Development of new radiation hard materials with high secondary electron emission yield for the nuclear industry

Abstract

Photomultiplier tubes (PMTs) play a crucial role in various research fields, particularly in particle physics, where they serve as highly sensitive detectors for light. These experiments often occur in environments with extreme radiation levels, exposing PMT components to prolonged radiation damage that can degrade their performance over time. A critical element in the PMT structure is the dynode system, which facilitates signal amplification through the secondary electron yield (SEY) mechanism. SEY is fundamental to enhancing the initial photoelectron signal, making it essential to the detection process. Therefore, materials used for dynodes must not only exhibit a high SEY but also possess significant radiation hardness to ensure long-term stability and minimize frequent replacements. In this study, Aluminum Oxide (Al2O3) and Magnesium Oxide (MgO) were selected for evaluation based on their potential to meet these criteria. The materials were subjected to controlled gamma irradiation from a Co-60 source at varying doses. Theoretical Monte Carlo simulations were conducted to assess the radiation damage in terms of displacements per atom (DPA), while SEY measurements were performed post-irradiation to quantify the impact of the radiation exposure on the SEY of the Al2O3 and MgO samples. SEY measurements were conducted at the 6 MeV Tandem Accelerator at the iThemba Laboratory for Accelerator-Based Sciences (LABS) in Johannesburg. The existing microprobe setup was adapted to enable electronic measurements of SEY from MgO and Al2O3 samples across different radiation doses. These adaptations allowed for precise quantification of SEY variations as a function of absorbed gamma radiation dose. The results indicate a clear inverse relationship between radiation dose and SEY: as the radiation dose increases, SEY decreases. This degradation is attributed to ionizing events induced by gamma rays, leading to the formation of vacancies and interstitial defects within the material. These structural defects alter the electronic properties of the dynodes, ultimately reducing their amplification efficiency (SEY). This study provides valuable insights into the radiation tolerance of dynode materials and highlights the importance of selecting robust materials to enhance the longevity and reliability of PMTs in radiation harsh environments.