| name | q-range-basics |
| description | Plan an experiment's accessible momentum-transfer (Q) range from wavelength, scattering angle, and detector geometry. Use when a user asks about choosing instrument configuration, Q_min/Q_max, or length scales probed in any neutron scattering experiment. |
| version | 2 |
| review | {"status":"pending","reviewer":null,"reviewed_on":null,"basis":[],"notes":"v2: restructured to required skill anatomy (Overview / When to Use / Process / Rationalizations / Red Flags / Verification). Existing technical content retained and reorganized.\n","approved_commit":null} |
| metadata | {"techniques":["SANS","diffraction","reflectometry","inelastic"],"tags":["Q","planning","geometry","wavelength"]} |
Q-range basics
Overview
The momentum transfer for elastic scattering is
$$ Q = \frac{4\pi}{\lambda} \sin\theta $$
where $2\theta$ is the scattering angle and $\lambda$ is the neutron wavelength.
When to Use
Use this skill when:
- You need to estimate accessible $Q_{min}$ and $Q_{max}$ for an experiment.
- You are choosing wavelength, detector geometry, or scattering-angle coverage.
- You need to convert between target real-space length scales and Q range.
Do not use this skill when:
- You need instrument-control scripting details for data acquisition.
- You need detailed reduction or fitting guidance.
Process
- Start from the real-space length scale of interest using $d \approx 2\pi/Q$.
- Choose candidate wavelength bands compatible with the source/chopper setup.
- Estimate scattering-angle coverage and solve for expected Q reach.
- Cross-check low-Q and high-Q limits against beamstop and detector edges.
- Confirm the chosen setup still covers all required science length scales.
Typical ranges
| Technique | Q range (Å⁻¹) | Length scale d ≈ 2π/Q |
|---|
| SANS | 0.001 – 0.5 | 10 – 6000 Å |
| Reflectometry | 0.005 – 0.3 | surfaces / multilayers |
| Diffraction | 0.5 – 20 | atomic bonds |
| Inelastic | instrument-dependent | varies |
Configuration checklist
- Start from the length scale you want to probe: $d \approx 2\pi/Q$.
- Choose $\lambda$ compatible with the source/chopper.
- Solve for $\theta$ (or detector distance) that puts your target $Q$
on the detector.
- Cross-check $Q_{min}$ from beamstop and $Q_{max}$ from detector edge.
Available tools (when bound)
If the agent has tools bound from this skill's scripts/tools.py, use them
for concrete numeric work:
compute_q(theta_deg, wavelength_aa) — Q from θ and λ.compute_d_spacing(q_inv_aa) — real-space length scale $d \approx 2\pi/Q$.half_angle(two_theta_deg) — convert 2θ to θ before calling compute_q.
Rationalizations
| Excuse | Rebuttal |
|---|
| "I can estimate Q in my head and skip explicit checks." | Small mistakes in angle or wavelength assumptions propagate directly into the accessible Q window and can miss the science target entirely. |
| "Beamstop only affects beamline setup, not science planning." | Beamstop geometry directly sets practical $Q_{min}$ and therefore the largest resolvable length scales. |
| "A single wavelength choice is enough for all target length scales." | Wider science goals often require balancing low-Q and high-Q coverage that may not be reachable with one wavelength/configuration. |
Red Flags
- Forgetting that $Q_{min}$ is set by the beamstop, not just $\lambda$.
- Using only long wavelengths when short-$d$ features are important, reducing
achievable $Q_{max}$.
- Solving Q limits without checking actual detector-angle boundaries.
- Mixing up $\theta$ and $2\theta$ in calculations.
Verification
