  The primary advantage of moderately superheated bubble chamber detectors is
their simultaneous sensitivity to nuclear recoils from WIMP dark matter and
insensitivity to electron recoil backgrounds. A comprehensive analysis of PICO
gamma calibration data demonstrates for the first time that electron recoils in
C$_3$F$_8$ scale in accordance with a new nucleation mechanism, rather than one
driven by a hot-spike as previously supposed. Using this semi-empirical model,
bubble chamber nucleation thresholds may be tuned to be sensitive to lower
energy nuclear recoils while maintaining excellent electron recoil rejection.
The PICO-40L detector will exploit this model to achieve thermodynamic
thresholds as low as 2.8 keV while being dominated by single-scatter events
from coherent elastic neutrino-nucleus scattering of solar neutrinos. In one
year of operation, PICO-40L can improve existing leading limits from PICO on
spin-dependent WIMP-proton coupling by nearly an order of magnitude for WIMP
masses greater than 3 GeV c$^{-2}$ and will have the ability to surpass all
existing non-xenon bounds on spin-independent WIMP-nucleon coupling for WIMP
masses from 3 to 40 GeV c$^{-2}$.
