Below we list some of the most common questions regarding proposals and beamtimes at NanoMAX and try to answer them.
There are multiple things to consider when choosing the photon energy for your experiment:
- What does the beamline offer?
- Do I have a specific beam size in mind?
- The size of the focal spot produced by the KB mirror system at the diffraction endstation depends on the chosen photon energy. If you have a maximum usable spot size in mind, you will have to choose a photon energy above a lower limit. For details click here.
- The focus spot size produced by Fresnel zone plates at the imaging endstation is independent of the chosen photon energy.
- Which elements do you want to see via X-ray fluorescence?
- To excite the X-ray fluorescence of any element of interested, you will have to choose a photon energy of at least slightly above the corresponding X-ray absorption edge. Example: If you want to see/measure the K-alpha XRF line of iron (6.405 keV), you will have to choose an incident beam photon energy above the K edge of iron at 7.112 keV.
- Which scattering / 2-theta angle do I need to reach in my diffraction experiment?
- Our Pilatus3-1M detector at the diffraction endstation has a fixed size and can only be placed a minimum distance from the sample. Hence the maximum 2-theta angle is limited. Depending on the d-spacing of the Bragg spots/rings you want to observe, you will have a lower limit on the photon energies that you can choose for your experiment.
- Do I need a very high flux?
- For both endstations the maximum flux is provided at around 8 keV. The further the used photon energy is from that, the fewer photons will be probing the sample in the same amount of time.
- The flux vs photon energy dependency curve for the diffraction endstation is shown here.
The correct answer is “that depends”. More specific, it depends on how many data points you intend to record, how long you will be exposing at each measurement position and how much time is used for moving between measurement positions.
- number of measurement positions:
- For a simple mapping experiment you would ideally make the step size match the beam size to illuminate the whole mapped area evenly.
- For example to measure a 10 um x 10 um area with a 100 nm beam and therefore 100 nm steps, you would need N = 100 x 100 = 10 000 scan points. Reducing the beam/step size or increasing the scan area will subsequenytly increase the number of data points to record accordingly.
- exposure time for every single scan point:
- This depends heavily on your sample and what you want to see in it and how many photons you will have to put on your sample to see what you want to see.
- As rough guidance (that might not be true for your sample):
- common exposure times for XRF mapping are between 10ms and 50ms
- common exposure times for XRD mapping are between 40ms and 200 ms
- time used up between scan points:
- At NanoMAX mapping experiments usually make use of continuous / fly-scans to minimize the unused time between exposures. A conservative estimate for the dead-time between exposures is 1 ms.
Now simply multiply to estimate the time needed for a single map.
Right now we do not provide any standard in situ capabilities (heating / cooling / gas environment / liquid cells / pressure / laser pulses ) from the beamline side.
That does not mean that such experiments are impossible to perform at NanoMAX. It means that such capabilities have to be provided by the user side. Many different sample environments have been used at the diffraction endstation in the past. If you intend to bring your own sample environment, contact the beamline staff well in advance of submitting your proposal so that feasibility questions can be discussed.
We are currently in the process of identifying the most requested capabilities and subsequently designing respective standard solutions for the beamline.