R. Medenwaldt

Conditions and Rates of Biotic and Abiotic Iron Precipitation in Selected Danish Freshwater Plants and Microscopic Analysis of Precipitate Morphology

Abstract This study compares the biotic precipitation of iron in the sand filters of a new freshwater plant, Astrup, with the abiotic precipitation of iron in the sand filters of a traditional freshwater plant, Forum, in the same area of Denmark. We have observed that a third freshwater plant, Grindsted, which was planned to precipitate iron in the traditional abiotic way is in fact precipitating iron biotically because of poor aeration and very low oxygen content of the raw water. The dominant iron-precipitating bacteria was Gallionella ferruginea in both Astrup and Grindsted. The morphology of the iron precipitates were investigated using light, X-ray, scanning electron and transmission electron microscopy. The physicochemical conditions governing precipitation and the precipitated iron sludge were also investigated. The biotically precipitated iron was shown to be oxidised and precipitated with a rate about 60 times faster than the traditional abiotic process in spite of the much poorer physicochemical conditions for the process. The faster kinetics indicate a catalytic activity due to the presence of exopolymers from Gallionella ferruginea . A model is proposed for the relationship between the differences in characteristics of the iron precipitate and the kinetics of the precipitation.

Channeling of high-energy particles in bent crystals - Experiments at the CERN SPS

Abstract During the latest decade, experiments have been performed at the CERN SPS to investigate the use of high-energy channeled nuclei in bent crystals for extraction, beam splitting and beam bending. An understanding of channeling in a bent crystal with extraction and deflection efficiencies for different energies, crystal types and ions has been developed. Furthermore, the long-standing question of radiation damage has been addressed with encouraging outcome. This makes extrapolations possible for the construction of, e.g., an extraction device for the LHC at CERN, RHIC at Brookhaven or new splitting elements in high-energy beams.We present the main results obtained and discuss existing and future applications of bent crystals in high-energy physics.

Measurement of the stopping power of silicon for antiprotons between 0.2 and 3 MeV

Our previous measurement of the stopping power of silicon for antiprotons has been extended down to 200 keV. The antiproton stopping power is found to be more than 30% lower than that for equivelocity protons at 200 keV. The “Z13 contribution” to the stopping power (the Barkas effect) is deduced by comparing the stopping power for protons and antiprotons. Comparisons to theoretical estimates are made.

Description of an x-ray microscope with 30 nm resolution

The Aarhus x-ray microscope has been in operation for some years and has evolved into a scientific user facility, the activities of which include a wide range of applications. A technical description of the microscope is given with emphasis on problems that have general character for high precision instrumentation and soft x-ray equipment.

Measurement of the antiproton stopping power of gold - the Barkas effect

Abstract The stopping power of gold has been measured for antiprotons in the energy range 0.2–3 MeV using a novel time-of-flight technique. The antiproton stopping power is found to be less than half the equivalent proton stopping power near the electronic stopping power maximum. In the high-energy limit the two stopping powers merge.

Measurement of pair production by high-energy photons in an aligned tungsten crystal

Abstract A new measurement has been made of the rate of pair-production in a 3.2 mm thick tungsten crystal, exposed to photons with energies in the range 10 to 150 GeV, for angles of incidence up to 10 mrad from the crystal axis. A strong enhancement of the pair-production rate is observed when the beam is aligned along the 〈100〉 crystal axis, as compared to a random orientation. This effect can be exploited in the NA48 CP-violation experiment by using a thin crystal rather than an amorphous material to convert photons, thus minimising the scattering of kaons in the converter.

Hard photon yields from (70–240) GeV electrons incident near axial directions on Si, Ge and W single crystals with a large thickness variation

Abstract The dramatic peak found in photon spectra from 150 GeV channelled electrons has for the first time been investigated for 70, 150, and 240 GeV electrons incident on crystals with thickness from 100μ (Si). Very pronounced variations for the high energy part of the photon spectra are found. In Si the photon peak is not found for lower energies and thin crystals. For well-aligned electrons in Si the yield is more than 160 times the Bethe-Heitler one. In general the peak in the photon spectra disaapears for incident angles larger than half the critical angle for channeling. Dramatic radiative energy losses are found along axial directions - in 0.6 mm 〈110〉 Ge a 150 GeV electron loses more than 60% of its total energy.

Detailed investigations of shower formation in Ge- and W-crystals traversed by 40–287 GeV/c electrons

Abstract The development of electromagnetic cascades along crystalline directions have for the first time been measured. As compared to random incidence the number of produced particles is enhanced more than 10 times along axial directions in Ge and increasing with particle energy. The critical angle for this strongly enhanced shower formation is around 1 mrad which means that the effect could be harnessed for high-resolution gamma-ray telescopes.

NEW RESULTS FROM THE CERN-SPS BEAM DEFLECTION EXPERIMENTS WITH BENT CRYSTALS

Abstract Results from five distinct bending experiments performed recently in the H8 beam at CERN are presented. Firstly, deflection of a positive pion beam at 200 GeV/c is compared to the “standard” 450-GeV/c proton beam for a bending angle of 3.1 mrad along the (111) plane in a 50 mm silicon crystal. Second, deflection of negative pions at 200 GeV/c is investigated for the same crystal, for incidence along the (111) plane as well as the 〈110〉 axis. Small deflection effects are seen, but no negative particles are bent through the full bending angle of the crystal. Third, the first results from beam deflection at high energy using a germanium crystal are shown. Slightly higher deflection efficiencies than for silicon are seen for large bending angles, but significantly smaller than expected for such a crystal with higher atomic number. Fourth, deflection efficiencies using a strongly irradiated silicon crystal have been measured for the first time, and a small reduction in efficiency is seen in the irradiated region. Finally, deflection of positive particles using axial alignment of a bent silicon crystal has been investigated at 450 GeV/c. Qualitatively similar behaviour as in previous experiments at 12 GeV/c is seen; the beam splits into several beams corresponding to the different planes, and even weak planes are observed.

Experimental investigations of hard photon emission from strong crystalline fields

Abstract For the first time very pronounced high-energy photon peaks have been measured in the radiation emission from 70, 150 and 240 GeV electrons incident at 0.1–1.0 mrad to the axis in diamond and Si crystals. The energy of the photons in the peaks is 0.7–0.8 times the particle energy with yields of 50 times the Bethe-Heitler one (in diamond). The peaks consist of single photons and are caused by the influence of strong crystalline fields on emission of coherent bremsstrahlung, emitted when the ultrarelativistic electrons cross the rows of atoms in a crystal plane. The effect should be envisaged as a source for nearly monoenergetic photons in the multihundred GeV-region.