The answers to the Space SE question Soyuz landing ground detection really surprised me. A gamma ray altimeter is used by the returning Soyuz capsule to detect the presence of solid ground immediately below the capsule at the last split-second (not foliage or other weak structures) to trigger the retro-rockets milliseconds before touch down to reduce the impact.
What kind of sensor is used to reliably detect gamma rays scattered from the Earth? It would have to be fairly sensitive because only a small fraction of the original gamma ray intensity would backscatter and find its way back to a small detector with enough energy to make it through the heat shield attenuation. It would also have to very reliably survive all of the vibrations and thermal excursions along the way.
Wikipedia (Russian): Гамма-лучевой высотомер