Nuclear Physics

1

atom – the smallest particle of an element that retains the chemical properties of that element

An atom is composed of…

• Nucleons

– Protons Subatomic

1

Protons

– Neutrons

• Electrons

Nucleons – protons and electrons in the nucleus

Subatomic Particles

Atom Basics

The number of protons determines the element

Neutrons and protons are about the same weight

A l t i b t 1/2000 th f t

2

An electron is about 1/2000 the mass of a proton

A teaspoon of nuclei would weigh more than a battleship!

The nucleus is held together by nuclear forces

Atom Basics

The electrons determine the size of the atom

Electrons move so fast in such a tiny area they make the atom seem solid (Like a moving fan

3

blade)

Size: If the nucleus was a marble… then the entire atom would be the size of a football field.

Sizes of atoms are measured in picometers 1,000,000,000,000 picometers = 1 meter

Subatomic Particles

Electron e- -1 ≈ 0

Symbol Charge Mass(u)

4

Proton

Neutron

p+

n0

+1

0

1

1

11

10

Reading a Periodic Table

3Li Symbol

Atomic Number “Z Number”

(number of protons)

5

LiLithium6.941

Symbol

Atomic Mass (weighted average)

Name

( p )

Basics of Atoms

1) A neutral atom has the same number of electrons as it has protons.

2) Atoms of the same element can have different d t diff t b f t

6

masses due to different numbers of neutrons.

3) Elements with different masses are called isotopes. Nuclide is a term for an isotope.

Nuclear Physics

2

Designating Isotopes

Mass Number = Protons + Neutrons

→ Mass Number – Protons = Neutrons

7

Example: an isotope of uranium has a mass of 235 and an atomic number of 92. How many neutrons?

235 – 92 = 143mass protons neutrons

Designating Isotopes

There are two common ways to present isotopes

Example: an isotope of uranium has a mass of 235 and an atomic number of 92.

8

Nuclear Symbol Hyphen Notation

Uranium-235U23592

massZ

How to remember Nuclear Symbol

Correct Incorrect

U23592 U92

235

9

The nuclear symbol should look like an easy subtraction problem to calculate neutrons.

U23592

143

Isotopes of hydrogen

Electron

NeutronProton

10

Protium Deuterium Tritium

H11 H2

1 H31

mass = 1 mass = 2 mass = 3

99.985% 0.015% Rare & radioactive

Heavy Water

Water composed of deuterium hydrogen is often labeled D2O rather than H2O. Because each molecule is heavier but still the same size, D2O is more dense.

I b f D O i li id H O ill i k!!

11

Ice cubes of D2O in liquid H2O will sink!!

Regular ice

D2O ice

$65 / 100g

Relative Atomic Mass – a separate unit of measurement is used for mass of individual atoms.

amu – Atomic Mass Unit (u)

12

1 amu = 1.660540 x 10-27 kg

The amu is based off 1/12th the mass of Carbon-12

Proton – 1.007276 u

Neutron – 1.008665 u

Nuclear Physics

3

Nuclear Force – force that acts between protons and neutrons.

This is about 100 times stronger than the l t ti f th t t

13

electromagnetic force that causes protons to repel one another.

Nuclear forces only act at a distance of about the radius of a proton.

Binding Energy – The difference in energy between individual nucleons and an assembled nucleus (always negative)

Bi di i t i ll i i l t

14

Binding energy is typically given in electron volts (eV) or million electron volts (MeV)

1 eV = 1.60217 x 10-19 J

Mass Defect - The difference in mass between the total of nucleons and the assembled nucleus.

Einstein related the binding energy to the mass defect.

E

15

Binding Energy for 1 u = 931.49 MeV

2mcE E = energy

m = mass defectc = 3 x 108 m/s

Find the binding energy of He in MeV

The mass of a helium-4 nucleus is 4.002603 u. There are 2 neutrons and 2 protons.

Find the nucleon masses Find the Mass Defect

Protons 2 x 1 007276 u 4 002603 u

42

16

Protons 2 x 1.007276 u 4.002603 u

Neutron 2 x 1.008665 u - 4.031882 u

4.031882 u - 0.029279 u

Convert using the binding energy of 1 u

E = (-0.029279 u)(931.49 MeV / u) = -27.273 MeV

1717

Binding Energy and Stability

The most tightly bound nucleus is iron-56.

Nuclear reactions for elements lighter than 56 will tend to increase the mass. Stars will transform hydrogen into helium carbon and other heavier

18

hydrogen into helium, carbon, and other heavier elements.

Nuclear reactions with for heavier elements will tend to decrease the mass. An atom of Uranium-238 will decay into thorium-232.

Nuclear Physics

4

Three Types of Decay

Alpha Emission – A helium nucleus (positively charged) is emitted from a large atom. This will drop the mass by four and the atomic number by two. The extra electrons from the large atom are quickly absorbed into the environment.

Beta Emission – There are three types of beta emission. This will not effect the mass number but will affect the number of protons or neutrons.

Gamma emission – This will involve the release of energy to stabilize the atom. This often accompanies alpha or beta decay.

19

Penetration Power of Radiation

20

Band of Stability

21

Alpha Emission ( )

22

Beta Emission ( )Electron Emission

23

Beta Emission ( )Positron Emission

24

Nuclear Physics

5

Beta Emission ( )Electron Capture

25

Gamma Emission ( )

This often accompanies alpha and beta decay. This releases excess energy without affecting

the mass or atomic number.Gamma EmissionBeta Decay

26

Thorium – 232 Decay Series

27 28

29

Half Life

Half Life is the time it takes for half of the atoms of an isotope to decay.

To find the amount of material remaining:

t

2/1

2

1AA o

t

A = amount Ao = initial amount

t = time t1/2 = half life 30

Nuclear Physics

6

31

Smoke Detectors

Smoke detectors use Americium-241 as an alpha source to continually ionize the air between two charged plates. This allows current to flow between them.

Smoke particles neutralize the air particles and stop the electric current This will set off the alarm

32

electric current. This will set off the alarm.

Nuclear Fission

Fission is the process of splitting an atomic nucleus Fission is the process of splitting an atomic nucleus into fission fragments of smaller atomic nuclei and into fission fragments of smaller atomic nuclei and neutrons. neutrons.

An atom that can be split called fissionable.An atom that can be split called fissionable.

Fissile nuclei a special type of fission that can have a Fissile nuclei a special type of fission that can have a chain reaction. Not all fissionable nuclei are fissile. chain reaction. Not all fissionable nuclei are fissile.

Fissile nuclei are generally Fissile nuclei are generally heavy atoms with large heavy atoms with large numbers of nucleons. numbers of nucleons. The nuclei are struck by The nuclei are struck by neutrons initiating the fission process. neutrons initiating the fission process.

33

Nuclear Fission

MeVnBaKrUn 2003 10

14156

9236

23592

10

Large amounts of energy are produced by fissionLarge amounts of energy are produced by fission

34

Nuclear Chain Reaction

A classic example of a fission reaction is that of UA classic example of a fission reaction is that of U--235 where stray neutron strikes an atom of U235 where stray neutron strikes an atom of U--235. 235.

It absorbs the neutron and becomes an unstable It absorbs the neutron and becomes an unstable atom of Uatom of U--236 This undergoes fission236 This undergoes fissionatom of Uatom of U 236. This undergoes fission. 236. This undergoes fission.

More neutrons are released in the reaction. These More neutrons are released in the reaction. These neutrons can strike other Uneutrons can strike other U--235 atoms to initiate 235 atoms to initiate their fission.their fission.

Uranium 238 on the other hand is not fissile Uranium 238 on the other hand is not fissile

35 36

Nuclear Physics

7

Natural FissionThe fission process is an a natural one as a French The fission process is an a natural one as a French researcher found a natural uranium reactor in Gabon, West researcher found a natural uranium reactor in Gabon, West Africa; it has been estimated to be over 2 billion years old.Africa; it has been estimated to be over 2 billion years old.

37

Nuclear Power Generation

38

Fuel RodsA fuel assembly consists of a square array of 179 to 264 fuel rods, and 121 to 193 fuel assemblies are loaded into an individual reactor. A reactor could have over 50,000 fuel rods storing 200 tons of uranium!!

39

Control Rods

Control rods slide in and out between fuel rods. They are used to moderate the nuclear reaction by absorbing neutrons. When completely inserted, they can absorb enough neutrons to stop the chain reaction.

Cadmium, Indium, Silver and a variety of rare earth metals are used for control rods.

40

Fusion

Fusion is a nuclear reaction where two light atomic nuclei fuse or combine to form a single heavier nucleus.

Due to the repulsive forces of protons, fusion will only p p , ytake place when nuclei have extreme high thermal energy. The proton-proton chain of the Sun requires temperatures at about 20,000,000 K !!!

The fusion reaction generates a tremendous amount of energy due to protons losing mass to form larger nuclei.

41

Fusion reactions do not occur naturally on our planet but are the principal type of reaction found in stars

The large masses, densities, and high temperatures of stars provide the initial energies needed to fuel fusion

Nuclear Fusion on the Sun:Proton-Proton Chain

stars provide the initial energies needed to fuel fusion

Fusion on the Sun involves turning four protons into a helium nucleus. This process gives off 25 MeV.

By comparison, one dynamite molecule can give of about 20 eV, or less than one millionth the energy.

42

Nuclear Physics

8

Nuclear Fusion on the SunProton-Proton Chain

43

Attempting Nuclear Fusion

Nuclear fusion has been made possible on Earth using a fission reaction for ignition.

Because temperatures required for fusion are so great, confining and harnessing power has not beenconfining and harnessing power has not been achieved.

Currently the most powerful nuclear devices are hydrogen bombs, also called thermonuclear bombs. These use a fission reaction to start the fusion process.

44

Little Boy – Messed up Hiroshima

45

Hiroshima Play Video

46

Little Boy

47

Little Boy

48

Nuclear Physics

9

Fat Man – Messed up Nagasaki

49

Fat Man

50

Fat Man

51

Mushroom Clouds

Cooler, Denser Air

Hot, Less Dense Air

52

Natural Mushroom Clouds

Mount Redoubt Eruption - Alaska 53

Hydrogen Bomb (Thermonuclear)

54

Nuclear Physics

10

Hydrogen Bomb Operation

55

Castle Bravo Cloud – Bikini Island 1954

56

Play Video

1 Mile

Castle Bravo Cloud – Crater

57

Relative Bomb Strengths

58

Fireball Size

59

Tsar Bomba – The Biggest About 57 megatons – 14 miles high

Play Movie

60

Nuclear Physics

11

Fission Process

61

Fission Chain Reaction

62

63

Energy Released

E = mc2

Speed of Light 3 x 108 m/s

Change in Mass

Energy

64

Castle Bravo Cloud – Bikini Island 1954

65