Determination of the positron diffusion length in Kapton by analysing the positronium emission

Doppler profile spectroscopy and Compton-to-peak ratio analysis have been used to study the positronium (Ps) emission from the Kapton surface as a function of the positron implantation energy E.Two different positions for the sample have been performed in the experiment.In the first case the sample and the Ge-detector are perpendicular to the positron beam. With this geometry the emission of para-positronium (p-Ps) is detected as a narrow central peak.In the second case, by rotating the sample 45° with respect to the beam axis, the emission of p-Ps is detected as a blue-shifted fly away peak. The implantation of the positrons is described by the Makhov profile, where we used the modified median implantation for polymers as given by Algers et al. [J. Algers, P. Sperr, W. Egger, G. Kögel, F.H.J. Maurer, Phys. Rev. B 67 (2003) 125404].Thermalised positrons can diffuse to the surface and may pick up an electron to be emitted as Ps. We found a thermal and or epithermal positron diffusion length L₊ = 5.43 ± 0.71 nm and L₊ = 5.51 ± 0.28 nm correspondingly for both cases, which is much more than the one found by Brusa et al. [R.S. Brusa, A. Dupasquier, E. Galvanetto, A. Zecca, Appl. Phys. A 54 (1992) 233]. The respective efficiency for the emission of Ps by picking up an electron from the surface is found to be f_pu = 0.247 ± 0.012 and f_pu = 0.156 ± 0.003.     

Development of the Low Energy Particle Toroidal Accumulator project

The Low Energy Particle Toroidal Accumulator (LEPTA), a positron storage ring with electron cooling, was constructed and put in operation at the Joint Institute for Nuclear Research (Dubna). The storage ring is a generator of directed beams of ortho-positronium (o-Ps) produced upon the recombination of the beam of positrons circulating in the storage ring with a single-pass electron beam. In 2004 the storage ring was put in operation with the circulating electron beam. The source of positrons of the positron injector was tested with a new radioactive source delivered from South Africa. The positron trap was put in operation for electrons. The electron cooling system was tested with a pulsed electron beam. The progress in commissioning LEPTA is described in this paper.     

Formation of defect structures in Au-polysilane interfaces probed by low energy positron beams

Formation and accumulation of defect structures at interfaces between polysilanes and vacuum-evaporated gold (Au) electrodes are discussed quantitatively by low energy positron annihilation spectroscopy. The size distribution of the defects at the interface is analyzed based on the values of ortho-positronium (o-Ps) lifetime (τ₃), and no effect of the evaporation process is observed in the polymer films. The intensity of o-Ps (I₃) indicates no considerable change before and after Au evaporation on dialkyl-substituted polysilanes, however, the values of I₃ is increased ∼20% in phenyl-substituted polysilane (PMPS) by the evaporation. The I₃ dependence on incident positron energy suggests the formation of the defects not only at an Au-PMPS interface but also in PMPS bulk phase as deep as 400 nm from the interface. Phenyl ring dissociation from the polymer backbone will play a significant role in the selective formation of the defects in PMPS. This is the first report on the direct measurement of defect structures at conjugated polymer-metal interface with non-destructive way, implying that electrode fabrication by vacuum evaporation affects the solid state structure of polymers.     

Some aspects of positronium physics

Some aspects of both theoretical and experimental study of the positronium system to probe physics beyond the Standard Model are reviewed. In particular, new experiments to search for the invisible decay of orthopositronium (o-Ps) with the sensitivity in the branching ratio Br(o-Ps → invisible) ≃ 10⁻⁸–10⁻⁷ are discussed. The experimental technique involves a specially designed high-efficiency pulsed slow positron beam, which is also applicable for other experiments with o-Ps in vacuum. Details of the beam design, as well as the first measurements results are presented. Possible applications of the slow-pulsed positron beam for materials research are discussed. The text was submitted by the authors in English.     

Orthopositronium annihilation and emission in mesostructured thin silica and silicalite-1 films

Mesoporous silica films and MFI-type pure silica zeolite films were investigated using slow positrons. Detection of the 3γ annihilation fraction was used as a quick test to estimate the emission of orthopositronium (o-Ps) into vacuum. Positronium time-of-flight (TOF) spectroscopy, combined with Monte-Carlo simulation of the detection system was used to determine the energy of o-Ps emitted from the films. Evidence for an efficient o-Ps emission was found in both the mesoporous and silicalite-1. A 3γ fraction in the range of 31-36 % was found in the films with the highest o-Ps yield in each type of porous material, indicating that 40-50 % of the implanted positrons form positronium in the pore systems with very different pore sizes. Time-of-flight measurements showed that the energy of the orthopositronium emitted into vacuum is below 100 meV in the film with 2-3 nm pores at 3 keV positron energy, indicating an efficient slowing down but no complete thermalization in the porous films of 300-400 nm thickness.     

Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders

Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 25° and 600 °C. Three positron lifetimes were found in these systems: a short-lived component (0.2 ns) due to positrons in the bulk; a component (0.45 ns) due to surface-trapped positrons; and a long-lived component (2 ns) ofo-Ps in the voids or the interfacial spaces of powders. Both bulk and surface positron lifetimes increase as a function of temperature. Correlations between the intensity of surface-trapped positrons and the surface area and between Ps formation and the surface area of graphite are found. The Ps formation probability increases as a function of temperature. A thermal desorption model interprets the emission process of Ps atoms from the surface of graphite to the vacuum and gives an activation energy of 0.23±0.02 eV.Preliminary results of this paper were presented at the March Meeting of American Physical Society, Los Angeles. Bull. Am. Phys. Soc.28, 347 (1983)     

Quantum beats in the 3γ annihilation decay of Positronium observed by perturbed angular distribution

We have applied conventional Time Differential Perturbed Angular Correlation (TDPAC) method to observe the anisotropy oscillations in the 3γ annihilation decay of polarized Positronium in a weak magnetic field. The effect, as predicted theoretically and experimentally demonstrated by Barishevsky et al. [V.G. Barishevsky, O.N. Metelitsa, V.V. Tikhomirov, Oscillations of the positronium decay γ-quantum angular distribution in a magnetic field, J. Phys. B: At. Mol. Opt. Phys.22 (1989) 2835], is induced by the coherent admixture of the m = 0 states of ortho-Positronium (o-Ps) and para-Positronium (p-Ps) in interaction with the magnetic field.The following experimental characteristics are to be considered:

(i)

the oscillation frequency corresponds to the difference in energy of the Ps atom levels in magnetic field and is proportional with H²;

(ii)

in a fixed geometry the modulation depth (oscillations amplitude) depends on the mean positron polarization;

(iii)

privileged angles of the polarization vector, magnetic field and detectors are required for optimizing the observed oscillations amplitude.

The normalized difference spectrum function (R(t)) obtained from time spectra measured in vacuum and in different gaseous atmospheres (Ar, H₂, N₂) have the oscillations amplitude constant and we conclude that the Ps atoms are not fully thermalized over a time interval of about 400 ns.The R(t) functions obtained for o-Ps annihilation decays, in dry air or Ar-O mixture, have the oscillations amplitude time dependent due, probably, to the paramagnetism of the Oxygen molecules.     

Positronium in molecular substances

The progress of research into the behavior of the positronium in molecular substances in the last several years is reviewed. The Ps atom can be treated as a solute in a liquid. The repulsive exchange force is the dominant force which reacts against the van der Waals attractive force. One of the most useful general relationships found is the one between the pick-off quenching rate and the surface tension of the liquid. The pick-off quenching rate is found, in general, to be an additive property of the functional groups of a molecule. The quenching of theo-Ps atom by an active quenching agent in an inert solvent has been found to be primarily diffusion controlled. Some deviations from the simple Arrhenius equation have been found. For gases, more theoretical work on the collision problems of Ps with atoms or molecules is needed. For amorphous molecular solids, it is likely that the property of pick-off quenching ofo-Ps is similar to that in liquids. However, in crystalline molecular solids, the situation is quite different. A few definite cases have been found where a Ps atom does not form or exist. Further work in this area should be very interesting. The foregoing leads us to believe that positronium or positron annihilation can be useful in the study of fundamental properties of molecular liquids and solids.     

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M_2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M_2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M_2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.     

Surface and bulk investigations at the high intensity positron beam facility NEPOMUC

The NEutron-induced POsitron source MUniCh (NEPOMUC) at the research reactor FRM II delivers a low-energy positron beam (E = 15-1000 eV) of high intensity in the range between 4 × 10⁷ and 5 × 10⁸ moderated positrons per second. At present four experimental facilities are in operation at NEPOMUC: a coincident Doppler-broadening spectrometer (CDBS) for defect spectroscopy and investigations of the chemical vicinity of defects, a positron annihilation-induced Auger-electron spectrometer (PAES) for surface studies and an apparatus for the production of the negatively charged positronium ion Ps⁻. Recently, the pulsed low-energy positron system (PLEPS) has been connected to the NEPOMUC beam line, and first positron lifetime spectra were recorded within short measurement times. A positron remoderation unit which is operated with a tungsten single crystal in back reflection geometry has been implemented in order to improve the beam brilliance. An overview of NEPOMUC's status, experimental results and recent developments at the running spectrometers are presented.     

New mechanism for positronium formation on a silicon surface

We describe experiments in which positronium (Ps) is emitted from the surface of p-doped Si(100), following positron implantation. The observed emission rate is proportional to a Boltzmann factor exp{-E(A)/kT}, which is dependent on the temperature T of the sample and a characteristic energy E(A)=(0.253±0.004) eV. Surprisingly, however, the Ps emission energy has a constant value of ～0.16 eV, much greater than kT. This observation suggests the spontaneous emission of energetic Ps from a short-lived metastable state that becomes thermally accessible to available surface electrons once the positron is present. A likely candidate for this entity is an electron-positron state analogous to the surface exciton observed on p-Si(100) c(4×2) by Weinelt et al. [Phys. Rev. Lett. 92, 126801 (2004)].     

Scattering effects of primary electrons in Co/Cu(1 1 1) measured by Auger; elastic and loss electrons

Electron energy losses were measured as a function of the incidence angle of the primary electron beam for the Co/Cu(1 1 1) adsorption system. The measurements performed for the clean and covered substrate reveal characteristic intensity maxima associated with the close packed rows of atoms, as it was observed in the so called directional Auger and directional elastic peak electron spectroscopy profiles. The incidence angle dependent signal of electron energy losses measured for the clean (Cu 3p_3/2) and covered (Co 3p_3/2) substrate gives the so called directional electron energy loss spectroscopy (DEELS) profiles which contain structural as well as chemical information. The scattering of primaries and different emission processes associated with electron energy losses, Auger, and elastically backscattered electrons are discussed. A change in the h_Cu (Cu M_2,3VV transition) Auger signal recorded during the continuous cobalt deposition shows that the growth mode is not a pure layer by layer type. The complete covering of the substrate by Co at higher coverages is confirmed by the comparison between experimental and theoretical ratios of the Auger peak heights.     

Construction of a positron microbeam in JAEA

We have developed a positron microbeam using magnetic lenses based on the commercial scanning electron microscope (SEM). A slow positron beam was generated using a handmade source with ²²Na and a solid neon moderator. The beam diameter was 3.9 μm on a target. Two-dimensional image of S parameter was successfully obtained. By introducing a beam pulsing section, positron lifetime measurement beam is also available.     

Evolution of vacancy defects in the surface layers of a metal irradiated with a pulsed electron beam

The distribution of vacancy defects in the surface layers of α-Fe after irradiation with a high-current pulsed electron beam is studied experimentally by unique nuclear-physical methods — low-energy positron annihilation, Rutherford backscattering (RBS), and proton-induced x-ray emission (PIXE). Regions with low local density, which are sources of crater formation on the surface of the irradiated sample, are observed by scanning a proton microbeam. Positron lifetime measurements reveal that as the electron beam power increases, nonequilibrium vacancies tend to be captured by carbon impurity atoms. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 618–622 (25 April 1997)     

Low-temperature measurements of the fraction of re-emitted positronium from a Cu(111)+S surface

A positron beam has been utilized to measure the positronium (Ps) fraction re-emitted from a Cu(111)+S sample from 40 K to 350 K for incident energies ranging from 0.5 to 5 keV. Our results at 525 eV incident energy are compared with two recent theories from 40 K to 850 K. The Ps fraction shows only a slight positive temperature dependence below room temperature and we conclude that the positron trapping rate and probability of Ps formation at the surface are largely independent of sample temperature for Cu(111)+S. In addition we present evidence that positrons in the bulk material are not localized in shallow traps at low temperatures for well-annealed high-purity single crystals of Cu and Al.     

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.     

To the application of the emission Mössbauer and positron annihilation spectroscopies for detection of carcinogens

Being the main cause of cancer, almost all chemical carcinogens are strong electrophiles, that is, they have a high affinity for the electron. We have shown that positron annihilation lifetime spectroscopy (PALS) is able to detect chemical carcinogens by their inhibition of positronium (Ps) formation in liquid media. Electrophilic carcinogens intercept thermalized track electrons, which are precursors of Ps, and as a result, when they are present Ps atom does not practically form. Available biophysical data seemingly indicate that frozen solutions model better an intracellular medium than the liquid ones. So it is reasonable to use emission Mössbauer spectroscopy (EMS) to detect chemical carcinogens, measuring the yield of ⁵⁷Fe²⁺ions formed in reactions of Auger electrons and other secondary electrons they produced with ⁵⁷Fe³⁺. These reactions are similar to the Ps formation process in the terminal part the positron track: e⁺+ e^? =>Ps. So EMS and PALS are complementary methods for detection of carcinogenic compounds.     

Magnetic and optical properties of self-organized InMnAs quantum dots

Ten layers of self-assembled InMnAs quantum dots with InGaAs barrier were grown on high resistivity (1 0 0) p-type GaAs substrates by molecular beam epitaxy (MBE). The presence of ferromagnetic structure was confirmed in the InMnAs diluted magnetic quantum dots. The ten layers of self-assembled InMnAs quantum dots were found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T_C=80 K. It is likely that the ferromagnetic exchange coupling of sample with T_C=80 K is hole mediated resulting in Mn substituting In and is due to the bound magnetic polarons co-existing in the system. PL emission spectra of InMnAs samples grown at temperature of 275, 260 and 240 °C show that the interband transition peak centered at 1.31 eV coming from the InMnAs quantum dot blueshifts because of the strong confinement effects with increasing growth temperature.     

Reflectivity modification of polymethylmethacrylate by silicon ion implantation

The effect of silicon ion implantation on the optical reflection of bulk polymethylmethacrylate (PMMA) was examined in the visible and near UV. A low-energy (30 and 50 keV) Si⁺ beam at fluences in the range from 10¹³ to 10¹⁷ cm⁻² was used for ion implantation of PMMA. The results show that a significant enhancement of the reflectivity from Si⁺-implanted PMMA occurs at appropriate implantation energy and fluence. The structural modifications of PMMA by the silicon ion implantation were characterized by means of photoluminescence and Raman spectroscopy. Formation of hydrogenated amorphous carbon (HAC) layer beneath the surface of the samples was established and the corresponding HAC domain size was estimated.     

The influence of UV-irradiation and support type on surface properties of poly(methyl methacrylate) thin films

Thin poly(methyl methacrylate) (PMMA) films were prepared by a solution casting on different supports (glass and aluminium plates with different gloss). UV-irradiation (λ = 254 nm) was used for polymer modification. Surface properties of PMMA were studied by contact angle measurements, attenuated total reflection infrared spectroscopy and optical microscopy. It was found that support type has no influence on surface properties of virgin PMMA, however, the changes in these properties were observed during UV modification of polymer film. The most efficient photochemical reactions appeared in sample placed on the rough Al, whereas the smallest effect was observed in polymer on the glass.