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

The ILL_BRISP Instrument

Time of Flight Neutron Spectrometer for Small Angle Inelastic Scattering BRISP



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 
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.

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.laz"
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


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

Generated automatically by McDoc, Peter Willendrup <peter.willendrup@risoe.dk> / Wed Sep 5 11:20:18 2018