Acquiring reflectivity data with the Bruker D8 Advance
This is a flexible instrument, the method described below is what is required only for the simplest reflectivity measurements. Bruker’s Refsim and HMI's Parratt32 can be used to model the data obtained (for more advanced simulations other programs are available). For more demanding applications either consult the manual or email the SAF.
To obtain a reflectivity curve it is first necessary to align the reflectometer to the sample, this is essential because data must be collected with the angle of incidence being equal to the angle of reflection if not you are collecting diffuse rather than specular data. The alignment needs to be very precise because the data is collected at small angles. Once the alignment is complete the data can be acquired in one of two programs either using immediate measurement (the simplest method this does not allow any experimental changes during the experiment) or job measurement (this is more sophisticated allowing both automatic absorber changes and manual slit/absorber changes).
Aligning the reflectometer to the sample
1) The detector and tube must first be set to be diametrically opposite each other. This is achieved using the D8 Immediate Measurement program from in the D8 directory. Set the scan type to detector scan, continuous, the scanspeed to 0.1 sec/step, the start value to –0.2 and the stop value to 0.2. Then place the 0.1mm slit in the first slit position and toggle the Det.Slit button to Out (this attenuates the beam, the cartoon adjacent to the button should show two beams not four passing the slit). Start the scan by selecting Data Collection. A peak of above 4000 counts and a full width at half maximum of less than 0.1o should be observed, allow the system to collect for about 2mins then press stop. To determine the peak position click the Zi button in the top row of buttons (or select Determine Zi Value in the Diffractometer menu) a Zi determination box will appear. Check that the Enter theoretical position box contain 0 and take a note of the New ZI-value, click Save and Send new ZI. A warning box appears All circles will be initialized (check crashing possibility)! click cancel (If you click OK the goniometer will be initialised, this is not necessary for every Zi adjustment but is probably worthwhile doing for your first sample). Then click the red goniometer button (Move All Drives) to reset both the tube and detector back to 0.
2) The sample must then be placed at the correct height; this is done using the rate meter (this is opened using the button with red and green lines and CPS underneath). Without the sample present and with the shutter open the red bar should be giving above 4000 counts. If the shutter is closed click the open button, if the shutter is apparently open but the counts are low <20 then click the close button and then the open button (in some cases the software has the wrong state of the shutter), if the counts are still low close the rate meter and click the red goniometer button (Move All Drives) to reset both the tube and detector back to 0, then reopen the rate meter if the counts are still low contact the SAF. Make a note of the count rate and click the vv Store vv button to record the free count rate in the green bar graph.
Place your sample on the table and lower the knife edge (take care not to crash it into your sample) set the digital dial gauge to 0 and raise your sample until you can only just see light along the complete length of the gap between it and the knife edge (adjust the tilt of the sample using the micrometer under the table if necessary to make it parallel with the knife edge). Now raise the knife edge clear of the sample and lock it into the up position. Using the ratemeter (you can leave this open while you open and close the cabinet doors) raise the sample until the count rate is halved (and hence the sample is in the center of the beam).
3) To enable you to make the beam parallel to your sample a rocking curve must be obtained. To do this set the scan type to rocking curve, continuous, the scanspeed to 0.1 sec/step, the start value to –0.6 and the stop value to 0.6. Start the scan by selecting Data Collection. A peak of about 2000 counts and a full width at half maximum of less than 1o should be observed, allow the system to collect for about 2mins then press stop. To determine the peak position click the Zi button in the top row of buttons (or select Determine Zi Value in the Diffractometer menu) a Zi determination box will appear. Check that the Enter theoretical position box contain 0 and take a note of the Current ZI-value, Calculated Peak position and New ZI-value, click Save and Send new ZI. A warning box appears All circles will be initialized (check crashing possibility)! click cancel.
4) Step 3 has only changed the angle of the tube and hence you now need to apply the same correction but in the other direction to the detector (e.g. if the tube current ZI value was 19.742 the calculated peak position was –0.1 and hence the new tube ZI value was 19.842 and the detector ZI value was 38.158 the new detector value must be set to 38.058). To do this a detector scan must be obtained as in step 1(use the parameters given in step 1), however the data does not have to be good as you will enter the values you need, once the peak is found click stop and the Zi button. Type the difference between the tube calculated peak position that you noted down and the detector calculated peak position into the Enter theoretical peak position box so that the New ZI-value reads what you have calculated (see example above). Then click Save and Send new ZI. A warning box appears All circles will be initialized (check crashing possibility)! click cancel.
5) Now you need to check the height again, click the red goniometer button (Move All Drives) to reset both the tube and detector back to 0 and open the ratemeter. This should read half of the count rate you noted without the sample present, if it does not adjust the sample height until does then repeat steps 3 to 5 until the calculated peak position obtained for the rocking curve is less than 0.001 and the count rate is half of the count rate without the sample present to within 10% and hence the beam is aligned to the sample.
Obtaining a reflectivity curve using Immediate Measurement
Lower the knife edge to within 0.2 mm of the sample (take care not to crash it into your sample). Then to obtain a reflectivity curve set the scan type to locked coupled, continuous and the scanspeed, start, increment and stop parameters to what you want for your sample. Insert the slit width you require and toggle the Det.Slit button to Out (this attenuates the beam by about a factor of 100, the cartoon adjacent to the button should show two beams not four passing the slit) complete a scan by selecting Data Collection. Check that the count rate does not exceed 1000 if it does increase the start value or decrease the slit width until it does not. Then toggle the Det.Slit button to In (the cartoon adjacent to the button should show four passing the slit), then complete a scan by selecting Data Collection. The data can be displayed on a linear scale by clicking the graph button with a y on it or on a logarithmic scale by clicking the graph button with lny on it (although this displays as base 10 logarithmic plot!). Once sufficient a number of scans have been collected for you to resolve the features you are interested in have been completed click Stop. To save the data select Save As from the File menu and choose a file name and then save it into your data directory.
Obtaining a reflectivity curve using the Job Measurement
If you do not already have .dql and .job files for your measurements copy the dql.dql and job.job files from the JOBfiles directory to your own and rename them. Then to change the scan parameters you need to edit the .dql file.
To edit a .dql file open the EditDQL program in the D8 directory and then using the Open command in the file menu open your .dql file. A warning Edit DQL box appears (Save changes to UNTITLED) click No. The Edit Qualitative Measurement Extended Instructions board should appear, if it does not open it using the Qualitative Extended command in the Measurement Setup menu (you can use the Qualitative Simple method but the extended version is more flexible, so is all that is considered here). Most items for reflectivity are grayed out, so only those you can modify are considered here:
Comment: Allows you to record a comment about this experimental method.
Scanmode: Allows you to either scan continuously with angle or collect data in angular steps.
User Task File: Not generally used but enables batch measurement and processing.
Delay time: This allows a delay before the measurement and between range changes to enable you to change slits/absorbers manually for different ranges.
Def. Generator: Must be left at 40 kV and 40 mA.
Electronics: Must be left at High Voltage = 947.61102 V, Amplifier Gain = 80.000, Base Level = 0.55097 V and Upper Level = 1.73044 V.
Oscillations: All grayed out.
Scintillation: Must be left as (1).
Range Entries: This board enables you to divide the scan into several separate ranges. The Loop Variables options for Scan axis both the Step size and Time/Speed boxes should be checked and in Miscellaneous the Det. Slit should be checked if you wish to measure both with and without the automatic absorber (in this case the Delay time can be set to zero unless you wish to manually change something in the beam path as well) and the kV and mA boxes should not be checked. The incr. row should contain all zeros (unless you want to increment a loop or range and then range and loops buttons just increment and copy either a range or a loop into measurement ranges). In the edit row type the start, stop, step size, time/speed and whether or not you want the Det. Slit in or out then click insert to put it into the experimental method or click on a predefined range and then edit and replace it. Rows can be removed either by cutting or deleting them. Once you have set up the experimental method you require the Total measurement time is displayed in the bottom left of the board (if this is too long reduce the time/speed parameters). Then click OK on the Range Entries board and OK on the Edit Qualitative Measurement Extended Instructions board and then click on save in the file menu and then close the EditDQL program.
To edit the .job file open the JOB Measurement program in the D8 directory, then open your .job file using the Open command in the file menu. Check in default directory line your directory is selected if it is not change it using the Change Default Directory command in the file menu. Change the name in the RAW-File box to be what you want to call your data, the Sample Identification box to any details you want to record about the sample and the Parameter File box so that it contains the path and name of your .dql file. Then save the .job file using the Save As command in the file menu. .
To run the measurement lower the knife edge to within 0.2 mm of the sample (take care not to crash it into your sample) and then click Execute Job in the JOB Measurement program. To view the data during the measurement open the Status Display program and it the data graph does not appear immediately, click the graph button in the top left of the window. Your data will automatically be saved in your directory into the .raw file you designated in the .job file.
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