The photograph on the left shows the probe which contains the Geiger–Müller tube. The opening is to allow the detection of beta radiation. With it closed, the Geiger counter will only detect gamma particles. In the open position, both gamma and beta radiation detection is possible. It is shown here in the open position.
The photograph on the right is a picture of the Geiger–Müller tube which is located inside the metal body of the probe in the picture on the left. This is now an expensive tube that is becoming extremely difficult to find today.
What is a Geiger–Müller tube?
The Geiger–Müller tube or "GM tube" is what I consider to be the heart of this Geiger counter. The GM tube derived its name from both Hans Geiger and Walther Müller. Hans Geiger invented the device in 1908, and then 20 years later, Walther Müller, while working with Hans Geiger, developed it even further. The GM tube is the receiving, or more accurately, the sensing element of a Geiger counter. The GM tube can detect a single particle of ionizing radiation, and will typically produce an audible click in the speaker of the Geiger counter for each particle detected. The GM tube consists of a tube that is filled with a low pressure (~0.1 atmospheric pressure) inert gas. This is usually neon gas, but helium and argon gas are used as well.
Between the electrodes inside of the GM tube is a potential difference of several hundred volts but there is no current flowing. The inside walls of the tube are either entirely metal or will have their surface coated with a conductor to form the cathode, while the anode is a wire passing up through the center of the tube. When ionizing radiation passes through the tube, some of the gas molecules will be ionized, thus creating both positive charged ions, and electrons. The strong electric field that is created by the GM tube's electrodes then accelerate the ions towards the cathode and the electrons towards the anode. The ion pairs will have gained sufficient energy to ionize further gas molecules through collisions on the way, creating an avalanche of charged particles. This results in a short, intense pulse of current which passes (or cascades) from the negative electrode to the positive electrode which is measured and then registered in the speaker of the Geiger counter.
Digging in a little deeper into the operation of the tube, it is, in short, a type of gaseous ionization detector that has an operating voltage in the Geiger plateau. Depending on the characteristics of the specific counter, the exact voltage range may vary. In the Geiger plateau region the potential difference in the counter is strong enough to ionize all the gas inside of the GM tube when triggered by the incoming ionizing radiation whether it is alpha, beta or gamma radiation.
Voltages below the Geiger plateau region will not be high enough to cause a complete discharge which results in what is known as a limited Townsend avalanche. In this state, the GM tube acts as a proportional counter where the output pulse size depends on the initial ionization that was created by the radiation. On the other end of the spectrum, a higher voltage will cause a phenomenon known as "quenching" in which the positively charged ions will be drawn to the cathode creating a continuous pulse in the counter.
Plectron instruction manual
Doug Moore's book detailing the restoration of the PRI-107B