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The VITO (Versatile Ion-polarized Techniques Online) project is a new experimental setup at the ISOLDE facility at CERN. VITO is a dedicated beam line for producing laser-induced spin-polarized beams of both, atoms and ions, and it has been commissioned in response to the continuously growing demand for the use of spin-polarized beams. The new VITO beam line is a modification of the formerly existing ultra-high vacuum beam line, connecting ASPIC (Apparatus for Surface Physics and Interfaces at CERN), and it has been under construction since the beginning of 2014. Once fully commissioned, VITO will open up numerous possibilities for carrying out multidisciplinary experiments in the areas of nuclear and solid state physics, fundamental interaction physics and biophysics. In its final stage the VITO beam line will provide three fully independent experimental stations: UHV chamber for material science applications, a P-asymmetry station where highly-polarized ions will be available, and a central open-end station suitable for travelling experiments. The VITO beam line will operate in two different modes providing either beams of spin-polarized atoms or ions, or non-polarized ion beams to all three end stations operating from 10(-10) mbar to 50 mbar. Recent experimental campaigns with stable and radioactive beams have allowed for testing VITO's constituent parts and have demonstrated 96% of ion beam transmission to the collection chamber installed on the central station. The first experimental results obtained with on-line Perturbed Angular Correlation (PAC) spectroscopy using Cu-68m ion-beams will be briefly discussed. (C) 2016 Elsevier B.V. All rights reserved.

Solid state physics (SSP) research at ISOLDE has been running since the mid-1970s and accounts for about 10%–15% of the overall physics programme. ISOLDE is the world flagship for the on-line production of exotic radioactive isotopes, with high yields, high elemental selectivity and isotopic purity. Consequently, it hosts a panoply of state-of-the-art nuclear techniques which apply nuclear methods to research on life sciences, material science and bio-chemical physics. The ease of detecting radioactivity—, We gratefully acknowledge many years of support given to the maintenance and upgrade of the solid state physics laboratories and provision of large scale equipment by the Federal Ministry of Education and Research (BMBF) through different grants, including 05K13TSA and 05K16PGA, from Fundacao para a Ciencia e a Tecnologia (FCT) project CERN-FIS-NUC-0004-2015 and Research Foundation Flanders (FWO) in collaboration with the University of Leuven (KU Leuven). ASF acknowledges the PhD grants support by FCT (SFRH/BD/84743/2012) and KU Leuven. ED-B and ARGC acknowledge FCT-PhD grants (SFRH/BD/95865/2013 and SFRH/BD/86386/2012.

One of the main obstacles to the technical application of the wide-gap semiconductor ZnO represents the difficulty to achieve reliable p-type doping of ZnO with group V elements (N, P, As, Sb) acting as acceptors located on O lattice sites. The theoretically proposed concepts of cluster-doping or codoping may lead to an enhanced and stable p-type conductivity of ZnO. We report on PAC results obtained by codoping experiments of ZnO by ion implantation using the donor 111In and the group-V acceptor Sb. The formation of In-Sb pairs has been observed. Based on these PAC results, there is no evidence for the formation of In-acceptor complexes involving more than one Sb acceptor. These results has been complemented by photoluminescence measurements.

We present a study of TiO2 single crystals from the point of view of the dopant atom that simultaneously behaves as the probing element. We used gamma-gamma time dependent perturbed angular correlations working with selected tracer elements (111In/ 111Cd, 111mCd/ 111Cd) together to investigate the different behavior of Cd and In dopants, particularly their interaction with point defects in the TiO2 lattice. Results show that the hyperfine interactions observed at 111Cd from 111In or 111mCd decay are quite different. 111In/ 111Cd results show a single site fraction characterized by a quadrupole frequency with asymmetry parameter similar to those observed for the same probe nuclei in bulk TiO2 oxides. Results for 111mCd/ 111Cd reveal two site fractions, one characterized by the same hyperfine parameters to those measured in bulk TiO2 and another fraction characterized by a quadrupole frequency and asymmetry parameters with higher values, as observed in thin TiO2 films and correlated with point defects. The results are discussed emphasizing the differences for Cd and In as defect traps on TiO2.

Using scanning photoluminescence spectroscopy (µPL) the diffusion of implanted Au atoms in p -type CdTe was investigated. After annealing at 800 K under Cd-rich atmosphere, uphill diffusion was observed in agreement with former radiotracer studies using the isotope 193Au. In addition, the µPL technique delivered information about the concentration profiles of intrinsic defects and residual impurities. The measurements support the model that describes uphill diffusion of acceptor dopants in terms of intrinsic defects, i.e. the Cd vacancy and Cd self-interstitial. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

One of the main obstacles to the technical application of the wide-gap semiconductor ZnO represents the difficulty to achieve reliable and sufficient p-type doping. The theoretically proposed concepts of cluster-doping or codoping should lead to an enhanced and stable p-type conductivity of ZnO. We report on PAC results obtained by codoping experiments of ZnO by ion implantation using the donor 111In and the group-V acceptors N and P. The formation of In-N and In-P pairs has been observed. Based on these PAC results, there is no evidence for the formation of In-acceptor complexes involving more than one N or P acceptor. Finally, first reports on PAC investigations using 77Br as donor in ZnO are presented.

In CdTe, the achievable n-type doping is limited by the formation of DX-centers. A characteristic feature of DX-centers is the ‘persistent photoconductivity (PPC)’ which is created by illumination at low temperatures and caused by a metastable state of the DX-center. The DX-center and the PPC effect in n-type CdTe are theoretically explained by the ‘large lattice relaxation model’. PAC measurements on In doped CdTe using 111In/111Cd and, in addition, resistivity measurements on the same samples have been performed. Below 150 K, the samples showed a PPC effect that was accompanied by an increase of about 20% of the carrier concentration. This effect is not accompanied by any changes of the observed EFG. Possible explanations of the EFG observed, originally assigned to the DX-center, will be discussed. Finally, first reports on the investigation of DX-centers in CdTe using the radioactive isotope 117Cd decaying to 117In are presented.

Radioactive atoms have been used in solid-state physics and in material science for many decades. Besides their classical application as tracer for diffusion studies, nuclear techniques such as Mossbauer spectroscopy, perturbed angular correlation, and emission channeling have used nuclear properties to gain microscopical information on the structural and dynamical properties of solids. The availability of many different radioactive isotopes as a clean ion beam at facilities like ISOLDE/CERN has triggered a new era involving methods sensitive for the optical and electronic properties of solids, especially in the field of semiconductor physics. Spectroscopic techniques like photoluminescence (PL), deep-level transient spectroscopy (DLTS), and Hall effect gain a new quality by using radioactive isotopes. Due to their decay the chemical origin of an observed electronic and optical behavior of a specific defect or dopant can be unambiguously identified. This contribution will highlight a few examples to illustrate the potential of radioactive isotopes for solving various problems connected to defects in semiconductor physics.

The structural properties of the nanocrystalline semiconductor ZnO (nano-ZnO) doped with the donor Indium were investigated by perturbed γγ angular correlation spectroscopy (PAC) and extended X-ray absorption fine structure measurements (EXAFS). Up to an average concentration of one In atom per nanocrystallite, PAC measurements show that about 12% of the 111In atoms are incorporated on substitutional Zn sites. At higher In concentrations, new In defect complexes are visible in the PAC spectra, which dominate the spectra if the average In concentration exceeds one In atoms per nanocrystallite. In addition, the local environment of Zn and In atoms in In doped nano-ZnO was investigated by EXAFS. The measurements at the K edge of Zn show that the crystal structure of nano-ZnO corresponds to bulk ZnO. In heavily In doped nano-ZnO the EXAFS experiments at the K edge of In exhibit an expansion of the first O shell about the In site. Since about four O atoms are detected in this first shell a substitutional incorporation of the In atoms in the ZnO lattice is suggested. The second shell to be occupied by Zn atoms as well as higher shells are almost invisible, which might have the same microscopic origin as the occurrence of defect complexes observed by PAC.

Radioactive atoms have been used in solid state physics and in materials science for decades. Besides their classical applications as tracers for diffusion studies, nuclear techniques such as Mossbauer spectroscopy, perturbedγγ angular correlation,β-NMR, and emission channeling make use of nuclear properties (via hyperfine interactions or emittedα orβ particles) to gain microscopic information on structural and dynamical properties of solids. During the last decade, the availability of many different radioactive isotopes as clean ion beams at ISOL facilities like ISOLDE/CERN has triggered a new era involving methods sensitive to the optical and electronic properties of solids, especially in the field of semiconductor physics. This overview will browse through ongoing solid state physics experiments with radioactive ion beams at ISOLDE. A wide variety of problems is under study, involving bulk properties, surfaces and interfaces in many different systems like semiconductors, superconductors, magnetic systems, metals and ceramics.

Radioactive atoms have been used in solid-state physics and in material science for many decades. Besides their classical application as tracer for diffusion studies, nuclear techniques such as Mofibauer spectroscopy, perturbed angular correlation, β-NMR, and emission channeling have used nuclear properties (via hyperfine interactions or emitted α- or β-particles) to gain microscopical information on the structural and dynamical properties of solids. During the last decade, the availability of many different radioactive isotopes as a clean ion beam at ISOL facilities like ISOLDE/CERN has triggered a new era involving methods sensitive for the optical and electronic properties of solids, especially in the field of semiconductor physics. Extremely sensitive spectroscopic techniques like deep-level transient spectroscopy (DLTS), photoluminescence (PL), and Hall effect gain a new quality by using radioactive isotopes: Due to their decay the chemical origin of an observed electronic and optical behavior of a specific defect or dopant can be unambiguously identified. This review will briefly introduce the experimental techniques used and browse through the ongoing experiments in solid-state physics using radioactive ion beams demonstrating the wide variety of problems under study involving bulk properties, surfaces and interfaces in many different systems like semiconductors, superconductors, magnetic systems, metals and ceramics.

The radioactive isotope 111mCd was implanted into BaTiO3 in order to measure electric field gradients (EFGs) with Perturbed Angular Correlation spectroscopy (PAC). It is possible to anneal the implantation induced lattice damage to a certain extent. Then, 111mCd probes are positioned at two sites with distinct axially symmetric EFGs characterized by the quadrupole coupling constants of 33.9(9) and 69.8(9) MHz. These EFGs can be assigned to 111mCd at the Ti- and the Ba-sites

The diffusion of Ag atoms in CdTe was investigated using the radiotracer 111Ag, which was introduced by implantation with an energy of 60 or 80 keV. The measured diffusion profiles are explained by assuming the existence of a repulsive interaction between Ag and residual Cu atoms causing a drift of the Ag atoms towards the centre of the crystal, which supposes the diffusion in a concentration gradient. This effect vanishes if the Ag concentration is increased and becomes more pronounced if the crystals are simultaneously co-doped with Cu.

The ferromagnetic spinels CdCr2Se4 and CuCr2Se4 were investigated by PAC (perturbed angular correlations) after implantation of the probes 111In(111Cd), 111mCd, 111Ag(111Cd) and 77Br(77Se). The site occupation of different probes was determined by magnetic and electric hyperfine interactions. Theoretical calculations of hyperfine parameters by the WIEN97 code gave satisfactory efg (electric field gradients) but the magnetic hyperfine fields are inconsistent with the experiments.

Electric field gradients have been measured at substitutional lattice sites in ternary semiconductors using Perturbed γ -γ Angular Correlation spectroscopy (PAC). The experimental results for AIBIIIC2 chalcopyrite structure compounds and • AIIB2 IIIC4 IV defect chalcopyrites are compared with ab nitio calculations. The latter were carried out with the WIEN code that uses the Full Potential Linearized Augmented Plane Wave method within a density functional theory. The agreement between experiment and theory is in most cases very good. Furthermore, the anion displacements in AgGaX2 -compounds (X: S, Se, Te) have been determined theoretically by determining the minimum of the total energy of the electrons in an elementary cell.

Single crystalline (0001) gallium nitride layers, capped with a thin epitaxial aluminum nitride layer, were implanted with magnesium and subsequently annealed in vacuum to 1150-1300 oC for 10-60 minutes. Photoluminescence (PL) measurements showed the typical donor acceptor pair (DAP) transition at 3.25 eV after annealing at high temperatures, which is related to optically active Mg acceptors in GaN. After annealing at 1300 °C a high degree of optical activation of the implanted Mg atoms was reached in the case of low implantation doses. Electrical measurements, performed after removing the AlN-cap and the deposition of Pd/Au contacts, showed no p-type behavior of the GaN samples due to the compensation of the Mg acceptors with native n-type defects.

In order to unambiguously identify the chemical nature of Cd and Ag related optical transitions in GaN, epitaxial GaN layers were implanted with the radioactive isotope 111 Ag which decays into stable 111 Cd . This chemical transmutation was monitored by photoluminescence (PL) spectroscopy. Being an element specific property, the half-life of this decay was used to establish the chemical assignment of the optical transitions to a specific defect. We found that the Ag related transitions consist of a series of four single lines (1.610, 1.600, 1.594, and 1.573 eV), each accompanied by two phonon replicas separated by 63 meV. Cd produces two PL bands centered at 2.7 and 3.2 eV. Additional Cd-related single transitions at 3.341, 3.328, and 3.249 eV have been observed. Exponential fits to the PL intensities yield half-lives of tAg1/2=(7.61±0.27) d and tCd1/2=(7.60±0.27) d, respectively, in good agreement with the half-life of 111 Ag of 7.45 d.

111 m Cd and 112 Cd ions have been implanted into GaN. With photoluminescence spectroscopy and perturbed γγ angular correlation spectroscopy (PAC) the reduction of implantation damage and the optical activation of the implants have been observed as a function of annealing temperature using different annealing methods. The use of N2 or NH3 atmosphere during annealing allows temperatures up to 1323 and 1373 K, respectively, but above 1200 K a strong loss of Cd from the GaN has been observed. Annealing GaN together with elementary Al forms a protective layer on the GaN surface allowing annealing temperatures up to 1570 K for 10 min.

Hydrogen radicals are decisive for the low-temperature growth and carbon doping of In 0.53 Ga 0.47 As in LP-MOVPE. This is demonstrated for the growth of highly p-type doped In 0.53 Ga 0.47 As layers with CCl 4 as dopant source. Perturbed angular correlation measurements (PAC) were used to investigate the passivation of acceptors by hydrogen in low-temperature grown In 0.53 Ga 0.47 As. Based on the above analysis an InP-based layer stack is developed which employs low-temperature growth of the base layer, high-temperature growth of the remaining HBT layers, and an in situ post-growth annealing under TMAs/N 2 ambient.

We report nuclear magnetic resonance on oriented nuclei (NMR-ON) measurements on 198 Au in a hcp Co single crystal which was magnetized with an external magnetic field B ext = 0−2 T applied at different angles with respect to the crystal c -axis. These experiments showed that the effective electric field gradient can be “tuned” continuously with the magnetic field, which opens new possibilities for the determination of small electric field gradients originating from an unquenched orbital momentum, and for the measurement of electric quadrupole moments of radioactive nuclei.

High‐concentration, mixed group‐III (In, Ga)/group‐V (P,As) ‐implanted silicon single crystals have been studied by combining Rutherford backscattering spectroscopy/channeling spectrometry, Hall/resistivity measurements, transmission electron microscopy, and perturbed γγ angular correlation. A variety of processes and physical properties of the implanted layers are found to be dramatically changed in the mixed compared to the single implants. These include solid‐phase recrystallization, redistribution of group‐III and group‐V atoms, phase nucleation and separation, lattice location of the impurity atoms, and electrical activation of the group‐V atoms. The cause of these changes on a microscopic level is shown to be a strong chemical interaction between the donor and acceptor atoms, resulting in the formation of AIIIBV chemical compounds. For the As/In and P/In in Si systems, the formation of moleculelike AIIIBV complexes with donor and acceptor atoms sitting in substitutional silicon lattice sites is dire...

The perturbed-gamma-gamma-angular-correlation technique (PAC) is a valuable tool for the characterization of defect complexes in crystalline materials. The features of the technique arising in the application to semiconductors are illustrated using the example of In-111-H pairs in Si. This system was studied in the course of an investigation of the passivation of acceptors in Si by H. The consequences of the chemical transmutation of the radioactive probe atoms are discussed. Using Schottky-diode structures, it is shown that PAC is well suited for use in combination with electrical techniques. The PAC technique, which works at any temperature and over a wide range of dopant concentrations, is also sensitive to different charge states of the defect complexes formed. A discussion of the results focuses on the influence of electronic parameters on the electric-field gradient measured at the In-111/Cd-111-H pairs in Si.

The formation of acceptor-donor pairs in germanium was studied by perturbed angular correlation spectroscopy. At the single acceptor In the donors P and As were trapped, resulting in In-P and In-As pairs as well as in In-P2 complexes. At the double acceptor Cd three P and two As correlated configurations were formed, i.e. Cd-P and Cd-As pairs, Cd-P2, Cd-As2 and Cd-P3 complexes. In Ge:Sb and Ge:Se the pairing of Cd acceptors with Sb or Se donors was detected. As predicted by calculations of the Coulomb binding energy the stability of the complexes decreases with increasing number of involved donor atoms, and a configuration formed at the double acceptor Cd reveals a higher stability than the analogous one at the single acceptor In. The weak temperature dependence of the corresponding field gradients indicates either electrical inactivity of Cd-donor complexes or the generation of shallow levels.

The local lattice environment of the donor In in CdS is investigated measuring the electric-field gradient at the site of the radioactive probe atom 111In by the perturbed gammagamma angular correlation technique. It is shown that implantation of In into CdS with subsequent annealing drives 100% of the In atoms to Cd lattice sites. Diffusion of In into CdS under S overpressure results in the formation of InCd-VCd pairs which seem to be responsible for the self-compensation of In donors in CdS (12 References).

Bridgman-grown, nominally undoped CdTe crystals were doped with Ag by implanting radioactive 111Ag. Photoluminescence spectra of the crystals show a donor-acceptor pair (DAP) line at 1.491 eV. The decrease of the intensity of this line with a half life of T1/2=(7.2±0.4) d is in good agreement with the half life of the β− decay of 111Ag to 111Cd of 7.45 d. This decrease is not caused by the aging behavior of Ag which was reported in the literature. The data show that the involved acceptor defect contains exactly one Ag atom and confirm the earlier assignment of the acceptor to the AgCd defect. Based on the DAP line at 1.491 eV, the spectra did not reveal a contamination of the CdTe crystals by stable Ag.

We report on experiments which unequivocal identify the chemical nature of optical transitions related to As (2.58 eV), Ge (3.398 eV) and Se (1.49 eV) found in the photoluminescence (PL) spectra of GaN. For this purpose epitaxial GaN layers were doped by ion implantation (60 keV, 3×1012 cm−2) with the radioactive isotopes 71As and 72Se. The isotope 71As (half-life 64.28 h) decays first into 71Ge (11.43 d), which finally transmutes into stable 71Ga. The isotope 72Se decays via 72As (26 h) into stable 72Ge. These chemical transmutations were monitored with photoluminescence spectroscopy (PL). The half-lives resulting from exponential fits on our PL data are in excellent agreement with the half-lives of the isotopes. Our experiments clearly show that in each case the luminescence center involves exactly one As, Ge or Se atom. In addition to this, the results imply that no optically active GaN antisite exists.

We report on experiments which observe on a microscopic scale the migration of isolated hydrogen in InP, GaAs, and InAs. Using the radioactive acceptor 117Cd, Cd-H pairs have been formed in these III-V semiconductors. After the decay of 117Cd to 117In, H is no longer bound to an acceptor and can diffuse freely. This diffusion has been observed by perturbed yy angular correlation (PAC) spectroscopy. At 10 K, the occupation of two different lattice sites by hydrogen has been observed. First results on the diffusion of hydrogen will be discussed.

Metal layers deposited on the surface of the semiconductor CdTe effect a shift of the concentration profile of Ag dopants in CdTe by several 100 μm during diffusion anneal at about 550 K. Radio-tracer experiments using 111Ag show that this effect occurs in the case of Al, Ni, Cu, and Au layers. A preliminary explanation relates the effect to an enhancement of the diffusion of Ag atoms in CdTe caused by Cd self-interstitials that are generated at the metal/semiconductor interface.

A systematic investigation of the behavior of Cd-implanted GaAs after rapid thermal annealing is presented. The use of various experimental techniques gives a detailed picture regarding the annealing process in the low-dose regime (10(12) and 10(13) cm-2) on a microscopic as well as on a macroscopic scale. Perturbed angular correlation experiments, using the radioactive probe Cd-111m, yield information on the immediate environment of the Cd implant on an atomic scale. Rutherford backscattering channeling and photoluminescence spectroscopy give complementary information concerning the overall damage level in the implanted layer, Hall measurements are used to determine the degree of electrical activation of the implanted Cd acceptors. The outdiffusion of the implanted radioactive Cd atoms is also investigated. The removal of defects in the next-nearest neighborhood of the Cd atoms takes place after annealing at 700 K and is accompanied by a general recovering of the crystal lattice. Between 600 and 900 K more distant defects are removed. The observed outdiffusion of about one-third of the dopant atoms after annealing above 600 K is discussed in context with their partial incorporation in extended defects. Although already at 700 K, 80% of the implanted Cd atoms are on substitutional lattice sites with no defects in their immediate environment, an annealing temperature in excess of 1000 K is necessary to obtain electrical activation of the implants. It is concluded that compensating defects, present in ion-implanted GaAs, are the reason for the significantly higher temperature required for electrical activation as compared to the incorporation of the dopants on defect-free, substitutional lattice sites.

Perturbed Angular Correlation (PAC) and Hall measurements were used to investigate the stability of the passivating Cd-H complex in GaAs after low energy H implantation (150 eV, 1014 cm−2) at 300 K. From the observed Cd-H pair formation and reduction of hole concentration it is deduced that about 10 % of the implanted H atoms form pairs with the Cd atoms. The influence of the dopant depth profile on the apparent stability in zero bias isochronal annealing experiments is reported. After H loading a reduction of carrier mobility is observed, which is stable up to about 400 K. The formation of Cd-H pairs after low energy H implantation into InP was studied by PAC. By measuring the fraction of pairs in an isochronal annealing experiment, the stability of the pairs is deduced yielding a dissociation energy of ED = 1.8 (1) eV.

The Cd-H complex in Cd-111m-doped GaAs implanted with low-energy (150-400 eV) hydrogen atoms is identified and studied by perturbed angular correlation spectroscopy using radioactive Cd-111m as a probe. By measuring the fraction of Cd-H pairs in an isochronal annealing experiment, the stability of the pairs is deduced yielding a dissociation energy of E(D) = 1.35(10) eV. After Cd-111m implantation but preceding the H loading, the GaAs samples have to be annealed at temperatures exceeding 900 K in order to form Cd-H pairs. These temperatures are in agreement with the temperature range required for electrical activation of Cd implants, suggesting that a Coulombic interaction is responsible for the formation of Cd-H pairs in GaAs.

Using the perturbed γγ angular correlation technique (PAC) the pairing of Cu with the radioactive acceptor atom 111In in Si is detected. Because of the identity of the electric field gradients the so-called X defect, observed after chemomechanical polishing of Si wafers and known of neutralizing acceptor atoms in Si, is identified as a Cu atom. It is also shown that as-delivered Si wafers already contain Cu atoms which neutralize acceptor atoms if the wafers are annealed at 1173 K.

Local fallout in the days following the accident at Chernobyl reached levels 30 to 40 times natural background at locations as far as 1500 km from the accident. In the absence of coordinated responses at the state and national levels, spontaneously generated local risk management efforts were undertaken in many places. We describe here our own local risk management activities in a locally hot region of southern West Germany. Based on the achievement of high credibility with the local population and the institution of effective protective action in the food chain, we argue that in future radiological disasters risk management should retain a strong decentralized component.

The presence of Cu atoms in p-type Si is detected via their characteristic electric-field gradients measured at the radioactive acceptor $^{111}\mathrm{In}$${/}^{111}$Cd by the perturbed \ensuremath{\gamma}\ensuremath{\gamma} angular correlation technique. Cu forms pairs with acceptor atoms, thereby electrically passivating them, and is present as a contamination in Si wafers. Using the experimentally known Cu diffusion data and taking into account the effect of ion pairing, a new activation energy of 0.15 eV is deduced, which is in accordance with the dissociation energy of 0.70 eV measured for Coulombic-bound-acceptor\char21{}Cu pairs.

The local environment of 111In implanted in CdTe has been studied by means of the perturbed angular correlation technique as a function of annealing conditions and temperatures. It is found that the damage correlated with the implantation process anneals at 200 °C. At 250–500 °C a new defect exists which is shown to be correlated with loss of Cd and can be inhibited when the escape of Cd can be suppressed. Outdiffusion of In starts at about 500 °C, and at 650 °C almost no In remains in the sample. These results agree with, and add to, published results on the annealing of In‐implanted CdTe obtained by other techniques.

La formation de complexe d'hydrogene accepteur dans Si est etudiee par l'intermediaire des gradients de champ electrique causes par les atomes H dans le site des atomes accepteurs 111 In radioactif

An overview and assessment of the nuclear accident at Chernobyl is presented. The authors have assembled data from throughout Europe to estimate upper bounds for the possible radiation releases from the accident, the exposures these may produce in humans, and the health consequences that may follow. Measurements of radioactivity in air, fallout, and milk and other food are included. Doses from the accident are compared with those from other sources of radiation exposure and a comparison is made of the annual risk of cancer from this and other causes.

The formation of molecule-like complexes, consisting of a defect and a radioactive111In atom, is studied using the perturbedγγ angular correlation technique (PAC). The complexes are characterized by their defect specific electric field gradients which also contain information on the geometry of the formed complexes. Whereas the complex is formed with the111In atom, its electric field gradient is measured after the decay of the radioactive111In atom to111Cd. Formation and dissolution of the molecule-like complexes is pursued for a variety of different conditions, such as sample temperature, dopant concentration and position of the Fermi level. In particular, the interaction of In atoms with the following defects in Si was investigated: Intrinsic defects, created by particle irradiation; substitutional donor atoms (P, As, Sb, Bi); and interstitial impurity atoms (Li, H, and an unidentified X defect); especially, the latter ones are known to passivate acceptor atoms in Si. Methodology and specific properties of the PAC technique will be illustrated with the help of these examples.

Perturbed angular correlation (PAC) and ion channelling techniques were used to study the trapping of vacancies by In atoms in a Cu-0.13 at.% In crystal after H+ irradiation. Four major defect complexes were observed; the fractions f1, f2, f7 and fT of In atoms which were associated with these complexes were measured as a function of annealing temperature and irradiation fluence. The complexes f1, f2 and f7 were observed only by the PAC technique; the coupling constants for these defects were vQ1=116 MHz, vQ2=181 MHz and vQ7=106 MHz, all having eta =0. The first two of these had previously been shown to be due to trapping of one and two vacancies. The third may be due to He trapping. A tetrahedral complex fT, consisting of an In atom in the tetrahedral interstitial site surrounded by four nearest-neighbour vacancies was observed by channelling but was invisible by the PAC method. Complexes, f1, f2, and fT were formed during annealing from 180-250K. The fractions f1 and f2 decreased strongly during annealing from 250-230K, while the tetrahedral complex was stable up to approximately 500K. The complex f7 appeared during annealing above 320K and continued to increase up to 600K. The fractions f1, f2 and fT were all reduced during 70K irradiations by the absorption of self-interstitial Cu atoms.

The local environment of 111In implanted into diamonds has been studied by means of the perturbed angular correlation technique as a function of annealing temperature and of implantation dose. It is found that even when the diamond is heated under vacuum to the highest possible temperature before graphitization occurs (2100 K), the implants are not driven to spherically symmetric sites in the lattice. This lack of annealing, if common to all heavy implants, may have serious consequences on the usefulness of doping of diamond by heavy‐ion implantation.

Acceptor‐defect and acceptor‐impurity complexes in GaAs have been studied by perturbed angular correlation spectroscopy. In n‐type GaAs close Cd‐S pairs were formed; the temperature dependence of the related electric field gradient indicates an electrically inactive Cd‐S complex. After a hydrogen plasma exposure of GaAs a distinct Cd‐defect complex has been observed. Similarly, plasma‐induced configurations were also formed in InP and GaP.

Precision measurements of correlation observables in nuclear beta decay constitute a sensitive tool to search for signatures of new physics beyond the standard electroweak model. Over the past decade, measurements in nuclear and neutron decays provided new constraints on the possible presence of exotic weak couplings as well as new tests of maximal parity violation and time reversal invariance. This presentation reviews recent achievements in the field and presents current efforts aiming to reach new levels of sensitivity.