CRG - Cold neutron time-focussing time-of-flight spectrometer

Guide hall n°1, cold guide H15


composite, vertically focussing pyrolitic graphite crystals

Incident wavelength in Å

4.1; 4.6; 5.1; 5.9


elastic energy resolution

at 4.14 Å

170 µeV

at 4.62 Å

120 µeV

at 5.12 Å

70 µeV

at 5.92 Å

50 µeV

max energy loss of neutrons

3 meV

max energy gain of neutrons

200 meV

max momentum transfer

2.6 Å-1

vertical divergence

100 mrad

beam size at the sample

3 x 5 cm2

flux at the sample at 4.1 Å

8.9 x 104 cm-2 s-1

Fermi chopper speed

3000 - 15000 rpm


total thickness 12 cm


337 elliptical 3He detectors

sample-to-detector distance

248 cm

angular range

10° ... 115°

vertical detection angle

+/- 15°

sensitive area

4 m2

total background

4.3 Hz

Sample environment

accommodating samples with 69 mm Ø

ILL orange cryostat

1.5 ... 310 K


50 ... 1500 °C

heating and cooling loop

100 ... 620 K


2.5 ... 600 K

dilution insert

min. 50 mK

vertical magnet

max. 2.5 T

Instrument description

SHARPER is a time focussing time-of-flight spectrometer designed for quasielastic and inelastic scattering for incident wavelengths in the range of 4 to 6 Å.

An intense beam is extracted from the H15 guide by a vertically focussing monochromator array. It consists of three composite pyrolytic graphite monochromators using the full height (20 cm) of the guide and focussing the beam at the sample position. In order to minimise the interference with the subsequent instruments, the monochromator can deliver only four wavelengths: 4.1; 4.6; 5.1; and 5.9 Å. The second order reflection from the graphite monochromator is removed by a beryllium-filter cooled at liquid nitrogen temperature.

To achieve the time-focussing condition, the beam is pulsed by a Fermi chopper. It has a small slot length to ensure a good transmission. The normal distance between the Fermi chopper and the sample is 38 cm. To prevent frame-overlap when the chopper is rotating faster than 7500 rpm, a suppressor chopper is placed before the Fermi chopper and rotates in phase with the latter at a lower speed. 

The secondary spectrometer consists first of an evacuated sample area. This allows us if necessary to remove the external wall of a cryostat or an oven. The vertical positioning of the sample box is achieved by a motor. A helium filled box between the sample and the detectors minimises the background. The sample box is equipped with an oscillating collimator which prevents parasitic reflections (from the cryostat walls for example) reaching the detectors. The detector bank is entirely covered with detector boxes, thus avoiding the inconvenience of moving the counters.