  Historically, there is little faith in particle tracking velocimetry (PTV) as
a tool to make quantitative measurements of thermal counterflow in He II, since
tracer particle motion is complicated by influences from the normal fluid,
superfluid, and quantized vortex lines, or a combination thereof. Recently, we
introduced a scheme for differentiating particles trapped on vortices (G1) from
particles entrained by the normal fluid (G2). In this paper, we apply this
scheme to demonstrate the utility of PTV for quantitative measurements of
vortex dynamics in He II counterflow. We estimate $\ell$, the mean vortex line
spacing, using G2 velocity data, and $c_2$, a parameter related to the mean
curvature radius of vortices and energy dissipation in quantum turbulence,
using G1 velocity data. We find that both estimations show good agreement with
existing measurements that were obtained using traditional experimental
methods. This is of particular consequence since these parameters likely vary
in space, and PTV offers the advantage of spatial resolution. We also show a
direct link between power-law tails in transverse particle velocity probability
density functions (PDFs) and reconnection of vortex lines on which G1 particles
are trapped.
