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. Obviously 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.
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 (see the user's manual for further details).
- Two sample sticks for low temperature use,
- A sample stick for use at high temperature,
- A sample stick that holds the standard Vanadium sample.
1.4K 4He Flow Insert
A simple flow insert for the top loading CCR with a base temperature of 1.4K (see image above)
This is 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 1300°C are achievable with a stability of less than a degree.
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 10 mm 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.
VeriCold (E18) Dilution Refrigerator
A high cooling power bottom loading dilution fridge with a base temperature of 20mK
For more information see the sample environment page or contact the instrument scientists.