摘要:

The energy loss, stopping profiles and backscattered fraction and energy distribution of positrons and electrons with incident energies up to 10 keV incident upon amorphous elemental metals are stochastically modeled within a Monte‐Carlo framework. Elastic Coulomb scattering is considered using the partial wave approximation for low incident energies; the screened Rutherford potential can be used at higher energies. Inelastic scattering is considered using a variety of energy loss mechanisms. K‐shell core excitations are handled using the formalism developed by Kolbenstvedt. Higher core level excitations are treated with Gryzinski’s excitation function. In the case of valence electron excitations, both single particle (electron–hole) and collective (plasmon) processes are considered. Valence interactions are modeled using the mean free path developed by Penn. Energy loss and angular scattering are treated with the random phase approximation using the Lindhard dielectric function. The incident particles are followed to an energy of 25 eV. In the case of positrons, no annihilation in flight is considered. Details of the model and the information it generates are presented.

ISSN:0094-243X    

DOI:10.1063/1.40202

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have performed Monte Carlo simulations of positron slowing down in Al for incident positron energies in the range 1–10 keV. We present results for the backscattering probabilities and implantation profiles. The backscattering probabilities are significantly higher than the experimental values obtained at University of East Anglia (UEA) by Bakeret al. However, the results for implantation profiles show good agreement with experimental results by the UEA group, and they indicate that the Makhov distribution, commonly employed in analysis of positron experiments, is not an accurate representation of the true profile. By comparing results obtained fo different models for the scattering cross sections we identify the transport cross section for elastic scattering and the stopping cross section for inelastic scattering as the parameters characterising the influence of the different scattering processes on the positron transport.

ISSN:0094-243X    

DOI:10.1063/1.40184

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

I will outline the recent work, in some cases extremely recent, done in Norwich on positron slowing down and thermalisation using both Monte Carlo simulation and the Boltzmann equation. I will also describe our recent efforts on the positron state at external surfaces and in voids and on trapping into the surface state.

ISSN:0094-243X    

DOI:10.1063/1.40199

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The implantation profileP(x) for 7 and 10 keV positrons in aluminium, and the median penetration depths for positrons in the energy range 5–10 keV, have been measured by monitoring the meanSparameter for a 0–800 nm thick wedge of aluminium deposited on a glass substrate. Because of the short positron diffusion lengths in both the aluminium and glass, and the uniform density of the sample,P(x) is essentially equal to the annihilation profileA(x), and is obtained by differentiating the S vs wedge thickness data. The results are distinctly non‐Makhovian, and are close in shape to the recent Monte Carlo results of Jensen and Walker. The measured median penetration depths vary as (42.3±3.6)EnA˚ (n=2.07±0.04;E=positron energy in keV), in marked contrast with the widely used form. The effect on these last measurements of positron diffusion to the Al‐glass interface is discussed, as are the consequences for the analyses of prositronium fraction adSparameter profiling data.

ISSN:0094-243X    

DOI:10.1063/1.40200

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Electron backscattering yields as a function of icident energy (T=5–35 keV) are reported for thick targets of Au, Ag, Cu, Al, and Be. The experimental values are in reasonable agreement with results of Monte Carlo computer simulations. Doubly differential distribution (intensity versus energy and angle) data for Au atT=35 keV, measured with a rotatable solid state detector, confirm that (i) the angular distribution follows a Lambert cosine distribution, and (ii) the most probable backscattered energy decreases (almost linearly) with increasing scattering angle.

ISSN:0094-243X    

DOI:10.1063/1.40201

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Results are presented from an investigation of the slowing down and thermalisation of positrons implanted into metals. We have solved the Boltzmann equation for the positron momentum distribution in a homogeneous medium allowing the positrons to scatter off electrons and phonons. We obtained both the time‐dependent and steady‐state solutions. Our results give a more detailed and accurate description of positron thermalisation than earlier work and we show that non‐thermal trapping is not important for positron studies of vacancies in metals.

ISSN:0094-243X    

DOI:10.1063/1.40203

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A guided tour of the literature to date on low‐energy positron diffraction is provided. Historical aspects of the literature are briefly stated. (AIP)

ISSN:0094-243X    

DOI:10.1063/1.40204

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The positron reemission microscope (PRM), originally proposed by Hulett, Dale and Pendyala, operates on principles fundamentally different from those utilized in existing microscopes and offers sensitivity and contrast not available in conventional microscopes. (AIP)

ISSN:0094-243X    

DOI:10.1063/1.40205

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A review is given on the results of PAES (positron annihilation induced Auger Electron Spectroscopy) studies to data, with a concentration on those results obtained at the University of Texas at Arlington. Low energy positions, trapped in a surface localized state annihilate with core electrons resulting in the emission of Auger electrons. The advantages of PEAS include: (i) the elimination of the very large secondary electron background, and (ii) increased surface selectivity. (AIP)

ISSN:0094-243X    

DOI:10.1063/1.40173

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Positron induced Auger electron spectroscopy (PAES) from a clean Ge(100) surface was studied as a function of temperature. Three low‐energy Auger peaks were detected at 50 eV, 90 eV and 100–150 eV, attributed to M2,3M4M4, M2,3M4V and M1M4M4Auger transitions, respectively. An estimated 4(±1)% of the surface trapped positrons annihilate with Ge 3p level electrons. The PAES yield from a Ge(100) surface is reduced at elevated temperatures, in accordance with an activation process found earlier in several Ps fraction experiments. A desorption model adopted from these studies does not accurately describe the PAES intensity at higher temperatures (≳500 °C), which levels off at 5% of the room temperature value. Possible sources for the discrepancy are discussed. On a Ge(100) surface, an upper limit for the Ps emission near the melting point is 97%. The error in calibration parameters due to the earlier assumption of 100% Ps emission from Ge surfaces seems to induce only small errors to the Ps fraction measurements.

ISSN:0094-243X    

DOI:10.1063/1.40174

出版商:AIP    

年代:1991

数据来源: AIP