User Guide –
1. What does VESPERS stand for?
VESPERS is a short form of “Very sensitive Elemental and Structural Probe Employing Radiation from a Synchrotron”. It is a hard X-ray microprobe beamline providing the techniques of X-ray fluorescence spectroscopy/mapping, X-ray diffraction/mapping, and X-ray absorption spectroscopy.
2. What kind of techniques are available on VESPERS?
In general, the techniques available on VESPERS are X-ray fluorescence spectroscopy/mapping, X-ray diffraction/mapping, X-ray absorption spectroscopy, and their combinations. Laue diffraction with polychromatic or board bandpass beam is of unique from this beamline; it provides a capability to do quick measurement on crystalline samples, therefore enables the mapping for grain/strain distribution, in-situ measurements, etc.
3. What kind of beams can be provided on VESPERS?
a. ΔE/E ~10-4 beam from double Si(111) monochromator;
b. ΔE/E ~1.6x10-2 (1.6%) beam from double multilayer monochromator;
c. ΔE/E ~10-1 (10%) beam from double multilayer monochromator;
d. Polychromatic or “Pink” beam.
4. What is the “pink” beam?
“Pink” beam represents a modified “white” beam from the source (bending magnet for VESPERS). Due to the low-energy cut-off from windows and the high-energy cut-off from X-ray mirrors, the spectrum of “white” beam from the source was truncated, therefore the term of “pink” beam is used.
5. What is the energy range of X-rays provided by VESPERS beamline?
a. 6.7-30 keV, for ΔE/E ~10-4 beam from double Si(111) monochromator;
b. 2-30 keV, for ΔE/E ~1.6x10-2 (1.6%) beam from double multilayer monochromator;
c. 2-30 keV, for ΔE/E ~10-1 (10%) beam from double multilayer monochromator;
d. 2-30 keV, for polychromatic or “pink” beam.
However, due to air/window attenuation and bending magnet source, the photon flux will be low at low and high energy.
6. The XRF map does not match to the optical image of my sample. What is the reason?
The XRF map measures the projection of the sample to beam direction. Depending on the experimental setup/geometry/sample-orientation, XRF map may look different comparing to the optical microscope image. It contains the depth/thickness information along the beam path. And hard X-ray usually can penetrate into the samples quite deep; this effect becomes more obvious with less-attenuated samples, such as biological samples.
7. What’s the requirement for sample preparation for XRF/XRD mapping?
Ideally, the sample should be flat (uniform in thickness) with a smooth surface, and thin (to better match optical image). However, if the concentration level is low for the element of interest, the thicker sample may become essential to obtain reasonable signals.
8. How to mount the sample for XRF or XRD measurement?
Several different shaped mounting pieces with magnetic mounting base are available on beamline. Usually, the sample needs to be stuck onto those pieces for mounting. It may help, if the sample is pre-mounted on a metal-free (free of elements of interest) slide for XRF, and amorphous (diffraction free) slide for XRD.
On beamline, there are some Kapton tapes and some metal-free plastic sheets available for immediate use.
9. How big is the field of view (FOV) for XRF/XRD mapping?
The sample stages used have a travel range of 40 mm in vertical, and 150 mm in horizontal. This defines the FOV of 40 mm x 150mm. However, we suggest the samples to be much smaller than that of “FOV”. When the micron-sized beam was used to run the mapping, the mm-sized area is very big! If the big-sized mapping is critically important, we suggest a coarse map of big area (with big step-size) followed by a fine map of small area (with fine step-size) to wisely utilize the beamtime.
10. I understand that, depending on the experimental focus, different experimental setup is necessary to optimize the measurement. What would be the consideration?
a. Techniques to be used, combination of techniques, needs to switch the techniques while keep beam on same spot;
b. Penetration power of beam into the sample, how well do you need to match the map to optical image, the effect of depth information;
c. Effect from scattering, e.g., Compton scattering from the sample, air scattering;
d. Effect from direct beam, etc.
This link listed pros and cons for the different setup for comparison.
11. What detectors are used for XRF measurement?
We use Vortex® silicon drift detectors (single-element and 4-element from Hitachi High-Technologies) for XRF measurement on microprobe site, and 4-element Vortex® or 13-element Ge detector (from Canberra/Mirion) on mm-beam site.
12. What detector is used for XRD measurement?
13. Do you have any online optical camera or microscope on beamline?
a. a camera to provide an inch-size view for the sample; plus
b. an optical microscope online with up to 500x magnification.
14. What do we need to do to move the sample into focus accurately?
To get the beam from mm-size to micron-size, a KB mirror system was used to heavily focus the beam. Therefore, to ensure the sample in focus is very critical. Otherwise, the beam size and location would not be accurate.
We provide two methods for the purpose: (1) a laser displacement sensor, and (2) online microscope. The former has an accuracy of ~10 micron, and the latter has depth sensitivity of ~10 micron.
15. How accurate is the beam marker on camera and microscope?
a. Cross-hair on camera view;
b. Red-circle on microscope view.
The latter is more sensitive due to high magnification. The accuracy of this marker is highly depended on how close the sample surface is to the focus. If the sample is in the focus, it would be better than 10 micron. However, if the sample surface is not flat, or sample surface off from focus after long travel, the beam marker may become less accurate. Therefore, to keep the sample in the focus is critical.