Geiger Mueller (GM) Detectors

The essential components of the geiger-mueller detector are its two collecting electrodes: the anode and cathode (the anode is positively charged with respect to the cathode). In most cases, the outer chamber wall serves as the cathode. The potential difference between the anode and cathode is usually in the 800 to 1200 volt range.

We have three basic GM detector designs: Side Window (cylindrical), End Window, Pancake

Side Window (thin/thick walled cylindrical) GM

A cylinder (often 446 stainless steel) serves as the cathode while a wire (usually tungsten) running along the central axis serves as the anode. The primary application of the side window GM is the measurement of gamma exposure rates. Nevertheless, its wall can be thin enough to permit higher energy betas (>300 keV) to be counted.

The density thickness of a typical thin walled cylindrical GM is 30 mg/cm2. The density thickness (aka aerial density) is a convenient way to describe the thickness of very thin materials it is the product of the material’s density (mg/cc) and its thickness (cm). To measure it, weigh one square centimeter of the material.

Pancake GM

The pancake GM tube is a truncated cylinder having the shape of a pill box. As is true of the end window GM tube, one end is covered with a thin (usually mica) window. The window thickness is 1.5 to 2.0 mg/cm2.

The shape of the anode is unusual since it has a circular configuration in a plane parallel to the entrance window. The primary function of the pancake GM is the detection and measurement of beta emitting surface contamination. It also responds to alpha particles and gamma rays.

End Window GM

As is the case with the side window GM, the cathode is a stainless steel cylinder. The anode is supported at one end and extends only part way along the tube axis. The tip of the anode is typically covered with a small glass bead. The window, covering one end of the tube, is usually made of mica and typically has a density thickness of 1.5 to 2.0 mg/cm2.

Like any Geiger Mueller detector, the end window GM responds to gamma rays. Nevertheless, the end window GM is most commonly used to count beta activity. The end window GM can also be used to count alpha particles.