Geiger Mueller (GM) Detectors
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.
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.
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.