3 Experimental Methods
Each detector has characteristics which make it suitable for different tasks. The backscattering detector has, in total, 720 discrete detector elements which may be used, decoupled, as a radially pixelated position sensitive detector. These elements are software linked to form 60 rings, these rings are designed to mirror the Debye- Scherrer rings produced by powder diffraction. In this manner the geometric aberration may be minimised. The data obtained in backscattering detector is of inherently high resolution. The effective upper cf-spacing limit o f the backscattering detector is approximately 5Â. This limit is a direct consequence o f the incident flux of the diffractometer which, at only modest intensity, extends to wavelengths of approximately 10Â. In order to measure longer J-spacing information, detectors at lower angles are vital. In these detectors, for a given (/-spacing the Braggs’ Law equation is satisfied by neutrons of shorter wavelength and therefore, on HRPD, of higher flux.
The 90° detector utilises a ZnS scintillator, which by virtue o f its peak height response can discriminate between neutrons and y radiation. This insensitivity to y rays is significant, as the backscattering detector is quite insensitive to this radiation. The
detector is comprised o f 6 modules each with 6 6 elements. Each module is positioned
on a constant radius from the Im sample position. As with the backscattering detector just mentioned, data may be collected in each of the 396 discrete elements but more
usually the detector is software configured into 6 6 radial segments.
The low angle detector utilities 16" ^He tubes as a detector. The HRPD low angle bank, currently houses 72 tubes which lie on a constant radius parallel to the through beam direction and are configured in 3 rows of 24 tubes. Again, similar software linking strategies outline above may be applied. The long secondary flightpath of the low angle bank, necessary in order to minimise angular divergence, requires that the large tank housing the detector be filled with Ar gas. The tank is therefore discrete from the other sample and detector tanks which are evacuated during diffraction measurements. The incident and transmitted beam intensity is monitored by two Davidson (1985) monitors situated at 93.50m and 96.74m from the moderator.
3 Experimental Methods
The characteristics of each detector bank are summarised below:
Backscattering 90° Low A ngle
Detector Specification
ZnS scintillator ZnS scintillator V2 lOatm He^ gas tubes
Geometry 60 rings: 7 < r , < 8.5cm Slab: 20 X 20cm 72 tubes: (20cm active length) 35.5 < r6o ^ 37cm 66 X 3mm elem ents 8 tubes/module 8 Octants: 4147cm^ 6 Modules: 24(X)cm^ 9 M odules: 1800cm^ Fixed Scattering
A ngle
1 6 O ° < 2 0 < 1 7 6 ° ( lm ) 87° < 20 < 93° 28° < 20 < 32°
Solid A ngle (Q ) 0.41 ste r (lm ) 0.08 ster 0.01 ster
Resolution (Ad/d) ~ 4-5 X 10 “ ~ 2 X 10^ ~ 2 X 10^ d-spacing range (30-
230m s)
~ 0.6 - 4.6Â ~ 0.9 - 6.6Â - 2 . 2 - 16.5Â
The powder diffractometer HRPD has been used in conjunction with a gas pressure cell (Chapter 3.3.3) and modified Orange cryostat to enable us to characterise the low temperature structure of TlFeCl] under hydrostatic pressure. More information on this experiment can be found in Chapter 4.4.1.
3.2,2.2 The Powder Diffractometer, PO LARIS, at the IS IS facility, Oxford.
As with HRPD, POLARIS has a variety of detector banks available for data collection, these are, ‘very low angle’, ‘low angle’, 90° and ‘back scattering’, as shown below.
3 Experim ental M ethods
Low angle detectors
7 ransmiltod beam monitor \ 90 d eg rees detectors Backscattering detectors Incident beam
Sam ple tank
8.5m collimator
11.5m coNimator
Long d-spAcing detectors
Figure 41: Schematic of the powder diffractom eter POLARIS at the ISIS facility^^®.
The POLARIS diffractometer is located on the D7 beamline at the ISIS facility, receiving a ‘w hite’ beam of neutrons from the ambient temperature water moderator. It has the possibility of incident wavelengths of 0.1 - 6.0 Â, with a corresponding incident energy range o f ~ 2meV - 8eV. Motor driven collimators allow the incoming
beam to be reduced, from a maximum size o f 40mm high x 20mm wide, to match the sample size. This eliminates any background scattering contamination from the sample environment equipment.
The characteristics of the detectors present are given below in tabular format.
Position Very low angle Low angle 90 degrees Backscattering
Type ZnS scintillator */2” ^He tubes ZnS scintillator 1” ^He tubes
No. o f detectors 80 80 216 58 20 range 13°-15° 28° - 42° 85° - 95° 1 3 0 °-1 6 0 ° Q(steradians) 0.009 0.046 0.48 0.29 Ad/d (%) 3 x 1 0 ^ 1 X 10 2 7 X 10 3 5 X 10^ Li (m) - 2 . 2 1 .7 2 -2 .6 5 -0 .8 0 .6 0 - 1.30 d range (Â ) 0 .5 - 2 1 .0 0 .5 - 8 .1 5 0 . 3 - 4 . 1 0.2 - 3.2
We utilised POLARIS for low temperature, structure determination studies, on the triangular lattice antiferromagnets CsFeCl^ and CsFeBr^ under hydrostatic pressure. A
3 Experimental Methods
modified Orange cryostat was used in combination with a gas clamp cell to obtain the necessary sample environment.
3.2.3 Flat Cone Diffractometer.
The flat cone technique is a modified case o f the Weissenberg technique which was developed for use with X-ray diffractometers in conjunction with photographic detectors. In the Weissenberg method a single crystal is rotated about an axis and thus the planes normal to this axis will diffract. The diffraction will take the form of a plane or cone shape i.e. all the reflections o f one reciprocal plane or layer are recorded along straight lines on a cylindrical film. If only one line is selected by filtering the image by placing a layer-line screen before the film, then a two dimensional lattice plane can be mapped on the two dimensional film by coupling the crystal rotation and the film rotation. This same procedure can be realised with a one-dimensional electronic multidetector that is placed along one layer line. For each rotational movement o f the single crystal a separate measurement is made, this results in a loss in resolution perpendicular to the layer line in comparison to the film method. However statistical analysis of the data allows this to be reduced to a minimum. There are distinct advantages to the fiat cone technique, firstly and most obviously the data collection rate will be many times that of a conventional TAS due to the fact that the data is being collected by a multidetector which can scan many Bragg peaks simultaneously, compared to a TAS which may only measure one point o f the scattering function at any one time. Thus this technique is well suited to the systematic search o f the intensity distribution in reciprocal space. This is especially true for the determination o f unknown lattices, e.g. in magnetically ordered crystals, or for the observation of diffuse scattering between Bragg points. The detector may also be tilted out of the plane of the experiment in order to establish scattering from different layers in reciprocal space. The principle of this method is described below.
The incoming neutron beam is reflected at the monochromator in the usual way and impinges onto the sample. The scattered neutrons are analysed by reflection at fiat crystal plates. These are orientated such that the neutrons are reflected out of the horizontal (experimental) plane into the vertical plane. A schematic of the setup of the fiat cone technique is shown below.
3 Experimental Methods
s A