The Geiger counter, a device based on the Geiger-Müller (G-M) tubes, is used to measure radioactivity. The tube is a sealed, low-pressure enclosure (glass or metal) filled with a low-pressure, inert gas such as helium, argon or neon. The principle works on the Townsend avalanche phenomenon, which involves changes in the voltage output from energized inert gas when radioactive particles or energies strike gas particles in the tube. These voltage changes, or spikes, can be measured. By counting the changes in the voltage (represented as "clicks"), it becomes possible to create an input into a variety of electronic devices such as speakers, LED's or even input into devices like the Arduino.
A Geiger-Counter consists of two distinct circuit sections: The high-voltage (HV) generator, and the Geiger–Müller (GM) tube interface. The high voltage section converts a relatively low-voltage source (usually around 9V DC) into a higher voltage (anywhere from 300V DC to 500V DC). The second section sets the voltage for the Geiger Tube based on the specs of the G-M tube being used and allows output of the voltage spikes in "clicks". The design used today uses the SBM-20 tube and its variants, which is one of the more common and easier-to-find G-M tubes available on eBay, through electronic wholesalers or included with some kits.
The CPM (Counts per Minute) or CPS (Counts per Second) what is recorded from the tube and it is then converted through a secondary output into a counter that can be used to manually or electronically convert to Seiverts (or more appropriately for the amateur experimenter, millisieverts. A good radiation dosage guide is listed here. Using a 74HC165 and a 555 Timer IC (sending the Geiger Counter click output into a 555 trigger) can make a pretty reliable cascading digit display on 7-element LED's. I won't go into specifics but more is included here. However, my focus was on interfacing a Geiger Counter with the Arduino.
There are lots of kits out there -- most of them relatively affordable and easy to assemble. Kits provide the advantage of a pre-defined set of components and a pre-designed circuit layout. Also, the component specs have already been tested, so when working with higher voltage along side components such as IC's, going with a known design at first is wise. Electronics Goldmine has a number of kits that include all components and some pretty easy-to-read instructions. I've assembled C8090 kit. It has a solid state SMD piezo component for low-voltage output clicks without requiring a separate counter.
However, if you are more experienced, you can look for the Arduino IDE Geiger Counter
as an option. It takes more expertise, however, it runs on a lower voltage and the Atmega-328 EEPROM which can be
programmed with an Arduino using a USB-TTL adaptor.
Arduino G-M Schematic: Open Source Published by RH Electronics
The Arduino kit has a few challenges, such as soldering the pin headers to the LCD readoutand soldering the chip mount holder. A good instructional video can be found on-line here. Practice first before assembling this kit, as it is pretty close-quarters in terms of the circuit board. The bonus is that an excellent example and instructions are provided here. Also, mounting the LCD display over complete Geiger counter circuit board involves being careful that the LCD display is not pushed all the way down becauuse the mount connections on the bottom of the LCD board will short with components on the main Geiger counter board. The ATmega-328 chip provided with kits are pre-programmed and are calibrated pretty well with any of the SBM-20 Geiger tubes. For calibrations some adjustment may be required with Potentiometer P2 and luckily, the instructions provided with the kit provide good explanation, especially if you intend on using other tubes or want much finer grained calibration. The instructions have both color photo images of assembled components and good user documentation.
Geiger-Muller tubes have limitations, especially when working with high end radiation. Tubes have limitations in terms of "dead rates", a bounce period after an event in which events are not counted -- which means that actual radiation count is an estimation and not a precise calculation. Also, tubes have a limited lifespan due to the breakdown of the inert gas over time -- especially dependent on how constantly exposed they are to events. For more on various types of G-M tubes, check out DIY Geiger Counter as a good starting point.
Potassium chloride, which is easy to find and relatively cheap, has a surprising level of radioactivity due to the presence of potassium isotope K-40. Some commercial smoke detectors contain small amounts of Americium-243, also a radioactive isotope. However, it is not as reliably found and more expensive. Granite countertops may contain some measurable level of radiation if they contain unique minerals which have higher-than-normal levels of low-level radiaoactive elements included. The sources I use in my experiments are radium watch dial hands (painted with radium many years ago so that they would glow in the dark). These can be purchased from some sources and represent very minor but measurable amounts of radiation.