A common method for measuring radioactive radiation is based on a counter tube according to the Geiger-Müller principle. This counter tube usually consists of a cathode (-) metal tube, which is filled with a noble gas – helium, argon or neon – under reduced pressure (about 200 hPa). Through the middle axis of this tube, a wire (usually tungsten) as the anode (+) is stretched. Between the cathode and the anode, a high voltage of several hundred volts is applied.
With a Geiger counter, ionizing radiation can be measured. Generally, ionizing radiation is any radiation capable of knocking out an electron from an atom or molecule. Such radiation is also based on radioactive substances, which is why one often speaks of “radioactive radiation.” Strictly speaking. However, only the radioactive material that emits ionizing radiation is not the radiation itself. When radioactive substances are decomposed, ionizing radiation is generated, which is subdivided into alpha radiation, beta radiation and gamma radiation. Alpha radiation and beta radiation consist of charged particles. Gamma rays consist of photons or quanta.
Geiger counter in action
Now, if an ionizing α- or β-particle or an ionizing γ-quantum penetrates the Geiger-Müller counter tube, this splits off when penetrating electrons from the atomic nuclei of the noble gas contained therein. Due to the high voltage in the counter tube, the electron (-) is pulled to the anode with great acceleration (+). The electron is accelerated so strongly that further particles are ionized upon contact with the electron and thus a whole cascade of secondary particles is formed, which lead to a “saturation” of the counter tube. These interactions create a chain reaction: Countless electrons reach the anode wire and the ionized gas becomes conductive for a short time. The circuit closes.
The electrons flow through the resistor to the positive pole. According to Ohm’s law, a tension arises over this resistance. This voltage is amplified electronically via a parallel connected amplifier and finally measured by a counter or acoustically reproduced via a loudspeaker by a click sound. This creates the typical crackling sound of a Geiger counter. Cracking signals that an ionizing particle has entered the Geiger-Müller counter tube.
What is the “dead time”?
After this gas discharge follows a short so-called “dead time,” in which the counter tube is unresponsive for further impulses: In the described chain reaction caused by the splitting off of the electrons correspondingly many positively ionized particles (+). The positive ions are attracted to the cathode (-) but move slower than the electrons. Thus, the cathode is briefly shielded from a large number of ions (in the range of 108 particles). By shielding the cathode, the voltage between the anode and the cathode in the Geiger-Müller tube drops drastically. As a result, newly ionized particles and electrons are no longer accelerated accordingly; the chain reaction stops. The circuit is interrupted and the counter no longer measures a signal. Thus, if a second ionizing particle enters the Geiger-Müller tube immediately after the first, it may not be registered. This interval is called “dead time” (depending on the design approx. 0.1-0.3 milliseconds). Only when the ion cloud has discharged at the cathode, a new measurement can be made. By special additives in the counter tube, such as halogens or alcohol vapor, the gas discharge is “deleted” again, so that the process can be triggered again. The dead time deviation of the measurement results can be approximately corrected using the following formula: By special additives in the counter tube, such as halogens or alcohol vapor, the gas discharge is “deleted” again so that the process can be triggered again. The dead time deviation of the measurement results can be approximately corrected using the following formula: By special additives in the counter tube, such as halogens or alcohol vapor, the gas discharge is “deleted” again so that the process can be triggered again. The dead time deviation of the measurement results can be approximately corrected using the following formula:
|M =||M ‘|
|1 – T * M. ‘|
M ‘= measured counting rate of the Geiger counter
T = dead time of the Geiger counter
M = corrected counting rate of the Geiger counter
Measurement of different types of radiation
By using special counting tube gases such as boron trifluoride, neutron radiation can also be detected due to the nuclear reaction taking place. Counter tubes according to the Geiger-Müller principle are used in the commercially available Geiger counters, which are used for the detection of radioactive radiation, for example in mining, in nuclear medicine and environmental and radiation protection. Many commercially available Geiger counters can either detect only alpha radiation or are only suitable for beta and gamma rays. With the Gamma-Scout Geiger counter, for example, the permeability can be restricted to the types of radiation β and γ with the aid of a 1 mm aluminum foil that can be pushed in front of the inlet opening. Optionally, a 3 mm thick aluminum foil can be pushed in front of the opening,
Units of measurement for radioactivity and radiation
The radioactivity of a substance is given by the number of particles decaying per second. Radioactivity = number of decays / second. 1 Becquerel corresponds to the decay of exactly one core per second.
A simple Geiger counter counts the number of received pulses or converts the number of pulses over a specific period into a pulse rate in CPS (counts per second). The longer the measurement, the more accurate the pulse rate can be determined.
Many Geiger counters can also measure the radiation exposure of the human body. The amount of radiation absorbed by the human body is given in Sievert (Sv), where 1 Sv corresponds to an absorbed dose of one joule per kilogram (J / Kg). This so-called equivalent dose is the energy absorbed by the body of the ionizing radiation. Since alpha, beta and gamma radiation have different detrimental effects on the body; the absorbed dose is multiplied by a correction factor that takes into account the biological effects of the respective types of radiation.