A counter is one of three main types of detectors that record different types of signals. The activity or intensity of radiation is measured in counts per second (cps), which expresses a rate of counts per unit time registered by a radiation monitoring instrument. In general, commonly used quantities are:
- counts per minute (cpm)
- counts per second (cps)
The best-known counter is the Geiger-Müller counter. In radiation counters, the generated signal from the incident radiation is created by counting the number of interactions occurring at the sensitive volume of the detector.
The unit of counts per second detected by a device is somehow proportional to the activity of a measured sample. But note that this proportionality is also determined by the distance between the detector and the sample and the detection efficiency.
What is one count
Let’s assume gaseous ionization detectors. A basic gaseous ionization detector consists of a chamber filled with a suitable medium (air or a special fill gas) that can be easily ionized. As a general rule, the center wire is the positive electrode (anode), and the outer cylinder is the negative electrode (cathode), so that (negative) electrons are attracted to the center wire. Positive ions are attracted to the outer cylinder. The anode is at a positive voltage for the detector wall. As ionizing radiation enters the gas between the electrodes, a finite number of ion pairs are formed. Under the influence of the electric field, the positive ions will move toward the negatively charged electrode (outer cylinder), and the negative ions (electrons) will migrate toward the positive electrode (central wire). Collecting these ions will produce a charge on the electrodes and an electrical pulse across the detection circuit. However, it is a small signal, and this signal can be amplified and then recorded as one count.
