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PH 103

PH 103. Dr. Cecilia Vogel Lecture 21. Review. Nuclei properties composition, N, Z, A binding energy. Outline. Nuclei a, b ,g decays Radiation damage exponential decay. Conservation and Nuclear Reactions. Charge is conserved in all nuclear reactions

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PH 103

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  1. PH 103 Dr. Cecilia Vogel Lecture 21

  2. Review • Nuclei • properties • composition, N, Z, A • binding energy Outline • Nuclei • a, b ,g decays • Radiation damage • exponential decay

  3. Conservation and Nuclear Reactions • Charge is conserved in all nuclear reactions • Ex: if a positive particle is emitted • nucleus must become less positive • Number of nucleons is conserved • for example, p can’t turn into a positron alone

  4. Conservation and Nuclear Reactions • Energy is conserved in all nuclear reactions • Remember that mass is a form of energy • & may be converted to or from other forms • In a nuclear process • if mass is lost, energy is released (exothermic) • if mass is gained, energy input is needed (endothermic) • All spontaneous processes are exothermic • such as all nuclear decays • In all nuclear decays, mass is lost

  5. Alpha Decay • Occurs in some heavy nuclei • Particle emitted is • alpha particle, a • which is a 4He nucleus • Parent nucleus loses 2 protons and 2 neutrons • So daughter nucleus has Z - 2, A - 4

  6. Alpha Decay • Parent nucleus loses 2 protons and 2 neutrons • So daughter nucleus has Z - 2, A - 4 • ex: 212Bi. • Look in Appendix B to see • it decays by alpha-decay. • Also find Z=83 (in appendix B or periodic table). • Daughter has Z - 2 = 81. • Look up -- this is Thallium. • A - 4 = 212 - 4 = 208. • Daughter is 208Tl

  7. Alpha Decay • Energy is conserved • mass energy is lost, • kinetic energy is gained by emitted alpha. • ex: 243Am.Daughter is 239Np • Use Appendix B for masses. • Initial mass: • mass of 243Am = • Final mass: • mass of 239Np = , mass of 4He = • total final mass = • Initial mass > final mass! • always true in decays

  8. Beta-minus Decay • Occurs in neutron-rich nuclei • Particles emitted are • e- and antineutrino, • (anti)neutrino has zero charge • mass very close to zero • Parent nucleus loses a neutron • but gains a proton • So daughter nucleus has Z + 1, same A

  9. Beta-minus Decay • Parent nucleus loses a neutron • but gains a proton • So daughter nucleus has Z + 1, same A • ex: 210Tl. • Find in appendix B • that it decays by b- • and that Z = 81. • Daughter has Z + 1=82 Lead. • same A=210 • Daughter is 210Pb

  10. Beta-plus Decay • Occurs in neutron-deficient nuclei • Particles emitted are • e+ and neutrino, • e+ is a positron, an anti-electron • Parent nucleus loses a proton • but gains a neutron • So daughter nucleus has Z - 1, same A

  11. Beta-plus Decay • Parent nucleus loses a proton • but gains a neutron • So daughter nucleus has Z - 1, same A • ex: 40K. • App B says b+ decay, • andthat Z= 19. • Daughter has Z - 1= 18 Argon. • same A=40 • Daughter is 40Ar

  12. Gamma Decay • Occurs in excited nuclei • nucleus is not in its ground state • Particle emitted is • a photon, • a very high energy photon • high frequency • gamma part of EM spectrum • Particle emitted has no charge, no nucleons • only takes away energy • So daughter nucleus is same isotope • in lower energy level

  13. Radiation Damage • Visible light • very little damage • yellows paper, fades dyes, etc • UV • sunburns, some ionization • Ionizing radiation, • a, b, g • energetic enough to ionize atoms • and ions are very reactive. • Damaging reactions occur in living tissue • Cells can be damaged, die, or become cancerous

  14. Measure of Damage • Damage depends on amount of energy absorbed by the tissue • more energy means more ionization, • so more damage • But if the energy is spread out, it is less damaging. • So what is important is • energy per unit mass • 1 rad = 0.01 J/kg • 100 rad = 1 J/kg = 1Gy = SI unit, but very big

  15. Measure of Damage • Damage depends on amount of energy • 1 rad = 0.01 J/kg • Damage also depends on type of radiation • Relative biological effectiveness, RBE=WR • = measure of how damaging radiation is • compared to 200-keV X-rays • alphas are more damaging than betas, which are more damaging than gammas • RBEa>RBEb>RBEg

  16. Measure of Damage • Damage depends on amount of energy • 1 rad = 0.01 J/kg • 1Gy = 1 J/kg • Damage also depends on type of radiation • dose in rem = dose in rad*WR • dose in sievert = dose in Gy*WR • For example, consider workers at the Fukushima Daiichi nuclear power plant: • some received doses >100 mSv • but none above Japan's guidance value of 250 mSv for exposure of emergency workers (source: Reuters)

  17. Penetrating Radiation • So then, why are gammas exciting? • Alphas are stopped by cardboard, skin • betas are stopped by sheet metal, rock • gammas are only stopped by thick lead! • There are lots of alpha emitters in the rocks • but, the alphas don’t penetrate to vital organs • mostly stopped by skin • exception: Radon is an alpha emitter • it’s worrisome, because it’s a gas

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