the physical universe: nucleus

Chapter 7

The Physical Universe, 11/e

Konrad B. Krauskopf, Prof. Emeritus of Geochemistry, Stanford Univ.Arthur BeiserISBN: 0072418265Copyright year: 2004

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 7

The Nucleus

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The Nucleus: Main Ideas Atomic Structure

Rutherford Model Nuclear Structure

Radioactivity Radioactive decay Half Life Radiation Hazards

Nuclear Energy Units Binding Energy

Fission and Fusion Elementary Particles

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The Atom and its Nucleus The atom was accepted as the building block

of matter More experiments were needed to better

understand the details of atomic structure…

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Very little was known The nucleus of the atom was a mystery until

Ernest Rutherford set out to investigate the inside of the atom

In 1911, Rutherford performed a series of experiments in which he bombarded a gold foil with energetic particles

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Rutherford’s discovery The relatively heavy particles passed

through the gold fold with very little deflection

Rutherford concluded that the nucleus must be VERY SMALL VERY MASSIVE (and thus DENSE) OVERALL POSITIVELY CHARGED

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Quick look at simple Atomic Structure All atoms are composed of

NEUTRONS PROTONS ELECTRONS

All atoms have a nucleus contains NUCLEONS That means PROTONS and NEUTRONS

These are “nucleons”

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Rutherford’s conclusion

MOST of the volume of the atom is

EMPTY SPACE Nucleus occupies 1 trillionth of the volume

this means the nucleus is very massive and

The mass of the proton is about 2000 times that of an electron

dense

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Atom Identity Atoms are identified by

the number of protons in the nucleus Atoms are arranged in the periodic table

according to increasing number of protons The number of protons in the nucleus is called the

ATOMIC NUMBER Simplest atom

H (Hydrogen)1 proton, 1 electron

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Isotopes Since an atom is identified by the number of

protons, changes to the number of protons result in a completely different atom

However, changes in the number of NEUTRONS results only a change in mass

These variations in neutron number for a specific atom are called ISOTOPES

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NUCLIDE A nucleus with a particular composition Noted by the symbol:

XAZ

The ‘X’ stands for the chemical symbol for an element Z is the atomic number (# of protons) A is the MASS NUMBER

(# protons +#of neutrons)

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Radioactivity Becquerel discovered accidentally:

Uranium produces penetrating radiation Pierre and Marie Curie did further

experimentation Discovered more materials that exhibited the

same behavior

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Radioactive elements Most elements do NOT have radioactive

isotopes About 2000 nuclides have been identified. Of

these only 256 do not undergo radioactive decay.

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Radioactive atoms What is “radioactivity”??

Radioactivity is the term given to the

spontaneous “falling apart” of an atomic nucleus

When the nucleus falls apart, LARGE AMOUNTS OF ENERGY

can be released…..

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Why do nuclei decay? Reasons for nuclear decay:

LARGE NUCLEUS The strong force that holds the nucleus together

acts only over very short distances Large nucleus large separation

The heaviest stable nucleus has 83 protons (Bismuth)

RATION OF NEUTRONS TO PROTONS If the ration is too large, or too small, the nucleus is

unstable

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Radioactivity

Types of decay:

Alpha () emissionBeta () emissionGamma () emissionElectron CapturePositron Emission

When these processes occur, a different element may be formed

The nucleus of an atom becomes UNSTABLE.As a radioactive nucleus begins to DECAY,

these particles/rays are emitted

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Five Decay Processes

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What’s the difference?? Alpha emission (lowest energy)

stream of particles that are identical to helium nuclei not from helium, but pieces of a larger nucleus of a “heavy”

atom Alpha emission can be stopped by a thin layer of skin

or paper emission of an alpha particle means

the nucleus of the decaying atomis reduced by 2 protons and 2 neutrons

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What’s the difference?? Beta Emission (medium energy)

when a neutron becomes a proton by emitting an electron

so, beta emission is actually a stream of energetic electrons

can be stopped by a thin sheet of aluminum or even thin layers of clothing

THE NUCLEUS CHANGES WITH THE LOSS OF 1 NEUTRON, GAIN OF ONE PROTON

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What’s the difference?? (cont.) Gamma Emission (highest energy)

Gamma rays originate inside the nucleus of heavy atoms

energy HIGHER than x-rays gamma rays will penetrate even into a thick sheet of

lead Gamma radiation is VERY DANGEROUS

NUCLEAR MAKEUP IS UNCHANGED

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Alpha Beta Gamma

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After Decay When an atomic nucleus is unstable, decay brings

the nucleus to a more stable state The final product of nuclear decay is a stable

element This may require numerous decay steps

Uranium 238 requires 8 alpha decays and 6 beta decays to eventually become Lead 206, a stable element

U23892 Pb206

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WHEN DO ATOMS DECAY? Any unstable nucleus EVENTUALLY decays For a collection of UNSTABLE nuclei, the

occurrence of DECAY is SPONTANEOUS RANDOM

You can’t tell which one will decay first, second, etc.

Furthermore, we cannot PREDICT what a half life will be based on the fundamental properties of the atom!

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BUT, we can MEAUSURE We can measure the decay as it happens THE PERCENTAGE OF NUCLEI THAT DECAY

EACH SECOND REMAINS CONSTANT Decay means the emission of Alpha, Beta, or Gamma

Radiation. The “TIME” of the decay is characterized by a

MATERIAL CONSTANT

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Characterizing Radioactive Elements What is a “half-life”??

The half life of a radioactive isotope is the time necessary for

half of any given quantity of the material to decay.

For example: If you had a collection of 100 radioactive atoms, and after 20 minutes, 50 of the atoms had undergone decay -we say the half life is 20 minutes

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More about half life The “half-life” of a radioactive isotope is the amount of

time needed for 1/2 of the material to DECAY

this time is the same for ANY AMOUNT MOST half-life times are too long to sit and measure

like Radium 226, half life = 1620 years! half-life is determined by measuring the RATE at which

decay occurs, and then EXTRAPOLATING to find out the half life

FAST rate -> SHORT half-lifeSLOW rate -> LONG half-life

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Here is how it goes Half life is really the AVERAGE TIME for a single

atom to decay Let’s say we start with 100 atoms of X with a half

life of 10 minutes After 10 minutes, we would have 50 X atoms After 20 minutes, we would have 25 X atoms After 30 minutes, we would have 12-13 X atoms After 40 minutes, we would have 6.5 X atoms

Each ‘time period’ of one half life, means half of the remaining atoms have PROBABLY undergone decay…

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Radiation Hazards All ionizing radiation is harmful to living tissue The hazardous effects of radiation exposure may

not be immediate The harmful effects of radiation may result in

GENERATIONAL damage to living organisms MOST radiation is “background”

This is natural and generally unavoidable exposure

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Radiation Exposure: Sources

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Measuring radiation exposure Exposure to radiation is defined in terms

of DOSAGE The unit for DOSAGE is

SIEVERTS (Sv) 1 Sv is the amount of any radiation that has

the same biological effects as those produced when 1kg of tissue absorbs 1 Joule of x-rays or gamma rays

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Typical Radiation doses Natural sources: 3mSv per year Medical and Dental x-rays: 0.6 mSv per

year Typical mammogram: 4 mSv per year Average per individual:

About 3.6mSv per year

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Dose Limits In many countries the recommended dose

limit is 20 mSv the Corresponds to estimated risk of cancer for 1 in

1000 In the U.S. the recommended dose limit is

set at 20 mSv The risk of radiation induced cancer is much

smaller than other hazardous activities such as smoking!

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Nuclear Energy Using the atomic nucleus as an energy

source provides an alternative to fossil based fuel sources

Using nuclear energy for good has been contrasted by using nuclear energy for mass destruction

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The atomic mass unit The atomic mass is small when compared

with everyday objects A new unit is required to describe the

small mass of the atomTHE ATOMIC MASS UNIT

(u)1 u = 1.6610-27 kg

Note: this is the approximate mass of a hydrogen atom

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The Electronvolt The most commonly used unit for atomic

physics is the

Electronvolt (eV)

1 eV = 1.60 10-19 Joules For example

It takes 14.5 eV to remove an electron from a neutral Nitrogen atom

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The MEGAelectronvolt

Because the energies involved in nuclear decay are large, the electron volt is not useful to describe nuclear energies

Instead the megaelectronvolt (MeV) is used 1 million eV = 1MeV 1MeV = 1.60 10-13 Joules

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What holds the nucleus together If you think about all those (+) protons being

jammed into such a small space, It seems like, since they have SAME CHARGE, they

would REPEL EACH OTHER!!! So, how is it that the DENSE, DENSE, DENSE

NUCLEUS of an atom stays together?? THERE MUST BE SOME ATTRACTIVE

FORCE BETWEEN NUCEAR PARTICLES THAT OVERCOMES THE NATURAL ELECTROSTATIC REPULSION…. This is called the STRONG NUCLEAR FORCE

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When a nucleus decays Energy is given off as the strong nuclear

forces work is done against strong nuclear forces

The energy is a result of the transformation of some of the mass in the nucleus to ENERGY

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Energy Equivalent The energy equivalent of the missing mass

of a nucleus is called the binding energy of the nucleus.

Nuclei with larger binding energies require more energy to break up the nucleus

Typical binding energies are large! For stable nuclei: 2.2 MeV to 1640 MeV

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Binding energy per nucleon

The ratio of the total binding energy with the number of nucleons

Gives a way to characterize the stability of a particular nucleus

When the ratio is analyzed graphically, remarkable conclusions can be made

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Two remarkable conclusions If a heavy nucleus were to be divided into

two smaller nuclei, the binding energy per nucleon of the two smaller nuclei WILL BE LARGER!

If two lighter nuclei were to be joined together, the binding energy per nucleon of the heavier nucleus WILL BE LARGER

The ‘center’ point is the most stable element: Iron 56

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Fission/Fusion means CONfusion?!? Fission: (heavy elements)

separation of an atomic nucleus accompanied by a large release of ENERGY

Fusion: (light elements)joining together of two small atomic nuclei accompanied by a large release of ENERGY

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Binding energy to Nuclear Energy NUCLEAR ENERGY

The binding energy per nucleon lead scientists to understand that changes in nuclear structure might be used to harness tremendous amounts of energy through

NUCLEAR FISSION

NUCLEAR FUSION

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Difficulties in harnessing nuclear energy The spontaneous chain reaction

When a nucleus begins to decay, several neutrons may be released.

These neutrons can cause neighboring nuclei to begin decay

This is GOOD, since it means that the decay reaction will continue

BUT, this causes difficulties due to the spontaneous nature—making the reaction difficult to control

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Chain reaction

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The Chain Reaction Too few neutrons: chain reaction ceases

quickly Too many neutrons: chain reaction is

uncontrollable, the reaction becomes volatile Energy release causes an explosion

This realization quickly led to the exploration of the use of nuclear energy for weapons

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Nuclear Fission Reactor First reactor built by Enrico Fermi at University

of Chicago in a squash court in the basement under an athletic

field... Fermi discovered that the way to safely control the

rate of the chain reaction was through the use of a MODERATOR element

carbon (graphite) rods

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Historic anniversary December 2, 1942 the first atomic reactor was brought to

criticality by Dr. Enrico Fermi in Chicago Illinois.

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KEY – 1 - : CRITICALITY Getting the chain reaction

means that neutrons emitted by one U235 will cause another U235 atom to fission, and so on.

A certain amount of mass is required in order for a CHAIN REACTION to occur CRITICAL MASS is the amount needed to

sustain this reaction.

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KEY –2-: MODERATION The reaction must be ‘slowed down’ in order to fully

exploit all the possible available nuclei Slower neutrons increase the probability for the U atom

fission. There are several ways to moderate the chain reaction

In Fermi’s reactor, graphite rods worked to SLOW DOWN the neutrons emitted.

In some modern reactors, water is used as the moderating medium

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Modern Nuclear Power Plants Most power plants use Uranium as the radioactive

source Uranium 238 atoms

The basic process uses the heat generated by the U decay to produce steam The steam is then used to turn turbines which produce

electric energy

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Nuclear Nuclear fuelfuel

Nuclear Nuclear fuelfuel

Super heated water (enclosed)

LAST, and VERY IMPORTANT is the COOLING of the whole system. This is the ONLY WATER THAT IS NOT

COMPLETELY ENCLOSED. Usually comes from a nearby lake or river, recirculated back into the river…

NUCLEAR DECAY PRODUCES HEAT

ENCLOSED water circulates around fuel—gets HOTHOTHOT

More ENCLOSED water is heated to boiling, producing steam,

which turns a turbine—causing the coils of an ELECTRIC GENERATOR to rotate---remember Ampere’s law?

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What about nuclear FUSION? Fusion reactions can release more energy

than fission reactions For example: Our Sun!

Three Issues HIGH temperature (>100 million C) High density of nuclei Nuclei stability

The nuclei must remain together long enough to make produce more energy than is required to fuse them

Achieving these three critical conditions is the quest of scientists around the world

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Smaller pieces of matter Electrons Protons Neutrons

Considered to be ‘elementary’ particles But, experiments have shown that nucleons can be

divided into smaller particles calledQUARKS

In addition, scientists have discovered numerous other ‘pieces’ While these small bits of matter may not be relevant to

the ordinary matter we deal with in our daily lives, they have led scientists to many discoveries about how nature works

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A word on ANTIMATTER Any ANTIMATTER particle has the

Same mass as its ‘regular’ matter partner OPPOSITE charge as its ‘regular’ matter

partner For example:

The antiparticle for the electron is called the positron

When an electron collides with a positron, gamma radiation is emitted WHILE THE MATTER UNDERGOES ANNIHILATION

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Fundamental Interactions By study of the fundamental interactions of

the building blocks of matter, scientists have devel0ped a theory that describes the basic interactions of all matter

This theory is summarized in terms of four fundamental forces of nature

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The Basic Four STRONG INTERACTION

Holds nucleons together ELECTROMAGNETIC INTERACTION

Produces electric and magnetic phenomenon WEAK INTERACTION

Causes beta decay; helps determine nuclear composition

GRAVITATIONAL INTERACTION Responsible for the attractive forces between all masses

in the universe

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Unification

The work of particle physicists and other scientists is to try to understand how these four fundamental forces might have, at one

point in time, been unified into a single force of interaction for all matter in the universe.

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Leptons and Hadrons Two broad categories for elementary particles

Leptons Hadrons

Not affected by strong interactionPoint particlesExample: electron, neutrino

Affected by the strong interactionWell defined sizesExample: protons and neutrons

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Six kinds of quarks The internal structure for hadrons! Six types

Up Down Top Bottom Charm Spin

Unlike any other particle, quarks can have a charge of less than e The charge of a quark is a fraction of e: 1/3e or 2/3e

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Let’s sum it up Hadrons are made from leptons

ONLY 6 different leptons 6 quarks, 6 leptons, SYMMETRY

as expected VERY SIMPLE Each particle has an ANTIMATTER

partner...

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In-Lecture Quiz Questions

Chapter 7

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In-Lecture QUIZ CHAPTER 7 Longer half life means ____________decay

rate

A. faster B. slower

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In-Lecture QUIZ CHAPTER 7 T/F Nuclear weapons are based on the idea

of nuclear fusion.

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In-Lecture QUIZ CHAPTER 7 Write down two types of nuclear reactors

commonly used to produce electric energy.

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In-Lecture QUIZ CHAPTER 7 What causes radioactivity?

A. Some atomic nuclei spontaneously collide and cause an explosion, emitting high energy particles or rays

B. When atomic nuclei are large, they spontaneously decay, emitting high energy particles or rays

C. When atomic nuclei are small, they sometimes bump into each other, emitting high energy particles or rays

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In-Lecture QUIZ CHAPTER 7 What are the three types of radioactive

emission? A. Alpha, Beta, Tau B. Alpha, Beta, Gamma C. Delta, Omega, Zeta

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In-Lecture QUIZ CHAPTER 7 Which types of emission result in a daughter

nucleus that is a different element than the ‘parent’? A. Only Gamma B. Only Alpha and Beta C. All emissions (alpha, beta, and gamma)

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In-Lecture QUIZ CHAPTER 7 The antimatter partner for every particle is

identical except that it has opposite_____. A. weight B. force C. mass D. charge

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In-Lecture QUIZ CHAPTER 7 (short essay) Why is the binding energy per

nucleon such an important result?

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In-Lecture QUIZ CHAPTER 7 The most prevalent source of radiation in

the average lifetime is due to: A. dental x-rays B. natural background radiation C. flying at high altitudes D. living next door to a nuclear power plant

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In-Lecture QUIZ CHAPTER 7 The atomic mass unit is defined as being

approximately equal to A. The mass of a helium atom B. The mass of a carbon nucleus C. The mass of a silicon atom D. The mass of a hydrogen atom

the physical universe: nucleus

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