Narrow beam emission of slow positrons from negative affinity surfaces

We have measured the angular distribution of the slow positrons (e⁺·) from negative work function (φ⁺) surfaces of Cu and Al bombarded by keV e⁺ in ultrahigh vacuum. In analogy with √-point electrons emitted from negative electron affinity surfaces, the majority (>50%) of the slow e⁺ leave the surface with energy ≈ φ⁺ and velocity within ～20° of the surface normal. This agrees qualitatively with the predictions of a simple 1D model.     

Electron and positron work functions of Cr(100)

We report measurements of the electron and positron work functions of submonolayer contaminated single crystal surfaces of Cr(100) in ultra high vacuum. The positron work function ø₊ is obtained by measuring the spectrum of slow positrons reemitted by the Cr(100) surface when it is bombarded with keV energy positrons. The electron work function ø_- is measured relative to Al(100) by comparing the target biases at which the slowest emitted positrons are recollected by the target. We obtain ø₊ = ?1.76(10) eV and ø_- = 4.46(6) eV for our Cr(100) surface using the value ø_- = 4.41(3) eV for Al(100) reported by Grepstad, Gartland and Slagsvold. The ø₊ value is in agreement with the ?2.2 eV calculated by Nieminen and Hodges. The positronium work function for Cr implied by these results is ?4.10(10) eV; the positronium negative ion (Ps^-) work function for this surface is calculated to be + 0.37(7) eV. A search for Ps^- showed that at a 90% confidence level less than one in 10³ thermalized positrons reaching the Cr surface are emitted as Ps^-. The slow positron emission spectrum was observed not to change over the 70–300 K range in contrast to recent theoretical predictions.     

Laser-cooled positron source

We examine, theoretically, the feasibility of producing a sample of cold (⩽4 K), high-density (≈10¹⁰/cm³) positrons in a Penning trap. We assume⁹Be⁺ ions are first loaded into the trap and laser-cooled to approximately 10 mK where they form a uniform density column centered on the trap axis. Positrons from a moderator are then injected into the trap along the direction of the magnetic field through an aperture in one endcap of the trap so that they intersect the⁹Be⁺ column. Positron/⁹Be⁺ Coulomb collisions extract axial energy from the positrons and prevent them from escaping back out the entrance aperture. Cooling provided by cyclotron radiation and sympathetic cooling with the laser-cooled⁹Be⁺ ions causes the positrons to eventually coalesce into a cold column along the trap axis. We present estimates of the efficiency for capture of the positrons and estimates of densities and temperatures of the resulting positron column. Positrons trapped in this way may be interesting as a source for antihydrogen production, as an example of a quantum plasma, and as a possible means to produce a bright beam of positrons by leaking them out along the axis of the trap. Contribution of the National Institute of Standards and Technology; not subject to US copyright.     

Testing of a cryogenic source of slow monochromatic positrons

The results of testing a cryogenic source of slow monochromatic positrons based on the ²²Na isotope, designed and constructed at JINR, are presented. A solid neon moderator was used to generate a monochromatic beam from the continuous β⁺ spectrum of 0–0.5 MeV, with solid neon being frozen to the base layer, which is cooled to the temperature of 5–7 K. Test source of the ²²Na isotope allows the forming of a slow positron beam of intensity 5.8 × 10³ part./s and the average energy 1.2 eV at a spectrum width of 1 eV. The efficiency of moderation is 1% of the total positron flux.     

On the production mechanism of the narrow positron lines observed in heavy-ion collisions

Following a suggestion of Bang et al. we deduce that the total energy transfer associated with the narrow positron lines observed in heavy-ion collisions is given approximately by the sum of the rest masses of the positron and the accompanied electron plus their kinetic energies. Since the same behaviour has been established previously for the production process of the dynamically induced positrons, our observation represents a strong hint that the origin of the e⁺ -lines may also be connected with the strong Coulomb field occuring in heavy-ion collisions.     

Production of intense positron beams at the VEPP-5 injection complex

A source of positrons allowing 5 × 10⁸ positrons accelerated to the energy of 70 MeV to be produced per pulse has been developed. The process of electron-positron pair production in an electromagnetic shower is used for production of positrons. The electromagnetic shower is generated in a tantalum target by a beam of 2 × 10¹⁰ electrons with energy 270 MeV. A high efficiency of positron collection (positron yield Y ≈ 0.1 GeV^?1) is ensured by a unique design of the matching device.     

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.     

Concept of a 6-T Penning trap at liquid-He temperature for the accumulation of positrons

High densities of ultra cold positrons are required for applications such as positronium production, scattering processes with atoms, surface analysis, cooling of highly charged ions and antihydrogen production. At the University of Aarhus, Denmark, an accelerator based slow positron source delivers about 5 × 10⁴ positrons within a 10 ns bunch at a repetition rate of 10 Hz. The energy spread is below 1 eV and the beam diameter is about 1 mm. The positron bunches shall be injected into a 6-T Penning trap at the temperature of liquid helium. The bunches can be captured at nearly 100% efficiency by a fast time variation of the trap potential. The cyclotron motion cools down by synchrotron radiation with a time constant of 80 ms. The axial motion can be cooled by coupling to the radial motion or by resistive cooling in a tuned circuit. By stacking of 100 pulses about 5 × 10⁶ positrons can be accumulated within 10 s. After this time most of the positrons have cooled down sufficiently that the trapping cycle can be started again. At the anticipated accumulation rate a positron plasma at the space charge limit should be obtainable within 1 h. This revised version was published online in July 2006 with corrections to the Cover Date.     

Toward the theory of electron and positron states in dielectric clusters

Analytical expressions for the binding energy of electrons and positrons in dielectric clusters, analyzed in this work, neglect the elastic effects. Therefore, we present the density-functional theory for neutral liquid clusters that experience the spontaneous deformation. Using the 1/R-expansion, R being the cluster radius, the exact analytical expressions for the size corrections to the chemical potential, surface tension, and atomic density are derived from the condition of mechanical equilibrium. The problem of calculating these corrections is reduced to calculating the quantities for a liquid with a flat surface. The size compression and tension of density occur in the 1/R and 1/R ² orders respectively. The sizes of charged rigid and elastic critical clusters, for which the electron or positron binding energy is close to zero, are calculated for Xe _N ^? , Kr _N ^? , Ar _N ^? , Ne _N ⁺ , He _N ⁺ . The calculations show significant contribution of self-compression to the binding energy of the excess electron in contrast to the positron.     

The decay of the vacuum in the field of superheavy nuclear systems

In the strong Coulomb field of a nucleus or quasimolecule withZ?172 a change of the QED vacuum has been predicted, signalled by the spontaneous emission of positrons if holes in theK-shell are available. The dynamical semiclassical theory of positron excitation in heavy ion scattering is presented and extended to collisions with nuclear contact. Interference patterns in the energy spectrum of the emitted positrons and, for sufficiently long reaction timeT, the emerging of a characteristic line at the position of the 1sσ-resonance is predicted. The position and width of the 1sσ-resonance is calculated in dependence on the nuclear configuration, the effect of electron screening is taken into account. Recent experimental indications [1, 2] for structures in the spectra of positrons emitted in U-U collisions at energies close to the Coulomb barrier are discussed in terms of spontaneous positron creation. The observed spectra could be explained under the assumption of long lasting nuclear reactions (T?4·10^?20 s) with a cross section in the millibarn region. Various consequences of this interpretation are discussed.     

Extremely cold positrons for antihydrogen production

The storage of extremely cold (4 K) antiprotons in a Penning trap is an important step toward the creation and study of cold antihydrogen. The other required ingredient, the largest possible number of comparably cold positrons, is still lacking. These would be recombined in a high vacuum with the trapped antiprotons, already stored at a pressure below 5×10⁻¹⁷ Torr, thereby avoiding annihilation of the antihydrogen atoms before they can be used in high accuracy measurements or in controlled collision experiments. In an exploratory experiment, positrons from a 18 mCi²²Na source follow fringing field lines of a 6 T superconducting solenoid through tiny apertures in the electrodes of a Penning trap to strike a tungsten (reflection) moderator. The positron beam is chopped mechanically and a lock-in directly detects a positron current of 2.5×10⁶e⁺/s on the moderator. The use of a moderator, unlike an earlier experiment in which < 100 positrons were confined in vacuum, should greatly increase the number of positrons trapped in high vacuum.     

The weak neutral charge and coherent photoproduction of lepton pairs

The production of e⁺e⁻ pairs in the nuclear electroweak field from intermediate energy circularly polarized photons is studied. The polarization asymmetry provides a measurement of the weak charge induced by neutral current interactions. The analysis is made for the double differential cross section corresponding to the energy spectrum and angular distribution of the outgoing positron. Predictions, within the standard model, of the parity violating asymmetry reach values near 10⁻⁴ at incident photon energies of 500 MeV and positrons in the backward hemisphere.     

Transmission of positrons with aβ + energy distribution through thin films

The transmission through Al foils of isotropically implanted positrons from a²²Na ⁺ source has been measured. It is shown that the transmission is reasonably well-described using an exponential profile once backscattering is accounted for, except for thicknesses below approximately 7 mg/cm². Below this thickness, the measured transmission is slightly less than that predicted by the exponential profile. Such a deviation has previously been observed for collimated positrons, suggesting that the implantation profile has no significant dependence on the spatial distribution of the incident positrons. This deviation is critical for the proper interpretation of positron lifetime experiments on thin films using a conventional positron lifetime spectrometer.     

Positron lifetime and 2D-ACAR studies of divacancies in Si

We have measured positron lifetime and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) distributions of Floating-Zone grown (FZ) Si specimens containing divacancies (V₂) with the definite charge states, V ₂ ⁰ , V ₂ ^–1 or V ₂ ^–2 from room temperature to about 10 K. These charge states are accomplished by an appropriate combination of dopant species, their concentration and irradiation doses of 15 MeV electrons. with reference to the currently accepted ionization level of divacancies. The positron lifetime of the negatively charged divacancy increases with temperature, while that of the neutral divacancy shows little change with temperature. The positron trapping rate, obtained from lifetime and 2D-ACAR measurements, increases markedly with decreasing temperature. This is found not only for the negative divacancies but also for the neutral divacancy. We need a model which explains this temperature dependence. The 2D-ACAR distribution from positrons trapped at divacancies shows nearly the same distribution for the different charge states, which differs considerably from the case of As vacancies in GaAs studied by Ambigapathy et al. We have observed a small but definite anisotropy in the distribution of trapped positrons in V ₂ ^– using a specimen containing oriented divacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Positron spectra from internal pair conversion observed in 238U + 181Ta collisions

We present new results from measurements and simulations of positron spectra, originating from ²³⁸U + ¹⁸¹Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e⁺-energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed γ-transitions ( E _γ∼ 1250-1600 keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed. Received: 27 April 2000 / Accepted: 14 September 2000     

The E166 experiment: Development of an undulator-based polarized positron source for the international linear collider

A longitudinal polarized positron beam is foreseen for the international linear collider (ILC). A proof-of-principle experiment has been performed in the final focus test beam at SLAC to demonstrate the production of polarized positrons for implementation at the ILC. The E166 experiment uses a 1 m long helical undulator in a 46.6 GeV electron beam to produce a few MeV photons with a high degree of circular polarization. These photons are then converted in a thin target to generate longitudinally polarized e ⁺ and e ⁻. The positron polarization is measured using a Compton transmission polarimeter. The data analysis has shown asymmetries in the expected vicinity of 3.4% and ∼1% for photons and positrons respectively and the expected positron longitudinal polarization is covering a range from 50% to 90%.      

DasK/β+-Verhältnis beim Zerfall J128→T128

In the decay of J¹²⁸ the intensity of a weak positron activity was measured to be (3.1±0.7)·10^?5 per disintegration. The remeasurement of the totalK capture was found to be 0.059±0.007 in agreement with the results of other authors. From this theK/β ⁺-ratio of the decay to the ground state of Te¹²⁸ is 1800±400. Theoretical calculations ofK/β ⁺ for allowed transitions show that the maximum energy of the positrons is 245±12 keV. In the decay of Rh¹⁰⁴ and Lu^176m no positrons more than 5·10^?6 could be detected.     

Positron surface states bound by the Newns-Barton potential

The binding energies and eigenfunctions of the surface states of positrons outside materials with negative positron affinity are considered. When the material is represented by a continuum hydrodynamic model with finite dispersion the potential takes the Newns-Barton form. The characteristic material parameter is B = ω_p/β where ω_p is the bulk plasma frequency and $\text{β ? √}\text{3}\text{5}\text{x}$ the Fermi velocity. For aluminium B ? 0.6a^?1 (a is the Bohr radius) and the Newns-Barton potential yields a ground state binding energy E⁽¹⁾ ? 0.46 ev - considerably smaller than the binding energy of the pure image (or non dispersive) potential for which E⁽¹⁾ ? 0.85 ev.     

Positron annihilation studies of heavy-ion induced radiation damages in stainless steels

Variable-energy (0–30 keV) positrons were used to study the depth distribution of heavy-ion induced vacancy type defects in the following specimens, SUS304 and SUS316 austenitic stainless steels, a SUS444 ferritic stainless steel and nickel metal, which were irradiated by 0.5 MeV He⁺, 2.0 MeV C⁺, 3.5 MeV Si²⁺ and 4.0 MeV Ni²⁺ ions up to 0.01 dpa at peak. Vacancy type effective residual defects (ERD) were evaluated from the line shape parameterS of Doppler broadened positron annihilation photon spectra. With increasing primary knockon atom (PKA) energy, a decrease of the vacancy type ERD was observed. The ERD differences among the specimens are discussed in comparison with theoretical predictions.     

Constraints on couplings of a fast decaying axion

Constraints are imposed on the couplings of an axion-like pseudoscalar particle φ that decays into e⁺e⁻ and γγ. The electron g−2 sets a severe bound not only on the φe⁺e⁻ coupling but also on the φγγ coupling. The most reliable bound on the φ coupling to the light quarks comes from the decay ∑⁺ → pe⁺e⁻ which was measured in a hydrogen bubble-chamber experiment with good sensitivity and resolution covering the entire range of e⁺e⁻ mass practically down to 2m_e. These upper bounds, together with a constraint from the decay K⁺→π⁺γγ, reinforce arguments against the particle-physics interpretation of the anomalous positrons in heavy-ion collisions, since the couplings are by orders of magnitude smaller than needed to produce e⁺e⁻ pairs at the observed level by known mechanisms.