Positron lifetimes in V3Si

From positron annihilation lifetime measurements on V₃Si single crystals we estimated that about 17.2% of the positrons annihilate with core electrons.     

Investigation of positron life times in metallic glasses Fe67Co18B14Si1 and Fe4.7Co70.3B10Si15 before and after crystallization

Positron life times in metallic alloys FeCoBSi of amorphous structures and after crystallization were investigated. The hypothesis that in investigated samples positrons annihilate not only with electron-gas electrons but with covalent-bond electrons as well, is put forward.     

Positron annihilation in gamma irradiated crystals

The angular correlation between the two gamma-quanta arising from positron annihilation in untreated and gamma-irradiated alkali halides is reported. For low level gamma irradiation the percentage of positrons annihilating with F-centre electrons is not very significant. When the F-centre concentration is increased to as much as 1.5 × 10¹⁷ per cm³, about 15 per cent of positrons annihilate with F-centre electrons. The angular correlation curve becomes progressively narrower with increase in F-centre concentration. Positrons annihilating in gamma-irradiated crystals reduce the F-centre considerably. The results of a systematic investigation on F-centre bleaching by positron annihilation are reported.     

Positron annihilation in ultrafine particles of Fe

Measurements of two-photon annihilation in ultrafine particles of Fe (～ 100 A in diameter) show that a considerable fraction of the positrons annihilate in the state of positronium. The emission of slow positrons from a metal surface can account for these results.     

Trapped Positrons and Electrons in the Inner Radiation Belt According to Data of the PAMELA Experiment

Measurements of secondary-electron and secondary-positron fluxes below the geomagnetic cutoff in near-Earth space were performed by means of the PAMELA magnetic spectrometer installed on board the Resurs-DK1 satellite launched on June 15, 2006, in an elliptical orbit of inclination 70° and altitude 350 to 600 km. This spectrometer permits measuring the fluxes of electrons and positrons over a wide energy range, as well as determining their spatial distributions to a precision of about 2°. A calculation of particle trajectories in the geomagnetic field makes it possible to separate electrons and positrons originating from cosmic-ray interactions in the Earth’s magnetosphere. The spatial distributions of quasitrapped, trapped, and short-lived albedo positrons and electrons of energy above 70 MeV in the radiation belt were analyzed. The ratio of the electron-to-positron fluxes and the energy spectra of the electrons and positrons in question are indicative of different productionmechanisms for stably trapped and quasitrapped secondary particles.     

Spin depolarization in Ni and Ce

Spin polarized electrons produced by polarized light in a Ge single crystal are depolarized by thin layers of Ni and Ce deposited on the surface of the sample. The mean free path for spin-exchange scattering is found to be (12.5 ± 1.5) Å in Ni and (3.2 ± 1) Å in Ce. The depolarization by Ce is found to be identical at 300 and 4 K. It is concluded that the occupancy of the 4? shell of the Ce atoms is similar at both temperatures.     

Die Reichweite von Elektronen und Positronen kleiner Energie in Kupfer,Silber und Gold

In continuation of previous work the range of 40 to 160 keV monoenergetic electrons and positrons in copper, silver, and gold is determined and the corresponding range-energy-relations are stated. In copper the positrons have a shorter range than electrons of the same energy, but in silver and gold they have a longer range, in disagreement with the values calculated byNelms 2. For the three absorption materials all the ranges of electrons can be described by the formula E^?=14,3 · (Z^4/3/A) · R^{0, 61}, and all the ranges of positrons by E⁺=29,7 · (Z/A) · R^{0, 60} (E in keV, R in mg/cm²). The ratio of the range of positrons to that of electrons for copper agrees rather well with the calculated one, but for silver and gold it differs considerably. This might be due to the increasing influence of the elastic scattering of the particles on nuclei.     

Positron annihilation study of the influence of doping on the 3<Emphasis Type="Italic">d</Emphasis> electron states in the Ni<Subscript>3</Subscript>Al intermetallic compound

The 3d electron states in Ni₃Al single crystals doped with Fe, Co, and Nb have been investigated using angular correlation of annihilation radiation (ACAR). The ACAR spectra contain information on the momentum distribution of valence electrons and strongly bound 3d electrons of the intermetallic compound. It has been established that the positrons in the Ni₃Al crystals predominantly annihilate in the nickel sublattice from delocalized states. The doping of the compound by the third element leads to a variation in the momentum distribution of Ni 3d electrons due to the change in the character of interatomic bonds. An analysis of the momentum distribution has demonstrated that the niobium atoms increase the covalent component of the chemical bond as compared to the binary compound due to the d _Nb-d _Ni hybridization. The doping with cobalt atoms also enhances the tendency toward the formation of the covalent bond. At the same time, iron atoms have a weak effect on the electronic structure of the intermetallic compound.     

Determination of positron beam parameters by various diagnostic techniques

Various diagnostic techniques have been applied at the neutron-induced positron source Munich NEPOMUC in order to determine the positron beam parameters such as intensity, beam shape and energy distribution. The positron beam intensity is determined by the detection of the annihilation radiation of positrons, which annihilate in a movable target. The use of a micro-channel plate (MCP) detector with a CCD-camera allows a direct measurement of the positron beam shape and the lateral resolved intensity distribution. At NEPOMUC a movable MCP-assembly inside the evacuated beam line enables a quick examination of the beam shape during operation. A retarding grid was mounted inside the homogeneous magnetic guiding field in order to determine the distribution of the longitudinal positron momentum, and hence estimate the energy distribution of the positrons.     

Theoretical study on the positron annihilation in Rocksalt structured magnesium oxide

Based on the atomic superposition approximation(ATSUP) and first-principles pseudopotential plane-wave methods,the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Boron’ski-Nieminen positron-annihilation-rate interpolation formula respectively.The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results.The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.     

Highly sensitive investigation of initial growth process of sodium films by multiple-attenuated-total-reflection spectroscopy

Multiple-attenuated-total-reflection spectroscopy (MATRS) is applied to highly sensitive measurement of small amount of metal adsorbed on a fused quartz substrate. A sodium film thinner than 0.01 Å in mean thickness or 10^?10 g/cm² in weight could be detected. MATRS brings information also about shape and electronic properties of small island particles. Sodium islands were predicted to have cap-shape and the number of free electrons per unit volume in sodium islands was found to decrease from the bulk value as the mean thickness decreases to less than 1 Å.     

Positron annihilation in Sn fine particles

It was found that positron annihilation mechanisms in Sn fine particles fall into two categories: annihilation with electrons inside the particles and that from particle surface trapped states.     

Kaluza-Klein dark matter

We propose that cold dark matter is made of Kaluza-Klein particles and explore avenues for its detection. The lightest Kaluza-Klein state is an excellent dark matter candidate if standard model particles propagate in extra dimensions and Kaluza-Klein parity is conserved. We consider Kaluza-Klein gauge bosons. In sharp contrast to the case of supersymmetric dark matter, these annihilate to hard positrons, neutrinos, and photons with unsuppressed rates. Direct detection signals are also promising. These conclusions are generic to bosonic dark matter candidates.     

A comparative study of electron and positron penetration in silicon dioxide

Many researches are devoted to the study of silicon dioxide, a material of great interest for its use in the micro-electronics industry. This paper aims to compare the behavior of electrons and positrons when impinging on silicon dioxide targets in order to investigate the differences and the similarities. In particular, the inelastic mean free path, the stopping power, the differential elastic scattering cross-section and the total and transport elastic scattering cross-section of electrons and positrons penetrating in silicon dioxide targets are compared in order to better understand their influence in determining the implantation profiles shapes, the mean range of penetration, the maximum range of penetration and the backscattering coefficient as a function of the primary energy of the incident particles.     

Über die Temperaturabhängigkeit optischer Eigenschaften kleiner Silberteilchen

The influence of low temperature on the spectral light absorption of small silver particles has been investigated in the spectral range between 3500 and 4500 Å. Measurements of the absorption coefficient were taken at 293, 77, and 4.2 K. For this purpose a method was developed to obtain silver particles (mean diameter between 20 and 300 Å) inside a matrix of polymethylmethacrylate. This highpolymer allowed measurements of optical transmission at low temperatures and the exact determination of form and size distribution of the inbedded particles by electron microscopy. Using the Kramers-Kronig-analysis modified by Kreibig the optical constants were obtained. The results show relatively good agreement with constants calculated for the Drude free electron gas taking into account temperature dependend electron collision frequency.     

Features of the Mechanism of Formation of the Radiation Belt of High-Energy Electrons and Positrons

New results of the PAMELA experiment revealed significant difference in the electron and positrons flux and fraction between trapped particles of the radiation belt and quasitrapped particles. A decrease in the ratio of the electron-to-positron fluxes both with increasing altitude and with decreasing energy was observed for the inner radiation belt, but this does not fit in currently used models. The residual atmosphere density in the trapping region for L ~ 1.15?1.2 is estimated on the basis of calculations of trapped-particle trajectories in the Earth’s magnetosphere. It is shown that processes leading to the energy loss for trapped electrons and positrons in the interactions with residual-atmosphere atoms play an important role in the formation of fluxes of these particles, but these processes cannot cause the reduction of the positron fraction in the total flux at energies below some 100 MeV. The role of the process of δ-electron production in the formation of the belt of trapped electrons and positrons is considered. Allowance for this process makes it possible to explain the above reduction at least partly.     

Effect of alkali leaching on the surface structure of Ni3Al catalyst

The surface structure of the alkali-leached single-phase Ni₃Al powder was investigated by X-ray diffraction, BET (Brunauer-Emmett-Teller) surface area analysis, electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction. It was found that fine Ni particles of several nm in diameter were formed on the outer surface layer of the Ni₃Al powder after the alkali leaching process. The surface of the Ni particles was covered with a thin layer of Ni oxides and hydroxide, Ni₂O₃, NiO and Ni(OH)₂, and these Ni oxides and hydroxide can be easily reduced by hydrogen to the metallic nickel that is catalytically active. The inside of the Ni₃Al powder remained as the original Ni₃Al ordered structure after alkali leaching. Having heat resistant properties, the Ni₃Al phase can serve as a support of the fine Ni particles and provide the structural and thermal stabilities to the fine Ni particles.     

Positron creation and annihilation in tokamak plasmas with runaway electrons

It is shown that electron-positron pair production is expected to occur in post-disruption plasmas in large tokamaks, including JET and JT-60U, where up to about 10(14) positrons may be created in collisions between multi-MeV runaway electrons and thermal particles. If the loop voltage is large enough, they are accelerated and form a beam of long-lived runaway positrons in the direction opposite to that of the electrons; if the loop voltage is smaller, the positrons have a lifetime of a few hundred ms, in which they are slowed down to energies comparable to that of the cool ( less, similar 10 eV) background plasma before being annihilated.     

Observations of Trapped Electrons and Positrons with E > 50 MeV in the Inner Radiation Belt by the PAMELA Magnetic Spectrometer

Geomagnetically trapped electrons and positrons with energy above 50 MeV were observed in PAMELA experiment on board Resurs DK satellite. The instrument consists of magnetic spectrometer, imaging electromagnetic calorimeter, time-of-flight system, anticoincidence and neutron detectors that provide unique particle identification and background rejection. PAMELA was collecting data since June 2006 till January 2016. The satellite orbit with initial altitude 350–600 km and inclination 70° crosses the inner radiation belt in South Atlantic Anomaly at L-shell ∼1.2. The trapped electrons and positrons were selected on the basis of a trajectory simulation in the Earth magnetic field. Features of the energy spectra of electrons and positrons at low energies are analyzed.

     

Quantum mechanical equations of motion of particles and spin in polarized media

Quantum mechanical equations of motion are obtained for particles and spin in media with polarized electrons in the presence of external fields. The motion of electrons and their spins is governed by the exchange interaction, while the motion of positrons and their spin is governed by the annihilation interaction. For particles with spin S ≥ 1, second-order terms in spin are taken into account. The equations obtained can be applied to describe the motion of particles and spin both in magnetic and nonmagnetic media.