Micro Collimators - MX2

The Micro collimators (available on MX2) are several electron microscope apertures made of Platinum-Iridium (95:5). These apertures reduce the beam size upstream of the sample by chopping away part of the beam, with an inevitable reduction of intensity.

Why and when to use the Micro Collimators

The background scattering of the non-crystalline material surrounding your crystal can have a profound (and negative) contribution on your I/sigma (signal to noise), affecting your data quality.

In the best case scenario your beam size matches your crystal size and background scattering is not a big issue. However if your crystal is smaller than the beam and the material surrounding your crystal diffracts or scatters X-rays, it is best to reduce the beam size to match the size of your crystal. This can be done using the micro collimators.

Additionally, you can use the micro collimators to target a specific portion of a crystal to check if a crack or a layered plate is only on part of your crystal. If your crystal suffers radiation damage but is large, then the collimators can reduce the diffracting cross-section of the crystal, distributing the radiation damage over a larger portion of the crystal.

How to use the Micro Collimators

Before you use the collimators, ask an MX staff member if the collimators are available and have been aligned recently. If you think you will be using micro collimators please request it on your EA to ensure it is aligned in beamline setup.

The collimator controls are on the first tab of the MX2 Shutterflux GUI:

Open

The micro collimator controls on the MX Shutterflux GUI

Open

The microcollimator apertures as seen through a microscope. The 300 um aperture is on the bottom right. Your guess is as good as mine for the others.

Select the size of the beam you would like to change to. Wait until the circle behind the label changes to yellow. For reference, the full beam is 25 x 15 um. Be aware that you will need to ensure your sample is centered and you will need to account for the flux lost in decreasing the beam size.

Rastering

Using the collimator on a very small crystal means that centering of the crystal is very important, especially since parallax errors are a bigger problem with small crystals surrounded vitrified (glassy) materials. To ensure that your crystal is centered in the beam, use the rastering tool. More information on rastering can be found here.

Flux loss

Expect a loss in flux when using the collimator, due to cutting down the beam size. Remember to adjust the attenuation in the collection tab to account for this loss of flux.