Artículos (Centro Nacional de Aceleradores)

URI permanente para esta colecciónhttps://hdl.handle.net/11441/10939

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Conceptual Design of a Scintillator-Based Fast-Ion Loss Detector for the Wendelstein 7-X Stellarator
(IEEE, 2022) Van Vuuren, Anton Jansen; Lazerson, S. A.; Leviness, A.; García Muñoz, Manuel; Gates, D.; Galdón Quiroga, Joaquín; Hidalgo Salaverri, J.; Rueda Rueda, J.; García Domínguez, Javier; Ayllón Guerola, Juan Manuel; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Universidad de Sevilla. Departamento de Ingeniería Mecánica y de Fabricación; European Union (UE); Ministerio de Ciencia e Innovación (MICIN). España
A conceptual design of a scintillator-based fast-ion loss detector (FILD) has been developed for the Wendelstein 7-X stellarator (W7-X). Simulations using the Monte Carlo codes ASCOT5 and BEAMS3D have been performed to calculate the expected flux of neutral beam injection (NBI)-generated fast hydrogen ions onto the conceptual detector probe head. These fast-ion loss fluxes have been calculated for several magnetic field configurations as well as probe insertion positions. At the maximum insertion position, both co- and counter-going losses with high incident pitch angles are observed; however, at retracted positions, only co-going fast ions reach the probe head. The FILDSIM code has been used to optimize the geometry of the detector entrance and collimating elements to achieve a wide velocity space coverage as well as a high velocity–space resolution. A synthetic FILD signal is calculated for the expected loss distribution via forward modeling using the instrument response function. The synthetic signal is found to largely retain the velocity space features of the loss distribution.
Plutonium Signatures in a Dated Sediment Core as a Tool to Reveal Nuclear Sources in the Baltic Sea
(American Chemical Society, 2023) López Lora, Mercedes; Olszewski, G.; Chamizo, E.; Tornquist, P.; Pettersson, H.; Eriksson, Mats; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Swedish Radiation Safety Authority (SSM); Ministerio de Ciencia, Innovación y Universidades (MICINN). España
Plutonium distribution was studied in an undisturbed sediment core sampled from the Tvären bay in the vicinity of the Studsvik nuclear facility in Sweden. The complete analysis, including minor isotopes, of the Pu isotope composition (238Pu, 239Pu, 240Pu, 241Pu, 242Pu, and 244Pu) allowed us to establish the Pu origin in this area of the Baltic Sea and to reconstruct the Studsvik aquatic release history. The results show highly enriched 239Pu, probably originating from the Swedish nuclear program in the 1960s and 1970s and the handling of high burn-up nuclear fuel in the later years. In addition, the 244Pu/239Pu atomic ratio for the global fallout period between 1958 and 1965 is suggested to be (7.94 ± 0.31)·10-5. In the bottom layer of the sediment, dated 1953-1957, we detected a higher average 244Pu/239Pu ratio of (1.51 ± 0.11)·10-4, indicating the possible impact of the first US thermonuclear tests (1952-1958).
Isospin Dependence in Single-nucleon Removal Cross Sections Explained Through Valence-core Destruction Effects
(Elsevier, 2023) Gómez Ramos, Mario; Gómez Camacho, Joaquín José; Moro Muñoz, Antonio Matías; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Ministerio de Ciencia e Innovación (MICIN). España; Junta de Andalucía
The discrepancy between experimental data and theoretical calculations in one-nucleon removal reactions at intermediate energies (quantified by the so-called “quenching factors”) and its dependence on the isospin asymmetry of the nuclei has been an open problem in nuclear physics for the last fifteen years. In this work, we propose an explanation for this long-standing problem, which relies on the inclusion of the process of core destruction due to its interaction with the removed nucleon. To include this effect, we extend the commonly used eikonal formalism via an effective nucleon density, and apply it to a series of nucleon knockout reactions. The effect of core destruction is found to depend strongly on the binding energy of the removed nucleon, leading to a significant reduction of the cross section for deeply bound nucleons, which reduces the isospin dependence of the “quenching factors”, making them more consistent with the trends found in transfer and (p,pN) reactions.
Defect Engineering of Silicon with Ion Pulses from Laser Acceleration
(Springer Nature, 2023) Redjem, Walid; Amsellem, Ariel J.; Allen, Frances I.; Benndorf, Gabriele; Bin, Jianhui; Bulanov, Stepan; Esarey, Eric; Feldman, Leonard C.; Ferrer Fernández, Francisco Javier; García López, Francisco Javier; Schenkel, Thomas; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Office of Science. EE.UU.
Defect engineering is foundational to classical electronic device development and for emerging quantum devices. Here, we report on defect engineering of silicon with ion pulses from a laser accelerator in the laser intensity range of 1019 W cm−2 and ion flux levels of up to 1022 ions cm−2 s−1, about five orders of magnitude higher than conventional ion implanters. Low energy ions from plasma expansion of the laser-foil target are implanted near the surface and then diffuse into silicon samples locally pre-heated by high energy ions from the same laser-ion pulse. Silicon crystals exfoliate in the areas of highest energy deposition. Color centers, predominantly W and G-centers, form directly in response to ion pulses without a subsequent annealing step. We find that the linewidth of G-centers increases with high ion flux faster than the linewidth of W-centers, consistent with density functional theory calculations of their electronic structure. Intense ion pulses from a laser-accelerator drive materials far from equilibrium and enable direct local defect engineering and high flux doping of semiconductors.
Spectrometric Performance of SiC Radiation Detectors at High Temperature
(Elsevier, 2024) Jiménez Ramos, María del Carmen; García Osuna, Adrián; Rodríguez Ramos, Mauricio; Viezzer, Eleonora; Pellegrini, G.; Godignon, P.; Rafí, J. M.; Rius, G.; García López, Francisco Javier; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Universidad de Sevilla. Departamento de Física Aplicada II; Ministerio de Ciencia, Innovación y Universidades (MICINN). España; Junta de Andalucía; Ministerio de Ciencia e Innovación (MICIN). España
In this work, we have investigated the performance of a 4H–SiC radiation sensor in the temperature range from 25 °C to 450 °C to explore its compatibility as detector of fast ion losses in plasma diagnostic of future nuclear fusion reactors. To simulate the escape of fusion-born alpha particles in D-T (deuterium-tritium) fusion plasmas, spectroscopic measurements were carried out in a vacuum chamber by irradiating the detector with a 3.5 MeV alpha beam from a Tandem accelerator. The detector was found to have an energy resolution ≤2% over the entire temperature range analyzed. Relevantly, the excellent spectrometric capabilities of the device have allowed us to calculate from experimental data, with unprecedented accuracy, the average energy required to create a single electron-hole pair in 4H–SiC as a function of temperature.
Description of continuum structures in a discrete basis: Three-body resonances and two-nucleon decays
(SciPost, 2020) Casal Berbel, Jesús; Rodríguez Gallardo, Manuela; Arias Carrasco, José Miguel; Gómez Camacho, Joaquín José; Fortunato, Lorenzo; Vitturi, Andrea; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Università degli Studi di Padova. Italia; Ministerio de Ciencia, Innovación y Universidades (MICINN). España; European Union (UE). H2020
Weakly bound and unbound three-body nuclei are studied by using the pseudostate method within the hyperspherical formalism. After introducing the theoretical framework, the method is applied first to the 9Be nucleus, showing a good agreement with the available data for its low-lying dipole response. Then, recent results on the structure and decay of the two-neutron emitters 26O and 16Be are presented. In particular, the role of the n-n correlation in shaping their properties is discussed.
A rotary and reciprocating scintillator based fast-ion loss detector for the MAST-U tokamak
(American Institute of Physics, 2018) Rivero Rodríguez, Juan Francisco; García Muñoz, Manuel; Martin, R.; Galdón Quiroga, Joaquín; Ayllón Guerola, Juan Manuel; Akers, R. J.; García Vallejo, Daniel; González Martín, Javier; Rodríguez Ramos, Mauricio; Sanchis Sánchez, Lucía; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; European Commission (EC); Universidad de Sevilla
The design and unique feature of the first fast-ion loss detector (FILD) for the Mega Amp Spherical Tokamak - Upgrade (MAST-U) is presented here. The MAST-U FILD head is mounted on an axially and angularly actuated mechanism that makes it possible to independently adapt the orientation [0◦ , 90◦ ] and radial position [1.40 m, 1.60 m] of the FILD head, i.e., its collimator, thus maximizing the detector velocity-space coverage in a broad range of plasma scenarios with different q95. The 3D geometry of the detector has been optimized to detect fast-ion losses from the neutral beam injectors. Orbit simulations are used to calculate the strike map and predict the expected signals. The results show a velocity-space range of [4 cm, 13 cm] in gyroradius and [30◦ , 85◦ ] in pitch angle, covering the entire neutral beam ion energy range. The optical system will provide direct sight of the scintillator and simultaneous detection with two cameras, giving high spatial and temporal resolution. The MAST-U FILD will shed light on the dominant fast-ion transport mechanisms in one of the world’s two largest spherical tokamaks through absolute measurements of fast-ion losses.
Accelerator Mass Spectrometry (AMS) in Radioecology
(Elsevier, 2018) García León, Manuel; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Ministerio de Economía y Competitividad (MINECO). España
Accelerator Mass Spectrometry (AMS) provides with an excellent sensitivity for the determination of radionuclides in the environment. In fact, conventional radiometric techniques can hardly compete with AMS in the solution of many problems involving the measurement of very low levels of radioactivity in Nature. For that reason, during the last years AMS has become a powerful tool for Radioecology studies. In this paper a review is done on the evolution of AMS applications to the measurement of environmental radioactivity and, therefore, its contribution to the understanding of radionuclide behavior in Nature.For that, the advantages of using AMS to determine key nuclides as 129I, 14C, Pu-isotopes and others in different natural compartments will be discussed. The content of the paper is illustrated with the contributions to these studies of the Spanish National Center for Accelerators (CNA) AMS systems.
Implications of the Harmonization of [18F]FDG-PET/CT Imaging for Response Assessment of Treatment in Radiotherapy Planning
(MDPI, 2022) Jiménez Ortega, Elisa Eugenia; Agüera Rodríguez, Raquel; Ureba Sánchez, Ana María; Balcerzyk, Marcin; Wals Zurita, Amadeo; García Gómez, Francisco Javier; Leal Plaza, Antonio; Universidad de Sevilla. Departamento de Fisiología Médica y Biofísica; European Union (UE); Junta de Andalucía; European Commission (EC). Fondo Europeo de Desarrollo Regional (FEDER)
The purpose of this work is to present useful recommendations for the use of [18F]FDG-PET/CT imaging in radiotherapy planning and monitoring under different versions of EARL accreditation for harmonization of PET devices. A proof-of-concept experiment designed on an anthropomorphic phantom was carried out to establish the most suitable interpolation methods of the PET images in the different steps of the planning procedure. Based on PET/CT images obtained by using these optimal interpolations for the old EARL accreditation (EARL1) and for the new one (EARL2), the treatment plannings of representative actual clinical cases were calculated, and the clinical implications of the resulting differences were analyzed. As expected, EARL2 provided smaller volumes with higher resolution than EARL1. The increase in the size of the reconstructed volumes with EARL1 accreditation caused high doses in the organs at risk and in the regions adjacent to the target volumes. EARL2 accreditation allowed an improvement in the accuracy of the PET imaging precision, allowing more personalized radiotherapy. This work provides recommendations for those centers that intend to benefit from the new accreditation, EARL2, and can help build confidence of those that must continue working under the EARL1 accreditation.
The Potential of 233U/236U as a Water Mass Tracer in the Arctic Ocean
(Wiley, 2022) Chamizo Calvo, Elena; Christl, M.; López Lora, Mercedes; Casacuberta, N.; Wefing, A.-M.; Kenna, Timothy C.; Ministerio de Ciencia, Innovación y Universidades (MICINN). España; Swiss National Science Foundation (SNFS)
This study explores for the first time the possibilities that the 233U/236U atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, 233U originates from global fallout while 236U carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the 233U/236U ratio provides a tool to identify water masses with distinct U sources. In this work, 233U and 236U were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their 233U/236U signature; and (b) Deep and Bottom Waters show extremely low 233U and 236U concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high 233U/236U ratios are interpreted as a relative increase of naturally occurring 233U and 236U and thus for gradually reaching natural 233U/236U levels in the deep Arctic Ocean. Our results set the basis for future studies using the 233U/236U ratio to distinguish anthropogenic and pre-anthropogenic U in the Arctic Ocean and beyond.
Development of Slowed Down Beams at the Fragment Separator for FAIR
(Jagiellonian University, 2011) Naqvi, F.; Boutachkov, P.; Górska, M.; Gerl, J.; Farinon, F.; Gregor, E.T.; González Álvarez, Marcos Aurelio; Wollersheim, H.J.; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
The feasibility studies of the slowed down beam setup involving deceleration of a 64Ni beam at 250 MeV/u to 13 MeV/u in a thick Al degrader was performed at the FRagment Separator (FRS) at GSI. The experimentally measured energy spread and the nuclear reaction yields in the degrader are in good agreement with simulations.
Destruction of the cosmic γ-ray emitter 26 Al in massive stars: Study of the key 26 Al ( n , p ) reaction
(American Physical Society, 2021) Lederer Woods, Claudia; Woods, P. J.; Davinson, T.; Kahl, D.; Lonsdale, S. J.; Aberle, O.; Chiaveri, Enrico; Cortés Giraldo, Miguel Antonio; Ferrer Fernández, Francisco Javier; Guerrero Sánchez, Carlos; Lerendegui Marco, Jorge; Quesada Molina, José Manuel; Sabaté Gilarte, Marta; Žugec, P.; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Austrian Science Fund (FWF). Austria; Consejo de Instalaciones Científicas y Tecnológicas (STFC). Reino Unido; Consejo Europeo de Investigación. Unión Europea
The 26 Al (n ,p) 26 Mg reaction is the key reaction impacting on the abundances of the cosmic y-ray emitter 26 Al produced in massive stars and impacts on the potential pollution of the early solar system with 26 Al by asymptotic giant branch stars. We performed a measurement of the 26 Al ( n , p ) 26 Mg cross section at the high-flux beam line EAR-2 at the n_TOF facility (CERN). We report resonance strengths for eleven resonances, nine being measured for the first time, while there is only one previous measurement for the other two. Our resonance strengths are significantly lower than the only previous values available. Our cross-section data range to 150 keV neutron energy, which is sufficient for a reliable determination of astrophysical reactivities up to 0.5 GK stellar temperature.
Determining B(E1) distributions of weakly bound nuclei from breakup cross sections using Continuum Discretized Coupled Channels calculations. Application to 11Be
(Elsevier, 2020) Moro Muñoz, Antonio Matías; Lay Valera, José Antonio; Gómez Camacho, Joaquín José; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Ministerio de Ciencia, Innovación y Universidades (MICINN). España; European Commission (EC). Fondo Europeo de Desarrollo Regional (FEDER); European Union (UE); Ministerio de Ciencia, Innovación y Universidades de España y fondos FEDER FIS2017-88410-P y RTI2018-098117-B-C21; Programa de investigación e innovación Horizonte 2020 de la Unión Europea Subvención No. 654002.
A novel method to extract the B(E1) strength of a weakly bound nucleus from experimental Coulomb dissociation data is proposed. The method makes use of continuum discretized coupled channels (CDCC) calculations, in which both nuclear and Coulomb forces are taken into account to all orders. This is a crucial advantage with respect to the standard procedure based on the Equivalent Photon Method (EPM) which does not properly take into account nuclear distortion, higher order coupling effects, or Coulomb- nuclear interference terms. The systematic and statistical uncertainties of this procedure are evaluated. The procedure is applied to the 11Be nucleus using two sets of available experimental data at different energies, for which seemingly incompatible B(E1) have been reported using the EPM. We show that the present procedure gives consistent B(E1) strengths, thus solving the aforementioned long-standing discrepancy between the two measurements.
Probing proton halo effects in the 8B+64Zn collision around the Coulomb barrier
(Elsevier, 2021) Spartà, Roberta; Di Pietro, Alessia; Figuera, Pierpaolo; Tengblad, Olof; Moro Muñoz, Antonio Matías; Martel Bravo, Ismael; Fernández García, Juan Pablo; Viñals, Silvia; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; European Union (UE); Istituto Nazionale di Fisica Nucleare (INFN) Italia; Science and Technology Facilities Council (STFC) Reino Unido
Proton halo effects in the 8B+64Zn reaction at an energy around 1.5 times the Coulomb barrier have been studied at HIE-ISOLDE CERN using, for the first time, the only existing postaccelerated 8B beam. This, together with the use of a high granularity and large solid angle detection system, allowed for a careful mapping of the elastic angular distribution, especially in the Coulomb-nuclear interference region. Contrary to what is observed for the one-neutron halo nucleus 11Be on the same target in a similar energy range, the analysis of the elastic scattering angular distribution shows only a modest suppression of the Coulomb-nuclear interference peak, with no remarkable enhancement of the total reaction cross-section. Inclusive angular and energy distributions of 7Be produced in direct reaction processes have also been measured. The comparison of these data with the results of theoretical calculations for the elastic and non-elastic breakup contributions indicate that both processes are important. Overall, the experimental data suggest a 8B collision dynamics at the barrier very different from the one of neutron halo nuclei, showing only modest effects of coupling to continuum. This behaviour can be interpreted as due to the presence of the additional Coulomb interactions halo-core and halo-target together with the presence of the centrifugal barrier felt by the valence proton of 8B.
The naturally occurring radioactivity of ‘scalar energy’ pendants and concomitant radiation risk
(Public Library of Science, 2021) Hassan, Halmat Jalal; Hashim, Suhairul; Sanusi, Mohamad Syazwan Mohd; Jamal, Mohamad Hidayat; Bradley, David Andrew; García-Tenorio García-Balmaseda, Rafael; Universidad de Sevilla. Departamento de Física Aplicada II
Forming part of a study of radiological risk arising from use of radioactive consumer products, investigation is made of pendants containing naturally occurring radioactive material. Based on use of gamma-ray spectrometry and Monte Carlo (MC) simulations, the study investigates commercially available ‘scalar energy pendants’. The doses from these have been simulated using MIRD5 mathematical phantoms, evaluation being made of dose conversion factors (DCFs) and organ dose. Metallic pendants code MP15 were found to contain the greatest activity, at 7043 ± 471 Bq from 232Th, while glass pendants code GP11 were presented the greatest 238U and 40K activity, at 1001 ± 172 and 687 ± 130 Bq respectively. MP15 pendants offered the greatest percentage concentrations of Th, Ce, U and Zr, with means of 25.6 ± 0.06, 5.6 ± 0.005, 1.03 ± 0.04 and 28.5 ± 0.08 respectively, giving rise to an effective dose of 2.8 mSv for a nominal wearing period of 2000 h. Accordingly, these products can give rise to annual doses in excess of the public limit of 1 mSv.
Human oocytes and zygotes are ready for ultra-fast vitrification after 2 minutes of exposure to standard CPA solutions
(Nature Research, 2019) Gallardo Molina, Miguel; Sáenz Cuesta, Jaime Luis; Risco, Ramón; Universidad de Sevilla. Departamento de Física Aplicada III
Vitrification of human oocytes and embryos in different stages of development is a key element of daily clinical practice of in vitro fertilization treatments. Despite the cooling and warming of the cells is ultra-fast, the procedure as a whole is time consuming. Most of the duration is employed in a long (8–15 minutes), gradual or direct exposure to a non-vitrifying cryoprotectant solution, which is followed by a short exposure to a more concentrated vitrifying solution. A reduction in the duration of the protocols is desirable to improve the workflow in the IVF setting and reduce the time of exposure to suboptimal temperature and osmolarity, as well as potentially toxic cryoprotectants. In this work it is shown that this reduction is feasible. In silico (MatLab program using two-parameter permeability model) and in vitro observations of the oocytes’ osmotic behaviour indicate that the dehydration upon exposure to standard cryoprotectant solutions occurs very fast: the point of minimum volume of the shrink-swell curve is reached within 60 seconds. At that point, intracellular water ejection is complete, which coupled with the permeation of low molecular weight cryoprotectants results in similar intracellular and extracellular solute concentrations. This shows that prolonging the exposure to the cryoprotectant solutions does not improve the cytosolic glass forming tendency and could be avoided. To test this finding, human oocytes and zygotes that were donated for research were subjected to a shortened, dehydration-based protocol, consisting of two consecutive exposures of one-minute to two standard cryoprotectant solutions, containing ethylene glycol, dimethyl sulfoxide and sucrose. At the end of this two-minute dehydration protocol, the critical intracellular solute concentration necessary for successful vitrification was attained, confirmed by the post-warming survival and ability to resume cytokinesis of the cells. Further studies of the developmental competency of oocytes and embryos would be necessary to determine the suitability of this specific dehydration protocol for clinical practice, but based on our results, short times of exposure to increasingly hypertonic solutions could be a more time-efficient strategy to prepare human oocytes and embryos for vitrification.
Impact of Gamma Radiation on Dynamic RDSON Characteristics in AlGaN/GaN Power HEMTs
(MDPI, 2019) Martínez, Pedro J.; Maset, Enrique; Martín Holgado, Pedro; Morilla García, Yolanda; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
GaN high-electron-mobility transistors (HEMTs) are promising next-generation devices in the power electronics field which can coexist with silicon semiconductors, mainly in some radiation-intensive environments, such as power space converters, where high frequencies and voltages are also needed. Its wide band gap (WBG), large breakdown electric field, and thermal stability improve actual silicon performances. However, at the moment, GaN HEMT technology suffers from some reliability issues, one of the more relevant of which is the dynamic on-state resistance (RON_dyn) regarding power switching converter applications. In this study, we focused on the drain-to-source on-resistance (RDSON) characteristics under 60Co gamma radiation of two different commercial power GaN HEMT structures. Different bias conditions were applied to both structures during irradiation and some static measurements, such as threshold voltage and leakage currents, were performed. Additionally, dynamic resistance was measured to obtain practical information about device trapping under radiation during switching mode, and how trapping in the device is affected by gamma radiation. The experimental results showed a high dependence on the HEMT structure and the bias condition applied during irradiation. Specifically, a free current collapse structure showed great stability until 3.7 Mrad(Si), unlike the other structure tested, which showed high degradation of the parameters measured. The changes were demonstrated to be due to trapping effects generated or enhanced by gamma radiation. These new results obtained about RON_dyn will help elucidate trap behaviors in switching transistors.
A view of the implanted SiC damage by Rutherford backscattering spectroscopy, spectroscopic ellipsometry, and transmission electron microscopy
(AIP Publishing, 2006) Battistig, G.; Khánh, N. Q.; Petrik, P.; Lohner, T.; Dobos, L.; Pécz, B.; García López, Francisco Javier; Morilla García, Yolanda; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
4H-SiC single crystalline substrates were implanted at room temperature with 150 keV Al+ ions using fluences of 4 1014, 1 1015, and 2 1015 cm−2 with current density of 2.5 A cm−2. The samples were subsequently annealed at 1100 °C in N2 for 1 h in order to analyze their structural recovery. The disorder induced in both sublattices by the Al+ ions was studied by backscattering spectrometry in channeling geometry with a 3.5 MeV He2+ beam. The results were compared with the optical properties of the samples measured by spectroscopic ellipsometry. In a previous work, we concluded that during the postimplantation annealing of a highly damaged SiC crystalline material the short distance order can be recovered, while the long distance disorder remains. We also presented the possibility to have grains of different polytypes oriented faraway from the original direction. Now, this alternative is confirmed by the cross-sectional transmission and high resolution electron microscopy studies, carried out to obtain information about the crystal structure.
Correlation between Mn oxidation state and magnetic behavior in Mn/ZnO multilayers prepared by sputtering
(AIP Publishing, 2007) Céspedes, E.; García López, Francisco Javier; García Hernández, M.; Andrés, A. de; Prieto, C.; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Compositional, microstructural, and magnetic characterization of ZnO 30 Å/Mn x n multilayers prepared by sputtering is presented to study the observed ferromagnetism in the Mn-ZnO system. The nominal Mn layer thickness, x, is varied from 3 to 60 Å, while the number of bilayers, n, is increased to maintain the total amount of Mn constant. Microstructure information was deduced from x-ray reflectivity, Mn oxidation state was determined by x-ray absorption spectroscopy, and magnetic properties were measured over a temperature range of 5–400 K. Magnetic behavior of these samples is found to be related to the Mn layer thickness x. Multilayers with x 30 Å exhibit ferromagnetism with a Curie temperature above 400 K, while mostly paramagnetic behavior is obtained for x15 Å. Magnetic behavior is discussed in terms of electronic and structural parameters of samples. Mn-ZnO interface effect is related to the ferromagnetic order of the samples, but it is not a sufficient condition. The essential role of the Mn oxidation state in the magnetic behavior of this system is pointed out. It is shown a correlation between the obtained ferromagnetism and a Mn oxidation state close to 2+.
Quantification of self-sputtering and implantation during pulsed laser deposition of gold
(AIP Publishing, 2008) Perea Folgueras, Ángel; Gonzalo, José; Budtz Jørgensen, C.; Epurescu, George; Siegel, Jan; Afonso Rodríguez, Carmen Nieves; García López, Francisco Javier; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
This work reports on the quantification of self-sputtering and implantation occurring during pulsed laser deposition of Au as a function of the laser fluence used to ablate the gold target. The experimental approach includes, on one hand, in situ electrical Langmuir and optical two-dimensional imaging probes for determining, respectively, ion and excited neutral kinetic energy distributions. On the other hand, it includes determination of the density of i ions reaching a substrate, and ii gold atoms deposited on a substrate as well as of a proportion of atoms that are self-sputtered. The experimental results supported by numerical analysis show that self-sputtering and implantation are both dominated by ions having kinetic energies 200 eV. They are a fraction 0.60–0.75 of the species arriving to the substrate for ablation laser fluences 2.7– 9.0 J cm−2. Self-sputtering yields in the range 0.60–0.86 are determined for the same fluence range.

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