page 1 1/4/2016 5:42 pmphys hon nuclear physics (ch.30) ch. 30: nuclear physics nucleus –nucleons...

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Nuclear Physics (Ch.30)

Ch. 30: Nuclear Physics• Nucleus

– Nucleons (A) = Protons (Z) + Neutrons (N)

– Mass and Atomic Numbers

– Number of protons & neutrons in nucleus is limited.

• Radioactivity is the decay of nuclei to more stable element via emission of “radiation” (α or β particles, rays, etc.).

• Half-Life (2n exponential decay)

• Isotopes - 3000 known nuclei, but only 266 stable ones!

• Radioactive Processes

– α, β, and γ-rays

– Natural radioactivity > At. No. 83

– Fusion (Joining) v. Fission (Splitting) of Atoms – both release energy.

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Nucleus: Particle Composition

• Z protons + N neutrons = A nucleons (1 – 10 fm 10-15diam.).

• 1920: Ernest Rutherford

– Bombarded Au foil with Alpha particles

– Most of atom is empty space with massive + charged nucleus.

• 1932: James Chadwick discovered neutron (bombarded Be with α).

• Isotope: same Z (# protons), different N (# neutrons).

– 15O and 16O … or … 12C and 14C … or … 238U and 235U …

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The Atom: Particle Properties

Particle Charge amu kg

Proton +1.6x10-19 1.007276 1.67x10-27

Neutron 0 1.008665 ~1.67x10-27

Electron –1.6x10-19 5.4858×10-4 9.11x10-31

Recall the 4 Models:

1. Single Indivisible Particle

2. Plum-Pudding

3. Planetary

4. Planetary-Quantum (Bohr Model)

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• Nuclear mass is slightly less than mass of constituent protons and neutrons due to nuclear binding energy.

• Mass is converted to energy when a nucleus is formed, E = mc2.

Nucleus: Binding Energy

• Binding energy per nucleon peaks at A = 56(~8 MeV/nucleon) and slowly decreases.

• Energy is released when a heavy nucleus (A~200) fissions into lighter nuclei near A~60.

Nucleon Number ABin

din

g E

ne

rgy

/ N

uc

leo

n (

MeV

)Peaks at Fe

(A = 56)Fission

(A ~ 200)

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Radioactivity: Historical Overview

• 1896: Becquerel accidentally discovered that a mysterious rock emitted invisible radiation onto a photographic plate.

• 1898: Marie and Pierre Curie discovered polonium (Z=84) and radium (Z = 88), two new radioactive elements.

• 1903: Becquerel and the Curie’s received the Nobel prize in physics for radioactive studies.

• 1911: Marie Curie received a 2nd Nobel prize (in chemistry) for discovery of polonium and radium.

• 1938: Hahn (1944 Nobel prize) and Strassmann discovered nuclear fission - Lisa Meitner played a key role!

• 1938: Enrico Fermi received the Nobel prize in physics for producing new radioactive elements via neutron irradiation, and work with nuclear reactions.

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Antoine Henri Becquerel

1/2 of the prize (France)

The Nobel Prize in Physics 1903

"in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity"

"in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel"

Pierre Curie France 1/4 of the prize

Marie Curie France 1/4 of the prize

Contributors to the Study of Nuclear Physics

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Recall: Nuclear Physics.

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Radioactive Decay Examples

= +

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Types of Nuclear RadiationALPHA () PARTICLE is identical to helium nucleus. It has 2 protons and 2 neutrons, mass number of 4 and atomic number of 2.

BETA () PARTICLE is a high-energyElectron. It has a negative charge andmass number of 0.

GAMMA () RAYS are high-energy radiation, like X-rays. They contain no mass or charge, only energy. λ = 10-10 to 10-15 m

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Alpha Emitters

Note: An Alpha particle has the same structure as a Helium nucleus.

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Alpha Emitters:226Ra ? + 4

2He

First, let’s figure out the identity of the new nucleus. How? Determine the atomic number: 88 – 2 = 86. The new element is Rn.

Next, figure out the mass number of the new nucleus: 226 – 4 = 222.

22688Ra 222

86Rn + 42He

88

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Beta Emitters

Note: During Beta Decay, a Neutron spontaneously changes to a Proton.

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Beta Emitters

• 146C X + 0

-1e

• To find the new mass # we take 14-0 = 14

• To find the new atomic # we take 6+1 = 7

• The element with atomic number 7 is Nitrogen

• 146C 14

7N + 0-1e

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Producing Radioactive Isotopes:

TRANSMUTATION is the process of changing one element into another (can be via bombardment OR emission of radiation).

This can be natural or artificial.

A stable atom can be bombarded with fast-moving particles, protons, or neutrons.

A radioactive isotope is called aRADIOISOTOPE.

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A stable atom can be bombarded with fast-moving particles.

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Radiation Exposure:

BACKGROUND RADIATION is theradiation that is in the environment.

Background radiation can come from food, building materials, cosmic rays, etc.

The air molecules in the atmosphere block out some cosmic rays.

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Radiation Exposure:Radiation sickness from too much radiation. Nausea, vertigo, and fatigue (side effects of chemotherapy). More exposure can lead to death. Exposure is measured by LD50 or lethal dose that is expected to cause death in 50% of the people receiving that dose.

To minimize problems, workers often wear badges to monitor the maximum permissible dose.

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Note: Chart displays average values of doses for common isotopes.

a basements of buildings

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Radioactivity

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Half-Life:The HALF-LIFE of a radioisotope is theamount of time it takes for half of the sample to decay.

A DECAY CURVE is a graph of the decayof a radioisotope (amount vs. time).

Some radioisotopes have long half-lives. For other radioisotopes, the half-life can be short.

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Note: Chart displays average values of half-life for common isotopes.

A(T) = Ao2-T/t

T/t = n, number of half-lives

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Note: Chart displays average values of half-life for common isotopes.

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Nuclear Physics (Ch.30)Chart based on individual isotopes, with average half-life estimates.

Radio-Isotopes Used in Medicine

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Fission and Fusion:FISSION is splitting a big atom into twosmaller atoms by bombarding with neutrons.

Energy is released according to Einstein’s equation: E = mc2. E is energy, m is mass, c is speed of light.

The fission process can continue until all of the available “big atoms” are gone. This is a CHAIN REACTION.

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A Nuclear Fission Reaction.+ 200 MeV

Unstable More Unstable

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Chain

Reaction:

A Chain Reaction

Uranium 235, struck with a neutron, and split into Krypton and Barium

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Fission and Fusion:FUSION is the combining of two small atoms into one bigger atom with release of energy.

More energy is released than fission.

Occurs continuously in the sun and stars

Requires temperature of 100,000,000C

Problem to reach and maintain this tempGood source of future energy – lots of H in oceanWaste products decay much faster than fission

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NUCLEAR FUSION : Joining atoms.

1H2 + 1H3 2He4 + 0n1 + 17.59 MeV

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Nuclear Power:This is a fission reaction. Mass of uranium is kept small and CONTROL RODS absorb neutrons to prevent chain reaction.

Problems are:•Public perception•Security•Hazardous radioactive waste that hashalf-life of thousands of years

*storage 2150 ft underground in NM10% of power in US from nuclear power

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Nuclear Physics (Ch.30)(A Nuclear Reactor Used to Boil Water for a Steam Turbine

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A Nuclear Reactor Used to Generate Electricity

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What Happened at Chernobl.

1. Steam Explosion.

3. Containment held.

2. Partial Meltdown.

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Radioactivity: Overview of Units• Activity: Becquerel (Bq) = 1 decay / s

1 curie (Ci) = 3.7×1010 decays / s (or Bq)(disintegration rate of 1g of radium)

• Ion Dose: Ionizing behavior of radiation is most damaging to us!Roentgen = 2.6×10–4 C/ kgair (or 0.0084 J/kg)

• Energy Dose: rad = 0.01 J/kg

Energy Dose for Human Health Considerations:rem = # rads × quality factor ( = 10 and = 1)

Dosages: 0.5 rem / yr = natural background5 rem / yr = limit for nuclear power plant workers500 rem = LD50 (50% die within a month)

750 rem = fatal dose (5000 rem = fatal within 1 week)

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Particle detectors:

1. Photographic Film.

2. Scintillation Screen.

3. Gieger-Muller Tube.

4. Cloud Chamber.

5. Bubble Chamber.

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The Nobel Prize in Physics 1939

Awarded to E.O. Lawrence,

Univ. of California, Berkeley

"for the invention and development of the Cyclotron and for results obtained with it, especially with regard to artificial radioactive elements"

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The Linear Accelerator was invented by John Cockcroft and E. T. S. Walton at the

Cavendish Laboratory, Cambridge, England, in the late 1920s

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Known Elementary Particles

Particle Symbol Charge Mass (amu)Proton p , 1H1 +1 1.007825

Neutron n 0 1.008665

Electron -1e0 , β- -1 0

Alpha 2He4 , α +2 4.0026

Positron +1e0 , β+ +1 0

Neutrino υ 0 0

Quark , +2/3 , -1/3 0u d

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Note: The neutron differs from a proton only by “d” “u” quarkreplacement!

The Discovery of Quarks!

u

Charge Q

Mass

+2/3 -1/3

~5 [MeV/c2] ~10 [MeV/c2]

Quark up down

d

uu

d

ProtonQ = +1

M=938 MeV/c2

du

d

NeutronQ = 0

M=940 MeV/c2

The Building Blocks of Protons and Neutrons.

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