Neutron flux

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	1 / (4piR^2) * exp(-Sigma*t) * B per source particle	Yes (inverse square law)
calculate_dose	Flux * ICRP-116 dose coefficient	Yes (inverse square law)

The same calculate_flux interface applies to neutrons; just create a neutron Source. The result is per source particle - apply an absolute strength via result.scale(strength=...). Pulsed neutron devices (e.g. an ICF burn) take a strength in neutrons per shot; steady-state generators take particles per second (or per hour, if you prefer the result in /hr).

Example

import rad_point_kernel as rpk

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

source = rpk.Source(particle="neutron", energy=14.1e6)
# Pulsed DT shot: strength in neutrons per shot, result in /shot
result = rpk.calculate_flux(layers=layers, source=source).scale(strength=1e16)
print(f"Flux: {result.flux} neutrons/cm2/shot")
print(f"Transmission: {result.transmission_fraction}")
print(f"Optical thickness: {result.optical_thickness}")
print(f"Distance: {result.total_distance_cm} cm")

Flux: 222891680.11106703 neutrons/cm2/shot
Transmission: 0.2857238342261427
Optical thickness: 1.2527295492622972
Distance: 1010.0 cm

The returned CalcResult has the same properties as in the photon case; see Flux (photon).

With a manual build-up factor

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_flux = 3.0
result = rpk.calculate_flux(
    layers=layers,
    source=source,
    buildup=rpk.BuildupModel.constant(B_flux),
).scale(strength=1e16)
print(f"Flux (B={B_flux}): {result.flux} neutrons/cm2/shot")
print(f"Applied build-up:  {result.buildup_factor}")

Flux (B=3.0): 668675040.333201 neutrons/cm2/shot
Applied build-up:  3.0

See Calculate build-up with MC for how to compute B from first principles.