From protons to neutrons

The neutron beam used by POSY2 is generated by the Intense Pulsed Neutron Source which consists of a particle accelerator, spallation target, and surronding moderators and reflectors. H- ions produced in an ion source are accelerated to 750 keV in a pre-accelerator and then to 50 MeV in a linac. The beam of H- ions are then magnetically steered to a Rapidly Cycling Synchrotron (RCS). At its entrance the H- pass through a carbon foil that strips off electrons. The resulting proton beam is accelerated to 450 MeV and strikes a uranium target thirty times a second.
The neutrons produced by the target are slowed down in a solid methane moderator with an effective mean temperature of 20 K.

A much more complete description on how IPNS generates neuttrons can be found here.

Data, the easy way

Easy to learn, easy to use.

POSY2 was designed to be a user instrument, hence one important aspect was kept in mind: simplicity of use. The instrument can be operated by a single user with only a few hours of “on hand” experience. Accordingly, moving parts have been completely minimized. The only moving part is a sample holder assembly on the goniometer which is computer controlled.

The detector is a fixed He3 gas linear detector which allows a maximum incident angle of 3 degrees at the sample position. This limits the maximum incident neutron momentum to ~0.1 Å^-1; such a range is ideal for studying polymer systems. The angle of incidence at the sample position is calibrated in terms of detector channels (at the reflected beam position); thus the accuracy in insertion and removal of samples do not affect the accuracy by which the angle is determined.

The scattering plane of the neutrons is horizontal— samples are placed in vertical geometry. Thus it is primarily designed for the study of solid- solid and solid-liquid interfaces in the vertical geometry. The standard dimensions of the sample holder are large enough for neutron reflectivity as well as allow for easy preparation of polymer layered samples which are homogenous; essential for reflectivity purposes. It is also possible to accommodate samples whose sizes are somewhat different from the standard.

As regards the operation of the instrument, the setting up of runs (data collection) is accomplished in only a few minutes. The user has to insert the sample, input the desired sequence of runs at different angles and start the runs (for a preset amount of time at a given angle).