Changes in the Nucleus

1
Changes in the Nucleus

• Chapter 28
• Addison-Wesley

2
Unit Objectives

• Understand radioactivity and radioactive decay
• Characterize alpha, beta, and gamma radiation
• Understand and calculate half-life
• Distinguish between fission and fusion, including
  bombs and power plants
• Determine sources of radiation in your life
• List common applications of radioactive isotopes

3
Introduction

• Nuclear reactions change the composition of the
  atoms nucleus
• Radioactivity is the natural decay of unstable
  isotopes
• Three ways of this occuring alpha, beta, gamma
• Small fraction of all natural isotopes are
  radioactive

4
Band of stability

• Certain numbers of protons and neutrons make a
  nucleus stable

5
Nuclear Glue

• Strong nuclear force, possessed by all neutrons
  and protons, hold nucleus together
• Protons also repel because of positive charge and
  therefore SNF must overcome this
• Neutrons behave like glue because they are neutral

6
Elements

• All elements above 83 (bismuth) are radioactive

7
Alpha Radiation

• It is a helium nucleus
• Has 2 protons and 2 neutrons
• Penetrates very little and is stopped by skin and
  paper
• Symbol

8
Beta Radiation

• High energy electrons
• How can this come from the nucleus?
• A neutron turns into a proton and spits an
  electron out
• Symbol ß or -10e
• Can penetrate clothes and damage skin
• Stopped by glass

9
Gamma Radiation

• Part of the Electromagnetic Spectrum (EM)
  handout and next slide
• Often accompanies alpha and beta decay
• No mass, just high energy
• Symbol ?
• Penetrates rapidly, you need several inches of
  lead for protection

10
Electromagnetic Spectrum
11
Cell Phones

• These operate at a wavelength size of 9 cm, in
  the range of microwaves
• Are they dangerous what do you think?
• No health effect has been proven
• Manufacturers must restrict limit of exposure to
  levels far below that found to cause damage to
  lab animals
• Some countries recommend limiting childrens use
• MUCH RISKIER Driving while talking on your cell
  phone!

12
Decay Equations

• When nucleus emits any radiation, it is said to
  decay.
• Equations reflect conservation of matter.
• Try a few you dont have to know these.

13
Half-Life

• Definition Time it takes for ½ of the atoms in
  a sample to decay.
• Practice Cesium-137 has a half-life of 30
  years. If you begin with an 8-gram sample, how
  much is left after 30 years? 60 years?

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Other direction

• Scientists know how much isotope a bone should
  have by mass.
• Example A scientist finds a bone which should
  have 1.2 grams of Cesium-137. It has 0.15 grams
  of the isotope. How old is it?

15
Radioactive Dating

• This is how fossils are dated. Several different
  isotopes are used to confirm results.
• This process can be very complicated.

16
Yearly Exposure to Radioactive Isotopes

• Take the survey to estimate your yearly exposure
• Lots of sources!
• Video http//www.teachersdomain.org/9-12/sci/phys
  /energy/subtopic_nucrad.html
• Handout isotopes in your body

17
Lung Exposure

• Smoking, radon gas
• Look at decay series for uranium
• Inhaled and stays in lungs because it is an
  a-emitter
• Causes lung cancer
• Radon Gas - http//www.teachersdomain.org/9-12/sci
  /phys/energy/subtopic_nucrad.html

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Medical Uses

• Radioactive tracers easy to follow in the body
• Tumor detectors rapidly dividing cells
  concentrate the radioactivity
• Attacking tumors large doses of the isotope
  could destroy the tumor
• Handout

19
Damage from Radioactive Exposure

• Handout
• Website
• http//www.nlm.nih.gov/medlineplus/radiationexposu
  re.html

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Transmutation

• Term used whenever one element becomes another
• Two ways
• natural radioactive decay of isotopes
• bombardment

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Bombardment

• Upper atmosphere
• Particle accelerators in labs
• Nuclear reactors or bombs

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Rutherford

• First person to do bombardment in a lab
• Equation on overhead
• All elements above 92 are radioactive and
  synthetic by bombardment

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Particle Accelerators

• Particles (usually neutrons) travel close to the
  speed of light
• Slam into other nuclei
• New elements, elementary particles discovered
  this way
• CERN is the largest in the world

24
Cyclotron
25
CERN accelerator

• 27 km circle on France/ Switzerland border

26
Fermi

• Fermi proposed this as a way to make larger
  elements
• Fission accidentally discovered during World War
  II
• Definition A larger nucleus is bombarded with a
  particle and it splits into smaller nuclei,
  releasing energy

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Fission

• Equation on board - handout
• Very small mass difference releases energy by
• E mc2
• 1 pellet of uranuium (less than 1 gram 3
  barrels of oil 1 ton of coal 17,000 cubic
  feet of natural gas

28
Chain Reaction
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Controlled Fission

• Nuclear Power Plant
• Control rods absorb excess neutrons
• Most are being decommissioned in the US today -
  why?
• Handout
• http//www.teachersdomain.org/9-12/sci/phys/energy
  /subtopic_nucrad.html

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Nuclear Power Plant
31
Uncontrolled Fission

• Nuclear bomb
• Fissionable material U-235 was first
• Triggered by high-speed electrons (polonium is
  common)
• Called A-bomb
• Ended WWII (read)
• http//inventors.about.com/library/weekly/aa050300
  a.htm

32
Fusion

• Opposite from fission
• Two smaller nuclei fuse to form a larger but more
  stable nucleus
• Hydrogen into helium (handout)
• This is the way sun produces energy

33
Fusion Facts

• Much more energy than fission
• No waste
• Cheap fuel
• So whats the problem?
• High temperatures (plasma)
• Almost impossible, very high cost
• Still trying one proposed in France

34
Fusion Bomb

• Also called H-bomb
• First developed in US in 1952 in a race against
  Russia - 750 times more powerful than Hiroshima
• http//www.teachersdomain.org/9-12/sci/phys/energy
  /subtopic_nucrad.html

35
Some interesting facts

• The 1954 Bikini bomb was the biggest man-made
  explosion until the USSR's 50-megaton test in
  1961.
• Three weeks after Bikini bomb it emerged that a
  Japanese fishing boat, called Lucky Dragon, was
  within 80 miles (129 km) of the test zone at the
  time. Its 23 crew were severely affected by
  radiation sickness.
• They were among 264 people accidentally exposed
  to radiation because the explosion and fall-out
  had been far greater than expected.
• The original natives were granted 325,000 in
  compensation and returned to Bikini in 1974. But
  they were evacuated four years later when new
  tests showed high levels of residual
  radioactivity in the region.
• Twenty-three nuclear tests were carried out at
  Bikini between 1946 and 1958.

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H-Bomb

• November 15, 1952