With the recent interest in Geiger counters I wanted to share one of my favorite items in the house. This is the Lionel CD V-700 Model 6b Geiger Counter which was used for measuring radiation, primarily in case of a massive nuclear attack on the United States or other friendly countries. The CD V-700 is a very sensitive Gamma and Beta radiation detector, so sensitive that they were considered ‘useless’ in case of a thermonuclear war. This is good for modern day experiments, disasters and ‘treasure hunting’ with a Geiger counter. As you can see on pictures further down the page this one hasn’t been in for calibration since 1993.

This is one of the classic Geiger counters you’re likely to see in older movies and it may bring back some memories to people who lived in the unsure nuclear ages. Even now can be uncertain times with the amount of nuclear energy and power we use in the world and with countries working every day to build a nuclear arsenal for themselves so they can improve their ‘street cred’ in international politics.

The CDV-700 is very simple to operate and has just a few major components. The primary sensing component is the Geiger-Muller tube. The Geiger-Muller tube or GM tube was originally created by Hans Geiger and then further improved with the assistance of Walther Muller ( Müller ). The operation of the Geiger tube is basic in principal. It is a vacuum tube with an electrode in the middle and an outer shell with a charge (voltage) differential between the shell and the electrode. When a radioactive particle passes through the vacuum between the outer shell and the electrode it creates a conductive path.

The pictures below show the Geiger-Muller tube of the CD V-700. The tube is contained inside of a steel housing and the steel housing has slots in it. If you rotate the outer housing of the steel housing 3 windows are opened removing shielding from the tube itself. With the windows closed the CD V-700 is sensitive to Gamma radiation. When the housing is rotated it will also pick up Beta radiation. The CD V-700 Model 6b is not capable of detecting alpha radiation.  Alpha radiation detector’s Geiger-Muller tubes usually have a window made from a material, normally Mica.

Inside of a Geiger Counter is a circuit which detects the conduction of electrical potential between the shell and the electrode and generates a click. In newer systems the pulse is fed into a pulse counting system which do the math and show the exposure on a digital display. In older systems like CDV-700 Model 6b Geiger counter the output pulse is tied to an analog averaging circuit which in it’s most simple concept is a capacitor which is charged by the pulse, and a resistor which provides the time base for decay. As a pulse charges the capacitor the stored energy in the capacitor increases, which also increases the reading the meters needle. The resistor slowly drains the capacitor causing the needle to drop back down. If multiple nuclear particles pass through the Geiger-Muller tube, then multiple charging pulses are applied to the averaging circuit, increasing the voltage on the capacitor and the needle reading as well. I’ve read that the Lionel Geiger detector has one of the best versions of the CDV-700 electronics as compared with Victoreen and other brands, yet others dispute that. Having only this one CD V-700, I can’t honestly say myself.

You can see the schematic of the CD V-700 Model 6b in the photo below. The schematic for the Geiger counter is included in the base of the housing for repair and maintenance of the device.

You can see how the electronics are implemented in the CD V-600 Geiger Counter with the easily accessible schematics and easy access to the batteries and the circuit board itself. Many of these Cold War era Geiger counters were designed with maintenance and serviceability in mind and you can tell. More modern Geiger counters do not contain any ‘user serviceable’ parts inside of them, but they are much smaller and more portable Geiger counters which fit in with today’s modern and compact lifestyles.

If you’re in the market for a Geiger counter you need to take into consideration the type of radiation you are looking to detect and how little of it you want to detect. Geiger counters come with sensitivity for Alpha, Beta and Gamma particles and if you have a Gamma and Beta only you may be missing out on the Alpha particles. If you’re buying this as a scientific curiosity you want a detector which can pick up all the different types of radiation. A lot of the newer and smaller Geiger counters are capable of doing this, but be sure to shop and research what you need.

When there’s not a particular reason for having a Geiger Counter you can use it for other things. I like to wander around and pick up rocks and see if they’re radioactive, I check various metal objects around the house. I remember reading articles quite a while ago where radioactive materials had accidentally been smelted with regular metals resulting in radioactive consumer products and metal stock, so I will occasionally check raw stock with it. Just use your imagination, there’s quite a bit of things to do. Sometimes though I just turn it on, close my eyes and listen to the background radiation while imagining and high velocity particle shooting it’s way through the Geiger-Muller tube and how the circuits functioning in response to each one.

One extra note about the CD V-700 Model 6b is that it comes with its own radioactive check source with taped to the side of the Geiger counter which allows you to check the functionality of the counter to make sure the electronics are working okay. The radiactive check source is a small piece of either Radium 266 or depleted Uranium. The half-life of some of these materials is short compared to the physical age of these older detectors so they may provide you with a quality check of functionality.


I’m currently selling this Lionel CD V-700 Model 6b Geiger counter on eBay to help fund my CNC Router Project sometimes small sacrifices are required to complete other projects.

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