APS / XOR XAFS beamline evaluation

The APS beamline evaluation was carried out by the Beamline Evaluation Group

Data Collection

Baseline Evaluation of XAFS Bending Magnet Beamlines (2006). All experiments were performed under “standard optimized operating conditions” for each beamline. Details are recorded in the Lab Book Notes.

Beamline Configuration

The following information was collected at each beamline:

• Monochromator (e.g., crystal, cut, geometry, encoding, motors, etc.)
• Harmonic Rejection (e.g., mirrors, coatings, detuning methods, etc.)
• Detectors (used during the evaluation)
• Counting Chain (e.g., amplifiers, VTF, scalers, etc.)
• Controls and Software (e.g., user interface, output file format, etc.)
• Feedback (if used, e.g., for beam stability)
• Synchrotron operating mode during test

Mono Calibration

Method: XANES scans of metal foil reference standards collected over a large energy range without recalibrating the monochromator between foils. (1 sec/point). Edge positions were measured using the first peak in the first derivative of XANES calculated at the beamline and using the BEAMLINE software. Measured energies were compared against nominal values as reported in Rev. Sci. Instrum., 67 (1996) 686.

• Sample details: Metal foils from EXAFS Materials (Joe Wong’s company). Set provided by M. Newville.
• XANES scan details:
  • -20 to 30 eV
  • step sizes 0.3, 0.4 and 0.8 eV for low, mid and high energies, as noted in table
  • 1 s/pt or less. Not critical.
• Record detector settings: per beamline.
• Record other settings: per beamline.

Energy Resolution

Method: Measure the full width at half maximum of the V_2O_5 pre-edge feature.

• Sample details: powder-on-tape prepared by Matt Newville.
• Scan details:
  • –100, -20, 5eV steps
  • –20, 30, 0.2 eV steps
  • 2.81, 8, 0.075 Å-1 steps
  • 0.5 s/pt (w/1 s/pt settling time)
• Record detector settings: per beamline.
• Record other settings: per beamline.

Base Noise Levels

Method: record intensity at 10 keV for 3 minutes. Record with beam off for 3 minutes. Record data with knife edge 1/2 way through beam Vertical for 3 minutes. Set delay time to 0 seconds.

• Record detector settings: per beamline.
• Record other settings: per beamline.

Detector Linearity

Method: scan a narrow slit across the beam horizontally to see how uniform the detector is from side to side. N.B., this test could not be performed on all beamlines.

• Record detector settings: per beamline.
• Record other settings: per beamline.

Harmonic Content

Method: Scan the energy around 6.66 keV through a Mo foil to look for emergent Mo XANES from the third harmonic. Scan parameters are the same as for the Vanadate, but with larger steps in the XANES region.

• Nominal edge position for Mo is 20,000 eV. Run a XANES scan with E0 = 6,667.
• 25 μm thick Mo foil from sector 20.
• Scan details:
  • –100, -20, 5 eV steps
  • –20, 30, 2 eV steps
  • 2.81, 8, 0.075 Å-1 steps
  • 0.5 s/pt (w/1 s/pt settling time)
• Record detector settings: per beamline.
• Record other settings: per beamline.

Data Quality

Method: collect transmission EXAFS of solutions with 0.1 edge step in ca. 2 absorption lengths of water. Solutions and transmission cells were prepared by Matt Newville using dilution calculations by Bruce Ravel. Solutions of zinc nitrate and cadmium nitrate were used.

• Scan details (typical)
  • –200, -20, 5 eV steps
  • –20, 30, 0.4 eV steps @ Zn, 0.8 eV steps @ Cd
  • 2.81, 16, 0.05 Å-1 steps
  • 1.0 s/pt
• Scan details for cadmium nitrate solution
  • –200, -50, 5 eV steps
  • –50, 30 eV in 1.5 eV steps @ Cd
  • 2.81, 16, 0.05 Å-1 steps, kweighted counting by factor of 1.5
  • 1.0 s/pt

Beamline Operations

Method: collect objective and subjective data on the overall experience from an "outside user" point of view.

• Practical limits on energy range for EXAFS (highest and lowest measured spectra)
• Ease of changing energy
• Availability of detectors
• Availability of special sample environments
• Ease of integrating APS Pool Detectors and Equipment
• Data collection software
• On-line data processing and analysis
• Known sources of systematic errors
• Tools
• Travelogue

Data Processing

Mono Calibration

Summary Plot of Monochromator Calibration experiments attachment:MonoCalibrationSummaryPlot.pdf

Summary Data for Monochromator Calibration experiments attachment:MonoCalibrationSummaryData.txt

Zn Solution (noisy data)

• Athena Project of Zn solution data:

The numerical results are shown below:

The values for edge step are given as a guide -- the actual solutions used were from different batches, and so some variation is expected.

The values for "epsilon_k" () are the automatic estimates from IFEFFIT, and should also be taken with a grain of salt. The plot of below clearly shows the data from 20-BM to be the best, but the data from 5-BM, 13-BM, and 33-BM seem equally noisy to me: