D17B Manual 1-1-01

GENERAL HELP

Starting MAD and LAMP

Instrument control for users is only possible from the control terminal

Starting with the instrument control program

You should have the prompt MAD > on the instrument control terminal. If not, (e.g. you see the prompt D17) start the instrument control program by just typing MAD. Note: it takes about 15 sec until the prompt MAD comes up. If MAD was not cleanly exited or if MAD is blocked, from another UNIX window type MADKILL before typing MAD again.

In another window type lamp to start the idl control/analysis program
click pad and start the macros (from buttons) d17data and d17spy. The displays can be made larger in the dial menus by selecting large dial
If lamp hangs up it can (mostly) be brought back to life by <crtl> c in the lamp window until the snooper> prompt appears in the unix window. At
the snooper prompt type
>retall
then
>lamp
if this does not work you will have to stop lamp and restart by typing lamp in the unix window

Summary of instrument variables

san rotation of sample

trs      translation of sample
phi     tilt of sample
sht     sample height
det     detector distance
dan    detector angle
s1w   background slit width
s1h    background slit height
s2w   first collimation slit width
s3w   second collimation slit width
s3h    second collimation slit height

For monochromatic and polarized neutrons only

col1    collimation arm angle
sol      in/out of sollar collimator
ros     rotation of sollar colimator
trm     translation of monochromator   (30=pol, 101=mono & 230=out)
rof      rotation of mono/pol (~1.76)
trf      translation of filter (101= in , 230=out)
rof     rotation of filter (~ 0.1)
tra     translation of analyser (-2-530mm)

Not motors

fl1      flipper in/out
b1      flipper current (use the lamp dial flipper to tune the currents)
b2      compensation current
att      attenuator (att n sets att value n. n=0-7)
chop open    chopper opening
chop speed chopper speed

To read a motor value:
MAD>san <return>
reads the current value of the motor san

To move a motor:
MAD>san x <return>
moves the motor san to the value x

To re-define a motors value without moving it:
MAD>par set san x <return>
re-defines the motor san to the value x without moving it

To scan a motor:
MAD>scan san n1 n2 n3 co n4 t a <return>
scans the motor san from n1 to n2 with a step n3, counts for n4 seconds and plots the total detector counts without saving each run. Leaving out the ëaí will save each run.

Automatic alignment of a sample

An alternative to the 'manual' method below there is now an automatic macro which will align the sample. Simply type in the LAMP window

>autoalign,a,b
where a is th sample length and b is the desired sample angle. The program will under-illuminate the sample, set trs to be zero at the correctly
aligned value and move and define the sample angle to be the desired value.

Manual method for sample alignment

1) Switch the motors san.trs,phi and sht to manual

2) Align the sample in phi and san by getting the reflection from the laser perpendicular to the beam on the cross on the wall.
Move sht so this laser stikes the sampl ein the middle. Align the sample in trs such that the laser parallel to the beam just strikes the reflecting surface.

3) Switch the motors san.trs,phi and sht back to program

4) Re-define the values for trs,san,phi and sht to be zero i.e.
MAD>par set san 0 <return>
MAD>par set trs 0 <return>
MAD>par set phi 0 <return>
MAD>par set sht 0 <return>

4) Run the alignment script file align.cmd with the command:
MAD>start align <return>

5) You should have a peak in the trs scan. If the peak value for trs is x then first move
negative of the to take up backlash:
MAD>trs (x-0.5) <return>
then move to the peak:
MAD>trs x <return>

6) Take a quick run to enable a calculation of the true angle between the beam ans the reflection surface, q
MAD>run 10 t

7) In lamp use the macro tth to calculate q
tth,n1,n2 <return>
where n1 is a run number of a direct beam run and n2 is the run number of the quick run just taken. Look in the blue window from which lamp was running and note the calulated value of q.

8) Re-define the value of san to be this calculated value (it should be close to 0.3)
MAD>par set san q
where q is the calculated value.
If the calculated value was more than 0.2 degrees from 0.3 then repeat steps 5 to 8

9) You are now ready to run. Trs zero is now means the beam strikes the middle of the reflection surface and san is equal to the angle between the beam and the reflection surface. To move the sample out of the beam move positive in trs.

Setting up a reflectivity measurement

1) What is the q range you wish to measure? qmin-qmax

2) Do you want high (constant 1% dT/T) or low resolution (1-10% dT/T)?

3) What is the active reflection area of your sample L=length along beam H=height

4) The useful wavelength range is 2.2-19A. This determines the angles needed to cover the desired q-range.
Using q=sin-1(ql/(4p)) the first angle is determined by qmin and lmax
q1= sin-1(qmin x lmax/(4p))
The final angle is determined by qmax and lmin
qf= sin-1(qmax x lmin/(4p))

Now if these two angles are sufficient for the entire q-range they will have to overlap in q

if the maximum q from the first angle , 4psin(q1)/lmax is greater than the minimum q from the final angle, 4psin(qf)/lmin then only these two angles are needed. If there is no overlap then an intermediate angle must be chosen that overlaps the q-range from both q1 and qf.

4) Assuming two angles were sufficient we need to calculate what collimation we need to under-illuminate the sample for each angle. If we keep the illumination constant we gain in flux at the price of looser resolution in the second angle. Use the program slitwit on d17sgi to calculate good values for s2w and s3w for each angle. The distance between the slits is 3.4m and the distance from the final slit to the sample axis is 210mm. Sample length is L

5) Usually a chopper opening of 1.2 is fine for the first angle. This corresponds to zero true chopper opening (the is an offset of 1.2 degrees) and means dT/T is constant at 1%. For the second angle if the low resolution option is chosen the chopper angle can be set to be 3-4 degrees. For high resolution both angles would use the 1.2 degrees value.

6) Set he height of the beam (s3h) to be 5mm less than the sample height H. You may have to set it twice.

7) With the sample out of the beam set the conditions for the first angle (opening, s2w and s3w) with the detector angle, dan set to 0.8 degrees. Take a quick run and make sure the the mean rate is less than 10,000 c/s. If so reduce the slits. If the rate is fine then count for an hour with these conditions.

to set a chopper opening:
MAD> chop open 1.2 <return>
it takes about 5 minutes to move from 1.2 to 3 degrees.

to read the chopper status
MAD>chop read <return>

8) For the second angle with the larger slit values and chopper opening the beam cannot be directly put on the detector. Rotate the oscillating attenuator, found just after the slit s3, into the beam. Switch on the power supply found under the focusing guide (red button) and the slit should oscillate up and down. This provides a wavelength independent attenuation of the beam. Set the chopper opening, s2w and s3w for the second angle conditions and measure for an hour.

9) Place the water sample in the beam making sure that it is the correct height. Remove the oscillating attenuator from the beam. Move dan to 30 degrees, s2w=s3w=3mm and open the chopper until you have a rate of ~1000c/s. Count for one hour.

10) Remove the water and mount the sample.

Scan types for monochromatic mode:

Angular resolution

Choose:

Constant relative angular resolution- Both collimation slits are opened for each sample angle (q-point) such as to keep the illumination and relative angular divergence constant.

Why? Maximises intensity for each q-point. Fractional q-resolution is constant.

But: Must make a calibration run of the direct beam of each q-point. This typically takes 30 minutes and can be used for many reflectivity scans.

Or:

Constant slits- Both slits are fixed at a value such that the sample is under-illuminated at the smallest theta in the scan.

Why? Only one direct beam measurement is required saving time and this does not rely on the use of attenuators.

But: Severe loss of intensity at high q as only a tiny fraction of the sample is illuminated. Relative angular resolution is good (and gets better at high q) but needlessly so as the q resolution will never be better that that given by the selector.

Detector movement

Choose:

Theta 2theta- For each q-point the detector angle is moved double the sample angle thus keeping the reflected beam on the same part of the detector.

Why? Reduces the effect of varying detector efficiency of the pixel lines.

But: If DAN cannot position better than 0.1 degrees (which is not normally the case) then there is no point as the reflection is not kept at the desired X pixel line.

Or:

Constant detector angle- Detector angle remains constant throughout the scan.

Why? If there is interesting SANS scattering in parallel with the reflectivity it is more convenient to analyse this with a fixed detector angle.

But: Maximum reflection angle (hence q) is limited by half the angle subtended by the detector from the sample position. Water calibration must be more reliable.

Measurement steps in q

Choose:

Resolution q steps- dq/q is known from the contributions of the selector (dlambda/lambda) and the angular resolution (dtheta/theta). Resolution steps means that the gap between 2 q points in the scan is dq.

Why? Whatís the point in taking more points in q that can be resolved by the instrument? Scan production programs do allow the step to be some multiple of dq.

But: At low q the step size may be smaller than the precision of some motors

Or:

Constant q steps- dq is a constant for the whole scan.

Why? There may regions of interest such as the critical edge, off specular scattering or bragg peaks where many points may be required even if they are superficial in terms of reflectivity resolution.

But: If there are no parts of the scan of special interest it is an enormous waste of time and neutrons.

6) Use the program SCAINMAIN.TK to create a scan command file

Simply type scanmain.tk at the prompt in a UNIX shell. The program allows you to make all the choices for that type of scan you require and FTP it to the instrument. From MAD it only remains to type

MAD> start NAME <CR> where name is the NAME.CMD file created by scanmain.tk

Useful programs

On the instrument control computer, Just type the name to start in lamp or idl

tof

Analyses TOF data to create R(q) with the background removed

spondle

To retreave a R(q) data file and read it into a workspace

spondle,'stuff.dat',w1,x1,e1

reads the file stuff.dat into workspace 1

tth

calculates 2 theta for a direct stored beam and reflection run

tth,numor1,numor2

where numor1 id the db run and numor2 is the reflection run. The reults is seen in the blue idl window outside lamp.

The following programs can be accessed from the Silicon graphic terminal by typing the program name at a UNIX shell prompt.

slitwit

Calculates the angular resolution and illumination of the sample given the instrument geometry, sample dimensions and reflection angle.

scanmain.tk

Derived from the old FORTRAN scanscam program to create a scan command file. Also creates a direct beam calibration file if desired for a variable slit scan. The resulting files are ftpíed to the instrument control computer at the push of a button.

slab_fit

Maximum entropy program for finding the scattering length density profile

from the reflectivity data with the minimum of assumptions. From D. Shiva RAL.

slab_refine

Refinement of a starting guess of the scattering length density profile from slab_fit or the user. From D. Shiva RAL.

INSTRUMENT VARIABLES

Scanning a motor

MAD > scan trs n1 step co n2 m a n3

where trs is the motor (this is the sample translation motor. N1 is the motor starting value, step is the coder step, co sets a count at each value for a preset of n2. M can be m or t for monitor or time presets, a is alignment mode and n3 is the total number of steps in the scan. With the ëaí then no data is stored but a listing of the detector counts in a box defined by par scan for each step is shown plus the coder value of any peak found is given. If the ëaí is omitted then each step is stored as a normal run file.

MAD will plot the total counts over all time bins and all detector pixels. Make sure the direct beam is not on the detector if you are scanning a reflection.

Input of identification parameters

Enter the parameter level.