Positron Moderator Research Articles

Slow positrons from a magnetic bottle

A magnetic bottle device with optimized sizes for high-efficiency positron moderation was manufactured and attached to a positron detection system based on two gamma detectors operating in coincidence to detect positron annihilation events. The detection system’s performance was tested with a 22Na calibrated source. Titanium foils with a natural isotope mixture that were 2-μm-thick were proton irradiated to produce 48V. Experiments with a stack of up to four 48V foils showed that the magnetic bottle trap achieved a 10-fold increase in the standard positron moderator efficiency of tungsten in transmission mode. However, the measured moderation efficiency of ∼0.4% was found to be in huge discrepancy with the value of 1.8% obtained by Gerchow et al. for a non-optimized trap. One possible reason for this discrepancy was pointed out, namely that Gerchow et al mainly detected moderated positrons but almost no fast/unmoderated positrons, which escape the magnetic bottle trap and should also reach the positron detector used. Our experimental and simulation results showed that the ratio of moderated to fast/unmoderated positrons that reach our detection system is about 1:7.

Optimization of a device for positron moderation based on a magnetic bottle

The design optimization of a magnetic bottle device for high-efficiency positron moderation is described herein. The interaction between positrons and matter, including positron tracking, was performed via GEANT and COMSOL simulation. Optimization was performed for a 10 kW power supply for the excitation of two magnetic coils and were established geometry sizes of the device, namely a chamber with a radius of 100 mm and a length of 300 mm. The irradiation of a Ti foil with a thickness of 1 ÷ 10 μm at the TR19 cyclotron for 8 h with a proton current of 10 μA can produce 48V with an activity of 6 ÷ 60 MBq. Given such a source of fast positrons for use in the optimized magnetic bottle device, a slow positron beam intensity of 2 ÷ 10×104 s−1 can be achieved.

Optimizing the moderator geometry and thickness for a reactor-based slow positron source

Monte Carlo radiation transport simulations were used to calculate the slow positron yields from tungsten foil positron moderators at a reactor-based slow positron source. The slow positron generation rates were coupled to a simple electrostatic extraction model to estimate the yield of extractable slow positrons from the source. These calculations were used to optimize the foil thicknesses, length, pitch, and extraction voltages.

Positron production using a 9 MeV electron linac for the GBAR experiment

For the GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN’s Antiproton Decelerator (AD) facility we have constructed a source of slow positrons, which uses a low-energy electron linear accelerator (linac). The driver linac produces electrons of 9 MeV kinetic energy that create positrons from bremsstrahlung-induced pair production. Staying below 10 MeV ensures no persistent radioactive activation in the target zone and that the radiation level outside the biological shield is safe for public access. An annealed tungsten-mesh assembly placed directly behind the target acts as a positron moderator. The system produces 5×107 slow positrons per second, a performance demonstrating that a low-energy electron linac is a superior choice over positron-emitting radioactive sources for high positron flux.

Defect Structure Determination of GaN Films in GaN/AlN/Si Heterostructures by HR-TEM, XRD, and Slow Positrons Experiments

The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e., thickness of layers, crystal structures, growth orientation, elemental diffusion depths, edge, and screw dislocation densities), within two GaN/AlN/Si heterostructures, that alter their efficiencies as positron moderators. The structure of the GaN film, AlN buffer layer, substrate, and their growth relationships were determined through high-resolution transmission electron microscopy (HR-TEM). Data resulting from high-resolution X-ray diffraction (HR-XRD) was mathematically modeled to extract dislocation densities and correlation lengths in the GaN film. Positron depth profiling was evaluated through an experimental Doppler broadening spectroscopy (DBS) study, in order to quantify the effective positron diffusion length. The differences in values for both edge and screw dislocation densities, and correlation lengths (Le, Ls) found in the 690 nm GaN film, were associated with the better effective positron diffusion length (Leff) of = 43 ± 6 nm.

Microscopy studies on magnetron sputtered thin films with preferred orientation for positron moderation

Doppler-broadening Spectroscopy (DBS) and positron annihilation lifetime spectroscopy (PALS) are two positron annihilation spectroscopy techniques used for identifying near surface vacancies and lattice dislocations. Positrons manifest a broad energy distribution of about several MeV and in order to generate a low-energy positron beam, the positrons need to pass through a material called “moderator” so that a narrow positron kinetic energy bandwidth can be obtained, without losing too much intensity. The moderator must possess certain properties, among which a large negative positron work function and a large branching ratio to free positrons. The first can be assessed to an intrinsic characteristic of the material, that is usually correlated to the material bandgap, and in order to attain the second, a defect free, epitaxial thin film should be obtained. Due to its 3.4 eV bandgap, GaN represents a very promising candidate for positron moderation and field assisted moderation, that might manage longer average positron diffusion lengths. The aim of the present study is to obtain semi-insulating GaN thin films by magnetron sputtering GaN on different substrates (ZnO, TiO2, Al2O3) with fewer impurities incorporated, which should hold promises of increasing the positron diffusion length in the material. Preliminary depositions made in this study are to be discussed, highlighting both their advantages and limitations derived from their structure characterization.

Calculation and tabulation of efficiencies for tungsten foil positron moderators

Monte Carlo radiation transport simulations were used to calculate the positron stopping profiles in tungsten positron moderator foils. Stopping profiles were numerically integrated with efficiency kernels derived from Green's function solutions of the 3D diffusion equation to determine the moderation efficiency in both the backscattering and transmission geometries. Stopping profiles and efficiencies were calculated for positron energies from 10 keV to 10 MeV and incident angles from 0° to 75°. The resulting efficiencies agreed with other calculated and measured values in the literature, especially when similar values of the positron diffusion length and surface emission branching ratio were used. Large discrepancies with some of the values reported in the literature are mainly attributed to differences in foil manufacture and surface condition—factors which are known to greatly influence the diffusion length—as well as work function and branching ratios. This work provides tabulated efficiencies for tungsten foil moderators that can be interpolated and integrated with a positron flux having arbitrary energy and angular distributions.

Defect imaging and detection of precipitates using a new scanning positron microbeam

We report on a newly developed scanning positron microbeam based on threefold moderation of positrons provided by the high intensity positron source NEPOMUC. For brightness enhancement a remoderation unit with a 100 nm thin Ni(100) foil and 9.6% efficiency is applied to reduce the area of the beam spot by a factor of 60. In this way, defect spectroscopy is enabled with a lateral resolution of 33 μm over a large scanning range of 19 × 19 mm2. Moreover, 2D defect imaging using Doppler broadening spectroscopy (DBS) is demonstrated to be performed within exceptional short measurement times of less than two minutes for an area of 1 × 1 mm2 (100 × 100 μm2) with a resolution of 250 μm (50 μm). We studied the defect structure in laser beam welds of the high-strength age-hardened Al alloy (AlCu6Mn, EN AW-2219 T87) by applying (coincident) DBS with unprecedented spatial resolution. The visualization of the defect distribution revealed a sharp transition between the raw material and the welded zone as well as a very small heat affected zone. Vacancy-like defects and Cu rich precipitates are detected in the as-received material and, to a lesser extent, in the transition zone of the weld. Most notably, in the center of the weld vacancies without forming Cu-vacancy complexes, and the dissolution of the Cu atoms in the crystal lattice, i.e. formation of a supersaturated solution, could be clearly identified.

Efficient positron moderation with a commercial 4H-SiC epitaxial layer

We have studied the properties of a commercially available 4H-SiC epitaxial layer and evaluated its potential application as an efficient positron remoderator. A remoderation efficiency of more than 65% has been measured for incident positrons with 1 keV energy. We have determined the work function and the energy distribution of the emitted slow positrons, a property which is essential for practical applications. Comparison of the positron moderation properties of the epitaxial layer with results from a n-type 4H-SiC single crystal, indicate that the epitaxially grown layer is a superior secondary moderator than its substrate counterpart.

Moderation and diffusion of positrons in tungsten meshes and foils

The efficiency of tungsten meshes and thin foils for moderation of fast positrons from 22Na has been investigated in transmission geometry and a fair agreement has been found with previous experimental results where directly comparable. For foils, the dependence on material thickness is found to be similar to the prediction of the Vehanen-Mäkinen diffusion model; however, the magnitude is 5–10 times lower. A broad consensus is observed between experiment and the results of a three-dimensional model developed in this work. For a given thickness, meshes are found to be generally better than foils by around a factor of 10 with a maximum efficiency (∼10−3) comparable to that achieved with thin single crystal foils, in accord with previous measurements and the results of the present model.

Optimisation of the thickness of the moderator for positron annihilation process study in Ar gas

To investigate the deceleration and annihilation processes of positron in noble gas, we attempted to optimise the thickness of the positron moderator. The results of the decreasing trend of free positron intensity and ortho-positronium intensity with the thickness, and the annihilation rate suggest that the energy of almost all the positrons passed through the metal foils was much above the positronium formation threshold of 9 eV. Total thickness of 0.1 g cm-2 of the moderator and the Ar gas was required to stop almost all the positron in Ar gas.

Positron Beam Characteristics at NEPOMUC Upgrade

In 2012, the new neutron induced positron source NEPOMUC upgrade was put into operation at FRMII. Major changes have been made to the source which consists of a neutron-γ-converter out of Cd and a Pt foil structure for electron positron pair production and positron moderation. The new design leads to an improvement of both intensity and brightness of the mono-energetic positron beam. In addition, the application of highly enriched 113Cd as neutron-γ-converter extends the lifetime of the positron source to 25 years. A new switching and remoderation device has been installed in order to allow toggling from the high-intensity primary beam to a brightness enhanced remoderated positron beam. At present, an intensity of more than 109 moderated positrons per second is achieved at NEPOMUC upgrade. The main characteristics are presented which comprise positron yield and beam profile of both the primary and the remoderated positron beam.

Annealing of a pre-assembled tungsten positron moderator by direct electron bombardment

Annealing of a pre-assembled tungsten positron moderator by direct electron bombardment

Flash lamp annealing of tungsten surfaces marks a new way to optimized slow positron yields

Tungsten in the form of a mono-crystalline foil with an optimum thickness of about 2 μm is often used as a positron moderator in mono-energetic positron beams with 22Na positron sources. The efficiency of such a moderator strongly depends on its prior heat treatment, i.e. an annealing procedure with considerable difficulty at temperatures of about 2000 °C under vacuum conditions. Flash lamp annealing (FLA) has been tested as new method to quickly anneal W foils in order to produce easy manageable, low-cost moderators with a high efficiency. With FLA, just the surface of a W foil is heated above the melting point (3422°C) within 1 to 3 ms, i.e. without melting the whole foil volume. In this way, a surface cleaning is reached connected with a considerable increase in the positron diffusion length. Conventional polycrystalline W foils of 9 μm ± 25% thickness, heat treated by FLA, were characterized and tested as positron moderators. First promising tests result in a moderator efficiency of ~3*10−4 and clearly demonstrate that FLA is also applicable to tungsten meshes.

Linac-based positron source and generation of a high density positronium cloud for the GBAR experiment

The aim of the recently approved GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment is to measure the acceleration of neutral antihydrogen atoms in the gravitational field of the Earth. The experimental scheme requires a high density positronium cloud as a target for antiprotons, provided by the Antiproton Decelerator (AD) – Extra Low Energy Antiproton Ring (ELENA) facility at CERN. We introduce briefly the experimental scheme and present the ongoing efforts at IRFU CEA Saclay to develop the positron source and the positron-positronium converter, which are key parts of the experiment. We have constructed a slow positron source in Saclay, based on a low energy (4.3 MeV) linear electron accelerator (linac). By using an electron target made of tungsten and a stack of thin W meshes as positron moderator, we reached a slow positron intensity that is comparable with that of 22Na-based sources using a solid neon moderator. The source feeds positrons into a high field (5 T) Penning-Malmberg trap. Intense positron pulses from the trap will be converted to slow ortho-positronium (o-Ps) by a converter structure. Mesoporous silica films appear to date to be the best candidates as converter material. We discuss our studies to find the optimal pore configuration for the positron-positronium converter.

A combined matrix isolation spectroscopy and cryosolid positron moderation apparatus

We describe the design, construction, and operation of a novel apparatus for investigating efficiency improvements in thin-film cryogenic solid positron moderators. We report results from solid neon, argon, krypton, and xenon positron moderators which illustrate the capabilities and limitations of our apparatus. We integrate a matrix isolation spectroscopy diagnostic within a reflection-geometry positron moderation system. We report the optical thickness, impurity content, and impurity trapping site structures within our moderators determined from infrared absorption spectra. We use a retarding potential analyzer to modulate the flow of slow positrons, and report positron currents vs. retarding potential for the different moderators. We identify vacuum ultraviolet emissions from irradiated Ne moderators as the source of spurious signals in our channel electron multiplier slow positron detection channel. Our design is also unusual in that it employs a sealed radioactive Na-22 positron source which can be translated relative to, and isolated from, the cryogenic moderator deposition substrate. This allows us to separate the influences on moderator efficiency of surface contamination by residual gases from those of accumulated radiation damage.

High-intensity and high-brightness source of moderated positrons using a brilliant γ beam

Presently large efforts are conducted towards the development of highly brilliant gamma beams via Compton back scattering of photons from a high-brilliance electron beam, either on the basis of a normal-conducting electron linac or a (superconducting) Energy Recovery Linac (ERL). Particularly ERL's provide an extremely brilliant electron beam, thus enabling to generate highest-quality gamma beams. A 2.5 MeV gamma beam with an envisaged intensity of 10^15 s^-1, as ultimately envisaged for an ERL-based gamma-beam facility, narrow band width (10^-3), and extremely low emittance (10^-4 mm^2 mrad^2) offers the possibility to produce a high-intensity bright polarized positron beam. Pair production in a face-on irradiated W converter foil (200 micron thick, 10 mm long) would lead to the emission of 2 x 10^13 (fast) positrons per second, which is four orders of magnitude higher compared to strong radioactive ^22Na sources conventionally used in the laboratory.Using a stack of converter foils and subsequent positron moderation, a high-intensity low-energy beam of moderated positrons can be produced. Two different source setups are presented: a high-brightness positron beam with a diameter as low as 0.2 mm, and a high-intensity beam of 3 x 10^11 moderated positrons per second. Hence, profiting from an improved moderation efficiency, the envisaged positron intensity would exceed that of present high-intensity positron sources by a factor of 100.

Thermal analysis of EPOS components

We present a simulation study of the thermal behaviour of the moderator and the electrostatic lens of the positron source EPOS at the Research Center Dresden-Rossendorf. The positron moderator and the upper part of the electrostatic extraction einzellens are directly exposed to the primary electron beam with an energy of 40 MeV and a current of 1 mA. Thus, it was necessary to investigate whether the construction can stand the high temperatures. As a result we found that thin moderator foils with a thickness less than 40 μm will not reach too high temperatures. The thickness can be varied up to 0.2 mm, so we adjust the temperature in order to anneal lattice defects. These defects are caused by radiation. The wall of the extraction lens which is made from a stainless steel tube must be distinctly thinned to 0.05 mm to avoid damage temperatures. The time-dependent simulation shows, that the final temperatures are reached after less than a minute.

Development of mini linac-based positron source and an efficient positronium convertor for positively charged antihydrogen production

We have installed in Saclay a facility for an intense positron source in November 2008. It is based on a compact 5.5 MeV electron linac connected to a reaction chamber with a tungsten target inside to produce positrons via pair production. The expected production rate for fast positrons is 5·1011 per second. The study of moderation of fast positrons and the construction of a slow positron trap are underway. In parallel, we have investigated an efficient positron-positronium convertor using porous silica materials. These studies are parts of a project to produce positively charged antihydrogen ions aiming to demonstrate the feasibility of a free fall antigravity measurement of neutral antihydrogen.

Development of a low-energy positron source and an efficient positron-positronium converter for positively charged antihydrogen production

We have installed in Saclay a facility for an intense positron source in November 2008. It is based on a compact 5.5 MeV electron linac and a reaction chamber with a tungsten target to produce positrons via pair production. The expected production rate for fast positrons is 4·1011 per second. The study of moderation of fast positrons and the construction of a slow positron trap are underway. We have also developed an efficient positron-positronium convertor at CERN. These studies are parts of a project to demonstrate the feasibility of an experiment to produce the positively charged antihydrogen ion for a free fall measurement of neutral antihydrogen.