Theoretical radiation exposure loss from radioactive particulate matter

While exploring the properties of radioactive particulates in the context of exposure to resuspended radionuclides, I could not determine whether the size of a radioparticulate played a role in the delivery of ionizing radition. Alpha-emitters are especially problematic in the environment as the alpha radiation delivers more of an effective dose to tissue compared to betas and gammas. The range of alpha particles in air is only a few centimeters, so surely it is less so within a solid particulate of a metal oxide such as AmO2 or UO2. I therefore put together a simulation using Geant4 Particle Transport Code to probe the flux of alpha radiation emitted from fine particulates at various respirable sizes.

Self-shielding of alpha flux

The alpha yield the AmO2 radioparticulate is presented Figure 1. At the lower end of fine particulate sizes, there is virtually no loss of alphas. Around 12-13 micrometers, the yield of alphas begin to drop by a few percent, then at the higher end the fractional loss becomes substantial.

Figure 1: Fractional exit of alpha particles emitted from Am-241 in AmO2 particulates.

Self-moderation of alpha flux

The alpha particles emitted from the particulate also lose kinetic energy as it collides with the structure of the particulate before leaving. This average exit energy for each particulate size is depicted in Figure 2. This loss appears to be linear the diameter of the particle. The uncertainty in this energy loss follows a heteroscedastic trend, wherein the standard deviation increases with particle size. This is likely due to the increasingly inescapable range of particulate which inner alphas must traverse in order to exit. At the lower limit of fine particulates, there is almost no loss in energy, but at the higher end the energy is more than halved.

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