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Neutron Capture Cross Sections of Selenium Isotopes

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Neutron Capture Cross Sections of Selenium Isotopes Neutron Capture Cross Sections of Selenium Isotopes Howard D. Dearmon Department of Physics Oregon State University 1 Oct 2018 Advised by Dr. Kenneth Krane In partial fulfillment of the requirements of PH403 Table of contents Abstract ....................................................................................................... 1 Introduction ................................................................................................. 2 Theory ......................................................................................................... 5 Methods .....................................................................................................10 Results ........................................................................................................18 Conclusion ..................................................................................................20 References ..................................................................................................21 Acknowledgements ....................................................................................22 Abstract The thermal cross sections and resonance integrals for neutron capture by 74,78,80,82Se were measured using the same procedure and equipment for all of the isotopes. Uncertainty and absence of accurate previous values for several isotopes introduces a need of measuring these parameters using similar techniques. This was done using OSU’s TRIGA reactor and four of its facilities: the fast pneumatic transfer facility, cadmium-lined in-core irradiation tube, G-ring in- core irradiation tube, and thermal column. These new measurements are made using high resolution detectors, improved parameter values and more consistent methods for each isotope, yielding more precise values than were made in past experiments. 1 Introduction In the neutron capture nuclear reaction, a neutron collides with an atomic nucleus, which either absorbs the neutron to form a heavier nucleus or scatters it away. The neutron capture cross section is defined as the probability that the nucleus of an atom will absorb a neutron incident upon it. This probability is expressed as an area and measured in units of barns (10-28 m2). Since neutrons carry no electric charge, they are more easily able to penetrate an atom’s nucleus than other charged particles. This absorption, being dependent on the neutron’s energy, occurs in stars and nuclear reactors. The capture cross sections are further divided into two measures, which are the effective cross-sections for the nucleus at different neutron energies; the thermal neutron capture cross section (σ) for the lower energy thermal neutrons , and the resonance integral (I) for higher energy neutrons. This resonance integral consists of summing up all the contribution of absorption peaks at specific neutron energies for a particular atom. Accurate measurements of neutron capture cross sections for different elements are crucial for applications such as neutron activation analysis, where the elements in a material capture neutrons and change to radioactive isotopes of that element and can be used to find the composition of bulk materials. This technique is used to measure the amount of selenium in things like crops and cereals due to its importance for a healthy immune system and protection against certain diseases and cancers [1]. In this series of experiments the neutron capture cross-section (NCCS) for the selenium isotopes 74,78,80,82Se, which are irradiated to produce the 2 radioactive isotopes 75,79m,81g,81m,83gSe, are measured. A list of the different isotope reactions is found in the following theory section. The neutron capture cross section of these selenium isotopes has been measured irregularly [2]; measurements for certain isotopes vary significantly from experimenter to experimenter and in some cases there is only one measurement or none. Even the values that appear accurate due to agreement with others were in some cases taken over 35 years ago and can be improved using current technology like improved high resolution solid-state Ge detectors and more accurate values of parameters like half-lives and isotope abundances. Past measurements studied only a couple of the isotopes at a time, making comparisons of their results with others less reliable than doing all 7 of them with the same sample and methods. It is with this reasoning that determining the NCCS for most of the selenium isotopes using the same procedure and technique at the same time is necessary to ensure accurate values. Several isotopes of selenium have metastable states; these metastable states are excited states that, due to greater spin difference between the metastable state and the ground state, are occupied for far longer than other excited states. This occupation time is on the order of minutes for 79m,81mSe, whereas regular excited states are occupied for around 1-10 ns [3]. These metastable states then decay and are fed back into the ground state, which increases the difficulty of finding the number of atoms in the ground state. The correction and its theory are explained in the following theory section. With this correction, the cross-section of isotopes like the 81gSe decay can be found instead of just the ratio of metastable state cross-section to the ground state cross- section. 3 To find the values of σ and I of the selenium isotopes several samples of selenium oxide containing the isotopes were irradiated by the OSU TRIGA reactor using the facilities described in the methods section. 198Au, 97,95Zr and 60Co samples were also irradiated along with the selenium. Since the cross-sections and resonance integrals of these elements are well known they can be used to determine the thermal flux and epithermal flux used to find the cross-sections and resonance integrals of the selenium isotopes; these fluxes will be explained in more detail in the following sections. Next the irradiated samples were put in front of the germanium detectors and the specific gamma-rays of each isotope are counted using a multi-channel analyzer computer program called MAESTRO. The activity is then calculated and used to find the cross-sections and resonance integrals; this is described in more detail in the theory and method sections. 4 Theory To find the cross-sections and resonance integrals for each of the selenium isotopes an indirect approach is used, as they are not quantities that can be directly measured. One parameter that can be used is the activity of each selenium sample. This can be measured using a high-resolution Ge detector to measure the gamma rays emitted from the radioactive isotope. This process is shown in Figure 1. Figure 1: Decay process for neutron capture 5 When an isotope is bombarded by neutrons with a high enough energy, its nucleus can sometimes capture this neutron and change into a new nucleus. This new isotope is in an unstable state and will release its extra energy to get to a more stable lower energy state. It does this by emitting high energy gamma rays while inside the reactor, bringing it down to what is known as a “metastable” state. This state, while above the ground state, is lower in energy than the initial higher energy resonance state from before and longer lasting as well. Finally the isotope transitions to the ground state by emitting more gamma rays; it is these gamma rays that are measured and used for calculating the neutron capture cross sections. Sometimes, instead of a metastable state, the isotope transitions to a radioactive ground state that beta decays into a daughter element like bromine or krypton. After this beta decay occurs the new element is in an excited state that decays to the ground state; the gamma rays it emits in this process are also measured. There are two parts that make up the total neutron capture cross section: the thermal neutron cross section, which is the cross section of the isotope when it absorbs a low energy neutron in the thermal temperature range, and the resonance integral, which is the sum of multiple resonance peaks due to the isotopes interaction with epithermal neutrons. Higher energy neutron interactions are neglected due to the probability of interactions being minuscule. Figure 2 illustrates the different areas of cross sections we look at. 6 Figure 2: Cross section of 74Se with respect to energy. The thermal cross section (below 1 eV) depends on the inverse velocity of the neutron 1/v, making it easier to calculate without having to integrate the many resonances like in the epithermal neutron range. We call this value the thermal cross section σ. The individual resonance peaks in the epithermal range can’t be individually isolated so they are summed up into an integral we call the resonance integral Ӏ. Since the neutron capture cross section can’t be directly measured we need to calculate it using certain equations and other parameters that can be directly 7 measured, mainly the thermal and epithermal fluxes. We begin by using the activity equation: -λt Activity=N0(σфth + Iфepi)(1-e ) (2.1) Here N0 is the number of atoms that is initially present in the sample material, фth is the thermal flux, фep is the epithermal flux and λ is the exponential decay constant. Neutron flux is defined as the amount of neutrons passing through a square centimeter area per second, usually measured in units of cm-2s-1. We take these measurements in three different types of locations inside the reactor: The Thermal Column where most of the neutrons captured are in the thermal
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