FIGARO

Guide hall n°1, ILL7, cold neutron guide H171

(Vertical Neutron Source)
guide section 80 x 30 mm2 (w x h)

Instrument Data

beam size at sample

40 x 10 mm2

incident wavelength range

2 - 30 Å

scattering plane

vertical

Figaro

Time-of-flight 

4 choppers

Frame overlap mirrors

[20Å] [30Å]

white beam flux at sample

~10n cm-2 s-1

angle undeflected beam

0.62˚

angles deflected beam

-2.5˚ < θ < 3.8˚ (horizontal)

constant wavelength resolution δλ/λ

[0.8-7.0%]

elastic Q-range (down)

0.0045 – 0.42 Å-1 (horizontal)

elastic Q-range (up)

0.0045 – 0.27 Å-1 (horizontal)

Detector

type

3He Tubular Aluminium Monoblock

sample-detector distance

1.0 – 2.8 m

size

500 x 250 mm2

resolution

2 x 7 mm2

dead-time

34 ns

 

Characteristics

FIGARO (Fluid Interfaces Grazing Angles ReflectOmeter) is a high flux, flexible resolution time-of-flight neutron reflectometer with a vertical scattering plane, which was commissioned at the ILL in April 2009. It is used for studies of thin films at the air/liquid, liquid/liquid and solid/liquid interfaces mainly in the fields of soft matter, chemistry and biology. Applications involve the study of surface behaviour of surfactants, polymers and other amphiphiles in adsorption troughs and the interactions of proteins or DNA with lipid monolayers on a Langmuir trough. No polarized neutron option is currently available nor is foreseen in the near future.

Special features of the instrument include the possibility to strike the interface from above or below in a wide q-range. With an incoming beam of wavelengths comprised between 2 and 30 Å, a q-range of 0.0045 to 0.42 Å-1 can be achieved for horizontal samples by using just two incoming angles of 0.62 and 3.8˚, and the q-range can be extended for solid samples by tilting them. Total measurement times are roughly similar to those used on the more established D17 reflectometer but, due to the effects of gravity on the neutron trajectories, FIGARO is less well suited for the study of relatively small solid/liquid interface cells (i.e. 5 cm or less). A particularly useful feature of FIGARO, however, is the ability to gain flux at constant resolution by using a chopper pair with a wide separation. Hence the instrument is well suited to the study of fast kinetic processes with time slices possible at low incident angles (surface excess) on the order of 1 s and at higher incident angles (structure) on the order of minutes.

The first component of the instrument is one of two frame-overlap mirrors to remove neutrons with wavelengths above 20 or 30 Å. Four choppers follow, with carbon fiber discs of 800 mm diameter and 45˚ aperture, independently rotating at a speed of up to 2000 rpm. The distance between the discs is such that 6 different wavelength resolutions can be obtained, ranging from 0.8 to 7.0%, by keeping zero projected chopper opening. Choppers are followed by two guide sections of M=4 supermirrors which deflect downwards or upwards to allow incoming angles of the neutron beam between 0.62 and 3.8˚. Six incident angles on the sample have now been fully commissioned. Next comes a 2-m collimation section, in order to eliminate off-specular scattering arising from the two supermirrors, comprising two slits and an absorbing guide on the top and bottom faces and M=4 supermirror sides to carry on the horizontal focussing down to 40 mm. A two dimensional multitube detector is positioned at a distance of about 3 m from the sample. It can move up and down to detect the reflected and direct beams at all angles. It consists of an aluminium plate with 64 square holes of 7 mm internal side and 2 mm resolution along the 250 mm length. This detector allows measurements of specular and off-specular reflectivity, and a GISANS option for samples which require horizontal geometry has also been implemented even counting times are relatively long. The sample stage is equipped with active and passive anti-vibration systems, x-y-z translation axis, goniometer for solid samples.

An optical sensor (Keyence, Japan) is used for the easy alignment of free liquid samples, and both a phase modulated ellipsometer (Beaglehole, New Zealand) and a Brewster angle microscope (Nanofilm, Germany) are available offline for users to make preparatory and complementary optical measurements.

A thorough technical reference paper on the instrument may be found HERE.