Loq technical information
LOQ normally operates at 25 Hz but 50 Hz operation over one half of its normal wavelength range is possible and sometimes advantageous (see the section on Instrument Setups). Please contact the instrument scientists if you think 50 Hz operation may be of use to you.
Technical Summary
| Incident wavelengths | 2.2 - 10.0 Å at 25 Hz, 2.2 - 6.7 Å or 6.3 - 10.0 Å at 50 Hz |
| Momentum transfer, Q | 0.006 - 0.24 Å-1 (main detector), 0.15 - 1.4 Å-1 (new high-angle bank |
| Dynamic range in Q | 40 (on main detector), 230 (simultaneous use of all detectors) |
Notes for proposers:
- Users are expected to provide their own sample cuvettes/cans. But where necessary, some quartz cuvettes are available for loan. Please discuss with the instrument scientists. ISIS does not endorse any particular manufacturer of quartz cuvettes but both Hellma (stock codes: 110, 120 & 404; material codes: QS/QX) and Starna Scientific (stock codes: 21 & 32; material code: Q) manufacture suitable products.
If the ratio of H:D present in a hydrogenous sample is more than 50% H the sample path-length (thickness) should not exceed 1 mm (or absorbtion, incoherent and inelastic scattering effects may manifest themselves). Otherwise path-lengths of 2 mm are acceptable. In predominantly deuterated and/or very dilute samples 5 mm path-lengths can be used. Exceptionally 10 mm path-lengths may be necessary.
The volumes required for different 1 mm path-length cells are approximately (minimum in brackets):
10 mm wide rectangular - 0.25 ml (0.15 ml)
20 mm wide rectangular - 0.55 ml (0.40 ml)
disc-shaped ('banjo') - 0.25 ml (0.25 ml)
The advantage of disc-shaped cells and the 20 mm wide rectangular cells is that a larger diameter neutron beam can be used, increasing the count rate on the sample, where appropriate.
- The new high-angle detector bank records scattering at angles out to 35 degrees but this range may be restricted by certain types of sample environment. Please consult the instrument scientists.
- For low temperature work down to 10 K we recommend the use of a closed-cycle refrigerator rather than an orange cryostat. This is because a CCR has fewer aluminium alloy thermal shields to scatter neutrons.
- The Goudsmit electromagnet provides at least 1.5 T over 50mm. 1.9 T has been achieved with smaller pole gaps.
- The variable-opening chopper is under computer-control. This permits, for example, automatic quasi-monochromatic running for inelastic effect measurements. Please note, however, that in this mode the time resolution is poor and count rates are low.
Instrument Parameters
| Isis Beamline | N5, viewing the 25 K liquid hydrogen (lower) moderator. |
| Primary flight path | Soller supermirror bender (24 mrad, to remove neutrons with wavelengths less than 2 Å), upstream scintillator monitor, aperture dial No 1, variable-opening (2 - 126 degrees) disc chopper, frame overlap mirror (removes neutrons with wavelengths greater than 12 Å), 3 m evacuated flight tube, sample position scintillator beam monitor, aperture dial No 2, final collimation tube. |
| Sample position | Around 11.1 m from moderator. Approximate size is 0.4 m (parallel to beam) by 1.5 m. No height restriction. Beam is approximately 0.63 m above base plate. Crane access possible (SWL 1000 Kg). Sample transmission scintillator monitor on motorised rack. Provided with water, helium and electrical services. Secondary, top-loading, in-vacuum sample position with limited access and services around 12.5 m from moderator (giving an approximate Q range of 0.01 - 0.34 Å-1). |
| Beam size at sample | Defined by aperture No 2 and final collimation. Between 2 - 20 mm diameter. Typically 8 mm diameter. |
| Neutron flux at sample | Dependent on collimation, ISIS accelerator performance and target type. Typical time-averaged flux is 2x10^5 cm-2 s-1 (ISIS TS1 at 40Hz, 160 uA 800 MeV proton beam, tantalum target). |
| Secondary flight path | Evacuated tank to main detector. |
| Detector | 3He-CF4 filled ORDELA "area" detector 15.15 m from moderator. Active area is 64 cm x 64 cm with 5 mm resolution. Detector mapping under software control. External, annular, high-angle, scintillator "area" detector bank 11.6 m from moderator. |
References
RK Heenan, J Penfold & SM King, SANS at Pulsed Neutron Sources: Present & Future Prospects, J Appl Cryst (1997), 30, 1140-1147
