| Counts scintillations.
Just in case that wasn't obvious enough, I'll elaborate. This device is often used in applications such as x-ray diffraction. It measures the number of incident X-rays.
Since humans (and many photosensors) cannot directly detect x-ray band radiant energy, a scintillator crystal is used to convert incoming x-rays to rays of visible light. This light is then passed into a photomultiplier tube, which amplifies its originally weak signal and sends a potential (voltage) to some kind of recording device. The voltage recorded is proportional to the intensity of the incident ray.
A sensor of this type may be passed about a sample in x-ray diffraction and correlated with location to find things like the bragg angle and such.
While a Geiger Counter can do the same thing, it's not as precise, as it is pretty much limited to detecting the presence of an incident ray and not the intensity. You might say, the scintillation counter is not as clumsy or random as a Geiger Counter. | 
