I can’t find the correct tag to this device.
Don’t even know if this is something that should be tag.
(It is a water tank, it has water inside, but it is used as a detector, not to storage water for later use)
Some thing like man_made=scientific_instrument, scientific_instrument=Cerenkov_detector or possibly radition_detector, & then radiation_detector=Cerenkov_detector. probably with lower=case for everything. Just cross-check with telescopes & use a similar principle but using scientific instrument. The advantage of using a fairly generic term here is that they will be specialised, might not be noticed etc, so a generic term might make it easier for others to map other oddities they come across.
These tags are not in use, see also man_made=monitoring_station for perhaps some other ideas.