Name of Machine used to measure intensity of nuclear radiation?

There are several types of machines used to measure radiation. The 'classic' is the Geiger Counter which dates back to the late 40's or 50's. It's quite efficient, especially in measuring α and ß Emissions and passably so on Γ Rays. It works by counting the electric arc discharges inside the counter sensing probe caused by impinging radioactive particles.

Slightly newer, is the Scintillation Counter which is far more accurate in Γ Ray work. The Scintillation Counter, in spite of it's impressive name is really a rather simple instrument. They are comprised of a Photoelectric coupled with a Crystal that emits a tiny flash of light when it is exposed to Γ Rays ... the stronger the Γ field, the more the flashes.

Those are 2 basic types of Radiation Monitors. The designs cited above are the 2 prototypical flavors.