I believe you’re thinking of neutrinos. Neutrons love bonking into things, and they’re really good at it. Too good; if the thing they’re bonking is too big they’ll just keep on bouncing, so you gotta bounce them off small stuff. Concrete has lots of small stuff bound up in it, so while there’s better shields (borax and paraffin slurry is a classic one) there’s few cheaper or easier to make shields than concrete.