G-M Pancake Detectors

The G-M "pancake" detector is perhaps the closest approach to a "universal" radiation detector that is currently on the market. It is easy to use, reasonably priced, and capable of detecting alpha, beta, and gamma radiation. As a result, it is among the radiation detectors that are most often bought and utilized. Almost all locations that handle radioactive materials, including commercial nuclear reactors, hospitals, research organizations, and machine shops, employ G-M pancake detectors. But this detector has limits that are all too frequently overlooked or misinterpreted. Therefore, this essay will go through every detail of the G-M pancake detector that the RSO would find useful.

Operational Guidelines

One of the earliest forms of radiation detectors still in use is the G-M counter. The initials "G-M" stand for Hans Geiger and Walter Müeller, who invented it in 1928. (Note: Calling a G-M counter or detector a "Geiger counter" identifies someone as being ignorant of nuclear physics.)

A G-M tube is a sealed metallic tube, often made of steel, that is filled with a counting gas. When a particle or photon interacts with a counting gas, it causes the release of one electron, causing it to ionize. The anode or positive terminal is a tiny wire or plate hung in the tube's center that is electrically isolated from the casing. Between the cylinder wall and the anode, a strong voltage is supplied (called the cathode, or negative terminal). Any radiation that interacts with the gas will result in an electrical pulse if the voltage is high enough, which may then be detected by a counting device. (For further details on how voltage influences the operation of all gas-filled radiation detectors, see the G-M area of the six-region curve illustrated on page 16).

Since practically every gas molecule in G-M detectors is ionized, the magnitude of this pulse is independent of the intensity of the radiation that produced it. This prevents direct isotope identification with GM detectors and prevents them from being able to electronically differentiate between alpha, beta, and gamma radiation.

Construction

Although the G-M tube has varied throughout time, its basic structure has not changed. To make G-M tubes more suitable for a given application, different counting gas formulas and G-M tube diameters and shapes are utilized. An example of such optimization is the G-M pancake detector.

The term "pancake" detector originated from the tube's actual form. It does like a little, thick pancake since it is roughly 2 inches wide and just over half an inch thick. The moniker works well to set it apart from the "hot dog," which is the other popular form of G-M tube. A thin covering of mica is used in place of steel on the G-M pancake tube's active "face". This "window" permits the passage of the majority of alpha and beta particles into the counting section of the tube where they may interact with the counting gas and be detected since mica has a lower density and thickness than the steel casing. Only the betas and alphas with the lowest energies are halted.