The Monitor 4 is a compact, analog radiation detector. A general-purpose survey meter capable of detecting alpha, beta, gamma, and x-rays over 3 selectable ranges.
Proven and reliable over decades of use in the industry, the Monitor 4 has evolved into one of the best analog radiation detectors of today. Simple and ergonomic, a red count light flashes and a beep sounds with each count detected. Battery check and silent operation with the flick of your thumb.
Halogen-quenched uncompensated GM tube with thin mica window 1.5-2.0 mg/cm2 thick.
The effective diameter of the window is 9.1 mm (.36 in.).
For more info: https://www.zabdiresources.com/monitor-4-analog-radiation-detector