Georgi Genov

Ice perfection and onset of anomalous preservation of gas hydrates

Anomalous preservation is the well-established but little-understood phenomenon of a long-term stability of gas hydrates outside their thermodynamic field of stability. It occurs after some initial decomposition into ice in the temperature range between 240 and 273 K. In situ neutron diffraction experiments reveal that the low-temperature on-set of this effect coincides with the annealing of stacking faults of the ice formed initially. The defective, stacking-faulty ice below 240 K apparently does not present an appreciable diffusion barrier for gas molecules while the annealed ordinary ice Ih above this temperature clearly hinders gas diffusion. This is supported by further experiments showing that the so-called ice Ic formed from various high-pressure phases of ice, gas hydrates or amorphous ices does transform fully to ordinary ice Ih only at temperatures near 240 K, i.e. at distinctly higher temperatures than generally assumed. In this light, some quite disparate observations on the transformation process from ice Ic to ice Ih can now be better understood. The transformation upon heating is a multistep-process and its details depend on the starting material and the sample history. This ‘memory’ is finally lost at approximately 240 K for laboratory time-scale experiments.

Experimental studies on the formation of porous gas hydrates

Abstract Gas hydrates grown at gas-ice interfaces were examined by electron microscopy and found to have a sub-micrometer porous structure. In situ observations of the formation of porous CH4- and CO2- hydrates from deuterated ice Ih powders were made at different pressures and temperatures, using time-resolved neutron diffraction data from the high-flux D20 diffractometer (ILL, Grenoble) as well as in-house gas consumption measurements. The CO2 experiments conducted at low temperatures are particularly important for settling the open question of the existence of CO2 hydrates on Mars. We found that at similar excess fugacities, the reaction of CO2 was distinctly faster than that of CH4. A phenomenological model for the kinetics of the gas hydrate formation from powders of spherical ice particles is developed with emphasis on ice-grain fracturing and sample-consolidation effects due to the outward growth of gas hydrate. It describes (1) the initial stage of fast crack-filling and hydrate film spreading over the ice surface and the two subsequent stages which are limited by (2) the clathration reaction at the ice-hydrate interface and/or by (3) the diffusive gas and water transport through the hydrate shells surrounding the shrinking ice cores. In the case of CO2-hydrate, the activation energies of the ice-surface coating in stage 1 are estimated to be 5.5 kJ/mol at low temperatures and 31.5 kJ/mol above 220 K, indicating that water molecule mobility at the ice surface plays a considerable role in the clathration reaction. Comparable activation energies of 42.3 and 54.6 kJ/mol are observed in the high temperature range for the reaction- and diffusion-limited stages 2 and 3, respectively.

BGO front-end electronics and signal processing in the MXGS instrument for the ASIM mission

This paper presents the Bismuth Germanate Oxide (BGO) front-end electronics design and signal processing in Modular X- and Gamma ray sensor (MXGS) instrument onboard the Atmosphere Space Interaction Monitor (ASIM) mission, funded by the European Space Agency. University of Bergen is responsible for the design and development of the detector layers and readout electronics for the MXGS instrument. The principal objective of the instrument is to detect Terrestrial Gamma ray Flashes (TGFs), which are related to thunderstorm activity. The digital pulse processing scheme used in the MXGS BGO detector gives it a significantly higher rate capability than what has been achieved in other instruments used in the study of terrestrial gamma flashes. The front-end electronics for the BGO detector layer in MXGS system also uses fewer components compared to conventional analog front-ends for BGO detectors, thereby increasing its reliability and projected lifetime in the harsh space environment. The MXGS instrument is expected to see about 1000 TGFs in a time period of one year.

Low-energy CZT detector array for the ASIM mission

In this article we introduce the low-energy CZT (CdZnTe) 16 384-pixel detector array on-board the Atmosphere Space Interaction Monitor (ASIM), funded by the European Space Agency. This detector is a part of the larger Modular X-and Gamma-ray sensor (MXGS). The CZT detector array is sensitive to photons with energies between 15 keV and 400 keV. The principal objective of the MXGS instrument is to detect Terrestrial Gamma ray Flashes (TGFs), which are related to thunderstorm activity. The concept of the detector array is presented, together with brief descriptions of its mechanical structure and electronics. A method for detector characterization is outlined and preliminary experimental results are shown.