  We build and test a single-photon detector based on a Si avalanche photodiode
Excelitas 30902SH thermoelectrically cooled to -100 deg. C. Our detector has
dark count rate below 1 Hz, 500 um diameter photosensitive area, photon
detection efficiency around 50%, afterpulsing less than 0.35%, and timing
jitter under 1 ns. These characteristics make it suitable for long-distance
free-space quantum communication links, which we briefly discuss. We also
report an improved method that we call long-time afterpulsing analysis, used to
determine and visualise long trap lifetimes at different temperatures.
