NuclearScience2 - N uclear Science Exp II-Beta Gamma...

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Nuclear Science Exp. II-Beta, Gamma Interaction 1 NUCLEAR SCIENCE RADIATION EXPERIMENT: Absorption of Beta Particles in Aluminum OBJECTIVES To study the absorption properties of beta particles. To observe the nature of the interaction of beta particles with aluminum metal To determine the half-value thickness and the mass absorption coefficient for beta particles with two different maximum energies in aluminum metal. THEORY Experimental evidence shows the beta particle to be identical with the electron. It has a rest mass of 9.1 x 10 -28 grams and a charge Q of 1.6 x 10 -19 coulombs. Thus, we conclude that the principal distinction between an electron and a beta particle is the source or origin. An electron emitted from a nucleus is called a beta particle. The velocity of a beta particle is dependent on its energy. Velocities range from zero continuously up to about 2.9 x 10 10 cm/sec, or nearly the velocity of light. Classically, the energy of the beta particle is given by the expression E β = 1 / 2 mv 2 This equation is quite useful for small values of v, but at higher velocities the following relativistic correction, as proposed by Einstein, is required E = mc 2 1 - v 2 c 2 Here, m is the mass of the particle, which is an invariant, v is the velocity of the particle, and c the velocity of light. Beta spectra - It has been determined experimentally that beta particles have a continuous energy spectrum and that a part of the decay energy is carried away by neutrinos. The shapes of these spectra differ among nuclides, the shape being a function of the type of nuclear transition. (Beta transitions are referred to as "allowed", "forbidden", etc. to indicate nuclear conditions related to stability.) Ra-E ( 210 Bi) which is frequently used as a standard, decays by a "forbidden" transition and hence has a spectrum that peaks in the low energy region (see Figure 1). The average energy of the beta particles lies between one-fourth and one-third of E m . In some instances sharp peaks are found in
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Exp. II-Beta, Gamma Interaction 2 the beta spectra. These are caused by beta emitters that produce conversion electrons. 137 Cs is a typical example. Energy(MeV) Emax Number of Beta Particles Figure - BETA SPECTRUM FOR Ra-E ( 210 BI) Beta-particle interactions with nuclei - A collision of a beta particle with an atomic nucleus involves a coulomb interaction in which the electron is sharply deflected in its path. If this interaction is elastic, the process is called Rutherford scattering, and the energy of the emergent beta particle is essentially equal to that prior to the collision. This can be deduced from the fact that the total kinetic energy for the colliding systems has not changed (the collision was elastic) and since the atom is at least several thousand times heavier than the electron, the recoil energy of the atom will be negligible. Rutherford scattering is primarily responsible for back-scattering of beta particles.
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This note was uploaded on 09/26/2011 for the course PHYSICS 106 taught by Professor Arubi during the Summer '11 term at UCLA.

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NuclearScience2 - N uclear Science Exp II-Beta Gamma...

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