Instrument description

Reactor hall, thermal beam H12
PG 002: 2.20 ... 3.60 Å
PG 004: 1.10 ... 1.80 Å
Cu 220: 0.85 ... 1.50 Å
Cu 111: 1.25 ... 2.24 Å
take-off angle 2θM 39 ° ... 65°
resolution δ>Ei / Ei 3 ... 6 %
flux on sample 5 x 105 cm-2s-1
background choppers νmax 5 000 rpm
Fermi chopper νmax 32 000 rpm
duty cycle 3 x 10-3
beam size on sample 2 x 4 cm2
primary collimation δθ

IN4C is a time-of-flight spectrometer used for the study of excitations in condensed matter. It works in the thermal neutron energy range (10-100 meV).

Primary spectrometer

The main components of the beam conditioning part are the two background choppers, the double curvature mono-chromator with four faces and the Fermi chopper.

The background choppers are rapidly pulsating beam shutters which act as a low-pass filter. Thus they eliminate from the beam most of the fast neutrons and gamma rays that would give background noise in the spectra. The modular shielding encloses the background choppers in separate compartments in order to cut off these undesired neutrons as early as possible.

A suitable energy is selected from the thermal neutron spectrum with the crystal monochromator. The monochromator, an assembly of 55 crystal pieces, concentrates the divergent incident beam onto a small area at the sample position. The full use of the available solid angle gives a high incident flux. The variable curvature of the monochromator is essential in controlling the time and space focussing conditions for optimal performance (see H. Mutka, Nucl. Instr. and Meth. A 338 (1994) 144).

The Fermi chopper rotates at speeds of up to 40000 rpm. It transmits short neutron pulses (10 ... 50 µs) to the sample. The time-of-flight of neutrons between the chopper and the sample (1 ... 5 ms) can be measured by using precise electronic circuitry.

Secondary spectrometer

The sample environment is designed to accommodate standard cryostats and furnaces. A radial collimator around the sample position is used to cut the scattering from the sample environment.

The secondary flight-path is in vacuum to avoid parasitic scattering of the transmitted neutrons.

The detector bank covers scattering angles of up to 120°. In addition to the 3He detector tubes (length 300 mm, width 30 mm, elliptical section, pressure 6 bar) a 3He filled multidetector (eight sectors with 12 radial cells each; outer diameter Å 60 cm) will allow us to observe forward scattering.

The time-of-flight spectra measured at various angles are further treated in order to obtain the scattering function S(Q,w) that is characteristic of the properties of the sample. One can extract from it the complete information on the energy and length scale of the dynamical phenomena of isotropic systems such as liquids, disordered materials as well as of non-dispersive (i.e. localised) excitations in molecules and magnetic ions. Otherwise orientationally averaged quantities are obtained, for example the density of vibrational states in polycrystals.