We used radiation sources and detectors at the steelworks for thickness gauge - you set them on either side of the plate and measure the lost and from that calc

We used radiation sources and detectors at the steelworks for thickness gauge - you set them on either side of the plate and measure the lost and from that calculate the steel thickness very accurately. I presume in a mine it could be used say on a conveyor belt where you measure the lost radiation to help determine the density or weight of the ore.