Things that will catch you out

Things that will catch you out

phi is not the scattering angle. It is 180° minus it. A detector at 170° means phi = 10. Use geometry.scattering_angle for the physical value.

Fitting windows must cover channels where the model has counts. Poisson likelihood is undefined where the model predicts zero, so those channels contribute neither to χ² nor to the gradient. A window reaching past the spectrum silently throws away most of its own evidence, and a parameter can sit motionless while the fit reports success:

warning: 81 of 81 windowed channels had zero predicted counts
  thickness[0]                     995.54     ← did not move

pyRUMP reports the count and warns; RUMP's manual mentions it in one line. Narrow the window and refit.

Thickness is areal density, 10¹⁵ atoms/cm². Converting to nanometres needs an assumed atomic density.

Straggling is off by default, matching RUMP. Set straggle=1.0 for the Bohr value. Note the in/out combination is an approximation and there is no Chu correction.

Absolute yields depend on charge, solid angle and efficiency, which are rarely known to better than a few percent. Use a normalisation window rather than trusting them — the worked example above is 9× out for exactly this reason.

Neighbouring heavy elements are not separable. In and Sn differ by 0.6 channels at 3 MeV; fit their ratio, not each independently.

Build the stopping registry once. It refits polynomials per beam energy, so recreating it inside a fit loop is slow for no reason.