Defects in Si-Rich SiO2 Films Prepared by Radio-Frequency Magnetron Co-sputtering Using Variable Energy Positron Annihilation Spectroscopy

Si-rich SiO2 films prepared by rf magnetron co-sputtering method are studied by slow positron beams. The nega- tively charge point defects （probably Pb centres or peroxy radicals） at the silicon nanocluster （nc-Si）/SiO2 interface are observed by Doppler broadening spectra. Coincidence Doppler-broadening spectra show that positrons have a higher annihilation probability with core electrons nearby oxygen atoms than silicon atoms. The formation of N-related bonds may be the reason for the prevention of the migration reaction of Si and 0 atoms, hence nc-Si formation is inhibited by annealing in nitrogen compared to in vacuum.     

Defects and hyperfine interactions in binary Fe-Al alloys studied by positron annihilation and Mössbauer spectroscopies

The defects, the behavior of 3d electrons and the hyperfine interactions in binary Fe-Al alloys with different Al contents have been studied by measurements of positron lifetime spectra, coincidence Doppler broadening spectra of positron annihilation radiation and Mössbauer spectra. The results show that on increasing the Al content in Fe-Al alloys, the mean positron lifetime of the alloys increase, while the mean electron density of the alloys decrease. The increase of Al content in binary Fe-Al alloys will decrease the amount of unpaired 3d electrons; as a consequence the probability of positron annihilation with 3d electrons and the hyperfine field decrease rapidly. Mössbauer spectra of binary Fe-Al alloys with Al content less than 25 at.% show discrete sextets and these alloys make a ferromagnetic contribution at room temperature. The Mössbauer spectrum of Fe₇₀Al₃₀ shows a broad singlet. As Al content higher than 40 at.%, the Mössbauer spectra of these alloys are singlet, that is, the alloys are paramagnetic. The behavior of a 3d electron and its effect on the hyperfine field of the binary Fe-Al alloy has been discussed.     

Nearby dark matter subhalo that accounts for the DAMPE excess

The precise spectrum of electrons and positrons at high energies plays an important role in understanding their origin.DArk Matter Particle Explorer has reported their first measurement of the e+e-spectrum from 25 GeV to4.6 TeV.This spectrum reveals a tentative peak at ～1.4 TeV,which requires a nearby source of e+e-.In this work,we study the properties of a large nearby dark matter subhalo that accouns for this spectral peak,such as its mass and distance from Earth,for different concentration models.We compare spectra of the sources in 3 FGL with that from DM annihilation in the subhalo,and find ten candidates that have comparable gamma-ray spectra.However,according to the N-body simulation,the probability that such a large subhalo is close enough to Earth is less than 0.1%.     

Identification of pore size in porous SiO2 thin film by positron annihilation

Positron annihilation lifetime and Doppler broadening of annihilation line techniques have been used to obtain information about the small pore structure and size of porous SiO2 thin film produced by sputtered Al-Si thin film and etched Al-Si thin film. The film is prepared by an Al/Si 75：25 at.-% （A175Si25） target with the radiofrequency （RF） power of 66 W at room temperature. A 5 wt.-% phosphoric acid solution is used to etch the Al cylinders. All the A1 cylinders dissolved in the solution after 15 h at room temperature, and the sample is subsequently rinsed in pure water. In this way, the porous SiO2 on the Si substrate is produced. From our results, the values of all lifetime components in the spectra of Al-Si thin film are less than 1 ns, but the value of one of the lifetime components in the spectra of porous SiO2 thin film is τ = 7.80 ns. With these values of lifetime, RTE （Rectangular Pore Extension） model has been used to analyze the pore size.     

Shell Correction at the Saddle Point for Superheavy Nucleus

The potential energy surface for superheavy nucleus has been studied within the framework of the constrained relativistic mean field theory, and the shell correction energy as a function of de[ormation has been extracted by the Strutinsky shell correction procedure. Contrary to the usual expectation, the shell correction energy at the saddle point is too important to be neglected, and it has essential contribution to the fission barrier in superheavy nucleus.     

Defect types and room-temperature ferromagnetism in undoped rutile TiO2 single crystals

Room-temperature ferromagnetism has been experimentally observed in annealed rutile TiO2 single crystals when a magnetic field is applied parallel to the sample plane.By combining X-ray absorption near the edge structure spectrum and positron annihilation lifetime spectroscopy,Ti3+-V O defect complexes(or clusters) have been identified in annealed crystals at a high vacuum.We elucidate that the unpaired 3d electrons in Ti3+ ions provide the observed room-temperature ferromagnetism.In addition,excess oxygen ions in the TiO2 lattice could induce a number of Ti vacancies which obviously increase magnetic moments.     

Scaling Properties of Runaway Electrons

Runaway electrons in tokamaks have been widely studied theoretically and experimentally. The runaway confinement time τ1 in ohmic and additionally heated tokamak plasmas presents an anomalous behavior when compared with theoretical predictions based on neoclassical models. Runaway electrons have received lately a great attention due to several reasons： （a） the possibility to study electromagnetic turbulence by measuring the runaway flux fluctuations and its energy spectra, and （ b ） the runaway electrons are powerful diagnostics capable of yielding valuable information on the actual distribution function of fusion experiments.     

Multi-Electron Effects in Attosecond Transient Absorption of CO Molecules

Using the fully propagated time-dependent Hartree–Fock method, we identify that both the dynamic core polarization and multiorbital contributions are important in the attosecond transient absorption of CO molecules.The dynamics of core electrons effectively modifies the behaviors of electrons in the highest occupied molecular orbital, resulting in the modulation of intensity and position of the absorption peaks. Depending on the alignment angles, different inner orbitals are identified to contribute, and even dominate the total absorption spectra. As a result, multi-electron fingerprints are encoded in the absorption spectra, which shed light on future applications of attosecond transient absorption in complex systems.     

Visible to deep ultraviolet range optical absorption of electron irradiated borosilicate glass

To study the room-temperature stable defects induced by electron irradiation, commercial borosilicate glasses were irradiated by 1.2 Me V electrons and then ultraviolet(UV) optical absorption(OA) spectra were measured. Two characteristic bands were revealed before irradiation, and they were attributed to silicon dangling bond(E'-center) and Fe3+species,respectively. The existence of Fe3+was confirmed by electron paramagnetic resonance(EPR) measurements. After irradiation, the absorption spectra revealed irradiation-induced changes, while the content of E'-center did not change in the deep ultraviolet(DUV) region. The slightly reduced OA spectra at 4.9 e V was supposed to transform Fe3+species to Fe2+species and this transformation leads to the appearance of 4.3 e V OA band. By calculating intensity variation, the transformation of Fe was estimated to be about 5% and the optical absorption cross section of Fe2+species is calculated to be 2.2 times larger than that of Fe3+species. Peroxy linkage(POL, ≡Si–O–O–Si≡), which results in a 3.7 e V OA band, is speculated not to be from Si–O bond break but from Si–O–B bond, Si–O–Al bond, or Si–O–Na bond break. The co-presence defect with POL is probably responsible for 2.9-e V OA band.     

Scaling Properties of Runaway Electrons in HL-1M Tokamak

1 Introduction Runaway electrons in tokamaks have been widely studied theoretically and experimentally. Runaway electrons have received lately a great attention due to several reasons： a） the possibility to study electromagnetic turbulence by measuring the runaway flux fluctuations and its energy spectra, and b） the runaway electrons are powerful diagnostics capable of yielding valuable information on the actual distribution function of fusion experiments.     

Contribution of valence s+p type electrons to the high momentum components of the electron-positron momentum density in gold

It is generally believed that the high momentum components (HMCs) of the angular correlation of annihilation radiation or Doppler broadening spectra map the positron annihilation with predominately core electrons, containing only a small fraction coming from localised d+f-type valence electrons. In present work, we study how far the contribution of valence electrons to the HMCs of the electron-positron (e-p) momentum density can be neglected. Calculations are performed for gold within both the independent particles model (IPM) and including the e-p correlation effects non-locally, using the weighted density approximation (WDA). In particular, the HMCs due to valence s+p-type electrons are compared with their core and d+f-type valence counterparts.     

Doppler broadening of positron annihilation photons in metals

The Doppler broadening of positron annihilation photons was measured in 17 metals. A model which considers the positron lifetime spectra in metals to be composed of terms for annihilation with conduction and core electrons and surface centers of low electron momentum is used to correlate calculated core annihilation rates with the Doppler lineshape. Ta metal was doped with defects with high energy implantations of¹⁴N⁺⁴ ions at variable doses. Differences in the Doppler linewidths were ascribed as being principally a reflection of the probabilty of annihilation with core electrons relative to annihilation with conduction electrons.     

High-momentum analysis in Doppler spectroscopy

Unique information about the chemical vicinity of positron annihilation sites is provided by the contribution of high electron momenta to the Doppler spectrum, since this momentum range is characteristic for the annihilation with core electrons and hence element specific. However, the corresponding energy region in the spectrum is overlaid by a huge background caused by the annihilation radiation itself and the Compton spectrum of other gamma lines having an energy above 511 keV. Usually these backgrounds are reduced by measuring both annihilation quanta in coincidence.By mathematically analyzing the background contributions, we open another possibility to obtain the high-momentum region employing one single germanium detector. A necessary precondition is employing either background-free positron beams or a low-background positron source, e.g. ⁶⁸Ge, instead of the widely used positron emitter ²²Na. The ⁶⁸Ge-source emits positrons with an endpoint energy of about 1.9 MeV, where as the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low.When analyzing spectra from metals and semiconductors according to the described background subtraction, the same information contained in the momentum range up to 35 × 10⁻³m₀c or beyond can be extracted, as if the spectra were measured employing a coincidence setup with two Ge-detectors.     

Gamma spectra resulting from the annihilation of positrons with electrons in a single core level

The first gamma spectra associated with the annihilation of positrons with individual core levels (Cu 3p and Ag 4p) are presented. The spectra were obtained by measuring the energy of gamma rays time coincident with Auger electrons emitted as a result of positrons annihilating with a selected core level. Relativistic calculations show good agreement with experiment over a limited range of momenta. However, statistically significant differences indicate that the measurements can provide an impetus to new calculations of many body effects in positron-core electrons annihilation.     

Background removal in auger electron spectroscopy: a new experimental approach

A review of the main available methods of background removal in Auger electron Spectroscopy and X-ray photoelectron Spectroscopy is given. The major features, assumptions and results of theoretical works, which form the basis of the present method, are presented. This method uses a convolution technique of the experimental spectrum with the single event loss function. It has been applied to Auger electron spectra (Si, Ag, Fe, Ni, Cu, Al). When Auger energy is sufficiently low (Si, Ag), it has been assumed that Auger electrons act as a secondary electrons source within a multiplet energy range. In every case results are satisfactory.     

Quantification of surface effects: Monte Carlo simulation of REELS spectra to obtain surface excitation parameter

The surface excitation effect is investigated by using the quantum mechanical frame work of complex self-energy of electrons which interact with a bounded semi-infinite medium. In the self-energy formalism, differential inverse inelastic mean free path (DIIMFP) has contributions from bulk and surface plasmons. Monte Carlo simulation of the interaction of electrons with a solid medium and surface has been performed. The surface excitation parameter (SEP) is then obtained from the simulated reflection electron energy loss spectroscopy (REELS) spectra. The calculated SEP results by Monte Carlo simulation are compared with the previous calculations of total surface excitation probability, which was estimated by a numerical integration of surface term of DIIMFP. The contribution merely due to surface excitations towards REELS spectra is extracted by subtracting the two Monte Carlo simulated REELS spectra that based on the two models of electron inelastic scattering, i.e. a full surface model (SM) and a pure bulk model (BM). The surface excitations found to be significant at low energy losses and diminish at higher energy losses whereas the bulk plasmon contributions show opposite behavior and are negligible at lower energy losses. The average number of surface excitations is then evaluated by the computation of ratio of the integrated surface contribution to the elastic peak. The calculated results for Ag are found to be reasonably in agreement with our previous results for total probability of surface excitations and other reported experimental data for SEP. Surface correction factor (SCF) is calculated using SEP for several metals and is compared with the reported ratio of SCF with Ni sample as reference.     

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.     

The Mössbauer effect and positron annihilation studies of Fe−Ni alloys

Room temperature measurements of⁵⁷Fe Mössbauer spectra as well as positron lifetime spectra and one-dimensional angular distributions of photons coming from the two-photon annihilation of an electron-positron pair were performed for Fe and Fe?Ni alloys containing up to 15 at % of nickel. The data were analyzed in terms of a rearrangement of s-like electrons due to alloying of iron with nickel. It was found that for the systems studied the mean density of 4s-like electrons is independent of the Ni concentration in the sample, whereas the density of these electrons at Fe atoms in the specimens is lower than that in the pure Fe. The result is discussed in terms of transfer of charge and of intra-atomic s-d conversion.     

On the broadening of the annihilation line

The broadening of annihilation line was studied with a Ge detector. The annihilation line was deconvoluted taking into account the instrumental resolution of 1.08 keV. The P_f in several metals are found. The contribution of core electrons to annihilation is deduced.