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IAEA International Atomic Energy Agency Radioactivity - 1 Radioactive Decay Day 1 – Lecture 4

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IAEA Objective To discuss radioactive decay principles and some pertinent terms To define the units to measure radioactive decay 2

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IAEA Content Activity Law of Radioactive Decay Half-Life Mean Life Decay Constant Units 3

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IAEA Radioactivity Radioactivity is the property of some atoms that cause them to spontaneously give off energy as particles or rays. Radioactive atoms emit ionizing radiation when they decay 4

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IAEA Naturally Occurring Elements In nature there are about 300 nuclides Majority of naturally occurring elements aye stable But A few of high atomic weight, from polonium (Z =84) onward Radium (88), Thorium (90), Uranium (92) consists entirely of unstable nuclides The unstable substances undergo spontaneous change, radioactive disintegration or radioactive decay at definite rates. 5

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IAEA 1 Bq = 1 disintegration per second Activity The amount of a radionuclide present SI unit is the Becquerel (Bq) 6

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IAEA Multiples & Prefixes (Activity) MultiplePrefixAbbreviation 1-------Bq 1,000,000Mega (M)MBq 1,000,000,000Giga (G)GBq 1,000,000,000,000Tera (T)TBq 1 x 10 15 Peta (P)PBq 7

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IAEA International Atomic Energy Agency Units Curie (Ci) = 3.7 x 10 10 dps Becquerel (Bq) = 1 dps 1 Ci = 3.7 x 10 10 Bq

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IAEA Non-SI Units QuantityOld UnitSI UnitConversion Activitycurie (Ci)becquerel (Bq)1 Ci=3.7 x 10 10 Bq Absorbed Doseradgray (Gy)1 rad = 0.01 Gy Equivalent Doseremsievert (Sv)1 rem = 0.01 Sv 9

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IAEA The Decay Constant is denoted by NOTE: Units on are Typically or sec -1 or “per second” Decay Constant 1time 1sec 10

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IAEA A = N where “A = activity” has units of disintegrations per second (dps or Bq) Activity Activity 11 If we have N atoms of a given radioisotope and the radioisotope has a known decay constant, the activity of this sample is given by the simple product of the number of atoms and the decay constant.

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IAEA The relationship between half-life and decay constant is: Half-Life and Decay Constant T ½ = 0.693 12

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IAEA Half-Life 13

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IAEA Half-Life RadionuclideHalf-Life Phosphorus-3214.3 days Iridium-19274 days Cobalt-605.25 years Caesium-13730 years Carbon-145760 years Uranium-2384.5 x 10 9 years 14

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IAEA Sample Problem A criticality accident occurs in a Uranium processing facility. 10 19 fissions occur over a 17-hour period. Given that the fission yield for 131 I is 0.03 and its half-life is 8 days, calculate the 131 I activity at the end of the accident. Neglect 131 I decay during the accident. 15

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IAEA Solution to Sample Problem Activity = N = x x ( 10 19 x 0.03) = 3 x 10 11 Bq 131 I 0.693 8 days 1 86,400 sec day -1 3 x 10 11 Bq 3.7 x 10 10 Bq/Ci = 8.1 Ci 131 I 16

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IAEA Differential Equation for Radioactive Decay = - N(t) dN dt 17 Rate of decay at a certain time is directly proportional to the number of radioactive atoms present at that time -ve sign is because of decay (decrease)

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IAEA Radioactive Decay Equation N(t) = N o e - t 18

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IAEA Expressing the equation in terms of activity: Radioactive Decay Equation N(t) = N o e - t A(t) = A o e - t where A(t) = activity at any time t and A o = the initial activity at time t = 0 or 19

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IAEA Radioactive Decay The amount of activity decayed away after “n” half-lives is given by A AoAo 1 - 20

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IAEA The amount of activity “A” remaining after “n” half-lives is given by Radioactive Decay A AoAo 1 2n2n = 21

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IAEA International Atomic Energy Agency Mean Life T M = 1.44 T 1/2 For some applications, as in the case of dosimetry of internally deposited radioactive material, it is convenient to use the average life of the radioisotope.

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IAEA A radionuclide has a half life of 10 days. What is the mean life? Sample Problem 23

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IAEA Solution to Sample Problem Mean Life = 1.44 T 1/2 = 1.44 x 10 days = 14.4 days 24

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IAEA Where to Get More Information Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009) International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002) 25

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