Intense Positron Beam Research Articles

Low-energy positrons of high intensity at the new positron beam facility NEPOMUC

In summer 2004 the NEutron-induced POsitron source MUniCh—NEPOMUC—of the new Munich research reactor FRM-II was set into operation at the nominal reactor power of 20 MW. With this novel facility one of the worlds most intense low-energy positron beam is now available. Intensity and positron beam profile measurements were performed at 1 and 30 eV, respectively. Within the first reactor cycle a yield of up to 10 8 moderated positrons per second was achieved. In the present arrangement of NEPOMUC's instrumentation the monoenergetic positron beam is magnetically guided to a coincident Doppler broadening (CDB) facility and to a positron-induced Auger electron spectroscopy (PAES) analysis chamber.

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.

Development of positron diffraction and holography at LLNL

A low-energy positron diffraction and holography spectrometer is currently being constructed at the Lawrence Livermore National Laboratory (LLNL) to study surfaces and adsorbed structures. This instrument will operate in conjunction with the LLNL intense positron beam produced by the 100 MeV LINAC allowing data to be acquired in minutes rather than days. Positron diffraction possesses certain advantages over electron diffraction which are discussed. Details of the instrument based on that of low-energy electron diffraction are described.

Magnetic focusing of an intense slow positron beam for enhanced depth-resolved analysis of thin films and interfaces

The intense reactor-based slow positron beam (POSH) at the Delft research reactor has been coupled to a Two-Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) setup. The design is discussed with a new target chamber for the 2D-ACAR setup based on Monte Carlo simulations of the positron trajectories, beam energy distribution and beam transmission in an increasing magnetic field gradient. Numerical simulations and experiment show that when the slow positron beam with a FWHM of 11.6 mm travels in an increasing axial magnetic field created by a strong NdFeB permanent magnet, the intensity loss is negligible above ∼6 keV and a focusing factor of 5 in diameter is achieved. Monte Carlo simulations and Doppler broadening experiments in the target region show that in this configuration the 2D-ACAR setup can be used to perform depth sensitive studies of defects in thin films with a high resolution. The positron implantation energy can be varied from 0 to 25 keV before entering the non-uniform magnetic field. 2D-ACAR depth-profiling results in He-irradiated Si obtained with the new setup are presented.

The development of the intense positron beam at Washington State University

A High Voltage Engineering K-4000 Van de Graaff ion accelerator has been acquired by Washington State University. This accelerator will be used to create a positron source through the 12 C(d, n) 13 N reaction. The accelerator is capable of accelerating protons or deuterons up to 4 MeV with a maximum beam current between 200 and 300 μA. During normal operation, a 3 MeV deuteron beam will impinge on a diamond or graphite target producing ∼3.3 Ci of 13 N from which a ∼1×10 7 s −1 monoenergetic positron beam will be created. In addition to creating positrons, the source will also emit a large number on neutrons (∼1.2×10 11 s −1), producing a serious radiation hazard. The development of this 13 N based positron beam is discussed including the expected positron yield and radiation shielding requirements.

Methods for defect characterisation in thin film materials by depth-selective 2D-ACAR

The advent of intense positron beams makes it possible to perform depth-selective 2D-ACAR (two-dimensional angular correlation of annihilation radiation) studies. The Delft POSH–ACAR setup employs a strong permanent magnet for focusing of the POSH beam on the sample, which leads to a ∼15% spread in implantation energy. The effects of this spread on positron depth-profiling data are discussed, and are shown to be consistent with Doppler experiments on Si(1 0 0) with a subsurface layer of nanocavities. A method is presented to decompose depth-selective 2D-ACAR spectra reliably into their various (layer) components. This is used to reveal strong positron trapping in the nanocavities in Si(1 0 0).

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.

Present and future positron-molecular scattering experiments

Progress to date (June, 1999) of the Oak Ridge positron scattering project is reviewed. Results from our positron time-of-flight mass spectrometer include ionization and fragmentation of some halomethanes, positronium formation below the threshold for double ionization of Ne and the heavier noble gases, and the production of HeH + by positron impact on mixtures of He and H 2. New directions for future experimental work are discussed. These entail the construction and testing of a new spectrometer which measures both the masses and recoil energies of charged products, including secondary electrons. We hope to use an intense positron beam, such as the one at Lawrence Livermore National Laboratory or a similar beam, as the ionizing agent for the new spectrometer, which contains a pulsed supersonic molecular beam. With such an instrument, a range of new phenomena can be studied.

A positron lifetime spectroscopy apparatus for surface and near-surface positronium experiments

A positron lifetime spectroscopy apparatus, which utilizes an intense pulsed positron beam generated by an electron linear accelerator, is presented. Using this apparatus we can measure positron lifetime spectra with variable energy, high count rate, wide measurable time range, and high peak-to-background ratio. These features are suitable for surface and near-surface positronium experiments.

Ion-implantation Generated Nanovoids in Si and MgO Monitored by High Resolution Positron Beam Analysis

AbstractThe formation of nanovoids in Si(100) and MgO(100) by 3He ion implantation has been studied. Contrary to Si in which the voids are generally almost spherical, in MgO nearly perfectly rectangular nanosize voids are created. Recently, the 2D-ACAR setup at the Delft Positron Research Center has been coupled to the intense reactor-based variable-energy positron beam POSH. This allows a new method of monitoring thin layers containing nanovoids or defects by depth-selective high-resolution positron beam analysis. The 2D-ACAR spectra of Si with a buried layer of nanocavities reveal the presence of two additional components, the first related to para-positronium (p-Ps) formation in the nanovoids, and a second one most likely related to unsaturated Si-bonds at the internal surface of the voids. The positronium is present in excited kinetic states with an average energy of 0.3 eV. Refilling of the cavities by means of low dose 3He implantation (1×1014 cm−2) followed by annealing reduces the formation of Ps and the width of the Ps peak in the ACAR spectrum. This width reduction is due to collisions of Ps with He atoms in the voids. In MgO, p-Ps formed with an initial energy of ~3 eV shows a final average energy of 1.6 eV at annihilation due to collisions with the cavity walls. Possibilities of this new, non-destructive method of monitoring the sizes of cavities and the evolution of nanovoid layers will be discussed.

Ion-implantation Generated Nanovoids in Si and MgO Monitored by High Resolution Positron Beam Analysis

ABSTRACTThe formation of nanovoids in Si(100) and MgO(100) by 3He ion implantation has been studied. Contrary to Si in which the voids are generally almost spherical, in MgO nearly perfectly rectangular nanosize voids are created. Recently, the 2D-ACAR setup at the Delft Positron Research Center has been coupled to the intense reactor-based variable-energy positron beam POSH. This allows a new method of monitoring thin layers containing nanovoids or defects by depth-selective high-resolution positron beam analysis. The 2D-ACAR spectra of Si with a buried layer of nanocavities reveal the presence of two additional components, the first related to para-positronium (p-Ps) formation in the nanovoids, and a second one most likely related to unsaturated Si-bonds at the internal surface of the voids. The positronium is present in excited kinetic states with an average energy of 0.3 eV. Refilling of the cavities by means of low dose 3He implantation (1×1014 cm-2) followed by annealing reduces the formation of Ps and the width of the Ps peak in the ACAR spectrum. This width reduction is due to collisions of Ps with He atoms in the voids. In MgO, p-Ps formed with an initial energy of ∼3 eV shows a final average energy of 1.6 eV at annihilation due to collisions with the cavity walls. Possibilities of this new, non-destructive method of monitoring the sizes of cavities and the evolution of nanovoid layers will be discussed.

Lithium nanocluster formation in Li+ - ion implanted MgO

AbstractPhoton absorption (PA), Positron Beam Analysis (PBA) and Neutron Depth Profiling (NDP) is applied to study the relation between photon absorption behavior and the precipitates formed by ion implantation and thermal annealing. Monocrystals of MgO(100) were implanted with 1.0×10166Li ions cm−2 at an energy of 30 keV. The samples were thermally annealed in air in steps up to 1200 K. After each step Doppler broadening Positron Beam Analysis (PBA) was applied to monitor the depth profile of the implantation defects. The evolution of the depth profile of lithium was followed with the aid of NDP. During the annealing there is hardly any change in the location of the lithium implantation peak at 150 nm (peak concentration 2 at. %). Only after annealing to 1200 K the majority of the lithium has left the crystal and optical absorption effects have disappeared. During annealing at 750 K an absorption band develops between 400 and 600 nm; at 950 K the maximum absorption is centered at 450 nm corresponding to Mie absorption and scattering by lithium nanoclusters. Positron beam analysis shows a considerable increase of annihilations with low momentum electrons in the implanted zone. A positron method for measuring electron momentum distributions (2D-ACAR) coupled to an intense positron beam gave evidence for the presence of semi-coherent metallic lithium inclusions.

Spatial sampling of crystal electrons by in-flight annihilation of fast positrons

Energetic, positively charged particles travelling along a low-index crystal direction undergo many highly correlated, small-angle scattering events; the effect of these interactions is to guide or ‘channel’ (refs 1,2,3,4,5,6,7,8) the particles through the lattice. Channelling effectively focuses positive particles into the interstitial regions of the crystal: nuclear collisional processes such as Rutherford backscattering are suppressed, while the number of interactions with valence electrons increases. The interaction of channelled positrons with electrons produces annihilation radiation that can in principle9,10,11,12 serve as a quantitative, spatially selective probe of electronic charge and spin densities within the crystal: in the interstitial regions, two-photon annihilation is enhanced relative to single-photon annihilation, because the latter process requires a nuclear recoil to conserve momentum. Here we report observations of single- and two-photon annihilation from channelled positrons, using a monoenergetic beam flux of 105 particles per second. Comparison of these two annihilation modes demonstrates the ability of channelled positrons to selectively sample valence electrons in a crystal. Useful practical implementation of the technique will require the development of more intense positron beams with fluxes approaching 107 particles per second.

A theoretical search for possible high efficiency semiconductor based field assisted positron moderators

The need to obtain more intense positron beams is an ever-pressing concern in experimental positron physics. The present paper explores one route of obtaining better conversion efficiencies, namely by use of the field assisted positron technique. The basic principles of field assisted moderation are first reviewed. Theoretical estimates of possible expected positron yields from various semiconductor based field assisted moderators are then presented, the general aim being to give some guidance to experimentalists seeking to test such potential moderator systems. The calculations are based on positron affinities calculated using density functional ab initio pseudopotential theory and are found to be in good accord with experimental data where such exist. The work suggests that wide-gap materials such as GaN, C and SiC tend to compress the positron wavefunction leading to higher energy positron band states that favor positron emission. However, these materials also suffer from short positron lifetimes and low densities. In contrast, materials such as GaP, GaAs and ZnSe are also favored as positron emitters albeit with lower emission energy, but these materials have the advantage of longer lifetimes and higher densities that favor more efficient moderation.

The SHI slow positron beam facility with a compact cyclotron

The SHI slow positron beam facility has been constructed. A compact cyclotron based slow positron beam production system was developed for the first time by Sumitomo Heavy Industries (SHI) in 1993. However, in the primary slow positron beam facility, the length of the slow positron beam line is very long (about 25 m) and the efficiency of the slow positron beam transportation is not so high. In order to solve the problem, we have constructed a new compact cyclotron and a new slow positron beam facility next to the primary ones. We have named it `The SHI Slow Positron Beam Facility.' The new facility is the first accelerator based `compact' slow positron beam facility realized by using a compact cyclotron. And the new facility is the first accelerator based slow positron beam facility that was planned to be constructed `before' the accelerator construction. On the other hand, a general accelerator based slow positron beam facility is very `large' and was planned to be constructed `after' the accelerator construction. The new slow positron beam facility has some improvements in annealing the moderator, cooling the target, transporting the beam, monitoring the beam and so on, compared with the primary facility. An intense slow positron beam has already been produced by making use of the 27 Al (p,n) 27 Si reaction in the new facility. Variable-energy positron lifetime spectroscopy with a new positron pulsing system has also been arranged in the new facility.

High intense positron beam at the new Munich research reactor FRM-II

The Munich Intense POsitron Source (MIPOS) facility for producing a high intense positron beam at the new Munich research reactor (Forschungs-Reaktor München II) FRM-II is presented. Positrons are generated by pair production of high energy prompt γ-rays from neutron capture in cadmium: 113 Cd(n,γ) 114 Cd . A cadmium cap will be located inside a beamtube at an undisturbed thermal neutron flux of 2×10 14 n cm −2 s −1. Model calculation showed that this would lead to a mean capture rate of 1.2×10 13 n cm −2 s −1. Thermal load resulting from absorbed γ-rays is expected to be less than 4 W cm −2. Inside the cadmium cap a structure of platinum and a stack of tungsten foils is placed for converting the γ-rays into positron–electron pairs. Platinum is used as converting material, since the cross section for pair production is even higher (+11%) than in tungsten. The maximum of the energy spectrum of the positrons produced is about 800 keV. The tungsten foils also act as moderator. The positrons will be accelerated by electric lenses and guided by magnetic fields. Various arrangements are tested to improve the efficiency of the system. After remoderation of the positron beam an intensity of about 10 9 slow positrons per second is expected.

Development of Texas intense positron source

The Texas Intense Positron Source (TIPS) is a reactor-based low-energy positron beam facility utilizing some novel techniques in positron beam production. This facility will be located at the University of Texas (UT) at Austin Nuclear Engineering Teaching Laboratory (NETL) and is being developed by UT Austin and UT Arlington researchers. TIPS will use a large area (total area of 900–1800 cm 2) 64Cu source to supply fast β + particles for subsequent moderation to form an intense monoenergetic positron beam in the energy range of 0–50 keV with an expected intensity of 10 8 e +/s. Natural copper will be neutron activated near the core of the NETL 1 MW TRIGA Mark II research reactor to produce the 64Cu isotope. The activated source will be transported to the moderator/remoderator assembly, outside the biological shield of the reactor. This assembly combines the primary moderation and posterior remoderation of the fast β + particles into one stage using solid Kr to produce a low-energy positron source of a few eV with a diameter of 8 mm. The low-energy positron beam is then extracted by an electrostatic modified SOA gun and after further acceleration to 5 keV, the beam is focused onto the object slit of a 90° bending magnet. After further focusing and another 90° bend, the beam enters the main accelerator/decelerator that transports the beam onto the target for experimentation. The components of TIPS have been manufactured and are currently being optimized. In this communication we present some of the details of the TIPS facility and furthermore briefly discuss its intended applications.

Two dimensional localization of electrons and positrons under high counting rate

The construction of two wire chambers for high counting rate application is reported. Each chamber includes three wire planes – one anode and two orthogonal cathodes – in which the wires operate as independent proportional counters. One of the chambers is rotated with respect to the other, so that four position coordinates may be encoded for a charged particle crossing both chambers. Spatial resolution is determined by the wire pitch: 1 mm for cathodes, 2 mm for anodes. 320 electronic channels are involved in the detection system readout. Global counting rates in excess of 10 7 events per second have been measured, while the average electron–positron beam intensity may be as high as 3×10 7 events per second.

Electric-field-assisted moderator for generation of intense low-energy positron beams

In this paper we reexamine an old idea of using an electric-field-assisted moderator (FAM) based on a rare-gas solid (RGS) to form intense low-energy positron beams. Contrary to common belief, the main body of a successful FAM should consist of a dirty (a high level of molecular impurities) RGS which will allow for a high ${e}^{+}$ drift velocity. On top of this layer is condensed a thin layer of a highy-purity RGS. When the ${e}^{+}$ enters this latter region its energy will heat up, and ${e}^{+}$ emission into vacuum will become possible despite a positive ${e}^{+}$ affinity of the RGS. Simple calculations show that the ${e}^{+}$ intensity can be increased by a factor of 100 over what can be achieved using a RGS moderator without an applied electric field and, equally important, the ${e}^{+}$ transverse energy is reduced to less than 0.3 eV. This leads to a gain in brightness by a factor of 1000.

Production and application of pulsed slow positron beam using an electron linac

Slow positron beam is quite useful for non-destructive material research. At the Electrotechnical Laboratory (ETL), an intense slow positron beam line has been constructed by exploiting an electron linac in order to carry out various experiments on material analysis. The beam line can generate pulsed positron beams of variable energy and variable pulse period. Various capabilities of the intense pulsed positron beam is presented, based on the experience at the ETL, and the prospect for the future is discussed.