The General Antiparticle Spectrometer (GAPS) is an Antarctic stratospheric balloon mission designed to provide unmatched sensitivity to low-energy (<0.25 GeV/n) cosmic-ray antiprotons, antideuterons, and antihelium nuclei as signatures of dark matter. The distinctive GAPS particle identification technique relies on measuring the energy loss along the track of an incoming antinucleus as it slows down and is captured into an exotic atom, and then detecting the de-excitation X-rays and the nuclear annihilation products. This measurement is realized using a Tracker composed of more than 1000 custom silicon strip detectors and a plastic scintillator time-of-flight (TOF) system instrumenting more than 40m^2. Together, these subsystems provide the velocity and energy resolution, stopping power, particle tracking, and X-ray identification necessary to distinguish rare antinucleus signals from the abundant positive-nucleus backgrounds, all within the constraints of a high-altitude mission. A multi-loop capillary heat pipe system has been developed to maintain the tracker operating temperature with significant mass and power savings over a conventional pump-based system. The first GAPS science payload flew for 25 days during the 2025/26 NASA Antarctic balloon campaign. We detail the design, integration, and commissioning of the payload prior to flight.

The accurate simulation of sub-GeV particle detectors is essential for interpreting experimental data and optimizing detector design. This work identifies and addresses several critical aspects in modeling such detectors, taking as a case study the High-Energy Particle Detector (HEPD-02), a space-borne instrument developed within the CSES-02 mission to measure electrons in the ∼3–100 MeV range, protons and light nuclei in the ∼30–200 MeV/n. The HEPD-02 instrument consists of a silicon tracker, plastic and LYSO scintillator calorimeters, and anticoincidence systems, making it a representative example of a complex low-energy particle detector operating in Low Earth Orbit. Key challenges arise from replicating intricate detector geometries derived from CAD models, selecting appropriate hadronic physics lists for low-energy interactions, and accurately describing the detector response—particularly quenching effects in scintillators and digitization in solid-state tracking planes. Particular attention is given to three critical aspects: the precise CAD-level geometry implementation, the impact of hadronic physics models on the detector response, and the parameterization of scintillation quenching. In this study, we present original solutions to these challenges and provide data–MC comparisons using data from HEPD-02 beam tests.

The China Seismo-Electromagnetic Satellite (CSES) mission is a joint China-Italy initiative focused on investigating Earth’s geophysical environment through non-imaging remote sensing from space. Its primary objective is to establish a constellation of satellites capable of continuously monitoring global electromagnetic fields, particle fluxes, and plasma parameters within the iono-magnetospheric system. Goals of this space program are the investigation of possible lithosphere-atmosphere-ionosphere-magnetosphere coupling mechanisms and their role in inducing perturbations in the upper ionosphere and the lower boundary of the radiation belts. Additionally, CSES contributes to space weather studies, including investigations of the magnetosphere, heliosphere, and galactic cosmic rays. Each satellite in the constellation carries multiple instruments to measure charged particles, electromagnetic fields, and plasma properties. CSES-01, launched in February 2018, remains operational. The second satellite, CSES-02, was launched on June 14, 2025, marking the transition to a multi-point observation capability. Italy contributed the High-Energy Particle Detector to both missions—HEPD-01 for CSES-01 and the upgraded HEPD-02 for CSES-02. These detectors are designed for precise measurements of electrons, protons, light nuclei, and transient gamma rays in the multi-MeV range. HEPD-02 significantly enhances energy resolution and extends the detection range when combined with the lower-energy instruments aboard CSES-02, enabling continuous coverage from 100 keV to 200 MeV. This article presents the scientific goals of the CSES program, with a focus on the role of HEPD-02 in energetic particle studies, both as a standalone detector and in synergy with the mission’s multi-instrument, multi-satellite framework.

This paper introduces TROPix, a parametric simulation tool designed to reproduce the output of silicon pixel detectors. Developed for the Direction Detector of the HEPD-02 detector on the second China Seismo-Electromagnetic Satellite (CSES-02), TROPix utilizes the GEANT4 toolkit for energy deposition and a parametric approach for charge generation and diffusion. The simulation process incorporates thermal noise and digitization, with all parameters fine-tuned using experimental beam-test data. Although TROPix is tuned for HEPD-02 data, it can also be adapted for use with other detectors employing pixel sensors, providing a versatile and computationally efficient solution for numerous applications. Results for Monolithic Active Pixel Sensors (MAPS) based on ALTAI chips are presented, highlighting TROPix’s ability to replicate the property of pixel clusters, defined as the number of pixels fired by the particle passage. These characteristics comprehend the dependencies on charged particle deposited energy and incident angle, as well as the typical shapes of clusters. Validation using beam test data acquired with ALTAI sensors demonstrates that TROPix reproduces the cluster size dependence on deposited energy, with agreement within 20%. For cluster shape occurrences, the simulated results align with the data within 25% of the total uncertainty.

Ionospheric electric field monitoring has an important role in the understanding of the terrestrial environment and in the study of the various couplings with both the outer space and the ground level. The China Seismo-Electromagnetic Satellite (CSES) mission consists of a constellation of spacecrafts equipped with several instruments able to provide a large set of information, especially for applications ranging from Space Weather to Ionosphere-Lithosphere coupling. In this work we present the main features of the new Electric Field Detector (EFD-02), developed for the CSES-02 satellite, together with its measurement concept and peculiarities. The characteristics of each subsystem are reported to show the state of the art of the instrument. Particular emphasis is given to the high-level performance and novelties of EFD-02, which significantly improve the scientific information about plasma dynamics. The main scientific objectives of ionospheric electric field monitoring for both the low and high latitudes relevant to EFD-02 capabilities are finally described.

During their propagation in the heliosphere, interplanetary coronal mass ejections (ICMEs) interact with galactic cosmic ray (GCR) particles, modifying their spectrum and driving anisotropies. We analyze the first large Forbush decrease (FD) of Solar Cycle 25 on 2021 November 3–5 by using multipoint in situ observations and neutron monitors to study the association between FD characteristics and ICME. We use the Grad–Shafranov reconstruction to infer the magnetic field configuration of the ICME. We model the neutron monitor response through primary spectrum and anisotropy. The primary spectrum is parameterized with the force-field approximation and the anisotropy is modeled through a spherical harmonic expansion. We optimize the model parameters during the FD by using ground-based observations provided by the worldwide neutron-monitor network. The model’s results are compared with space-based measurements of the differential proton flux measured by the HEPD-01 detector on board the CSES-01 satellite and of the integral counts of both the High-Energy Particle Detector (HEPD-01) and the High Energy Telescope on board the Solar Orbiter. Anisotropy develops during the ICME passage, within the magnetic flux rope (MFR) and is found to be bidirectional. The force-field parameterization of the primary GCR fluxes based on ground-based measurements is found to be in very good agreement with spacecraft observations in the sub-GeV range. The GCR anisotropy obtained by fitting the model to ground-based observations is consistent with interplanetary magnetic field observations. The results suggest that the local magnetic field has a substantial axial component that is aligned to the MFR axis, and determines the GCR anisotropy at the typical neutron monitor energies.

This article describes the innovative system performing the trigger and the readout of the photomultiplier tubes (PMTs) for the high-energy particle detector (HEPD) onboard the second satellite of the China Seismo Electromagnetic Satellite (CSES) mission. The second HEPD is designed to measure cosmic rays covering an energy spectrum ranging from a few megaelectronvolts to several hundreds of megaelectronvolts. This high-precision detector consists of different subsystems: a solid-state tracker, a segmented trigger, a calorimeter composed of a tower of plastic scintillators, and two layers of lutetium-yttrium oxyorthosilicate (LYSO) crystals, along with a containment detector. The data acquisition (DAQ) process for the trigger, calorimeter, and containment detector is carried out by a single electronic board, relying on two Weeroc CITIROC application-specific integrated circuits (ASICs), which are being utilized in space for the first time. This board also generates and manages trigger signals for the entire detector. It effectively captures signals with distinct timing characteristics from plastic scintillators and inorganic crystals. Given the wide range of particle fluxes encountered during CSES's orbit, adaptability of the trigger generation system becomes crucial, and DAQ is optimized to ensure consistent energy spectra measurement with a substantial duty cycle. The trigger system of the second HEPD implements concurrent trigger patterns and the ability to select DAQ strategies based on orbital zones and the presence of impulsive events. Each trigger configuration is designed to meet scientific demands concerning the field of view and the characteristics of the particles reaching the detector, with prescaling settings fine-tuned accordingly. In addition to monitoring particle bursts, trigger configurations specific to gamma rays will be tracked in 5-ms intervals to measure photon fluxes in the energy range from 2 to 20 MeV and provide sensitivity for impulsive events, such as gamma-ray bursts (GRBs). This article provides a comprehensive account of the design criteria, the architectural choices specifically adapted for space applications, and the original trigger management strategy. Additionally, this article presents an in-depth analysis of the performance of the trigger system and comprehensive results from laboratory and beam tests conducted on the qualification and flight models of second HEPD.

The High-Energy Particle Detector-02 (HEPD-02), onboard the second China Seismo-Electromagnetic Satellite (CSES-02), introduces-for the first time in a space application-a tracker designed entirely using monolithic active pixel sensors (MAPS). The MAPS selected for the HEPD-02 tracker were developed as part of CERN's R&D efforts for the realization of the ALICE Inner Tracking System (ITS). The readout system has been optimized, significantly reducing power consumption. The performance of the MAPS has been validated for HEPD-02's scientific objectives through beam tests using particle and ion beams in the tens to hundreds of MeV energy range. This article details the design principles and structure of the tracker. The integration procedure is outlined step by step, with a discussion of the challenges encountered at each stage and the strategies adopted to address them. The description concludes with an analysis of production yields and the criteria used for selecting components for the flight and qualification models. In addition, an overview of the space qualification activities is provided. The launch of CSES-02 is scheduled for mid-2025. Following an initial in-orbit commissioning phase, the technology will achieve technology readiness level 9, making it available for future space missions.

The High-Energy Particle Detector (HEPD-02) is one of the scientific payloads of the China Seismo-Electromagnetic Satellite (CSES-02). The HEPD-02’s main purpose is to characterize the particle environment in the Earth’s vicinity, identifying sudden changes in the fluxes and correlating them with solar and terrestrial phenomena. Additionally, HEPD-02 also has capabilities in detecting Gamma-Ray Bursts. At the core of HEPD-02, a tower of scintillation counters made of plastic and LYSO crystals is able to recognize electrons in the range between 3 and 100 MeV, protons and nuclei between 30 and 200 MeV/n. Plastic scintillators covering the calorimeter on five sides allow to reject particles entering from the top and not completely absorbed within its volume. In this work, the design of the HEPD-02 is reviewed in comparison to its predecessor, HEPD-01, highlighting the innovations of the new design. The design of each scintillation counter type has been fully validated through a campaign of prototype realization, testing, and characterization. The production of the scintillation counters, including the PMT selection process, is also discussed. Finally, the performance of the counters is compared with simulations, showing an agreement of within 20% with the expected performance, thereby meeting expectations.

The intricate behavior of particle acceleration and transport mechanisms complicates the overall efforts in formulating a comprehensive understanding of solar energetic particle (SEP) events; these efforts include observations of low-energy particles (from tens of keV to hundreds of MeV) by space-borne instruments and measurements by the ground-based neutron monitors of the secondary particles generated in the Earth atmosphere by SEPs in the GeV range. Numerous space-borne missions provided good data on the nature/characteristics of these solar particles in past solar cycles, but more recently-concurrently with the rise toward the maximum of solar cycle 25-the High-Energy Particle Detector (HEPD-01) proved to be well suited for the study of solar physics and space weather. Its nominal 30-300 MeV energy range for protons can enlarge the detection capabilities of solar particles at low Earth orbit, closer to the injection limit of many SEP events. In this work, we characterize three SEP events within the first six months of 2022 through spectral and velocity dispersion analysis, assessing the response of HEPD-01 to >M1 events.

Cosmic rays' interactions with the residual atmosphere surrounding the Earth produce a variety of particles, like electrons, positrons, protons, anti-protons, and Helium nuclei that can be observed below the local geomagnetic cutoff. In this work, we present new measurements of downward-going, albedo proton fluxes with kinetic energy in the range ∼40–∼250 MeV, performed by the High-Energy Particle Detector (HEPD-01) on board of the China Seismo-Electromagnetic Satellite - CSES-01 - at an altitude of ∼500 km. Employing a dedicated trajectory-tracing simulation routine, the protons collected by HEPD-01 are classified into quasi-trapped (QT), long lifetime (≳10 s) particles concentrating in the equatorial region of the Earth, and un-trapped (UT), distributed at all latitudes; the latter includes both precipitating short lifetime particles (UTS) and pseudo-trapped long lifetime (UTL) populations, abundant in the so-called penumbra regions. The temporal trend of re-entrant protons between 2018 and 2022 is also reported, assessing the stability of such population during the data-taking period of HEPD-01; this highlights their independence from the long-term modulating effect of the solar activity.

The High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite, located on a Sun-synchronous orbit at 500 km of altitude with an inclination of 97 degrees, features a dedicated logic counting low-energy event rates, which proved sensitive to intense Gamma-Ray Burst (GRB). The present work reports a comprehensive analysis of signals induced by GRBs in the event-rate data collected between 2018 August and 2022 June. After accurately modeling the background rate as observed in different passages of the satellite over the same geographical area, we detected significant deviations to be compared with observations of GRB candidates from other observatories. The analysis revealed 12 statistically significant excesses, that have been associated with GRB 181222B, GRB 190114C, GRB 190129B, GRB 190305A, GRB 190928A, GRB 200412B, GRB 200422A, GRB 200826B, GRB 201009A, GRB 210702A, GRB 211211A, and GRB 220624A. We report light curves for 0.3-50 MeV photons, comparing them with findings from other space telescopes. The catalog of observations is published, complete of GRB observation time, duration, integrated counts, and fluence.

In this paper we report the detection of five strong gamma-ray bursts (GRBs) by the High-Energy Particle Detector (HEPD-01) mounted on board the China Seismo-Electromagnetic Satellite, operational since 2018 on a Sun-synchronous polar orbit at a ∼507 km altitude and 97° inclination. HEPD-01 was designed to detect high-energy electrons in the energy range 3–100 MeV, protons in the range 30–300 MeV, and light nuclei in the range 30–300 MeV n −1 . Nonetheless, Monte Carlo simulations have shown HEPD-01 is sensitive to gamma-ray photons in the energy range 300 keV–50 MeV, even if with a moderate effective area above ∼5 MeV. A dedicated time correlation analysis between GRBs reported in literature and signals from a set of HEPD-01 trigger configuration masks has confirmed the anticipated detector sensitivity to high-energy photons. A comparison between the simultaneous time profiles of HEPD-01 electron fluxes and photons from GRB190114C, GRB190305A, GRB190928A, GRB200826B, and GRB211211A has shown a remarkable similarity, in spite of the different energy ranges. The high-energy response, with peak sensitivity at about 2 MeV, and moderate effective area of the detector in the actual flight configuration explain why these five GRBs, characterized by a fluence above ∼3 × 10 −5 erg cm −2 in the energy interval 300 keV–50 MeV, have been detected.

The Electric Field Detector (EFD-02) on board the second China Seismo-Electromagnetic Satellite (CSES-02) will measure the electric field components at a Low Earth Orbit (LEO) over a wide frequency band (DC - 3.5 MHz) and with 1 mu V/m sensitivity in the Low Frequency band. EFD-02 will measure the voltage differences between pairs of probes installed at the tips of four booms deployed from the satellite. In this article, we describe the Zynq System on Chip (SoC)-based digital hardware subsystem dedicated to signal processing, and the selected implementation strategy, which successfully complied to the specific requirements of the space mission. Furthermore, we present a comprehensive overview of the assessed instrument performance.

GRB 221009A is a long gamma-ray burst among the most energetic and nearest (z = 0.151) detected so far. The energy fluence of the burst was so large to cause ionization of the upper layers of Earth's atmosphere and also observable signals in satellite-borne particle detectors. Electron signals, with the same GRB time development, can arise from the interaction of energetic photons with the particle detector and support structures. This effect was previously reported for the HEPP-L on board the China Seismo-Electromagnetic Satellite. We searched for the same effect on the particle detectors on board five POES and MetOp satellites. Electron signals in coincidence with the gamma-ray emission of the burst were found in three satellites, which were well illuminated by the GRB. The properties of the found electron signals are reported and discussed.

The General Antiparticle Spectrometer (GAPS) is an upcoming balloon mission to measure low-energy cosmic-ray antinuclei during at least three ~35-day Antarctic flights. With its large geometric acceptance and novel exotic atom-based particle identification, GAPS will detect ~500 cosmic antiprotons per flight and produce a precision cosmic antiproton spectrum in the kinetic energy range of ~0.07-0.21 GeV/n at the top of the atmosphere. With these high statistics extending to lower energies than any previous experiment, and with complementary sources of experimental uncertainty compared to traditional magnetic spectrometers, the GAPS antiproton measurement will be sensitive to dark matter, primordial black holes, and cosmic ray propagation. The antiproton measurement will also validate the GAPS antinucleus identification technique for the antideuteron and antihelium rare-event searches. This analysis demonstrates the GAPS sensitivity to cosmic-ray antiprotons using a full instrument simulation and event reconstruction, and including solar and atmospheric effects.

The General Antiparticle Spectrometer (GAPS) Antarctic long duration balloon mission is scheduled for launch during the austral summer of 2024-25.Its novel detection technique, based on exotic atom formation, excitation, and decay, is specifically designed for the detection of slow moving cosmic antiprotons and antideuterons.Such antinuclei are predicted by a wide variety of allowed dark matter models, as well as other astrophysical theories like primordial black holes.There are two main components of the GAPS instrument: a large-area tracker and a surrounding time-of-flight system (TOF).The combination of these two systems allows GAPS to effectively differentiate between species of negatively-charged antinuclei and determine the energy deposition, velocity, and trajectory of particles interacting with the detector.This contribution will focus on the TOF, which determines the velocity of the incoming antiparticle and provides the trigger to the experiment.We will give an overview of the TOF detector, an explanation of relevant electronics, and a report on its construction and preliminary performance.The TOF is composed of 160 thin plastic scintillator paddles ranging in length from 1.5 to 1.8 meters.At each paddle end, signals from six silicon photomultipliers are combined to produce two copies of the resulting waveform: one to form the trigger and one for data readout.This design is optimized for low mass and fast data acquisition while still maintaining good light collection.

Compilation of papers presented by the GAPS Collaboration at the 38th International Cosmic Ray Conference (ICRC), held July 26 through August 3, 2023 in Nagoya, Japan.

This paper presents the project Comprehensive spAce wEather Studies for the ASPIS prototype Realization (CAESAR), which aims to tackle the relevant aspects of Space Weather (SWE) science and develop a prototype of the scientific data centre for Space Weather of the Italian Space Agency (ASI) called ASPIS (ASI SPace Weather InfraStructure). To this end, CAESAR involves the majority of the SWE Italian community, bringing together 10 Italian institutions as partners, and a total of 92 researchers. The CAESAR approach encompasses the whole chain of phenomena from the Sun to Earth up to planetary environments in a multidisciplinary, comprehensive, and unprecedented way. Detailed and integrated studies are being performed on a number of well-observed “target SWE events”, which exhibit noticeable SWE characteristics from several SWE perspectives. CAESAR investigations synergistically exploit a great variety of different products (datasets, codes, models), both long-standing and novel, that will be made available in the ASPIS prototype: this will consist of a relational database (DB), an interface, and a wiki-like documentation structure. The DB will be accessed through both a Web graphical interface and the ASPIS.py module, i.e., a library of functions in Python, which will be available for download and installation. The ASPIS prototype will unify multiple SWE resources through a flexible and adaptable architecture, and will integrate currently available international SWE assets to foster scientific studies and advance forecasting capabilities.

Time-resolved measurements of differential fluxes of low energy charged particles, trapped in the magnetosphere, are interesting for Space Weather characterization and to study the coupling between the lithosphere and magnetosphere, allowing the investigation of the possible correlations between seismic events and particle precipitations from Van Allen Belts. The project of a compact (10x10x10cm$^3$) particle spectrometer, the Low Energy Module (LEM) as part of the Zirè instrument on board the NUSES space mission is shown. The LEM will be able to perform measurements of energy, direction, and composition of low energy charged particles down to 0.1 MeV kinetic energy. The particle identification capability of the LEM relies on the $\Delta$E-E technique performed by thin silicon detectors. To fulfill the size and mass requirements of the whole mission, the particle direction measurement is based on the "active collimation" technique. The detection concept and the expected LEM performances will be summarized.