Updating wavelength in PowderDiffraction

[sbanerjee23]

I'm trying to calculate a powder diffraction pattern, but am unable to update the wavelength in the global dictionary. I am able to update other keys, and add a wavelength key and pass it to fpsettings, but this has no effect on the output pattern. This script is based off the xrayutilities_experiment_Powder_example_Iron example and I'm currently running it inside a jupyter notebook. I tried some alternatives: running this as an executable in a directory alongside a xrayutilities.conf file, and also passing a wl=#.## argument directly to PowderDiffraction() but am not having any luck. Any advice?

%matplotlib inline

import matplotlib.pyplot as plt
import numpy as np
import xrayutilities as xu
from xrayutilities.materials.cif import CIFFile
from xrayutilities.materials.material import Crystal

ni_cif = CIFFile('./stru/Ni.cif')
ni_crystal = Crystal(name="Ni", lat=ni_cif.SGLattice())

#X-ray wavelength is 0.1866 Angstroms
settings = {'global': {'wavelength': 0.1866e-10,
                      'dominant_wavelength': 0.1866e-10,
                      'diffractometer_radius': 0.010101}}

ni_powder = xu.simpack.Powder(ni_crystal, 1)
pd = xu.simpack.PowderDiffraction(ni_powder, fpsettings=settings)

tt = np.arange(1, 200, 0.01)
inte = pd.Calculate(tt)

print(pd)

fig, ax1 = plt.subplots(1, 1, figsize=(8,8))

ax1.plot(tt, inte)
ax1.set_xlim(0,200)

Here is a partial output of the printed settings dictionary:

Settings: {'absorption': {'absorption_coefficient': 100000.0}, 
'global': {'wavelength': 1.866e-11, 
'dominant_wavelength': 1.5405929759615019e-10,
'equatorial_divergence_deg': 0.5,
'diffractometer_radius': 0.010101}}

[dkriegner]

unfortunately the setting of wavelength is currently a bit complicated and not very well documented

if you use a settings variable as shown below it should work.

settings = {'global': {'diffractometer_radius': 0.50101},
            'emission': {'emiss_wavelengths': (0.1866e-10,),
                         'emiss_intensities': (1.0,),
                         'emiss_gauss_widths': (3e-14,),
                         'emiss_lor_widths': (3e-14,), 
                        }}

I will think about how to make this more intuitive and flexible. Note that all these settings at the moment must be of type tuple.
also check:

• your diffractometer radius -> its very small in your example!
• two theta above 180 might cause troubles.

[dkriegner]

@sbanerjee23 please let me know what you think of the latest changes.

it is now possible to set the wavelength/energy for PowderDiffraction in the following ways

1. during the class construction in one of the following two ways:

pd = xu.simpack.PowderDiffraction(Fe_powder, wl=1.1) # unit Angstrom
pd = xu.simpack.PowderDiffraction(Fe_powder, en=10000) # unit eV

or

settings = {'emission': {'emiss_wavelengths': 0.1866e-10}} # unit meter
pd = xu.simpack.PowderDiffraction(Fe_powder, fpsettings=settings)

2. after class construction in one of the following two ways

pd = xu.simpack.PowderDiffraction(Fe_powder)
pd.wavelength = 1.1 # unit Angstrom
pd.energy = 10000 # unit eV

or

pd = xu.simpack.PowderDiffraction(Fe_powder)
settings = {'emission': {'emiss_wavelengths': 0.1866e-10}} # unit meter
pd.update_settings(settings)

Note that now also in emiss_wavelength one can just specify a float. What is still not possible is to use dominant_wavelength which I consider a "read-only" option.

The inconsistency of the units (meter vs. Angstrom) is a historic problem which I am aware of but currently would like to keep as it is.