DP-70 was a chemical dosimeter used in the Polish army during the Cold War. A dosimeter is a device used to measure the amount of radioactive radiation absorbed by a body, in this case, by soldiers.
DP-70 has a form of a small metal cylinder (to protect the device in hard, combat conditions) that covers an ampule with a transparent liquid. This substance change color proportionally to absorbed radioactive radiation.
It's a simple device, but has two major drawbacks - it can't be used by colorblind persons and is not precise. Due to those two factors, the device was replaced by more modern DKP-50 dosimeter.
It can be bought for around two euro, unfortunately it's designed to measure ranges present during nuclear war, so it's far too insensitive to make any experiments with environmental radioactivity.