Unclear quantitative scale factor in mass_fractal model

[gnsmith]

Describe the bug
I was not sure if this was a SasView or a SasModels issue.

I was attempting a quantitative calculation of the scattering intensity from the mass_fractal model, which parameterizes the scale in terms of an arbitrary scale_factor, a sphere volume, and a Delta SLD. The description says that it is unitless, but according to the definition it should have the units 1/cm^-1 (to give an absolute scale). I looked into the sasmodels code in SasView but could not find anything that helped guide me to defining the units.

Using reasonable volume fractions, volumes, and SLD differences, I get a scale of 7e+5 1/cm, but this gives a calculated forward scattering intensity of like 10^12 1/cm, which seems unrealistic.

To Reproduce

1. Choose the model Shape Independent/mass_fractal
2. Change the default parameters to a scale of 7e5, a radius of 2000, a fractal dimension of 2, and a cutoff of 10000.
3. Compute and note the scattering intensity.

Expected behavior
Most SasView models implement SLDs separately, calculate the volume during fitting, and then the scale is the volume fraction. This model defines the scale differently, but says it is a unitless parameter. I would expect that my calculated scale should give a reasonable scattering intensity. I am doing this to simulate scattering, not fit it, so getting the quantitative scale correct is important.

Screenshots

SasView version (please complete the following information):

• Version: 6.1.1

Operating system (please complete the following information):

• OS: Windows 11

Installation method (please complete the following information):

• Installed: Windows installer

Additional context
Add any other context about the problem here.

[smk78]

Looking at mass_fractal.py I do note the comment: