Build Your Own Geiger Counter

Build Your Own Geiger Counter

Digital meter adapter

 

Build a fully functional Geiger Counter capable of measuring the three primary forms of radiation; alpha, beta and gamma radiation. The counter is sensitive enough to detect background radiation. In addition you can enhance the basic Geiger Counter by adding a Digital Meter Adapter, RS-232 Adapter with free Windows Radiation monitoring program or a Randam Number Generator. The windows 98/XP radiation program is free and available for downloading, more on this later.

The geiger counter described here may be purchased as a kit, or you may purchase the key components seperately and build the Geiger Counter from scratch. The kit simpilifies the construction. Images SI Inc. has many different geiger counter kits available for purchase. The kit described here is the GCK-02 Kit.

The Geiger Counter produces an audible click and blinks a LED each time it detects a radioactive particle. Typically the counter clicks 10-20 times a minute due to normal background radiation. While the device is sensitive enough to measure background radiation, it is not suitable for measuring radon gas. There are Radon gas detectors that use an activated charcoal filter that are easy to use and more accurate.

Radioactivity

Radioactivity is the spontaneous emission of energy from the nucleus of certain atoms. The most familiar radioactive material is uranium.

There are three forms of energy associated with radioactivity; alpha, beta and gamma radiation. The classifications were originally determined according to the penetrating power of the radiation, see Figure 1. Our Geiger Counter can detect the three types of radiation; alpha, beta and gamma radiation.

Figure 1
Figure 1. Penetration strength of radiation

Alpha rays are the nuclei of helium atoms, two protons and two neutrons bound together. Alpha rays have a net positive charge. Alpha particles have weak penetrating ability, a couple of inches of air or a few sheets of paper can effectively block them.

Beta rays were found to be electrons, identical to the electrons found in atoms. Beta rays have a net negative charge. Beta rays have a greater penetrating power than Alpha rays and can penetrate 3mm of aluminum.

Gamma rays are high-energy photons. This has the greatest penetrating power being able to pass through several centimeters of lead and still be detected on the other side. Thick lead is needed to attenuate gamma radiation.

Detecting Radiation – The Geiger Mueller Tube

Geiger Mueller tubes are simple devices that detect and measure radioactivity. The original design by H. Geiger and E.W. Mueller in 1928 hasn’t change very much. The basic sensor functioning remain the same.

A cut away drawing of a typical Geiger Mueller (GM) tube is shown in Figure 2. The wall of the GM tube is a thin metal (cathode) cylinder surrounding a center electrode (anode). The metal wall of the GM tube serves as the cathode of the GM Tube. The front of the tube is a thin Mica window sealed to the metal cylinder. The thin mica window allows the passage and detection of the weak penetrating alpha particles. The GM tube is first evacuated then filled with Neon, Argon plus Halogen gas.

Figure 2 Figure 2. Cross-section and function of typical Geiger Mueller tube

Our GM tube is put into an initial state (ready to detect a radioactive particle), by applying + 500-volt potential to the anode (center electrode) through a ten mega ohm current limiting resistor. A 470K-ohm resistor is connected to the metal wall cathode of the tube and to ground. The top of the 470K resistor is where we see our pulse signal whenever a radioactive particle is detected.

In this initial state the GM tube has a very high resistance. However, when a radioactive particle passes through the GM tube, it ionizes the gas molecules in its path and creates a momentary conductive path in the gas. This is analogous to the vapor trail left in a cloud chamber by a particle. In the GM tube, the electron liberated from the atom by the particle, and the positive ionized atom both move rapidly towards the high potential electrodes of the GM tube. In doing so they collide with and ionize other gas atoms, creating a momentary avalanche of ionized gas molecules. And these ionized molecules create a small conduction path allowing a momentary pulse of electric current to pass through the tube allowing us to detect the particle.

This momentary pulse of current appears as a small voltage pulse across the 470 K ohm resistor. The halogen gas quickly quenches the ionization and the GM tube returns to its high resistance state ready to detect more radioactivity.

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