Instrument Description

Reactor hall, thermal beam IH3
 
Monochromator

Double focusing – horizontal variable focusing.

Incident beam area at the monochromator: 6 x 6 cm2.
Monochromator surface: 20.8 x 8.6 cm2

20 crystals in a 4 x 5 matrix: crystal size 4 x 2 cm2, mosaic spread 0.4° (PG) and 0.25° (Cu)

crystal
PG(002)
Cu(111)
PG(004)
d-spacing (Å)
3.355(nominal)
2.087
1.677(nominal)
λ0 (Å)
1.977(expt.)
1.28  (expt.)
0.989(expt.)
E0 (meV)
20.9 (expt.)
49.9 (expt.)
83.6 (expt.)
Collimation

Soller Collimator                α1 = 0.4°
700 mm length - 5 mm slit spacing
16 Gd2O3 coated Kapton sheets, 75 µm thick

Honeycomb Collimator       α2 = 0.4°
- 2000 mm length, Gd2O3-coated aluminum
- honeycomb arrangement of converging tubes of hexagonal section
- 3 different collimators for convergence at 2**, 4(recommended), 6 m from the sample position
- collimators are maintained under vacuum and selected by means of a 4-sector revolver
- one sector is used for coarse collimation using 2 Cd diaphragms (in: 81x81 mm2, out: 67x67 mm2)

Choppers
Background chopper
Fermi chopper
5000 rpm max
15000 rpm max
Sample
Beam area at the sample depending on the monochromator and honeycomb collimator adjustement
Sample chamber diameter 500 mm, height 550 mm
Silicon windows for vacuum separation.
Vacuum level in the sample chamber:          

10-4 mbar
Orange MAXI cryostat:
T=1.5-300 K
sample access diameter: 100mm

Ancillary equipment

Furnace:
T=300-1500K
Sample access diameter: 40mm

Detectors
PSD Reuter-Stockes 3He multi-tube assembly
100 3He filled (15 bar) tubes, φ=1/2" 1m length
Available detection area 2.1 m2
Detector arrangement

128 central tubes: diameter 12.7 mm, length 1118 mm
Side wings of 16 tubes each: diameter 12.7 mm, length 600mm

Pixel size

Height: 0.43 mm Width: 12.7 mm

              0.86 mm

Distances, useful ranges and resolution (approximate values)
Monochromator – Fermi chopper 5540 mm

Fermi chopper – Sample

1000 mm
Sample – Detector Dsd (variable) 2000-6000 mm
E0(meV) Dsd Energy Transfer
E (meV)
Angular and Qel ranges Resolution
DE / E0 DQ-1)
20.9 meV 2 m
4 m
6 m
± 18
± 14
± 12

1° - 15°
1° - 15°
1° - 9°

0.05-0.8 Å-1
0.05-0.8 Å-1
0.05-0.5 Å-1
3%
2.9%
2.4%
0.02
49.9 meV 2 m
4 m
6 m
± 50
± 45
± 35

1° - 15°
1° - 15°
1° - 9°

0.09-1.3 Å-1
0.09-1.3 Å-1
0.09-0.8 Å-1
3.8%
3%
2.7%
0.03
83.6 meV 2 m
4 m
6 m
± 80
± 70
± 60

1° - 15°
1° -15°
1° - 9°

0.1-1.7 Å-1
0.1-1.7 Å-1
0.1-1.0 Å-1
4.5%
3.2%
3.1%
0.04
** Please ask the instrument responsible for this configuration

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.