Chapter 3 in Massa

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Chapter 3 in Massa

• X-rays
• What are they?
• How are they produced?
• How do x-rays interact with matter?
• How does the nature of the crystal effect that
  interaction?

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Homework Problem 3
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Wilhelm Röntgen (1845-1923)?
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Crook's Tubes
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Crook's Tubes
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Crooks Tubes

• Produced cathode rays
• Cathode rays were particles
• Cathode rays did not penetrate matterthey
  were not observed outside the tube.
• Cathode rays were charged.

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The Discovery of X-rays in 1895

• Röntgen was working in a very dark lab with a
  Crook's tube.
• Across the room was a watch glass containing
  Ba2Pt(CN)4
• Whenever the Crook's tube was on the watch glass
  glowed.
• These were a new mysterious ray
• Named x-rays

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What could you do with X-rays?
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What were x-rays?

• Were they waves or particles?
• What was their wavelength (energy)?
• How were they produced?
• What more could be done with them?
• A question that wasn't asked until much later is
  how safe are they?

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There are two types of x-rays
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Discrete Lines
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X-rays
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Modern X-ray Tube
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Problems

• Most of the electrons are simply conducted by the
  target. This produces a huge amount of heat in a
  small area.
• For example, a power supply might provide 20ma at
  50Kv or 1Kw of power. Of this 97 will become
  heat.
• Obviously it is important that there be a good
  vacuum between the filament and the anode or
  there will be arcing.

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One Solution to the heat Problem

• We can better dissipate the heat if we spin the
  target
• This is a rotating anode generator
• In this case the vacuum must be kept by pumps
• Need a seal to hold 1x10-8mm vacuum and allow
  anode to spin at 3000-6000 rpm.

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Rotating Anodes

• Expensive to purchase
• Expensive to maintain
• 12 fold increase in beam intensity
• Various beam diameters are possible
• The main x-ray source at Purdue is a rotating
  anode generator.

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Need Even More Intensity

• Want 2 to 3 orders of magnitude or more.
• Want tunable wavelength
• Want very small beam size
• Use a synchrotron.
• Accelerate electrons to near the speed of light
  and have them circle around a large circle
• Only standing waves of the circle diameter will
  be allowed

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The Advanced Photon Source
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Which Wavelength to Use

• Generally use Cu 1.5418Å or Mo 0.71073Å
• The longer the wavelength the farther apart the
  diffraction spots are in space. For large unit
  cells like macromolecules use Cu.
• Cu produces more x-rays and the detectors have a
  higher efficiency in measuring them.
• Mo is not as absorbed as Cu. Best for heavy
  element problems.

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Homework