Detection and uses of radioactivity

Geiger counters are often called survey meters in the industry. 

Normally, a sample of argon gas will not condnct a cnrrent when an electrical potential is applied. However, the formation of ions and electrons produced by the passage of the high-energy particle allows a momentary cnrrent to flow. Electronic devices detect this current flow, and the number of these events can be counted. Thus the decay rate of the radioactive sample can be determined. 

Unless otherwise noted, all art on this page is Cengage Learning 2014. 

Brigham Young researcher Scott Woodward taking a bone sample for carbon-14 dating at an archaeologicai site in Egypt. 

Radioactive nuciides are often called radionuclides. Carbon dating is based on the radionuciide iC. 

Another instrument often used to detect levels of radioactivity is a scintillation counter, which takes advantage of the fact that certain substances, such as zinc sulfide, give off light when they are struck by high-energy radiation. A photocell senses the flashes of light that occur as the radiation strikes and thus measures the number of decay events per unit time. 

Radiocarbon dating is based on the radioactivity of the nuclide isC, which decays via /3-particle production  

Carbon-14 is produced in the atmosphere when high-energy neutrons from space collide with nitrogen-14  

A schematic representation of a Geiger-Muller counter. The high-energy radioactive particle enters the window and ionizes argon atoms along its path. The resulting ions and electrons produce a momentary current pulse, which is amplified and counted. 

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Autoradiography is the method of choice for detection and quantification of radioactive labeled samples in electrophoresis gels, blotting, or hybridization filters using X-ray films. The main drawback of X-ray films is a limited linear range of image density with respect to the amount of radioactivity. 

All the instruments for the detection and measurement of radioactivity are based on the physical processes of radiation interaction with matter. The understanding of these processes has led to the development of many types of radiation detectors. The methods used for the measurement of radionuclide concentrations in various matrices are numerous. Some are better then others, but the best is always a combination of several techniques. 

Nuclear Stability and Radioactive 19.4 Detection and Uses of 19.6 Nuclear Fission and Nuclear Fusion... 

Over the last 30 years or so the detection of radioactivity directly on TLC plates has taken dramatic leaps forward. Prior to the introduction of radiation detectors, the classical method used for the detection and quantitation of radioactivity on a plate involved exposure to x-ray film as the first step. This could take from a few hours up to one or two months, and this technique only located the radioactivity. The second step after location was quantitation, which was achieved by removing the zone of interest, either by scraping the silica gel off or by cutting away if the plates were aluminum- or plastic-backed, followed by liquid scintillation counting. Such a procedure is extremely labor intensive and is limited in terms of accuracy and resolution (see above). 

The use of radioactive tracers was pioneered by Georg von Hevesy, a Hungarian physical chemist, who received the Nobel Prize in 1943 for his work on radioactive indicators (1). Radioisotopes have become indispensable components of most medical and life science research strategies, and in addition the technology is the basis for numerous industries focused on the production and detection of radioactive tracers. Thousands of radioactive tracers have been synthesized and are commercially available. These are used worldwide in tens of thousands of research laboratories. 

The methods for detection and quantitation of radiolabeled tracers are deterrnined by the type of emission, ie, y-, or x-rays, the tracer affords the energy of the emission and the efficiency of the system by which it is measured. Detection of radioactivity can be achieved in all cases using the Geiger counter. However, in the case of the radionucHdes that emit low energy betas such as H, large amounts of isotopes are required for detection and accurate quantitation of a signal. This is in most cases undesirable and impractical. Thus, more sensitive and reproducible methods of detection and quantitation have been developed. 

In 1987, CL started to be applied in DNA hybridization assays as an alternative to the use of radioactive tags. These assays are based on the specificity of a binding process that of DNA strands for each other. An unknown DNA can be identified with the Southern blot method in which the strands of the analyte are separated and allowed to interact with labeled probe DNA strands on nitrocellulose filter paper. If the label on the probe is detected, the DNA can be identified and, in some cases, quantitated. Conventionally, radioactive tags were used be-... 

A prerequisite to pharmacokinetic/pharmacodynamic studies is the availability of a sufficiently selective and sensitive assay. The assay must be capable of detecting and accurately quantifying the therapeutic protein in the presence of a complex soup of contaminant molecules characteristic of tissue extracts/body fluids. As described in Chapter 7, specific proteins are usually detected and quantified either via immunoassay or bioassay. Additional analytical approaches occasionally used include liquid chromatography (e.g. HPLC) or the use of radioactively labelled protein. 

Antibody molecules have no inherent characteristic that facilitates their direct detection in immunoassays. A second important step in developing a successful immunoassay, therefore, involves the incorporation of a suitable marker . The marker serves to facilitate the rapid detection and quantification of antibody-antigen binding. Earlier immunoassay systems used radioactive labels as a marker (radioimmunoassay RIA) although immunoassay systems using enzymes (enzyme immunoassays EIA) subsequently have come to the fore. Yet additional immunoassay systems use alternative markers including fluorescent or chemiluminescent tags. 

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