|Keywords:||Low temperature physics; Physics; Condensed matter physics|
|Full text PDF:||http://pqdtopen.proquest.com/#viewpdf?dispub=10012481|
This dissertation is an account of the first calorimetric detection of individual He*2 excimers within a bath of superfluid 4He. When superfluid helium is subject to ionizing radiation, diatomic He molecules are created in both the singlet and triplet states. The singlet He molecules decay within nanoseconds, but due to a forbidden spin-flip the triplet molecules have a relatively long lifetime of 13 seconds in superfluid He. When He* 2 molecules decay, they emit a ~15 eV photon. Nearly all matter is opaque to these vacuum-UV photons, although they do propagate through liquid helium. The triplet state excimers propagate ballistically through the superfluid until they quench upon a surface; this process deposits a large amount of energy into the surface. The prospect of detecting both excimer states is the motivation for building a detector immersed directly in the superfluid bath. The detector used in this work is a single superconducting titanium transition edge sensor (TES). The TES is mounted inside a hermetically sealed chamber at the baseplate of a dilution refrigerator. The chamber contains superfluid helium at 100 mK. Excimers are created during the relaxation of high-energy electrons, which are introduced into the superfluid bath either in situ via a sharp tungsten tip held above the field-emission voltage, or by using an external gamma-ray source to ionize He atoms. These excimers either propagate through the LHe bath and quench on a surface, or decay and emit vacuum-ultraviolet photons that can be collected by the detector. This dissertation discusses the design, construction, and calibration of the TES-based excimer detecting instrument. It also presents the first spectra resulting from the direct detection of individual singlet and triplet helium excimers.