Nuclear Physics and Radioactivity Online Introduction to

Nuclear Physics and Radioactivity

Online Introduction to Nuclear Physics • http: //www. sciencejoywagon. com/physicsz one/lesson/12 nuclear/intronuc. htm • Online lesson on nuclear decay http: //207. 10. 97. 102/chemzone/lessons/11 n uclear/nuclear. htm • Nuclear Fusion http: //ippex. pppl. gov/ippex/About_fusion/I NDEX. HTML

Protons and Neutrons • Atomic nuclei are made of protons and neutrons • Proton is positive mp = 1. 6726 x 10 -27 kg • Neutron is neutral mn = 1. 6749 x 10 -27 kg • Both called nucleons

Courtesy Lawrence Berkeley Laboratory

Different types of Nuclei Are Called Nuclides • Protons and neutrons are nucleons • Atomic number Z is number of protons • Atomic mass number A is protons plus neutrons • Neutron number N = A – Z • Nuclide symbol ZXA

Usually A and Z are on the left Courtesy Lawrence Berkeley Laboratory

What is 7 • • • 15 ? N Chemical element? Nitrogen 7 Atomic number? Atomic mass number? 15 Neutron number? 8 Pronounced? Nitrogen Fifteen

Properties • Atomic properties determined by number of electrons • Nuclei with certain atomic number but different neutron number are called isotopes • Most elements have many isotopes

Nuclear Masses C 12 has mass 12. 000000 u • 6 • Neutron 1. 008665 u • Proton 1. 007276 u • Neutral hydrogen atom 1. 007825 u • By E = mc 2 1 u = 1. 6605 x 10 -27 kg = 931. 5 Me. V/c 2 Try this yourself

2 Rest Masses in Me. V/c • Electron 0. 51100 • Proton 938. 27 • Neutron 939. 57 H 1 atom 938. 78 • 1 • Is hydrogen more or less massive than proton and electron together? • How can you explain this?

Binding Energy • Energy holding the nucleus together • Stable Nucleus called a bound state • Mass of stable nucleus less than sum of masses of protons and neutrons in it • It takes energy to break it apart • Binding energy is negative

Example of 2 • • 4 He 2 x mn = 2(1. 008665 u) = 2. 017330 u 2 x 1 H 1 = 2(1. 007825 u) = 2. 015650 u Sum 4. 032980 u Measured 2 He 4 mass = 4. 002602 u (With electrons) • Difference 0. 030378 u • Must use 1 H 1 instead of p to balance electrons

2 4 continued He • 0. 030378 u x 931. 5 Me. V/c 2/u =28. 3 Me. V • Total binding energy of nucleus • Energy that must go into nucleus to split it into separate nucleons Comparison: binding energy of electron in hydrogen atom is 13. 6 e. V. What does that tell you?

Average Binding Energy per Nucleon

Four Forces of Nature (in order of decreasing strength) • • Strong Electromagnetic Weak Gravity The strong force holds the nucleus together. It is very short range compared to electric and gravity

Radioactivity • Some nuclei change disintegrate into pieces whose total mass is less than mass of nucleus • Called radioactive decay • Discovered by Bequerel in 1896 (U) • Curies found Ra and Po Pitchblende sample

Marie and Pierre Curie • She coined term “radioactivity” • Both won Nobel prize • Pierre killed crossing street • Marie gets his teaching Job at Sorbonne-first Woman to teach there in 650 Years. Later she dies of anemia.

Three Kinds of Radioactivity • Alpha (a) – Positively charged – Least penetrating. Paper stops it • Beta (b) – Negatively charged – ½ cm Aluminum stops it • Gamma (g) – Uncharged, released as photons of light – Most penetrating. Thick lead may not stop it.

Neutron Emission A fourth type of Radioactivity- emits no charged particles, just releases a neutron from the nucleus. By far the most damaging of any type, takes 3 ft of lead or a massive amount of concrete to stop it.

Which Way Will It Bend? Magnetic field in x x x Radium source Lead block

Alpha Decay • Nucleus gives off a particle - 2 He 4 • Z decreases by 2 • A decreases by 4 Ra 226 -->86 Rn 222 + 2 He 4 • 88 • Rn is different element Graphics courtesy of Centennial of Discovery of radioactivity http: //web. ccr. jussieu. fr/radioactivite/english/accueil. htm

Energy in a Decay Energy released is (Mp – Md – ma) c 2 (Mp – Md – ma) = mass defect Mp is mass of parent 88 Ra 226 Md is mass of daughter 86 Rn 222 Energy appears as KE of a particle and daughter (recoil energy) Compare the energy of the a particle with that of the recoiling daughter. • • • What is true about their momenta and directions?

Conservation Laws in Nuclear Processes • • • Total energy is conserved Momentum is conserved Charge is conserved Angular momentum is conserved Number of nucleons (plus anti-nucleons) is conserved

You Find Out • What does Americium 241 decays into • Use your periodic table at back of text Answer 93 Np 237 Neptunium Application Am 241 is used in smoke detectors • 95

Smoke Detector Ionization Chamber Americium source inside Courtesy “How Stuff Works” Smoke particles decrease flow of ionization current

Beta Decay C 14 --> 7 N 14 + -1 e 0 + (anti)neutrino • 6 e 0 is electron (same as b-) • -1 • Z increases by 1; electron from nucleus • A does not change • Occurs for neutron heavy isotopes

Wolfgang Pauli What is a Neutrino? • Massless*, neutral particle that travels with the speed of light (hypothesized by Pauli in 1930) • Incredibly penetrating - passes through Earth • Required to be emitted in beta decay in order that momentum and energy be conserved(beta energies are not unique) • Observed in 1956 by Reines and Cowan • Symbol is n(nu) with bar over it - antineutrino *There is some evidence that the neutrino has a tiny non-zero mass

Positron (Beta+) Decay Ne 19 --> 9 F 19 + e+ + n • 10 • e+ is positron(anti-electron) • Z of nucleus decreases by 1 • A does not change • Occurs for neutron light isotopes

Courtesy Stanford Linear Accelerator Lab Question: What is true about the directions of the daughter nucleus, beta, and neutrino?

Electron Capture • Occurs when nucleus absorbs an orbital electron. Example: Be 7 + e- --> 3 Li 7 + n • 4 • Z of nucleus decreases by 1 • A does not change • Electron disappears and one proton becomes a neutron • X-rays are given off as electrons jump down

Fermi’s Theory • Explained beta decay and EC in terms of a new “weak” force • Fermi was last “double threat physicist; great theorist and experimenter.

Gamma Decay • Emitted when excited nucleus jumps down to a lower energy state NA* ZNA + g • Z • Gamma and x-ray are same, high energy photon

What is true about the momenta of the daughter nucleus and the gamma ray? Courtesy Lawrence Berkeley Laboratory

Review • There are stable nuclides (isotopes) and unstable (radioactive ones) • Stable means mass of pieces is more than that of whole nucleus. • Unstable means opposite • MOST isotopes are NOT stable; they undergo one form of decay or another

Radioactive Decay Law • Decay is random process • no. decays in short time DN = -l. NDt • N = N 0 e-lt by integration • Decay constant is called l (rate of decay) • Number of decays per second also proportional to e-lt “exponential function” • Half life = time for half of original sample to decay = 0. 693/l • Link for decay simulation

Exponential Decay Curve N = N 0 -lt e

Question • A sample contains about 1000 nuclei of a certain radioisotope. The half life is four minutes. About how many nuclei will remain after 16 minutes? • Hint: make a table Answer: about 62 nuclei

Decay Table Number of Half Lives 1 Fraction of Nuclei Remaining 1/2 2 1/4 3 1/8 4 1/16

Randomness of Decay • No way to tell which nucleus will decay when • Actual number that decay varies around a most probable number • Uncertainty is proportional to

Decay Series • A chain of successive decays Starting with U 238

Radioactive Dating • n + 7 N 14 6 C 14 + p provides continual supply of carbon 14 at about rate of decay C 14 --> 7 N 14 + -1 e 0 + antineutrino • 6 • When organism dies no more supply so ratio of carbon 14 to 12 decreases – with 5730 yr half life • Useful for dating objects up to 60, 000 years old

Nuclear Reactions • Transformation of one element into another is called transmutation. • Sought unsuccessfully by Alchemists • Usually happens in collision • Rutherford(1919) discovered in 2 He 4 + 7 N 14 8 O 17 + 1 H 1

Conservation Laws in Nuclear Reactions • • • Momentum Energy Charge Nucleon(Baryon) Number – heavy particles Lepton Number – light particles

Example: slow neutron reaction n 1 + 5 B 10 3 Li 7 + ? • 0 • Answer 2 He 4 which is also called an • Alpha particle • Challenge: Given speed of helium atom 9. 30 x 106 m/s find the – Velocity and KE of the lithium atom – Hint: what is initial momentum of the system?

Nuclear Fission and Fusion • In fission a large nucleus breaks apart releasing energy • In fusion light nuclei merge to form a heavier nucleus and energy is released.

Nuclear Fission • Uranium nucleus absorbs neutron and splits in two • Easier to do with 92 U 235 than common U 238 92 • Discovered Germany 1938 • Dangerous time

Courtesy students at Illinois Math and Science Academy

Nuclear Chain Reaction Courtesy Nuclear Energy/Nuclear Waste. Chelsea House Publications: New York, 1992.

Above All, Fission Produces Heat

Application: Nuclear Power Plant

How Control Rods Moderate Reaction http: //www. npp. hu/mukodes/anim/sta 1 -e. htm

Diablo Canyon Nuclear Plant – PG&E Power Output 1100 MW each Domes are 215 feet high Courtesy Jim Zim

Ranch Seco Nuclear Plant Near Sacramento • Shut down in 1989 • De-commissioning still underway • Planned completion 2011

Three Mile Island Nuclear Plant • • Partial meltdown, March 28, 1979 50% of reactor core destroyed or melted Hydrogen bubble forms inside containment Metropolitan Edison lies about radiation release • Situation stabilized without injuries

Meltdown Scene Chernobyl Nuclear Plant Unit 4 Operating Power 3. 2 GW Thermal, 1 GW electrical Estimated number of radiation victims = 3. 2 million 400 times more radioactivity was released than in the explosion of the Hiroshima Atomic Bomb

2001 Power Crisis Strikes California • Nuclear Power plant proposed for Alameda Point, Alameda • What do you think?

Application: Atomic Bomb Explodes When Critical Mass Assembled Little Boy High explosive Fat Man High explosive Plutonium 239 (uses implosion)

Fission Bombs • Destructive Force about 20, 000 tons of TNT • 1945: Hiroshima and Nagasaki destroyed • 100, 000+ civilians killed

Nuclear Fusion • Light nuclei come together (fuse) to form heavier nucleus • Mass of product greater than sum of pieces • Large energy release • Powers the Sun • Used to make H-bombs – “thermonuclear bombs”

Powering the Sun H 1 + 1 H 1 1 H 2 + e+ + n 0. 42 Me. V • 1 H 1 + 1 H 2 2 He 3 + g 5. 49 Me. V • 1 He 3 + 2 He 3 2 He 4 +1 H 1 + 1 H 1 12. 86 Me. V • 2 • Proton-proton chain powers the sun • Net effect: 4 protons combine to form one helium nucleus

Condition for Fusion • Product needs more binding energy than reactants • Reactants must be heated to millions of degrees to get close enough for nuclear reaction to be possible(very hot plasma) • Overcome coulomb repulsion • Nuclear forces very short range

Reactions for Controlled Fusion • • • H 2 + 1 H 2 1 H 3 + 1 H 1 4. 03 Me. V 1 H 2 +1 H 2 2 He 3 + n 3. 27 Me. V 1 H 2 + 1 H 3 + 2 He 4 + n 17. 59 Me. V 1 H 1 is proton 1 H 2 is deuteron (deuterium - stable) 1 H 3 is triton (tritium, half life 12. 3 years) 1

Question • How can you recognize a fusion reaction? Makes lighter elements into heavier ones Releases energy

Fusion Reactors – The Allure Extract Fuel from Water Courtesy Princeton Plasma Physics Laboratory

Fusion Reactors – The Challenge • Need to create conditions at center of a star • Need to contain bulk amounts of plasma at temps above 20 million degrees • Need to get more energy out than you put in • Need to demonstrate on commercial scale

Possible Design

Tokamak – Magnetic Confinement in a Hollow Doughnut (Torus) Courtesy Princeton Plasma Physics Laboratory