Calibrating the TOF option

Determining D0

Determining DET

Determining the opening offset

Determining POFF

Log of TOF parameters

CALIBRATING THE TIME-OF-FLIGHT ON D17
R. Cubitt, A. R. Wildes and G. Fragneto
v. 2.2        August 25, 2009

NOTE:  There is a LAMP/GEORGE macro which will carry out this procedure automatically.  It requires that the standard Fe/Ti multilayer is mounted and aligned.  Then the calibration routine can be launched using the macro: d17calopen

NOTE:  Once you're happy that the TOF calibration has been correctly executed, be sure to:

• Edit 'init_para.cmd' in the /users/d17 directory.  This command file is run every time MAD starts, and it sets the relevant TOF parameters that are saved in all the MAD data files
• Edit the logfile at the end of this page
• Print out the new values and stick them up above the instrument computer.
  

Some useful constants:
        K = 3956 (for wavelength in Angstroms and speed in metres/sec)

•    Determining D0
D0 is the distance from the first chopper to the sample.  UNLESS THERE IS A SERIOUS INSTRUMENT REBUILD you can assume that this is a constant.  THIS REPRESENTS THE ONE THING YOU CAN ASSUME TO BE CONSTANT.

•    Determining DET
DET is the distance between the sample and the detector.
The correct distance for DET can be determined geometrically, INDEPENDENTLY of all other parameters (except D0).
1.    Put a Si substrate at the sample position, find a reflection in TOF mode.
2.    Open S3H so that you have plenty of fly-past, try to put the fly past around the middle of the detector.
3.    Select a part of the TOF spectrum which is NOT influenced by REFRACTION
4.    Bin the data to a two-dimensional data set, fit a Gaussian to each of the two peaks (the reflected peak and the fly-past peak) and note the pixel difference.
5.    Change DET and repeat ~5 times, covering the whole range of DET (1100 – 3400 mm). 
6.    Plot the pixel difference as a function of DET.  This should be a straight line.  DET = 0 should converge to the sample position.  Any DET intercept NOT equal to zero is an offset.  Subtract this offset by driving to a position that you know and resetting the value of DET using the command:        MAD>par set det
EXAMPLE:
    You find an offset = –10
-    Drive to 1990 (in reality, corresponds to 2000)
-    Enter the command:
MAD> par set det 2000
NOTE:  This method assumes that DET is driving correctly and reproducibly.  If DET is NOT driving correctly, the line of pixel difference vs. DET will NOT be straight!

•    Determining the opening offset
The opening offset is the nominal phase angle such that there is no direct line-of-sight between the first and second choppers.
1.    Put the Fe/Ti multilayer (large substrate, 100 repeats of (50/50) bilayers). At the sample position and find a reflection.
2.    Choose a small phase for the choppers.  Note the TOF parameters!
3.    Measure the time-of-flight position of the monochromator peak at a number of different positions of DET.
4.    Plot the time channel position of the peak as a function of DET (in metres).  Make sure that the time channels start counting from 0, not from 1.
5.    The gradient will give you the wavelength following the equation:

     gradient = λ / (channel width * K)

λ in Angstroms
channel width must be in seconds!
6.    Calculate the opening for which dt is zero.
open(dt=0) = – (λ * cht * 360) / (K * chopper period)
    cht is the distance between the choppers (in metres), typically ~0.087m. 
chopper period in seconds (1000 = 0.060 s).
7.    Fix DET, change the opening using the command:
MAD> chop speed 1000 open x
and measure the intensity as a function of opening.  This should follow a straight line, the x-intercept is equivalent to having dt=0  for the given wavelength.
8.    The opening for having dt=0 is then given by the equation:
opening offset = xintercept + (λ * cht * 360) / (K * chopper period)
EXAMPLE:
    Wavelength determined to be 5.078 Å
    cht taken to be 0.087 m
    chopper period = 0.06 s
    No intensity seen for chopper opening at xintercept = 0.417
    Opening offset = 0.417 + (5.078*0.087*360)/(3956*0.06)  = 1.087 (example by RC, 25.02.04)
   
NOTE ADDED: AW  25.08.09.  MAD has been changed a little so that the requested chopper opening accounts for the open offset.  The value above should be added to the old opening offset to get the new opening offset.

•    Determining POFF
POFF a parameter attributed to the first chopper disk. 

The beam is defined by the trailing edge of the first chopper and the leading edge of the second chopper.

The first chopper holds a magnet which is radially directly below the centre of the opening.  The chopper housing holds a pickup.  The instrument clock is set from when the magnet passes the pickup.

Because the clock is set from the first chopper, the trailing edge of the this disk can be referred to as the beam defining edge.

When the magnet and the pickup are aligned, POFF is defined as being twice the angle between the beam defining edge and the 'chopper open' position. 

The angle between the magnet and the beam defining edge is 180-22.5=157.5 degrees.  Depending on where the pickup is positioned, there will be an offset.  As an example, if POFF = 285, the angle that the beam defining must rotate from when the pickup detects the magnet = 285/2 = 142.5 degrees. This means than the pickup must be 157.5-142.5 = 15 degrees from the vertical in the anti-rotation direction.

POFF can be derived using an equation using the DET vs. Channel number (see Opening Offset  above). Each measurement will give a value for POFF, and the scatter in the values will give a statistical uncertainty for POFF.
Use the following equation for each measurement:

POFF = (channel number + n electronic delay + 0.5 - λ * [D0 + DET]/[K * channel width])

           * (2 * channel width * 360) / (chopper period)

           + opening requested - open offset
Note:
a)    n electronic delay is not a time but a number of channels (e.g. in TOF parameters, delay = 23300 µs and time per bin = 57 µs, then electronic delay = 23300/57 = 408.77)
b)    Make sure that the channel number has been derived from data where the minimum time channel is zero!  If this is not done there will be a systematic error in the calculation of POFF!

Alternatively, a trial-and-error can be used:
1.    Assume that the method for calibrating the opening offset has been correctly done.  The only free variable for a correct wavelength calibration is now POFF
2.    Measure the reflectivity from the multilayer at a series of incoming angles SAN.  Measure also the main beam.
3.    Start COSMOS and run the data reduction for the multilayer reflectivity.  Change the machine parameters to the correct values for the opening offset.  Change the value of POFF in the machine parameters until the Bragg peaks for the multilayer match.

A second method which is prone to error is:
1.    Follow the procedure for setting the opening offset.
2.    Choose to phase the choppers with the opening offset
3.    Measure the position of the peak as a function of different chopper speeds, same opening offset.  Plot the result as a function of chopper period (in seconds).
      This should follow a straight line, and POFF is given by the gradient according to the equation:
 

0.5 * POFF = 360 * channel width * gradient

    NOTE: the chopper shouldn’t be run faster than 1000rpm as you’re not allowed to.  Try to run as slowly as you can (e.g. 800rpm), although be aware that the choppers might have difficulty phasing.
THIS METHOD IS PRONE TO ERRORS!  Ideally, you would have a range of points over the whole range of chopper speeds down to zero.  As it happens, you end up with points all bunched up around 900 rpm and the gradient is very prone to error!

At this point, all parameters should correlate and you should be able to reproduce D0 with the equation:

D0 = K/λ * (channel width * opening offset - electronic delay + (chopper open * chopper period)/360)

You should also be able to take any time-of-flight spectra for the monochromator and calculate the correct wavelength based on the parameters that you’ve calculated.

Please update this table after every TOF calibration!