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Title: Integration and optimisation of sensitive bolometric detectors with pulse-tube cryocoolers
Author: Walker, Gareth Robert
ISNI:       0000 0004 2750 0945
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2007
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The integration of 300 mK semiconductor bolometers and 4 K hot-electron bolometers with commercial pulse tube cryocooler (PTC) systems has been investigated. The process also involved the integration of a two-stage 4He-3He adsorption refrigerator with the PTC to produce a cryogen free sub-Kelvin system. Thermometry electronics with an onboard computer and an internet interface have been developed that allow for remote and/or automatic operation. Methods to reduce excess noise seen in the 300 mK bolometer have been investigated to demonstrate the possibility of using high sensitivity bolometric detectors in this system for astrophysics and Earth observation instruments. In order to minimise the microphonic induced noise and thermal fluctuations from the pulse tube operation an AC biasing circuit was used. The origin of excess noise components in the bolometer readout have been identified and quantified as a function of both mechanical and electrical configurations of the cold components. With the optimum configuration thermal fluctuation noise, rather than microphonic induced noise dominates, reducing the bolometer sensitivity by approximately two times compared to the same device operated in a liquid helium cooled cryostat. Clearly the performance could be improved by increasing the thermal isolation of the sorption refrigerator from the pulse tube temperature oscillations. The hot-electron bolometer behaved nominally up to 6 kHz when operated in the cryogen free cryostat. Dynamic audio frequency noise deteriorated the detector's performance above 6 kHz and has been attributed to the movement of the helium gas through the PTC's valves and orifices during its thermodynamic cycle. Restrictions of the signal analyser mean that it is unclear whether this microphonic noise is present above 1 MHz.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available