re: Impact debris...hmmm.
But the assumption here is homogenous mixing, while in reality the vaporized impact material wll recondense and fall on top, as on the Earth.
None the less the amounts the amounts are very small... and the deposition long, given the Moon's smaller gravity.
Larger craters on the Earth will provide a good test of detectors before engagining in instrument re-egiineering them for proble use, and then running up launch costs.