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Chapters 39 & 40

Radioactivity and Nuclear

Physics

The Atomic Nucleus

• Nucleons—the protons (+ charge) and neutrons (0 charge) in the nucleus of an atom.

– Neutrons and protons have close the same mass (neutron is slightly larger)

– Nucleons are 2,000 times more massive than electrons.

– Protons and electrons have charges that are equal in magnitude but opposite in sign.

– The # of protons = the # of electrons.

• The # of protons determines the chemical properties of the atom.

• Strong force—the attractive force that holds the nucleus together.

– Strong only over short distances

– Neutrons increase the attractive strong force and prevent protons from electrically repelling one another.

• The more protons there are, the more neutrons that are needed

Radioactive Decay

• Radioactive—term used

to describe the

spontaneous decay of

atomic or subatomic

particles.

– The more protons that are in a nucleus, the more neutrons you need to hold it together.

– For elements over 83 protons, the addition of extra neutrons cannot stabilize the nucleus

Types of decay

• All elements above bismuth (83) decay in some way.

• There are three types named after the first three letters in the Greek alphabet.

• Alpha α

• Beta β

• Gamma χ

Neutron stability

• One factor the effects stability of nucleus is the instability of the neutron

• A lone neutron will spontaneously decay into a proton and an electron.

• If you have a bunch of neutrons, about half of them will decay in about 11 minutes.

• A lone neutron is radioactive

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Alpha Rays

• Have a positive

charge

• a stream of

particles made of

two protons and

two neutrons (He)

– Alpha particles

• Can be stopped by

a sheet of paper

Alpha Decay

• Parent atom is the original atom

• It breaks into the daughter

nucleus through a process

called transmutation

• This can happen over and over

until a stable nucleus is found.

Beta Ray

• A stream of

electrons

• emitted from a

nucleus when a

neutron forms a

proton and an

electron

• can be stopped

by a sheet of Al

Gamma Ray

• Basically massless energy--photons

• High frequency electromagnetic energy

• Emitted when nucleons jump in nuclear energy levels

• Stopped by a thick layer of lead

Radioactive Isotopes (39.4)

• Isotopes—atoms of an element that differ

in their number of neutrons.

– Hydrogen has 3 isotopes

• The common isotope—1 proton

• Deuterium—1 proton, 1 neutron

– Heavy water—water molecules that contain

deuterium

• Tritium—1 proton, 2 neutrons

– Tritium is unstable and undergoes beta decay.

Summary

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Isotope Symbols (p. 613)

• Information is about the nucleus only

(nothing about the electrons).

• Bottom number is protons.

• Top number is protons + neutrons.

Isotopes -

Same Protons

Different Neutrons

Radioactive Half-Life (39.5)

• Half-life—the time needed for

half of the radioactive atoms to

decay.– Rates of radioactive decay appear to be

absolutely constant.

– ½ life can be calculated from the rate of

disintegration.

• The shorter the ½ life, the faster it

disintegrates.

• Geiger counters measure the rate of

disintegration.

– Radium-226 has a ½ life of 1,620 years.

– Uranium-238 has a ½ life of 4.5 billion years.

Large Nuclei – what

happens to them?

Half-Life

• The “pile” doesn’t get smaller.

• The unstable nuclei don’t disappear, they

change into another type of nucleus.

• The pile becomes less radioactive, not

smaller (unless the new element is a gas!)

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Half Life Lab- Carbon 14 Dating

Isotope Symbols (p. 613)

• Information is about the nucleus only

(nothing about the electrons).

• Bottom number is protons.

• Top number is protons + neutrons.

Transmutation of

Elements

• When you give off an alpha or

beta particle, a new element is

made.

• Consider this for U-238

Natural Transmutation of Elements

• Transmutation—the changing of one

element into another.

– The emission of an alpha or beta particle

from the nucleus is one cause of

transmutation.

– Alpha particle emission causes the

atomic # to decrease by two

– Beta particle emission causes the atomic

# to increase by one.

• Consider the transmutation of

Uranium into Thorium

Alpha Decay

• Figure p. 617; decay of uranium 238 into

thorium 234

– Number of P ↓2

– Mass Number ↓4

– Number of Neutrons ↓2

– Notice that the mass numbers balance and the

atomic numbers balance.

• A helium ion spontaneously emitted to

achieve a more stable nucleus (state)

Beta Decay

• The thorium above, is also radioactive and

transmutes into protactinium (bottom p.617)– Number of P ↑1

– Mass Number = (stays the same)

– Number of Neutrons ↓1

• A neutron emits an electron (a beta particle)

becoming (changing into) a proton

• Still get a new element

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U-238 cycle (shown on next slides)

1. Undergoes alpha emission to

become Th-234

2. Th-234 undergoes beta

emission to become Pa-234

3. Pa-234 undergoes beta

emission to become U-234

• U-234 Th-230 etc.

1. Decay of U238

• Uranium is

element 92

• Alpha decay –

loses 2P and 2N

• New atomic

number is 90 (this

is Thorium)

• New mass is 234

p. 619

2. Decay of Th234

• Thorium is

element 90

• Beta decay – 1P

becomes 1N & 1e-

• New atomic

number is 91 (this

is Protactinium)

• Mass is still 234

3. Decay of Pa234

• Protactinium is

element 91

• Beta decay – 1P

becomes 1N & 1e-

• New atomic

number is 92 (this

is Uranium)

• Mass is still 234

1. Decay of U234(a new example)

• Uranium is

element 92

• Alpha decay –

loses 2P and 2N

• New atomic

number is 90 (this

is Thorium)

• New mass is 230

p. 619

Writing Nuclear

Equations

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Some Other Useful Symbols

He4

2

e0

1−

n1

0

Alpha Particle

Beta Particle

Neutron

What has to remain the same on the two

sides of a chemical equation?

Rules

• Total Number of Nucleons

remains constant

• Charge is Conserved

Alpha Decay of Pu-239

Beta Decay Gamma Decay

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Carbon Dating (39.8)

• The earth’s atmosphere is bombarded by

cosmic rays (mostly protons).

– Most protons capture an electron to form

hydrogen atoms in the upper atmosphere.

– Neutrons travel longer distances and may be

captured by the nucleus of a nitrogen atom.

+N14

7 C14

6 H1

1+n1

0

– Less than one-millionth of 1% of the Carbon in the

atmosphere is carbon-14.

Carbon Dating– C-14 joins with oxygen to form carbon dioxide,

which is taken up by plants and organisms

that ultimately consume them.

– C-14 is a beta-emitterC

14

6 N14

7 e0

-1+

– A fixed ratio of C-14 to C-12 is maintained in an organism’s

body as long as it is alive.

– The longer an organism is dead, the less C-14 left in its

remains.

– The ½ life of C-14 is 5,730 years.

– Fluctuations in the production of C-14 result in dates that have

an uncertainty of 15%.

Radioactive Tracers

• Radioactive isotopes of all

elements have been produced by

bombarding the element with

neutrons and other particles.

• Tracers are radioactive isotopes

that can be used to measure the

rate of some process of interest.

– Uptake of fertilizer by plants

– Metabolic processes within the body.

Radiation and You (39.11)

• Radioactive decay warms the center of the earth.

• Helium comes from alpha particles that were once shot out of radioactive nuclei.

• Most radiation we are exposed to comes from outer space.

– The atmosphere deflects much of this radiation.

– We are bombarded most by neutrinos.

• The most common high-speed particles

• Have near zero mass and no charge

• Billions pass through your body each second

– About once per year a neutrino triggers a nuclear reaction in your body.

• Most pass completely through the earth

• Gamma radiation is the most dangerous and comes from radioactive materials

– Causes genetic mutation

• Beta particles also can cause genetic mutation.

Nuclear Fission (40.1)

• Nuclear fission—the splitting of atomic nuclei.

• In all known nuclei, the nuclear strong forces dominate over the repulsive electrical force.

• If a uranium nucleus is elongated, the electrical force takes over causing it to split.– The absorption of a neutron by a uranium nucleus

supplies enough energy to cause this elongation.

– Between 2 and 3 neutrons are produced in most nuclear fission reactions.

– These neutrons can cause the fissioning of 2 or 3 other nuclei, releasing between 4 and 9 additional neutrons.

– This may lead to a chain reaction.

Chain Reactions

– A chunk of U-235 smaller than a baseball would still not fission.• Too many neutrons would find their way to the surface

before striking a U-235 atom.

– Critical mass—the amount of mass for which each fission produces, on average, one additional fission event.• Subcritical mass—one in which the chain reaction dies

out.

• Supercritical mass—one in which the chain reaction builds up explosively.

n1

0 Kr91

36 +

• Fission occurs mainly for U-235.

– Makes up 0.7% of the uranium in pure uranium metal.

– U-238 absorbs neutrons without fissioning.

+ U235

92 Ba142

56 + 3( n1

0 )

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Chain Reaction

The Nuclear Fission

Reactor (40.2)• About 21% of electric energy in the U.S. is

produced by nuclear fission reactors.

• 3 main components to a fission reactor

– Nuclear fuel combined with a moderator to slow down

neutrons.

• Fuel = Uranium, with its fissionable isotope U-235 enriched

to ~3%.

• Moderator = graphite, a pure form of carbon, or water.

– Control rods

• Usually made from cadmium or boron, which readily absorb

neutrons.

• Control how many neutrons from each fission event are

available to trigger additional fission events.

– Water used to transfer heat from the reactor to the

generator.

Nuclear Fission ReactorWhy doesn’t this happen

in Uranium Deposits?

• This type of chain reaction only

occurs with the rare U-235 (0.7%

of natural U)

• U-238 will absorb the neutrons

not allowing the chain reaction.

• U-238 can “snuff out” the

reaction

Energy Released

• The energy released by an atom of U is about 7 million times that of a molecule of TNT

• KE of fragments and neutrons, and gamma radiation.

Explosions

• If the chunk of U-235 were the size of

a baseball, an enormous explosion

would result.

• If the chunk were smaller, there

would be a chance that many

neutrons would escape the surface

before hitting another.

• We have a critical mass that we

need.

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Nuclear Bomb!!!

• Start with two subcritical masses.

Neutrons reach the surface too

readily to have an explosion.

• Now force the two masses together

in a small area (use TNT)

• the combined mass is supercritical

and fission occurs.

• BOOOM!!!!

Plutonium

• U-239 is created when U-238 absorbs a

neutron.

• U-239 emits a beta particle and forms

Neptunium-239 )(1/2 life = 2.3 days).

• Np-239 emits a beta particle to form

Plutonium-239 )(1/2 life = 24,000

years).

• Pu-239 can be easily separated from

uranium.

The Breeder Reactor

• When mixed together, the fissioning of Pufrees neutrons that convert U-238 into more Pu-239.– Produces useful energy

– Breeds more fission fuel

• After a few years of operation, breeder reactors breed twice as much fuel as they start with.

• This is like refilling a gas tank with water

and making gas.

Mass-Energy Equivalence

(40.5)• Mass and energy are equivalent.

– “E = mc2”

– Mass is like a super storage battery.

– When mass decreases, the stored up energy is released.

• A nucleon inside a nucleus has less mass than its rest mass outside the nucleus.

– For Uranium, the difference = 0.7%

– The binding energy is greatest for iron

• The mass difference is related to the “binding energy” of the nucleus.

– This represents the amount of work it would take to disassemble the nucleus.

Nuclear Fusion

• The steepest part of the hill is from H-->Fe

• If we could stick two atoms together, the release in energy would be enormous!!

• Nuclei are positively charged. They would have to combine at a very high speed to stick (aka very high temp)

• thermonuclear fusion

Fusion

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The SunFusion vs Fission

• Fusion does not have chain

reactions to control (no big

boom)

• No pollution

• Produced He (yay balloons!)

• No deadly products from

reaction

• The next slides contain

additional information for

chapter 40

Radiation and you!

• It is all around you!

• Most of it comes from nature,

cosmic and minerals.

• X-rays can give a great portion

• Nuclear fallout, power plants,

etc.

• Nothing to really worry about.

Fission

• The Splitting of

Atomic Nuclei

• Think about the

forces involved

in the nucleus

• Strong force &

electromagnetic

force

Nuclear Fission

Reactors

• 21% of US power

is nuclear

• One Kg of

Uranium is as

affective as 30

freight-car loads

of coal.

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Nuclear Reactor Elements of a reactor

• Nuclear Fuel: U-235 enriched to about 3%.

• Moderator to control the reaction (may be graphite or water)

• Control Rods- can be moved in and out of the reactor to control neutron multiplication. Usually made of cadmium or Boron to absorb neutrons safely.

• Water is heated and used to turn generator.

Nuclear Waste

• When U splits into smaller atoms, they still have too many neutrons.

• Said to be neutron-rich.

• This makes them radioactive.

Plutonium

• Can be made from transmutation

of Uranium and separated from

the Uranuim by chemical means.

• Has a very long half-life and is

very dangerous to humans

(cancer)

• Used in Breeder reactions!

One Million Years for the

Energy to Reach the Surface p. 639

p. 640

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Controlling Nuclear

Fusion

• To get things this hot we need a

container to hold them. It would

melt through anything that we

have.

• Use magnetic fields to suspend

dueterium that is heated with

lasers.