---
title: "Following energy exchange with neutron flight times"
description: "Choppers select incident neutrons. Detector positions and arrival times reveal how scattering changes their energy and direction."
canonical_url: "https://sandboxphysics.com/stories/instruments/pulsed-neutron-time-of-flight/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/instruments/pulsed-neutron-time-of-flight/"
last_modified: "2026-11-13"
---

[How physics instruments measure](https://sandboxphysics.com/stories/instruments/)

# Following energy exchange with neutron flight times

Send particles over a known distance and the faster ones usually arrive first. A neutron time-of-flight spectrometer uses that idea to measure speed, then compares neutron energies before and after a sample. The difference comes from energy exchanged with the material.

Sandbox Physics · 2026-11-13

## A pulse supplies a departure interval

A spallation source sends energetic protons into a heavy-element target to release neutrons. Collisions in a moderator slow the neutrons into an energy range useful for experiments.

A pulsed source releases neutrons with many speeds over a short interval. Faster neutrons move ahead as they travel; slower neutrons fall behind. After a known distance, arrival time helps separate their speeds.

The start of the clock matters too. Emission lasts a finite time, and electronics introduce delays. Independent clocks and known-path references calibrate these relationships.

Sources: [1] [2]

## Rotating slots select a velocity range

A neutron chopper is an absorbing disk with an opening. As the disk spins, the slot briefly crosses the beam and lets neutrons through. The rest strike absorbing material.

Open two slots at suitable times and only a range of velocities passes both. Their known separation and opening-time difference constrain the selected speed. Narrower openings shorten the accepted interval and reduce the number delivered to the sample.

Sources: [2] [3]

## A neutron can leave slower or faster

Elastic scattering changes direction with approximately unchanged neutron kinetic energy. Inelastic scattering also exchanges energy. A neutron can deposit energy and excite a vibration in the sample. An already excited sample can give energy back to a neutron.

A direct-geometry spectrometer constrains incident energy first. The incident conditions estimate the sample arrival interval; the subsequent flight reveals outgoing energy. Their difference defines the energy delivered to the sample. A positive value means sample energy gain; a negative value means neutron energy gain.

$$
W=E_i-E_f
$$

Incident minus outgoing kinetic energy defines energy delivered to the sample. Outgoing energy requires the flight path and time after scattering.

Sources: [1] [3]

## Detector position belongs in the measurement

Detectors surround the sample and record which cell was hit and when. Position supplies the scattering direction. Direction and energy together help investigate the spatial scales of atomic motion.

A detector cell has width and thickness, and the sample occupies a finite volume. Neutrons reaching one cell can travel different distances. Slow neutrons also fall under gravity. These differences broaden sharp energy features and belong in the instrument response used for comparison.

Sources: [1] [2]

## One arrival can have more than one origin

A slow neutron may still be travelling when the source emits its next pulse. The same arrival could then come from an earlier slow neutron or a later fast one. This is pulse overlap.

An additional chopper can remove some origins. Events that survive must still be compared with the actual source pulses, opening intervals and detection times. Several compatible origins require several retained energy possibilities.

Sources: [1]

## Known samples reveal the instrument response

An elastic reference tests time zero and instrumental broadening. Known energy-transfer references also test the inelastic readout. Empty-holder and dark measurements estimate background; an independent monitor records the incident exposure.

Sample and background are finite counts. Subtraction can leave negative values in individual cells. Retaining them preserves the noise information. When references drift or origins remain ambiguous, the original events and allowed ranges still provide useful measurements.

Sources: [1] [3]

## [Trace pulses, slots and energy exchange](https://sandboxphysics.com/labs/neutron-triple-axis-spectrometer/?instrument=pulsed-neutron-tof)

Narrow the slots, then open the pulse-overlap demonstration and compare arrivals with allowed energy ranges.

A supplied pulse beam, finite choppers and a finite-level sample, with gravity, detector gaps and source-pulse ambiguity retained.

## Sources and further reading

1. [Copley & Udovic · Neutron Time-of-Flight Spectroscopy (1993)](https://nvlpubs.nist.gov/nistpubs/jres/098/jresv98n1p71_A1b.pdf)

   Read printed page 72 on flight times and known incident energy, and pages 83–84 on accessible regions, chopper correlations and flight paths. Anticipated 1993 performance is not used as a present facility specification.

2. [ORNL · ARCS instrument components](https://spectroscopy.ornl.gov/ARCS/Instrument-components.html)

   Read component roles in velocity selection, guides, choppers and sample delivery. Teaching geometry, timings and material parameters are separately supplied.

3. [ANSTO · Pelican time-of-flight spectrometer](https://www.ansto.gov.au/our-facilities/australian-centre-for-neutron-scattering/neutron-scattering-instruments/pelican)

   Read pulse formation with a rotating chopper, arrival-time energy measurement and the elastic/inelastic distinction.

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atomic spacing](https://sandboxphysics.com/stories/instruments/powder-xray-diffractometer/) [How Small-Angle X-ray Scattering Reads a Particle Halo](https://sandboxphysics.com/stories/instruments/small-angle-xray-scattering/) [Why X-ray fluorescence depends on sample thickness](https://sandboxphysics.com/stories/instruments/xray-fluorescence-spectrometer/) [Why an AFM makes a bump look wider](https://sandboxphysics.com/stories/instruments/atomic-force-microscope/) [How a small indentation separates hardness from elasticity](https://sandboxphysics.com/stories/instruments/nanoindenter/) [How Differential Scanning Calorimetry Reads a Melting Peak](https://sandboxphysics.com/stories/instruments/differential-scanning-calorimeter/) [How thermogravimetry separates mass loss from the gas around it](https://sandboxphysics.com/stories/instruments/thermogravimetric-analyzer/) [How a diamond anvil cell reads pressure in ruby 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## Keep reading

How physics instruments measure · Reading path

### How physics instruments measure

1. [Article **How a vernier reads between millimetres**](https://sandboxphysics.com/stories/instruments/length-metrology/)
2. [Article **Air helps hold up the object on a balance**](https://sandboxphysics.com/stories/instruments/precision-balance/)
3. [Article **How Cavendish Measured the Attraction Between Spheres**](https://sandboxphysics.com/stories/instruments/cavendish-balance/)
4. [Article **Which pressure does a liquid column measure?**](https://sandboxphysics.com/stories/instruments/manometer/)
5. [Article **The cup absorbs heat too**](https://sandboxphysics.com/stories/instruments/calorimeter/)
6. [Article **Rotational rheometers need both torque and speed**](https://sandboxphysics.com/stories/instruments/rotational-rheometer/)
7. [Article **Why the air beyond a pipe changes its resonance**](https://sandboxphysics.com/stories/instruments/resonance-tube/)
8. [Article **How a seismometer reads the ground through motion inside its case**](https://sandboxphysics.com/stories/instruments/seismometer/)
9. [Article **Why does a meter change the circuit?**](https://sandboxphysics.com/stories/instruments/electrical-meters/)
10. [Article **How a null detector reveals a resistance**](https://sandboxphysics.com/stories/instruments/resistance-bridges/)
11. [Article **Why a capacitor can behave like an inductor**](https://sandboxphysics.com/stories/instruments/lcr-impedance/)
12. [Article **Where does the slow wave on an oscilloscope come from?**](https://sandboxphysics.com/stories/instruments/digital-oscilloscope/)
13. [Article **How a lock-in amplifier finds a signal buried in noise**](https://sandboxphysics.com/stories/instruments/lock-in-amplifier/)
14. [Article **Where a small peak in a spectrum can come from**](https://sandboxphysics.com/stories/instruments/spectrum-analyzer/)
15. [Article **How a network analyzer separates the cable from the device**](https://sandboxphysics.com/stories/instruments/vector-network-analyzer/)
16. [Article **When a slower quartz crystal can measure added mass**](https://sandboxphysics.com/stories/instruments/quartz-microbalance/)
17. [Article **How an echelle spectrograph gives overlapping colors a second direction**](https://sandboxphysics.com/stories/instruments/echelle-spectrograph/)
18. [Article **How total brightness reveals many infrared colors**](https://sandboxphysics.com/stories/instruments/ftir-spectrometer/)
19. [Article **Measuring a Solution’s Concentration with Light**](https://sandboxphysics.com/stories/instruments/uv-vis-spectrophotometer/)
20. [Article **How an ellipsometer reads thin-film thickness from reflected light**](https://sandboxphysics.com/stories/instruments/spectroscopic-ellipsometer/)
21. [Article **How a polarimeter reveals mixed polarization**](https://sandboxphysics.com/stories/instruments/mueller-polarimeter/)
22. [Article **How individual photons reveal a fluorescence lifetime**](https://sandboxphysics.com/stories/instruments/tcspc-fluorescence-lifetime/)
23. [Article **Brillouin Microscopy Listens to Sound with Light**](https://sandboxphysics.com/stories/instruments/brillouin-microscope/)
24. [Article **How a white sphere compares a lamp’s total output**](https://sandboxphysics.com/stories/instruments/integrating-sphere-radiometer/)
25. [Article **How powder rings reveal atomic spacing**](https://sandboxphysics.com/stories/instruments/powder-xray-diffractometer/)
26. [Article **How Small-Angle X-ray Scattering Reads a Particle Halo**](https://sandboxphysics.com/stories/instruments/small-angle-xray-scattering/)
27. [Article **Why X-ray fluorescence depends on sample thickness**](https://sandboxphysics.com/stories/instruments/xray-fluorescence-spectrometer/)
28. [Article **Why an AFM makes a bump look wider**](https://sandboxphysics.com/stories/instruments/atomic-force-microscope/)
29. [Article **How a small indentation separates hardness from elasticity**](https://sandboxphysics.com/stories/instruments/nanoindenter/)
30. [Article **How Differential Scanning Calorimetry Reads a Melting Peak**](https://sandboxphysics.com/stories/instruments/differential-scanning-calorimeter/)
31. [Article **How thermogravimetry separates mass loss from the gas around it**](https://sandboxphysics.com/stories/instruments/thermogravimetric-analyzer/)
32. [Article **How a diamond anvil cell reads pressure in ruby light**](https://sandboxphysics.com/stories/instruments/diamond-anvil-cell/)
33. [Article **How diamond fluorescence reveals a magnetic-field direction**](https://sandboxphysics.com/stories/instruments/nv-center-magnetometer/)
34. [Article **Reading a magnetic field in the rhythm of atoms**](https://sandboxphysics.com/stories/instruments/optically-pumped-magnetometer/)
35. [Article **Keeping a laser in time with atoms**](https://sandboxphysics.com/stories/instruments/optical-lattice-clock/)
36. [Article **Why a Kibble balance needs two experiments**](https://sandboxphysics.com/stories/instruments/kibble-balance/)
37. [Article **How a dilution refrigerator keeps carrying heat away**](https://sandboxphysics.com/stories/instruments/dilution-refrigerator/)
38. [Article **Why a bigger vacuum pump may barely help the sample**](https://sandboxphysics.com/stories/instruments/ultra-high-vacuum-system/)
39. [Article **How microwaves read a superconducting qubit**](https://sandboxphysics.com/stories/instruments/superconducting-qubit-readout/)
40. [Article **How current leaves a voltage across a thin sheet**](https://sandboxphysics.com/stories/instruments/four-probe-hall/)
41. [Article **How an ion flight becomes a mass spectrum**](https://sandboxphysics.com/stories/instruments/time-of-flight-mass-spectrometer/)
42. [Article **How four straight rods select ion mass**](https://sandboxphysics.com/stories/instruments/quadrupole-mass-filter/)
43. [Article **How a rapidly reversing field can hold ions**](https://sandboxphysics.com/stories/instruments/paul-ion-trap-mass-spectrometer/)
44. [Article **How a cyclotron keeps a proton on the accelerating beat**](https://sandboxphysics.com/stories/instruments/cyclotron-synchrocyclotron/)
45. [Article **Why the drift tubes in an RF linac get longer**](https://sandboxphysics.com/stories/instruments/radio-frequency-linac/)
46. [Article **The directions hidden inside a small beam spot**](https://sandboxphysics.com/stories/instruments/beam-diagnostics/)
47. [Article **How defocusing magnets help a beam stay in a ring**](https://sandboxphysics.com/stories/instruments/synchrotron-storage-ring/)
48. [Article **Why a collider cannot keep every collision**](https://sandboxphysics.com/stories/instruments/collider-trigger-daq/)
49. [Article **Getting synchrotron light onto a sample**](https://sandboxphysics.com/stories/instruments/synchrotron-beamline/)
50. [Article **How a free-electron laser gathers electrons into a brighter beam**](https://sandboxphysics.com/stories/instruments/xray-free-electron-laser/)
51. [You are here **Following energy exchange with neutron flight times**](https://sandboxphysics.com/stories/instruments/pulsed-neutron-time-of-flight/)

1 more group

### Other articles that use the same experiments

Each experiment from this article that another series also uses, followed by the articles there.

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2. [Article **Neutron scattering: where atoms sit and how they move**](https://sandboxphysics.com/stories/probe-evidence/neutrons-structure-and-motion/)