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

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.
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

The ILL_BRISP Instrument

Identification

Description

Input parameters

Links

The `ILL_BRISP` Instrument