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?

Homework Problem 3

Wilhelm Röntgen ( )‏

Crook's Tubes

Crooks Tubes Produced cathode rays Cathode rays were particles Cathode rays did not penetrate matter—they were not observed outside the tube. Cathode rays were charged.

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 Ba 2 [Pt(CN) 4 ] Whenever the Crook's tube was on the watch glass glowed. These were a new mysterious ray Named x-rays

What could you do with X-rays?

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?

There are two types of x-rays

Discrete Lines

X-rays

Modern X-ray Tube

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.

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 -8 mm vacuum and allow anode to spin at rpm.

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.

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

The Advanced Photon Source

Which Wavelength to Use Generally use Cu Å or Mo Å 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.

Homework