Room: AAPM ePoster Library
Purpose: To understand the impact of neutrons and their secondaries on the performance of a pre-clinical prompt gamma (PG) Compton camera (CC).
Methods: A Monte Carlo (MC) model of the CC imaging system (consisting of two detection stages containing a total of 64 Cadmium Zinc Telluride (CZT) crystals) was constructed using Geant4 (v10.p03). With the MC model, simulations of a 175 MeV proton pencil beam incident on a 30 x 10 x 10 cm high-density poly-ethylene phantom were performed. The MC model tracked all secondary particles interacting with the CZT crystals, specifically any particle with an energy loss greater than zero. For each interaction in the CC, the particle type, position, energy deposition, time stamp was stored in a ROOT output file. The reaction history of each interacting particle was also stored in order to determine whether it was produced by a neutron interaction. The resulting interactions were split into non-neutron secondaries (NNS) and neutron secondaries (NS), and their impact on the energy deposition, timing and PG scatters recorded by the CC were investigated.
Results: The majority of the interactions in the CC were gammas (82%), 73% of which were NNS while 28% were NS events. Neutron interactions events produced 99% of the ions and 77% of the secondary protons. More PG scatters (23%) are produced for NNS than NS (15%). NNS interactions had an average energy deposition of 0.86 MeV compared to 0.77 MeV per NS interaction, although NS protons deposited an average of 21.5 MeV. For prompt (< 20 ns) particle emission, a 2.5 ns cutoff will eliminate 32% of the NS particles.
Conclusion: Secondaries produced from neutron interactions in the CC have a negative impact on data and image quality. Further development of methods to separate these neutron secondaries from PG interactions in the CC is needed.