In answer to a DFAQ (depressingly-frequently asked question), a geiger counter measures radiation (such as the nuclear kind), it's that doohicky that makes the eerie clicks that click more rapidly when the radiation exposure becomes more intense.
To equip my solar rovers with some geological tools, I built a the world's first Geiger counter Mindstorms sensor. It detects Beta and Gamma radiation.
Topmost picture: At Brickcon, I set it up with a Mindstorms-operated Technic printer graphing the radiation level that the geiger counter was detecting. In front of the geiger counter, I put a rotating tray with four arms, each arm holding something radioactive, and the public could operate the controls to spin the tray in order to put each of the different items under the sensor.
Items included roadside granite, a household smoke detector, sand from near the first atomic explosion 60 years ago, and a naturally occurring uranium-bearing mineral.
This is the geiger counter, mounted inside a modified Lego rock-raiders laser brick. The tube at the front contains the geiger-muller tube. The transparent stud on top contains a light that blinks with each event (click).
This is the printer, graphing the results of the radiation readings.
These are the controls, allowing people at the convention to operate the display and watch how the printer responds when they bring different items near the geiger counter.
Holes cut in the underside of the tube allow greater beta radiation sensitivity - unlike gamma, beta radiation is only weakly penetrating, so the plastic if left intact would partly shield the geiger tube from some of it.
Legendary work! For this you should get MOC of the day! Not only have you harnessed the will to modify your Lego but you have also gone unique with this design. Make a set of instructions for people to use(meaning me and all MOCers) if we can't get a hold on school supplied Geiger Counters. I've tried raiding, asking, even getting a prac activity aprroved! Nothing works...