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Unformatted text preview: Physics 8B Lab 10 Radioactivity rev 4.0 Lab 10 Radioactivity Introduction It is possible for cancer to be induced by radiation. To protect people working in occupations where they may be exposed to radiation in an ongoing way, limits of exposure are regulated. Radiographers and dentists need to minimize exposure to X rays, while people using radio-nuclides (e.g. molecular biologists sequencing DNA) are also interested in ensuring that their exposure is below what is considered a safe upper limit. The limit in the US is currently 5mSv per year. Increasing the distance from the radiation source, and/or shielding can limit exposure to radiation. The shielding affects of increasing the distance from the source to the detector (which may be you), and placing materials between the source and the detector, rely on the radiation interacting with the air or a shielding material. There are three processes in which the photons of radiation interact with matter, thereby losing energy: (1) Compton scattering - ray photons collide with free electrons, or valence electrons which are loosely bound. In the collision part, the energy and momentum of the photon is transferred to the electron. The scattered photon leaves with less energy than before the collision. The probability of this happening depends on the number of free electrons in the path of the photon. (2) Photoelectric effect - When a ray photon collides with a tightly bound electron it is possible for all of the photon energy to be transferred to the electron. This occurs mainly with electrons in the inner shells of heavy atoms, like lead. At a given photon energy the probability increases in proportion to the fourth power of the atomic number, Z 4 . The probability also depends on the number of such electrons in the path of the photon. (3) Pair production - It is possible for a ray photon to be transformed into an electron and a positron as it moves in the strong electric field close to the nucleus. The photon energy must be greater than the energy equivalent of the mass of the electron and of the positron, i.e. 1.022MeV. 137 Cs is a beta emitter, the - decay of which leads, in 94% of decays, to the first excited state of the 137 Ba nucleus, at an excitation energy of 0.662MeV. The subsequent de-excitation involves emission of a 0.662MeV ray. The remaining 6% of - decays go directly to the ground state of 137 Ba. In this lab we will be detecting the 0.662MeV rays produced from the de-excitation of 137 Ba. 137 55 CS 137 56 BA* + -1 e + 137 56 Ba* 137 56 Ba + where * indicate an excited state. Physics 8B Lab 10 Radioactivity rev 4.0 Part I: Radiation - Distance Dependence The intensity of electromagnetic radiation, including radiation, is defined as the power that strikes a unit area. Because energy is always conserved, the total energy passing through a sphere of any given radius centered on the source is the same. passing through a sphere of any given radius centered on the source is the same....
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