J. Räisänen

PROTON-INDUCED THICK-TARGET GAMMA-RAY YIELDS FOR THE ELEMENTAL ANALYSIS OF THE Z = 3-9, 11-21 ELEMENTS

A systematic study of the relative thick-target yields of prompt γ-rays following proton bombardment has been carried out at Ep=1, 1.7 and 2.4 MeV for the elements Z=3–9, 11–21. The relevant spectra for each element are depicted and a table of the most suitable γ-rays for elemental analysis are given. Depending on the strength of the reaction cross section, the sensitivity limits of the detection vary from ppm to a few percent under practical measuring arrangements. Particular aspects of the measuring techniques are discussed.

New approach to energy loss measurements

Abstract A new approach to energy loss measurements is proposed. In the same experiment electronic stopping force (power) in gold, nickel, carbon, polycarbonate and Havar for 40 Ar, 28 Si, 16 O, 4 He and 1 H ions in the energy range 0.12–11 MeV/u has been measured. In this paper we give the full results for gold, nickel, and carbon and for 40 Ar, 16 O, 4 He and 1 H ions. Good agreement of the measured stopping force values for light ions with literature data is interpreted as the positive test of the experimental technique. The same technique used with heavy ions yields agreement with the published data only for energies above 1 MeV/u. At lower energies we observe progressively increasing discrepancy. This discrepancy is removed completely as soon as we neglect pulse height defect compensation. This observation makes us believe that the majority of the published results as well as semi-empirical calculations based on them (like the popular SRIM) may be in error at low ion energies. Procedures to evaluate foil quality and to determine pulse height defect for charged ions in semiconductor detectors are given. Our improved MCP-based time pick-off detector is described in detail.

Self-diffusion of silicon in molybdenum disilicide

The self-diffusion of silicon in single crystal MoSi2 was studied by means of a radiotracer technique using the short-lived radioisotope 31Si (half-life ), which was produced and implanted into the samples at the ion-guide isotope separator on-line device at the University of Jyväskylä in Finland. Diffusion annealing and subsequent serial sectioning of the specimens were performed immediately after the radiotracer implantation. In the entire temperature region investigated (835–1124u2009K), the 31Si diffusivities in both principal directions of the tetragonal MoSi2 crystals obey Arrhenius laws, where the diffusion perpendicular to the tetragonal axis is faster than parallel to it. In previous studies the same features were observed for the 71Ge diffusivities in MoSi2, except that these are somewhat higher than those of 31Si. Furthermore, it is noteworthy that in MoSi2 the diffusivities of 31Si and 71Ge are orders of magnitude faster than the diffusivity of 99Mo. This large difference suggests that silicon diffusion and molybdenum diffusion are completely decoupled and that silicon diffusion takes place exclusively on the silicon sublattice. Literature data on the phase growth of MoSi2 are in accordance with the present results on the 31Si diffusivities; Monte Carlo simulations of the correlation effects of vacancy-mediated diffusion on the silicon sublattice of MoSi2 lead to their rationalization.

A novel method for obtaining continuous stopping power curves

Abstract A new method has been developed for obtaining continuous stopping power curves in transmission geometry. In the method both the incident energy of the particle and its energy after passing through the sample foil are extracted directly from the semiconductor detector. Full range of energies is measured simultaneously eliminating step-by-step measurements and providing continuous data. A time-of-flight (TOF) spectrometer provides unambiguous matching of relevant particle groups from the run with and without absorber. Suitable energy distribution of incident particles was achieved by choosing the right thickness and tilting angle of a scattering foil. The method is very fast and reliable. Sample results for stopping power values for 40 Ar in Au, Ni, and polycarbonate are presented and compared with SRIM-2000 calculations. It is proposed to adopt a suitable standard for specific energy loss measurements to avoid possible discrepancies in energy calibration. For instance, Au or C would be good reference materials and all the new high precision data should be quoted not only in absolute values but also relative to the standard material that was measured with the same set-up. Our new method provides sufficient accuracy and speed to make such an approach feasible.

Simultaneous wide-range stopping power determination for several ions

A new procedure to extract simultaneously continuous stopping power curves for several ions and several absorbers over a wide energy range and with statistical errors reduced to negligible level is presented. The method combines our novel time-of-flight based method with the capability of our K130 cyclotron and ECR ion-source to produce the so-called ion cocktails. The potential of the method is demonstrated with a 6.0 MeV/u cocktail consisting of 16O4+, 28Si7+ and 40Ar10+ ions. The stopping power in polycarbonate in the energy range of 0.35–5 MeV/u has been determined with absolute uncertainty of less than 2.3% and with relative below 0.2%. The results are compared with literature data and with SRIM2000 parameterisation including cores and bonds corrections.

Magnetic Properties of Polycrystalline Bismuth Ferrite Thin Films Grown by Atomic Layer Deposition.

The atomic layer deposition (ALD) method was applied to grow thin polycrystalline BiFeO3 (BFO) films on Pt/SiO2/Si substrates. The 50 nm thick films were found to exhibit high resistivity, good morphological integrity, and homogeneity achieved by the applied ALD technique. Magnetic characterization revealed saturated magnetization of 25 emu/cm(3) with temperature-dependent coercivity varying from 5 to 530 Oe within the temperature range from 300 to 2 K. Magnetism observed in the films was found to change gradually from ferromagnetic spin ordering to pinned magnetic domain interactions mixed with weak spin-glass-like behavior of magnetically frustrated antiferromagnetic/ferromagnetic (AFM-FM) spin ordering depending on the temperature and magnitude of the applied magnetic field. Antiferromagnetic order of spin cycloids was broken in polycrystalline films by crystal sizes smaller than the cycloid length (∼60 nm). Uncompensated spincycloids and magnetic domain walls were found to be the cause of the high magnetization of the BFO films.

Influence of surface topography on depth profiles obtained by Rutherford backscattering spectrometry

A method for determining correct depth profiles from samples with rough surfaces is presented. The method combines Rutherford backscattering spectrometry with atomic force microscopy. The topographical information obtained by atomic force microscopy is used to calculate the effect of the surface roughness on the backscattering spectrum. As an example, annealed Au/ZnSe heterostructures are studied. Gold grains were observed on the surfaces of the annealed samples. The annealing also caused diffusion of gold into the ZnSe. Backscattering spectra of the samples were measured with a 2 MeV 4He+ ion beam. A scanning nuclear microprobe was used to verify the results by measuring backscattering from grains and from areas of the samples where no grains had been formed during annealing.

Sculpturing Nanowires with Ion Beams

Mass production in the close and sub-10-nm feature size regime is hindered by the lack of available and suitable tools capable of delivering the required levelofwell-controlledminiaturization. Low-energy ion beams provide a great degree of control when applied to prefabricated nanostructures for gradual downsizing and shaping. The unique sculpturing capability exhibited by energetic ions on the nanometer scale in forming threedimensional (3D) nano-objects enables fabrication of continuousnanowires, of virtuallyanymaterial,macroscopically long, and with an effective diameter of less than 10 nm. Newdevelopments in surface nanostructuring are expected to meet the emerging needs of industry in miniaturization, which currently faces serious difficulties in upholding the trend, particularly in microelectronics. The availability of effective tools for reliable mass production of complex composite nanoscale structures is a necessary prerequisite for successful industrial exploitation of the dramatic developments in nanoscience. Future success in developing 10-nm and sub-10-nm technologies will depend to a great degree on how well the process of nanostructuring is controlled on the nanoscale. In this size regime, morphological non-homogeneities characteristic to thematerial forming thebodyof ananostructure become comparable with its size. Any structural imperfections that are of little concern at the macroscale become absolutely critical when approaching the 10-nm size regime. In nanostructures bearing some functionality, that is, nanodevices, a nanowire is a necessary attribute forming an elemental link enabling charge transfer. The 10-nm feature size domain is of great interest for both fundamental research in the field of low-dimensional nanostructures and developing new nanoelectronic applications where nanostructuring enables new functionalities. New, exiting physics is expected at this scale due to the quantum size effects that, for example, may open a new route towards quantum communication and computation. It was predicted theoretically that a superconducting loop with embedded 10-nm-diameter nanowire

Polycarbonate, Mylar and Havar stopping powers for 1.0-3.25 MeV/nucleon 40Ar-ions

Stopping powers of polycarbonate, Mylar and Havar for 1.0-3.25 MeV/nucleon 40Ar-ions have been determined by the transmission method in two geometries. The stopping power values were obtained within uncertainty of 2.1-4.5% for the various materials. The present results are compared with the predictions obtained by the most commonly used procedures employed in obtaining stopping powers. These include the Northcliffe and Schilling model, semi-empirical parametrization of Ziegler et al (SRIM2000) with and without the cores and bonds model and the Hubert et al formulation. SRIM2000 values were in good agreement in case of Mylar and Havar, on average within 3% of present results. For polycarbonate the differences were less than 6% on average. The cores and bonds (CAB) model improved the parametrization values slightly. The Northcliffe and Schilling model and the Hubert et al formulation both yielded values within 5% or less for Mylar and polycarbonate. For the Havar the Hubert et al formulation and the present results disagreed by 10% on average.

Elastic scattering cross sections for 6Li and 7Li scattering by aluminum, silicon and titanium below 12 MeV at angles of 140∘ and 170∘

Abstract Elastic scattering cross sections for 6Li and 7Li scattering by natural aluminum, silicon and titanium have been measured in the energy range of 4–11 MeV. Scattering angles were 140∘ and 170∘ for 6Li and 140∘ for 7Li. The threshold energies above which Rutherford backscattering becomes invalid have been determined. Above this threshold the cross sections decrease rapidly, rendering backscattering analysis impractical. For 7Li scattering by Al and Si the observed threshold values were higher than for 6Li scattering. This behavior was reversed for scattering by Ti. The findings were systematic for both scattering angles. The present cross section and threshold data have been compared with the available literature data. The experimental threshold energies are compared with results obtained by analytical models based on nuclear potential perturbation calculations. For all targets studied, the experimental 6Li threshold energy values exceed the calculations. In the case of 7Li the comparison yields less systematic results.