Positron Moderator Research Articles

Slow positron moderator studies at Brandeis – Collaborative work with Karl F. Canter

AbstractThe collaborative research work carried out with Karl F Canter on slow positron moderators at Brandeis University is presented. A compact electrostatic positron beam has been designed and constructed for the determination of slow positron yield of single crystalline W(100) moderator foil with provision for in‐situ heating and surface characterization. Moderation yield is found to gradually increase with successive heating cycles, which is also correlated with the removal of surface and bulk carbon‐vacancy complexes. The work on inert Ar gas moderator is also briefly discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN–a new material for positron moderation

GaN with its wide bandgap might be of interest as a positron moderation material in much the same way as SiC is. To investigate this, positron beam experiments have been performed to establish the diffusion behaviour and surface branching of positrons implanted with energies varying from 0 to 25 keV into an epitaxially grown layer of semi-insulating GaN on a sapphire substrate. The measured diffusion length of the positrons amounted to 19.3 ± 1.4 nm. The surface branching ratios were as follows: 48% positron emission, 12% positronium formation and 40% trapping at the surface. The positron workfunction was shown to be negative with a value of 2.4 ± 0.3 eV. The materials feasibility for positron moderation and its possible use in field assisted moderation is discussed.

Spectrometer for the investigation of temperature dependent Ps formation and material dependent moderation efficiency

AbstractThe moderation efficiency of the commonly used positron moderators, such as tungsten or solid rare gases, is very low and amounts to less than 10‐2. For this reason a novel spectrometer was developed that enables the determination of the positron workfunction ϕ+ and the temperature dependent moderation efficiency ε of various sample materials. An additional feature of the experimental setup is the ability to investigate the temperature dependence of positronium (Ps) formation. Either with three BGO detectors the emitted gamma‐rays of the ortho‐Ps decay can be detected in coincidence or the decay spectrum can be measured with a high resolution Ge detector. The layout of this novel spectrometer and simulations are presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Performance of a slow positron beam using a hybrid lens design

The University of Hong Kong positron beam employs conventional magnetic field transport to the target, but has a special hybrid lens design around the positron moderator that allows the beam to be focused to millimeter spot sizes at the target. The good focusing capabilities of the beam are made possible by extracting work-function positrons from the moderator in a magnetic field free region using a conventional Soa lens thus minimizing beam canonical angular momentum. An Einzel lens is used to focus the positrons into the magnetic funnel at the end of transportation magnetic field while at the same time bringing up the beam energy to the intermediate value of 7.5 keV. The beam is E × B filtered at this intermediate energy. The final beam energy is obtained by floating the Soa–Einzel system, E × B filter and flight tube, and accelerating the positrons just before the target. External beam steering saddle coils fine tune the position, and the magnetic field around the target chamber is adjusted so as to keep one of the beam foci always on the target. The system is fully computer controlled. Variable energy-Doppler broadened annihilation radiation (VEDBAR) data for a GaN sample are shown which demonstrate the performance of the positron beam system.

Compact positron beam for measurement of transmission moderator efficiencies and positron yields of encapsulated sources

We have constructed a simple positron beam for measuring both the efficiencies of positron moderator foils and the positron yields of encapsulated positron beam sources. The slow-positron emission rate from the moderator is determined from the positron annihilation radiation. This technique is also applied to measure source yields. In addition to the coincidence technique, the source yield is determined by measuring electric currents generated by the annihilating positrons. The performance of the system is demonstrated with different tungsten moderator foils before and after heat treatments, and several Na 22 sources with primary activities in the range 1–50 mCi.

Influence of defect-impurity complexes on slow positron yield of a tungsten moderator: Positron annihilation, Auger, and SIMS study

Polycrystalline tungsten foil annealed at successively higher temperature up to $\ensuremath{\sim}2300\mathrm{K}$ has been investigated for slow positron moderation yield. LINAC-based intense positron beam lifetime studies have revealed that reemitted slow positron and positronium fractions gradually improve upon high-temperature annealing. So as to correlate the presence of defects and chemical impurities with the improvement in slow positron yield, positron annihilation, Auger electron spectroscopy, and secondary ion mass spectroscopy (SIMS), studies have been carried out on virgin and high-temperature $(\ensuremath{\sim}2300\mathrm{K})$ annealed W foils. The positron beam S parameter shows a large value throughout the sample depth corresponding to the virgin sample, while it is lower for annealed samples. This indicates that, as compared to virgin sample, the annealed W sample has a lower concentration of vacancylike defects. Auger studies revealed that in virgin state the surface is fully contaminated with carbon, while the annealed foil shows prominent W peaks. Corroborative SIMS concentration profiles have indicated that the carbon content is much lower in an annealed sample over a large depth region. From these studies, it is concluded that improvement in slow positron yield upon high-temperature annealing is obtained due to the removal of the surface tungsten-carbide layer as well as carbon-vacancy complexes present throughout the sample depth.

Positron source based on neutron capture

A positron beam based on absorption of high-energy prompt γ-rays from thermal neutron capture in 113Cd was installed at a neutron guide of the High Flux Reactor at the ILL in Grenoble. The positron source consists of platinum foils acting as γ–e +e −-converter and positron moderator. After acceleration to 3 keV by electrical lenses the positrons were magnetically guided through the beamline. Measurements were performed for various source geometries. The beam diameter and moderation characteristics such as positron work function, moderation efficiency and degradation were determined as well. The results lead to an optimised design of the in-pile positron source which will be implemented at the Munich research reactor FRM-II.

A novel set-up for positron scattering in gases

A very low-energy positron spectrometer for scattering measurements in gases is presented. The apparatus uses electrostatic optics, two stages of positron moderation, magnetic focusing in the scattering chamber and a pseudo-random time-of-flight correlation technique.

Intense positron source at the Munich research reactor FRM-II

The principle and the design of the in-pile positron source at the new Munich research reactor FRM-II are presented. Absorption of high-energy prompt γ-rays from thermal neutron capture in 113Cd generates positrons by pair production. For this purpose, a cadmium cap is placed inside the tip of the inclined beam tube SR11 in the neutron field of the reactor, where an undisturbed thermal neutron flux up to 2×1014 n cm-2 s-1 is expected. At this position the flux ratio of thermal to fast neutrons will be better than 104. Monte Carlo calculations showed that a mean capture rate in cadmium between 4.5 and 6.0×1013 n cm-2 s-1 can be expected. Inside the cadmium cap a structure of platinum foils is placed for converting γ-radiation into positron–electron pairs. The heated foils also act as positron moderators to generate monoenergetic positrons. After acceleration to 5 keV a positron beam is formed by electric lenses and guided by magnetic fields. In the primary positron beam an intensity up to 1010 slow positrons per second is expected.

First platinum moderated positron beam based on neutron capture

A positron beam based on absorption of high energy prompt γ-rays from thermal neutron capture in 113Cd was installed at a neutron guide of the high flux reactor at the ILL in Grenoble. Measurements were performed for various source geometries, dependent on converter mass, moderator surface and extraction voltages. The results lead to an optimised design of the in-pile positron source which will be implemented at the Munich research reactor FRM-II. The positron source consists of platinum foils acting as γ−e +e −-converter and positron moderator. Due to the negative positron work function moderation in heated platinum leads to emission of monoenergetic positrons. The positron work function of polycrystalline platinum was determined to 1.95(5) eV. After acceleration to several keV by four electrical lenses the beam was magnetically guided in a solenoid field of 7.5 mT leading to a NaI-detector in order to detect the 511 keV γ-radiation of the annihilating positrons. The positron beam with a diameter of less than 20 mm yielded an intensity of 3.1×10 4 moderated positrons per second. The total moderation efficiency of the positron source was about ε=1.06(16)×10 −4. Within the first 20 h of operation a degradation of the moderation efficiency of 30% was observed. An annealing procedure at 873 K in air recovers the platinum moderator.

Experimental determination of positron-related surface characteristics of 6H-SiC

The positron work function of 6H-SiC was determined to be −2.1±0.1 eV from an analysis of the energy spectrum of positrons reemitted from the surface. The positron reemission yield, highest in the sample inserted into vacuum after atmospheric exposure and cleaning with ethanol, was significantly reduced after sputtering with 3 keV, 125 μA min Ne+ ions. The yield was not recovered even after annealing at 900 °C, presumably due to the stability of sputter induced defects. Sputtering at lower energies caused a smaller decrease in the reemission yield that was largely recovered after annealing at 850 °C. Analysis using electron induced Auger electron spectroscopy and positron-annihilation-induced Auger electron spectroscopy indicated that the surface was Si enriched after sputtering and C enriched after subsequent annealing. Values of positron diffusion length and mobility in the unsputtered material were extracted from the dependence of the reemission yield on the beam energy. The application of SiC as a field-assisted positron moderator is discussed.

Investigation of positron work function and moderation efficiency of Ni, Ta, Pt and W(1 0 0)

Intense slow positron beams are essential for depth dependent measurements using positron annihilation technique. For this purpose efficient positron moderators are searched for. In this study, the moderation efficiency and positron work function of polycrystalline Ni, Ta, Pt, and of W(1 0 0) were investigated. The slow positron beam facility at Technical University of Munich is operated with a 6 mCi 22Na source and a W(1 0 0) foil (1 μm thick, in situ annealed at 2600 K) as moderator in transmission geometry. For the material dependent measurements, the tungsten foil was replaced by 1 μm thick foils of Ni, Ta and Pt. The energy distribution of transmitted positrons could be measured by variation of the acceleration voltage. The transmitted positrons were magnetically guided by a curved beam tube to a beam catcher. There, the yield of slow positrons was determined by detection of the 511 keV annihilation quanta. In order to characterise surface contamination and adsorbate concentration, X-ray induced photoelectron spectra (XPS) were recorded for each sample.

A high-efficiency positron moderator using electro-polished tungsten meshes

Positron moderators made with stacks of electro-polished tungsten meshes have been tested. The wires of the meshes were thinned to about 10 μm in diameter by an electro-polishing treatment. The moderation efficiency was measured to be 7.5×10 −4 for 12 overlapping meshes.

Improved tungsten moderator structures for slow positron beams

Tungsten filaments from commercial incandescent light bulbs have been tested as slow positron moderators. The filaments were annealed by electric current in the neutral gas atmosphere of the bulbs at or above the normal operating temperature of the lamp. The total efficiency of the positron source/moderator assembly was found to be as high as 4×10 −4. This value is almost doubled at high extraction voltage (3000 V). Neither the annealing temperature (i.e. the heating current) nor the filament thickness (related to the nominal power of the lamp) affected significantly the moderation efficiency, most probably due to the relatively thick filaments used and the difficulties in controlling the source geometry. It is suggested that the moderation efficiency can be further improved by using structures with thinner tungsten patterns, optimised configuration and extraction electric field.

Investigation of positron moderator efficiency by the time of flight technique using isotopic sources

The paper describes a technique for investigation of efficiency of positron moderator mock-ups that are in development for the slow positron source on SPring-8 hard synchrotron radiation. The investigation is performed by the time of flight technique using isotopic sources. Results of the multiwire moderator's mock-up efficiency measurements are provided. The results from measurements are compared against the results of Monte-Carlo simulation of this mock-up's efficiency.

Conceptual design of a slow positron source based on a magnetic trap

A unique 10.3 T superconducting wiggler was designed and manufactured at BINP SB RAS. The installation of this wiggler in the SPring-8 storage ring provides a possibility to generate a high-intensity beam of photons (SR) with energy above 1 MeV (Ando et al., J. Synchrotron Radiat. 5 (1998) 360). Conversion of photons to positrons on high- Z material (tungsten) targets creates an integrated positron flux more than 10 13 particles per second. The energy spectrum of the positrons has a maximum at 0.5 MeV and the half-width about 1 MeV (Plokhoi et al., Jpn. J. Appl. Phys. 38 (1999) 604). The traditional methods of positron moderation have the efficiency ε= N s/ N f of 10 −4 (metallic moderators) to 10 −2 (solid rare gas moderators) (Mills and Gullikson, Appl. Phys. Lett. 49 (1986) 1121). The high flux of primary positrons restricts the choice to a tungsten moderator that has ε≈10 −4only (Schultz, Nuc. Instr. and Meth. B 30 (1988) 94). The aim of our project is to obtain the moderation efficiency ε⩾10 −1. We propose to moderate the positrons inside a multi-stage magnetic trap based on several (3–6) electromagnetic traps that are connected in series. Magnetic field of the traps grows consecutively from stage to stage. We propose to release the positrons from the converter with the use of an additional relativistic electron beam passing in synchronism with the SR pulse in the vicinity of the converter. The average electron beam energy and current are 1–2 MeV and 100 mA, respectively. The electrical field of the beam is high enough to distort the positron paths by an amount comparable with the Larmor radius. The further drift of the positrons to the trap axis will occur due to the strengthening of the magnetic field. The magnetic field amplitude of adjacent traps varies in time in the antiphase and increases from 0.9 T in the first stage to 6 T in the last one. The positron transition from stage to stage takes place at the moment of the field equalization. The removal and longitudinal confinement of the positrons is provided by the electrostatic switches that separate the stages. Mixing of the positron velocity components occurs due to the scattering on the neutral gas. The gas pressure is sustained at a level of 10 −6 Torr inside the third stage of the trap and the positron energy decreases down to 1 keV here. The last stage of the device is the Malmberg–Penning trap with a 6 T stationary magnetic field (Malmberg and Driscoll, Phys. Rev. Lett. 44 (10) (1980) 654). The positrons accumulate inside it in 1 s approximately and cool down to energy less than 1 eV.

Lifetime measurements with a scanning positron microscope.

First lifetime results obtained with a scanning positron microscope will be presented. A pulsed positron beam with a variable energy from 0.5 to 20 keV, with a spot diameter of 2 microm, can be electronically scanned over an area of 0.6x0.6 mm(2). This beam is formed after a double-stage stochastic cooling (moderation) of positrons emitted from a radioactive isotope. Included in the system is a conventional scanning electron microprobe for surface analysis. Three-dimensional positron lifetime spectra of a GaAs sample with a small surface scratch reveal the range due to the mechanical damage.

Positron re-emission from electro-deposited NiW

Measurements on re-emitted positrons from electro-deposited NiW show the alloyto be an excellent candidate for the positron moderator in a new generation ofpositron beam systems in which hitherto unattainable geometries can be usedeither to accommodate novel beam designs and/or to generate more intense beamsof positrons. After annealing at 1350 °C the moderation efficiency of the NiWis found to be ~8×10-4, comparable to that commonly reported forpolycrystalline W. It emits positrons of energies up to ~3 eV, a valuecharacteristic of the work-function emission of positrons from W. There-emitted positron energy spectrum contains features characteristic of bothNi and W, suggesting that a degree of surface segregation takes place onannealing. The remoderation efficiency of the alloy is stable after a declineof approximately 10% in the first two days after annealing in situ.

A very low-energy apparatus for positron scattering on atoms and molecules

An apparatus for very low-energy positron scattering measurements in gases is presented. The apparatus uses electrostatic optics, two stages of positron moderation, magnetic focusing in the scattering chamber and a pseudo-random time-of-flight correlation technique.

High-efficiency multi-wire tungsten moderator for slow positron source on SPring-8 hard synchrotron radiation

The paper provides results of numeric simulations of in-target positron production process, processes of moderation, thermalization, diffusion, and reemission of positrons in high-efficiency multi-wire moderator made of tungsten monocrystalline wire with regular wire spacing. The paper looks into dynamics of slow positrons in the moderator's vacuum gaps taking into account of external fields. The feasibility of using multi-wire moderator with non-regular structure - multi-layer “wire felt” moderator is discussed. According to maximal estimate the multi-wire moderators can reach very high efficiency of fast-slow positron transformation ∼10 −2. Using such moderator the intensity of slow positron source on hard synchrotron radiation of SPring-8 can reach the level of ∼10 11 e +/s.