10: The nucleus, radioactivity and nuclear medicine

1. 10: The nucleus, radioactivity and nuclear medicine You only have to know what I talk about in class

2. Nuclear reactions • Emphasis so far on interpretation of chemical behavoir in terms of the electronic structure of cmpds. Form bonds by transfer of e-’s or sharing of e pairs. • This chapter are interested in the chemistry of the nucleus.

3. Nucleus contains protons and neutrons (to a chemist) and most of the mass of the atom. • Protons and neutrons collectively called nucleons. Specific nucleus called a nuclide(isotope). • AQ ZA=mass no.=no. of protons+no. of neutrons Z= no. of protons in the nucleus = charge on nucleus = atomic no. Q = symbol for element.

4. Isotopes • 12C 13C 14C • 1H 2H 3H • Some isotopes are stable, others are not. • Stable isotope persists indefinitely.

5. Do not copy • Unstable isotopes are radioactive: on a random basis, one atom of a collection suddenly emits a simpler particle and/or energy of very high frequency (energy) and changes into a different nucleus. The energy emitted is capable of breaking chemical bonds.

6. 2 kinds of radioactive isotopes: • 1. natural: occur in nature (Becquerel 1896) Every element after Bi is radioacitive. • 2. induced: man-made: brought about by particle bombardment of a nucleus (Rutherford in 1919) • All elements with atomic nos. higher than 83 are radioactive. All isotopes of 43Tc and 61Pm are radioactive.

7. Particles that are emitted in radioactive decay (natural) • 1. alpha particle: 24He or 24a very energetic helium nucleus (no e-s) • a. very ionizing in matter • b. +2 charge 24He2+ • c. low penetrating power, relatively slow moving (10% speed of light)

8. 2. beta emission (rays, particles) : -10b, -10e stream of high speed electrons emitted by nucleus; move at 90% speed of light • How do you get an electron in the nucleus. • neutron decays into a proton and an electron • a. -1 charge, “zero” mass • b. greater penetrating power, less ionizing power than alpha particle

9. 3. gamma ray: g : electromagnetic radiation of very short wavelength; pure energy • a. zero charge, zero mass • b. very penetrating, highly energetic • c. most radioactive decays emit gamma rays as well as other particles

10. Other particles emitted • 4. neutrons 01n • a. uncharged • b. produce cell damage due to ionizing effects of neutrons colliding with protons in cells in body.

11. 5. protons 11p 11H • a. +1 charge • 6. positron 10b, 10e • a. +1 charge, “zero” mass • b. antimatter of beta particle

12. Ionizing radiation • Alpha, beta and gamma radiation are ionizing radiations. Leave trail of ions in their wake. • Alpha and beta not bad unless injested--cause skin and eye damage • Gamma can penetrate body and cause internal damage.,

13. 10.2: Balancing nuclear reactions • 1. In nuclear transformations charge is conserved: sum of atomic nos. of reactants = sum of at. nos. of products • 2. Mass is not conserved (E=mc2) but there is no change in the total mass no. (# of nucleons) sum of mass nos. of reactants = sum of mass nos. of products

14. Let’s balance nuclear reactions • 614C  X_+ 714N • 92239U  X + -10e • 511B 37Li + X

15. 10.24 Write a nuclear reaction to represent radium-226 decaying to radon-222 and X.

16. Nuclear transmutation--the alchemists’ dream • Bombard a nucleus with other particles (high energy) frequently in particle accelerators. In this you produce radioisotopes artificially--create new ones • 46106Pd(, p)47109Ag • 10.26: Complete 92238U + 714N  X + 601n

17. Radioactive decay rates • Radioactive decay follows first order kinetics: rate = k[isotope] • The half-life of a given isotope is an identifying characteristic of that isotope. • Half-life (t1/2): time required for one-half of given quantity of a substance to undergo change and is independent of how much of the isotope you start with. Isotopes have their own half-life.

18. Carbon-14 has a half-life of 5730 years. How much of a 500g sample will remain after 17,190 years. • If a patient is administered 10 ng of technetium-99m (half-life 6 hours) how much will remain 2 days later?

19. Isotope half-life • 238U 109yr • 218At 1.4s • 210Bi 5 d • 60Co 5.26y • 131I 8.07d (overactive thyroid) • 90Sr 28.1yr

20. 10.3: properties of radioisotopes • Nuclear stability and structure: • What allows the packing of protons (+ charge) in such a small region as the nucleus? • nuclear diameter about 1 x 10-12 cm; density of nucleus about 2 x 1014 g/cm3. • This corresponds to a density of 220,000,000 tons/cm3

21. Thought to be neutrons that allow packing of so concentrated a charge in such a small volume. • No nucleus of 2 or more protons without neutrons. (1H) • Some more facts about nuclear stability: Nuclei with 2,8,20,50,82 protons or neutrons and with 126 neutrons more stable than nuclei with other nos. • Binding energy: energy that holds protons, neutrons together in nucleus

22. Nuclei with even nos. of neutrons and protons are more stable than those with odd nos. • protons neutrons No. stable isotopes odd odd 4 (2H, 14N) odd even 50 even odd 53 even even 157

23. Nuclear binding energy • Nuclear binding energy is the energy required to break up a nucleus into its component protons and neutrons. This energy represents the conversion of mass to energy that occurs during an exothermic nuclear rxn.

24. Dating based on radioactive decay • Radiocarbon dating:: • C-14 is produced in nature by 714N + 01n 614C + 11H • and decays by 614C 714N + -10 • In living matter the amt of C-14 remains constant. After a plant dies the amt of C-14 decreases. This can be used to date the matter in question.

25. 10.4 Nuclear fission • Nuclear fission: heavy nucleus (mass no . 200) divides into form smaller nuclei of intermediate mass and one or more neutrons with release of a large amt of energy

26. For example: 92235U + 01n g135I + 97Y +401n 139Ba + 94Kr + 301n 131Sn + 103Mo + 201n 139Xe + 95Sr + 201n etc • Initiated with slow neutrons (speed of air molecules at room temp)

27. Nuclear chain rxn: self-sustaining sequence of nuclear fission rxns • Critical mass: minimum mass of fissionable material required to generate a self-sustaining nuclear chain rxn (chain propagates so more neutrons are generated than are absorbed or lost to outside--rxn proceeds at ever increasing rate--can become runaway)

28. subcritical Critical mass

29. Nuclear reactors • Makes use of U-235 fission reaction • Slow neutrons split U-235 more efficiently than fast ones: use a moderator to slow down the neutrons produced in the rxn: non-toxic, inexpensive, not turn radioactive with neutron bombardment, fluid (so can be used as coolant) --water works well

30. Use enriched U-235 (3-4% in U2O3) • Control rate of rxn by controlling no of neutrons allowed to react: control rods made of Cd or B (absorb neutrons) • Reactor: short of critical mass--chain rxn to continue at a slow, usable rate, but not runaway. Reactors: not atomic bomb capability--but dangers from radioactivity.

31. 113Cd + 1n g114Cd + g

32. Nuclear fusion • Combining of 2 lightweight nuclei to form heavier ones--large release of energy • Requires high temperatures--thermonuclear rxns

33. In sun: 1H + 2H g3He 3He + 3He g4He + 21H 1H + 1H g2H + 10b • Fusion reactors: fuels cheap, inexhaustible, little dangers from radioactivity • Problems: Haven’t figured out how to get more energy back than put in: for fusion need temps ~100 million degrees C and way of containing atoms in small region of space for fusion