Nuclear Physics and Radioactivity

Online Introduction to Nuclear Physics

• http://www.sciencejoywagon.com/physicszone/lesson/12nuclear/intronuc.htm

• Online lesson on nuclear decay http://207.10.97.102/chemzone/lessons/11nuclear/nuclear.htm

• Nuclear Fusion http://ippex.pppl.gov/ippex/About_fusion/INDEX.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 7N15 ?

• Chemical element?

• Atomic number?

• Atomic mass number?

• Neutron number?

• Pronounced?

Nitrogen

7

15

8

Nitrogen Fifteen

Properties

• Atomic properties determined by number of electrons

• Nuclei with certain atomic number but different neutron number are called

• Most elements have many isotopes

isotopes

Nuclear Masses

• 6C12 has mass 12.000000 u

• Neutron 1.008665 u

• Proton 1.007276 u

• Neutral hydrogen atom 1.007825 u

• By E = mc2 1 u = 1.6605 x 10-27 kg =

931.5 MeV/c2

Try this yourself

Rest Masses in MeV/c2

• Electron 0.51100

• Proton 938.27

• Neutron 939.57

• 1H1 atom 938.78

• 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 2He4

• 2 x mn = 2(1.008665 u) = 2.017330u• 2 x 1H1

= 2(1.007825 u) = 2.015650u• Sum 4.032980u• Measured 2He4 mass = 4.002602u (With electrons)

• Difference 0.030378u

• Must use 1H1 instead of p to balance electrons

2He4 continued

• 0.030378u x 931.5 MeV/c2/u =28.3 MeV

• 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 eV. 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 teachingJob at Sorbonne-firstWoman to teach there in 650Years. Later she dies of anemia.

Three Kinds of Radioactivity

• Alpha ()– Positively charged– Least penetrating. Paper stops it

• Beta ()– Negatively charged– ½ cm Aluminum stops it

• Gamma ()– uncharged– Most penetrating. Thick lead may not stop it.

Which Way Will It Bend?

Magnetic field in x x x

Lead block

Radium source

Alpha Decay

• Nucleus gives off particle - 2He4

• Z decreases by 2

• A decreases by 4

• 88Ra226 -->86Rn222 + 2He4

• Rn is different

elementGraphics courtesy of Centennial of Discovery of radioactivity http://web.ccr.jussieu.fr/radioactivite/english/accueil.htm

Energy in Decay

• Energy released is (Mp – Md – mc

• (Mp – Md – m mass defect

• Mp is mass of parent 88Ra226

• Md is mass of daughter 86Rn222

• Energy appears as KE of particle and daughter (recoil energy) Compare the energy of the 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 93Np237 Neptunium

Application

•95Am241is used in smoke detectors

Smoke Detector

Courtesy “How Stuff Works”

Ionization Chamber

Americium source inside

Smoke particles decrease flow of ionization current

Beta Decay

• 6C14 --> 7N14 + -1e0 + (anti)neutrino

• -1e0 is electron (same as -)

• Z increases by 1; electron from nucleus

• A does not change

• Occurs for

neutron heavy

isotopes

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 nu with bar over it - antineutrino

Wolfgang Pauli

*There is some evidence that the neutrino has a tiny non-zero mass

Positron (Beta+) Decay

• 10Ne19 --> 9F19 + e+ +

• 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:

•4Be7 + e- --> 3Li7 +

• 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

• ZNA* ZNA +

• Gamma and

x-ray are same,

high energy photon

Courtesy Lawrence Berkeley Laboratory

What is true about the momenta of the daughter nucleus and the gamma ray?

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 N = -Nt

• N = N0e-t by integration

• Decay constant is called rate of decay)• Number of decays per second also proportional to

e-t “exponential function”• Half life = time for half of original sample to

decay = 0.693/• Link for decay simulation

                                                   

Exponential Decay Curve N = N0e-t

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

Fraction of Nuclei Remaining

1 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

n

Decay Series

• A chain of successive decays

Starting with U 238

Radioactive Dating

• n + 7N14 6C14 + p provides continual supply of carbon 14 at about rate of decay

•6C14 --> 7N14 + -1e0 + antineutrino

• 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

2He4 + 7N14 8O17 + 1H1

Conservation Laws in Nuclear Reactions

• Momentum

• Energy

• Charge

• Nucleon(Baryon) Number – heavy particles

• Lepton Number – light particles

Example: slow neutron reaction

•0n1 + 5B10 3Li7 + ?

• Answer 2He4 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 92U235 than common 92U238

• 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/sta1-e.htm

Diablo Canyon Nuclear Plant – PG&E

Courtesy Jim Zim

Power Output 1100 MW each

Domes are 215 feet high

Ranch Seco Nuclear PlantNear 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 SceneChernobyl 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 BombExplodes When Critical Mass Assembled

Fat Man

(uses implosion)

Little Boy

High explosive

High explosive Plutonium 239

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

• 1H1 + 1H1 1H2 + e+ + 0.42 MeV

• 1H1 + 1H2 2He3 + eV

• 2He3 + 2He3 2He4 +1H1 + 1H1 12.86 MeV

• 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

• 1H2 + 1H2 1H3 + 1H1 4.03 MeV

• 1H2 +1H2 2He3 + n 3.27 MeV

• 1H2 + 1H3 + 2He4 + n 17.59 MeV

• 1H1 is proton

• 1H2 is deuteron (deuterium - stable)

• 1H3 is triton (tritium, half life 12.3 years)

Question

• How can you recognize a fusion reaction?

Makes lighter elements into heavier ones

Releases energy

Fusion Reactors – The Allure

Courtesy Princeton Plasma Physics Laboratory

Extract Fuel from Water

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