D. Hofmann

Ultrahigh vacuum analyzer for convoy electron measurements

Abstract A new parallel plate electrostatic electron energy analyzer for measurements under ultrahigh vacuum (UHV) conditions is described. The construction and characteristic data of the spectrometer are given and typical convoy electron spectra are shown.

Secondary electron emission from entrance and exit surfaces of thin clean foils bombarded with H+, C+, and O+

Abstract We have measured secondary electron emission from thin, clean foils of C, Al, Ti, Ni, and Cu bombarded with H+, C+, and O+, at energies between 0.3 and 1.2 MeV/amu for protons and 20 to 80 keV/amu for the heavy projectiles. We observe in some but not all cases a proportionality between the secondary electron yield and the stopping power, but such a proportionality seems to break down at the lower projectile energies used by us. The yields emitted in forward direction are generally somewhat larger than those emitted backwards. The results are discussed in the frame of an extended Sternglass model.

Elemental composition of different types of wood

Abstract For the use of wood as an environmental sensor the average or normal elemental composition of wood has to be known (here with Z > 12). Since we find with PIXE that the elemental composition depends on the species, environmental influences can only be deduced indirectly from deviations from average elemental concentrations, which have to be known for each examined species. The average elemental composition was calculated from the concentrations of more than 10 samples from almost as many sites for beech and oak. Other species were also analysed but not from more than three different sites. Significant correlations between certain elemental concentrations were found (in particular between Ca and Mn), also studied was the influence of the growing site on the elemental concentrations.

Molecule transmission and convoy electron production by fast projectiles in thin solids

Abstract Zero degree convoy electron production of isotachic (1.7 MeV/u) zero electron projectiles (H+, He2+), one electron projectiles (H+2, He+) and two electron projectiles (H+3) have been studied as function of thin solid foil transient time t. Convoy electron velocity distribution shape and yield Y(t) are correlated to projectile species transmission probabilities T. Model calculations are presented and discussed.

VELOCITY EFFECT IN SECONDARY ELECTRON EMISSION BELOW AND ABOVE THE ELECTRONIC STOPPING POWER MAXIMUM

We studied secondary electron emission yields γ from a carbon surface with helium projectiles near the electronic stopping power maximum (0.2–2 MeV). We ask the question: “Does γ depend on the electronic stopping power (dE/dx)e only or is there an additional dependence on the projectile velocity VP?” A velocity effect is indeed found for backward emission. The results are discussed in the framework of recent electron transport and track models and compared with the yield of secondary ions from the same collision system.

Secondary-ions from atomic collision processes in solid surfaces

Abstract Using time-of-flight technique (TOF) we investigated the secondary ion emission from an uncleaned carbon surface induced by fast helium bombardment near the maximum of electronic stopping power ( d E d x ) e . We examined the production of secondary ions dependend on projectile properties only, as specific projectile energy ( 50 keV/u ≤ E P m P ≤ 500 keV/u ) and incident charge state (0 ≤ qi ≤ 2), but not on target properties, as surface conditions. The “pre-equilibrium stopping power” plays a major role in secondary ion production in the examinated projectile velocity regime.

The influence of solid surfaces on low-energy convoy electron emission

The influence of solid surfaces on the emission of low-energy convoy electrons (8 eV<or=Ee<or=13 eV) from different target materials (C, Al, Cu, Ag, Sm, Gd and Au) after Kr+ impact (1.4 MeV<or=Ep<or=2.2 MeV) has been studied for the first time under ultrahigh-vacuum conditions. It is found that the convoy electron yield increases with decreasing residual surface coverage if the surface is cleaned by dynamic sputtering. The convoy electron yield reaches a saturation value which depends on the work function of the target; the convoy electron yield increases with decreasing work function. This indicates that the transmission of convoy electrons is influenced by both surface contamination and the surface potential barrier.

Experimental study of signal enhancement by matrix effects in PIXE analysis

Abstract In PIXE spectra, secondary fluorescence by X-rays, produced in the target itself, may be one of the main reasons for incorrect analytical results. In our study, we determine the X-ray enhancement in PbGe sandwich targets on a thick matrix (Si) in the projectile energy range 2.0 ≤ EH+ ≤ 3.0 MeV.

Charge state analysis of heavy ions after penetration of uncleaned and sputter cleaned conducting surfaces

Abstract The evolution of the charge state distribution of fast ions inside a solid is of basic interest in various research fields as stopping power measurements etc. The existing models for the charge state evolution differ in the treatment of the projectile-exit-surface interaction, which has a strong influence on the final charge state distributions. We measured the charge state distributions for C+, N+, and O+ (30 ≤ E/M ≤ 130 keV u ) impact on thin C, Cu, and Au foils, where the surface properties were modified by sputter cleaning. The measurements show a pronounced change of the mean projectile charge state to lower values in the case of sputter cleaned surfaces. This result underlines the importance of the projectile-surface interaction for the generation of the outcoming charge state distribution.

In vitro nucleosome assembly with plant histones

Nucleosome; DNA‐histone interaction; Nucleosome reconstitution; Core histone; (Plant)