Spectral performance of single-channel plastic and GAGG scintillator bars of the CUbesat Solar Polarimeter (CUSP)

TL;DR

The paper presents the CUbesat Solar Polarimeter (CUSP), a dual-phase Compton polarimeter designed to measure hard X-ray polarization from solar flares in the 25–100 keV band. It details the instrument concept (plastic scatterers and GAGG absorbers with MAPMT and APD readouts) and reports laboratory single-channel spectral performances for both detector chains, including energy calibration, dynamic range, and crosstalk, as well as the verification of coincidence logic. Key findings show feasible spectral response with development-board electronics and highlight the need for dedicated flight electronics to reach the target 1.17 keV Compton depositions and to manage noise and pile-up effects at high rates. The work establishes the feasibility of CUSP’s spectro-polarimetric approach and outlines the path toward a flight prototype and eventual launch, which could yield time-resolved polarization measurements to constrain flare acceleration regions and magnetic configurations with broad implications for heliophysics and space weather forecasting.

Abstract

Our Sun is the closest X-ray astrophysical source to Earth. As such, it makes a formidable case study to better understand astrophysical processes. Solar flares are in particular very interesting as they are linked to coronal mass ejections as well as magnetic field reconnection sites in the solar atmosphere. Flares can therefore provide insightful information on the physical processes at play on their production sites, but also on the emission and acceleration of energetic charged particles towards our planet, making it a formidable forecasting tool for space weather. While solar flares are critical to understanding magnetic reconnection and particle acceleration, their hard X-ray polarization -- key to distinguishing between competing theoretical models -- remains poorly constrained by existing observations. To address this, we present the CUbesat Solar Polarimeter (CUSP), a mission under development to perform solar flare polarimetry in the 25-100 keV energy range. CUSP consists of a 6U-XL platform hosting a dual-phase Compton polarimeter. The polarimeter is made of a central assembly of four 4x4 arrays of plastic scintillators, each coupled to multi-anode photomultiplier tubes, surrounded by four strips of eight elongated GAGG scintillator bars coupled to avalanche photodiodes. Both types of sensors from Hamamatsu are respectively read out by the MAROC-3A and SKIROC-2A ASICs from Weeroc. In this manuscript, we present the preliminary spectral performances of single plastic and GAGG channels measured in the laboratory using development boards of the ASICs foreseen for the flight model.

Figures (15)

• Figure S1: Left: Partial CAD design of CUSP's hard X-ray polarimeter showing its sensitive parts. Right: Schematic working principle of a Compton polarimeter. The incoming photon is Compton scattered in a segment of the detector, depositing 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 for determining the azimuthal scattering direction of the primary photon.
• Figure S2: Left: Diagram showing the laboratory setup used to characterize the CUSP scatterer. The MAROC-3A development board is reading out the MA-PMT used to read out the plastic scatterer. Right: Plastic scintillator bar wrapped using PTFE tape and coupled to an MA-PMT channel. A $^{109}$Cd source is placed on top of the scintillator and the assembly is placed in a dark box.
• Figure S3: Left:$^{55}$Fe, $^{109}$Cd, and $^{241}$Am spectra measured with an EJ-204 scintillator bar using the unipolar fast shaper, a preamplifier gain of 0.2, and a bias voltage of 840 V. Right: Same spectra measured using MAROC-3A's bipolar fast shaper 1 at a bias of 820 V and with a preamplifier gain of 0.15.
• Figure S4: Left:$^{241}$Am spectra measured using the unipolar fast shaper with a bias of 840 V with three kinds of plastic scintillators. Right: Scintillation spectra of various PVT-based scintillators convolved with the quantum efficiency of the R7600-03-M16-Y002 MAPMT from Hamamatsu.
• Figure S5: Left: Spectra measured by the 16 MAPMT channels, with channel 1 coupled to a wrapped plastic scintillator with an $^{55}$Fe on top, and all other channels covered by an opaque jig (see Figure \ref{['fig:scatterers_setup']}, right). The effect of optical crosstalk through the MAPMT's entrance window can be seen in the spectra of the channels neighboring channel 1. Right: Crosstalk map showing an optical crosstalk of up to 10.9 % in the direct neighbors and non-significant crosstalk to non-neighboring channels. The channel ID and crosstalk value are indicated on each bin.