Neutron transmission

Three levels of calculation, each building on the previous:

Function	What it computes	Considers geometry?
calculate_transmission	Material attenuation only (exp(-Sigma*t))	No
calculate_flux	S / (4piR^2) * exp(-Sigma*t) * B	Yes (inverse square law)
calculate_dose	Flux * ICRP-116 dose coefficient	Yes (inverse square law)

calculate_transmission computes the fraction of uncollided neutrons that pass through a set of layers. Like the photon version, it returns a float between 0 and 1 representing exp(-Sum(Sigma_r,i * t_i)).

Single layer example

Calculate neutron transmission through 10 cm of iron at 14.1 MeV (D-T fusion):

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)
frac = rpk.calculate_transmission(layers=layers, source=source)
print(f"Transmission: {frac}")

Transmission: 0.2857238342261427

Comparing shield thicknesses

import rad_point_kernel as rpk

iron = rpk.Material(composition={"Fe": 1.0}, density=7.874)
source = rpk.Source(particle="neutron", energy=14.1e6)

for thickness in [5, 10, 20, 50]:
    layers = [rpk.Layer(thickness=thickness, material=iron)]
    frac = rpk.calculate_transmission(layers=layers, source=source)
    print(f"{thickness:>3d} cm iron: {frac}")

  5 cm iron: 0.5345314155652058
 10 cm iron: 0.2857238342261427
 20 cm iron: 0.0816381094448883
 50 cm iron: 0.001904286756949739

Notes

• Neutron removal cross sections come from ENDF/B-VIII.0.
• The calculation uses the removal cross-section approximation, which is most accurate for fast neutrons and deep penetration problems.
• For spectra with multiple energy lines, see Source spectra.