Pocket-size α,β,γ Geiger counter

Ionizing radiation is everywhere.

As you read this, there could be a number of ionizing radiation sources influencing the cells in your body. Radon 222, for example, is a gas and the most prevalent source of background radiation most will encounter. With a half life of 3.8 days, radon 222 is a natural step in the decay chain of Uranium found in the Earth’s crust.

Radon-Decay-Chain
Uranium – 238 decay chain – image from coxcolvin.com

Radon is also considered to be the second leading cause of lung cancer after smoking. As you can see above, when Radon decays into Polonium-218 it releases alpha particles. If radon gas were present in your lungs during this decay period, those decaying alpha particles can cause free radical damage to cells and possibly induce mutations in DNA as well. This process is also completely imperceptible to human senses.

So how do we detect radioactive decay? There are a number of laboratory experiments which allow humans to observe the presence of significant radioactivity. For example: when radiation was first discovered, French physicist Henri Becquerel was experimenting with phosphorescent salts and photographic plates (panes of glass coated with a silver halide that decomposes when exposed to sufficient energy). He noticed that some of the salts would cause reactions with the plate even when separated by a black piece of paper that blocked all visible light. Becquerel continued experimenting until he was able to establish that the reaction on the photographic plates had nothing to do with phosphorescence but was instead a result produced only with Uranium compounds and metal.

Becquerel’s experiment indicates that radioactive particles are of high energy and can be detected using a material or process sensitive to energy. In fact, film was and still is used as a type of dosimeter for monitoring personal radiation exposure. However, film dosimeters cannot be reused and do not provide real time feedback.

This leads us to perhaps the most ubiquitous and recognizable type of radiation detector; the Geiger counter.

ludlum_model_3
Ludlum Model 3 – a gold standard for industrial survey meters

A Geiger counter is a device intended for real time monitoring of ionizing radiation. Well designed units are robust, portable, and can be optimized for the magnitude and types of ionizing radiation to be measured.

Geiger counters use a Geiger–Müller (GM) tube as the radiation sensor. GM tubes are typically cylindrical in shape, made of thin metal or glass, and contain a gas mixture which is primarily a noble gas (helium, neon, or argon). The gas surrounds an anode, which is then energized with high voltage to create an electric field inside the tube.

gmtube_cutaway
drawing of a GM tube and an example detection event – image from imagesco.com

When ionizing radiation enters the GM tube, free electrons in the gas are excited by the energy of the radioactive particle. This causes a Townsend discharge; an event where free electrons released by incident radiation cause the surrounding gas molecules to free more electrons. The strong electric field in the tube causes the electron collision and release events to accelerate and effectively “avalanche”, resulting in electrical conduction from the tube anode (positive) to the cathode (negative, usually the body of the GM tube itself).

That discharge event is called a “count”; a singular radiation detection event by the GM tube. Different models of tubes will have different count rates (usually measured in CPM – counts per minute) in the same radiation field. Generally speaking, smaller tubes are less sensitive due to the smaller internal volume of the tube.

Now that we have a basic understanding of ionizing radiation and Geiger counter detection principals, I’d like to introduce a little gizmo I assembled:

20200430_114031

This is a complete, portable geiger counter designed by my friend Mr. Henry Carl Ott. The entire device fits into a tube which is approximately 5″ long by just over 1″ diameter. I was fascinated with the prototype when he first revealed the project, and I finally had the pleasure of assembling one with PCBs he provided to me.

The tube in this counter is made by LND Inc, model #712. The 712 tube is time tested design which provides good sensitivity in a small package. It can also detect alpha radiation as it has a mica window on one side. You can read about its specifications on LND’s website: click here.

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Mica end window of the LND 712

Overall, I really enjoyed building this little detector and using it is a breeze. The rotary encoder user interface allows for easy interaction with menus to adjust parameters such as beeping, blinking, geiger tube voltage, battery monitoring, and more. The mini LCD screen is also in color, which is a nice touch.

geiger_gif
using a shard of “radioactive red” fiestaware as a radiation source

The form factor, performance, and usability are (in my opinion) superior to most commercial geiger counters. This is only the first revision, and I know Carl is working on both software and hardware improvements for possible future iterations. While this design is only a prototype and not for sale, please do contact myself or Carl (hcarlott@gmail.com) so that we can gauge interest in this project moving forward. Should we receive suitable levels of engagement, a small production run of test tube sized geiger counters is a possibility!

Update 5/10/2020: A brief demonstration video of the complete prototype assembled in this article:

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