Positron Beamline Research Articles

Temperature dependence of positron annihilation lifetime in near-surface and bulk of room-temperature ionic liquid observed by a slow positron beam

We obtained temperature dependence up to 150 °C of the triplet positronium (ortho-Ps) lifetime in N,N,N-Trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) using the vertical slow positron beamline installed at AIST. The ortho-Ps lifetimes at the near-surface and bulk regions of TMPA-TFSI were measured. The lifetime difference between the near-surface and bulk at 150 °C, 130 °C above the melting temperature, suggested that the molecular oriented structure at the surface remained. The ortho-Ps lifetimes at the near-surface and bulk became shorter as the temperature increased. On the contrary, the ortho-Ps lifetimes became longer as the temperature increased when other molecular liquids were employed except for water.PACS/topics: 78.70.Bj.

Effect of neutron–photon converter materials (Cd, Gd, and Sm) on the positron production in a reactor-based slow positron beamline

The change in the amount of positron production when Cd cap, installed as a neutron–photon converter, is replaced with Gd or Sm caps was simulated by Monte Carlo calculations in the slow positron beamline at the Kyoto University Research Reactor, especially focusing on the change in the amount of positron production induced by neutrons emitted from a reactor core. Based on the simulation results, the amount of positron production induced by neutrons obtained using Gd and Sm caps was estimated to decrease to (69±4)% and (54±3)%, respectively, compared with the amount obtained using Cd cap. Meanwhile, Cd and Gd caps with a thickness of 1 mm were evaluated to have almost the same burn-up lifetime. Therefore, a Cd cap is the best choice, while a Gd cap, which has a higher melting point, could be an alternative if the structure of the positron source makes effective cooling difficult.

Low energy electron scattering from pyridine using a Surko trap and beam

This paper presents measurements of low energy electron scattering from pyridine. The low energy positron beamline at the Australian National University was used for these measurements, with a change in operational parameters allowing for the measurement of electron scattering processes. We have collected data for the low energy total cross section for electron scattering, as well as measurements of the differential cross sections for electrons up to 3 eV impact energy. The operation of the beamline will be briefly outlined and data are compared to R-matrix and Schwinger multichannel theoretical calculations, as well as previous experimental data.

Positron scattering from pyrazine

The present paper provides a joint experimental and theoretical study of positron scattering from pyrazine. Experimental data were measured employing a low-energy positron beamline, and covered an energy range from 1 to 79 eV. Cross sections were measured for total scattering, total elastic scattering, positronium formation, direct ionization, and the sum of the total discrete electronic-state excitation processes. In addition, measured total electronic excitation cross sections for the individual ${1}^{1}{B}_{3u},$ ${1}^{1}{B}_{2u},$ ${1}^{1}{B}_{1u},$ and ${2}^{1}{B}_{2u}$ states are also reported. Finally, experimental elastic differential cross sections (DCSs), here in the energy range $1--20$ eV, are also presented. Results from two theoretical approaches are given: the R-matrix approach and the independent atom method with screening corrected additivity rule and interference terms. Where a comparison between our experimental and theoretical results could be made, the level of accord varied from being quite good to marginal depending on the actual scattering process under consideration. Last, a comparison between the present elastic DCS results and those from our earlier study on positron scattering from pyrimidine [Palihawadana et al., Phys. Rev. A 88, 012717 (2013)] showed a somewhat unexpected level of agreement.

Design and development of radio frequency system for pulse positron low energy beam and its electron beam trials

Details of the design and development of radio frequency (RF) system at 37.5 MHz and 150 MHz and initial test results of electron beam trials of a pulsed positron beam line being developed at BARC, Mumbai are presented in this paper. The RF part of the beam line is based on the conventional positron pulsing technique of chopper-pre buncher-buncher arrangement. Picosecond (ps) pulsing is achieved by a 150 MHz quarter wave resonator, which times focuses the chopped positron beam onto the sample. The chopper and pre buncher, preceding the buncher, are designed to operate at one-fourth of the frequency of the buncher. The reflection type grid chopper uses sinusoidal waves riding on a DC offset to chop the incoming beam into less than 2 ns pulses. These various structures were excited by the RF power coupled from an overall two frequency RF system. This RF system comprises of amplifiers at 37.5 MHz and 150 MHz that derive a single common input from an arbitrary frequency generator (AFG) to reduce the timing distortion, their respective impedance matching units and RF coupling lines made of RF components used for amplitude and phase adjustments and then, feeds the RF power to these structures. The RF system was initially tested on a prototype setup before incorporating in the main pulsed positron beam line. RF interference suppression techniques like filtering and shielding were used to ensure minimum interference between RF signals and the various detector electronics. Before positron beam trials, electron beam trials were performed in the positron beam line. A final pulse width 280 ps was measured with an electron beam.

A comparison of experimental and theoretical low energy positron scattering from furan.

This paper presents a joint experimental and theoretical study of positron scattering from furan. Experimental data were measured using the low energy positron beamline located at the Australian National University and cover an energy range from 1 eV to 30 eV. Cross sections were measured for total scattering, total elastic and inelastic scattering, positronium formation, and differential elastic scattering. Two theoretical approaches are presented: the Schwinger multichannel method and the independent atom method with screening corrected additivity rule. In addition, our data are compared to corresponding electron scattering results from the same target with a number of significant differences observed and discussed.

Reduction of background radiation effects for positron lifetime measurements in the slow positron beamline at the Kyoto University Research Reactor.

Positron annihilation lifetime spectroscopy (PALS), which is recognized as one of the major analytical methods of positron annihilation spectroscopy, can directly detect information related to the size of vacancy-type defects from lifetime values. PALS measurements performed under high background radiation have been previously reported. It is well known that coincidence techniques such as age-momentum correlation (AMOC) measurements are effective for the background reduction, but count rates decline significantly. In this study, a preliminary experiment was performed to reduce the influence of the background radiation without the coincidence technique in the pulsing system of the Kyoto University research Reactor (KUR) slow positron beamline. This experiment involved the introduction of a gate circuit for the background radiation discrimination using a dynode signal from a single scintillation detector (photomultiplier). After introducing the gate circuit, the time resolution and the lifetime value of Kapton were 308 ps and 388 ± 3 ps, respectively, with count rates of ∼400 counts/s at a KUR 5 MW operation. In the AMOC measurement, the time resolution and the lifetime value of Kapton were 297 ps and 380 ± 7 ps, respectively, with count rates of ∼40 counts/s at a KUR 5 MW operation. When the single detector with the gate circuit was used, the count rate was ∼1 order of magnitude higher than those of the AMOC measurements, while the time resolutions of the two methods were comparable.

Elastic and inelastic scattering of positrons from neon

SynopsisData will be presented for positron scattering from neon. As a “simple” target, neon provides a useful target for the testing and exploration of our fundamental understanding of scattering processes. The low-energy, high-resolution positron beamline based at the Australian National Universitywas used to measure elastic differential cross sections and total electronic and ionization cross sections for low energy scattering of positrons from neon. The experimental data presented will be compared to several recent theoretical calculations.

Contribution of cadmium to the total amount of positron creation in a reactor-based slow positron beamline

In the slow positron beamline at the Kyoto University Research Reactor (KUR), positron creation was enhanced by increasing the gamma-ray intensity at the positron source via the reaction of 113Cd(n,γ)114Cd. To achieve this, a cadmium (Cd) cap was attached to the positron source, surrounding it, and thus, without intentional cooling, the temperature was able to reach near the melting point of Cd via nuclear heating. In this study, the degree to which the Cd cap contributes to the quantity of positron creation was estimated by using the Monte Carlo calculation code PHITS (Particle and Heavy Ion Transport code System), which simulates radiation transportation and interaction with matter. As a result, the number of positrons created was found to become 2.0±0.1 times higher by using the Cd cap at the KUR slow positron beamline.The use of the Cd cap was confirmed to be significantly effective for enhancing positron creation.

An overview of the design of ELIPS—A new slow positron beam line

Abstract In this work, the progress made by the positron group from ELI-NP in the design of a new slow positron beam line is summarized. The rays of a brilliant gamma beam will be used to produce a slow positron beam of high intensity. The main results of the simulations, done for the geometry optimization of the Converter/Moderator Assembly (CMA) for γ -to-slow e + conversion, are reported. By using the γ -beam with energy of up to 3.5 MeV and intensity of 2 . 4 × 1 0 10 γ s − 1 , the slow e + beam can be obtained with a maximum intensity of ∼ 2 × 1 0 6 s−1 with tungsten or platinum CMA. Solid neon moderation is applicable, and a successful implementation will result in a slow e + beam of intensity > 1 0 7 s − 1 . The CMA can be operated with a central hole ( ⌀ = 2 . 5 mm) in the foils in parasitic mode, i . e . , simultaneously with other γ -beam experiments, at the cost of reduced intensity by 18%. Furthermore, details on the slow positron transport and the spectroscopic systems, which are under construction, are provided.

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

Evaluation of the optical performance of a brightness enhancement system developed for the KUR slow positron beamline

A slow positron beamline using a nuclear reactor is under development at the Kyoto University Research Reactor (KUR). A brightness enhancement system was designed to reduce the initial positron beam size (~30 mm) below typical specimen sizes (< 10 mm) and keep the intensity of the beam as high as possible. After installation of the brightness enhancement system at the beamline, we tested its optical performance using an electron beam. The experimental result indicated that the system would have enough capability to obtain a small-sized beam suitable for positron measurements.

Evaluation of a positron-beam-pulsing system in KUR reactor-based positron beam facility

The pulsing performance of the positron-beam-pulsing system of the slow positron beamline at Kyoto University research Reactor was tested using electron beams trimmed by 5-mm- or 15-mm-diameter apertures. Electron pulses with full width at half maximum of 143 ps were obtained with a 5-mm-diameter beam which corresponds to the diameter of the brightness-enhanced positron beam. This indicates the pulsing system has a sufficient pulsing performance for positron annihilation lifetime measurements.

Defects and Charging Processes in Li-Ion Battery Cathodes Studied by Operando Magnetometry and Positron Annihilation

A brief report is given on recent studies of the atomistic processes during charging of battery cathode material LixCoO2 by means of magnetometry and positron annihilation. A set-up for operando magnetometry is implemented which, for the first time, allowed to continuously monitor the distinct variation of the magnetic susceptibility χ of LixCoO2 which occurs during consecutive charging and discharging cycles. The variation of χ with Li+ content in the concentration range 1&gt;x≥0.77 arises from a variation of the electronic density of states and from electronic correlation effects. The χ (x)-behaviour for x&lt;0.77 shows that oxygen is involved in the charging process. Positron annihilation reveals vacancy-type defects on the Li-sublattice, the size of which increases with Li-extraction. Indication for Li-reordering at the reversibility limit of Li extraction is found which correlates with χ (x)-variations in this concentration regime. First measurements on LixCoO2 thin-films performed at the positron beam line NEPOMUC of FRM II at the Heinz Maier-Leibnitz neutron source will be presented.

Development of a mono-energetic positron beam line at the Kyoto University Research Reactor

Positron beam facilities are widely used for solid state physics and material science studies. A positron beam facility has been constructed at the Kyoto University Research Reactor (KUR) in order to expand its application range. The KUR is a light-water-moderated tank-type reactor operated at a rated thermal power of 5MW. A positron beam has been transported successfully from the reactor to the irradiation chamber. The total moderated positron rate was greater than 1.4×106/s while the reactor operated at a reduced power of 1MW. Special attention was paid for the design of the in-pile position source to prevent possible damage of the reactor in case of severe earthquakes.

Positronium formation from porous silica in backscattering and transmission geometries

The Aarhus positron beam line is utilised to measure the positronium formation fraction from porous silica thin films created by the Glancing Angle Deposition technique. The highest formation fraction found from these studies in a backscattering geometry is (57.7 ± 1.0)% in good agreement with other measurements. In transmission mode, the maximum positronium output is found to be (12.5 ± 0.5)%. These are the first measurements of positronium formation in transmission of a porous silica thin film, a starting point for future attempts to optimise the positronium formation in transmission.

Present status of the low energy linac-based slow positron beam and positronium spectrometer in Saclay

A new slow positron beamline featuring a large acceptance positronium lifetime spectrometer has been constructed and tested at the linac-based slow positron source at IRFU CEA Saclay, France. The new instrument will be used in the development of a dense positronium target cloud for the GBAR experiment. The GBAR project aims at precise measurement of the gravitational acceleration of antihydrogen in the gravitational field of the Earth. Beyond application in fundamental science, the positron spectrometer will be used in materials research, for testing thin porous films and layers by means of positronium annihilation. The slow positron beamline is being used as a test bench to develop further instrumentation for positron annihilation spectroscopy (Ps time-of-flight, pulsed positron beam). The positron source is built on a low energy linear electron accelerator (linac). The 4.3 MeV electron energy used is well below the photoneutron threshold, making the source a genuine on-off device, without remaining radioactivity. The spectrometer features large BGO (Bismuth Germanate) scintillator detectors, with sufficiently large acceptance to detect all ortho-positronium annihilation lifetime components (annihilation in vacuum and in nanopores).

Development of a vertical slow positron beamline at AIST

Development of a vertical slow positron beamline at AIST

In-situ (C)DBS at high temperatures at the NEPOMUC positron beam line

We report on the current status of the sample heating stage in the CDB-spectrometer at the NEPOMUC positron beam line. The currently installed new sample heating is described in detail and various design aspects are discussed briefly. As an exemplary application, the positron diffusion at high temperatures in Ge was investigated by a depth dependent evaluation of both the Doppler broadening of the annihilation line and free Ps annihilation at the surface. It was confirmed that the temperature dependence of the positron diffusion is extraordinarily strong above 670 K.

Experimental and theoretical cross sections for positron collisions with 3-hydroxy-tetrahydrofuran

Cross section results from a joint experimental and theoretical investigation into positron scattering from 3-hydroxy-tetrahydrofuran (3H-THF) are presented. Total and positronium (Ps) formation cross sections have been measured from 1 to 190 eV using the positron beamline at the Australian National University, which has an energy resolution between 60 and 100 meV. The total cross section (TCS) and the elastic and total inelastic integral cross sections in the energy range between 1 and 1000 eV have been computed within the Independent Atom Model using the Screening Corrected Additivity Rule approach. In addition, we have calculated elastic differential cross sections at selected incident energies. Our computations represent the first theoretical results reported for this target species, while our measured Ps formation cross sections are also novel. Comparison of the present TCS with the previous results from the University of Trento shows a good level of agreement at the lowest energies. We also provide a comparison between the present cross sections for 3H-THF and those from our earlier study on the parent molecule tetrahydrofuran.