D20

High-intensity two-axis diffractometer with variable resolution

Reactor hall (ILL5), level C, thermal beam H11

Monochromators for lower take-off angles
	HOPG	(002)	Cu	(200)
take-off angle 2θ	λ/Å	Φ/n s-1cm-2	λ/Å	Φ/n s^-1cm^-2
26°	-	-	0.82	2.4 x 10⁷
28°	-	-	0.88	3.2 x 10⁷
30°	-	-	0.94	4.3 x 10⁷
42°	2.41	4.2 x 10⁷	1.30	9.8 x 10⁷

Germanium monochromator for higher take-off angles
Ge(113)	(113)	(224)	(115)	(117)	(119)	(337)	(335)
2θ	λ/Å	λ/Å	λ/Å	λ/Å	λ/Å	λ/Å	λ/Å
65°	1.83	-	1.17	0.85	-	-	-
90°	2.41	-	1.54	1.12	0.88	0.98	1.22
120°	-	2.40	1.88	1.37	1.07	1.20	1.49
ωM/°	0	-10	9	14	16	-6	-15

Monochromator	Geometry	Vertical focus	Take-off	Reflections
Cu(200)	transmission	fixed	26-30°	200
Cu(200)	transmission	fixed	42-46°	200
HOPG(002)	reflection	fixed	42-46°	002
Ge(113)	reflection	variable	63-122°	hhl

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

Collimation
primary collimation α1	natural 27' Soller collimators 10'/20'
secondary collimation α2	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 controlled by the sample diameter

³He microstrip gas-detector (PSD)
angular range in 2θ	153.6 deg.
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 (620 mm with radial oscillating collimator ROC) x 500 mm height from table to beam center
Sample Environment:	Uses:
Vacuum vessel (620 mm diameter)	RT experiments
Orange cryostat with 50 mm V tail	1.9 - 320 K
Furnace with V heating element of 20, 30 or 40 mm diameter	< 1100 C
Eulerian cradle	RT texture experiments
xy translators (250 mm)	RT strain experiments

Instrument Description

Neutron optics and monochromators

A pyrolitic graphite HOPG (002) monochromator in reflection position with fixed vertical focussing offers a wavelength of 2.4 Å at a take-off angle of 42˚. A preceding pyrolitic graphite filter of 6 cm thickness in the incident beam suppresses second harmonics. The transmission at 2.4 Å is about 70%.

A copper monochromator Cu (200) in transmission geometry with fixed vertical focussing provides wavelengths 0.82, 0.88, 0.94 Å at take-off angles from 26˚ to 30˚. A second copper Cu (200) monochromator, with optimised fixed vertical focussing 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 reduction of the natural divergence (27') of the incident polychromatic beam down to α1 = 10' or 20'.

A variable vertical focussing Germanium monochromator (113) gives increased resolution at higher take-off angles and several out of plane reflections are also accessible, as listed in the table. The resolution approaches Δd/d = 10^-3 with a flux of the order of 10⁷ n 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 0.15 m high. The PSD is filled with 3.1 bars ³He and 0.8 bar CF4 and has a detection gap of 53 mm. For this large PSD, micro-strip gas chamber (MSGC) technology has been developed: chromium is sputtered onto the polished surface of electronically conducting glass plates. The chromium is etched to create conductive micro-strip electrodes (alternately 4 cathodes 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 has an 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 a very homogeneous response and a very high stability, a high gaseous amplification with a relatively low high voltage (750 V) between anode and cathode, and the possibility of very high counting rates because of the small distance 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 signal first passes the discriminator threshold for a particular cell, its neighbouring cells are prevented from counting the same event a second or third time for 1.5 µs. 2.5 µs after having passed the threshold a cell may count again an event. Therefore, the total dead time is about 5.5 µs (in three different cells). Because of this parallelism, the counting rate is only limited to about 50k s^-1 per cell (dead time 27.5%). The amplifiers and discriminators are regrouped by 32 in one of the 48 boxes directly plugged on the rear of the detector.

The data acquisition system (DAS) has a parallel input for up to 1600 cells. The dead time between two successive diagrams is 160 ms in conventional sequential data acquisition mode. This allows recording a series of short diagrams, typically a few seconds each, to observe irreversible kinetic phenomena. If the kinetic process is much faster and reversible, one can reproduce it many times in a cyclic way, and the DAS works in stroboscopic mode. Today, up to 256 complete diagrams or slices can be recorded, with a minimum active counting time window (open gate) of 1 µs inside each slice of at least 30 µs (so a maximum slice frequency of 33 kHz). The dead time between slices is negligible with only about 20 ns.