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Prototype Development and Calibration of the CUbesat Solar Polarimeter (CUSP)

Nicolas De Angelis, Abhay Kumar, Sergio Fabiani, Ettore Del Monte, Enrico Costa, Giovanni Lombardi, Paolo Soffitta, Andrea Alimenti, Riccardo Campana, Mauro Centrone, Giovanni De Cesare, Sergio Di Cosimo, Giuseppe Di Persio, Alessandro Lacerenza, Pasqualino Loffredo, Gabriele Minervini, Fabio Muleri, Paolo Romano, Alda Rubini, Emanuele Scalise, Enrico Silva, Davide Albanesi, Ilaria Baffo, Daniele Brienza, Valerio Campamaggiore, Giovanni Cucinella, Andrea Curatolo, Giulia de Iulis, Andrea Del Re, Vito Di Bari, Simone Di Filippo, Immacolata Donnarumma, Pierluigi Fanelli, Nicolas Gagliardi, Paolo Leonetti, Matteo Merge, Dario Modenini, Andrea Negri, Daniele Pecorella, Massimo Perelli, Alice Ponti, Francesca Sbop, Paolo Tortora, Alessandro Turchi, Valerio Vagelli, Emanuele Zaccagnino, Alessandro Zambardi, Costantino Zazza

TL;DR

The paper addresses the need to measure the polarization of solar flare hard X-rays in the band $25$–$100$ keV to constrain magnetic reconnection and particle acceleration. It presents the CUSP concept, a dual-phase Compton polarimeter on a 6U-XL CubeSat, combining a central plastic-scintillator scatterer array read by MAPMTs with a surrounding GAGG absorber layer read by APDs, both managed by MAROC‑3A and SKIROC‑2A ASICs. Phase B laboratory tests using $^{55}$Fe, $^{241}$Am, and $^{109}$Cd sources demonstrate energy calibration capability and linear response from scattering and absorption channels, with deposition energies in the target range and APD gains around $100$. The results validate the polarimeter design and establish a path toward a larger 16-scatterer by 32-absorber prototype and a flight-model, with planned calibration at accelerator beamlines or synchrotron facilities to finalize performance. The work lays the foundation for time-resolved solar flare polarimetry that can illuminate flare geometry and particle acceleration processes relevant to space weather forecasting.

Abstract

The space-based CUbesat Solar Polarimeter (CUSP) mission aims to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP will allow to study the magnetic reconnection and particle acceleration in the flaring magnetic structures of our star with its unprecedented sensitivity to solar flare polarization. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed to develop new CubeSat missions. It has been proposed as a constellation of a two Cubesat mission to monitor the Sun for Space Weather, and will proceed with a single-satellite asset in its baseline implementation. In the frame of CUSP's Phase B study, that started in December 2024 for a 1-year period, we present the development status of this dual-phase polarimeter. Preliminary laboratory results using two chains of acquisition will be discussed. The first chain of acquisition, based on the Hamamatsu R7600 multi-anode photomultiplier tubes coupled to plastic scintillator bars and read out by the MAROC-3A ASIC, is used to detect the Compton scattering of incoming photons. On the other hand, GAGG crystals coupled to avalanche photo-diodes with a readout based on the SKIROC-2A ASIC are used to absorb the scattered photons. By reconstructing the azimuthal scattering direction for many incoming photons, one can infer the linear polarization degree and angle of the source. We will discuss the calibration results obtained with our prototype detector by using well-known radioactive isotopes, allowing us to assess the performances of our detector over the full 25-100 keV energy range.

Prototype Development and Calibration of the CUbesat Solar Polarimeter (CUSP)

TL;DR

The paper addresses the need to measure the polarization of solar flare hard X-rays in the band – keV to constrain magnetic reconnection and particle acceleration. It presents the CUSP concept, a dual-phase Compton polarimeter on a 6U-XL CubeSat, combining a central plastic-scintillator scatterer array read by MAPMTs with a surrounding GAGG absorber layer read by APDs, both managed by MAROC‑3A and SKIROC‑2A ASICs. Phase B laboratory tests using Fe, Am, and Cd sources demonstrate energy calibration capability and linear response from scattering and absorption channels, with deposition energies in the target range and APD gains around . The results validate the polarimeter design and establish a path toward a larger 16-scatterer by 32-absorber prototype and a flight-model, with planned calibration at accelerator beamlines or synchrotron facilities to finalize performance. The work lays the foundation for time-resolved solar flare polarimetry that can illuminate flare geometry and particle acceleration processes relevant to space weather forecasting.

Abstract

The space-based CUbesat Solar Polarimeter (CUSP) mission aims to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP will allow to study the magnetic reconnection and particle acceleration in the flaring magnetic structures of our star with its unprecedented sensitivity to solar flare polarization. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed to develop new CubeSat missions. It has been proposed as a constellation of a two Cubesat mission to monitor the Sun for Space Weather, and will proceed with a single-satellite asset in its baseline implementation. In the frame of CUSP's Phase B study, that started in December 2024 for a 1-year period, we present the development status of this dual-phase polarimeter. Preliminary laboratory results using two chains of acquisition will be discussed. The first chain of acquisition, based on the Hamamatsu R7600 multi-anode photomultiplier tubes coupled to plastic scintillator bars and read out by the MAROC-3A ASIC, is used to detect the Compton scattering of incoming photons. On the other hand, GAGG crystals coupled to avalanche photo-diodes with a readout based on the SKIROC-2A ASIC are used to absorb the scattered photons. By reconstructing the azimuthal scattering direction for many incoming photons, one can infer the linear polarization degree and angle of the source. We will discuss the calibration results obtained with our prototype detector by using well-known radioactive isotopes, allowing us to assess the performances of our detector over the full 25-100 keV energy range.

Paper Structure

This paper contains 8 sections, 1 equation, 4 figures, 1 table.

Table of Contents

  1. INTRODUCTION: SOLAR FLARES AND POLARIZATION
  2. SOLAR FLARE POLARIMETRY WITH CUSP
  3. Compton Polarimetry
  4. The CUbesat Solar Polarimeter
  5. CUSP PROTOTYPE DEVELOPMENT AND CALIBRATION
  6. Experimental Setup
  7. Spectral Measurements with Single Channel
  8. CONCLUSIONS & OUTLOOK

Figures (4)

  • Figure 1: Left: Schematic working principle of a Compton polarimeter. The incoming photon is Compton scattered in a segment of the detector, deposition some energy which is converted into scintillation optical light and collected by a photosensor at the extremity of the scintillator bar. The scattered photon is then absorbed in a different segment of the instrument, which allows to determine the azimuthal scattering direction of the primary photon. Right: CAD design of CUSP's hard X-ray polarimeter sensitive parts.
  • Figure 2: Left: Diagram showing the experimental setup used to test single channels of CUSP's scatterer and absorber using development boards of the flight ASICs. Right: Pictures of a wrapped GAGG bar coupled to an APD with an $^{241}$Am source on top (top) and of a wrapped plastic bar coupled to a MAPMT channel with a $^{109}$Cd source on top (bottom).
  • Figure 3: $^{55}$Fe, $^{241}$Am, and $^{109}$Cd spectra acquired using a scatterer channel (EJ-204 plastic+MAPMT) using two types of fast shaper in the ASIC (unipolar fast shaper, left; bipolar fast shaper, right).
  • Figure 4: Spectra acquired using an absorber channel (GAGG+APD) with various radioactive isotopes having emission features in CUSP's energy range, namely $^{57}$Co, $^{241}$Am, $^{155}$Eu, $^{129}$I, and $^{109}$Cd.