Papers by Keyword: Positron Annihilation

Abstract: The PULSTAR nuclear reactor at North Carolina State University is a 1-MWth open-pool reactor and a center for irradiation and examination of materials. Among the different facilities based on this reactor, an intense positron beam facility has been established and operational since 2009, providing a positron beam reaching 6 × 10⁸ e⁺/s and two positron beam spectrometers. This facility has been recently upgraded in several aspects in terms of sample manipulation and heating/cooling capabilities, as well as coincidence Doppler Broadening Spectroscopy (CDBS) capability, enabled by a new sample changer and target chamber with new digital MCA systems. Currently, both beam and bulk spectrometers can perform CDBS and Positron Annihilation Lifetime Spectroscopy (PALS) with varying temperature settings. In addition, environmental control has also been added to the bulk system, where the sample pressure and humidity can be adjusted together with temperature. These systems have been utilized to study a variety of nuclear-related materials, such as metal alloys and oxide compounds, that had undergone surface and bulk damages induced by radiation. Such materials are widely used as structural and sensor materials related to nuclear reactors where microstructural damage is critical in determining their performance and failure modes. Some of the studies also demonstrated the unique advantage of Positron Annihilation Spectroscopy (PAS) over other traditional characterization techniques.

Abstract: The centers of bismuth (Bi) in silicon are being scrutinized as the defect qubits for mostly developed integrated electronics including its photonic component and we have applied the positron annihilation lifetime spectroscopy (PALS) to gain deeper insight into symmetry of the Bi impurity center whose configuration was modified by 15 MeV proton irradiation. It was revealed that hyperfine (hf) and super-hyperfine (shf) interactions of the nuclear and electron spin systems of the bismuth impurity center, ²⁰⁹Bi (J = 9/2), with the regular ²⁹Si (J = 1/2) atoms of silicon delay the essentially local event of emitting of a couple of annihilation gamma–quanta from within the crystal cell which comprises Bi impurity atom (J is the nuclear spin). This phenomenon is observed under increasing occupancy of Bi donor ground and excited states, in contrast to a profoundly enriched ²⁸Si (J = 0) material (so-called “semiconductor vacuum”) where content of ²⁹Si (J = 1/2) isotope was suppressed up to the value of ≈ 50 ppm. The many-body exciton-like states comprising a polyelectronic exciton {e⁻e⁺e⁻h} at Bi donor center are suggested for interpreting the data. The proton irradiation leads to acquiring by Bi impurity atom of an open volume ( V_op ) which is splitted in [V_op – Bi] complex. This defect possessing of D_3d symmetry dominates in the irradiated material. Being thermally stable up to ≈ 370 °C, [V_op – Bi] complex is annealed at ~ 470 – 500 °C. These data agree well with the results of ab intio cluster calculations performed on the basis of LDA-KKR formalism for exploring both the energy gain and symmetry of Bi–vacancy complex.

Abstract: The development of algorithms and simulation codes as well as the ever increasing comput ing power available have have enabled many-body simulations to emerge as a viable alternative to the two-component density-functional theory to study positron states and annihilation in solids, defects and on solid surfaces. One practical many-body method is quantum Monte Carlo. This article will aim to present the practitioners of positron annihilation methods the basic ideas of the variational and diffusion quantum Monte Carlo, provide examples of their past and recent applications in positron physics of atomic/molecular and solid-state systems as well as an outlook into the future.

Abstract: The change in the positron annihilation lifetime (PAL) of vacancy clusters before and after electrolysis hydrogen charging was determined using PAL measurements in electron-irradiated F82H. The experimental change indicated 8 hydrogen atoms were trapped in vacancy clusters; whereas the theoretical calculation resulted in approximately 14 atoms. As the samples were left at room temperature for 5 min until the start of the PAL measurements, the de-trapping effect of hydrogen atoms was also considered; approximately 13 hydrogen atoms were captured at each vacancy cluster. The PAL decreased after annealing at 148 K, which could not be explained theoretically. Therefore, further experiments and discussions are needed to obtain a precise change in the PAL of vacancy clusters containing hydrogen atoms in F82H.

Abstract: The well-known and often acceptable radiation tolerance of ferritic/martensitic (f/m) steels can be severely diminished when neutron irradiation is accompanied by the production of helium. The presence of helium in the irradiated materials changes the kinetics of the nucleation, recombination, and clustering of the radiation-induced defects. High production rates of helium may lead to a non-negligible volumetric bubble swelling at relatively low temperatures. Extrapolation of the knowledge gained from neutron irradiation experiments to fusion or spallation environments is additionally complicated due to the unknown and comprehensive effects of dpa rate, temperature, the presence of sinks in the crystal lattice and others. To improve the understanding of the microstructure and irradiation parameters effects, close attention must be paid to the early stages of the radiation damage. It is expected that the pre-existing vacancy-type defects, attributed to lattice distortion at the grain/subgrain boundaries and oxide-matrix interfaces, are effective sinks for primary defects and helium, i.e. they control the formation and growth of helium-vacancy agglomerations. This early-stage radiation damage, however, cannot be captured by conventional transmission electron microscopy, and thus other experimental techniques are called for. One of the most perspective experimental approaches to investigate small vacancy-type defects, with a high sensitivity to confined helium, is to utilize positron annihilation spectroscopy (PAS). In particular, two spectroscopy techniques, positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) of the annihilation line, can be beneficially used for the characterization of helium-vacancy clusters. This paper reviews the recent positron annihilation spectroscopy characterization of various irradiation experiments involving helium. Mainly two types of irradiation experiments are addressed, helium implantation and spallation neutron source irradiation experiments. Discussion is aimed at the potential of PAS in the early-stage formation of helium bubbles and the investigation of the effects of irradiation parameters in defect production and accumulation.

Abstract: Research in the field of modern battery materials demands characterization techniques which allow an inspection of atomistic processes during battery charging and discharging. Two powerful tools for this purpose are magnetometry and positron-electron annihilation. The magnetic moment serves as highly sensitive fingerprint for the oxidation state of the transition metal ions, thus enabling to identify the electrochemical ”active” ions. The positron lifetime on the other hand, is sensitive to open volume defects of the size of a few missing atoms down to single vacancies providing an unique insight into lattice defects induced by charging and discharging. An overview will be given on operando magnetometry studies of the important class of LiNiCoMn-oxide cathode materials (so-called NMC with Ni:Co:Mn ratios of 1:1:1 and 3:1:1) as well as of sodium vanadium phosphate cathodes. First operando positron annihilation studies on a battery cathode material (NMC 1:1:1) demonstrate the capability of this technique for battery research.

Abstract: Porous ZnO were synthesized with soft template method using zinc acetate Zn (CH3COO)₂·2H₂O as precursor and block copolymer F127 as the surfactant. Nitrogen adsorption-desorption measurements indicate that the ZnO sample contains large pores with mean diameter of about 30 nm. However, both small-angle X-ray diffraction and transmission electron microscope measurements indicate that the pore ordering is missing. Positron lifetime measurements reveal two long lifetime components in the porous ZnO. The longest lifetime τ₄ (75 ns) corresponds to ortho-positronium (o-Ps) annihilation in large pores. The pore size estimated from τ₄ is about 10.6 nm. This is much smaller than that estimated from Nitrogen adsorption-desorption measurements. In addition, the intensity I₄ is only about 2.2%. This is probably due to the chemical quenching and/or inhibition of positronium formation induced by ZnO, which reduces o-Ps lifetime and intensity, and leads to under estimation of the pore size.

Abstract: Pure MgO, ZrO₂ and mixture MgO/ZrO₂ nanocrystals were annealed in air from 100 to 1200°C. Variation of the microstructure and defects was investigated by positron lifetime spectroscopy and X-ray diffraction. The experiment results showed that the average positron lifetime of mixture MgO/ZrO₂ was more larger than that of single phase MgO and ZrO₂, and decreased with the increasing annealing temperature. Thermal annealing below 600°C, the movement of grain boundaries mainly led a reduce of the number of microvoids, and vacancy defects began to recover due to the growth of MgO nanoparticles after annealing between 600 to 900°C. Furthermore, ZrO₂ nanoparticles began to grow above 900°C, meanwhile the recovery of vacancy and vacancy clusters in MgO/ZrO₂ nanoparticles are restrained because of synergic effect between MgO and ZrO₂ nanoparticles.

Abstract: Late Professor Stewart initiated and shaped the International Conference on Positron Annihilation (ICPA) series. As a first-generation experimental positron-annihilation scientist, he made full use of the angular correlation of annihilation radiation (ACAR) method. He applied this method to study Fermi surfaces of metals, positron wave-functions in crystals, positron-electron and -phonon many-body interactions, and the vacancy formation energy in solids. He also studied with this method positronium in liquids and solids (T. Hyodo, J. Phys. Conf. Series, 618 (2015) 012002). All these studies enjoyed by Professor Stewart will long be remembered by the positron study community.

Abstract: We report on an assessment of the position resolving Hamamatsu-R3292 series photo-multiplier tubes with respect to their use for Angular Correlation of Annihilation Radiation (2D-ACAR) spectroscopy. The PMTs were coupled with a segmented scintillator and mounted at the Munich 2D-ACAR spectrometer. A series of measurements were performed in order to determine the energy and position resolution as well as the efficiency of the setup. Although a position resolution of Δ_xFWHM=1.98 mm was achieved, further improvements are needed.