To optimise the high neutron flux and rapid data collection rates several instruments have custom-built automatic Sample Changers, (SCs). Their prime advantage being that multiple samples can be loaded in a single operation, in the case of the TOSCA SC up to 24. The SCs are normally controlled by Command Files which will have been programmed to automatically place the pre-loaded samples in a particular sequence.
Currently at ISIS six instruments have their own SC; POLARIS, CRISP, TOSCA, SANDALS, GEM, and LOQ. While each has the same basic functionality they differ from one another in the detail of their mechanical construction to suit instrument and detector geometry.
The TOSCA and CRISP sample changers have been designed for cryogenic use, operating down to 20K and 1.5K respectively. The CRISP SC also operates in a 1.5Tesla horizontal magnetic field.
POLARIS Sample Changer is an ambient temperature changer having a 20-sample carousel. Samples are loaded into the circular carousel in 8mm dia. vanadium cans. A precision rotation stage selects the sample while stepper-motor driven plunger inserts it into the beam.
Polaris Sample Changer diagrammatic view
CRISP Sample Changer has 4 planar sample plates/trays which can be tilted by +-2o with a precision of 0.001o. The SC assembly is part of a special cryostat centre-stick. The functioning part, shown left, fits inside the cryostat tail. Two stepping motors, which operate the SC's vertical movement - between samples, and the sample tilt angle are at the upper end of the centre-stick at ambient temperature. The tail has sapphire windows to allow initial laser alignment of the samples.
TOSCA Sample Changer schematic
This is a cryogenic Sample Changer which operates in the TOSCA cryogen-free top-loading cryostat. The 24 flat-plate sample containers are hung from a series of rods which link across between the 2 transport chains. The diagram shows the motor drive mechanism and the lowermost sample, the one which will be in the beam. The distance between the two is of the order 1400 mm.
The base temperature on the lowest sample is ~20K.
GEM Sample Changer utilises a 700mm travel proprietary translation table rigidly mounted on a standard 500mm diameter SE flange. The triangular cross-sectioned sample carrier has been designed to just pass through the gap between the instrument's back scattering, 140o - 170o and 90o detector banks. Up to 10 samples, in 5mm dia. Vanadium tubes, are located into slots on the sample carrier.
SANDALS Sample Changer.
Sample cells are mounted on the aluminium frame in a 5-high x 3-wide array. The frame is built of hollow section to permit externally pumped cooling fluid to circulate and regulate the sample temperature. The frame and sample array is manipulated by a combination of linear and rotary stages to place the sample in the neutron beam.
Diagrammatic view: click to view large image.