Slow Positron Research Articles

High Efficiency Cyclotron Trap Assisted Positron Moderator

We report on the realisation of a cyclotron trap assisted positron tungsten moderator for the conversion of positrons with a broad keV–few MeV energy spectrum to a mono-energetic eV beam with an efficiency of 1.8(2)% defined as the ratio of the slow positrons divided by the β+ activity of the radioactive source. This is an improvement of almost two orders of magnitude compared to the state of the art of tungsten moderators. The simulation validated with this measurement suggests that, using an optimised setup, even higher efficiencies are achievable. A novel method for the production of 48V high-activity thin foil positron sources based on a solid target station at a medical cyclotron was developed. This is an improvement of more than one order of magnitude with respect to standard methods.

Study on surface structure of plasma‐treated polydimethylsiloxane (PDMS) elastomer by slow positron beam

In this paper, the variations in surface structure of polydimethylsiloxane elastomers before and after argon plasma treatments have been investigated by X‐ray photoelectron spectroscopy, slow positron beam, and scanning electron microscope. An inorganic silica‐like layer was probed by X‐ray photoelectron spectroscopy after 3 minutes or longer time of treatments, and the sample surface turned into totally hydrophilic. Short time (1 and 2 min) plasma exposure mainly removed preexisting low molecular weighted (LMW) siloxanes on sample surface. By using slow positron beam, the thicknesses of silica‐like layer for 3‐, 5‐, and 10‐minute–treated samples were estimated to be around 30, 66, and 91 nm, respectively. Beneath the silica‐like layer, a loose polymeric structure was also detected, which was ascribed to the accumulation of LMW siloxanes. Scanning electron microscope images showed that the silica‐like layer cracked after 10 minutes of plasma treatment, which provided direct diffusion pathways for LMW siloxanes. Hence, 10‐minute–treated sample showed rather low organic composition near surface. Slow positron beam provides valuable depth profile information for evaluating the surface aging condition of polydimethylsiloxane composite.

Study of corrosion-related defects of zirconium alloys with slow positron beam

The corrosion behavior of Zr-4 and N5 alloy specimens corroded in 0.01 mol/L LiOH aqueous solution at 360 °C/18.6 MPa and in super heated steam at 400 °C/10.3 MPa for 1, 3 and 14 days were investigated by slow positron beam based Doppler broadening spectroscopy. Results showed that there was an evident interfacial layer with pre-existed vacancies and voids in uncorroded Zr-4 specimens, while in uncorroded N5 specimen, the interfacial defect layer can not be identified or a thin interfacial layer was only contained. When the specimens were corroded in super heated steam at 400 °C/10.3 MPa for a few days, the existence of the interface layer in the Zr-4 specimen would delay the diffusion rate of the oxygen atoms and decelerated the oxidation rate of the corrosion process. However, at very early stage of the corrosion, as Zr-4 and N5 specimens were corrded in 0.01 mol/L LiOH aqueous solution at 360 °C/18.6 MPa, the effect of Li+ accelerated the diffusion rate of the oxygen atoms, while the effect of the interface defect layer became a minor effect.

Physical models and primary design of reactor based slow positron source at CMRR

Abstract Slow positron facilities are widely used in material science. A high intensity slow positron source is now at the design stage based on the China Mianyang Research Reactor (CMRR). This paper describes the physical models and our primary design. We use different computer programs or mathematical formula to simulate different physical process, and validate them by proper experiments. Considering the feasibility, we propose a primary design, containing a cadmium shield, a honeycomb arranged W tubes assembly, electrical lenses, and a solenoid. It is planned to be vertically inserted in the Si-doping channel. And the beam intensity is expected to be 5 × 10 9 e s + / s .

4p-YG-10 低速陽電子ビームを利用した金属多層Ni/Hfの非晶質化過程の評価

4p-YG-10 低速陽電子ビームを利用した金属多層Ni/Hfの非晶質化過程の評価

Subnanopore structural change of time-elapsed silica PECVD films elucidated by slow positron annihilation and ellipsometric porosimetry

Subnanopore structural change of time-elapsed silica PECVD films elucidated by slow positron annihilation and ellipsometric porosimetry

Experimental study on Ti K shell ionization cross sections induced by 8-9.5 keV positrons

Due to lack of experimental data of the inner shell ionization cross sections induced by low-energy positron, advanced theoretical models developed in recent years cannot be correctly evaluated, and the application of slow positron beam technique is greatly limited. Here we present the method of obtaining reliable experiment data of atomic inner-shell ionization cross section by positron impact. In this work, the slow positron beam device is used to generate 8-9.5 keV positron beams impacting on a pure thick Ti target, and the silicon drift detector (SDD) is adopted to collect the X-ray spectra produced by positrons impacting on thick Ti target, and the incident positron numbers are obtained by applying an HPGe detector to on-line collect annihilation photons. Then the experimental characteristic X-ray yields of Ti K shell impacted by 8-9.5 keV positron could be acquired. Meantime, the simulated characteristic X-ray yields are acquired by the PENELOPE program simulating the experiments. In the comparison between the experimental yields and the simulated yields based on two sets of different inner shell ionization cross section database in the PENELOPE code, i.e. the optical data model (ODM) and the distorted-wave Born approximation model (DWBA), there is a large difference between the simulated data from the ODM theoretical model and the experimental values, while the simulated yields from the DWBA theoretical model are in good agreement with the experimental results. Accordingly, a correction factor is introduced to modify the DWBA theoretical model database which is used in the PENELOPE, and then the experimental process is re-simulated. When the simulated yields and the experimental yields are in the highest consistence, the reliable Ti K shell ionization cross sections impacted by 8-9.5 keV positron could be obtained. The biggest advantage of using this method to obtain atomic inner-shell ionization cross section impacted by positron is that the effects of the multiple scattering of incident positrons in the thick target, from the bremsstrahlung and annihilation photons, and other secondary particles on the experimental characteristic X-rays do not need calculating (the calculation method that has been developed previously cannot give the more correct result about the contribution of the multiple scattering of incident positrons, from the bremsstrahlung and annihilation photons, and other secondary particles to characteristic X-rays).

Surface Carbonization of GaN and the Related Structure Evolution during the Annealing Process**Supported by the National Natural Science Foundation of China under Grant No 51402013, the National Key Research and Development Program of China under Grant No 2016YFE0133200, and the European Union’s Horizon 2020 Research and Innovation Staff Exchange Scheme under Grant No 734578.

To explain the stabilization mechanism of the carbon-ion-implanted GaN under the diamond growth environment, the luminescence characteristics and structure evolution correlative with sites’ carbon atoms located for high-fluence carbon-ion-implanted GaN are discussed. GaN is implanted with carbon ion using fluence of 2 × 1017 cm−2 and energy of 45 keV. Then the implanted samples are annealed at 800°C for 20 min and 1 h under the N2 atmosphere. The luminescence characteristics of carbon-ion-implanted GaN are evaluated by photoluminescence spectrum at wavelength 325 nm. The lattice damage of GaN is characterized by Raman spectrum and the corresponding vacancy-defect evolution before and after annealing is measured by slow positron annihilation. The results show that most of the carbon atoms will be located at the interstitial sites after carbon ion implantation due to the weak mobility. As the implanted samples are annealed, strong yellow luminescence is emitted and the vacancies for Ga (VGa) are reduced resulting from the migration of interstitial carbon (Ci) and formation of complexes (CGa and/or CGa-Ci) between them. As the annealing time is prolonged, the carbon ions accommodated by the vacancies are saturated, vacancy clusters with carbon atoms appear and the concentration of CGa diminishes, which will have an adverse effect on the diamond film nucleation and growth.

Mesoporous titania films investigated by positron annihilation based on a slow positron beam

Tunable mesoporous titania (TiO2) thin films were synthesized via a sol-gel method using an amphiphilic triblock copolymer F38 as the structural template. The dependence of crystalization, pore morphology and interconnectivity of TiO2 films on the weight ratio of F38 was studied by wide-angle X-ray diffraction, field emission scanning electron microscopy and Doppler broadening of positron annihilation radiation spectroscopy based on a slow positron beam. By loading more F38, the crystallization of TiO2 films is enhanced, accompanied by a decrement in oxygen vacancies/grain boundaries. Smaller and isolated mesopores are formed in the films prepared with F38 less than 15 wt%. The pore percolation occurs when the weight ratio of F38 is up to 20 wt% and larger and interconnected worm-like pores are formed.

Effects of displacement damage and helium production rates on the nucleation and growth of helium bubbles – Positron annihilation spectroscopy aspects

Fe-12 wt% Cr model alloy samples were implanted by 250 keV He2+ ions to three different fluencies (3 × 1017, 9 × 1017 and 1.5 × 1018 cm−2) at T < 100 °C. In a depth profile manner, the implantation impact according to defined peak profile was investigated using variable energy slow positrons, with the primary focus on the 2–13 dpa region. The obtained data were compared to published data on Optifer IX steel samples [1] irradiated in the frame of a two-years irradiation program of the Swiss Spallation Neutron Source. Bi-modal defect distribution represented by two defect components in positron lifetime spectrum reveals two distinct helium bubbles growth mechanisms. While at the lower helium production rate of the spallation environment, the bubbles grow primarily by migration and coalescence, at the high production rates of helium in the implanted samples, the results indicate this growth is driven by Ostwald ripening mechanism. A competitive growth process via emission of interstitial atoms (clusters) is discussed in terms of low-temperature He implantations.

Effect of Ultraviolet Absorber on Photo-Degradation of Epoxy Coating Studied by Slow Positron Beam

Effect of Ultraviolet Absorber on Photo-Degradation of Epoxy Coating Studied by Slow Positron Beam

Design of an Acceleration Gap for Brightness Enhancement in a Reactor-Based Slow Positron Beamline

Design of an Acceleration Gap for Brightness Enhancement in a Reactor-Based Slow Positron Beamline

Calculated cross sections for elastic scattering of slow positrons by silane

In this work we investigate elastic collisions of low-energy positrons with silane (${\mathrm{SiH}}_{4}$). We employed the Schwinger multichannel method to calculate integral and differential cross sections for impact energies up to 10 eV. The calculations were performed within the static plus polarization approximation. We carried out a systematic study employing different schemes to account for the polarization effects of the target due the presence of the incoming positron. We investigate how the inclusion of extra functions in different extra (chargeless) centers affects the calculated cross sections and the physical phenomena such as the Ramsauer--Townsend minimum and the virtual state formation. Our results are compared with available experimental total cross sections and with integral and differential cross sections computed with a model correlation-polarization potential. In particular, our integral cross section agrees well with the experiment at low energies and our differential cross sections agree well with the results from previous calculations.

Microstructure, defect structure and hydrogen trapping in zirconium alloy Zr-1Nb treated by plasma immersion Ti ion implantation and deposition

The effect of low energy plasma immersion ion implantation and deposition of titanium on microstructure, defect structure and hydrogen trapping in zirconium alloy Zr-1Nb was studied. Defect structure and distribution were analyzed by Doppler broadening using slow positron beam. The surface microstructure after modification is represented by nanostructured Ti grains with random orientation. The gradient distribution of titanium as well as vacancy type defects were analyzed. The concentration of vacancy type defects is rising with increasing bias voltage. Gas-phase hydrogenation of the Ti-modified Zr-1Nb alloy was performed at 400 °C for 60 min. The strong interaction of hydrogen with vacancy type defects was demonstrated. Two different changes in the defect structure after hydrogenation were observed: when a titanium film is formed on the surface (after deposition at 500 V) hydrogen trapping occurs with the formation of titanium hydride phases, while in the implanted layer (deposition at 1000 and 1500 V) hydrogen is trapped due to interaction with vacancy type defects. The physical basis of Ti diffusion and its influence on the evolution of defect structure after surface modification and hydrogenation were discussed.

Defect structure of oxygen-vacancy clusters in O18 and self ion implanted Fe(100) crystal by ion channeling and ab-initio study

Lattice site position of O18 in the presence of vacancy defects in Fe(100) crystal is measured by ion channeling and the corresponding oxygen-vacancy cluster configuration is predicted with Density Functional Theory (DFT) calculations. Energy dependent dechanneling and Slow Positron Doppler broadening analysis show the defects to be vacancy dislocation loops. In O18 ion implanted Fe, O18 is found to be in tetrahedral interstitial position. If self ion is irradiated together with O18 to increase vacancy defects, O18 is found to be trapped at 1.2 Å away from the lattice position along <111>. From DFT calculations, similar lattice site of O is predicted with a displacement of 1.1 Å along <111> for the interaction of oxygen with ½ <111> vacancy loop structure. Upon increasing vacancy defects further by increasing the fluence of self ions, O18 is found to shift near octahedral interstitial site displaced by 0.6 Å along <100>. DFT calculations show a similar displacement of 0.46 Å along <100> for the interaction of oxygen with <100> vacancy loop structure. Our combined experimental and DFT studies provide strong evidence of O trapping at vacancy dislocation loops and O trapping site is sensitive to ½ <111> and <100> type vacancy dislocation loops.

Some initial results of simulating a positron beam system by using SIMION

Slow Positron Beam (PB) is an important device in the study of positron physics and techniques, especially in material research. For the purpose of conceptual designing a PB system, we have simulated a PB system with the parameters of an existing system – SPONSOR, using SIMION software. The simulation results have been compared with the SPONSOR published results. The effect of magnetic field in controlling beam trajectory has been investigated in the preaccelerated and accelerated stages. The simulation results of using steering coils to adjust the beam trajectory are also presented in this report.

Investigation of the Implantation profile and average stopping depth of slow positrons in Magnesium Sulfide MgS

Investigation of the Implantation profile and average stopping depth of slow positrons in Magnesium Sulfide MgS

Positron Annihilation Studies of the Near-Surface Regions of Niobium before and after Wear Treatment

The positron annihilation studies of well-annealed niobium samples after dry sliding or sandblasting are reported. These treatments modify surface and subsurface regions introducing crystalline defects, which are extended beneath the surface of the sample at a depth of about 140 µm for the highest applied load of 50 N. These defects can be identified as dislocations with jogs or vacancies near dislocations. The obtained depth profile of mean positron lifetime reflects the defect depth distribution and exhibits the characteristic plateau near the worn surface, which disappears when lowering of the applied load. A similar depth profile was obtained also for niobium samples after sandblasting. Slow positron beam studies revealed that at the surface of the samples after dry sliding or other treatments some defects that localize positrons are present. Additionally, a large value of positron diffusion length of about 266(33) nm was found for well annealed niobium.

Effect of free volume and formation mechanisms of polyamide layers on nanofiltration membrane

Abstract Amine monomers of different chemical structures and multiple reactive functional groups, namely tris(2-aminoethyl)amine (TAEA), triethylenetetramine (TETA), and 1,3-diaminopropane (DAP) was reacted with acyl chloride monomers such as 1,3,5-benzenetricarbonyl chloride (TMC) and suberoyl chloride (SC) to synthesize polyamide thin film composite membranes through interfacial polymerization. The effect of membrane formation mechanisms on nanofiltration was investigated. A variable monoenergy slow positron beam was used to explore the differences in the free volume diameters of polyamide layers composed of different chemical structure. The results revealed that the polymerization degree, free volume diameter, and selective layer thickness of the polyamide layers were strongly correlated with each other. Specifically, a high polymerization degree induced a small free volume diameter and a thin selective layer. The cross-linked TAEA-TMC polyamide layer exhibited a free volume diameter of 3.9 A and was the thinnest (168 nm) among the synthesized membranes, whereas the linear DAP-SC polyamide layer was the thickest (873 nm) with a free volume diameter of 4.7 A. Finally, a stability test on the TAEA-TMC/PAN showed a slight fluctuation during the 36 h filtration, salt rejection maintained stability at around 97% and water flux of 22.3 L m −2 h −1 .

Vacancy like defects and hardening of tungsten under irradiation with He ions at 800 °C

The irradiation induced hardening and vacancy like defects are investigated as a function of irradiation fluences with 200keV He ion implantation at 800°C in this work. Nano-indentation tests show the irradiation induces hardening and the hardness values increase with increasing irradiation fluences. Doppler broadening spectroscopy analyses of slow positron annihilation find that a large amount of vacancy like defects are produced under irradiation. When the He/dpa ratio is less than about 2.2%He/dpa, the vacancy like defects include mainly empty vacancy clusters and loops, and HenVm complexes. When the He/dpa ratio is more than about 2.2%He/dpa, the vacancy like defects become mainly HenVm complexes with increasing irradiation fluences. Meanwhile, these complexes increase in size and He/V ratio while the irradiation fluences increase. Based on the Orowan hardening mechanism, we discuss the relationship between irradiation induced hardening and defects under indentation, especially the contribution of HenVm complexes to the hardening.