Lecture 36 Chapter 22, Sections 6-7 Nuclear Applications. Radioactive Dating Tracers Food Irradiation

Transcription

1 Lecture 36 Chapter 22, Sections 6-7 Nuclear Applications Radioactive Dating Tracers Food Irradiation

2 Effects of Radiation Radiation Damage Radiation that passes through matter rips off electrons ionization One α particle can generate more than 10 5 cations The cations are often chemically reactive. Immediate health effects The formation of ions destroys living cells. Cells that divide most rapidly tend to be the most easily damaged bone marrow, white blood cells, blood platelets, the lining of the GI tract, the cells in the gonads Long-term effects Alteration of DNA Genetic mutations of offspring

3 Tissue Penetration

4 Effects of Radiation on DNA Oxidation Breakage of DNA strand Chemical modification of bases Methylation of bases Hydrolysis of bases γ Damage can be induced by gamma rays, X- rays, UV-light and reactive oxygen species generated by normal metabolic processes If one strand is damaged, this can usually be repaired Double strand damage is harder to repair

5 Radiation Shielding Because alpha particles are massive easily stopped ~ 1 mm of solid material (paper, cloth, skin) Beta particles penetrate much farther mm of dense solid (plastic, metal, concrete) Gamma rays are difficult to shield Several meters of concrete/metal Several cm of lead

6 Applications of Radioactivity Dating using Radioactivity Each nuclide has a constant half-life Products of radioactive decay are well known Therefore, radioactive nuclides can be used to estimate the age of materials (if similar to half-life) Tracers Most common use today in medicine and research Allows one to track products of specific chemical reactants

7 Radioactive Dating Example The only radioactive isotopes found naturally on earth have very long half-lives: Nuclide t ½ (years) 238 U various pathways 206 Pb K 40 Ar + β Rb 87 Sr + β Those with slightly shorter half-lives no longer exist 146 Sm Pb I Therefore, earth between 10 8 and years old ( years)

8 40 K 40 Ar Example A rock made long ago contained only K (Ar would escape) Over the years, K decays into Ar This new Ar is trapped in the rock So, total moles of K (N) plus moles of Ar tell us how many moles of K must have existed originally (N 0 ) Since we know the half-life of 40K, we can find the age of the rock N 0 ln = N t ln 2 t 1 2

9 A rock from mars has a 40K/40Ar molar ratio of : 1. What is the age of the rock? Hint: If there is 1 mol of Ar, how much K is there? t ½ for 40 K is yrs. 25% years 25% years 25% years 25% years N 0 ln = N t ln 2 t

10 14 C is a little different Carbon Dating It is made continuously in upper atmosphere in equilibrium with radioactive decay of 14 C always a constant amount present in atmosphere Taken in by plants so that all living things contain a specific ratio of 14 C / 12 C This ratio is N 0 = When something dies, it stops taking in fresh carbon No new 14 C existing 14 C decays (t ½ = 5730 years) Ratio of 14 C / 12 C tells when organism stopped respiring N N 0 = measured ratio

11 In 1988, carbon dating on the material in the shroud of Turin revealed a 14 C / 12 C ratio of According to this measurement, when were the plants that make up the material in the shroud harvested? (Normal ratio ) N ( 14 0 t ln 2 C half-life 5730 years) ln = N t 1 20% 1. About B.C. 20% 2. About 0 A.D. 20% 3. About 700 A.D. 20% 4. About 1000 A.D. 20% 5. About 1300 A.D

12 Radioactive Tracers Applied in fields that range from geology to medicine. An isotopically enriched sample is introduced into some dynamic process and the course of the process can be followed. Useful in biology to measure growth Also used to follow chemical reactions Melvin Calvin discovered how carbon is turned from CO 2 into sugars in plants (Calvin Cycle) Grew plants in 14 CO 2 and analyzed extracts for 14 C content Nobel prize 1961 B.S. Michigan Tech 1931 Brent Krueger uses his office 1997

13 Food Irradiation Beta particles or gamma rays bombard material Kills most bacteria and insects Common for spices Becoming common for fresh fruits & veggies Also for meats e.g. kills the trichinosis bug Irradiated food is identified with label But, no real standards Meaning of label varies Label may be gone soon

14 Which Mushrooms would you eat? Non-irradiated Irradiated

15 Today Finish CAPA #21 Seminar on Transition Metal Complexes (Monday for us) Schaap :00 Read Benign Hamburger Monday Discuss Benign Hamburger a bit along with transition metals Turn in write-ups on Wednesday Wednesday = review, Friday = review Also Wed last seminar Scott Feller from Wabash More cool lipid stuff