Cal Poly Pomona PHY 432L - Natural Radiation (15 pages)

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Natural Radiation



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Natural Radiation

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Pages:
15
School:
California State Polytechnic University, Pomona
Course:
Phy 432l - Nuclear Physics Laboratory

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Natural Radiation There are a few radioisotopes that exist in our environment Isotopes that were present when the earth was formed and isotopes that are continuously produced by cosmic rays can exist today if they have long enough half lives Here we will discuss 5 of these isotopes Four were produced when the earth was formed 40 K half life of 1 277 109 years 238 U half life of 4 51 109 years 232 T h half life of 1 4 1010 years and 235 U half life of 7 1 108 years 14 C half life 5280 years is continuously produced in the upper atmosphere by cosmic rays entering the earth 40 K Most of the potassium found on earth is the stable 39 K However a small fraction 0 0117 of all potassium is 40 K 40 K is radioactive with a half life of 1 277 109 years 40 K decays in the following way 40 K has an 89 28 chance to undergo beta decay to the ground state of 40 Ca and a 10 72 chance to undergo electron capture to 40 Ar When electron capture occurs it is almost always to the excited state of 40 Ar 99 53 of the time From the excited state 40 Ar decays to the ground state emitting a gamma particle with energy 1460 83 KeV Thus when 40 K decays there is a 89 28 chance a beta particle is emitted and a 10 72 0 9953 10 67 chance a gamma is emitted Although 0 0117 seems like a small amount it means that one out of every 8500 potassium atoms is radioactive This is a large amount The average 70 Kg man has around 140 grams of potassium in his body So the number of radioactive 40 K nuclei in his body is N umber of 40 K nuclei 140g 6 02 1023 0 000117 2 47 1020 40g 1 which is a lot of nuclei The activity due of 40 K decays in the average person can be calculated using the relation activity A N or A N ln2 A 2 47 1020 ln 2 decays 4240 9 1 277 10 365 24 3600 sec sec 2 This amount of activity is equal to 4240 37000 0 11 Ci Perhaps we should wear a radioactive sign around our necks Fruits and vegetables can have as much as 0 4 potassium and an average soil sample contains 2 potassium Thus one Kg of



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