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Unformatted text preview: 22.01 Introduction to Ionizing Radiation Test #2 Professor J.C. Yanch 6 November, 2008 Name ____ SOLUTIONS _ _______ Instructions: This examination consists of 7 questions. Points awarded for each correct answer are noted on the left side of each question. A total of 100 points will be awarded. Please write all answers on the exam paper. If an answer extends to the back of page, please indicate that this is so. This is a closed book examination. 1. 252 Cf is an artificial radionuclide that decays by spontaneous fission 3% of the time. (a) Define spontaneous fission and write a balanced nuclear equation for the fission of one atom of 252 Cf. [Make an assumption regarding the precise nature (i.e. Z and A or N) of the decay products.] Spontaneous fission occurs without the addition of energy to the nucleus (e.g. by absorbing a neutron). The californium nucleus splits into tow fragments and 2-3 neutrons are emitted. There is a large number of possible combinations of decay products (fission fragments). Example: E Z Zr Ce Cf + + + neutrons 110 40 140 58 252 98 10 (b) Explain, using words and/or nuclear equations , how we can make Californium. We start with the heaviest natural element, 238 U, and bombard it intensely with neutrons. The neutron-rich products will eventually undergo - decay which will increase A. etc. Np U n U 239 93 239 92 238 92 +- Neutron absorption and - decay continues until Z increases to 98. (c) A parallel beam of neutrons incident on heavy water ( 2 H 2 O) produces a 4 MeV deuteron ( 2 H) in the straight-ahead direction. What was the energy of the neutron? We use: Q max = E 2 ) ( 4 M m mM + setting Q max to 4 MeV and solving for E 4 MeV = E (MeV) 2 ) 1 2 ( ) 1 2 4 ( + x x E = 4.5 MeV 2. a) Describe, using words and diagrams, the process of Compton scattering. Page 1 of 7 A photon interacts with a free electron. Emerging from the reaction is a longer wavelength (i.e. lower energy) photon and an energetic electron (a Compton electron). [The diagram should indicate the out-going particles with relevant scattering angles, and .] b) Carefully describe how the particle(s) leaving the interaction lose(s) energy in material. What energy loss mechanism(s) is/are most likely if the material is low Z (eg. water)? From a Compton scattering interaction we have a Compton electron and a Compton- scattered photon. The electron will lose energy via elastic collisions with electrons and by bremsstrahlung but collision losses will be far greater since Z is low. The photon will lose energy most likely by further Compton scatters since both photoelectric absorption and pair production are less likely in a low Z medium....
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