S18
CRG-B - Thermal neutron interferometer
Guide hall n°1, thermal guide H25 |
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Neutron interferometer with USANS option |
Monochromator | |
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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 |
Interferometry | |
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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 | 90% |
wavelength spread Δλ/λ | 2.4% |
beam divergency | 0.75° |
Ultra small-angle neutron scattering (USANS) | |
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Bonse-Hart camera | |
type | 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 | |
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polariser type | double magnetic prism deflection |
polarisation | > 99.5 % |
beam cross section | 1 x 1 cm2 |
Instrument description
Monochromator
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.
Interferometer
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.
USANS
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.