## 1 Introduction  
Solar flares are sudden and intense releases of energy originating in the Sun's Corona, caused by a rearrangement of the magnetic field lines via magnetic reconnection. These powerful eruptive events emit a broad spectrum of electromagnetic waves, including X-rays and ultraviolet light [1]. Along with electromagnetic radiation, solar flares also accelerate charged particles, such as protons and electrons, which can travel through space and reach the Earth's magnetosphere. Moreover, solar flares are also very often associated with the release of huge structures of magnetic field trapping charged particles that travel in interplanetary space and are known as coronal mass ejection (CME) [1; 2; 3; 4]. The impact of solar flares on human technology is significant [5; 6; 7]. While the Earth's magnetic field and atmosphere allow shielding from a lot of the harmful charged particles and electromagnetic radiation, the high-energy components can still cause relevant geomagnetic storms that disrupt satellite communications, interfere with power grids, and cause inaccuracies in GPS systems. The release of energy in the Earth's atmosphere can also induce its expansion, increasing the drag on satellites and leading to potential operational challenges and reduced lifespans for these spaceborne assets. Additionally, solar flares can pose a radiation hazard to astronauts in space and passengers on high-altitude flights near the poles. Understanding and predicting solar flares is crucial for mitigating their effects on our technological infrastructure [8]. By monitoring solar activity and studying the mechanisms behind these powerful phenomena, scientists aim to develop better predictive models and protective measures [9]. This proactive approach helps to safeguard critical systems, ensuring the reliability and resilience of our technology-dependent society. In particular, X-ray polarimeters are valuable instruments capable of determining the properties of the magnetic fields originating the solar eruptions and measuring also the directivity [10] and beaming properties of accelerated particles. The anisotropic distributions of electrons accelerated in ordered magnetic fields produce non-thermal emissions above 10-20 keV that are expected to be significantly polarized on the basis of various models [10; 11; 12; 13; 14; 15]. To this end, the CubeSat Solar Polarimeter (CUSP) project [16] aims to develop a constellation of two CubeSat satellites orbiting the Earth to significantly contribute to the current and future network of infrastructures useful for Space Weather monitoring activities such as the ASPIS (ASI SPace Weather InfraStructure) project of the Italian Space Agency (ASI) [17]. CUSP is a project funded by the Alcor program of ASI. Specifically, it will utilize Compton scattering polarimeters to measure the linear polarization of solar flares in the hard X-ray band (25-100 keV). The objective is to develop compact polarimeters, capable of flying on CubeSats, without optics, and achieving a minimum detectable polarization (MDP [18; 19]) of 10.2% for an M5.2 class flare [16]. To achieve this, the CUSP payload will consist of a dual-phase Compton scattering polarimeter to perform coincidence measurements between signals from plastic and inorganic scintillators. Specifically, the light emitted by the plastic scintillators will be read out by multianode photomultipliers (MAPMTs), and the scattered photons will be absorbed by Gd\({}_{3}\)Al\({}_{2}\)Ga\({}_{3}\)O\({}_{12}\) (GAGG) inorganic scintillators, with the resulting light signal read out by an array of 32 avalanche photodiodes (APDs). GAGG scintillators are chosen to maximize the photon absorption probability, thanks to their high effective atomic number [16], while platic scintillators are chosen to maximize scattering probability. Due to the very short implementation time required for this mission, well-known components, already used for this kind of application, have been selected.