SXD, THE SINGLE CRYSTAL FACILITY AT ISIS

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SXD, THE SINGLE CRYSTAL FACILITY AT ISIS

J. Forsyth, C. Wilson, A. Stringer, J. Howard, O. Johnson

To cite this version:

J. Forsyth, C. Wilson, A. Stringer, J. Howard, O. Johnson. SXD, THE SINGLE CRYS- TAL FACILITY AT ISIS. Journal de Physique Colloques, 1986, 47 (C5), pp.C5-143-C5-147.

�10.1051/jphyscol:1986519�. �jpa-00225836�

Colloque C5, supplgment au no 8, T o m e 47, aoOt 1986

SXD, THE SINGLE CRYSTAL FACILITY AT ISIS

J.B. FORSYTH' , ^C.C. WILSON", A.M. STRINGER*. , ^J.A.K. HOWARD"

and 0. JOHNSON"

'The. Rutherford Appleton Laboratory, Chilton, Didcot,

GB-Oxon OX11 OQX, Great-Britain

" ~ e p a r t m e n t of Inorganic Chemistry, The University, GB-Bristol BS8 ITS. Great-Britain

RGsurnB - Le diffractornetre de monocristaux SXD est equip6 d'un detecteur &

localisation spatiale 21 2 dimensions du type "Anger Camera". Vu que l'instru- ment travaille en temps de vol, les donnees se presentent sous forme de tr&s

grandes matrices tridimensionnelles. Une partie des techniques utilisees pour visualiser ces donnees et en analyser le contenu en information est d6crite.

Une description du detecteur et de son mode de fonctionnement sont donnees egalement.

Abstract - The SXD has a two-dimensional posirion-sensitive detector of the Anger Camera type / I / . Since the instrument employs the time-of-flight technique, this results in very large, three-dimensional arrays of data. Some of the processes being reviewed for displaying and analysing the information contained within such arrays will be described and space will also be devoted to discussion of the design and operation of the detector.

I - INTRODUCTION

The SXD is the single-crystal diffraction instrument under development at the Spallation Neutron Source in the U.K. (ISIS). The PSD is of the Anger Camera type, the design and functioning of which is outlined below. On a pulsed neutron source, this leads to the mapping out of a volume of reciprocal space, the third dimension being time, which converts directly to wavelength.

Construction of the hardware is almost complete, although the detector still has to undergo an extensive test program, before it can be used for experimental trials of the instrument as a whole. Software for all stages of the experiment and subsequent data analysis is under development also, although some areas have progressed more than others. In particular the program for instrument control and overall management of data collection is essentially fully operational. The data-acquisition electron- ics in the current configuration limit the number of raw data which can be recorded, so that the spatial coordinates are digitised into a grid 6 4 x 6 4 , facilitating a maximum of about 500 time channels. It is hoped eventually to increase the amount of histogramming memory available to match the design specification of the Anger Camera, viz: 1 2 8 x 1 2 8 spatial coordinates and a maximum of 700 time channels.

The strategy of data reduction and the associated tasks has been studied at some length. It may be the subject of a review after practical experience, but a reason- ably comprehensive plan has been drafted and this is included below, together with a description of the programs and files that are utilised in the process. An inter- active graphics program has been written to enable a user to assess visually the data in a variety of ways and a small number of low-level utility programs have become necessary for diagnostic purposes. These will also be described briefly.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986519

C5-144 JOURNAL DE PHYSIQUE

I1 - ^{THE ANGER CAMERA}

Figure 1 shows a schematic representation of a part of the PSD. THe complete det- ector has an aperture of 333 x 338 rmn and contains 49 photomultiplier tubes in a honeycomb arrangement, as shown. A scattered neutron arrives in the scintillation glass ( 2 ) which contains Lithium activated with Cerium and participates in a nuclear reaction producing a burst of photons. Light travelling back towards the sample is reflected and diffused by a layer of aluminium oxide ( 1 ) . An air gap ( 3 ) is left between the scintillation glass and the dispersion glass ( 4 ) to prevent total inter- nal reflection which would otherwise occur at the interface.

With this arrangement, only light rays with an angle of incidence less than 40" are transmitted, so that a cone of light falls upon the photomultiplier tubes. This is depicted in the diagram. By comparing the signals from each tube, it is possible to calculate where the bursts of light originated. This process and digitising and storing the count takes approximately 1 us, and the spatial resolution is about 2-3 mm.

Figure 1. The Anger Camera

INST. UPT 4 Deft USERS. DAT

SPECTRA.DAT

1

TCB.DAT

-

DETECTOR.DAT

WIRING. DAT

I

1

SXDnnnnn

.

A data-collection run is set up by issuing either a "Changet' or "Load" command to the Instrument Control Program (ICP). "Change" ailows modification of the main parameters held in a file INST. UPD, using 'Deft' (Data Editing via Full screen Techniques), while "Load" places the information along with a complete description of the instrumental configuration from the -.DAT disc files into the Current Run Parameter Table (CRPT). USERS.DAT contains information specific to one run (e.g.

machine angle settings). TCB.DAT (Time Channel Boundaries) is a loop-up table for converting the event time in clock pulses into a time channel, or "bin"

.

three files are similar look-up tables used by the electronics to assign memory locations to digitised positional coordinates on the detector.

Neutron counts, either from the Data Acquisition Electronics (DAE) or calculated by the simulation program, SIMX, are appended to the CEPT and the whole is written out to a disc file SXDnnnnn.RAQ on completion of a run number nnnnn.

Data Assessment PZAKSXD

SXD00001.RAIJ

-

.

I

PEAKINT Peal;

I

INDEXXD.DAT RAFSXD

.

? RAFSXD

.\\

IV - ^{PROGRAM SUMMARY}

SIMX: This program performs an advanced simulation of single-crystal time-of-flight neutron diffraction data, producing a raw data file that is as close as possible to

that expected from an actual experiment. The calculation includes a structure- factor evaluation from a postulated sample, using the Cambridge Crystallographic

C5-146 JOURNAL DE PHYSIQUE

Subroutine Library / 2 / and estimates the background arising from the incoherent scattering from the sample and air in the flight path. The geometry of the machine and a particular run set-up are both used in conjunction with an orientation matrix, UB, to determine precisely which reflections fall within the volume of reciprocal

space mapped out. The intensity is then scaled to allow for wavelength-dependent factors (namely the relative incident flux and the detector efficiency) from the user supplied look-up tables. A realistic temporal pulse profile is included together with a full calculation of the resolution as a function of position in reciprocal space and estimated beam divergences / 3 / . As a final feature, the predicted counts are given a randomised contribution using Poisson Statistics.

"Data" files produced in this way are obviously very useful for conducting trials on programs written to analyse real experimental data.

PEAKSXD: This is a fairly rudimentary program which searches for local maximum points within a raw data file. A peak is simply defined as any maximum point in a 3 x 3 x 3 pixel grid, and its centre is estimated by flttlng a 3-D paraboloid function to the point and its six nearest neighbours. The program operates on one SXDnnnnn.RAW file and outputs a corresponding SXDnnnnn.PKS file, which contains a list of the interpolated peak positions and a crude estimate of their significance.

It has been based on 'PEKSER' 1 4 1 , from the Argonne National ~aborator~, with a new section patched in accessing a raw data file of this form.

LATFIT: This program war. converted from the Dl9 software (I.L.L., Grenoble, France)

/5/ cope with variable-wavelength data. Essentially it attempts to solve for a trial UB matrix, starting with a list of measured diffraction geometries read from SXDnnnnn.PKS. It will also assign indices to peaks using the trial matrix and append them to a single file called 1NDEXED.DAT.

RAFSXD: Again this has been adapted from a Dl9 program, RAFD19 / 6 / , to handle var- iable wavelength data. This program will perform a least-squares refinement of the UB matrix, and a set of machine offsets, using the starting values from RAFSW.DAT and the diffraction information in 1NDEXED.DAT.

PEAKINT: / 7 / Further work remains to be done before this program is available for routine use, but its function is to use the refined UB matrix and to extract inte- grated intensities from the original raw data files. This is also based on a Dl9 program. For constant-wavelength data collection with an area detector, the third dimension is provided by o-scanning. This results in Bragg peaks being ellipsoids in reciprocal space. In time-of-flight work the peak shapes are less regular and in particular, possess long tails due to the temporal spread of wavelengths that occurs in the moderation process. Preliminary work suggests however, that the ellipsoidal model will be satisfactory for defining an integration envelope to give well deter- mined intensities, so this will be retained in the program for the present. How the

integrated intensities will be disposed of, and what will follow this stage has yet to be decided, but it is apparent that further programs for data reduction and correction will be required.

EUCLID: An interactive program to transform coordinates of a reciprocal lattice point between different frames of reference; for example, x, t, z to h, k, 9,. This is useful for checking transformation in other programs and possibly for calculating a group of settings for some specific observation, e.g. measuring one structure factor at different wavelengths.

BOX: Another interactive program which spools a printout of a local 3-D grid of

-

RLPLOT: This program has been designed to be on-line during run-time to extract reciprocal lattice rows or planes from raw data, with optional interpolation between data points, and to display them on a graphics terminal or to produce a hard copy.

Two examples are reproduced in the figures below, using the data taken from a simulation for a Nickel crystal.

interpolation. photograph.

Consideration of diffraction geometry in reciprocal space for the time-of-flight technique shows that the layers of constant x, z or t in rhe data histogram do not correspond to planes in reciprocal space. For this reason, the extraction of rows or planes of reciprocal-lattice points is achieved by setting up a grid in terms of Miller indices (real values, rather than integer values) as defined by the user.

This grid is converted point by point to x, z, t coordinates to give appropriate sub- scripts for the data array (this requires an orientation matrix). An effective neutron count can then be derived, either by interpolation between local counts or, for a rough and ready plot, by simply using the nearest data point. The latter option is desirable since full interpolation for a large two-dimensional grid and the implied amount of data accessing, constitutes a lengthy computational process which hinders use of the program as a run-time diagnostic.

ACKNOWLEDGEMENTS

Together with the groups from Bristol and the R.A.L. the following people have con- tributed a great deal to this work: Dr. R.F.D. Stansfield (Edinburgh), and

Dr. C. Wilkinson (London).

REFERENCES

/I/ Forsyth, J.B. and Lawrence, R.T.. private communication.

/ 2 / Brown, P.J. and Matthewman, J.C., Rutherford Appleton Laboratory Report

Rt 81-063 (1981).

/3/ Wilkinson, C. This Volume, and personal communication.

/ 4 / Schultz, A.J. et al., Argonne National Laboratory Program.

/5/ Pilotti, M.U., Stansfield, R.F.D., Wilkinson, C. and Khamis, H.W., ILL Internal Technical Report No. 85PI25T (1985).

161 Filhol, A., ILL Internal Technical Report. In preparation (1986).

171 Wilkinson, C. and Khamis, H.W., Position-Sensitive Detection of Thermal Neutrons, (Convert, P. and Forsyth, J.B., Eds.), Academic Press, New York, 1983,

pp.358-364.