q-range-basics

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

1. Start from the real-space length scale of interest using $d \approx 2\pi/Q$.
2. Choose candidate wavelength bands compatible with the source/chopper setup.
3. Estimate scattering-angle coverage and solve for expected Q reach.
4. Cross-check low-Q and high-Q limits against beamstop and detector edges.
5. 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

1. Start from the length scale you want to probe: $d \approx 2\pi/Q$.
2. Choose $\lambda$ compatible with the source/chopper.
3. Solve for $\theta$ (or detector distance) that puts your target $Q$ on the detector.
4. 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

• Target length scales are mapped to required Q range.
• Candidate wavelength and angle coverage are explicitly stated.
• $Q_{min}$ and $Q_{max}$ are both cross-checked against geometry constraints.
• Any assumptions about beamstop or detector limits are documented.