Dosimeters are rugged, precision instruments about the size of a pocket fountain pen, which are used to measure absorbed doses or quantities of gamma (X-ray) radiation. this is different from a Geiger counter. A Geiger counter shows the intensity of radiation at the time that you are looking at the meter. This does not give you the amount of radiation that you were exposed to over a period of time or the "dose" that you received.
This is a picture of a dosimeter scale.
Geiger counter readings are shown as MR/HR or R/HR this means that if you stay in an area that reads 1 R/HR after one hour you will get a dose of 1 R. When a Dosimeter gives a reading of 1 R it means that you already received a dose of 1 R.
Geiger counters detect radiation by using a tube called a Geiger Mueller tube. A high voltage is impressed on the elements of the tube. Whenever gamma Ray's enter the tube they ionize the gas in the tube and the voltage is discharged causing a signal ( or pulse )that is detected by the circuitry and causes the meter to register. If the Gamma rays are constant the meter will register the same amount. If the radiation changes the meter will register a different amount either higher or lower. As you can see this will show you the intensity of the radiation but not to the total amount that you receive.
Dosimeters on the other hand register the total amount of radiation that enters the device. Before dosimeters can be used to measure gamma radiation, the self-contained fiber voltmeter and ion chamber (electrometer) must be charged to approximately 165 volts D.C. this is called "Zeroing". Each time a particles of radiation enters the dosimeter it will discharge a small amount of the voltage charge. After enough radiation entered the dosimeter the line will move across the scale indicating the amount of dose received.
The picture above shows a charger that is used to zero the Dosimeter before it is used. The charger can also be used to read the Dosimeter. The Dosimeter has a viewing lens at one end and a glass port at the other so when you look into the lens at a light source you will see the scale as shown above. The vertical line will indicate the total exposure that you received. The charger has a built in light so that you can see the scale.
Here are some of the sizes that Dosimeters come in and their general application.
0 - 20mR (measures background radiation)
0 - 200 mR (Low-range) Measures background radiation
0 - 5R (Recommended for those near nuc. power plants)
0 - 20R (Recommended for those near nuc. power plants)
0 - 100R (Recommended for those handling radioactive materials)
0 - 200R (Recommended for terrorist/war concerns