J.P.F. Sellschop

Light volatiles in diamond: Physical interpretation and genetic significance

Abstract Natural diamond is characterised in terms of features in the infra-red and ultra-violet spectra. Additionally electron spin resonance, X-ray diffraction and topography, cathodoluminescence, mechanical and electrical measurements have been used to give more detail to such specification. It had been concluded that the major impurity in diamond was nitrogen and hence most physical phenomena have been interpreted as a manifestation of the mode(s) of occurrence of nitrogen. From neutron activation analysis studies some 58 elements have been identified in diamond, many of course at trace levels. It has been shown that these data reveal a distinctive trace and minor element chemistry for diamond. Recently ion beam analyses have quantified the role of nitrogen in diamond characterisation. Most recently ion beam analysis has revealed that hydrogen, nitrogen and oxygen are all major impurities in diamond. Quantitative studies have been made using (19F, α), (α, n) and (3He, p) reactions. High temperature annealing in ultra-high vacuum conditions results in a substantial increase in the hydrogen measured as well as in the shape of the depth profile. Hydrogen is released from defect centres and diffuses rapidly through the diamond. Some of these hydrogen atoms are trapped at defect sites which are concentrated near surface as a result of the ion beam bombardment. A lesser response to the annealing treatment is found for oxygen and the smallest change for nitrogen. These ion beam data lend independent support to our earlier interpretation of the neutron activation data that all diamonds contain defects distributed fairly uniformly and consisting of sub-microscopic inclusions, the elemental composition of which suggests that each is a magma droplet from the upper mantle in which the diamond crystallized. The water-richness of the magma is an essential feature of the diamond genesis conditions.

Northern Kalahari groundwaters: Hydrologic, istopic and chemical studies at Orapa, Botswana

Most students of the Kalahari thirstland have been in agreement that any groundwater encountered should be fossil as no rain recharge could occur in the vast sand-covered regions at present. This conclusion is challenged in the present study during which intensive observations were made over three years on 27 boreholes and several shallow wells, situated in the sand-covered area of Orapa, in the northern Kalahari. Three aquifers are of importance in the region: a phreatic aquifer in the sandy Kalahari beds; a heterogenous aquifer in the underlying highly jointed basalt and a confined aquifer in the deeper-lying Cave sandstone. Rest level observations suggest that the phreatic Kalahari beds aquifer and the confined Cave sandstone aquifer are interconnected by occasional joints and solution zones in the intermediate basalt. The chemical composition of the various waters is in good agreement with the hydrological model. Tritium and 14C measurements show clearly that the phreatic aquifer is constantly recharged by rain water. The confined Cave sandstone aquifer, on the other hand, is devoid of tritium and devoid of, or low in 14C, indicating that this aquifer is not leaking, or leaking very slowly and water exchange with the upper aquifers is negligible. It is felt that intensive abstractions from this aquifer may, however, provide space for active recharge. Stable-isotope determinations rule out any recharge from pans, lakes or the river system in the north. Recharge occurs by direct rain infiltration, without any intermediate evaporation.

Neutron induced reactions in silicon semiconductor detectors

Abstract Total cross sections for the 28 Si(n,γ) 25 Mg and 28 Si(n,p) 28 Al reactions have been measured for neutron energies between 12.80 and 16.24 MeV, for the ground and first four resolved excited states of 25 Mg and the unresolved pairs consisting of the ground and first excited, and second and third excited states of 28 Al. The neutron energy resolution was less than 30 keV. The active volume of silicon semiconductor detectors was used both as target and detector.

The distribution of hydrogen in polycrystalline CVD diamond

Abstract The location of the hydrogen prevalent in chemical vapor deposition (CVD) diamond has long been of much interest, not least because of the information it reveals about the H-driven growth mechanism. We have used micro-scanning elastic recoil detection analysis to map the hydrogen distribution in three dimensions in polycrystalline CVD diamond. The interface between two CVD layers, one grown with, and one grown without oxygen in the growth mixture has been studied for its hydrogen concentration. An upper limit on bulk hydrogen concentration has been determined. The possibility of hydrogen trapping in the bulk is also discussed.

Isotopic production cross sections in proton-nucleus collisions at 200 MeV

Intermediate-mass fragments from the interaction of

Experimental investigations of hard photon emission from strong crystalline fields

^{27}\mathrm{Al}

Quantification of hydrogen in solids by two methods of ion beam analysis

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Channeling in diamond at high depth resolution

^{59}\mathrm{Co}

Ion implantation of carbon in diamond

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Diffusion characteristics of hydrogen in diamond

^{197}\mathrm{Au}