Geiger Counter

Listens for radiation. Hold it and it clicks: a few clicks a second with nothing near, more the stronger the field. It reads the field at you before your armour, so it tells you why to put lead on, not how much got through.

Crafting

Glass tubing on top, redstone in the middle with an iron plate either side, copper wire under it.

Reading it

Carry one and instruments read radiation as a figure in Ru. Hold it and the same figure shows on your hand card. The figure comes from counting the clicks of the last five seconds, so it jitters the way a real counter does, and a weak source can read as nothing for a moment. Listen as much as you look.