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Investigation of In-Flight Annihilation of Positrons Using Coincidence Doppler Broadening Spectroscopy

Authors: Leon Chryssos, Christoph Hugenschmidt

Abstract: Doppler Broadening Spectroscopy (DBS) of the positron electron annihilation line allows the detection and analysis of defects in materials. DBS uses the fact that during the annihilation momentum has to be conserved and is therefore transferred from the annihilating positron-electron pair to the annihilation gamma quanta. As a sufficient approximation the positron is assumed to be thermalized in the solid when it annihilates with an electron. In that case, the dominant fraction of the transferred momentum originates from the electrons. However, when implanting high-energy positrons a small fraction of positrons (typicylly less than 1%) will annihilate with electrons before thermalizing. This process of in-flight annihilation can be differentiated from annihilation of thermalized positrons by Coincidence DBS (CDBS). We used the CDB Spectrometer at the NEutron induced POsitron source MUniCh (NEPOMUC) in order get a deeper understanding of in-flight annihilation in several materials of different densities and core annihilation probabilities. We utilized a ²²Na source as positron emitter which not only provides insight into the thermalization process of positrons, but also extends the capabilities of the CDB spectrometer.

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Spin-Polarized Positron Annihilation Spectroscopy: Principles and Applications in Materials Study

Authors: Atsuo Kawasuso

Abstract: In the summer school of PSD2024, focusing on spin-polarized positron annihilation spectroscopy in materials science, I reported the historical background and its possibility concerning current spintronics field, the basic principles, and prospects. Here, as a memorandum, I mainly summarize the basic principles, which can be relatively well-formulated, with some remarks.

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The Science Enjoyed by Professor Alec T. Stewart

Authors: Toshio Hyodo

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.

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A Preliminary Study of Oxidation Behavior of Mo-50Re Alloy at Different Temperature Using Positron Annihilation Lifetime Spectroscopy

Authors: Zhe Jie Zhu, Wei Yang, Jian Jian Shi, Tong Guang Zhai, Yi Chu Wu

Abstract: The oxidized Mo-50Re alloys in air at 573 K and 873 K for various times were investigated by X-Ray diffraction and positron annihilation lifetime spectroscopy. The results indicated that main orthogonal phase MoO₃ together with small amount of monoclinic phase Mo₈O₂₃ were formed on on the surface of the specimens, and the oxide film of specimens oxidized at 873 K were much thicker than that of specimens oxidized at 573 K. Meanwhile, when Mo-50Re specimens oxidized in air at 873K, the defect’s size within interface layer of the specimens was larger, and the oxide film on the surface of the specimens contained much more defects. Faster oxidation process were observed occured at 873K, which was likely due to the formation of larger-size interfacial defects.

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Positron Annihilation Spectroscopic Studies of Cu₂Te Thermoelectric Material

Authors: Wen Na Ge, Ju Ping Xu, Run Ye, Jian Dang Liu, Bang Jiao Ye

Abstract: Positron annihilation lifetime spectroscopy (PALS) and coincident Doppler-broadening spectroscopy (CDBS) have been used for investigating the evolution of vacancy-type defects in the thermoelectric material Cu₂Te which annealed at different temperatures. The results of PALS show that a fraction of positrons has got annihilated at the surfaces and the sample which annealed at 450 °C has the highest concentration of surface defects. The average positron lifetime and the S parameter have the same trends which gradually increase with the increase of the annealing temperature. This change implies that the total concentration of the defects has been changed with the change of the annealed temperatures. The results of the CDBS ratio spectrum and S-W plot indicate that the defect species have no change after annealing at different temperatures.

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Asymmetric Positron Interactions with Chiral Quartz Crystals?

Authors: J. David van Horn, Fei Wu, Gerald Corsiglia, Yan Ching Jean

Abstract: We have studied the interaction of the positron with chiral left-or right-handed quartz crystals. In Doppler-broadening experiments, using a mono-energetic positron beam there is a differential depth profile for positrons implanted in LH or RH z-cut quartz as identified by a shape parameter (S). Further, in bulk positron annihilation lifetime spectroscopy (PALS) experiments, the lifetime (τ₂) attributed to free annihilation of the positron interacting with the chiral lattice exhibits a larger value for the LH quartz, and the associated intensity (I₂) is also significantly different—RH quartz is consistently 10% greater than the LH crystal. The τ ₃ lifetime and its intensity, I₃, attributed to positronium interacting with defects in the quartz, also appears to exhibit differences between the enantiomeric sets of crystals. These observations may demonstrate chiral recognition using a positron annihilation technique, pave the way for a broad range of positron experiments, and may help inform hypotheses of chirality recognition, selection, or induction by beta radiation.

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Positron Manipulation and Positronium Laser Excitation in AEgIS

Authors: Sebastiano Mariazzi, Ruggero Caravita, Stefano Aghion, Claude Amsler, Akitaka Ariga, Tomoko Ariga, Germano Bonomi, Philippe Bräunig, Johan Bremer, Roberto S. Brusa, Luis Cabaret, Massimo Caccia, Fabrizio Castelli, G. Cerchiari, Karl Chlouba, Simone Cialdi, Daniel Comparat, Giovanni Consolati, Andrea Demetrio, Lea di Noto, Michael Doser, Alexey Dudarev, Antonio Ereditato, Craig Evans, Rafael Ferragut, Julian Fesel, Andrea Fontana, Ola K. Forslund, Sebastian Gerber, Marco Giammarchi, Angela Gligorova, Sergei Gninenko, Francesco Guatieri, Stefan Haider, Helga Holmestad, Torkjell Huse, Ine L. Jernelv, Elena Jordan, Thomas Kaltenbacher, Alban Kellerbauer, Mitsuhiro Kimura, Torsten Koetting, Daniel Krasnicky, Vittorio Lagomarsino, Pierre Lansonneur, Patrice Lebrun, Sebastian Lehner, Joanna Liberadzka, Chloe Malbrunot, Lisa Marx, Viktor Matveev, Zeudi Mazzotta, Giancarlo Nebbia, Patrick Nedelec, Markus Oberthaler, Nicola Pacifico, Davide Pagano, Luca Penasa, Vojtech Petracek, Ciro Pistillo, Francesco Prelz, Marco Prevedelli, Luca Ravelli, Laura Resch, Benjamin Rienäcker, Ole M. Røhne, Stefan Rosenberger, Alberto Rotondi, Michele Sacerdoti, Heidi Sandaker, Romualdo Santoro, Paola Scampoli, Fiodor Sorrentino, Michal Spacek, James Storey, Izabela M. Strojek, Gemma Testera, Ingmari Tietje, Stefan Vamosi, Eberhard Widmann, Pauline Yzombard, Sandra Zavatarelli, Johann Zmeskal

Abstract: Production of antihydrogen by using the charge exchange reaction, as proposed by AEgIS (Antimatter Experiment: gravity, Interferometry, Spectroscopy), requires the formation of a dense cloud of positronium atoms excited to Rydberg states. In this work, the recent advances in AEgIS towards this result are described. Namely, the manipulation of positrons to produce bunches containing more than 10⁸ particles and the laser excitation of positronium to Rydberg states, using n=3 as intermediate level, are presented.

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Radiation-Induced Defects in Si after High Dose Proton Irradiation

Authors: Yurii V. Funtikov, Leonid Yu. Dubov, Yurii V. Shtotsky, Sergey V. Stepanov

Abstract: Experiments on investigation of the radiation defects produced as a result of high energy proton irradiation of single crystal Si wafers are carried out. Parameters of the proton irradiation facility are presented. It is shown that the most efficient radiation defect formation correlates with the position of the Bragg peak of ionization losses. LT spectra were measured just after irradiation and then after keeping Si samples during 3 months of at room T. We did not observe any variation of the number density of the defects, except for the 7th wafer, where most part of protons was stopped. An efficient annealing of the vacancy-type defects starts at temperatures slightly lower than 100 °C (during 10 min). Annealing at about 700 °C leads to recovering of the monoexponrntial shape of the LT spectra.

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Study of Gamma Irradiation Effect on Unplasticized PVC by Positron Annihilation

Authors: Bao Yi Wang, Jing Yang, Xing Zhong Cao, Long Wei, Run Sheng Yu

Abstract: Positron annihilation technique is used to study the gamma irradiation effect on unplasticized PVC (UPVC). Positron annihilation lifetime spectroscopy (PALS) results show that both the o-Ps lifetime and intensity decrease with the increase of gamma irradiation dose. By comparing Coincidence Doppler Broadening (CDB) results of four typical polymers using different reference sample, it is confirmed that CDB ratio curve can truthfully reflect the momentum difference of electrons which positrons annihilate with. The CDB results of gamma-irradiated UPVC using non-irradiated UPVC rather than commonly chosen LDPE as a reference sample show obvious oxygen peak, together with FTIR results, verify the appearance of carbonyl groups around the free volume.

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On the Limits of Sensitivity of PALS to Incorporated Free Volume Holes in Polymeric Membranes and Sorbents: Effect of sc-CO₂ in Polyhexafluoropropylene

Authors: Victor P. Shantarovich, Valentin Bekeshev, Nikolai Belov, Irina Kevdina, Mikhail Filimonov, Irina Ronova, Aleksandr Nikolaev

Abstract: Positron annihilation lifetime spectroscopy (PALS) is a recognized instrument for the studies of size-distribution of nanopores (intrinsic free volume holes FVH) in polymers, particularly membrane materials. The limits of this application in the case of “alien” pores, produced by swelling of poly (hexafluoro propylene) PHFP in the gas (CO₂) in super-critical (sc-) state are discussed. The obtained conclusions are controlled by measurements of low temperature gas (N₂) sorption (LTGS) and by comparison of the data with the results on permeation of various gases through the PHFP membrane. Attention is payed to relaxation with time of the newly created FVH in the PHFP membrane.

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Papers by Keyword: Positron