| Moderator | Liquid-methane (CH4) at 105 K (poisoned with Gd at 2.5 cm) | ||||
| Incident energy | 7 – 1000 meV | ||||
| Energy resolution | Depends on the choice of Fermi chopper, incident energy and energy transfer
D hw /Ei = 1.5-8 % |
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| Primary flight path | 11.739 m | ||||
| Secondary flight path | 4.020 m | ||||
| Fermi chopper | 10.050 m
50-600 Hz phased to ISIS pulse ± 0.1 μs 4 chopper packages are commonly used on MARI optimised for different incident energy and resolution requirements
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| Detectors | Low-angle bank
4.020 m from sample 256 10 atm 3He tubes in 8 fold array covering 3 – 13º scattering angle |
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| High-angle bank
4.020 m from sample 10 atm 3He tubes covering 12º – 135º scattering angle |
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| Monitors | Monitor 1: 07.000 m from moderator Monitor 2: 10.297 m from moderator Monitor 3: 17.559 m from moderator |
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| Flux at sample | [× 104 n cm-2 s-1] | ||||
| 7 meV | 25 meV | 100 meV | 500 meV | ||
| G-Chopper 4% ΔE/E | 0.5 | 1.4 | 0.9 | n/a | |
| S-Chopper 5% ΔE/E | 0.5 | 1.5 | 0.9 | 1.3 | |
| A-Chopper 2.5% ΔE/E | n/a | n/a | 0.3 | 0.4 | |
| R-Chopper 4% ΔE/E | n/a | 0.8 | 0.5 | 0.7 | |
In this section
Mari
Mari is a chopper spectrometer with a wide angular coverage and energy range. It is is ideal for studying phonon densities of states in polycrystalline and disordered systems, and crystal field excitations in magnetic materials. Mari also has the lowest instrumental background of ISIS's chopper spectrometers, making it highly sensitive despite the relatively low incident neutron flux.
Instrument scientists
To find out more about each team member, click the + symbol.
Technical information
Mari features a unique vertical scattering geometry, with detectors forming an arc below the sample position due to space constraints. This design limits the types of sample environment equipment that can be used. Mari has a dedicated CCR with a hot-stick that can handle temperatures from 5 – 500 K, a dedicated Orange cryostat, which can be cooled to 1.8 K, and a dedicated furnace that covers the temperature range 20 – 1000°C. Mari cannot use any cryomagnet or the 3He or dilution inserts.
As with other direct geometry time-of-flight spectrometers, there is flexibility to choose operating conditions that optimise count rate, resolution and dynamic range. Typically, the instrument is run to maximise flux with a relatively coarse energy resolution of 4% of incident energy. Mari can cover an incident energy range from 5 – 2000 meV. A typical high-flux RRM mode setting (with the chopper rotating at 200 Hz) yields neutron pulses with incident energy of 100 meV (ΔE = 5 meV), 23meV (ΔE = 0.7 meV) and 10meV (ΔE = 0.25 meV), whilst in high-resolution mode (chopper at 400 Hz), the flux (around 2 ×104 n cm-2 s-1) is reduced by half, and the resolution improved to 3meV (Ei = 100 meV), 0.5 meV (Ei =23 meV) and 0.2 meV (Ei = 10 meV), respectively.
Related resources
Instrument parameters, energy resolutions and fluxes
Sample environment
The vertical detector geometry on Mari has profound implications for the optimal sample geometry and the design of sample environment equipment.
Samples with a slab geometry require large self absorption corrections, and although these are not difficult to perform it is possible to largely avoid them completely by using samples with a cylindrical cross-section. It is advantageous to fill as much of the beam as possible, however, a solid cylinder of 50 mm diameter is impractical and so the recommended sample geometry on Mari is an annulus of 45 mm in diameter 45 mm in length and a thickness sufficient to give a 10% scatterer.
The standard sample cans used on Mari are thin walled and made from aluminium, with an external diameter of 43 mm and a wall thickness of 0.1 mm. These can be sealed with indium if necessary and if they are to be cooled, we recommend that they are sealed under helium so that they do not crush at low temperatures. The helium also acts as an efficient exchange gas.
Because of the unusual detector geometry, much of the sample environment used on Mari is unique to the instrument. The following list describes the sample environment equipment suitable for use on Mari.
4 K – 600 K Top Loading Closed Cycle Refrigerator (CCR)
The CCR is a top loading system that allows samples to be changed without the necessity of bringing the main sample tank to air. The design is such that the absolute amount of aluminium in the beam is minimised and the IVC and 1st stage shield are domed to match the Mari detector geometry and minimise shadows and spurions. The sample space is filled with a low pressure He exchange gas, this improves sample cooling. The sample changing routine is similar to that of a standard orange cryostat.
- Two sample sticks for low temperature use
- A sample stick for use at high temperature
- A sample stick that holds the standard vanadium sample
1.4 K 4He Flow Insert
A simple flow insert for the top loading CCR with a base temperature of 1.4 K.
The MARI Furnace Tmax = 1000°C
This is a special low mass furnace where the thin niobium heating element acts as the outer wall of the sample can. The geometry is horizontal and cylindrical and the background very low. By varying the number of niobium heating elements temperatures in the range between 100°C and 1000°C are achievable with a stability of less than a degree.
McWhan Clamped Cell
The McWhan cell uses pre-stress alumina inserts to archive pressures of up to 20 kbar. The sample sizes are of the order of 4 mm in diameter and 10mm long. The geometry is much better suited to Mari as the cylinder axis is horizontal and internal boron-nitride collimation reduces the scattering from the alumina to a reasonable level. However, it is not possible to pressurise in-situ, and it takes several hours to cool the whole cell once it is on the instrument.
For more information, contact the ISIS Sample Environment team or the Mari instrument scientists.
Software
- Mantid – RAW file access, data reduction and scripting with Python, including crystal field fitting.
- Mantid MSlice – Viewing processed data in |Q| and energy.
- Matlab – Viewing data with MSlice, and other legacy software
Recent publications
Instrument reference
All publications and datasets based on experiments using Mari should cite that the data is collected by DOI: 10.5286/isis.instrument.8362. Experiment DOIs follow the format 10.5286/ISIS.E.RBXXXXXXX, where XXXXXXX is the 7-digit experiment (RB) number and these can be viewed via the Data Gateway.