I'm pretty sure the receiver can't be passive. If the receiver doesn't work due to power failure or malfunction, what good is a functioning transmitter?
It would be interesting to know why they decided to go this way instead of a more conventional radar. Soyuz is already full of RF equipment for communications and docking procedures. Adding a landing radar probably wouldn't add that much to overall complexity. It might be that gamma backscatter is more consistent over expected landing surfaces than microwave reflectivity.
Also, I wonder how they shield their detector from direct gamma rays from source so that they don't overwhelm the weak backscatter signal. They can't have much in the way of heavy shielding.
It could theoretically land anywhere on the planet in an accident situation, some freaked out country could fire up the radar jammers, would be terrible bad luck to land next to a weather radar, even if nothing bad happens it'll have Russian military aircraft loitering around the LZ and now you have the technical problem of proving every Russian aircraft radar ever made won't interfere with the landing system. I would guess if the rockets fire early the astronauts will be severely hurt (killed?) on landing so not firing early would be a priority. They didn't make it triple redundant for fun...
Also without any information its very hard to tell if this is just good old fashioned ALARA at work, or if its a real threat. On one hand a powerful source would require a smaller detector and size and mass are expensive in spacecraft. Then again the crew needs shielding from the source and they're using gammas so source intensity is also not cheap. My guess is its a combination of ALARA principle at work combined with some first responders might have radiological monitoring and that will detect it, causing a freak out unless previously warned.
