CRG-B thermal neutron interferometer S18

Guide hall n°1, thermal guide H25

Neutron interferometer with USANS option


two crystals mounted on a computer controlled support

type I

silicon perfect block crystal

reflecting planes

[111], [220], [113], [331]

wavelenght range

0.6 Å < λ <4 Å

type II

channel-cut silicon perfect silicon crystal

reflecting planes

[220], [331]

wavelenght range

1.6 Å < λ <2.9 Å

Beam cross section

2 x 5 cm2


Large perfect Si crystal interferometers of different designs

coherent beam-separation

2 - 5 cm

enclosed area

up to 100 cm2

path lengths

10-21 cm

performance of a [220] skew symmetric interferometer at 1.84 Å with beam area 1x1 cm2

flux in front of interferometer

16000 n cm-2 s-1

flux in O-bean and H-beam

7000 n cm-2 s-1

contrast in O-beam


wavelength spread Δλ/λ


beam divergency


Ultra small-angle neutron scattering (USANS)

Bonse-Hart camera


6-fold [220] Bragg reflection with tail suppression

peak intensity

10000 n cm-2 s-1

angular resolution

0.01 sec arc

momentum resolution

1.5·10-5 Å-1

signal to background ratio

> 105

Polarised neutron option

polariser type

double magnetic prism deflection


> 99.5 %

beam cross section

1 x 1 cm2


Instrument description


Two perfect crystal silicon monochromators are avialable.

  • The block crystal monochromator offers three different surfaces with crystal orientation [220], [111] and [113] respectively. The Bragg angle can be tuned from 27° to 55°. Typical usage: Interferometry
  • The channel-cut monochromator offers a [220] reflection in triple bounce geometry optimized for 30° Bragg angle. The triple Bragg reflection leads to a sharper rocking curve by suppressing the tails of the peak. Typical usage: USANS and polarized interferometry 

Most experiments use the [220] reflection with 30° or 45° Bragg angle. The former corresponds to 1.9Å wavelength where our thermal beam line offers maximal intensity. The latter corresponds to 2.7Å wavelength and is chosen for certain interferometers for geometric reasons.


Several interferometers with different shapes and sizes are available. 

  • 30° small symmetric, 90% contrast
  • 30° big symmetric, 70% contrast
  • 30° standard skew-symmetric, 65% contrast
  • 30° double loop
  • 45° single loop, 40% contrast
  • 45° double loop

A typical interferogram at 30° Bragg angle is recorded in about 10 to 15 minutes. This time can increase considerably if the beam is attenuated by samples, slits, spin polarizers etc.


The resolution given by the instrumente rocking curve without sample is in the order of 0.0003° = 1" = 5 µrad. A momentum transfer down to Q = 2 10^-5 Å^-1 can be measured, corresponding to structure sizes up to 30µm. Towards larger Q values (smaller structure sizes) the range is limited only by the strength of the scattering signal. A typical geological sample needs several hours to be measured at Q = 10^-3 while it is a matter of minutes at Q = 10^-4. For larger Q values one should consider SANS rather than USANS.