Add primary photon fluence at distance R from point source

src/pg_rad/physics/fluence.py

@@ -0,0 +1,37 @@
+import numpy as np
+
+
+def phi_single_source(
+        r: float,
+        activity: float | int,
+        branching_ratio: float,
+        mu_mass_air: float,
+        air_density: float,
+        ) -> float:
+    """Compute the contribution of a single point source to the
+    primary photon fluence rate phi at position (x,y,z).
+
+    Args:
+        r (float): [m] Distance to the point source.
+        activity (float | int): [Bq] Activity of the point source.
+        branching_ratio (float): Branching ratio for the photon energy E_gamma.
+        mu_mass_air (float): [cm^2/g] Mass attenuation coefficient for air.
+        air_density (float): [kg/m^3] Air density.
+
+    Returns:
+        phi (float): [s^-1 m^-2] Primary photon fluence rate at distance r from
+        a point source.
+    """
+
+    # Linear photon attenuation coefficient in m^-1.
+    mu_mass_air *= 0.1
+    mu_air = 0.1 * mu_mass_air * air_density
+
+    phi = (
+        activity
+        * branching_ratio
+        * np.exp(-mu_air * r)
+        / (4 * np.pi * r**2)
+    )
+
+    return phi