Radiation_0212_handout

Radiation_0212_handout - Radiation and Radioactivity This...

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PHYS 0212 Radiation and Radioactivity 1 Radiation and Radioactivity This experiment has four parts: 3. 137 Ba Decay Half-life 1. Counting Statistics 2. Gamma ( γ ) Ray Absorption Half-length and shielding 4. Dosimetry P N N P N N P In the Optical Spectroscopy lab we learned that an atom consists of light electrons orbiting a dense, heavy nucleus made of protons and neutrons. In this lab we will concentrate on the nucleus itself. PHYS 0212 Radiation and Radioactivity 2 N P Proton – positive charge, mass = 938.3 MeV/c 2 Neutron – no charge, mass = 939.6 MeV/c 2 e Electron – negative charge, mass = 0.511 MeV/c 2 2 Em c = A Z X Chemical symbol X = Mass number protons neutrons A = =+ Atomic number protons Z = = Number of neutrons A Z = Example: Cobalt 60 27 Co 27 protons 33 neutrons Isotope – Nuclei with the same number of protons (Z) but different numbers of neutrons (A). 58 27 Co 59 27 Co 60 27 Co 61 27 Co 62 27 Co 2 2 MeV E m c c == PHYS 0212 Radiation and Radioactivity 3 Beta ( β ± ) decay – Nucleus emits beta particle (electron or positron), medium shielding. 214 214 82 83 e Pb Bi e ν →+ + 26.8 minutes τ = 137 * 137 56 56 Ba Ba 2.55 minutes = 238 234 4 94 92 2 Pu U He 87.7 years = Alpha ( α ) decay – Nucleus emits alpha particle (Helium-4), maximally ionizing, light shielding. Gamma ( γ ) decay – Nucleon within the nucleus drops to a lower energy level and emits a gamma particle (electron or positron), heavy shielding. Three Basic Types of Nuclear Decay Gamma decay often occurs after alpha or beta decay because the daughter nucleus is left in an excited state. 137 137 * - 55 56 e Cs Ba e + 137 56 Ba + 60 60 * - 27 28 e Co Ni e + 60 28 Ni + 30.07 years = 2.55 minutes = 5.271 years = 10.467 minutes = PHYS 0212 Radiation and Radioactivity 4 Counting Statistics Many radiation experiments are counting experiments in which a detector simply counts the number of particles that pass through it during a fixed interval of time. Detector N = 1 N = 2 N = 3 N = 4 N = 5 N = 6 N = 7 N = 8 N = 9 N = 10 0.0 s t ∆ = 0.5 s 1.0 s 1.5 s 2.0 s 2.5 s 3.0 s 3.5 s 4.0 s 4.5 s 5.0 s N = 0 10 2.0 Hz 5.0 s N R t = Rate: Because the particles are emitted at random, the number of particles that pass through the detector in a time interval t will vary from one interval to the next. 5.0 s t ∆ = Trial 1: N = 10 Trial 2: N = 9 Trial 3: N = 11 Trial 4: N = 10 Trial 5: N = 11 Trial 6: N = 11 Trial 7: N = 11 Trial 8: N = 10 Trial 9: N = 9 Trial 10: N = 9 Trial 11: N = 11 Trial 12: N = 10 Trial 13: N = 11 Trial 14: N = 12 Trial 15: N = 9 Trial 16: N = 10 Trial 17: N = 10 Trial 18: N = 9 Trial 19: N = 10 Trial 20: N = 10 Trial 21: N = 9 10 avg N = ( ) 10 3.2 N σ
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PHYS 0212 Radiation and Radioactivity 5 () 1 N NN NNN N σ δ == = A radioactive source produces particles at random so if a detector is functioning correctly the distribution of counts will be Gaussian (Bell curve) with a standard deviation of N Fractional Uncertainty: If the standard deviation is not then the detector is not functioning correctly.
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This note was uploaded on 08/17/2008 for the course PHYS 0212 taught by Professor Naples during the Spring '08 term at Pittsburgh.

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Radiation_0212_handout - Radiation and Radioactivity This...

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