Reducing Radiation Damage in Microcrystals

1
Reducing Radiation Damage in Microcrystals

• John Cowan
• Synchrotron Radiation Source

2
Radiation Damage

• Primary damage caused by X-rays.
• More or less unavoidable.
• Secondary damage caused by electrons produced by
  the X-rays.
• Reduced by freezing
• Tertiary damage, the combined effect of the first
  two on the lattice.

3
Henderson, 1990

• Predicted X-rays of about 2?107 Gy will always
  destroy the crystalline diffraction from protein
  crystals
• This corresponds to about 1.6?1016 incident
  photons/mm2 of 8keV energy
• Only about 10 secs on most intense sources.

4

• Assumptions
• Photon absorption is Low
• Photo-electron absorption is High
• What happens to the photo-electron for very thin
  specimen?

5
Casino

• Monte Carlo Simulation of Electron Trajectory in
  Solids.
• Research group of Prof. Raynald Gauvin,
  Université of Sherbrooke, Québec, Canada.
• A good approximation.
• Some limitations on the geometry.

6
(No Transcript)
7
Absorbed Energy
8
Penetration Depth
9
10 micron Crystal
10
5 micron Crystal
11
Background Reduced with He
Use of helium (B,D) rather than nitrogen (A,B) in
the cold gas stream gives less background
12
Reduce Solvent
13
Mounting Crystals
10 mm Crystals on EMBL micro-diffractometer
14
Mounting Crystals
Carbon Coated e-m Grids
Lacey Support Films
15
Crystals Mounted on 14.2.
700 microns across, 25 micron holes.
MiTeGen Micromesh Mounts
16
Experimental Set Up
x,y scanning
em Grid
holey carbon

amorphous ice
thin carbon film?
Helium Gas 100K
protein crystal
Diffracted x-rays
Detector
Electron shower
X-ray Beam
Lacey film
Em grid
17
Colin Nave and Mark Hill
Journal of Synchrotron Radiation, 2005, 12,
299-303.
Will reduced radiation damage occur with very
small crystals.