Custom AmBe neutron source implementation

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Hello everyone,

I am trying to reproduce a customized AmBe neutron source in OpenMC that I previously implemented in MCNP and FLUKA, and I would like to confirm whether my approach is correct.

In MCNP, the source is defined as:

SDEF par=n erg=d1 rad=d2 pos=-15 0 0 ext=d3 axs=1 0 0 vec=1 0 0 dir=d4

where:

  • d1 is the energy distribution
  • d2 and d3 define a cylindrical spatial distribution (radius and axial extent)
  • d4 is the angular distribution (anisotropic along the x-axis, roughly parabolic with a maximum around 90°)

In FLUKA, I implemented the same source via a custom routine, sampling:

  • energy from a tabulated spectrum,
  • position uniformly in a cylinder aligned with the x-axis,
  • direction with a non-isotropic distribution based on cos(θ) with respect to the x-axis.

I translated this into OpenMC as follows:

# Energy distribution

AmBeSource = openmc.IndependentSource()
lstEnergy = np.array([...])        # energy in eV
lstProbability = np.array([...])   # associated probabilities
AmBeSource.energy = openmc.stats.Tabular(lstEnergy,lstProbability,'histogram')

# Cylindrical spatial distribution (aligned along x-axis)

r = openmc.stats.PowerLaw(0.0, r1, 2.0)
phi = openmc.stats.Uniform(0.0, 2*np.pi)
z = openmc.stats.Uniform(xmin, xmax)
AmBeSource.space = openmc.stats.CylindricalIndependent(r=r,phi=phi,z=z,origin=(-15.0, 0.0, 0.0))
AmBeSource.space.reference_uvw = (1.0, 0.0, 0.0)  # cylinder axis along x

# Angular distribution

mu_vals = np.array([...])
p_vals = np.array([...])
mu_dist = openmc.stats.Tabular(mu_vals,p_vals,interpolation='linear-linear')
phi_dist = openmc.stats.Uniform(0.0, 2*np.pi)
AmBeSource.angle = openmc.stats.PolarAzimuthal(mu=mu_dist,phi=phi_dist,reference_uvw=(1.0, 0.0, 0.0))
AmBeSource.particle = 'neutron'

# Settings

settings = openmc.Settings()
settings.run_mode = 'fixed source'
settings.source = AmBeSource
settings.export_to_xml()

Does this implementation correctly reproduce the combined energy, spatial, and anisotropic angular distributions of the original MCNP/FLUKA source?

Any feedback or suggestions would be greatly appreciated.

Thank you in advance!

Giorgia