Detector performance often depends on external parameters, such as electrical noise or unwanted secondary characteristics. The Advanced Technologies Service possesses the expertise to design measurements or experiments to characterize these effects, modeling them and providing methods to understand their dynamics and, where possible, mitigate their impact.
Designing the HERMES satellite experiment required studying the luminescence (afterglow) produced in a GAGG:Ce scintillator crystal exposed to ionizing radiation. It is an X-ray and gamma-ray detector in which each scintillator is coupled to two SDD cells positioned in front of the crystal relative to the direction of the incoming photons. Discrimination between the two types of photons is based on whether or not there is a coincidence between the signals from the two cells: gamma-ray photons interact within the scintillator, illuminating both cells, whereas X-ray photons are absorbed by an individual SDD sensor.
In addition to the radiation of interest, the scintillators are also exposed to charged particles trapped in the Van Allen belts; consequently, the luminescence of the GAGG:Ce crystal is modulated over time based on the satellite’s orbit. Regarding photon measurement, the crystal’s luminescence acts as an additional component of the SDD sensor’s dark current, degrading energy resolution in proportion to its intensity. To evaluate this phenomenon, tests were conducted by exposing a sample crystal to a proton beam of varying intensities at the TIFPA cyclotron in Trento.
The figure shows a comparison between the luminescence measured during the test and the response of a semi-empirical model of traps present in the crystal. This model enabled the estimation of the in-orbit evolution of the afterglow contribution to the SDD cell current, demonstrating that the energy resolution requirement would be met.
