscopy; Coincidence Doppler broadening; Silicon; Brazilian quartz; Silicon divacancy; SiO2/Si interface;

A personal overview is given of the advances in positron beam studies of solids and surfaces presented at the 10th International Workshop on Positron Beams, held in Doha, Qatar, in March 2005. Solids studied include semiconductors, metals, alloys and insulators, as well as biophysical systems. Surface studies focussed on positron annihilation-induced Auger electron spectroscopy (PAES), but interesting applications of positron-surface interactions in fields as diverse as semiconductor technology and studies of the interstellar medium serve to illustrate once again the breadth of scientific endeavour covered by slow positron beam investigations.
Keywords: Solids; Surfaces; Positron beams;