Excitations Group
03 Oct 2023



We operate some of the world's leading neutron spectrometers, able to measure magnetic and atomic excitations over a wide energy and momentum-transfer range.

​​​​​Excitations and Polarized Neutron Group 2016

​​About the Excitations Group

​Inelastic neutron scattering can provide rich information about atomic motions and magnetic interactions. Neutrons are a weakly interacting probe that do not perturb the system under study, making the results directly comparable to theory.

Our science programme spans a wide range of areas from the investigation of model magnetic systems through to the dynamics of disordered materials and quantum fluids and the dynamics of hydrogen in chemical and biological systems.

On this page you will find details of the members of the group, a summary of the characteristics of the instruments, a summary of the sample environment that is available on each instrument, and a comparison of the flux as a function of energy for the instruments.


Recent proposal rounds have seen significant number of beamtime proposals rejected by the panel for similar reasons. The Excitations group and Facility Access Panel wish to help users to submit good proposals so would point out the following reoccurring issues of missing information in proposals that is detrimental to their scores, together with straightforward remedies:

  • Lack of evidence that a large single crystals exists or that a sufficient number of co-aligned crystals can be aligned for the experiment. This can be resolved by including a picture (with scale bars)
  • Lack or unclear presentation of relevant structural and physical property characterisation of the materials to be studied. This can be resolved by presenting diffraction and physical properties characterisation of your samples in the proposal.
  • Lack of details regarding what will be measured during the experiment which makes it challenging or in some cases impossible for the viability and merit of the proposal to be assessed during review. This can be resolved by proposers contacting ISIS staff prior to proposal submission.


Main Contact

Other Contacts​

The Instruments

The excitations group operates five in instruments:

LET - a cold neutron spectrometer, for single crystal, powder and liquid samples

MAPS - a thermal neutron spectrometer, for single crystal and powder samples

MARI - a cold/thermal neutron spectrometer, for powder and liquid samples

MERLIN - a thermal neutron neutron spectrometer, for single crystal and powder samples

ALF - a diffractometer and test beamline, for rapid characterisation and alignment of crystal samples

All the instruments (except ALF) are direct geometry instruments where the incident neutron's energy is monochromated using one or more choppers.  Alll of the instruments ​can run in repetition-rate multiplication mode, where several incident neutron energies are used simultaneously, though the flexibility of this option varies across the instruments.

Here is a summary table that shows all of the instruments' characteristics​ in one place:


MARI has a wide angular coverage and a wide energy range and good resolution making it ideal for the study of phonon densities of states in crystalline and disordered systems, and crystal field excitations in magnetic materials.  MARI is the only chopper spectrometer at ISIS not to be equipped with a pixilated detector array using position sensitive detectors.  This makes MARI the instrument of choice for studies of polycrystalline and powdered samples, and liquids.  MARI also boasts the lowest instrumental background of the ISIS suite of chopper spectrometers, making the machine highly sensitive the incident neutron flux has recently (late 2018) been boosted significantly by the installation of a supermirror neutron guide like the ones used on MERLIN and MAPS.  


MAPS has a large array of position sensitive detectors at low angles making it ideal for the study of high energy excitations in single crystals.  A key feature of MAPS is its long secondary flight path (6m) which results in high energy resolution, a feature important for both single crystal magnetism and molecular spectroscopy experiments alike.  The high energy resolution of MAPS, and high energy transfer coverage at low Q makes the instrument ideal for complementary measurements of high energ​y excitations in molecular systems alongside TOSCA.


MERLIN is a high intensity, medium energy resolution spectrometer that would complement the high-resolution MAPS spectrometer.  MERLIN’s design exploits recent advances in technology with a m=3 supermirror guide to enhance flux, as well as 3 metre long position-sensitive detectors in a vacuum, making it ideal for the study of single crystals. The detector bank covers a massive π steradians of solid angle with an angular range from –45o to +135o in the horizontal plane, and ±30o in the vertical plane. This allows large swathes of Q,ω space to be accessed in a single run. MERLIN is able to run in repetition-rate multiplication (RRM) mode, allowing users to simultaneously measure with several incident energies.


The LET spectrometer is the latest addition to the inelastic suite at ISIS and has been in the user programme since 2011.  It utilises the high flux of cold neutrons on TS2 for studying low energy excitations in the region from 10s of µeV up to around 25 meV.  LET employs repetition-rate multiplication (RRM) - utilizing the whole time frame by allowing many incident energies at once and analysing them all separately.  LET supports a wide range of energy resolutions and a large position sensitive detector array allowing studies of crystals or polycrystals and liquids. LET was also built with a non-magnetic environment and a larger sample flange to allow for extreme sample environment equipment including neutron polarisation.

​Important note about polarised neutrons on LET: if you wish to submit a proposal that uses this capability it is essential that you contact the LET instrument scientists well before the proposal deadline in order to discuss technical feasibility.​


ALF is crystal alignment facility to make it possible to co-align multi-crystals, check the quality of crystals or simply pre-align a crystal prior to an experiment. You can request beam time on an application for beam time for any of the ISIS instruments. 

Data analysis software

Information on software used for data analysis can be found on the sidebar of this page. The IDAaaS (ISIS Data Analysis as a Service) platform has virtual machines that have much of the software commonly used for Excitations data analysis. IDAaaS can be accessed here: https://isis.analysis.stfc.ac.uk 

Magnetic Field Availability on the Instruments

There are three cryomagnets at ISIS used in high field experiment on the chopper spectrometers.  Each one provides a vertical field which is homogeneous over a large sample volume.  Each magnet may be used with a dilution or 3He insert for low temperature experiments.  We summarise below what magnets are available on which instruments, and the maximum allowable field

for each one.​

The 9T "Chopper" Magnet

Please con​tact the instrument teams if you would like to use this magnet on any instrument other than LET.

The 14T Diffraction Magnet

This has been successfully used on LET for experiments which require fields > 9T.  While almost the full horizontal scattering plane is viewable with this magnet, the out-of-place coverage is limited to ±7°. This magnet has been repaired (as of late 2021) so is now available for request on LET only. Please contact the LET instrument team if you wish to submit a proposal that requires this magnet.

The 7.5T Magnet

This is very occasionally used  - it has been used before on MAPS and MERLIN.  Aluminium tails are available for spectroscopy experiments.  The scattering angle coverage of this magnet is very limited, with a vertical coverage of ± 6°.


​Comparison of the flux and resolution of the instruments

​​Medium resolution, thermal neutrons

Below we compare the flux as function of Ei for MERLIN, MARI and MAPS for several different resolution options (quoted as a fraction of Ei, so 0.04Ei when Ei=50meV is an elastic resolution of 2meV) with different Fermi c​hoppers.





​High resolution, thermal neutrons




​Cold neutrons

​LET is the group's cold neutron spectrometer. Depending on the width of the slits in the disk of chopper 5 that are used, the instrument can be run in "high flux", "intermediate", or "high resolution" mode. We show the flux and resolution for selected chopper frequencies in each of these modes below