This page provides practical guidelines and examples for planning common HODACA measurements.
The appropriate scan range, step size, counting condition, and temperature sequence depend on the sample and scientific objective. The examples below are intended as starting points rather than fixed measurement conditions.
For SPICE command syntax and macro examples, see the SPICE Command Reference and Macros & Examples pages.
HODACA has 24 analyzer–detector channels arranged at 2° intervals. A single A2 setting therefore samples the scattering angle at approximately 2° intervals.
When the scattering is sharp or its position is not known precisely, measurements at two A2 positions separated by 1° are generally recommended. The two measurements interleave the analyzer channels and provide approximately 1° angular sampling, reducing the risk of missing a narrow feature between neighboring channels.
For finer sampling, four A2 settings separated by 0.5° can be used.
Example
A2 = 64.0, 64.5, 65.0, and 65.5°
Data measured with fine angular steps can later be rebinned or integrated over wider angular intervals during analysis. Angular regions that were not measured, however, cannot be recovered afterward.
A C2 step of approximately 1° is a useful standard choice for many measurements.
0.1–0.2°
Useful for sharp Bragg peaks, peak centering, and detailed alignment scans. The appropriate step should be chosen according to the observed peak width; several points across the peak are needed for reliable determination of the peak position and width.
0.5°
Useful for fine mapping or when the scattering varies rapidly with angle.
1°
A practical standard choice for general measurements.
2°
Useful for broad scattering, preliminary measurements, or rapid surveys.
The optimum step size depends on the width and shape of the scattering being measured. Fine angular steps can later be combined into wider bins during analysis, but unmeasured angular regions cannot be recovered afterward.
Using smaller A2 or C2 steps increases the number of measurement points and motor movements.
When the counting time per point is long, the additional motor movement time may be relatively small. When short counting times or small mcu values are used, however, motor overhead can become a significant fraction of the total measurement time.
For this reason, very fine angular steps should not be used automatically. Choose the step size according to the required angular resolution and the available measurement time.
Because A2 and C2 are instrument angles, it is not always straightforward to determine which region of reciprocal space will be covered by a proposed A2–C2 map.
Before setting up an A2–C2 map, it is recommended to use the ScanSim function in ASYURA to visualize the expected scan coverage and determine appropriate A2 and C2 ranges.
This is particularly useful before starting a long mapping measurement.
Important — check cryostat hoses before C2 scans
When a cryostat is installed, always test the full planned C2 motion before starting the measurement. Confirm that the cryostat hoses do not become excessively bent, stretched, twisted, or caught during the motion.
As a rule, a single C2 scan spanning more than 140° should not be performed with the cryostat hoses connected.
If a wider angular range is required, divide the measurement into separate C2 scans. Reposition and re-suspend the cryostat hoses manually as necessary before starting each scan segment.
Even for a scan narrower than 140°, divide the scan if the hose arrangement becomes unfavorable anywhere within the planned range.
Wide-range C2 scans should not be started unattended until the complete planned motion has been checked.
Before starting a long or unattended measurement, physically check the extreme instrument positions required by the planned scan.
In particular, for measurements covering a range of A2 and energy transfer, check the limiting combinations of the planned A2–E range, including the highest incident-energy condition and both ends of the A2 range.
Confirm that the spectrometer, sample-environment equipment, cryostat hoses, and other attached components have sufficient clearance from surrounding equipment and structures.
Do not assume that a scan is mechanically safe simply because the central scan position is accessible.
When appropriate, check the corner conditions of the planned A2–E range before starting the full scan.
For production measurements after sample alignment, counting with MCU (monitor count unit) is generally recommended.
The beam monitor count rate varies with beam conditions and also depends on the incident neutron energy. Monitor-based counting therefore provides a more useful normalization for production measurements than simply counting for a fixed amount of time.
In HODACA measurements, 1 mcu is typically adjusted so that it corresponds to approximately one minute of counting near the elastic condition (E = 0 meV).
A practical procedure is:
drive e -0.2
drive e 0
count preset monitor 100000
After the count finishes, read the actual counting time (time-act) in seconds.
The MCU value corresponding to approximately one minute of counting can then be set directly in SPICE using:
mcu 100000/[time-act value]*60
SPICE evaluates the arithmetic expression and stores the calculated value as the MCU setting.
For example, if time-act = 20 s:
mcu 100000/20*60
sets the MCU value to 300000.
Finally, check the setting using:
count preset mcu 1
Near E = 0 meV, the actual counting time should then be approximately one minute.
Important: The MCU setting is cleared when a new experiment is started with the begin command. After changing the experiment number, always check and reset the MCU value before running scans with preset mcu.
The MCU value is normally determined near E = 0 meV, where the monitor count rate is relatively high.
When the energy transfer is changed, the incident neutron energy and monitor count rate also change. Therefore, preset mcu 1 does not necessarily correspond to exactly one minute at other energy transfers.
For example, a point that takes approximately 60 s near E = 0 meV may require 70 s or longer at another energy transfer if the monitor count rate is lower.
This behavior is expected: MCU counting keeps the requested monitor count approximately constant rather than keeping the counting time constant.
When estimating the duration of a measurement, do not simply multiply the number of points by the nominal MCU value in minutes.
The actual measurement time also includes changes in monitor count rate with energy, motor-motion time, settling time, temperature changes, and other scan overhead.
This difference can become significant for maps containing many points or when the counting time per point is short.
When a macro loop uses decimal step sizes, the nominal final value may occasionally be excluded because decimal values cannot always be represented exactly in floating-point arithmetic.
For example, if the intended sequence is:
64.0, 64.5, 65.0, 65.5°
it is safer to write:
loop a=64.0,65.51,0.5
rather than:
loop a=64.0,65.5,0.5
The slightly larger upper limit ensures that the intended 65.5° point is included even if the internal floating-point comparison evaluates the calculated final value as slightly larger than 65.5.
This small offset should only be used to ensure inclusion of the intended final point; it does not change the actual 0.5° scan step.
Mechanical backlash can affect not only C2 but also other spectrometer axes, including the monochromator axes A1 and C1.
When practical, approach an important scan position from a consistent direction rather than moving directly to the target from an arbitrary previous position.
For example, before a C2 scan starting at −61°:
drive c2 -62
scan c2 -61 -57 0.5 preset mcu 12
Similarly, when setting the energy transfer to E = 0 meV, a typical procedure is:
drive e -0.2
drive e 0
This provides a consistent final approach of the monochromator axes to the requested energy.
The appropriate pre-position depends on the intended scan direction and must always remain within the allowed motor and energy ranges.
Before starting an unfamiliar scan, check the calculated positions and scan conditions whenever possible.
For scans supported by ScanSim feature in ASYURA, use it to verify:
scan start and end points
number of measurement points
calculated motor positions
accessibility of the requested reciprocal-space region
energy-transfer conditions
Example:
scansim h 1.02 0.98 -0.002 k 0 l 0 preset time 10
After checking the result, run the corresponding scan command.
Before starting a long or unattended measurement, check:
sample and sample environment
UB matrix and scattering plane
requested Q and energy-transfer ranges
A2 and C2 ranges and step sizes
motor limits and accessibility
counting condition
temperature sequence and stabilization condition
scan titles and experiment information
expected total measurement time
For long macro sequences, divide the experiment into manageable sections when practical. This makes it easier to restart the experiment after an interruption and to modify later measurements without repeating completed scans.