  We report here on measurements of the reflectivity and loss tangent measured
in the W-band (80-125 GHz) and D-band (125-180 GHz) in two samples of float
zone silicon with intrinsic stoichiometry - one irradiated by neutrons, which
increases the resistivity by introducing crystalline defects, and the other
unperturbed. We find a loss tangent $\tan(\delta)$ of 2.8e-4 and 1.5e-5 for
neutron-irradiated silicon and intrinsic silicon, respectively, both with an
index of refraction of 3.41. The results demonstrate the applicability of
silicon as a warm optical component in millimeter-wave receivers. For our
measurements, we use a coherent reflectometer to measure the Fabry-Perot
interference fringes of the reflected signal from dielectric slabs. The depth
of the reflection nulls provides a sensitive measurement of dielectric losses.
We describe the test setup which can also characterize scattering and
transmission, and can provide detailed characterization of millimeter wave
materials.
