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Quite a lot of the posts on this weblog are about radiation. To summarize some of the information I decided to write this post.

Geiger Counters

If you want to detect some radiation it is the easiest to buy a simple geiger counter.  On eBay various geiger counters are sold for varying prices. Also check out the links at the end of the article. Possible options are:

Cheap Russian counters such as the DSRB-88. Most of these counters just click and flash a light when radioactivity is detected. These are fine to find radioactive items in stores and compare the items in your collection. These can be had on E-Bay for €/$ 60 or more. Shop around and don’t pay too much for such simple devices.

Surplus and “classic counters” such as the Civil Defense CDV-700 and the Ludlum Model 3.  These are professional, rugged counters. They can be fitted with alternative probes and have analog metering with switchable ranges which enables more accurate measurements. These counters are also quite big and have roomy enclosures which makes it easy to mount alternative sockets and additional circuitry.  CDV-700’s can be bought for less than $100 but make sure that you buy a working counter. Don’t buy stuff from sellers that claim to be unable to test the counter. A working counter fitted with batteries should always detect some background radiation. If it doesn’t, look for other offers. Ludlum counters are more professional and rugged and have a corresponding higher price tag. Pay attention to the geiger tube power supply voltage. Most geiger tubes run on voltages around 400V, but counters like the CDV-700 output 900 volts. To connect lower voltage tubes you’ll need the following circuit.

“Modern” geiger counters such as the Gamma Scout, Terra-P, Inspector Alert, etc. These counters can be really expensive but also have nice features such as data logging, LCD display, long running time on batteries and tubes capable of detecting alpha radiation in addition to the usual beta/gamma radiation. In my opinion these counters are overpriced, similar features can be had much cheaper by building your own counter. However, if you want a complete solution and have money to burn these counters can be a proper choice.

Watch out: A lot of people sell high range, ionisation chamber meters as geiger counters. These meters won’t detect low activity samples and background radiation. They were made during the cold war to survey areas after a nuclear attack. If the needle on these meters moves it is either defective or WW III just started. Examples of these meters are the Civil Defense CDV-715, 717 and the German Total TTL6109A.

DIY counters

On the Internet a lot of schematics of geiger counters can be found in addition to kits with PCB’s and parts included.

Simple Geiger Counter is a schematic which mostly uses standard components. It’s made for generating counting pulses for microprocessors but it can be easily turned into the classic geiger counter by omitting the 555 pulse shaper and replacing it with this clicker circuit.

DIY Geiger Counter is a very nice Arduino based geiger counter with excellent features and extensibility. It enables you to build a simple counter, but adding features like an LCD, menus, data logging is very easy if you have some experience in electronics.  I have used it to successfully build a weather station with geiger counter capabilities.

A different DIY option is ionisation chambers. These detectors work with a chamber with two electrodes, like a geiger tube. The difference is that there are no pulses but only the current caused by particles and gamma rays ionising the air and causing a very small current to flow between the electrodes. This current is amplified by a FET or darlington transistor and can be read out on a DMM or a analog meter. Check out Charles Wenzel’s excellent page complete with explanation and circuits

Tubes

Geiger counters need geiger tubes to detect radiation. These tubes are available on eBay and other online marketplaces. When buying tubes pay attention to the following:

A lot of tubes have a thin metal wall on the outside. These tubes will only detect hard beta and gamma radiation. Don’t worry, they will usually detect most radioactive sources like uranium glass and radium clocks. When they have a large volume they will also detect background radiation easily. Good examples of tubes of this kind are the Russian SBM-20 and SI29-BG tubes. You can get them for about $/€15-20. I especially like the SI29-BG because it is small but quite sensitive.

There are also tubes that have a glass wall. These tubes are sensitive to hard beta and gamma when they have a large volume and thin enough walls, but there are also small glass tubes like the SI-3BG that are really deaf and only suited for testing purposes or high radiation environments. If you feel like experimenting, multiple insensitive tubes can be connected in parallel to create a more sensitive “virtual tube”. Not all glass tubes are bad. some such as the German RFT MKD VA-Z-115.1 are quite sensitive and are highly recommended in my opinion.

The best tubes in my opinion are tubes with a mica window. These tubes can detect (soft) beta, gamma and (often) alpha radiation. There are end window tubes wich have a small mica window and so called pancake tubes which are flat and have a large mica window. These are the most sensitive tubes. Common Russian tubes of this type are the round SI-8B, SI-13B and the rectangular SBT10/11. Unsurprisingly they are also the most expensive and sought after. Most pancake tubes sell for $100 or more. However, a probe with even a small pancake tube can be of high value. It easily detects granite countertops, small amounts of potassium-40 and any object made of porcelain. You can “swipe” areas and objects (your hands!) for contamination.

Scintillation detectors

When it comes to radiation detection, scintillation counting is one of the more advanced technologies an amateur can get access to. Scintillation detectors usually only detect gamma radiation, but are able to discern between different energies of gamma rays. This is really interesting because this makes it possible to see what kinds of isotopes are present in a sample, as almost any decay reaction sends out gamma rays of which the energies are known.

Scintillation probes can be found online but are quite expensive. You will also need a (geiger) counter which can supply a scintillation probe with the right voltage and sufficient current. Some Ludlum and modified CDV-700’s have this capability. Keep in mind that counters only provide a gross count, so you can only detect the gross amount of gamma rays for a sample or environment.

With a computer, a proper high voltage power supply and a scintillation probe you can identify isotopes by performing gamma spectroscopy. Nowadays there is software available which enables you to perfom this measurement without buying an expensive multichannel analyser. To couple a scintillation probe with a PC’s soundcard you can use this schematic, and this software to perform pulse analysis and displaying of gamma spectrums.

In the future I will dedicate a separate blog post about this subject as this is only a short explanation of scintillation detection.

Sources

Well, what is a Geiger counter with nothing to detect? Some common sources that can be checked out:

Radium clocks can be found in antique shops all over the world. Be careful, it is best not to open clocks and watches that contain this material. The radioactive paint has often flaked, causing radioactive dust to accumulate on the clock face and glass. Radium in quantities present in clocks and watches is readily detected as Radium is highly radioactive and sends out lots of alpha radiation and beta radiation from its decay products. Just take your counter everywhere and you will encounter this source.

Uranium can be bought online as mineral specimens sutch as autunite, uranophane etc. Also a large accessible source of this material is uranium glass, also known as Vaseline or depression glass which glows brilliantly green under a UV torch (alwasys handy to have). A less common source is porcelain with uranium glazing such as Fiestaware. There is even Dutch porcelain with uranium glazing.

Thorium is found in older gas mantles, various thorium containing minerals such as thorianite and monazite. Also found in arc welding rods where it is still legal to use this material.

Artificial isotopes and elements such as Americium-241 and Cesium-137 can also be found when looking properly. Americium is found in smoke detectors but is often phased out in favor of optical smoke detectors. Radioactive cesium can be found in various radio tubes and spark gaps. Cesium is a handy source because you can calibrate a spectrometry setup with it as it has two distinct peaks in the gamma spectrum.

Radio tubes in general, especially old ones can contain all kinds of isotopes, Radium-226, Cesium-137, Cobalt-60, Krypton-85 and Tritium. Not all these isotopes are easily detected but in a big pile of tubes there should be something to find of interest. Look for the so called spark gap tubes and rectifiers.

And lastly, some low level sources that can be found practically anywhere without people even realising it. Kitty litter, instant coffee, granite, porcelain all contain amounts of Thorium and/or uranium that can be detected with a sensitive pancake probe.

Some links to get you started

These are forums/mailinglists which contain a LOT of information:

Yahoo Groups: Gammaspectrometry , CDV-700 Club, GeigerCounterEnthusiasts

Fusor.net Neutron and radiation detection subforum

To buy simple geiger counters, check out:

eBay: Geiger Counters

Helmut Singer sells decent priced, checked German geiger counters without eBay troubles. Look for the FHT40 and SV500.