In general, the Geiger counter and also the ionization chamber are types of gas ionization detectors. To recap, the nucleus of an atom (the nucleus) is surrounded by electrons in orbit, like planets around a sun. Electrons have a negative charge and normally cancel out an equal number of positively charged protons in the nucleus. But if an electron absorbs energy from radiation, it can be ejected from its orbit.
This action is called ionization and creates an ion pair, a negatively charged free electron and a positively charged atom. Geiger counters detected gamma rays by ionization, which they produced in a gas-filled tube. Although the Geiger counter previously originated as a heavy instrument in the laboratory, shortly after 1934, Ellsworth, at the Geological Survey of Canada (GSC), built a “portable” version (9.5 kg) suitable for use in field measurements (Haycock, 195). Radiometric techniques in Earth sciences (outside the laboratory) probably began with the search for radioactive minerals at that time, but exploration of uranium deposits after 1943 greatly expanded the use of Geiger counters.
Operating at hundreds of volts between an anode and a cathode, an electric field swept through ionization to produce a pulse of current and voltage that could be counted. The detection efficiency (ratio of recorded counts to the number of gamma rays that hit the detector) was approximately 2%, which is extremely low. There was no ability to distinguish differences in gamma ray energies and only the total count rate (TC) was measured. Dead time (time to process a pulse, during which no other gamma rays (pulses) can be counted) was also considered high at about 0.1 ms. One application where Geiger tubes are still used today is in the gamma ray recording of wells in wells that penetrate uranium deposits.
There, the level of gamma radiation is so high that the inefficient detector is an advantage, avoiding saturation (overload) effects in more efficient detectors. The pulse amplitudes of all types of ion chambers are relatively small. In theory, the maximum amplitude of the signal accumulated from ion pairs produced by the interaction of, for example, an alpha particle in air along its path within the chamber is in the order of 10−5 V. Such a signal can be processed, but rather sophisticated electronic systems are required. The pulses of a single-photon interaction are a hundred times smaller, and successful and accurate amplification is difficult and sometimes even impossible.
Internal amplification within the detector volume, which is described in the section of this chapter dealing with proportional counting tubes, helps to overcome these problems. Whole body count is performed in Helsinki, Liviisa and Olkiluoto control groups annually. Lapps that constitute a risk group for the incorporation of radiocesium are monitored in cooperation with the University of Helsinki. Two commonly used methods are gamma ray recording and neutron recording. In the first case (gamma ray recording), natural radiation from the rock is used, while in the second case (neutron recording), a neutron source is used to excite the release of radiation from the rock.
The neutron source is usually a mixture of elements (of which beryllium and radium have been commonly used) and the method is a means of determining relative porosity or rock formations. An additional benefit of γ-ray recording is that the method helps define narrow formations, such as shale. Density can now be recorded with a new technique that uses radioactivity (density recording). The instrument consists of a gamma ray source of radioactive cobalt and a Geiger counter as a detector, which is protected from the source. The rock formation is bombarded with γ rays, some of which scatter from the formation and enter the detector.
The degree to which the original radiation is adsorbed is a function of the density of the rock. Test well sampling is another important method used in the search for oil (core sampling). Well data obtained from examining formation samples taken from various depths in the well are of considerable value in deciding future exploratory work. These samples can be cores, which have been taken from the well by a special core extraction device, or drill cuttings filtered from circulating drilling mud. The primary purpose of examining the sample is to identify the various strata in the well and compare their positions to the standard stratigraphic sequence of all sedimentary rocks found in the specific basin in which the well has been drilled. The water is treated before it is supplied as drinking water.
The goal of water treatment is to provide aesthetically acceptable and hygienically safe water to everyone. Water purification is the removal of contaminants (or reduction to an acceptable level) of untreated water to produce drinking water that is pure enough for the most critical intended uses (most common being human consumption). Water can be used for drinking, industrial use, medical facilities and many other desired end uses. For all of them, water treatment is essential.
A certain amount of treatment may also be required for used water before it can be returned to the ecosystem to ensure there is no adverse ecological impact. Ultraviolet water purification uses ionizing radiation to sterilize water. UV-C rays consist of light of wavelengths that fall within the range of 280 and 100 nm of the electromagnetic spectrum. The penetrating power of UV-C light is large enough to break DNA and RNA chains, killing harmful pathogens and disease-causing microbes inside the water.
It is environmentally friendly, since there are no chemicals involved. Plus, it doesn't add any flavor or smell. Irradiation is a similar practice that uses gamma radiation to sterilize medical equipment. Since this method only kills disease-causing microbes (bacteria and viruses), the water does not leak out of benign material and additional treatment is needed to make it drinkable.
It is worth noting that there is no residual radiation left after a substance is exposed to electromagnetic radiation, so no reading from a Geiger counter would be necessary.
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