One of the great advantages of I22 is its high flux but this can also present problems for some experiments. In some situations it may be necessary to make use of I22's attenuators to protect your sample or the detector from radiation damage. If radiation damage is likely to adversely affect your experiment, you may want to consider applying attenuators before opening the shutter. These can always be removed later if required.
RAPID, the 2D gas-filled SAXS detector has been designed to operate with high levels of incident flux over extended periods, however there is a saturation threshold of counts on the detector. Exposure of greater that 5 x 106 counts/s globally could cause damage to the detector. For Pilatus this is somewhat higher, 1 x 106 per pixel. We ask that you start your experiment with a short (1s) exposure and then close the shutter. Check the detector readings on the NCD Status window in the GDA, this is available on most perspectives. If the reading is within the exposure limit for your sample, you can open the shutter and continue with your experiments. If the global count rate is too high, please apply an attenuator and then take another 1s exposure. If the detector receives too much flux an automated routine will now shut the shutter to protect the detector. This will then ask you to introduce some attenuation!
There are several attenuators available for use on I22. They are positioned in the optics hutch and can be operated remotely using the GDA. A motor called d6filter allows you to move the attenuators in and out of the beam.
To check the current state of the attenuator, use the following command:
GDA will read back which attenuator is in the beam or read back 'Clear' if no attenuation is in use.
To use an attenuator, use the following command (the quotes are important):
pos d6filter 'nameofattenuator'
Wait until the motor has finished moving. GDA will confirm that the attenuator is in position.
Please remember to record which attenuators have been used if you are intending to calculate sample ransmissions or subtract backgrounds. Don't forget to remove attenuation when no longer needed!
pos d6filter 'Clear'
Note: Thin Al foils provide limited attenuation for strongly scattering samples at X-ray energies above 10keV. Please ask a beamline scientist about suitable attenuation strategies for your experiment.
Using attenuation. The above plot shows the X- ray transmissions for our attenuators. The molybdenum and Tungsten are much less transparent to X-rays, and therefore more useful for higher energies (shorter wavelengths). An expanded version below 5% is shown below
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