Assign _3_solution_fall07

Assign _3_solution_fall07 - Solutions Assignment#3 PCS229...

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PCS229 Solutions - Assignment #3 Fall2007 1 Questions 1 – 6 (modified from the 2007 MCAT Exam) are based on the following information: The decay of a radioactive sample releases energy, which can affect human tissue. Human tissue may be made to absorb radiation intentionally for diagnostic purposes, such as the use of radioactive iodine to study thyroid or kidney function, or technetium to scan for a brain tumor. However, the exposure of human tissue to radiation may also be nondeliberate, such as that caused, for example, by naturally occurring sources such as cosmic rays impacting the atmosphere, radioactive elements in the ground, or potassium-40 in the diet, or by artificial sources in laboratories and nuclear power facilities. Radiation ionizes the atoms absorbing the energy, which can directly damage DNA strands in a cell; damage to DNA strands can also be caused by the attack of hydroxyl radicals produced by the ionization of water molecules in the cell. The energy deposited by ionizing radiation per unit mass of body tissue is called the absorbed dose, D; the unit of absorbed dose is the rad. By definition, 1 rad is equal to 100 ergs per gram, where 1 erg is a non-Sl unit of energy equal to l0 -7 joule. Since 1.6 x l0 -19 J = 1 eV, 1 rad is equivalent to 6.25 x l0 10 MeV per kilogram. The biological effect of radiation is not simply a matter of measuring the absorbed dose, however. Different types of radiation have different effects and are therefore assigned different radiation weighting factors. For example, β particles and γ -rays deliver their energy over a long path, and thus deposit a relatively small amount over any small region; the radiation weighting factor, w R , for these radiations is only 1. By contrast, the value of w R for α particles is 20. So, to determine the overall effect of radiation on body tissue, the absorbed dose is multiplied by the radiation weighting factor to give the dose equivalent: R E w D D × = When D is expressed in rads, the dose equivalent is given in rems.
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