Chapter 3 in Massa

X-raysWhat 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

3. A molecule crystallizes in a monoclinic cell with a=15.168 b=10.348c=15.847 alpha=90.0 beta-100.035 and gamma=90.0 An oxygen atoms hascoordinates of (1.09730, 0.49006, 0.71629) and a carbon atom islocated at (1.07567, 0.42273, 0.65279). Calculate the carbon-oxygen bonddistance. What is the probable bond order.

The formula for a vector length can be simplified for a monoclinic crystal to

d2=a2x2 + b2y2 + c2z2 + 2acxzcos(β)x=(1.09730-1.07567)=0.02133 y=0.06733 z=0.06350d*d=230.068*0.00045497+107.081*.0045333+251.127*.0040323+2*15.168*15.847*.02133*.06350*-0.174250=1.395

d=1.181Å This is a carbon-oxygen double bond!

Wilhelm Röntgen (1845-1923)

Crook's Tubes

Crook's Tubes

Crooks Tubes

Produced cathode raysCathode rays were particlesCathode 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

This deceleration of a high energy electron produceswhite radiation called Bremsstrahlung.

This is a process not described by quantummechanics

If the energy of the electron is too low it is simplyrepelled.

Discrete Lines

Obviously this energy spectrum differs for eachelement. Thus, different elements have different x-ray spectra. By firing an electron beam into a solidand analyzing the x-rays the elements present canbe determined (usually F and greater).

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 1x10-8mm vacuum and

allow anode to spin at 3000-6000 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 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.

Homework

4. Using the transformation matrix given in lesson 3, convert the coordinatesfor the oxygen and carbon atom abovem into Cartesian coordinates. Recalculate the bond distance. (1/14/08)

This is due Friday 1/18/2008

Read Chapter 3.

If you have not done so read Chapter 2