Absorption of Beta and Gamma Rays

Absorption of Beta and Gamma Rays - Michael Lin Partners...

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Michael Lin Tuesday Section Partners: Josh Narciso, Bryant Rolfe Due Date: 04/11/07 Absorption of Beta and Gamma Rays Michael Lin The objective of this experiment was to study the behavior of beta and gamma rays passing through matter, to determine the endpoint energy of beta decay by measuring the range of beta particles from a given source, and to calculate the absorption coefficient in lead of the gamma radiation from a radioactive source . The maximum beta energy for the radioactive isotope Thallium-204 was measured to be 0.558 +/- 0.085 MeV, which was a 27.03% error from the actual value of 0.765 MeV. The absorption coefficient in lead of the gamma radiation from a Cesium -137 radioactive source was measured to be 1.118 +/- 0.071, and the corresponding energy of the gamma rays was 0.65 +/- 0.05 MeV. This result was in good agreement with the actual value for the energy of gamma rays, 0.662 MeV. INTRODUCTION Nuclei heavier than lead (and sometimes isotopes of lighter nuclei) have a probability of decaying spontaneously into smaller nuclei and one or more other light particles. The products of radioactive decay are separated into three main categories: alpha, beta, and gamma particles. Alpha particles are decay products that are also nuclei of helium atoms, consisting of two protons and two neutrons. Beta decay occurs usually when a nucleus consists of more neutrons than it can maintain in a stable state – the nucleus emits an electron, which also corresponds to the conversion of a neutron to a proton. Beta decay usually results in emission kinetic energies of the electrons of up to several million electron volts. Gamma particles are emitted in gamma decay when a nucleus is left in an excited state, usually after alpha or beta decay – the decay of the nucleus to a lower energy state releases a photon to conserve energy. Gamma ray photons typically also contain energy on the order of MeV (million electron volts). In radioactive nuclei, the source for emission of beta particles is the neutrons. Neutrons are stable in terms of electric charge, but not in terms of radioactivity – a typical free neutron outside of a nucleus will decay into a proton, electron, and neutrino within fifteen minutes. When a neutron decays, the mass energy of the neutron is used in creating the decay products (proton, electron, and neutrino). Because protons are the most massive of the three, most of the energy goes into creating the proton. Any remaining energy after the formation of the three decay products is converted into kinetic energy of the particles. The energy of the electron emitted during beta decay depends on the relative absorptions of kinetic energy – if the electron is created but absorbs no energy, its energy is simply E = m e c 2 ; if the electron absorbs all the kinetic energy, its energy is E = (m n – m p )c 2 , which is the energy of the neutron minus the energy of the created proton (energy of neutrino is negligible, so the difference must represent the kinetic energy and the energy in creating the electron).
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