Exam_2_2009_Solutions

Exam_2_2009_Solutions - 22.01 Radiation Effects and Uses...

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22.01 Radiation Effects and Uses Quiz #2 Professor J.C. Yanch 12 November 2009 Name ___ SOLUTIONS ___________ Instructions: This examination consists of 7 (seven) 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. (a) Sketch a typical x-ray tube, labeling the components important to x-ray production. Show: anode cathode voltage difference vacuum (i.e. glass envelope/walls) (b) Describe how the x-ray tube generates x-rays. Be thorough. Assume a voltage of 100 kV. 14 Electrons “boil off” a heated filament. The freed electrons are then attracted to the anode through a potential difference of 100 kV, thereby attaining an energy of 100 keV. In the anode the electrons lose energy via: 1. elastic collisions with orbital electrons . This gets dispatched as heat. Also we’ll see some characteristic x-rays as orbital vacancies are created and filled. 2. Bremsstrahlung . Electrons are deflected in the field of the nucleus, generating x-rays with energies dependent on the degree of deflection. We will see characteristic lines superimposed on the bremsstrahlung spectrum. (c) What information is provided by a medical x-ray image , and on what physical basis is this information obtained? - a map of the densities of materials - structure/anatomy - physical basis: variation in photon interaction probabilities leading to differential attenuation in the body as a function of density. 1
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2. Using one type of emission imaging as an example, briefly describe the differences between x-ray imaging (radiography) and emission imaging in terms of how an image is formed and what kind of information is provided in the image. 10 - source is inside body, not outside - source is a radionuclide, not x-ray tube - body’s physiology takes radionuclide to a specific area of body - we image position of RN in body - thus we image body function 2
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3. A dentist places an x-ray tube near the face of a patient to obtain an x-ray image of the teeth. The operating voltage of the tube is 1.0 x 10 5 V. Without filtration, considerable low energy (assume 20 keV) x-rays are incident on the skin. (a) Approximately what fraction of beam power is emitted as x-rays, and what happens to the rest of it? 22
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Exam_2_2009_Solutions - 22.01 Radiation Effects and Uses...

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