Gamma-ray astronomy with spaceborne detectors: HERD

Javier Rico

The IFAE Gamma-ray group started in 2020 participating in the design of HERD, a next-generation detector on board the China Space Station, for the study of high-energy cosmic rays and gamma rays. The IFAE group has the responsibility of providing HERD with advanced capabilities for Gamma-ray Astronomy by the design of a dedicated low-energy gamma-ray trigger system, to provide it with sensitivity to gamma rays in the range between ~100 MeV and 10 GeV with unprecedented angular and energy resolutions, and exploiting its capabilities as a wide field of view monitoring detector, complementary to CTA and other multi-messenger instruments in the study of transient events.

INTRODUCTION

HERD is the flagship scientific experiment proposed for the China Space Station (CSS, see Fig. 1). HERD is slated to commence space operations not earlier than 2028, with an operational lifespan of at least 10 years. The CSS will provide the necessary resources, adhering to specifications such as a mass budget of 4 t, maximum power consumption of 1.2 kW, envelope dimensions measuring 2.3×2.3×2.6 m3, and an average downlink requirement of 100 Mbps.

Figure 1: Artistic view of HERD installed on the CSS.

Instrumentation and Performance

HERD (Fig. 2) consists of a 3D imaging calorimeter (CALO), facilitating precise identification and energy measurement of cosmic rays (CRs). The CALO is surrounded on its five exposed faces by the fiber tracker (FIT), serving as the target for gamma-ray pair conversion. The plastic scintillator detector (PSD) acts as an efficient veto in gamma-ray identification. Additionally, a Silicon Charge Detector is employed for the precise determination of the absolute electric charge of incoming CRs. Positioned on a lateral side, a Transition Radiation Detector contributes to the CALO energy calibration.

Figure 2: Sketch of the HERD payload. The exploded view (right) shows the main HERD subsystems.

HERD will efficiently measure the incident direction, energy, electric charge, and nature of cosmic nuclei between 30 GeV and few PeV, cosmic electrons in the range between 10 GeV and 100 TeV, and gamma-rays above 100 MeV. The wide acceptance will be accompanied by excellent performance in terms of energy resolution (about 1% above 200 GeV for electrons and gamma rays and of about 20% for protons and nuclei from 100 GeV to 1 PeV), angular resolution (expected to be better than 0.1deg for electrons or gamma rays of 10 GeV), and proton/electron discrimination power (inefficiency lower than a factor 10-6). With these figures of merit, HERD guarantees direct observations of different CR species and gamma rays with unprecedented accuracy over a wide energy range.

IFAE Contribution and Recent Developments

Our group joined the HERD project in 2020 and proposed using the triggering capabilities of the FIT in a new, advanced “ultra-low-energy” gamma-ray (ULEG) trigger for HERD, and to provide the associated trigger electronics for the FIT and trigger/veto and readout electronics for the PSD. In addition, we oversee the studies to assess HERD’s potential for gamma-ray astronomy enabled thanks to the ULEG trigger.

We have designed, produced, and tested the electrical and functional models (EFMs) for FIT and PSD electronics systems (see Fig 3). The EFMs comprised a blend of commercial and custom-made components, offering performance below the final detector’s requirements (e.g., absence of space-qualified components, limited number of channels matching the needs for the current FIT and PSD prototypes, etc.). The EFMs served as demonstrations of the ULEG trigger’s feasibility, showcasing IFAE’s hardware capabilities. Additionally, they facilitated the accumulation of invaluable expertise in integrating our hardware into the HERD system, managing interfaces with various components (e.g., SiPMs, ASIC, central trigger system), and early problem identification and resolution in our designs. Both EFMs, and their integration as part of the ULEG trigger were successfully tested in the beam test campaigns performed at the CERN’s PS and SPS accelerators during August, September, and October 2023, together with the rest of the HERD prototype.

Figure 3: FIT and PSD prototypes during the HERD beam test at PS accelerator at CERN in October 2023

More recently, our group carried out an extensive effort to optimize, calibrate, and configure the silicon photomultipliers and readout ASIC of the FIT and PSD at the IFAE laboratories (Fig 4). This work allowed for a detailed characterization of these components, evaluating essential parameters such as noise levels, calibration constants, thresholds, dynamic range, linearity, and response time.

The refined electronic system was then integrated into a scaled-down prototype of the FIT, which underwent successful testing during a high-energy ion beam campaign at the CERN SPS accelerator in November 2024. The collected data confirmed the FIT’s spatial resolution and charge identification performance, marking a key milestone for the project. Furthermore, these advancements pave the way for the potential application of the fiber tracker technology in other astroparticle physics space missions.

In addition, we have produced detailed simulations of gamma-ray interactions in HERD, using the Géant4-based HerdSoftware framework, which contains a full HERD detector model while allowing for basic geometrical configuration (subdetector dimensions, number of detector planes, etc.). Thanks to that, we have been able to characterize HERD generic performance for gamma-ray detection in terms of effective area, energy resolution, angular resolution, background intensity, etc., as a function of the energy and gamma-ray incident direction.

Figure 4: IFAE optical setup for the characterization, calibration and optimization of the SiPM and BETA ASICs for the PSD and FIT detectors.