The Detection and Measurement of Radioactivity

1 Identify the function of a Geiger counter and describe how it operates. 

There are several ways to detect radioactivity. Perhaps the most obvious, but not necessarily the most convenient, is exposing photographic film, the very property that led to its discovery (Fig. 20.3). Another is the cloud chamber, an enclosed container 

Ionizing radiation passes through the thin window... 

How do Geiger counters and scintillation counters measure radioactivity Precisely, what do they count Do they count radiations or do they count something that is proportional to radiation Suggest units for radioactivity as it would be measured by either of these counters. 

Film badge. Workers in industries where exposure to radiation is known to be above background levels are required to monitor their cumulative exposure dosage. 

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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. 

Certain interactions with matter of the radiation accompanying the decay of unstable nuclides (a- and /9-particles, y rays) are the basis for the detection and measurement of radioactivity These include photochemical processes, by which a radioactive sample placed in close contact with photographic emulsion causes blackening of the latter upon development (autoradiography) gas ionisation and the deriving production of current pulses that can be analysed and measured by suitable devices excitation of orbital electrons of special molecules, either in a crystalline form or in solution, with subsequent emission of light pulses to be converted into electric current by a photoelectric detector (scintillation)... 

Particle detectors are instruments designed for the detection and measurement of sub-atomic particles such as those emitted by radioactive materials, produced by particle accelerators or observed in cosmic rays. They include electrons, protons, neutrons, alpha particles, gamma rays and numerous mesons and baryons. Most detectors utilize in some way the ionization produced when these particles interact with matter. 

DIFFERENCES BETWEEN IMAGING MODALITIES FOR THE DETECTION AND MEASUREMENT OF DEPOSITED RADIOACTIVITY... 

As an analytical method activation analysis determines the concentration of an elemental component of a sample material by inducing radioactivity in an isotope, or isotopes, of that element by means of a nuclear particle bombardment. The detection and measurement of the... 

Activation analysis methodology is quite similar to other instrumental analysis methods that use energy sources of either light, heat, X rays, or electricity to irradiate a material to bring about the emission of characteristic radiations. The detection and measurement of these radiations can then be used to indicate the amount of an elemental species in the material. Activation analysis requires a source of nuclear particles, such as neutrons, charged particles, or gamma rays, to bombard (or irradiate) the sample material to make it radioactive. 

After the irradiation remove each sample portion from its bombardment container in preparation for the detection and measurement of the specific radioactivity induced in the element of interest. Two methods of preparation are usually followed (o) A simple transfer of the radioactive material to a counting vial and the use of a direct (or nondestructive) technique of radiation detection and measurement and (b) the use of chemical separations to isolate the radioelement free from all other induced radionuclides prior to the detection and measurement of its radiations. 

Contamination on the hands can be detected with Geiger Muller or proportional counters. For detection and measurement of air-borne contamination, filters may be used. The activity on the filters can be measured continuously by passing the filters througli a proportional counter. Radioactive contamination on laboratory benches, instruments or floors can be detected by the simple method of wiping the surface to be checked for contamination is wiped with a piece of filter paper, and the filter paper is measured with a suitable detector for a, p or y activity. 

The methods used for the detection and quantification of radionuclides and labeled compounds are determined by the type of emission (jS or y), the energy of the radiation, and the efficiency of the system by which it is measured. Commercially available instrumentation is generally used, with exception of some research work and highly specialized routine analytical application. Detection of radioactivity can be achieved in all cases with the Geiger-Miiller counter. However, for the weaker-emitting radionuclides, i.e., H, and large amounts of radionuclides are required for detection of a signal. In most cases this is both undesirable and impractical. 

NAA is the most common form of activation analysis. The activation reaction is induced by the interaction of a neutron with the nucleus of an analyte element. Depending on the energy of the incident neutron and the reaction cross sections of the target elements, different types of reactions can take place, leading to activation products as described in O Sect. 30.2. This reaction is commonly followed by the measurement of a nuclide-characteristic de-excitation step (radioactive decay). It is this characteristic gamma-ray decay that is commonly used in the detection and determination of the element of interest. 

Radiation dose rates, radiation doses and radioactive substances in systems and rooms at the plant and releases of radioactive material should be monitored by this instrumentation. Air monitoring systems should be provided to detect radioactive material in the air of the rooms and in the ventilation systems. Radiation measurements should be made on process streams to... 

The method has been used for thyroid gland and other tissues, and for blood, bile, and urine. The preparation of the sample is different for each material, but all other steps are identical, including chromatographic separation, identification of spots, and detection and measurement of the radioactivity. 

The bacterial CAT enzyme catalyzes the transfer of acetyl groups to chloramphenicol from acetyl coenzyme A (acetyl CoA). In a typical assay, this reaction is monitored with relabeled chloramphenicol After separation by thin-layer chromatography (TLC), the acetylated and nonacetylated forms can be distinguished by autoradiography, and quantitation is achieved by isolating the forms and measuring their radioactivity in a scintillation counter. Quantitative CAT assays have been performed on Drosophila tissue culture and dissociated cell extracts (Di Nocera and Dawid 1983 Benyajati and Dray 1984 Thummel et al. 1988 Krasnow et al. 1989 Ye et al. 1997). CAT can also be detected with commercially available antibodies. In addition, a nonradioactive CAT assay exists that utilizes a fluorescent chloramphenicol derivative (Molecular Probes). 

Three treated cats were sacrificed 0.5, 1, 2, 5, and 10 days after treatment. Radioactivity in urine and feces collected over the 10-day period accounted for 28% and 19% of the applied dose, respectively, but no radioactivity was detected in expired air. Radioactivity in analyzed tissues reached maximal levels at 24 hours (accounting for 8.7% of the applied dose). These data are inadequate for quantitative measurements of the extent of dermal absorption of TOCP, because a significant traction of the applied radioactivity was not accounted for in the analysis, and some of the TOCP may have been ingested by the cats during grooming. 

These are unstable forms of elements which decay by the emission of radiation. A radioactive isotope of an element behaves chemically in the same way as the non-radioactive form, but its radiation may be detected and measured by a suitable instrument. In the rubber industry radioactive isotopes are used in beta ray thickness gauges, in studying the precise role of sulphur in vulcanisation, in the speedy determination of tread wear in tyres, etc. See Beta Rays. 

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