View source for Scanning Probe Diffraction Quick Reference

Back to [[Diffraction]]

This reference is intended for general users, notify staff of necessary revisions or to request more detail<br />
Prior to this section it is expected that sample alignment has been performed ([[Sample Alignment Quick Reference]])<br />
All MEDM windows below can be found from the main beamline MEDM screen, to bring up this screen double click the desktop icon "26id MEDM" or type "start_epics" on a terminal screen<br /><br />
'''To process a new value in any MEDM command field this window MUST be selected (left click or double click) and the mouse MUST remain pointed into that field until the user enters the new value AND hits return'''<br />


'''1) Setting up scan parameters'''<br />
If necessary call up the scan1 and scan2 windows from the main beamline screen (found under SCANS-DCM on the bottom left), then hit the bottom right More button twice on each window<br />
Remove prior positioners by double clicking the drive field until the text is highlighted, backspace, then return<br />
Add a new positioner by middle clicking on the intended motor's command value field, holding the middle button down, dragging to the drive field in the scan window, then release the middle button, left click, then return<br />
Select scanning range by EITHER adjusting the Start and End values OR adjusting the Center and Width values<br />
Select scanning steps by EITHER adjusting the Step Size value OR adjusting the #PTS value on the top right of the window<br />
For additional positioners in a single linear scan (ie to make a theta/two theta scan) click the green Positioners title box, and follow the above steps to modify the fields for positioners 2-4<br />
For a two dimensional scan, follow the above steps to add outer loop positioners and scanning ranges into the scan2 window (do not add any detectors or modify the detector triggers in this window)<br />

NOTES:<br />
i) Make sure the flags below the scan parameter fields are set to LINEAR, RELATIVE, and PRIOR POSITION (to set up other types of scans consult staff)<br />
ii) Make sure the number of active positioners (in parentheses to the right of the green positioners button) is correct, and that additional positioners from previous scans are removed<br />
iii) Make sure the positioner settling time is appropriate for the slowest motor, usually 0.2-0.5 sec - in many cases (soft motors, picomotors, or undulators) this should be at least 1 second<br />
iv) Do not use the "Clear positioners" button, this sets the scan type to "absolute" for all positioners<br />
v) An X next to the drive field should briefly flash if the new positioner is added correctly, and the units field should update to the new motor units<br />
<br />
'''2) Adding detector triggers'''<br />
Remove unneeded detector triggers by double clicking the pink value field until the text is highlighted, backspace, then return<br />
To activate scaler counters, select 32ch Scaler from the Scalers tab in the bottom right of the main beamline screen, drag the Count button to a DetTrigger field (then left click in the window, return)<br />
To activate fluorescence detectors, select 16-element SDD from the EDS tab in the middle left of the main beamline screen, drag the Erase/Start button to a DetTrigger field (then left click, return)<br />
To activate the CCD detector enter 26id:ccd.AcquireCLBK in a DetTrigger field<br />
Set the acquisition time of all three of these objects to the same value -if the CCD shutter is in use increase the exposure time for the scaler counter and fluorescence detector by ~0.2sec above the CCD exposure time<br />

NOTES:<br />
i) If saving images from the CCD make sure the change directory triggers are in the before scan and after scan placeholders in the scan1 window (consult with staff if these are absent)<br />
ii) If the NPI has thermally equilibrated it's convenient to add 26idcnpi:coarse:allstop to an unused DetTrigger field to ensure no coarse motors are left on<br />
iii) Make sure the scaler counter is set to single shot (that autocount is turned OFF)<br />
<br />
'''3) Adding detector channels'''<br />
To bring up an overview of current active detectors click on "DCM scan record detectors" on the NPI tab in the middle left of the main beamline screen (this screen cannot modify detector channels)<br />
To modify detector channels click on the green Detectors title box and bring up the appropriate subwindow for the detector channel range (1-10, 11-20, ..., or 51-60)<br />
Remove unneeded detectors by double clicking the pink value field of one of these subwindows until the text is highlighted, backspace, then return<br />
To add a new detector channel middle click on any process variable, drag to an available detector value field, then left click, return<br />

NOTES:<br />
i) Do not add more than 60 detectors<br />
ii) For more detail reference [[scanning probe detector channel descriptions]]<br />
<br />
'''4) Scanning the Nanoprobe Instrument'''<br />
The coordinate system for all NPI motors is a right hand rule about the beam propagation direction: +Z is downstream, +Y is up (vertical), +X is outboard (horizontal - pointing away from the ring wall)<br />
The usual alignment procedure calls for coarse scanning of the sample X/Y, and then fine scanning of the focusing optic X/Y - remember if both are scanned negative to positive these scans will appear inverted from each other in both axes<br />
When the sample is positioned at a diffraction angle, the horizontal scanning motion will travel further relative to the surface of the sample than the same vertical motion - apparent feature size will be compressed in X<br />
For an aspect ratio corrected square 2D scan of side length L that will appear like a top down microscope view of surface features, set Hybrid Y scan width = L in the outer loop, and set Hybrid X scan width = L*sin(theta) in the inner loop<br />
Due to the diffraction angle a horizontal X scan will also affect the Z positioning of the sample surface relative to the focal plane<br />
If the value of L*cos(theta) is greater than the depth of focus in the above scan, (more generally if for any scan (X scan width)/tan(theta)>DOF) ADD Focus Z as a second positioner to the X scan (Z scan width = L*cos(theta) OR (X scan width) / tan(theta))


NOTES:<br />
i) Once an object is found, you may want to re-verify the focus on this object (follow the example scan in part 5 of [[Sample Alignment Quick Reference]])