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Coupled neutron and photon dose

A fast neutron source inside a shield generates secondary photons through inelastic scatter and neutron capture. For a complete dose estimate you need both the primary neutron dose and the secondary photon dose. There are two ways to get this:

  • Coupled Monte Carlo via compute_buildup with a -coupled-photon quantity - the most accurate option.
  • Analytical PK estimate via calculate_secondary_photon_dose_rate - derives the secondary-gamma source per layer from neutron fluence and capture cross-sections, then transports the gammas to the detector. Faster and no MC required, but less accurate.

Coupled MC

import rad_point_kernel as rpk

iron = rpk.Material(composition={"Fe": 1.0}, density=7.874)
layers = [rpk.Layer(thickness=10, material=iron)]

source = rpk.Source(particle="neutron", energy=14.1e6)
results = rpk.compute_buildup(
    geometries=[layers],
    source=source,
    quantities=["dose-AP", "dose-AP-coupled-photon"],
)

r = results[0]
S = 1e12
neutron = r.mc["dose-AP"] * S
gamma = r.mc["dose-AP-coupled-photon"] * S
print(f"Neutron dose:    {neutron} Sv/hr")
print(f"Secondary gamma: {gamma} Sv/hr")
print(f"Total:           {neutron + gamma} Sv/hr")

Neutron dose:    1049.6225549402122 Sv/hr
Secondary gamma: 9.497871028800132 Sv/hr
Total:           1059.1204259690123 Sv/hr

With a manual build-up factor

calculate_secondary_photon_dose_rate accepts neutron_buildup as a BuildupModel. If you already have a neutron build-up factor from tabulated data or a prior run, pass it in:

import rad_point_kernel as rpk

iron = rpk.Material(composition={"Fe": 1.0}, density=7.874)
layers = [
    rpk.Layer(thickness=1000),
    rpk.Layer(thickness=10, material=iron),
]

source = rpk.Source(particle="neutron", energy=14.1e6)

B_neutron = 2.5
result = rpk.calculate_secondary_photon_dose_rate(
    source_strength=1e12,
    layers=layers,
    source=source,
    geometry="AP",
    neutron_buildup=rpk.BuildupModel.constant(B_neutron),
)
print(f"Neutron dose (B={B_neutron}): {result.neutron_dose_rate} Sv/hr")
print(f"Secondary gamma dose:       {result.secondary_photon_dose_rate} Sv/hr")
print(f"Total dose:                 {result.total_dose_rate} Sv/hr")

Neutron dose (B=2.5): 0.09925677904954962 Sv/hr
Secondary gamma dose:       2.022015119830285 Sv/hr
Total dose:                 2.1212718988798347 Sv/hr

Why the manual B is neutron-only

The secondary-gamma pathway is a derived quantity: neutron fluence reaches each material layer, capture and inelastic-scatter cross-sections determine how many secondary photons are born there, and the gammas are transported to the detector analytically. A user-supplied build-up factor only makes sense on the neutron leg - that is where "some of the scattered neutrons also reach this layer" is a first-class thing you tabulate. The secondary-photon leg has no analogous user-facing B input; its attenuation and geometry are built into the capture-gamma model itself. If you need to correct the secondary-photon side independently, prefer the Coupled MC path instead.

Notes

  • Coupled transport only makes sense for a neutron source; photon sources do not produce secondary neutrons in this tool.
  • dose-AP-coupled-photon is available alongside any irradiation geometry suffix: dose-PA-coupled-photon, dose-ISO-coupled-photon, etc.

Next: extracting build-up factors from MC

The compute_buildup call above tallies neutron and secondary-photon doses for one specific geometry. The same machinery can produce reusable build-up factors across a range of thicknesses, energies, or layer stacks - numbers you can plug back into calculate_flux or calculate_dose as BuildupModel.constant(B) on later runs without re-simulating. Calculate build-up with MC walks through that workflow.