A Nano-Scale Monte-Carlo Comparison of a Gadolinium Nanoparticle Under Thermal Neutron and Photon Irradiation
Abstract
Purpose
To quantify the dose deposition structure and production of secondary radiation caused by neutron capture on a gadolinium nanoparticle and evaluate the comparison to that caused by an incident photon beam.
Methods
Geant4 was used to perform Monte Carlo simulations of both thermal neutron and 100 keV photon beams incident on a 1 mm spherical water volume, with and without a 25 nm radius gadolinium nanoparticle (GdNP) placed in the center. Soft tissue was used in place of water for a neutron/no nanoparticle case to account for the correct nuclear secondaries. Gean4’s DNA physics is used to model electromagnetic interactions at the track structure level in water while G4EmLivermorePhysics was used in the nanoparticle volume and soft tissue. DICEBOX – a gamma-ray de-excitation simulation tool – is used in place of Geant4 photoevaporation libraries for gadolinium secondaries.
Results
Preliminary simulations of the dose deposited per primary particle and within a few micrometers of GdNP show an approximately order of magnitude increase in case of incident thermal neutrons when compared to incident photons. The secondary particle creation with the energy below 100 keV and close vicinity of GdNP is enhanced by a factor of ~50. In simulations with no GdNP, the absorbed dose per primary particle is significantly lower for incident neutrons compared to incident photon.
Conclusion
Gadolinium neutron capture shows a high propensity for local dose increase, as well as a significant increase in low energy, high-LET secondary radiation. These both increase lethality of nanoparticles within cancerous cells and allow for an overall reduction of neutron fluence providing an avenue for mitigating secondary gamma dose. Future research aims at quantifying the impact of this gamma flux in macroscopic volumes.