{"id":912,"date":"2018-03-20T16:35:58","date_gmt":"2018-03-20T16:35:58","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/?post_type=chapter&p=912"},"modified":"2018-09-19T15:17:59","modified_gmt":"2018-09-19T15:17:59","slug":"11-3-units-of-radioactivity","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-monroecc-orgbiochemistry\/chapter\/11-3-units-of-radioactivity\/","title":{"raw":"11.3 Units of Radioactivity","rendered":"11.3 Units of Radioactivity"},"content":{"raw":"
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Learning Objective<\/h3>\r\n
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  1. Express amounts of radioactivity in a variety of units.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
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    In Section 11.2 \"Half-Life\"<\/a>, we used mass to indicate the amount of radioactive substance present. This is only one of several units used to express amounts of radiation. Some units describe the number of radioactive events occurring per unit time, while others express the amount of a person\u2019s exposure to radiation.<\/p>\r\n

    Perhaps the most direct way of reporting radioactivity is the number of radioactive decays per second. One decay per second is called one becquerel (Bq)<\/span><\/strong><\/span>. Even in a small mass of radioactive material, however, there are many thousands of decays or disintegrations per second. The unit curie (Ci)<\/span><\/strong><\/span>, now defined as 3.7 \u00d7 1010<\/sup> decays per second, was originally defined as the number of decays per second in 1 g of radium. Many radioactive samples have activities that are on the order of microcuries (\u00b5Ci) or more. Both the becquerel and curie can be used in place of grams to describe quantities of radioactive material. As an example, the amount of americium in an average smoke detector has an activity of 0.9 \u00b5Ci.<\/p>\r\n\r\n

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    Note<\/h3>\r\n

    The unit becquerel<\/em> is named after Henri Becquerel, who discovered radioactivity in 1896. The unit curie<\/em> is named after Polish scientist Marie Curie, who performed some of the initial investigations into radioactive phenomena in the early 1900s.\u00a0 Note that 1 Ci = 3.7 x 1010<\/sup> Bq.\r\n<\/span><\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

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    Example 4<\/h3>\r\n

    A sample of radium has an activity of 16.0 mCi (millicuries). If the half-life of radium is 1,600 y, how long before the sample\u2019s activity is 1.0 mCi?<\/p>\r\n

    Solution<\/p>\r\n

    The following table shows the activity of the radium sample over multiple half-lives:<\/p>\r\n\r\n

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    Time in Years<\/th>\r\nActivity<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
    0<\/td>\r\n16.0 mCi<\/td>\r\n<\/tr>\r\n
    1,600<\/td>\r\n8.0 mCi<\/td>\r\n<\/tr>\r\n
    3,200<\/td>\r\n4.0 mCi<\/td>\r\n<\/tr>\r\n
    4,800<\/td>\r\n2.0 mCi<\/td>\r\n<\/tr>\r\n
    6,400<\/td>\r\n1.0 mCi<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

    Over a period of 4 half-lives, the activity of the radium will be halved four times, at which point its activity will be 1.0 mCi. Thus, it takes 4 half-lives, or 4 \u00d7 1,600 y = 6,400 y, for the activity to decrease to 1.0 mCi.<\/p>\r\n\r\n<\/div>\r\n

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    Skill-Building Exercise<\/h3>\r\n
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      A sample of radon has an activity of 60,000 Bq. If the half-life of radon is 15 h, how long before the sample\u2019s activity is 3,750 Bq?<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n \r\n

      Other measures of radioactivity are based on the effects it has on living tissue. Radioactivity can transfer energy to tissues in two ways: through the kinetic energy of the particles hitting the tissue and through the electromagnetic energy of the gamma rays being absorbed by the tissue. Either way, the transferred energy\u2014like thermal energy from boiling water\u2014can damage the tissue.<\/p>\r\n\r\n<\/div>\r\n

      The rad\u00a0<\/span><\/strong><\/span>is a unit equivalent to a gram of tissue absorbing 0.01 J:<\/p>\r\n1 rad = 0.01 J\/g<\/span><\/span>\r\n

      Another unit of radiation absorption is the gray (Gy):<\/p>\r\n1 Gy = 100 rad<\/span><\/span>\r\n

      The rad is more common. To get an idea of the amount of energy this represents, consider that the absorption of 1 rad by 70,000 g of H2<\/sub>O (approximately the same mass as a 150 lb person) would increase its temperature by only 0.002\u00b0C. This may not seem like a lot, but it is enough energy to break about 1 \u00d7 1021<\/sup> molecular C\u2013C bonds in a person\u2019s body. That amount of damage would not be desirable.<\/p>\r\n

      Predicting the effects of radiation is complicated by the fact that various tissues are affected differently by different types of emissions. To quantify these effects, the unit rem<\/span><\/strong><\/span>\u00a0(an acronym for roentgen equivalent, human) is defined as<\/p>\r\nrem = rad \u00d7 factor<\/span><\/span>\r\n

      where factor<\/em> is a number greater than or equal to 1 that takes into account the type of radioactive emission and sometimes the type of tissue being exposed. For beta particles, the factor equals 1. For alpha particles striking most tissues, the factor is 10, but for eye tissue, the factor is 30. Most radioactive emissions that people are exposed to are on the order of a few dozen millirems (mrem) or less; a medical X ray is about 20 mrem. A sievert (Sv) is a related unit and is defined as 100 rem.<\/p>\r\n

      What is a person\u2019s annual exposure to radioactivity and radiation? Table 11.3 \"Average Annual Radiation Exposure (Approximate)\"<\/a> lists the sources and annual amounts of radiation exposure. It may be surprising that fully 82% of the radiation exposure is from natural source, sources we cannot avoid. Fully 10% of the exposure comes from our own bodies\u2014largely from 14<\/sup>C and 40<\/sup>K.<\/p>\r\n\r\n

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      Table 11.3<\/span> Average Annual Radiation Exposure (Approximate)<\/strong><\/h5>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
      Source<\/th>\r\nAmount (mrem)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
      radon gas<\/td>\r\n200<\/td>\r\n<\/tr>\r\n
      medical sources<\/td>\r\n53<\/td>\r\n<\/tr>\r\n
      radioactive atoms in the body naturally<\/td>\r\n39<\/td>\r\n<\/tr>\r\n
      terrestrial sources<\/td>\r\n28<\/td>\r\n<\/tr>\r\n
      cosmic sources<\/td>\r\n28<\/td>\r\n<\/tr>\r\n
      consumer products<\/td>\r\n10<\/td>\r\n<\/tr>\r\n
      nuclear energy<\/td>\r\n0.05<\/td>\r\n<\/tr>\r\n
      Total<\/strong><\/td>\r\n358<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n
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      Note<\/h3>\r\n

      Flying from New York City to San Francisco adds 5 mrem to your overall radiation exposure because the plane flies above much of the atmosphere, which protects us from most cosmic radiation.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

      The actual effects of radioactivity and radiation exposure on a person\u2019s health depend on the type of radioactivity, the length of exposure, and the tissues exposed. Table 11.4 \"Effects of Short-Term Exposure to Radioactivity and Radiation\"<\/a> lists the potential threats to health at various amounts of exposure over short periods of time (hours or days).<\/span><\/p>\r\n\r\n<\/div>\r\n

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      Table 11.4<\/span> Effects of Short-Term Exposure to Radioactivity and Radiation<\/strong><\/h5>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
      Exposure (rem)<\/th>\r\nEffect<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
      1 (over a full year)<\/td>\r\nno detectable effect<\/td>\r\n<\/tr>\r\n
      \u223c20<\/td>\r\nincreased risk of some cancers<\/td>\r\n<\/tr>\r\n
      \u223c100<\/td>\r\ndamage to bone marrow and other tissues; possible internal bleeding; decrease in white blood cell count<\/td>\r\n<\/tr>\r\n
      200\u2013300<\/td>\r\nvisible \u201cburns\u201d on skin, nausea, vomiting, and fatigue<\/td>\r\n<\/tr>\r\n
      >300<\/td>\r\nloss of white blood cells; hair loss<\/td>\r\n<\/tr>\r\n
      \u223c600<\/td>\r\ndeath<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

      One of the simplest ways of detecting radioactivity is by using a piece of photographic film embedded in a badge or a pen. On a regular basis, the film is developed and checked for exposure. A comparison of the exposure level of the film with a set of standard exposures indicates the amount of radiation a person was exposed to.<\/p>\r\n

      Another means of detecting radioactivity is an electrical device called a Geiger counter\u00a0<\/span><\/strong><\/span>(Figure 11.2 \"Detecting Radioactivity\"<\/a>). It contains a gas-filled chamber with a thin membrane on one end that allows radiation emitted from radioactive nuclei to enter the chamber and knock electrons off atoms of gas (usually argon). The presence of electrons and positively charged ions causes a small current, which is detected by the Geiger counter and converted to a signal on a meter or, commonly, an audio circuit to produce an audible \u201cclick.\u201d<\/p>\r\n\"\"\r\n

      Figure 11.2 Geiger Counter Schematic.\u00a0 Attribution: By Svjo-2 [GFDL (http:\/\/www.gnu.org\/copyleft\/fdl.html) or CC BY-SA 3.0 (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0)], from Wikimedia Commons.<\/div>\r\n \r\n
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      Concept Review Exercise<\/h3>\r\n
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        What units are used to quantify radioactivity?<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n

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        Answer<\/h3>\r\n
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          [reveal-answer q=\"522426\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"522426\"]the curie, the becquerel, the rad, the gray, the sievert, and the rem\u00a0[\/hidden-answer]<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

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          Key Takeaway<\/h3>\r\n<\/div>\r\n
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