Pulse-Shape Analysis for Solar Energetic Particle Discrimination around Mars

The SP@M (Solar Particles @ Mars) instrument, part of the M-MATISSE mission payload, is designed to measure solar energetic particles (electrons ranging from 30 keV up to 1 MeV and ions from 30 keV up to 10 MeV) using a compact detection strategy. Instead of conventional detector stacks, SP@M relies on a single 1.5 mm-thick silicon detector, where particle identification is achieved through pulse-shape analysis. This technique exploits the dependence of signal rise time on the depth of energy deposition and charge-collection dynamics. A dedicated modeling framework combining Monte Carlo simulations and charge-transport tools was first developed to pre-dimension the prototypes. It provided first-order predictions of rise time distributions for electrons, protons and alpha particles under over-depletion regime. Experimental characterizations were then performed with <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">133</sup>Ba and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">207</sup>Bi conversion electrons, and a tri-alpha source (<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">239</sup>Pu, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup>Am, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">244</sup>Cm), using controlled irradiation on both the junction and ohmic sides of the detector. The results show that the irradiation side has a critical impact: ohmic-side irradiation systematically yields longer and more dispersed rise times, enhancing species separation. For deposited energies above 600 keV, electrons and alpha particles are clearly discriminated, while below this threshold the overlap prevents reliable separation at 4.0 ns sampling. These investigations demonstrate the feasibility of PSA for charged-particle discrimination in thick silicon detectors and highlight both its potential and limitations.