A beam of unknown massive particles, dark matter, passes through the earth and through many experiments attempting to measure its interactions.  Two figures of merit define the sensitivity of these experiments to a possible interaction: the minimum detectable interaction rate (background), and the minimum detectable interaction energy (threshold). My studies of the thresholds and principle backgrounds of PICO bubble chambers and SuperCDMS cryogenic detectors have relied on understanding the principles of fluid dynamics, nuclear physics, electron microscopy, physical chemistry, solid-state band structures, and x-ray crystallography.  These studies have led to the development of a caliation program and a sensitivity model for SuperCDMS SNOLAB, and to the ability to design ton-scale PICO detectors. These two technologies lead the world in the ability to discover low-mass dark matter and spin-dependent interactions with dark matter. The University of Montreal is set to take a lead role in developing and building large PICO bubble chambers, where control of neutron backgrounds will be critical to achieve the detector's potential.  The University of Montreal may also have a role in the development of new ultra-low threshold cryogenic detectors and their caliation using the Tandem accelerator and other local facilities.  Understanding detector backgrounds and thresholds presents a continuing challenge to dark matter searches.