D1B

D2B

D20

High-intensity two-axisdiffractometer with

variable resolution D20

Applications

NEUTRONSFOR SCIENCE

Selected examplesKineticsSelf-propagating high-temperaturesynthesis (SHS)

In situ neutron diffraction of the SHS ofTi3SiC2with 0.9 s time resolution was performed onD20, while heating from 850 °C to 1050 °C at100 °C /min.The diffraction patterns figure 1 indicate an a-b phase transition in Ti at 870 °C, pre-ignitionreactions (formation of TiCx), the formation ofa single intermediate phase (Si substitutedTiC)0.5 s after ignition, a delay of 2 s with heatingto in excess of 2000 °C followed by the rapidnucleation and growth of Ti3SiC2, which iseffectively complete within 5 s.The pre-ignitionreactions are exothermic and provide theextra heat for sustained SHS to occur. Noamorphous contribution to the diffraction pat-terns was detected and, as such, it is unlikelythat a liquid phase plays an important role inthe SHS. Using lattice parameters and knownthermal expansion data, the ignition tempera-ture was estimated at 901 ± 8 °C and the com-bustion temperature at 2320 ± 50 °C.

ThermodiffractometryPhase transitions (crystallisation,amorphisation, dehydration, etc.)

MagnetismDetermination of magnetic structures and

observation of weak magnetic peaks

KineticsDynamic studies in solid state chemistry

(concrete hardening, batteries, self-propagating high temperature synthesis,

clathrate formation, etc.)

Multi-stroboscopyDynamic studies on particle orientation in

flowing systems (rheology) and relaxation oflaser-induced excited states

TextureQuantitative texture analysis of composite

materials, archaeological and geologicalsamples, and bone

Very small samplesHigh-pressure prepared samples, in situ

experiments at extreme conditions (P or T)and isotopically substituted materials

Highly absorbing samplesMaterials containing natural

Sm, Li, Gd, Eu, B, etc...

Disordered systemsCo-ordination spheres in solids and liquids

PhysisorptionStructure of adsorbed solid layers

1

Absorbing samples

High intensity and the availability of shortwavelengths make it possible to record pow-der patterns of highly absorbing samples inannular sample holders (1 mm in the beam).Using a wavelength of 0.804 Å, the first fullstructure analysis of europium deuterides(natural Eu) with a reliable Eu-D bond distan-ce determination were done on D20 forEuMg2D6 and EuMgD4.

Stroboscopy

Stroboscopic acquisition was used for rheolo-gic investigations into the orientation of flator rod-like particles (clays, hematite), suspen-ded in water under conditions of laminarflow, turbulent flow and with a shear gradient.

2

Disordered SystemsVariation of structure factorof mercury with density

The isothermal density derivative of liquidmercury was measured by a difference experi-ment on D20.A high count rate and time sta-bility were essential requirements, fulfilled byD20. Diffraction patterns were collected atroom temperature at 3 bar and 2 kbar, showinga density change of 0.79 %. This experimentrepresented the first reliable determination ofthe density dependence of S(Q) of mercury atconstant temperature.

The users effected dynamic studies at startand stop of flow in pipe flow geometry using atube where the pumping direction changedevery 10 s. In order to study the dynamics ofdisordering and realigning of Ni(OH)2 plate-lets after flow reversal, every two cycles of thelatter served as the synchronisation signal fora cycle of 20 s containing 54 slices of 390 ms.After 17 cycles (6 min) the accumulated coun-ting rate of the in-plane peak (showing thedegree of orientation) was sufficient to showthe dynamic behaviour (Fig. 2).

D20 is a very high intensity diffractometer equipped with a large microstrip detector. As aresult, it opens up new possibilities for real-time experiments on very small samples as thecomplete diffraction pattern at 1536 positions, covering a scattering range of 153.6°, can beobtained in a few seconds. The variable monochromator take-off angle (up to 120°)increases flexibility, providing high-resolution over the complete diffraction pattern at thehighest take-off angle with a neutron flux approaching 107 n cm-2 s-1 and a high fluxapproaching 108 n cm-2 s-1 with medium resolution at lower take-off angles.This instrument was upgraded as a part of the ILL’s pre-Millennium Programme.

Instrument description

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Instrument Data

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20

location reactor hall, level C, thermal beam H11

monochromators HOPG (002) Cu (200)

take-off angle 2u l/Å f/n cm -2 s -1 l/Å f/n cm -2 s -1

26 o - - 0.82 3.4 x10 7

28 o - - 0.88 4.5 x10 7

30 o - - 0.94 5.9 x10 7

42 o 2.41 4.2 x10 7 1.30 9.8 x10 7

Ge(113)take-off angle (115) (117) (119) (337) (335)

2u l/Å l/Å l/Å l/Å l/Å

65 o 1.17 0.85 - - -90 o 1.54 1.12 0.88 0.98 1.22 120 o 1.88 1.37 1.07 1.20 1.49 vM/o 9 14 16 -6 -15

monochromator geometry vertical focus take-off reflections

Cu(200) transmission fixed 26-30 o 200 Cu(200) transmission fixed 42-46 o 200 HOPG(002) reflection fixed 42-46 o 002 Ge(113) reflection variable 63-120 o hhl

l/2 contamination at l=1.3Å about 0.3 % l/2 at l=2.4Å removed by graphite filters

collimation primary collimation ~1 natural 27'

Soller collimators 10'/20'

secondary collimation ~2 adjustable slits between monochromator and sample

mono-sample distance. 3.2 m sample-detector dist. 1.47 m max. beam size at sample 30 mm w x 50 mm h

resolution is also limited by sample diameter

3He microstrip gas-detector (PSD) angular range in 2u 153.6°number of detection cells 1536 sample-detector distance 1.471 m detector height 0.15 m

stroboscopic measurements min. time slice 30 µs (33 kHz) max. time slice 2 s min. counting time (window) per slice 1 µs max. number of slices 256

dedicated sample environment sample space - 750 mm diameter x 500 mm height

sample environment usesvacuum vessel (620 mm diam.) RT experiments

orange cryostat with V tail 1.7 - 300 Kfurnace (V heater) < 1150 C

furnace (Nb heater - background!) < 1500 Ceulerian cradle RT texture experiments

xy translator (250 mm) RT strain experiments

web: www.ill.fr/YellowBook/D20/

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Neutron optics and monochromators A pyrolytic graphite HOPG (002) monochromator in reflection geometry, with fixed vertical focusing, offers awavelength of 2.4 Å at a take-off angle of 42˚. A 6-cm thick pyrolytic graphite filter in the incident beam sup-presses second harmonics; the transmission at 2.4 Å is about 70%.A copper monochromator Cu (200) in transmission geometry, with fixed vertical focusing, provides wavelengths0.82, 0.88 and 0.94 Å at take-off angles from 26˚ to 30˚. A second copper Cu (200) monochromator, with opti-mised fixed vertical focusing for a take-off angle of 42˚, gives a wavelength of 1.3 Å at 42˚. At this wavelength,the monochromatic beam has its highest flux of about 9.8 x 107 n cm-2 s-1. Soller collimators allow a reduction

of the natural divergence (27') of the incident polychromatic beam down to ~1 = 10' or 20'.A variable vertical focusing germanium monochromator (113) gives increased resolution at high take-off anglesand several out of plane reflections are also accessible, as listed in the table. The resolution approaches

Dd/d = 10-3 with a flux of the order of 107 n cm-2 s-1 at the sample position, depending on the take-off angle.

The position sensitive detector (PSD)The PSD housing of aluminium provides a detection zone about 4 m long by 150 mm high.The PSD is filledwith 3.1 bars 3He and 0.8 bar CF4 and has a detection gap of 53 mm. For this large PSD, micro-strip gas cham-ber (MSGC) technology has been developed: chromium is sputtered onto the polished surface of electroni-cally conducting glass plates and then etched to create conductive micro-strip electrodes (alternately 4 catho-des and 4 thin anodes per detection cell). The detection plates have each 32 cells of 2.568 mm (0.1˚) each,covering in total 3.2˚. The current PSD covers 153.6˚, as 48 plates were mounted. Each cell of one plate hasan independent output from the detector through a metal-ceramic plug. The major interests of the micro-strip detection system for the instrument D20 are the precise and perfectly stable geometry, resulting in avery homogeneous response and a very high stability, a high gaseous amplification with a relatively low highvoltage (750 V) between anode and cathode, and the possibility of very high counting rates because of the smalldistance between anode and cathode (170 µm) giving a fast evacuation of the positive ions.Each cell is connected to an amplifier, followed by an anti-coincidence logic (CLET): after the amplifier signalfirst passes the discriminator threshold for a particular cell, its neighbouring cells are prevented from countingthe same event a second or third time for 1.5 µs. 2.5 µs after having passed the threshold a cell may againcount an event.Therefore, the total dead time is about 5.5 µs (in three different cells). Because of this paral-lelism, the counting rate is only limited to about 50 k s-1 per cell (deadtime 27.5 %). The amplifiers and discriminators are regrouped by 32in one of the 48 boxes directly plugged on the rear of the detec-tor.The data acquisition system (DAS) has a parallel input for upto 1600 cells. The dead time between two successive fra-mes is20 ns. This allows the recording of a series ofshort frames, typically a few seconds each, toobserve irreversible kinetic phenomena. Ifthe kinetic process is much faster andreversible, one can reproduce it manytimes in a cyclic way, and the DASworks in stroboscopic mode.Today, up to 256 completediagrams or 'slices' canbe recorded, with aminimum activecounting time'window' (open'gate') of 1µs insideeach slice of at least 30µs (so a maximum slicefrequency of 33 kHz).

Instrumentlayout