Radioactivity and Nuclear Energy

Chapter 19

All about the nucleus

• The nucleus is really small.– 1/100,000 the radius of an atom

• The nucleus is really heavy.– Density of 1.6 x 1014 g/cm3

• There is a lot of energy in the nucleus!– A nuclear reaction has more than 1,000,000 times

the energy as a chemical reaction

How did we find this out?

• Wilhelm Conrad Roentgen– Took the first X-ray in

1895

• Revolutionized the field of physics and chemistry with his discovery

Photo from http://www.accessexcellence.org/AE/AEC/CC/historical_background.php

Henri Becquerel

• Discovered radioactivity• Put a chunk of uranium ore in a drawer with

some photographic plates.• Plates were developed by the uranium• Becquerel determined that the ore must be

emitting energy spontaneously• Also determined that these particles were

charged.– The radiation was deflected by a magnet

Marie and Pierre Curie

• Discovered 2 different radioactive elements, radium and polonium.– Painstakingly extracted from pitchblende, a

uranium ore.

• Pioneers in the field of radioactivity.• Both had rather tragic deaths.

Our buddy Rutherford!

• Discovered the alpha and beta particles.• Did extensive work on the structure of the

atom and nuclear transformations

What exactly is radioactivity?

• Radioactivity is the spontaneous decomposition of a nucleus producing 1 or more particles.

• Types of radioactive decay:– Alpha– Beta– Gamma– Positron– Electron capture

A few reminders about nuclear equations.

• Conservation of Mass applies!• Mass number on Left=Mass number on right.• Atomic Number on left = Atomic number on

right• Isotope Notation:• Mass # (A)• Atomic # (Z)

€

ZAX

Alpha decay ()

• Most common form of radioactive decay– Heavy Radioactive Nuclides.

• An alpha particle is a helium nucleus– What would be the symbol for a helium nucleus?

Beta decay () (beta-particle production)

• Particle is an electron (symbolism for an electron?)

• A neutron has transformed into a proton, the nucleus spits out an electron.

• Atomic number Increases by 1• Mass number remains the same

Positron Production

• A positron is a particle with the same mass as an e-, but with a positive charge.

• 2211Na --> 0

1e + 2210Ne

• The production of a positron appears to change a proton into a neutron.

• Mass # stays the same• Atomic # decreases by 1

Electron Capture

• Electron capture is the process in which an electron is captured by the nucleus.

• 20180Hg + 0

-1e --> 20179Au + 0

0

Gamma Decay

• Gamma () rays are high energy photons• Gamma emission occurs when the nucleus

rearranges• No loss of particles from the nucleus• No change in the composition of the nucleus

– Same atomic number and mass number• Generally occurs whenever the nucleus undergoes

some other type of decay

Practice Problems

• Write the nuclear equation for the following radioactive decay processes:

1. Thorium-232 decays by alpha emission2. Radium-228 decays by beta emission3. Cobalt-60 decays by beta emission4. Americium-241 decays by alpha emission

Radioactive Decay Series

• Can’t achieve a stable nucleus with just one radioactive particle released.

Nuclear Bombardment

• Nuclear transformation – change of one element to another

• Bombard elements with particles

• Use particle accelerators

Other Symbols that are useful

• Proton: 11p

• Neutron: 10n

• Positron: 01e

• Deuterium: 21d or 2

1H

• Tritium: 31H

Sample Problems

• Write the nuclear transformation reaction described in the problems below:

1. Nitrogen-14 plus helium-4 results in the formation of another element and the release of an proton.

2. Aluminum-27 plus some other particle results in the formation of phosphorus-30 and the release of a neutron.

Large Hadron Collider

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are needed to see this picture.

Detecting Radioactivity

• Geiger Counter

Half-Life

• Half-Life is the time required for half of a radioactive sample to decay.

• Most active=shortest half-life• Least active=longest half-life• Uranium-238: 4.5 x 109 years• Protactinium-234: 1.2 minutes

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

time (half-lives)

Radioactive Decay

Sample Problems

• Gold-198, which has a half-life of 2.7 days, is used as an implant for cancer therapy. For an implant containing 50 micrograms of Au-198, how much remains after 8.1 days?

Using Radioactivity

• Carbon-14 Dating• compare the amount of C-14 to C-12• C-14 radioactive with half-life = 5730 yrs.• while living, C-14/C-12 fairly constant

– CO2 in air ultimate source of all C in body– atmospheric chemistry keeps producing C-14 at

the same rate it decays• once dead, C-14/C-12 ratio decreases• limit up to 50,000 years

The Shroud of Turin• Thought to be the burial

cloth of Christ• First documented in 1355.• Carbon dating done in 1988

by three separate laboratories– Shroud date: 1260-1390

• Cloth tested was not part of the original

• Fire in 1500’s may have altered the carbon content

• Authenticity still in question

QuickTime™ and a decompressor

are needed to see this picture.

QuickTime™ and a decompressor

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are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

The Real Power, Nuclear Power

• Discovered in late 1930’s.• Bombard U-235 with neutrons• Releases 2.1 x 1013 J

– 26 million times more energy than combustion of methane!

• Fission: Splitting heavy nuclei into smaller nuclei

n3 Kr BaU n 10

9236

14156

23592

10 ++→+

Releases 2.1 1013 J/mol uranium-235

A Chain Reaction

How do we harness this energy?

The nuclear reactor

Nuclear Fusion

• Process of combining 2 light nuclei • Produces more energy per mole than fission • Powers the stars and sun

A few problems with fusion…

• Strong nuclear force only effective at short distances• Nuclei are both positively charged

– Must overcome electrostatic repulsion

• Need temperatures of about 40 million K for fusion to work– Many technical problems still exist– Not clear if technically or economically feasible

THE H-BOMB

• AKA Thermonuclear bomb

• Atomic bomb surrounded by fusion materials– Lithium fissions to

helium and tritium– Tritium and deuterium

fuse• Temps around

400,000,000 degrees C

Medical Apps. Of Radioactivity

• Radiotracers• Radioactive nuclides

that can be introduced into organisms and traced for diagnostic purposes.

Positron emission tomography (PET scan)

• Designed to find cancer cells

• Uses sugar molecules tagged with a radioisotope that emits positrons

• Positron + electron = annihilation

• Cancer cells grow faster than normal, need more sugar, build-up of radiation at cancer site

Problems with radioactivity

• Factors Determining Biological Effects of Radiation

• Energy of the radiation • Penetrating ability of the radiation • Ionizing ability of the radiation