Rochford, James H; (2003) Development of cryogenic semiconductor microcalorimeters for spectroscopic x-ray astronomy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis assesses the feasibility of using Neutron Transmutation Doped Germanium (NTD Ge) as the thermistor element of a microcalorimeter that has the potential to be used to create a very large high spectral resolution x-ray imaging array. A number of numerical simulations of a single microcalorimeter system have been created. These models are capable of estimating the overall system performance of the complete microcalorimeter system. The models include the non-ideal effects incurred by the system bandwidth, non-linearity of the detector response during an x-ray event, the electric field across the detector and electron-phonon decoupling. The inclusion of these effects accounts for the observed poor performance of current experimental NTD Ge detectors. The model predictions have been experimentally verified down to 50mK. The models predict that an energy resolution comparable to that required by future spectroscopic arrays can be achieved using a small highly doped NTD Ge thermistor operating at l0mK. The construction of new thermal link schemes for individual detectors that simplify the creation of an array is investigated. It is shown that it is potentially feasible to use the boundary resistance of the thermistor bonded directly to the heat sink as a thermal link. Two novel array readout methods are presented; both of these significantly reduce the number of channels required to readout an array. One scheme uses diodes to constrain the voltage pulses produces by a pixel to remain in its own row and column of the array. The second scheme uses two thermistors to readout each pixel, this is shown to greatly reduce the number of wires into the array itself, thus reducing the thermal load to levels compatible with future space borne adiabatic demagnetisation refrigerators. It is shown that these detectors can be formed into moderately sized arrays. It is ultimately concluded that the construction of very large arrays using these particular detectors may prove impractical due to their monolithic nature.

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