[ Identification | Description | Input parameters | Output parameters | Links ]

# The `ILL_BRISP` Instrument

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
- Version: 0.2

## Description

    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.
    Careful 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. | "NULL"
inc | str | Sample incoherent specification (use laz, lau or Sqw file, or NULL to scatter isotropically, using cross sections read from the coherent file) | "V.laz"
container | str | sample container material. Thickness is .2 mm. Use NULL, Al or Nb. | "NULL"
LSD | m | Distance sample-detector | 4.5

## Links

- Source code for `ILL_BRISP.instr`.
- The BRISP spectrometer at the ILL BRISP

[ Identification | Description | Input parameters | Output parameters | Links ]