Fine-tuning simulation of radioactive particulate self-attenuation

Geant4 is a Monte Carlo simulation which requires a set of probable outcomes to process radiation events. These sets are called “physics lists” and are necessary to figure out what particles will do as time goes on. As it turned out, the necessary alpha particle physics list was not properly defined in the previous simulation outcome. When this was adjusted, the range of alphas in the particulate shrunk to the order of a few micrometers, which can be seen in Figure 1 for a 20 micron particulate.

Figure 1: Screenshot of visualization of many alpha events in a 20 micron AmO2 particulate.

Simulations were rerun using 100 thousand events and looped through a larger set particle sizes. The physics list fix also created new results of alpha yield and average exiting energy (Figs. 2 and 3). The yield dropoff is far steeper, with 10% loss by 2.5 microns and 90% loss by 10 microns. The average exit energy also exhibits interesting behavior across particle sizes, centered around the alpha-range. Leading up to the alpha-range, the average energy sharply decreases- while nearly all are exiting at this point with the rare multi-scatterer, an increasing number are initially present in a more central area which alphas deposit energy into before exiting. At the particle size of the alpha-range onward, there is an increasing number of central alpha particles which cannot escape the sphere, even traveling directly outward. As this occurs, the average energy gradually shifts away from the contributions of the central decays and therefore increases towards the average energy provided by the outer “skin” of the particle (the outer radial region less the alpha-range from the edge).

Figure 2: Fractional exit of alpha particles emitted from Am-241 in AmO2 particulates.
Figure 3: Average exiting energy of 5.486 MeV alpha particles emitted from Am-241 in AmO2 particulates.