User Guide for the Polaris Powder Diffractometer at ISIS
Contents
1. gt gt is displayed On a workstation a window labelled GKS will be created as an icon where graphics will appear In order for GENIE to read and display ISIS data its location either in the DAE or from a RAW file must be defined with the ASSIGN command followed by the data source To look at the data in a RAW file use the run number e g for run 16361 gt gt ASSIGN 16361 lt cr gt To look at the current run use DAE in place of the run number gt gt ASSIGN DAE lt cr gt As mentioned in sections 3 2 and 4 2 data from the current run are stored in the POLARIS DAE which can only be accessed by the POL FEM Therefore ASSIGN DAE will only work from sessions logged on to POL Once the location of the data has been defined the contents of any spectrum e g s33 section 4 4 can be plotted with the DISPLAY command gt gt DISPLAY S33 lt cr gt On the workstation the data will be displayed in the GKS window as a plot of counts microsecond against time of flight On a PERICOM terminal the screen will toggle to graphics mode before displaying the data To exit from the graphics display on the PERICOM terminal and return to the gt gt prompt press the lt Return gt key In GENIE a number of workspaces denoted W n are defined which may be used to store a single spectrum or more commonly to store the result of some arithmetic operation involving several spectra The current GENIE set up on POLARIS allows users a c2. The value of the product Lsin 0 required for this conversion is obtained from a calibration of POLARIS using the NBS standard silicon sample SRM 640b One of two procedures is used to normalise the data depending upon whether the data were collected during the current ISIS cycle or during a previous cycle For current data only the run number is required but for old data the name of the command file appropriate to the ISIS run cycle during which the data were collected must be determined 4 4 1 Data from the Current ISIS Cycle To normalise data from a given detector bank in GENIE type the name of the focusing command file for that bank The command file prompts first for the run number and then for the number of a GENIE workspace into which the final spectrum will be placed For example to focus and normalise the data from run number 11916 into workspace 1 gt gt AFOCUS lt crs or BFOCUS or CFOCUS or EFOCUS gt gt Enter run number 11916 lt cr gt or DAE gt gt Enter workspace number not 15 1 lt cr gt To normalise the data currently being collected type DAE instead of a run number n b this is ONLY possible when logged onto the POL FEM Note that workspace number 15 is used by the command file as a temporary store while focusing and normalising the data and should not be selected to contain the focused data The data may then be viewed with the DISPLAY command as described in the previous section gt gt DISPLA
3. Crystallography Subroutine Library CCSL have been written for the analysis of ISIS time of flight powder diffraction data Full details on the use of these programs are given in the report RAL 92 032 Profile Analysis Of Neutron Powder Diffraction Data At ISIS copies of which can be obtained through an instrument scientist The CCSL program recommended for structure refinement using POLARIS data TF14LS requires two input files The diffraction data file has the extension DAT and contains the normalised experimental diffraction profile corrected for sample absorption effects if necessary The crystal data file has the extension CCL and contains the structural unit cell space group atomic co ordinates etc instrumental total flight path average scattering angle zero point line shape and line width and least squares parameters required to construct the calculated profile and control the refinement The instrument parameters in the crystal data file have been determined empirically using data collected from NBS standard samples The diffraction data file is in ASCII format and is generated from the binary data stored in a GENIE workspace with the TF_FILE command gt gt TF_FILE lt cr gt gt gt Enter number of workspace containing the data 1 lt cr gt Enter data filename without extension C16628 lt cr gt Enter minimum t o f limit usec 2000 lt cr gt Enter maximum t o f limit usec 19550 lt cr gt The crystal da
4. arranged to have an approximately constant resolution As a result the individual detector elements are not time focused and this has important consequences in the data normalisation routines section 4 4 By partial differentiation of equation 2b it is possible to derive an expression for the Ad d resolution of any detector Rid A9 s cot 8 AF ery Inspection of equation 3 shows that for a given 48 the contribution to Ad d from the angular uncertainties increases as the scattering angle decreases becoming infinite at 20 0 while it is zero at 20 180 In the very low angle B bank the 0 term dominates and constant resolution is obtained by arranging the detectors so that the A0cof0 term is constant For detectors of identical size this is achieved by positioning the detectors along a straight line parallel to the incident neutron beam In practice however the detector modules in the B bank are angled 10 away from this ideal arrangement in order to increase the solid angle At large scattering angles the contribution of the A0cot0 term is small and the resolution is dominated by time uncertainties The detectors in the backscattering C bank are arranged in a straight line inclined to the scattered neutron beam and all intersect approximately the same fraction of the Debye Scherrer cones to give a constant resolution AND line shape across the bank The poorer resolution of POLARIS compared with HRPD is due to larger At
5. indicate the running status of the rotary pump Pirani and Penning gauges for monitoring the pressure in the sample tank and switches for operating the turbo pump are mounted in the rack above the vacuum control unit PUMP ON PUMP OFF START SAMPLE TANK VACUUM C Figure 3 The POLARIS Sample Tank Vacuum Control To evacuate the sample tank lift up the transparent cover and press the green switch labelled START on the control box This closes the vent valve and starts the rotary pump The green PUMP ON light on the control box will illuminate and the Pirani gauge will show the pressure in the sample tank dropping After a short delay the orange VAC ON warning lamp in the sample position enclosure will illuminate providing an additional indication of the status of the sample tank vacuum The turbo pump need only be used when pressures of lt 10 mbar are required e g when using a closed cycle refrigerator and is started by depressing the right hand switch on the panel between the Pirani and Penning pressure gauges To release the sample tank vacuum lift up the transparent cover and press the red switch labelled STOP on the control unit This stops the rotary pump and opens the vent valve The red PUMP OFF light will illuminate and the Pirani gauge will show the pressure in the sample tank rising If the turbo pump has been inuse it should first be switched off by pressing the right hand switch on the panel between the two pres
6. left red or BACKWARD right blue on the front of the control unit The carousel will rotate one position clockwise or anticlockwise as reguested Once it reaches the next position the LED display will indicate the number of the position reached this may not indicate until the carousel has rotated past position number 1 To return the sample changer to computer control follow the instructions given above in F 3 F 5 Command File Operation of the Sample Changer The commands detailed in section F 3 for positioning samples in the beam using CAMAC can along with the normal instrument control commands be used in a DCL command file to enable data to be collected automatically from several samples over an extended period of time without any user intervention An example of a small section of such a command file is given below set noon disable all error checking write sysSoutput write sysSoutput JE write sysSoutput Positioning sample 2 camac dir pol move 2 change title Lil 08Mn1 16Co0 7504 x 0 25 2 0426g 35x8mm begin waitfor 150 uamps end write sysSoutput write sysSoutput i write sysSoutput Positioning sample 3 camac dir pol move 3 change title LiMnCo04 soln route 2 1322g 35x8mm begin waitfor 150 uamps end write sysSoutput write sysSoutput Y write sys output Positioning sample 4 camac dir pol move 4
7. stores not only the x y o y information but also all the parameters associated with that workspace experiment title users names flight paths scattering angle etc This file is written using the WRITE command gt gt WRITE OPEN W1 AL203 NOR lt cr gt followed by gt gt WRITE CLOSE lt cr gt Here the contents of GENIE workspace W1 are written to the file AL203 NOR The WRITE CLOSE command is necessary to ensure that all the data are written to the file and the file is closed The GENIE intermediate format file is read back into GENIE with the READ command gt gt READ W2 Al203 NOR lt cr gt gt gt DISPLAY W2 lt cr gt Here the file AI203 NOR is read back in to GENIE workspace W2 and when plotted all the information contained in the original workspace is preserved It is also possible to write the contents of a workspace to an ASCII file e g for use in an alternative data analysis package or for transport to another computing facility with the SHOW DATA command gt gt SHOW DATA Wn OUT filename lt cr gt where n is the number of the workspace containing the data The OUT filename qualifier is optional and allows the name of the file to be specified If omitted a default filename of FOR001 DAT is used Alternatively use of OUT TT will result in the output listing being directed to the screen Once a normalised diffraction pattern or indeed any other GENIE image has been displayed on the graphics
8. symbols and logical names used in POLARIS data analysis including the GENIEINIT command file which initialises the GENIE data display and manipulation program section 4 3 and sets up a series of symbols which point to command files used in data analysis This LOGIN COM file will also determine whether you have logged on to a VAX or an Alpha computer and set the logical name POL_PROGS to point to the directory containing the executable files for VAX or Alpha processors respectively Because the POLARIS DAE can only be accessed by the POL FEM if you wish to examine the current run you will have to be logged on to POL When working from your own user account the SET HOST command can be used to connect to the POL FEM SET HOST POL lt cr gt This will connect you to the POL FEM and prompt you to log on which you should do with the username POL Username POL lt cr gt followed by Password lt cr gt Users can obtain the current POL password from an instrument scientist When using one of the workstations located in the DAC another instrument cabin or building R3 it may be necessary to specify the terminal from which you are working in order that the GENIE graphics window is opened correctly This is done with the SET DISPLAY command before starting up GENIE SET DISPLAY CREATE NODE nodename TRANS TCPIP lt cr gt or SET DISPLAY CREATE NODE xxx_nnnnnnn TRANS LAT lt cr gt where nodename is either the VMS cluster node name of
9. the workstation which you are using or a TCP address xxx xxx xx xX The LAT name xxx nnnnnnn if required will be displayed in the session manager window on the workstation which you are using To prevent the FEM from becoming overloaded users attempting to log on to POL with their own username may find they are denied access The SET HOST command can be used to connect to the HUB from any ofthe sessions running on the FEM SET HOST ISISA lt cr gt 4 3 GENIE During the course of an experiment it will be desirable to inspect the diffraction pattern whilst a run is still in progress and also to compare it with the patterns from previous runs This can all be done with the GENIE data display and manipulation program Normally GENIE will be running on one of the terminals in the POLARIS cabin but it can also be run from any of the terminals in the DAC or in the offices in building R3 Full details of the commands available in GENIE are given in the GENIE manual Report RAL 86 102 PUNCH Genie Manual and Appendix 2 of the PUNCH manual Only those commands necessary to perform a basic assessment of the data will be described here To start up GENIE simply type GENIE lt cr gt when working on a terminal other than the POL FEM it may be necessary to type GENIE DEV MOTIF The terminal will clear the GENIE header will be displayed at the top of the screen and several lines of text will scroll up the screen before the GENIE prompt
10. 993 08 18 18 318 const T mode 6506 1 0000 0 2 2629178 2629179 30205916 17 65 2704 5 4 8 2 Useful GENIE Commands a PLOT2 Also available PLOT3 PLOT4 This plots two or three or four workspaces on a single graphics screen It is useful where a comparison of two or more data sets is required gt gt PLOT2 lt cr gt or PLOT3 or PLOT4 gt gt Enter number of workspace at top 1 lt cr gt gt gt Enter number of workspace at bottom 1 lt cr gt gt gt Enter binning 1 lt cr gt gt gt Enter lower x limit 1 0 lt cr gt gt gt Enter upper x limit 2 0 lt cr gt If lt cr gt is entered in response to any of these prompts default values are taken on a PERICOM terminal the lt Return gt key must be pressed after the first plot to return the screen to text mode from graphics mode The y limits of the second plot are usually chosen to be identical to the default values for the first plot if a comparison between the relative intensities of spectra in two different workspaces is required b TPLOT This plots the information contained in the sample environment log file either for the current run or for a given run number gt gt TPLOT lt cr gt gt gt Give run number 16518 lt cr gt or DAE lt cr gt gt gt Give workspace 1 lt cr gt Give start date time lt cr gt Give finish time lt cr gt Give Se block name TEMP lt cr gt or TEMP1 lt cr gt Please give units K or C K
11. AW Increments the run number nnnnn Sets DAE state to SETUP CHANGE Enables the contents of the CRPT to be modified section 3 6 PAUSE Suspends data collection by DAE Sets DAE state to PAUSED RESUME Resumes data collection by DAE Sets DAE state to RUNNING UPDATE Copies data in DAE memory to CRPT STORE Writes contents of CRPT to file POLnnnnn SAV ABORT Stops data collection by DAE Sets DAE state to SETUP 3 5 The Data Acquisition Electronics DAE During the course of a run diffraction data from POLARIS are accumulated in the Data Acquisition Electronics DAE as a series of spectra each spectrum containing a histogram of neutron counts versus time of flight for a given detector The DAE has four operating modes listed in Table 3 and the current DAE mode is displayed at the top of the POLARIS dashboard section 3 3 At the end of each run the data in the DAE are copied to the CRPT which is then written to a file named POLnnnnn RAW on the POL FEM At the end of each run the run number nnnnn is incremented automatically Shortly afterwards each RAW file is automatically archived onto optical disk Table 3 POLARIS DAE Operating Modes SETUP Data are not being collected and the CRPT may be changed before starting a new run section 3 6 RUNNING Data are being collected and accumulated in the DAE PAUSED Data collection is temporarily suspended by the user WAITING Data collection
12. C lt cr gt This prompts for a choice of printer from those situated in building R55 in the DAC No 2 or the GEM cabin No 3 or in building R3 in rooms 1 38 No 0 or 2 9 10 No 1 creates the postscript format file DEC_POSTSCRIPT DAT and submits this file to the desired printer queue to be printed 4 5 Absorption Corrections Sample absorption effects are wavelength dependent and can be very significant for some elements and isotopes A routine is available which will take a normalised POLARIS diffraction pattern stored in a GENIE workspace and correct the data for sample absorption Before running the program the sample mass and dimensions must be measured the number density of scattering units in the sample calculated and the total scattering and total absorption at a neutron velocity of 2200ms cross sections per scattering unit determined A worked example for alumina Al2O3 is given below Mass of can sample 31 63 g Mass of empty can 19 98 g Mass of sample 11 65 g Diameter of sample can 0 952 cm Radius of can r 0 476 cm Height of sample h 5 5 cm Atom mass amu Os Oa Al 27 1 503 0 231 O 16 4 232 0 0002 os is the scattering cross section per atom in barns and og is the absorption cross section per atom in barns at a neutron velocity of 2200ms Values for these parameters for most elements and isotopes can be found in the tables in the POLARIS cabin orin the article by Sears V F Sears Neutron Scatte
13. F The Polaris 20 Position Automatic Sample Changer This Appendix gives instructions for operating the Polaris 20 position automatic sample changer The same procedure can be followed both for mounting or removing sample cans when the sample changer is off the beamline and for changing samples when the unit is located in the Polaris sample tank F 1 Description of Sample Changer The Polaris automatic sample changer figure F 1 holds a maximum of 20 thin walled cylindrical vanadium sample cans mounted on connecting rods in a carousel A 20 position gearbox rotates the carousel into a position whereby a linear actuator can push on the connecting rod to lower the sample into the neutron beam and retract it out of the beam afterwards The complete assembly is mounted on a circular aluminium plate the Tomkinson flange which sits atop the Polaris sample tank carousel sample can Figure F 1 Photograph of the Polaris sample changer The sample changer can be operated either manually from a control unit near the Polaris sample enclosure MANUAL mode or from the POL FEM through the CAMAC system AUTO mode The mode of operation is determined by the setting of a switch on the control unit F 2 Sample Can Mounting The 20 positions on the sample changer are numbered from 1 to 20 Each position is labelled both on the side of the carousel and on the axle on top of the 20 position gearbox The distance between the lower surface of the carouse
14. User Guide for the Polaris Powder Diffractometer at ISIS RI Smith and S Hull USER GUIDE FOR THE POLARIS POWDER DIFFRACTOMETER AT ISIS 1 INTRODUCTION 1 1 OVERVIEW 1 2 INSTRUMENT DESIGN 1 3 PRINCIPLE OF THE DETECTOR ARRANGEMENT 2 PERFORMING AN EXPERIMENT ON POLARIS 2 1 SAMPLE SAFETY ASSESSMENT 2 2 LOADING A SAMPLE 2 3 SAMPLE TANK EVACUATION 2 4 OPENING THE POLARIS BEAMLINE SHUTTER 3 DATA COLLECTION 3 1 OVERVIEW 3 2 THE POLARIS FEM 3 3 THE DASHBOARD 3 4 THE INSTRUMENT CONTROL PROGRAM ICP 3 5 THE DATA ACQUISITION ELECTRONICS DAE 3 6 THE CURRENT RUN PARAMETER TABLE CRPT 4 DATA ASSESSMENT 4 1 THE PUNCH SYSTEM 4 2 ACCESSING POLARIS DATA 4 3 GENIE 4 4 NORMALISING POLARIS DATA 4 4 1 Data from the Current ISIS Cycle 4 4 2 Data from Previous ISIS Cycles 4 4 3 Useful GENIE Commands 4 5 ABSORPTION CORRECTIONS 4 6 PROFILE REFINEMENT 4 6 1 CCSL Based Software 4 6 2 Examining the Profile Fit 4 6 3 GSAS 4 7 RETRIEVING ARCHIVED POLARIS DATA 4 8 MISCELLANEOUS POLARIS ROUTINES 4 8 1 Useful DCL Commands 4 8 2 Useful GENIE Commands APPENDIX F THE POLARIS 20 POSITION AUTOMATIC SAMPLE CHANGER F 1 DESCRIPTION OF SAMPLE CHANGER F 2 SAMPLE CAN MOUNTING F 3 CAMAC OPERATION OF THE SAMPLE CHANGER AUTO MODE CO OO NOO WO Oo 10 10 11 12 13 13 13 14 15 16 16 16 18 19 19 20 21 21 22 22 23 25 25 25 26 F 4 MANUAL OPERATION OF THE SAMPLE CHANGER MANUAL MODE F 4 1 To raise or lower t
15. Y W1 lt cr gt 4 4 2 Data from Previous ISIS Cycles First before running GENIE it is advisable first to check that the RAW file is still on the POL disk with the DIRECTORY command DIR POL DATA POLnnnnn RAWc lt cr gt If the file does not appear in the directory listing it can be retrieved from the optical disk archive system using the RESTPOL command as described in section 4 7 The data normalisation procedure is then similar to that given above for the current cycle except that the full name of the command file which selects the normalisation runs and perhaps a different detector configuration appropriate to that cycle must be given explicitly by the user gt gt OPOL COMMAND AFnnnnn COMeccr gt or BFnnnnn etc In this case nnnnn is the four or five figure number of the silicon calibration run collected during the same ISIS cycle as the old data A listing of all the available GENIE command files can be found from a directory of POL COMMAND F COM In general use the number which is closest to but lower than the run number of the data to be normalised In case of any doubt an instrument scientist will be able to advise 4 4 3 Useful GENIE Commands It is often desirable to save the information contained in a GENIE workspace to a file so that the data can be accessed quickly and efficiently at a later date One option available allows the data to be written in binary format to a GENIE intermediate format file which
16. ad Go 1 4 Finally to operate the sample changer either interactively or from a command file use the command Pol gt Gcamac dir pol move n 1 lt ns 20 note control is not returned to the keyboard until the selected sample position has been lowered which will take 3 minutes To allow the operation of the sample changer to be checked after it has been left unattended all the screen responses during normal AUTO operation are written also to a log file which can be inspected with the commands Pol gt type pol data sample changer log or Pol gt search pol data sample changer log string where string could be your run title or a date in d mmm yyyy format e g 5 nov 2003 F 4 Manual Operation of the Sample Changer MANUAL mode The sample changer can be operated manually from its control unit rather than from the POL FEM e g when loading or unloading samples or if CAMAC operation returns error messages Select MANUAL operation using the AUTO MANUAL switch on the left hand front panel of the sample changer control unit F 4 1 To raise or lower the sample Press the button marked RAISE upper yellow or LOWER lower green on the front of the control unit The linear actuator will move the sample up or down as reguested F 4 2 To rotate the carousel to a desired position Note that the carousel can ONLY be rotated when the actuator is in the retracted out of beam position Press the button marked FORWARD
17. ator are blocked by either 1 the nimonic alloy during and immediately after the short period of time when the proton pulse arrives at the ISIS neutron target or 2 the tail cutter at times later in the frame when slow neutrons passing the chopper would not reach the detectors before the end of the 20 000 us period between ISIS pulses Nimonic alloy block ee BaC tail va cutter Oy Direction of rotation Figure G 1 Schematic diagram of the Polaris chopper The nimonic alloy block stops the very highest energy fastest neutrons produced by the spallation process from travelling any further down the beamline Although these neutrons are of little use in a diffraction experiment they can be slowed down and scattered when they reach the sample position or any component near the sample position such that they give rise to unwanted background signals in the detectors This background suppression is of benefit primarily where bulky sample environment equipment is being used such as pressure cells furnaces and cryostats The tail cutter carries out two tasks It prevents frame overlap where the slow neutrons from one ISIS pulse are overtaken by the fast neutrons from the next pulse giving rise to erroneous extra intensity at the beginning of each frame It also acts as a further background suppressing device by preventing delayed neutrons from travelling down the beamline to the sample position these are neutrons emergin
18. cattering angle 20 and for this reason each bank is treated separately when normalising the data Because each of the detectors in a particular bank has approximately the same resolution all the detectors in that bank can be summed or focused into a single GENIE workspace to give a significant improvement in the counting statistics POLARIS data normalisation is carried out in three discrete steps through a set of GENIE command files one command file for each detector bank see also Appendix C First the spectra from all the detectors in a given bank are focused into a single GENIE workspace which is then divided by the incident monitor counts to normalise to counting time Next an instrument background spectrum is subtracted and finally the data are divided by a vanadium spectrum The vanadium spectrum which has been corrected for absorption and multiple scattering effects normalises the data to the energy distribution of the incident neutron flux taking into account the variation in detector efficiency with neutron energy The background and vanadium data sets used here are collected at the beginning of each ISIS cycle and are made available for use in normalising all POLARIS data during that cycle Because the detectors within each bank are not time focused section 1 3 they cannot be summed directly in time of flight Instead the data are converted to a d spacing scale see equation 2b section 1 3 as the spectra are summed in GENIE
19. change title LiCr0 11Mn1 8904 3 1476g 45x8mm begin waitfor 150 uamps end change title LiCr0 15Mn1 8504 1 1935g 30x6mm begin waitfor 150 uamps end exit Technical support is available through the Polaris instrument scientists Ron Smith ext 5683 Steve Hull ext 6628 or ISIS electronics technicians Dennis Cowdery ext 6765 mobile 1757 Jon Bones ext 5411 mobile 1750 At weekends or during silent hours please first contact the ISIS Main Control Room ext 6789 Appendix G The Polaris Chopper Note the settings on the Polaris chopper control electronics are FIXED There are NO user adjustable parameters This Appendix contains the bare minimum of information reguired to restart the chopper in the unlikely event that a trip in the power supply causes the chopper to stop G 1 Description ofthe Polaris Chopper A chopper is a device for preventing some unwanted portion of the polychromatic white neutron beam produced in the ISIS target from travelling down the beamline to the sample position The Polaris chopper is designed to remove high energy fast neutrons which have not been slowed sufficiently in the moderator to be of use in a diffraction experiment It consists of a rotor spinning at 50Hz onto which is mounted a small nimonic alloy block and a larger B C tail cutter The chopper rotation is synchronised with the ISIS accelerator so that neutrons emerging from the moder
20. data file name title etc In particular the bank number relates to a series of entries in a file known as the instrument parameter file which contains flight path scattering angle and line shape parameters for the detector bank in which the data were collected The POLARIS instrument parameter file filename 3BANKS POL contains parameters appropriate to the A C and E detector banks bank numbers 3 2 and 1 respectively and can be found in the directory pol diskO pol 4 7 Retrieving Archived POLARIS Data To restore RAW data files which have been deleted from the disk on the POL FEM onto the scratch disk the RESTPOL command can be used Note however that this command may not execute correctly on VMScluster nodes other than ISISA RESTPOL lt cr gt Use to restore raw and log data files to scratch disk polmgr restore Restored files are not guaranteed to stay for more than seven days Which file type Enter raw or log gt raw lt cr gt Enter run number for first file gt 11917 lt cr gt Enter run number for last file gt 11919 lt cr gt kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk A restore subprocess has been spawned do not log out until this subprocess has completed Online restores should appear in scratch disk polmgr restore within about 10 minutes Offline restores may take much longer up to 24 hours To check your process type show proc subproc kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk
21. et on the door to the POLARIS sample position for the duration of the experiment At the end of an experiment before being removed from the ISIS experimental hall all samples which have been irradiated in the neutron beam must be monitored by the ISIS Health Physics Group ext 6696 2 2 Loading a Sample During data collection samples are normally held in thin walled cylindrical vanadium sample cans These vary in length from 50mm to 75mm and have diameters ranging from 5mm to 11mm When working with air sensitive samples they can be fitted with Teflon O rings for measurements at room temperature Cu O rings for use in furnaces or indium seals for low temperature measurements For data collection at room temperature the sample cans are mounted on a candlestick so that the centre of the sample is 314mm from the lower face of the aluminium plate on the candlestick figure 2 neutron beam centre line Figure 2 Sample alignment on POLARIS candlestick Access to the POLARIS sample position is through the interlocked door at the front of the enclosure on top of the POLARIS blockhouse When the POLARIS shutter is closed section 2 4 this door can be opened and the top of the sample tank is approximately 300mm below floor level The POLARIS sample tank accepts all standard ISIS sample environment apparatus such as cryostats closed cycle refrigerators furnaces goniometers pressure cells etc The sample mounting procedure for each
22. ext contains several references to appropriate manuals where more detailed information may be found 1 2 Instrument Design POLARIS is located on ISIS beamline N7 and receives a polychromatic white beam of neutrons from the ambient temperature water moderator The sample position on POLARIS is 12 0m from the moderator and the incident and transmitted neutron flux is monitored by two low efficiency glass scintillator detectors positioned 4m before the sample position and 2 5m after it The maximum beam dimensions at the sample position are 40mm high x 20mm wide but two sets of stepper motor driven collimators in the incident beamline allow the beam size to be reduced to match the sample size or to minimise the background from sample environment equipment More detailed information on the use of this collimation can be provided by the instrument scientists See Appendix D POLARIS has a total of 434 He gas and ZnS scintillator detectors arranged into four separate banks two at low angles one at 20 90 and one at backscattering angles These are referred to as the A B E and C banks respectively Details of these detector banks are given in Table 1 with a schematic diagram of POLARIS shown in figure 1 Table 1 POLARIS detector configuration Figure 1 Schematic diagram of the POLARIS diffractometer bank position low angle low angle backscattering 90 degrees label A B C E detec
23. g from the moderator with actual velocities much greater than the apparent velocities determined by their arrival time at the chopper Delayed neutrons are fast neutrons resulting from nuclear reactions in the ISIS neutron target and generally are only produced when the target material is uranium G 2 Polaris Chopper Operation The Polaris chopper is driven by an electric motor which receives its power from a Polaron Cortina 30kW power supply situated on the floor of the ISIS experimental hall at ground level directly below the Polaris cabin An optical sensor on the chopper rotor enables the phasing of the chopper rotation relative to the proton beam extraction from the ISIS synchrotron to be controlled thereby ensuring that the chopper is closed at t zero If the chopper phasing drifts outwith preset limits such that the data collected may be corrupted a signal is sent to the Polaris DAE instructing it to ignore or veto that frame of data This procedure is controlled by a series of electronics cards mounted in a crate located in the left hand rack in the rear section of the Polaris cabin The only user relevant piece of information displayed on the control panel is the frequency in Hz at which the chopper is rotating which is indicated by the red LED display on the card labelled Frequency Meter The Polaris chopper operates at 50Hz if the display reads any number other than 50 then the chopper is either in the process of stopping or s
24. he Dashboard The POLARIS instrument dashboard Figure 5 shows details of the instrument configuration for the current experiment and will normally be displayed in the Supervisor window on the workstation see section 3 4 If the dashboard is not present it can be displayed on any terminal logged onto the POL FEM as username POL with the STATUS command STAT ON lt cr gt to switch the dashboard off type STAT OFF At the top of the dashboard display the current DAE state RUNNING SETUP etc see section 3 5 and run number are shown The remaining information on the dashboard gives information on the user Sample run time frame proton pulse count present and accumulated proton beam current the incident beam monitor counts and also displays values of any sample environment parameters being monitored by the CAMAC sample environment control program is RUN 16428 RB Number 0 Wed 27 Nov 1996 14 23 03 TEMP LOGG User Calibration Tel x5683 x6620 raw gt Title NBS Silicon Detector Tests TEMP 1 LOGG raw gt Current rm time 0 03 05 43 MONITOR Good Raw frames 499894 499895 Spectrum L Current Total uA 195 47 502 2 Fron mms 2000 0 DAE memory used Bytes 11584332 To mms 2199 3 438 Spectra 6596 Channels Counts 5177044 Figure 5 The POLARIS Dashboard 3 4 The Instrument Control Program ICP Data collection on POLARIS is controlled by the Instrument Control Program ICP which runs on the POL FEM This progra
25. he extension CCL The REVI command is available to facilitate this REVI C16228 lt cr gt This starts the EDT editor and reads in the CCN file At the end of the editing session typing lt Ctrl Z gt exits from the editor and automatically writes the new file with the extension CCL 4 6 2 Examining the Profile Fit The PRO output file contains the observed and calculated diffraction profiles which can be inspected in GENIE in order to examine the profile fit The PLS command loads the observed calculated and difference profiles into GENIE workspaces 20 21 and 22 the background profile into workspace 23 and the positions of reflection markers for up to three crystalline phases into workspaces 24 25 and 26 gt gt PLS lt cr gt gt gt Enter filename max 6 characters C16228 lt cr gt gt gt Enter lower and upper limits lt cr gt gt gt Enter binning 1 lt cr gt Entering lt cr gt in response to the lower and upper time of flight limits causes default values corresponding to the minimum and maximum times of flight in the DAT file to be used Once the profiles have been read into GENIE using PLS they can then be examined in smaller sections with the PLC command gt gt PLC lt cr gt gt gt Enter lower and upper limits 3000 5000 lt cr gt gt gt Enter binning 1 lt cr gt If desired the data in workspaces 20 to 26 can be converted to lattice a spacing with the U D command units in d spacing and
26. he sample F 4 2 To rotate the carousel to a desired position F 5 COMMAND FILE OPERATION OF THE SAMPLE CHANGER APPENDIX G THE POLARIS CHOPPER G 1 DESCRIPTION OF THE POLARIS CHOPPER G 2 POLARIS CHOPPER OPERATION G 3 RESTARTING THE POLARIS CHOPPER 27 27 27 27 30 30 31 31 1 INTRODUCTION 1 1 Overview The POLARIS instrument at ISIS is a high intensity medium resolution powder diffractometer The intense neutron flux on POLARIS combined with a large detector solid angle provides a high count rate allowing experiments to be performed with short counting times or with very little sample This has been exploited during kinetic experiments e g studies of chemical reactions or phase transitions where diffraction patterns are collected while a furnace or cryostat is heating or cooling the sample and in diffraction from very small sample volumes down to 1 mm The detector banks at 20 90 are particularly important for studies of samples contained in complex environments such as under high pressures or in reaction vessels By providing suitable collimation of the incident and scattered beams contamination of the diffraction pattern from the sample with signals from the surrounding sample environment apparatus can be eliminated This User Guide gives a short description of the POLARIS diffractometer and provides the basic information reguired to perform a powder diffraction experiment To keep the guide as brief as possible the t
27. hoice of up to 26 workspaces For example to store the sum of spectrum 5 plus spectrum 34 divided by spectrum 1 in workspace 1 and then display the result the commands would be gt gt W1 S5 S34 S1 lt cr gt followed by gt gt DISPLAY W1 lt cr gt Further GENIE commands allow the user to modify the plot alter the limits change the units etc fit functions to peaks in the data etc and full details of these are given in the GENIE manuals mentioned earlier To exit from GENIE type gt gt EXIT lt cr gt 4 4 Normalising POLARIS Data The data collected in each of the detectors on POLARIS are stored as a series of histograms of neutron counts versus total time of flight in which the time of flight axes are split into a large number of bins bounded by time channel boundaries TCBs defined by entries in the CRPT Once in the DAE and the RAW file at the end of the run each histogram is referred to as a spectrum GENIE allows the user to inspect and manipulate these spectra e g sum them together convert from neutron time of flight to lattice d spacing etc The correspondence between the spectrum number n and the position of the detector on POLARIS is determined by the wiring of the detector electronics and by data tables in the CRPT The current parameters and spectrum numbers for each detector are listed in Appendix A As discussed in sections 1 2 and 1 3 the Ad d resolution of the POLARIS detector banks varies with s
28. is temporarily suspended by the FEM this may occur if a furnace or cryostat drifts outside limits defined in the CAMAC sample environment control program or while it heats or cools to a requested temperature 3 6 The Current Run Parameter Table CRPT The Current Run Parameter Table CRPT is an area of memory in the ICP which holds a number of parameters associated with the current experiment such as sample details information on users names and institutions and the instrument configuration times of flight of the time channel boundaries monitor spectrum mapping of individual POLARIS detectors into spectra in the GENIE data visualisation program section 4 3 etc It also contains space into which the contents of the DAE can be copied atthe end of or during a run At the end of a run the contents of CRPT are written to a RAW file on the FEM Atthe beginning of an experiment it may be necessary to change some ofthe contents ofthe CRPT This should be done in consultation with an instrument scientist The command CHANGE should be typed at the Supervisor window when the DAE is in SETUP mode CHANGE lt cr gt The screen will clear and display the first of six pages which detail the current parameters in the POLARIS CRPT The values in the various fields are altered using the DEFT screen editor full details of which are contained in section 5 1 4 of the PUNCH manual The and cursor keys are used to move from one field to an
29. ise through 90 until it is horizontal At this stage users should check that the four red LED s labelled GUARD LINE on the orange interlock indicator box item C figure 4 are lit indicating that all interlocks are in place and microswitches are closed Any attempt to open the shutter without all four LED s being lit will immediately shut down the ISIS accelerator The POLARIS beamline shutter is then opened by lifting up the transparent cover and pressing the OPEN button on the neutron beam shutter control box item D figure 4 When the shutter is open the master key cannot be removed from the interlock box and there is no access to the interlocked areas PROTON BEAM OFF NEUTRON BEAM SHUTTER CONTROL GUARD LINE ry 2 B OBSCORRECT DOORSCLOSED m BEAMLINE OPERATION AL SAFE SHUTTER CLOSED Figure 4 The POLARIS beamline shutter control and interlock system To close the shutter lift up the transparent cover and press the CLOSE button on the shutter control unit Once the shutter has closed the key labelled N7 M can be removed from the interlock box and returned to the key rack enabling one of the keys labelled N7 A to be removed to allow access to the sample position The status of the POLARIS shutter is indicated by one of the four lamps on the neutron beam shutter control unit showing open opening closing or closed In common with all ISIS beamlines POLARIS shutte
30. k Subprocess user01_1 has completed System responses to the restore requests are written to a file RESTLOG LOG in the user s current default directory Once the subprocess has completed the restore requests will appear on one of the two archive restore queues online or offline This can be checked with the RSTATUS command RSTATUS lt cr gt Online restores will be returned after approximately 10 minutes although this can be as long as an hour or two if there are many files queued to be restored Offline restores may take as long as one day to be returned However be warned files on the scratch disk have only a limited lifetime and will be deleted again after 2 weeks Because the directory scratch disk polmgr restore is included in the definition of the logical POL_DATA all restored files can be read by GENIE without further user intervention 4 8 Miscellaneous POLARIS Routines 4 8 1 Useful DCL Commands a ISISNEWS Periodically the ISIS Main Control Room crew will broadcast messages giving information on the current operational status of ISIS to all terminals logged on the VMScluster e g scheduled liquid transfers in the methane moderator estimates of beam on times during repairs to ISIS etc As well as being broadcast to the terminals these messages are also stored in a log file which can be inspected at any time using the ISISNEWS command Within the ISISNEWS routine there are four separate topics Current_stat
31. l and the centre of the neutron beam is 90mm figure F 3 A selection of cans and spacers are provided which allow a wide range of sample sizes to be centred in the neutron beam Figure F 3 View ofthe carousel and a sample can with mounting dimension marked WHEN LOADING AND UNLOADING SAMPLES THE SPANNER PROVIDED MUSTBE USED WHEN TIGHTENING THE SPACERS TO THE CONNECTING RODS TO PREVENT THE CONNECTING ROD FROM TWISTING AND DAMAGING THE TEFLON COVERS ON THE UPPER HALF OF THE CAROUSEL The carousel is able to rotate continuously in both a clockwise and anticlockwise direction and the control software allows samples to be lowered into the beam in any order or even miss some samples positions F 3 CAMAC Operation of the Sample Changer AUTO mode 1 First select AUTO operation using the AUTO MANUAL switch on the left hand front panel of the sample changer control unit 2 Next initialise the sample changer control hardware at any DECTerm window on the POL FEM type Pol gt cset sample log display SAMPLE block appears on dashboard Pol gt cset sample init sample changer controller display will read init If a sample is in the lowered position it will be raised and the carousel then will rotate to position 1 WAIT UNTIL THIS OPERATION HAS COMPLETED may take 3 4 minutes 3 Then initialise the software type Pol gt cset sample 0 01 initialises software and lowers sample position 1 sample changer controller display will re
32. lt er gt Which log column do you want 2 or 3 def 3 lt cr gt The SE sample environment block name usually refers to either TEMP or TEMP1 If more than two temperatures are being monitored then e g TEMP2 TEMP3 etc can also be plotted The units determines the labelling in K or C on the y axis of the plot The log column refers to either the raw mV reading column 2 or the converted temperature in Kelvin or degrees Centigrade column 3 c GAUSS This is used to fit a Gaussian function to a single peak displayed in GENIE In general GEC is to be preferred see below although GAUSS can be used for fitting data from the B bank detectors where the line shape is dominated by uncertainties in the scattering angle and conseguently is well approximated by a Gaussian function gt gt GAUSS lt cr gt gt gt Enter xmin 2 3 lt Cr gt gt gt Enter xmax 2 6 lt Cr gt If only lt cr gt is entered in response to the prompts for xmin and xmax the cursor appears on the plot and the fitting region can be selected by moving the cursor to the left ofthe peak and typing L then moving the cursor to the right of the peak and typing R d GEC This is identical to GAUSS except that the functional form used to fit the peak is a convolution of a Gaussian which models angular contribution to the line shape and an exponential decay term which models the moderator contribution to the line shape gt gt GEC lt cr gt Appendix
33. m is used to start and stop data collection modify instrument parameters and change sample environment conditions through the CAMAC control program Communication between the ICP and CAMAC allows data collection to be suspended temporarily while a furnace or cryostat reaches its setpoint and can also be used to suspend data collection automatically e g if the temperature drifts away from limits defined by the user The instrument control commands recognised by the ICP along with their functions are listed in Table 2 To prevent unauthorised control of the instrument the commands in Table 2 will only be accepted if issued from a session logged on the POL FEM with username POL and having the process name supervisor Normally the window running this supervisor process will be displaying the POLARIS dashboard section 3 3 will have its prompt displaying Supervisor gt and will have Supervisor displayed in its title bar and icon title Any attempt to control POLARIS from another process will result in the error message Failed to execute the command STATUS 148 Likely cause Instrument under control of another terminal Table 2 POLARIS ICP Commands BEGIN Clears DAE memory section 3 5 Sets parameters in DAE to those specified by CRPT section 3 6 Instructs DAE to start data collection Sets DAE state to RUNNING END Stops data collection by DAE Copies data in DAE memory to CRPT Writes contents of CRPT to file POLnnnnn R
34. n 1 lt cr gt 4 DATA ASSESSMENT 4 1 The PuNCH System The PUNCH VMScluster at ISIS links all the instrument FEMs and the HUB cluster node name ISISA allowing these computers and their files to be accessed from the terminals situated in the DAC in the ISIS experimental hall R55 and the offices in Building R3 as well as from the instrument FEMs In addition the HUB may also be reached from outside RAL via JANET the Internet and DECnet 4 2 Accessing POLARIS data The procedures for routine inspection of POLARIS data will be described in the following sections 4 3 and 4 4 Except while carrying out an experiment on POLARIS computationally intensive procedures such as data analysis and structure refinement should not be run on the POL FEM but should be carried out on other machines in the VMScluster such as the HUB cluster node name ISISA For this purpose users will reguire a user account which can be arranged by one ofthe instrument scientists Users who have performed previous experiments at ISIS may already have their account set up with default settings to access data from another instrument If so an instrument scientist can arrange for these to be amended In order to ensure that all the commands necessary to analyse POLARIS data are set up when logging in users should edit their LOGIN COM file so that it contains the command line OPOLSDISKO POLMGR COMMANDJLOGIN COM This will ensure the correct definition of the
35. of these is beyond the scope of this guide and details are available from ISIS User Support Group staff and instrument scientists After removal from the neutron beam all samples which have become activated due to neutron irradiation must be stored in a controlled area which for POLARIS is the POLARIS active samples cupboard situated close to the interlocked enclosure Portable y and B y radiation detectors are available on the beamline for monitoring samples after they have been removed from the beam It should be noted atthis point that the vanadium sample cans themselves become activated emitting B radiation for approximately 72 hour after removal from the neutron beam Handling of any sample having an induced activity gt 75uSv h including any activity arising from the sample can must be carried out under the supervision of the ISIS duty officer ext 6789 2 3 Sample Tank Evacuation Attenuation of the incident and scattered neutron beam during data collection is reduced by evacuating the sample tank to a nominal pressure of 0 1 mbar with a rotary pump Also fitted is a turbo pump which will achieve a cryogenic vacuum of 1 0 mbar necessary when operating a closed cycle refrigerator The rotary pump is operated from the control unit located to the left of the shutter control system beside the steps down to the instrument figure 3 Two push button switches control the rotary pump and a solenoid operated vent valve while two warning lights
36. other while the lt and gt cursor keys and the delete key allow corrections to be made The fields are altered simply by typing the appropriate characters or numbers into the field Normally users will only need to change details on the first page and any changes to subsequent pages must be done only in consultation with an instrument scientist If the field prompts with toggle data type the key on the right hand keypad should be pressed until the field displays the required option To exit from the editor and write the new parameters to the CRPT press the key marked PF 1 on the right hand keypad then press E on the keyboard If you wish to guit and leave the CRPT unchanged then press PF1 followed by Q During the course of a run it is possible to change the experiment title without using the DEFT editor This is done with the CHANGE TITLE command which can either be typed at the keyboard or given from a command file regardless of the state of the DAE CHANGE TITLE NiMgSi04_1100_C_In_RALF1_Run_1 lt cr gt Note that the new title must be a continuous string of characters hence the _ symbol connecting different words Note also that any lower case characters will be translated into upper case in the new title An alternative which allows the use of spaces and lower case characters uses triple sets of quotation marks at the beginning and end of the new title CHANGE TITLE NiMgSiO4 1100 C in RAL F1 Ru
37. r open conditions are indicated by the following signals i illumination of the BEAM ON lamps on top of the POLARIS sample position enclosure and on the target station wall and ii the fluorescent lighting in the POLARIS blockhouse changing from white to blue and illumination of blue lighting in the vicinity of the sample position If any difficulties are encountered with the shutter or interlock system the user should immediately contact an instrument scientist or the ISIS Main Control Room ext 6789 The remaining keys labelled N7 A in the interlock key rack allow access to other restricted areas along the beamline for example the POLARIS blockhouse or chopper pit Under normal circumstances access to these areas is restricted to instrument scientists and ISIS User Support Group staff In the event that users require access they must contact either an instrument scientist or the ISIS Main Control Room ext 6789 In the unlikely event that any personnel become trapped inside an interlocked area shutter open conditions will be indicated as described above In such a case the ISIS beam is switched off by pressing either of the Proton Beam Off buttons which are located inside the POLARIS blockhouse on the beam stop wall and outside the blockhouse above the orange interlock indicator box on the beamline shutter control panel Figure 4 3 DATA COLLECTION 3 1 Overview Data collection on POLARIS is controlled by the Instrument Con
38. rating system Along with all the other ISIS instrument FEMs it is linked to the central VAX 4500 4300 computer which is known as the HUB to form a MScluster Within the VMScluster the node name of the POLARIS FEM is POL and the node name of the HUB is ISISA One advantage of the VMScluster is that each computer has direct access to the disk storage area of any other computer in the cluster so that users logged on to the HUB can access data files stored on the FEM Note however that the POLARIS DAE can only be accessed from the POL FEM The Motif X windows environment operating on the FEM allows users to run several sessions on the screen simultaneously each logged onto POL independently and performing a different task e g controlling the instrument inspecting the experimental data performing preliminary data analysis etc To open a new terminal window on the workstation use the left mouse button to select the DECTerm option from the Applications pull down menu in the Session Manager window This will generate a new window which will automatically log on to the POL FEM with username POL Two useful options on the workstation shuffle up and shuffle down are in a pull down menu accessed by pressing the left hand mouse button while the pointer is in a background region of the screen These allow windows to be pushed behind others or pulled to the front without changing the window to which keyboard input is directed 3 3 T
39. ring Lengths and Cross Sections Neutron News 1992 3 26 First calculate the mass of one Al 03 scattering unit in grams m 26 982 x 2 15 999 x 3 6 022x10 1 694x10 g Next calculate the sample volume V nth 3 141 x 0 476 x 5 5 3 914cm Then calculate the number of scattering units cm N V weight of sample mass of scattering unit x volume 11 65 1 694x10 x 3 914 1 757 x 10 cm Finally calculate the total scattering cross section per scattering unit Osc 2 X 1 50343 x 4 232 15 70 barns and the absorption cross section per scattering unit Gab 2 X 0 23143 x 0 0002 0 4626 barns at a neutron velocity of 2200ms The program is run from GENIE with the CORRECT command gt gt CORRECT lt cr gt gt gt Enter workspace containing uncorrected data 1 lt cr gt gt gt Enter workspace to contain corrected data 2 lt cr gt Enter cylindrical radius cm 0 476 lt cr gt Enter number density of scattering units cm 3 1 757E22 lt cr gt Enter scattering cross section barns 15 70 lt cr gt Enter absorption cross section barns 2200m sec 0 4626 lt cr gt The program determines the POLARIS detector bank in which the data were collected calculates and applies the absorption correction for that scattering geometry and passes the corrected diffraction pattern back to the given GENIE workspace 4 6 Profile Refinement 4 6 1 CCSL Based Software Several programs based on the Cambridge
40. screen it is possible to write the image to a file which can then be printed on one of the laser printers transferred to a remote site using FTP etc The file is written with the KEEP HARD command and is given the filename DEC device DAT where device is a code word for the file format gt gt KEEP HARD lt cr gt The default device is POSTSCRIPT but this can be changed either by adding the qualifier HARD device when starting GENIE or from within GENIE itself with the SET HARD command used before typing KEEP HARD e g GENIE HARD HP7475 lt cr gt and gt gt SET HARD HP7475 lt cr gt are equivalent This alternative device type HP7475 creates HPGL format files which are particularly useful because they can be incorporated into Word documents and furthermore they can be viewed and edited in Word as a Microsoft Drawing or a Microsoft Word 6 0 Picture A more detailed list of available device types can be obtained through ISIS Computer Support To printa POSTSCRIPT format file on one of the ISIS laser printers the command PLASER is available PLASERn DEC_POSTSCRIPT DAT lt cr gt where n is the number of the laser printer A command file has been written which will save a graphic image and print it out on a laser printer automatically Once the image has been created on the screen type HC on a PERICOM terminal it will be necessary first to leave the graphics screen by hitting lt cr gt before typing HC gt gt H
41. sure gauges After 3 4 minutes the pressure in the sample tank should reach atmospheric pressure the VAC ON warning lamp in the sample position enclosure will extinguish and the sample candlestick can be removed In order to achieve a cryogenic vacuum in the sample tank a thin window is fitted in the incident beam line upstream of the sample tank When changing samples users must ensure the sample tank has reached atmospheric pressure before attempting to remove the candlestick from the sample tank Should any attempt be made to remove the candlestick while there is still a partial vacuum in the sample tank indicated by the VAC ON warning lamp in the sample position enclosure there is a risk that the pressure difference created across this window will cause it to tear 2 4 Opening the POLARIS Beamline Shutter Before the POLARIS beamline shutter can be opened the interlocked door to the sample position enclosure must be closed and bolted The interlock key labelled N7 A then can be removed from the bolt lock and inserted into the vacant position in the shutter control system key rack item A figure 4 situated on the panel above the beam stop beside the steps down to the POLARIS sample position enclosure When all seven keys labelled N7 A are present in this key rack the key labelled N7 M with a dark red tag attached may be removed and inserted into the green shutter interlock box item B figure 4 where it is turned clockw
42. t values At is approximately equal on the two instruments but on HRPD t is a factor of 10 greater due to the larger flight path giving a resolution 10 times better However POLARIS has the advantage of much higher intensities particularly at short neutron wavelengths The d spacing and A ranges given in Table 1 refer to the average values covered by the whole bank In the case of for example the A bank longer d spacings up to 11 1A corresponding to a minimum Q of 0 54 can be accessed by considering only the detectors at lowest angles 28 26 although obviously there will be a decrease in the counting statistics However it should be noted that these longer d spacings will be recorded by the B bank detectors albeit with poorer resolution and a lower count rate 2 PERFORMING AN EXPERIMENT ON POLARIS 2 1 Sample Safety Assessment All users complete a Sample Record Sheet as part of their application for ISIS beam time the details on which are used to carry out a safety assessment This will give comments if any from the ISIS Safety Section concerning possible chemical or radiological hazards associated with the sample Any instructions concerning sample handling etc must be followed Before the beginning of the experiment the user should collect their sample safety assessment sheet from the filing cabinet in the Data Assessment Centre DAC in the ISIS experimental hall Building R55 after which it should be displayed in the pock
43. ta file is generated from one of a series of template files Xnnnnn CCL where X is the Polaris detector bank A B C or E and nnnnn corresponds to a silicon calibration run number which can be found in the directory pol calib silicon The appropriate silicon run number nnnnn is determined in a manner similar to that described in section 4 4 2 for focusing data from previous ISIS cycles Take a copy of the file Xnnnnn CCL with run number closest to but lower than the run number of your own data To run the profile refinement program simply type the program name followed by the input file name without any extension and an option for batch or interactive running TF14LS C16228 BATCH or INTERACTIVE for interactive running During structure refinement the appropriate extensions are assigned automatically to the input and output data file names In general three output files will be of interest the ine printer file extension OUT the profiles file extension PRO and the updated crystal data file extension CCN After examining the profile fit section 4 6 2 and reading through the OUT file the next stage in the structure refinement procedure will be to introduce further variable parameters into the least sguares This can be done by taking the CCN file containing the refined values of the variable parameters from the previous cycles of least sguares editing the least sguares information and writing a new data file with t
44. tarting or if the display reads zero the chopper has stopped G 3 Restarting the Polaris Chopper In the event that the chopper stops spinning contact a Polaris instrument scientist During silent hours or if an instrument scientist cannot be located the ISIS main control room ext 6789 will try to contact personnel from ISIS chopper group there is a limited level of out of hours support available for choppers Two panels of warning lamps one on the Polaron Cortina power supply the other on the chopper control crate in the Polaris cabin provide indications of faults in the power supply and control electronics respectively Please take a note of the state of these warning lamps Provided that none of the warning lamps on the chopper contro crate are illuminated an attempt can be made to restart the chopper from the Cortina power supply contact the ISIS main control room for assistance Under no circumstances should any attempt be made to restart the chopper until it has stopped spinning
45. then the profiles displayed using PLC gt gt U D W20 lt cr gt and W21 W22 etc To convert the data back to time of flight use the U T command units in total time of flight The positions of the allowed Bragg reflections can be displayed as vertical bars on the difference plot using the P T command plot using tics gt gt P T W24 ymin ymaxc lt cr gt or W25 or W26 This will draw vertical bars between the given y axis intensity limits at each of the x axis time of flight or d spacing positions 4 6 3 GSAS Also available for profile refinement using ISIS data is GSAS A C Larson and R B Von Dreele GSAS General Structure Analysis System Report LAUR 86 748 Los Alamos National Laboratory 1994 Please note that this software is not supported by ISIS staff and that any problems should be addressed to the program authors Also no guarantee can be given that the instrument parameter file below is kept up to date The data files reguired by GSAS are in a different format to those used by the CCSL based routines The ASCII format GSAS standard powder data file containing the normalised diffraction profile is generated from the data stored in a GENIE workspace with the GSAS FILE command gt gt GSAS FILE lt cr gt This prompts for the number of the workspace containing the normalised diffraction profile before scrolling through several screens of information asking for a detector bank number time of flight limits
46. tor type He ZnS He ZnS no of elements 2 x 40 80 4x 20 80 2 x 29 58 6 x 36 216 L m 1 72 2 65 2 2 0 65 1 35 0 80 20 range 28 lt 20 lt 42 13 lt 20 lt 15 130 lt 20 lt 160 83 lt 20 lt 97 Q ster 0 046 0 009 0 29 0 48 Ad d 1x10 3x10 5x10 7x10 a range A 0 5 8 3 0 5 21 6 0 2 3 2 0 2 4 0 Q range AT 0 75 12 6 0 3 12 6 2 0 31 4 1 5 31 4 1 3 Principle of the Detector Arrangement For a given Bragg reflection nan n 2dsing AR AAL T SAA 1 where is the neutron wavelength h is Planck s constant m is the neutron mass tis the total time of flight L is the total flight path from moderator to sample to detector dis the interplanar spacing and 20 is the Bragg scattering angle Substitution for h and m and conversion to appropriate units yields tius 505 56 L m sin 6 d A 3 HA ESTO or Lim sin8 000 2b From equation 2a it can be seen that if the product Lsin 0 is identical for all the detector elements in a bank then neutrons scattered from a given crystallographic plane hkl will reach each of the detectors in that bank with exactly the same time of flight Such an arrangement of detectors is said to be time focused e g see C G Windsor Pulsed Neutron Scattering Taylor amp Francis London p237 On POLARIS however the detectors are arranged into four discrete resolution focused banks whereby all of the elements within each bank are
47. trol Program ICP which runs on a computer known as the Front End Monitor FEM situated in the instrument cabin The instrument settings pertaining to the run in progress are stored on the FEM in a file called the Current Run Parameter Table CRPT and during a run the diffraction data are accumulated in the Data Acquisition Electronics DAE At the end of each run the contents of the DAE are copied to the CRPT which is then written to a file POLnnnnn RAW on the FEM known as a RAW file where nnnnn is a five figure run number Shortly afterwards the RAW file is archived onto optical disk The FEM itself has a limited amount of disk space so RAW files are deleted periodically from the disk These files can of course be restored from the optical disk so that data analysis can be performed at a later date The CAMAC control program runs on the FEM and provides a means of monitoring and changing sample environment parameters e g temperature etc by typing commands to the instrument computer More detailed information on CAMAC is given in section 5 2 of the PUNCH User Manual a copy of which is kept in the POLARIS cabin and which can also be obtained from the Computer Support office Room 1 38 Building R3 3 2 The POLARIS FEM The POLARIS Front End Monitor FEM is used to control both data collection and sample environment parameters The FEM is a Digital Alphastation 500 266 computer situated in the POLARIS cabin which runs the VMS ope
48. us Notes Summary and Unavailability To view the messages in the log file use the Current_status topic option ISISNEWS Current_status can be abbreviated to ISISNEWS C b WAITFOR This command is very useful when included in a command file It interrogates the DAE and will cause the command file to wait at a particular point until the desired number of microAmp hours or ISIS frames or monitor counts has been reached before continuing on to the next command line The format of the command is WAITFOR nnn UAMPS or nnn FRAMES or nnn COUNTS where nnn is an appropriate integer c POLPARS This command interrogates a single RAW file or a series of RAW files and returns selected run information either to the screen or to a user defined file POLPARS lt cr gt RUN PARAMETER SUMMARY FOR POLARIS DATA Output to a file default is the screen Y N Y lt cr gt Enter name for output file summary dat lt cr gt Enter first run number 10226 lt cr gt Enter last run number lt cr gt default lt cr gt makes last run first run SEARCHING FOR FILE pol data pol10226 raw FILE OPENED The NBS Standard Silicon Sample run number user was start date time finish date time no of spectra acguisition mode number of time channels time channel increment TOF window ms GFC RFC Monitor counts run duration in hrs run duration in UA Hr 10226 Commissioning 5 OCT 1993 14 39 20 6 OCT 1
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