1. PH 103 Dr. Cecilia Vogel Lecture 21

2. Review Outline  Nuclei  properties  composition, N, Z, A  binding energy  Nuclei   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,   which is a 4 He 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: 212 Bi.  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 208 Tl

7. Alpha Decay  Energy is conserved  mass energy is lost,  kinetic energy is gained by emitted alpha.  ex: 243 Am. Daughter is 239 Np  Use Appendix B for masses.  Initial mass:  mass of 243 Am =  Final mass:  mass of 239 Np =, mass of 4 He =  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: 210 Tl.  Find in appendix B  that it decays by  -  and that Z = 81.  Daughter has Z + 1=82  Lead.  same A=210  Daughter is 210 Pb

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: 40 K.  App B says  + decay,  andthat Z= 19.  Daughter has Z - 1= 18  Argon.  same A=40  Daughter is 40 Ar

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,    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=W R  = measure of how damaging radiation is  compared to 200-keV X-rays  alphas are more damaging than betas, which are more damaging than gammas  RBE  >RBE  >RBE 

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*W R  dose in sievert = dose in Gy*W R  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