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Here is, to my knowledge, the simplest functioning Geiger counter that you can build. This one uses a Russian-made SMB-20 Geiger tube, driven by a high-voltage step-up circuit robbed from an electronic fly swatter. It detects beta particles and gamma rays, emitting a click for every radioactive particle or gamma ray burst it detects. As you can see in the above video, it clicks every few seconds from background radiation, but really comes to life when radiation sources such as uranium glass, thorium lantern mantles, or americium buttons from smoke detectors are brought near. I built this counter to help me identify radioactive elements that I need to fill out my element collection, and it works great! The only real drawbacks of this counter is that it isnt very loud, and it doesnt compute and display the amount of radiation it is detecting in counts per minute. That means that you dont get any actual data points, just a general idea of radioactivity based on the amount of clicks you hear.
While there are various Geiger counter kits available on the net, you can build your own from scratch if you have the right components.
Let’s get started!

The Geiger counter (or Geiger-Müller counter) is a radiation detector developed by Hans Geiger and Walther Müller in 1928. Today, just about everyone is familiar with the clicking sounds it makes when it detects something, often regarded as the “sound” of radiation. The heart of the device is the Geiger-Müller tube, a metal or glass cylinder filled with inert gasses held under low pressure. Inside the tube are two electrodes, one of which is held at a high voltage potential (usually 400-600 volts) while the other is connected to electrical ground. With the tube in a resting state, no current is able to jump the gap between the two electrodes inside the tube, and so no current flows. However, when a radioactive particle enters the tube, such as a beta particle, the particle ionizes the gas inside the tube, making it conductive and allowing current to jump between the electrodes for a brief instant. This brief current flow triggers the detector portion of the circuit, which emits an audible “click”. More clicks means more radiation. Many Geiger counters also have an ability to count the number of clicks and compute counts per minute, or CPM, and display it on a dial or readout display.

Let’s look at the operation of the Geiger counter another way. The key principal of Geiger counter operation is the Geiger tube, and how it sets up a high voltage on one electrode. This high voltage is like a steep mountain slope covered in deep snow, and all it takes is a tiny bit of radiation energy (akin to a skier going down the slope) to set off an avalanche. The ensuing avalanche carries with it much more energy than the particle itself, enough energy to be detected by the rest of the Geiger counter circuit.

Since it’s probably been a while since many of us sat in a classroom and learned about radiation, here is a quick refresher.

Matter and the Structure of the Atom

All matter is composed of tiny particles called atoms. Atoms themselves are composed of even smaller particles, namely protons, neutrons, and electrons. Protons and neutrons are clumped together in the center of the atom - this part is called the nucleus. Electrons orbit the nucleus.

Protons are positively charged particles, electrons are negatively charged, and neutrons carry no charge and are therefore neutral, hence their name. In a neutral state, every atom contains an equal number of protons and electrons. Because protons and electrons carry equal but opposite charges, this gives the atom a neutral net charge. However, when the number of protons and electrons in an atom is not equal, the atom becomes a charged particle called an ion. Geiger counters are able to detect ionizing radiation, a form of radiation that has the ability to transform neutral atoms into ions. The three different kinds of ionizing radiation are Alpha particles, Beta particles, and Gamma rays.

Alpha Particles

An alpha particle consists of two neutrons and two protons bonded together, and is the equivalent of the nucleus of a helium atom. The particle is generated when it simply breaks off of an atomic nucleus and goes flying. Because it doesn’t have any negatively charged electrons to cancel out the positive charge of the two protons, an alpha particle is a positively charged particle, called an ion. Alpha particles are a form of ionizing radiation, because they have the ability to steal electrons from their surroundings, and in doing so transforming the atoms they steal from into ions themselves. In high doses, this can cause cellular damage. Alpha particles generated by radioactive decay are slow moving, relatively large in size, and because of their charge cannot pass through other things easily. The particle eventually picks up a few electrons from the environment, and in doing so becomes a legitimate helium atom. This is how almost all of the earth’s helium is produced.

Beta Particles

A beta particle is either an electron or positron. A positron is like an electron, but it carries a positive charge. Beta-minus particles (electrons) are emitted when a neutron decays into a proton, and Beta-plus particles (positrons) are emitted when a proton decays into a neutron.

Gamma Rays

Gamma rays are high energy photons. Gamma rays are located in the electromagnetic spectrum, up beyond visible light and ultraviolet. They have high penetrating power, and their ability to ionize comes from the fact that they can knock electrons off of an atom.

The SMB-20 tube, which we will be using for this build, is a common Russian-made tube. It has a thin metal skin that acts as the negative electrode, while a metal wire running lengthwise through the center of the tube serves as the positive electrode. In order for the tube to detect a radioactive particle or gamma ray, that particle or ray first must penetrate the thin metal skin of the tube. Alpha particles are generally unable to do this, as they are usually stopped by the walls of the tube. Other Geiger tubes that are designed to detect these particles often have a special window, called the Alpha window, that allow these particles to enter the tube. The window is usually made of a very thin layer of mica, and the Geiger tube must be very close to the Alpha source in order to pick up the particles before they are absorbed by the surrounding air. Sigh So that’s enough about radiation, let’s get to building this thing.”

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