Unit 08 Nuclear Structure Unit 08 Nuclear Structure Slide 1
The Plan Nuclear Structure Nuclear Decays Measuring Radiation Nuclear Power Plants Major Nuclear Power Accidents New Possibilities for Nuclear Power Unit 08 Nuclear Structure Slide 2
Definitions ~10-14 m Size of atom ~ 10-11 m (1000 times bigger) Unit 08 Nuclear Structure Slide 3
Definitions Nucleon all the things inside a nucleus, the protons and neutrons Element determine by number of protons (or electrons) Every uranium atom as 92 protons Atomic Number (Z) number of protons Z = 92 for uranium Unit 08 Nuclear Structure Slide 4
Definitions Mass Number (A) - number of protons plus neutrons Isotope a sub-set of an element with a specific number of neutrons 235 uranium 235 U 92-143 neutrons and 92 protons 238 uranium 238 U 92-146 neutrons and 92 protons Unit 08 Nuclear Structure Slide 5
Stable Nuclei Ends here?? Unit 08 Nuclear Structure Slide 8
How do you make radioactive material? 235 U 92 Xe + 1 94 n 236 140 U * + 0 92 54 38 Sr + 1 n 1 n 0 + 0 Unit 08 Nuclear Structure Slide 9
Nuclear Decays Nuclear Decays One or more particles emitted from nucleus May or may not turn into different element e.g. 4 2 He 227 223 Th 90 Ra 88 Different than fission! Notice the numbers have to add up Unit 08 Nuclear Structure Slide 10
Three Types (Different Effects!) Alpha Helium nucleus emitted 227 223 Th Ra + 4 He 90 88 2 Why He emitted? Binding Energy Unit 08 Nuclear Structure Slide 12
Decay Types Beta Beta electron or positron emitted 14 Ca 14 N 6 7 22 22 Na Ne 11 10 + e - + n + e + + n neutron turns into proton Why neutrino? Required for energy conservation Energy of emitted positrons Massless? Unit 08 Nuclear Structure Slide 13
Decay Types Gamma Gamma photon emitted from the nucleus 60 * Co 27 Co 60 27 + g Gamma rays from nucleus very short wavelength 0.1 to 10MeV (as opposed to ~20eV from atomic decay) No change in charge Unit 08 Nuclear Structure Slide 14
Gamma Rays Higher frequency Longer waves Enough energy to damage cells Unit 08 Nuclear Structure Slide 15
How does it decay? 140 Xe 140 Cs 54 1.3s 55 1.0m 140 56 Ba 12.7d 140 57 La 1.6d 140 58 Ce 94 38 Sr 1.2m 94 39 Y 18.8m 94 40 Zr 94 41 Mo Unit 08 Nuclear Structure Slide 16
How does it decay? Unit 08 Nuclear Structure Slide 17
Decay Chain alpha beta Series of decays, not just one. Unstable thorium 232 Stable lead 208 Unit 08 Nuclear Structure Slide 18
Summary Alpha (He nucleus) Most deadly Easiest to block Beta (electron or positron) Gamma (photon from nucleus Least deadly Toughest to block Unit 08 Nuclear Structure Slide 19
Measuring Radioactivity Different ways to measure radioactivity depending on need and/or available equipment. Can be very confusing. In general: 1) Activity Just count decay (Curie) 2) Energy deposited in some material (Rad) 3) Energy deposited in human body (REM) Unit 08 Nuclear Structure Slide 20
Activity Just count number of decays per second. R = DN Dt Simplest equipment (e.g. Geiger counter) Units Becquerel (bq) 1 decay/sec Curie (Cu) 3.7X10 10 decay/sec (1g of Ra) Unit 08 Nuclear Structure Slide 21
Example 11.1 Activity A Geiger counter is held 6cm from a radioactive sample and gives a reading of 3200 counts of a period of 2 minutes. Assuming the Geiger counter covers 1/8 of the total area, what is the activity of the sample? Unit 08 Nuclear Structure Slide 22
Measuring Radiation Amount Ionized Roentgens (R) amount of radiation that produces 3.3 X 10-10 Coulomb in 1cm 3 of air at 0 o C and 760mm of pressure. Or 1R = 2.58X10-4 C/kg of air Now energy of radiation becomes involved Important for biological effects. Unit 08 Nuclear Structure Slide 23
Measuring Radiation Absorbed Dose Rad (Radiation Absorbed Dose) amount of radiation that deposits 100 erg/gram of material or 0.01 J/kg of material Depends on the material absorbing radiation Internationally being replaced by Gray (Gy) 1Rad = 0.01Gy Unit 08 Nuclear Structure Slide 25
Absorbed Dose in Humans Roentgen Equivalent Man (rem) takes into account amount of harm done to human body More deadly radiation has higher Relative Biological Effectiveness (RBE) or Q factor International units are Siverts (Sv) 100 Rem = 1 Sv Unit 08 Nuclear Structure Slide 27
Absorbed Dose in Humans X-rays have Q = 1 (by definition) medium energy neutrons Q=2.5 (1rad of neutrons 2.5 more harmful to humans than 1rad of x-rays.) Alpha particles Q=20!!! (very deadly but easy to stop) Unit 08 Nuclear Structure Slide 28
Example 11.4 Effect on Human A beam of protons is directed at a 0.015kg cancer tumor. The particles have an energy of 5MeV and a relative biological equivalent of Q=4. If 1.6X10 10 particles emitted every second, what is the biologically equivalent dose delivered to the tumor in 25 seconds? Unit 08 Nuclear Structure Slide 29
Example 8.7 Radiation Effects A 0.5kg biological sample receives of dose of 456 rad from neutrons with an RBE of 6.2. a) How much energy is absorbed by the sample? b) What is the effective dosage in REM? c) If this same effective dosage was delivered with a rays which have an RBE of 13, what would be the dosage in rad? Unit 08 Nuclear Structure Slide 30
Allowed Human Dose Average annual dose 0.62rem (~50% natural, 50% medical) Nuclear Regulatory Commission (NRC) Limit for general public 5rem (50mSv) Nuclear Regulatory Commission (NRC) Limit for Nuclear power plant 0.1rem (1mSv) above background Child/pregnant women = 0.5rem Unit 08 Nuclear Structure Slide 31
Dose Effects on Humans Low does limits somewhat controversial Based on high dose cancer rates 50rem blood count change 320rem LD50/60 with no medical 800rem - mortality Linear No Threshold Model (LNT) 2 bottles of aspirin will kill you, so 2 aspirin kills you a little?? Unit 08 Nuclear Structure Slide 33
Most conservative always best? What is more harmful to the people 15 miles from the Fukushima power plant: Having to evacuate their home or Radiation from plant? (~0.0001rem/hr) Unit 08 Nuclear Structure Slide 34
Activity Just counting Curies Becquales Summary Absorbed Dose - Energy Deposited Roentgens Rad Gray Biological Equivalent Dose Damage to Humans Sv rem Unit 08 Nuclear Structure Slide 35
Half Life T 1/2 Average amount of time it takes for half the sample to decay Also average amount of time it takes for the decay rate to drop to half Unit 08 Nuclear Structure Slide 36
Example 11.4 Barium-122 has a half-life of 2 minutes. A fresh sample weighing 80 g was obtained. If it takes 10 minutes to set up an experiment using barium- 122, how much barium-122 will be left when the experiment begins? Unit 08 Nuclear Structure Slide 37
Example 11.5 Carbon-14 has a half-life of 5730 years and is used to date archaeological objects. A fresh charcoal made from a tree contains carbon-14 which will give a radioactive count of 13.60 disintegrations per minute per gram of carbon. Prehistoric cave paintings were found in Spain. A piece of charcoal found in the ancient cave in Altamira, Spain gave 1.70 disintegrations per minute per gram of carbon. From this information, determine the age of the cave paintings. Unit 08 Nuclear Structure Slide 38