# The `ILL_BRISP` Instrument *McStas: Time of Flight Neutron Spectrometer for Small Angle Inelastic Scattering BRISP* ## Identification - **Site:** ILL - **Author:** E. Farhi and N. Formissano [formisan@ill.fr] - **Origin:** ILL - **Date:** June 4th, 2009 ## Description ```text BRISP is a new concept thermal neutron Brillouin scattering spectrometer which exploits the time-of-flight technique and is optimized to operate at small scattering angles with good energy resolution. Keywords in the design of the BRISP spectrometer were : Thermal neutron energies: allowing for investigations in systems characterized by sound velocities up to 3000 m/s (three different incident energies between 20 and 80 meV are presently available). Easy small-angle access: enabling low-Q spectroscopy with thermal neutrons. Elastic wavevector transfer values Qel as low as 0.03 Å -1 at 20 meV incident energy can be reached. The position of the two-dimensional detector can be adjusted to cover different small-angle ranges between 1° and 15°. Time-of-Flight technique: for an efficient data collection allowing also for accurate neutron measurements as a function of external parameters such as temperature, pressure and magnetic field. Carefull optimization of monochromator-collimators-Fermi chopper: leading to 0.5 meV energy resolution and 0.02 Å-1 Q resolution in a typical configuration (20 meV incident energy and 4 m sample-detector distance), along with acceptable counting rates (flux at the sample 104 n s-1 cm-2). For this purpose, innovatory solutions were specially developed for some of the BRISP components. Main components: a Soller collimator defining the beam impinging on the monochromator, with a collimation angle of 0.4° two focusing multi-crystal monochromators, PG and Cu(111), that allow for the selection of three incident energies in the range from 20 to 80 meV. Fixed/variable curvatures are adopted in/outside the Brisp vertical scattering plane. a disk chopper used for background reduction and selection of the desired monochromator reflection through proper phasing with the Fermi chopper. three honeycomb converging collimators [1] to define the incident beam on the sample with a collimation angle of 0.4°, and to optimize convergence at three detector positions (2, 4, 6 m from the sample). A coarse resolution option is also available, without honeycomb collimator. a Fermi chopper producing short neutron pulses which enable the time-of-flight analysis. a high-vacuum sample chamber possibly equipped with 1.5-300 K MAXI Orange cryostat (100 mm) and 300-1900 K furnace a ~2 m2-area position sensitive gas detector (3He) whose distance from the sample can be varied between 2 and 6 m in order to access the required Q-range. A huge vacuum tank hosts the detector. An elastobore – polyethylene shielding surrounds the vacuum tank to reduce the environmental background. the long vacuum line ensures an under-vacuum neutron flight path from the background chopper to the detector. Configurations: crystal d-spacing (Å) lambda0 (Å) E0(meV) PG(002) 3.355(nominal) 1.977(expt.) 20.9 (expt.) Cu(111) 2.087 1.28 (expt.) 49.9 (expt.) PG(004) 1.677(nominal) 0.989(expt.) 83.6 (expt.) In this model, the sample is a plate of thickness e=4 mm, surrounded by an Al or Nb container, inside an Al shield (phi=10 cm). ``` ## Input parameters Parameters in **boldface** are required; the others are optional. | Name | Unit | Description | Default | |------|------|-------------|---------| | DM | Angs | Monochromator d-spacing. Use 3.355 for PG002, 1.677 for PG004 and 2.087 for Cu111. | 3.355 | | coh | str | Sample coherent specification (use laz, lau or Sqw file, or NULL to disable). Sample is a 5x5 cm plate, e=4 mm. | "V.lau" | | inc | str | Sample incoherent specification (use laz, lau or Sqw file, or NULL to scatter isotropically, using cross sections read from the coherent file) | "NULL" | | container | str | sample container material. Thickness is .2 mm. Use NULL, Al or Nb. | "NULL" | | LSD | m | Distance sample-detector | 4.5 | | Frequency | Hz | Disk-chopper frequency | 0 | | LCC | m | Distance between the Disk-chopper and the Fermi-chopper | 0 | | DELAY | s | Fermi phase wrt. Disk-chopper opening | 0 | ## Links - [Source code](ILL_BRISP.instr) for `ILL_BRISP.instr`. - The BRISP spectrometer at the ILL BRISP ---