Two new installations for non-destructive sample analysis: PIXE and PGAA

A new Proton Induced X-Ray Emission (PIXE) set-up has been designed and built at the Tandem Accelerator of the University of Cologne. The PIXE installation is used for the determination of elemental composition of thin samples or sample surfaces. The set-up was calibrated and tested with various types of samples. The experiments have been automated and the standard analogue based acquisition system has been replaced by a digital one based on the XIA DGF-4C modules. A small Peltier-cooled XFlash detector has been generally in use and brings many advantages when compared with common Si(Li) detectors. A scanning device can be used for macro-scanning of the surface of PIXE samples and the 2D elemental distribution can be determined. The absolute analysis of the sample composition with a commercial program GUPIX is described. Representative analysis of various samples is presented and compared to literature values or to the results of the PGAA method. The PIXE installation is now ready for routine use. A new Prompt Gamma-Ray Activation Analysis (PGAA) facility is now being designed for the research reactor FRM-II in Garching by Munich. The PGAA instrument at FRM-II will use a beam of cold neutrons for the determination of the elemental and isotopic composition of a sample bulk. A number of Monte Carlo simulations were performed to obtain the most convenient beam guide geometry for the cold neutrons; first to keep the neutron flux as high as possible and also to find out, how to focus the neutrons to a small spot of 1x1 mm^2. The divergence of the neutron beam was also part of the study. The challenging task is to make the PGAA facility flexible for four different instrumental set-ups: standard PGAA, Position-Sensitive PGAA, Cold Neutron Tomography and an Ge Array for nuclear structure experiments. The expected parameters of the PGAA facility at FRM-II are then compared to the former one at Paul Scherrer Institute (PSI) in Switzerland. In close cooperation with the PGAA group at the Budapest Neutron Centre (BNC) in Budapest, interesting geological samples were measured and analyzed. Small amounts of samples (100 - 300 mg) were analyzed to demonstrate the reliability of the PGAA analysis even for such cases. The results of the analysis for geological standards and meteorites, both either as stones or as homogeneous powders are presented and discussed. PIXE and PGAA analysis for the same samples were performed, the results were compared and conclusions about when PIXE and PGAA are competitive and when complementary are given. In case of PGAA, comparison with Instrumental Neutron Activation Analysis (INAA) is also discussed

Increasing the dynamic range for the analysis of boron in PGAA

Prompt gamma activation analysis (PGAA) is especially sensitive for elements with high neutron-capture cross sections, like boron, which can be detected down to a level of ng/g. However, if it is a major component, the high count rate from its signal will distort the spectra, making the evaluation difficult. A lead attenuator was introduced in front of the HPGe-detector to reduce low-energy gamma radiation and specifically the boron gamma rays reaching the detector, whose thickness was found to be optimal at 10mm. Detection efficiencies with and without the lead attenuator were compared, and it was shown that the dynamic range of the PGAA technique was significantly increased. The method was verified with the analyses of stoichiometric compounds: TiB2, NiB, PVC, Alborex, and Alborit

Increasing the dynamic range for the analysis of boron in PGAA

Prompt gamma activation analysis (PGAA) is especially sensitive for elements with high neutron-capture cross sections, like boron, which can be detected down to a level of ng/g. However, if it is a major component, the high count rate from its signal will distort the spectra, making the evaluation difficult. A lead attenuator was introduced in front of the HPGe-detector to reduce low-energy gamma radiation and specifically the boron gamma rays reaching the detector, whose thickness was found to be optimal at 10 mm. Detection efficiencies with and without the lead attenuator were compared, and it was shown that the dynamic range of the PGAA technique was significantly increased. The method was verified with the analyses of stoichiometric compounds: TiB2, NiB, PVC, Alborex, and Alborite

Stability of 10B4C thin films under neutron radiation

AbstractThin films of 10B4C have shown to be very suitable as neutron-converting material in the next generation of neutron detectors, replacing the previous predominantly used 3He. In this contribution we show under realistic conditions that 10B4C films are not damaged by the neutron irradiation and interactions, which they will be exposed to under many years in a neutron detector. 1μm thick 10B4C thin films were deposited onto Al or Si substrates using dc magnetron sputtering. As-deposited films were exposed to a cold neutron beam with fluences of up to 1.1×1014cm−2 and a mean wavelength of 6.9Å. Both irradiated and as-deposited reference samples were characterized with time-of-flight elastic recoil detection analysis, scanning electron microscopy, transmission electron microscopy, X-ray photoemission spectroscopy, and X-ray diffraction. We show that only 1.8ppm of the 10B atoms were consumed and that the film composition does not change by the neutron interaction within the measurement accuracy. The irradiation does not deteriorate the film adhesion and there is no indication that it results in increased residual stress values of the as-deposited films of 0.095GPa. From what is visible with the naked eye and down to atomic level studies, no change from the irradiation could be found using the above-mentioned characterization techniques

Nanocrystalline silicon: lattice dynamics and enhanced thermoelectric properties

Silicon has several advantages when compared to other thermoelectric materials, but until recently it was not used for thermoelectric applications due to its high thermal conductivity, 156 W K−1 m−1 at room temperature. Nanostructuration as means to decrease thermal transport through enhanced phonon scattering has been a subject of many studies. In this work we have evaluated the effects of nanostructuration on the lattice dynamics of bulk nanocrystalline doped silicon. The samples were prepared by gas phase synthesis, followed by current and pressure assisted sintering. The heat capacity, density of phonons states, and elastic constants were measured, which all reveal a significant, ≈25%, reduction in the speed of sound. The samples present a significantly decreased lattice thermal conductivity, ≈25 W K−1 m−1, which, combined with a very high carrier mobility, results in a dimensionless figure of merit with a competitive value that peaks at ZT ≈ 0.57 at 973 °C. Due to its easily scalable and extremely low-cost production process, nanocrystalline Si prepared by gas phase synthesis followed by sintering could become the material of choice for high temperature thermoelectric generators