Eval15 Determine Beam Parameters

The description of the properties of the primary beam are controlled by various numbers. Important ones are:

These describe the divergence and the size of the beam. It is not possible to measure these parameters directly. The shape and size of reflections on the detector are the result of a convolution of beam, wavelength, crystal and detector properties. The crystal effects can be eliminated by recording the primary beam with appropriate generator settings and the use of an attenuator.

The following procedure can be used to determine an optimal value for focus dist and collimatordiameter.

Firstly, the width of the primary is determined with a recording of the (attenuated) diract beam on the detector. With a simulation in eval15, the effect of the pointspread on the width of the beam can be determined and corrected for. By collecting the width of the beam at various detector distances the divergence of the beam can be calculated.

Then, with eval15, the simulated primary beam can be analyzed in a similar way at various detector distances. By comparing the experimental widths with the simulated ones optimal values for focus dist and collimatordiameter can be determined.

Determine the effect of the pointspread
Collect some images of the direct (attenuated) beam at different detector distances
Determine the width of the beam at each detector distance
Calculate the beam divergence
optimize focus dist
optimize collimatordiameter

Pointspread

The effect of the pointspread on the width of a reflection can be monitored with eval15. The only thing needed is a boxfile with reflections. The projection windows will be used. These 3 windows show, by default, the projections of the observed slices (Hor-Rot, Hor-Ver, Rot-Ver). The content of these projections may be changed into any of the slices.

Create the script pntspr.pic
```
draw profilehor on
zoom proj hor-ver projection
\alias ps projection simulated go
\alias pp projection pointspread go
```
After executing this script, a horizontal profile will be drawn in the projection windows (in orange). A gaussian will be fitted to these profiles (in brown). The aliases ps and pp can be used to show the simulated or pointspread projections. In the terminal window, the fitted parameters will be shown. FWHM (Full Width at Half Maximum) is the interesting one.
eval15 @pntspr
file n m
select a boxfile and browse until a suitable reflection is found. A suitable reflection will have (almost) no alpha1-alpha2 split and will be not too weak and not too strong.
ps
Record the FHWM of the simulated beam.
pp
Record the FHWM of the pointspread beam. The difference in FHWM of the simulated and pointspread profiles is the extend of the profile width due to the pointspread.

This all assumes that pointspreadgamma is set to an acceptible value. You may want to change this parameter and execute ps and pp to monitor the influence.

Collect

set the generator
install an attenuator
mount the collimator
set detector distance to 40 mm
collect an image for 5 seconds
repeat this for various (60, 80, 100, 120) detector distances
repeat all for different collimators.

Observed width

Create the script beam.vic
```
profilefit on
noglobalmax
plot
x2m 1 0 0
zoomfactor 10
zoomaroundmm hormm vermm
profile hormm-3 vermm hormm+3 vermm
!profile hormm vermm-3 hormm vermm+3
```
x2m 1 0 0 calculates where the laboratory vector (1,0,0) hits the detector. The resulting coordinate is saved in hormm and vermm, and these are used in the succeeding commands (for more information, see \evaluate).
view
read beam001
read the first image.
@beam
draw the profile and fit the gaussian. Note the FWHM.
read beam002
read the next image (or use next or nextscan)
@beam
Draw and fit the profile
repeat these last 2 steps for all images.

Divergence

Subtract the pointspread contribution from all observed FWHM's. For each collimator, make a graph of the corrected FWHM versus detector distance. A line can be fitted through these points, and the beamsize at distance zero can be extrapolated.

Focus dist

Now we will use eval15 to determine focus dist and collimator diameter. A boxfile is needed to set all experimental variables. The observed data from the boxfile will not be used.

eval15
file m n
Select any reflection.
sampleplot beam on
Draw the beam profile in the detector window
xtalscale 5 The crystal should be larger than the collimator
zoom det
Optionally zoom the detector window, the window where the beam profile will be drawn.
beampos 0
Examine the beam profile at the position of the crystal
beamgridsize
set the number of horizontal and vertical pixels in the beam profile window
beampixelsize
set the size (in mm) of each beam profile pixel.
go
One simulation. Modify beamgridsize and beampixelsize until the beam profile fills 25% of the window.
draw beamprofilehor on
Fitt a gaussian to the simulated beam profile
go
Start simulation, fitt the gaussian, and note FWHM
beampos -40
Examine the beam profile 40 mm behind the crystal. Note the minus sign.
go
Start simulation, fitt the gaussian, and note FWHM
repeat this for the various detector distances.
fdist 40
Change focus distance

Collimator diameter

Eval15
Eval15 commands