Problem 4 Activation Analysis (10 points) Activation analysis is one experimenta
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Problem 4 Activation Analysis (10 points) Activation analysis is one experimental technique for measuring the flux in a system. The basic idea involves irradiating a suitable material for some time period in the reactor where, via neutron capture (or some other neutron reaction), a radioactive nuclide is produced. Since the production rate of the radioisotope is proportional to the flux level, R = N sigma c phi. measuring the activity of the radioactive nuclide after it has been removed from the reactor should allow one to indirectly determine the neutron flux. As an example, consider the activation of gold in a thermal flux. Au-197 captures a thermal neutron to form radioactive Au-198 which has a half-life of 2.7 days. The Au-197 thermal capture cross section is about 85 barns (after proper averaging over the thermal region). A gold foil with a mass of 0.05 g is exposed for 45 minutes in the beam port region of a reactor operating at constant power. The activity of the foil was measured to be 333 Bq 20000 decays/mm 26 hours after removal from the reactor. The goal of the experiment was to measure the thermal flux at the location of the foil activation. With the above information, perform the following: 1. Describe the basic processes that are involved here -- that is, what is really happening? In doing this, make a well-labeled sketch (or preferably a formal plot) of the expected Au-198 activity versus time, including both the irradiation period and the time interval after removal from the reactor. 2. Based on your understanding of the problem from your description in Part a, estimate the value of the thermal flux in the beam port region based on the measured Au-198 activity. Be sure to identity any assumptions made as part of your analysis. Be formal...Explanation / Answer
Neutron activation analysis (NAA) is a nuclear process used for determining the concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The method is based on neutron activation and therefore requires a source of neutrons. The sample is bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well known. Using this information, it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it. A particular advantage of this technique is that it does not destroy the sample, and thus has been used for analysis of works of art and historical artifacts. NAA can also be used to determine the activity of a radioactive sample.
If NAA is conducted directly on irradiated samples it is termed Instrumental Neutron Activation Analysis (INAA). In some cases irradiated samples are subjected to chemical separation to remove interfering species or to concentrate the radioisotope of interest, this technique is known as Radiochemical Neutron Activation Analysis (RNAA).
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