Choosing an instrument

Let’s modify the simulation to show how a 100 Å nickel film might look if measured on the SNS Liquids reflectometer:

(Source code, png, hires.png, pdf)

This model is defined in nifilm-tof.py

The sample definition is the same:

from refl1d.names import *

nickel = Material("Ni")
sample = silicon(0, 5) | nickel(100, 5) | air

Instead of using a generic probe, we are using an instrument definition to control the simulation.

instrument = SNS.Liquids()
M = instrument.simulate(
    sample,
    T=[0.3, 0.7, 1.5, 3],
    slits=[0.06, 0.14, 0.3, 0.6],
    uncertainty=5,
)

The instrument line tells us to use the geometry of the SNS Liquids reflectometer, which includes information like the distance between the sample and the slits and the wavelength range. We then simulate measurements of the sample for several different angles T (degrees), each with its own slit opening slits (mm). The simulated measurement duration is such that the median relative error on the measurement \(\Delta R/R\) will match uncertainty (%). Because the intensity \(I(\lambda)\) varies so much for a time-of-flight measurement, the central points will be measured with much better precision, and the end points will be measured with lower precision. See Pulsed.simulate for details on all simulation parameters.

Finally, we bundle the simulated measurement as a fit problem which is used by the rest of the program.

problem = FitProblem(M)