Physics
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........54l&link_type=abstract
Thesis (PH.D.)--STANFORD UNIVERSITY, 1993.Source: Dissertation Abstracts International, Volume: 53-11, Section: B, page: 5792.
Physics
Phonon Physics
Scientific paper
This work describes the development of cryogenic phonon-mediated particle detectors for dark matter searches and neutrino detection. The detectors described in this work employ transition-edge sensors, which consist of a meander pattern of thin-film superconductor on a silicon substrate. When phonons from a particle interaction in the crystal impinge on the sensor in sufficient density, sections of the line are driven normal and provide a measurable resistance. A large fraction of the thesis describes work to fully characterize the phonon flux from particle interactions. In one set of experiments, ~25% of the phonon energy from 59.54 keV gamma-ray events was found to propagate "ballistically" (i.e. with little or no scattering) across a 300 μm thick crystal of silicon. Gamma-rays produce electron recoils in silicon whereas with dark matter and neutrino experiments nuclear recoils are also of interest. Two experiments were done to measure the ballistic component that arises from neutron events, which interact via nuclear recoil. Measurements indicate that the fraction of energy that is ballistic is ~50% greater for nuclear recoils than for electron recoils. Two novel detectors were fabricated and tested in an attempt to improve the sensitivity of the detectors. In the first detector, relatively large Al pads were linked by 2 μm wide Ti lines in a meander pattern. Phonons impinging on the Al pads create quasiparticles which diffuse in the Al pad until they are trapped in the lower gap Tl links. The sensitivity of the detector was found to be increased by this "funneling" action. A second detector was built that incorporates 0.25 mu m wide lines defined by direct electron-beam exposure of the photoresist. If the superconducting line is sufficiently narrow, single phonons are capable of driving sections normal which should improve the sensitivity and linearity of the detector. Unfortunately, this detector did not display improved sensitivity which is probably due to the fact that isolated normal regions cool too quickly to be detected. An improved detector is being developed that uses alternating links of two different superconductors to trap the heat in the links long enough for the resistance to be measured. The design and fabrication of the experimental apparatus, including the detectors, the cryostat, and broad -band cryogenic preamplifiers is described. A detailed phonon propagation Monte Carlo simulation and data analysis simulations are presented. Finally, future plans for the development of more sensitive detectors and further phonon physics experiments are discussed.
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