Instrument description

Neutron spin-echo spectroscopy measures the mobility of atoms or spins in a sample over time. For a spin-echo measurement we exploit the fact that a neutron has a spin which reacts to magnetic fields. In a magnetic field the spins rotate like a clock with a speed that is proportional to the strength of the magnetic field. If the structure in the sample changes within the time frame given by this neutron clock we see a decrease in the spin-echo signal. The signal is proportional to the so-called intermediate scattering function S(Q,t). A spin-echo measurement consists in measuring the change in the signal as the time is scanned (the time is proportional to the applied magnetic field).

For a spin-echo spectrometer it is crucial that all neutrons will see the same magnetic fields, independent of their paths through the spectrometer. For most existing spectrometers, like IN11 and IN15, this is achieved by constructing large coils which are placed with their central axis parallel to the beam axis. This has strongly limited the detector area and the data rate. In the WASP spectrometer we are using coils whose central axis is vertical and centered on the sample position. These coils create a field that is symmetrical about the sample and the same for all incoming and scattered neutrons on a horizontal plane. Therefore, we will be able to create a large bank of detectors with a solid angle of at least 150 x 2.5 degrees. WASP will have a data rate that is at least a factor of 50 higher than on comparable existing spectrometers.

The sample stage will take all (non-magnetic) ILL standard sample environments including orange cryostats, cryo-furnaces and 3He cryostats. A non-magnetic high temperature oven (300...780 K) will also be available.

EXPECTED PERFORMANCE