Skip to content

Photon 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)

calculate_flux returns flux per source particle, with inverse-square-law spreading over the total distance and an optional build-up factor. Apply an absolute source strength via result.scale(strength=...); the unit of the result follows the unit of the strength you supply (particles/sec gives /s, particles/shot gives /shot, etc.).

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="photon", energy=662e3)
result = rpk.calculate_flux(layers=layers, source=source).scale(strength=1e12)
print(f"Flux: {result.flux} photons/cm2/s")
print(f"Transmission: {result.transmission_fraction}")
print(f"Optical thickness: {result.optical_thickness}")
print(f"Build-up factor: {result.buildup_factor}")
print(f"Distance: {result.total_distance_cm} cm")

Flux: 252.00537358450575 photons/cm2/s
Transmission: 0.0032304454589905237
Optical thickness: 5.735135238256533
Build-up factor: 1.0
Distance: 1010.0 cm

The returned CalcResult object has these properties:

  • flux - flux at the outer surface; equals geometry * transmission * B (per source particle until you call .scale)
  • transmission_fraction - exp(-Sigma*t)
  • optical_thickness - Sum(Sigma_r,i * t_i)
  • buildup_factor - B (1.0 if no build-up model given)
  • total_distance_cm - total distance from source to detector
  • source_strength - the strength the result has been scaled by (1.0 = unscaled)

With a manual build-up factor

If you already have a build-up factor from tabulated data or a prior run, pass it via BuildupModel.constant. See Calculate build-up with MC for how to compute B yourself.

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="photon", energy=662e3)

B_flux = 2.0
result = rpk.calculate_flux(
    layers=layers,
    source=source,
    buildup=rpk.BuildupModel.constant(B_flux),
).scale(strength=1e12)
print(f"Flux (B={B_flux}): {result.flux} photons/cm2/s")
print(f"Applied build-up:  {result.buildup_factor}")

Flux (B=2.0): 504.0107471690115 photons/cm2/s
Applied build-up:  2.0