EXPERIMENTAL TECHNIQUES.
3.2 T h e S20 diffractom eter.
T h is special beam instrument was originally installed at the Institute Laue- Langevin in 1973, as a simple monochromatic topography station with a crude option for using polarised neutrons. Since then the instrument has been moved up the H 24 beam by approxim ately three metres, thus increasing the intensity by a factor o f three, whilst retaining the relatively 'clean' beam conditions characteristic o f the end o f a long beam guide. T h e gentle curvature o f the beam guide eliminates fast neutrons o r gamma rays and th e beam profile has a characteristic weak divergence (which can be reduced still further by collimation with a diaphragm at the end of the beam line).
A s as result o f a series o f improvements S20 is now an autom ated tw o axis diffractom eter and high resolution polarized neutron topography station. A schematic
illustration of the main features o f die present instrument are given in Figure 3. la with the particular adaptations employed in neutron topography being illustrated in Figure 3.1b.
The instrument is such that the incoming white beam is incident on a preselected monochromator producing a diffracted monochromatic beam in accordance with Bragg's law , as previously described in chapter 2. The monochromator carousel allows a choice o f four different monochromators giving a wavelength range from 0.9 A to 2.4 A. For polarised neutrons the monochromator is a magnetically saturated H eusler Alloy on a 111 reflection. However, in unpolarized work there is a choice between a Cu 220, Cu 111 and silico n 111 m onochromators, all o f which show different resolution and intensity responses as a function o f wavelength. These characteristics will be described in section 3 .6 o f this chapter.
The resulting monochromatic beam is subjected to tw o stages o f collimation, the firs t being a fixed circular hole o f diameter 3 cm at the exit o f the monochromator housing, and the second, a variable slit (normally being critically reduced to the sample size) situated-10 cm from the sample position. The sample itself is housed in a specially designed tail o f a closed cycle helium flow cryostat ("D isplex"), which provides a temperature range from 1 IK to 300K with an accuracy better than ± 0.05K.
The samples studied on S20 are normally mounted on a thin rigid aluminium p late by a small amount o f GE varnish at one extremity o f the sample. The sample is then surrounded by a cadmium fiducial mark, to aid the aligning and centering o f the crystal. A second mounting system allows the possibility o f applying a temperature gradient across th e sample by clamping the opposite extremity o f the sample to a thermally isolated section below the cold head o f the cryostat, as illustrated in F igure 3.2. The temperature gradient can be modified by applying a current across the 100 O hm heating resistance situated below the sample. The warm edge o f the sample is clam ped between two flush sheets o f aluminium. Absolute care must be taken not to apply too much pressure to the sam ple, and thus introduce strain which could affect the crystalline quality o f these near
Figure 3.1a: A schem atic plan view of the in stru m en t S20 Monochromator, housing Monochromator carousel Incoming white beam Beam stop
Figure 3.2: T h e sample holder fo r applying a th e rm a l gradient a cross th e sample Copper attachments
to Goldfinger '---Cadmium fiducial
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Insulator ^ — - G.E.vamish x I fixation point Aluminium sample 1 clamp '" s a m p l e Platinum resistance thermometer Thermally strapped thermometry leads©A®
;istance I 1 r " " " tD V /© ia/4c ^ • n . Thermal insulatorInsulated leads 100 Ohm heating resistance
F igure 3.3: A schem atic rep resentation o f th e flux guidance system in th e vertical electro m ag n ets
H Copper coldfinger* H Wound copper coils ■ Soft iron flux guides □ insCTion
perfect, thin, single crystal plates. In fact this poor thermal contact proved instrumental in producing a stable, reproducible temperature gradient across the sample.
External to the cryostat there is die facility to apply both vertical and horizontal magnetic fields b y means o f two air cooled electromagnets. In the case o f a horizontal Held, the geometric restrictions imposed by the need for the highest resolution possible necessitated the development of a series o f adjustable polepieces, which allowed variable fields o f up to 0.3 Tesla to be applied over several hours. The introduction o f magnetic flux guides in the vertical magnet was initially hindered by the solid copper 'cold finger* at the end of die second stage refrigeration device onto which the sample is mounted. This problem was solved by hollowing out the central chamber and inserting a piece of soft iron inside the coldfinger. A second soft iron piece was attached to the base o f the 77K shield, thus providing a more effective flux guide in the vertical direction, as illustrated in Figure 3.3. In this configuration a field o f - 0.2 Tesla could be continuously applied.
The diffracted neutron beam is detected by a 3He detector which has an efficiency in excess o f 96% over the wavelength range considered in these investigations. The deadtime related to the pre-amplification o f the signal from the detection o f neutrons is o f the order o f lps.
The instrument is driven by a P D P 11-73 Cubus machine processor through a Camac crate, to give automated omega and theta scans. W hilst the detector was normally used in the plane there was the facility to move o ff axis by ± 25 degrees but until now this has remained an uncoded, manual movement, and as such has been seldom used. 3 .3 Polarised neutrons.
Neutrons leaving a field saturated Heusler alloy 111 reflection will be polarised in the opposite direction to that of the applied field. It is imperative that this degree o f polarisation be retained until the neutrons impinge on the sample. Depolarisation effects are prevented by incorporating a sufficiently large guide field to overcome the parasitic
field effects at the partially iron made m onochrom ator housing. This w adiabatically rotate the polarisation in accordance with the following inequality a by Mezei (1979), and adapted to this particular case by El Kadiri in 1986:
0)lB >
8 K X m D (65)
where, Wl is the Larmor Frequency, h is Planck's constant, m the neutron mass, X the wavelength, and D is the distance required for the field to rotate through 180 degrees. Figure 3.4 shows a schematic illustration o f the apparatus used to adjust the direction of the polarisation of the neutrons on S20. The first guide field directs the polarisation along the axis o f a solenoid. The polarisation is then rotated into the vertical plane by two permanently magnetised ferrite sheets around w hich a soft iron circuit has been constructed. In the absence o f any current in the flipper coils the polarisation will remain in this direction until it passes into the final rotation field which rotates the polarisation into the horizontal plane. The field at A is roughly O.OlTesla, which when considering D to be of the order o f 10 cm, allows the previous inequality to be satisfied by a factor of around 20. The neutron polarisation is "down" (i.c. antiparallel to the guide field), over the whole path.
In order to change the polarisation o f the beam a current is applied to the two flipper coils. The first flipper coil produces a vertical field to nullify the effect o f the surrounding guide fields, after which the neutrons magnetic moment is free to process around the horizontal field produced by the second flipper coil. By placing a second magnetically Heusler alloy (on the 111 reflection) at the sample position these two fields can be critically adjusted to give an optim um flipping ratio. The polarisation is now "up", (i.e. parallel to the guide field). It will be held in the vertical plane until it is rotated, (remaining "up") into the horizontal plane b y the horizontal guide field. Thus the neutron polarisation will be almost parallel and anti-parallel to the scattering vector o f the
Figure 3.4: A schem atic r ep re se n ta tio n of th e m ethod of neutron p o larisatio n used on S20.
a) F lipper Coils O ff
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Ferrite sheets Double coil
(Vertical Guide field) flipper Cryostat Miochromator
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f t
© AI
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Monochromator HousingI I I ®
Sample b) Flipper C oils O nf Polarisation direction o f the monochromatic neutron beam ^ Direction o f the applied guide fields
samples used in these investigations. This satisfies the condition for observing chirality domains as described in equation (35).
The facility exists on S20 to rotate the final polarisation either vertically or horizontally perpendicular to the beam direction by use o f two guide fields constructed from pairs o f magnetic ferrite sheets sim ilar to those illustrated in figure 3.4. The polarisation can also be rotated along the beam direction by applying a sufficiently large field to a solenoid, which surrounds the neutron beam with its axis along the neutron beam. This ability to rotate the polarisation o f the incident neutron beam allows a great deal o f flexibility for the initial sample orientation.