CHAPTER 2 FUNDAMENTAL RADIATION CONCEPTS PAGE I ...

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Number of atomsNNto etNtNoetNOTE - the decay time and half-life must be in the same units of time.The e -t term indicates that the radioactive atoms decay exponentially. This equation,N t = N o e -t , is called the decay equation. The figure below plots the decay equation forP-32, which has a decay constant of 0.049 1 / day , starting from one million atoms. Thenumbers on the y-axis would change depending on the initial number of atoms, and thenumber on the x-axis would change depending on the radionuclide, but the overall shapeof the curve is the same for all radionuclides.Radioactive Decay of P-32100000090000080000070000060000050000040000030000020000010000000 20 40 60 80 100 120 140 160Time (days)If we were to substitute into the decay equation the time, T 1/2 , it takes for the reduction ofa quantity of radioactive atoms to half of the original, we get:1NT 2N oN Ne1 T1 / 22 o oe 1 T1 / 2212 1/ 221n1 T 1n1n 22-8 RSSC FUNDAMENTAL RADIATION CONCEPTS 072011
1n2T1/ 2ln 2T1/2 ln 2 0.693 0.693T1/ 2Thus, the decay constant () can be calculated for any radioactive nuclide from its halflife.VI.RADIOACTIVITY UNITSThe number of atoms, N, remaining at a particular instant in time is given by:A = Nwhere A is the activity, defined as the instantaneous number of atoms decaying per unittime. The activity is directly determined by the quantity of radioactive material in asample, and is generally a much more convenient quantity to use than the number ofradioactive atoms. The traditional unit for activity is called the curie (Ci).1 curie = 3.7 x 10 10 disintegrations per second (dps)OR1 curie = 2.22 x 10 12 disintegrations per minute (dpm)Because the curie is a very large quantity, fractions of the curie are often used:1 millicurie = (mCi) = 2.22 x 10 9 dpm = 10 -3 curies1 microcurie = (Ci) = 2.22 x 10 6 dpm = 10 -6 curies1 nanocurie = (nCi) = 2.22 x 10 3 dpm = 10 -9 curies1 picocurie = (pCi) = 2.22 dpm = 10 -12 curiesSince radioactive material is measured in units of activity, the decay equation now takesthe form:A = A o e -tRSSC FUNDAMENTAL RADIATION CONCEPTS 072011 2-9
Page 1 and 2: CHAPTER 2FUNDAMENTAL RADIATION CONC
Page 3 and 4: electrically neutral. An element is
Page 5 and 6: particles (0.511 MeV/photon). See p
Page 7: The Chart of the Nuclides lists all
Page 11 and 12: Ionization:Excitation:The process r
Page 13 and 14: particle approaches the nucleus of
Page 15 and 16: The primary loses part of its ener
Page 17 and 18: Applications of Pair ProductionAgai
Page 19 and 20: Example: What is an individual's eq
Page 21: IX.CONVERSION OF TRADITIONAL UNITS

CHAPTER 2 FUNDAMENTAL RADIATION CONCEPTS PAGE I ...

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