Nuclear Physics. Final Paper and Presentation

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Grace Casey
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1 Nuclear Physics Phys 1020, Day 20: Nuclear Weapons Blmfld 16.1 Reminders: All paper drafts in Review and midterm next week Office hours 1 Final Paper and Presentation Opportunity to explain some physics to a non-scientist using everyday object/phenomenon as an example Paper: 4-5 pages double spaced Hand in via SafeAssign by 11am, Tuesday 4/20 40 points Must complete to pass course Presentation: 3 min presentation, 1-2 min of questions In lab sections, week of 4/20 Bonus presentations in class 4/27 Digital projector (and laptop), overhead projector, chalk board 10 (+5) points 1

2 Sample paper outline - Introduction - Brief description of topic you re exploring - Background physics (the broad area of physics this falls into) - Discussion: More detailed description of the topic Physical principles that it employs Further applications of the area / physics - Conclusion: What are the next big steps in this field? How might someone expand on this work you ve done? Presentation - Don t try to cover everything - Pick one aspect of paper and explain in detail. - Don t read directly from your paper - Make sure your laptop/files connect/run smoothly Further instructions and grading rubric on course website Nuclear physics covers so many topical everyday life issues Local: Should I ask my realtor about radon? What about walking my dog at the rocky flats nature reserve? Did the radiation leak at NIST 2 years ago endanger you? International: Was the Manhattan project necessary? How can we stop rogue states acquiring a nuclear bomb? Should the world build more nuclear power plants? Basic knowledge of nuclear physics important for everyone 2

3 Structure of an atom and nucleus Nuclear forces and stored energy Nuclear fission - Alpha decay (spontaneous) - Fission bomb (neutron induced fission) Nuclear Fusion Nuclear Physics Radioactivity - Alpha, beta and gamma radiation - Why its bad for you Other interesting stuff that we won t have time for - Nuclear power - Nuclear medicine Sadly, (or perhaps much to your relief), demos will be limited Recipe- how to make an atom: Proton Neutron Electron Ingredients: 1 pinch protons 1 pinch neutrons 1 cup of electrons Charge C Zero C Mass kg kg kg 1. Mix protons and neutrons thoroughly. 2. Bake at 100 million degrees until sticks together to form solid dense nucleus (about s). 3. Frost with lightly with fluffy layer of negative electrons. 4. Chill before serving! atom size: Radius of nucleus is 10,000 times smaller than nucleus-electron distance 6 3

4 Elements different types of atom Each element has a unique number of protons An atom has the same number of electrons as protons (electrically neutral) Different numbers of neutrons make different isotopes of the same element - same chemical properties (electrons do the same stuff) - different nuclear properties - Isotopes defined by mass number: 235 U, 238 U hydrogen 1 p deuterium 1 p, 1n helium 2 p, 2 n Uranium p, 146 n Hydrogen isotopes Note: Atomic radii not well defined. Oxygen has 8 protons, 8 neutrons: Building Nuclei Consider a nucleus with 7 protons, 7 neutrons (nitrogen atom)..what if we want to add another proton to make oxygen (8 protons)? What will we need to do to get proton stuck to nucleus: a. just give it a little push so it will hit nucleus dead on and it will drift towards nucleus and stick. b. the closer it gets, the harder you have to push, will take lots of work c. you ll need to push really hard at first and then less as you get closer d. you ll have to push the proton towards the nucleus with a fixed amount of force (constant force). 8 4

5 Building What will we need to do to get proton stuck to nitrogen nucleus: b. the closer it gets, the harder you have to push, will take lots of work.. Lots of energy 7 positives One positive Big repelling force Bigger repelling force Force on proton given by Coulomb s law (k = Coulomb s constant): In general: Force = k x charge of object #1 x charge of object #2 (distance between objects) 2 F = k x charge of nucleus x charge of proton (distance nucleus-proton ) 2 The smaller the distance, the larger the force! 9 Potential energy curves Represent energy needed to bring charged particles together. Imagine throwing proton so starts out going towards nitrogen nucleus with a lot of speed (lots of kinetic energy): - Repelling force from nucleus slows it down - Proton s kinetic energy converted into electrostatic potential (stored) energy, as it gets closer to nucleus Potential energy Kinetic energy separation distance, r Gravity energy analogy. At center have lots of electrostatic energy proton wants to fly away Like ball at the top of a hill has gravitational energy and wants to roll down 10 5

6 Electrostatic potential energy of proton and N nucleus Potential energy Kinetic energy Separation (r) Potential energy = k x charge N nucleus. x charge of single proton at separation (separation distance) distance of r Charge of 1 proton = 1.6 x C; Charge of 7 protons=11.2 x C So at m away ~ radius of nucleus, Potential energy = (8.99 x 10 9 N m 2 /C 2 ) x (11.2 x C) x (1.6 x C) (10-15 m) = 1.61 x Joules = 10 million electron Volts. (1 electron volt = energy gained by electron moving through 1 Volt diff.= 1.6 x J) 11 What would the potential energy curve (potential energy vs. separation distance) for an electron and a proton look like? a. This curve would be flat, not going up or down b. Would look like curve for 2 protons except upside down (so going down as they got closer instead of up) c. Would be the same as the curve for 2 protons 12 6

7 Potential energy 3/28/2010 What would the potential energy curve (potential energy vs. separation distance) for an electron and a proton look like? b. Would look like curve for 2 protons except upside down (so going down instead of up) 0 Separation distance (r) Electron and proton get pulled together into a potential energy hole. We must SUPPLY energy to separate electron and proton. What about neutrons? no charge so no electrostatic interactions 13 Analyzing shape of potential energy curves between 2 particles: PE (1) Pushed apart very little, Potential energy not changing along top separation Particles repel Positive stored energy PE (3) PE separation Pushed apart hardest, Potential energy changing rapidly (2) Particles attract Energy required to separate Force on particles related to GRADIENT (SLOPE) of potential energy curve 14 7

8 So how can a bunch of positively charged protons stick together? Two different types of forces involved - Electrostatic forces - Repulsive - Only affects charged particles (protons) - Long range (each protons repels all other protons in nucleus) - Nuclear force - Attractive - Affects protons and neutrons (nucleons) in same way - Short range only bonds nucleons that are touching - Overwhelms electrical force when protons and/or neutrons (nucleons) are REALLY close together Hopper toy analogy Spring legs - like electrical force, pushes man away from base over large distance. Suction cup - like nuclear force, bonds man to base but only strong when in contact (Or nuclear force like double-sided tape - only works if in objects in close contact.) Why is the nuclear force so different? That is the way nature is! 15 Complete potential energy curve for proton approaching nucleus Electrostatic repulsion + PE PE r r Nuclear energy scale GIGANTIC compared to chemical energy. Why? Simple coulomb s law: F= k (charge 1)(charge 2) r 2 Nuclear attraction Chemistry: - Forces between electrons and protons on distance scale of atomic size (> m). Complete nuclear potential PE r Nuclear forces: - Forces between nucleons on distance scale of nuclear size - 10,000 times smaller than atom - Forces 100,000,000 times bigger because of 1/r 2. - Lots more potential energy stored!!! 16 8

9 Binding energy per nucleon Small Large Binding energy per nucleon Small Large Why is binding energy increasing with mass number for small? Why is binding energy decreasing with mass number for large? 9

10 Small Consider how forces in nucleus change when we go from 4 He (2 protons) to 7 Lithium (3 protons) 4 He 7 Li Electrostatic repulsion: - Protons feel repulsion of 1 more proton - Electrostatic repulsion increases Nuclear attraction: - All nucleons have more nucleons in contact - Nuclear binding increases A LOT Average effect More strongly bound nucleus Large Consider how forces in nucleus change when we go from 235 U (92 protons) to 237 Np (93 protons) Electrostatic repulsion: - All 90+ protons feel repulsion of 1 more proton - Electrostatic repulsion increases ALOT Nuclear attraction: - Only a couple of nucleons at edge increase number in contact - Nuclear binding increases only a little Average effect Less strongly bound nucleus 10

11 What is this curve useful for? Max binding energy for medium Small Large There are 2 types of nuclear bombs fission and fusion. Which elements should I choose for each type of bomb (be ready to explain why)? a) Small for both b) Medium sized for both c) Large for both d) Fission small, fusion large e) Fusion small, fission large What is this curve useful for? Max binding energy for medium Small Large There are 2 types of nuclear bombs fission and fusion. Which elements should I choose for each type of bomb (be ready to explain why)? a) Small for both b) Medium sized for both c) Large for both d) Fission small, fusion large e) Fusion small, fission large 11

12 Fission and fusion Fission Fusion Energy is conserved. For bomb we must RELEASE energy stored in nucleus Each nucleon in product(s) must be more tightly bound (in bigger potential energy hole) than in original nucleus(i) E = mc 2. Mass of product(s) less than mass of original nucleus(i) Fuse small Fission (break) large To release energy and create explosion Fission of large Spontaneous - Radioactive (nuclear) decay Neutron induced - Fission bomb - Nuclear power 12

13 No. of radon atoms 3/28/2010 Spontaneous fission alpha decay Alpha particle (2p + 2n) spontaneously breaks away from nucleus Nucleus is changed from one element to another 2P-2N Quantum mechanical tunneling: Small particles not localised (fuzzy) Occasionally a outside range of nuclear binding force Has lots of electrostatic PE Converts PE to KE and runs for it - a tunnels out of potential well - Never had enough energy to climb out Potential energy curve for alpha particle and product nucleus 25 Radon example of alpha decay 4.5 billion years 238 U 234 Th + a 222 Rn 3.8 days 218 Po + a T 1/2 Time Half life: Time for half of in sample to decay Decay time of any particular nucleus is unknown and random Radon exposure: Radon is a gas Biggest cause of public exposure to nuclear radiation Occurs where lots of 238 U in soil (like round here) Accumulates in basements pump it out 2 nd biggest cause of lung cancer 13

A useful simulation alpha decay http://phet.colorado.edu/simulations/sims.php?sim=alpha_decay 27 Potential curves for alpha decay from different Tunneling difficulty = width x depth of tunnel E E E 1.

14 A useful simulation alpha decay 27 Potential curves for alpha decay from different Tunneling difficulty = width x depth of tunnel E E E 1. Hard - takes long time, - billions of years! r r r 2. Medium 3. Easy Takes millionths of a second! How much energy released? a. 1 most, 2 second, 3 least b. 2 most, 1, 3 least c. 3 most, 2, 1 least d. 3 most, 1, 2 least 28 14

15 Potential curves for alpha decay from different Tunneling difficulty = width x depth of tunnel E E E Energy released 1. Hard - takes long time, - billions of years! r r r 2. Medium 3. Easy Takes millionths of a second! How much energy released? a. 1 most, 2 second, 3 least b. 2 most, 1, 3 least c. 3 most, 2, 1 least d. 3 most, 1, 2 least energy released is potential energy at bottom of crater 29 15

Nuclear Weapons (and Energy)

Nuclear Weapons (and Energy) the how, the what and why? Phys 1020, Day 25: Questions? Finish buoyancy Nuclear Weapons Blmfld 16.1 Reminders: work on projects 1 Air vs. Helium Balloon F buoyancy SAME VOLUMES