lecture_7 - R a d i o a c t i v e D a t i n g Nucleus Decay...

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Unformatted text preview: R a d i o a c t i v e D a t i n g Nucleus Decay Product Half Life Sm 147 Nd 143 106 Gyr Rb 87 Sr 87 48.8 Gyr Th 232 Pb 208 14.4 Gyr U 238 Pb 206 4.47 Gyr K 40 Ar 40 1.25 Gyr U 235 Pb 207 0.70 Gyr I 129 Xe 129 15.7 Myr Al 26 Mg 26 717,000 yr Cl 36 Ar 36 301,000 yr Kr 81 Br 81 210,000 yr C 14 N 14 5730 yr H 3 (tritium) He 3 12.43 yr Lattimer, AST 248, Lecture 7 p. 1/17 E x a m p l e Sample contains three minerals A, B, C Minerals have different chemical abundances and different Rb/Sr ratios When the rock solidified, the isotopic ratios of each mineral had equal values of Sr 87 /Sr 86 as per line Then For each decay of Rb 87 , a Sr 87 nucleus is produced, so the points move in a 45 northwesterly direction as time proceeds Today, the isotopic ratios of each mineral lie on the line Now The older the sample, the larger the angle becomes tan = t/t 1 / 2 The angle is 45 after 1 half life when t = t 1 / 2 Lattimer, AST 248, Lecture 7 p. 2/17 R a d i o a c t i v i t y i n o u r L i v e s Radioactivity in nature has three components: Primordial from before the creation of the Earth Cosmogenic produced by cosmic rays Artificial due to human activity Source Dose (USGS) mrem/yr Inhaled (Mostly Radon 200 Other ingested 39 Terrestrial radiation 28 Cosmic radiation 27 Cosmogenic 1 Artificial (medical) 50 Artificial (consumer products) 9 Artificial (coal + nuclear) 1 Total 355 A rem measures the equivalent dose and is equal to the absorption of 100 erg per g of material times a quality factor that depends on the radiation At an altitude of 1 mile, add 27 mrem to total On the Colorado Plateau, add 63 mrem to total Add 1 mrem for each 1000 miles traveled by jet Add 7 mrem if you live in a stone, brick or concrete building Add 1 mrem if you watch too much TV Lattimer, AST 248, Lecture 7 p. 3/17 C a r b o n D a t i n g C 14 originally on the Earth has long since decayed Cosmic rays bombard nitrogen nuclei in atmosphere at a fairly steady rate and convert some of them to C 14 Rate of production by cosmic rays balances rate of decay, building up a steady-state abundance of C 14 The abundances of C isotopes: C 12- 98.89%, C 13- 1.11% and C 14- 0.00000000010% C 12 /C 14 = 1 trillion: This is the ratio found in living tissue Dead tissue has loss of C 14 by radioactive decay: C 14 ( t ) = C 14 (0) e t/ 8266yr e a r th s c i .o r g / fo s s i ls /g e o t im e / r a d a te / r a d a te .h tm l A r n o ld & L ib b y ( 1 9 4 9 ) Lattimer, AST 248, Lecture 7 p. 4/17 N u c l e a r H y s t e r i a a n d I r r a t i o n a l F e a r s Closing of nuclear accelerator at Brookhaven National Lab in 1997 due to small tritium leak equivalent to amount used in an exit sign....
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This note was uploaded on 01/25/2012 for the course AST 248 taught by Professor Walter during the Spring '08 term at SUNY Stony Brook.

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lecture_7 - R a d i o a c t i v e D a t i n g Nucleus Decay...

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