Phonon-mediated particle detection using superconducting tungsten transition-edge sensors

Abstract

This thesis describes the development of several superconducting tungsten thin film based particle detector technologies. The initial motivation for this work was the construction of detectors sensitive to dark matter and neutrino scattering events. These technologies also show promise in other applications, including high resolution x-ray spectroscopy. The detectors described here consist of a tungsten thin film deposited on a silicon substrate. When an incident particle scatters in the silicon crystal, it deposits energy in the form of phonons which propagate to the surface of the crystal where they are absorbed in the tungsten thin film. The superconducting film is biased at or near its transition temperature. Changes in the resistance of the film are measured. The superconducting titanium transition-edge sensors previously developed by our group exhibit a threshold phonon energy density below which no signal is detectable. This threshold density poses severe restrictions on resolution, energy threshold, and absorber mass. In order to overcome these limitations, several new technologies were developed. In each case, a superconducting film with a sharp transition well below that of titanium (~ 380 mK) is necessary. To this end superconducting W films were developed with ~ 1 mK wide transitions at 70 mK. Before this work W thin films always exhibited transition temperatures > 600 mK. The first technology described here consists of a W thin film patterned into a 1 μm wide line 1.6 m long in a meander pattern. The line is biased at a constant current, and is temperature biased near the middle of its superconducting transition. When an event deposits energy in the W film, the resulting voltage pulse is measured with a cryogenic FET. A quantum efficient sensor is also described in which the heat capacity of individual, thermally isolated film segments biased just below their transition have heat capacities small enough that individual phonons drive them normal. The most promising technology discussed here is a novel sensor in which the temperature of the superconducting W film is held constant within its transition by an electrothermal feedback process. Energy deposited in the film by a particle interaction is removed by a reduction in the feedback Joule heating. This mode of operation leads to substantial improvements in resolution, linearity, dynamic range, and count rate. The sensor consists of a low impedance W film pad that is voltage biased. Particle interactions cause current pulses that are measured with a DC SQUID array. The fundamental limits on the energy resolution of this detector are analyzed, and found to be below the rms thermodynamic energy fluctuations in the film, and better than any existing technology operating at the same temperature, count rate, and absorber heat capacity. The enhancement of this electrothermal feedback technology with quasiparticle trapping is also explored. In this approach, superconducting Al thin film pads are placed in electrical contact with W lines. When phonons enter the Al film, they create quasiparticles which diffuse into the W lines on times of ~ 100 ns. Once in the W films they are rapidly thermalized. This enhancement allows the instrumentation of large surface areas with smaller W heat capacity. Using the quasiparticle trap enhanced electrothermal feedback technology, an energy resolution of < 400 eV FWHM is measured for 6 keV x-rays interacting on the backside of a 1mm thick silicon substrate. This sensitivity is sufficient for the construction of a dark matter detector, which will begin this year. Finally, the application of these technologies to other problems, including high resolution x-ray spectroscopy, infrared bolometry, and the resolution of individual low energy (~ 1 eV) photons is described.