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The X-$γ$ detector onboard the POEMMA-Balloon with Radio payload

Matteo Battisti

TL;DR

The work presents the X-γ detector designed for the POEMMA-Balloon with Radio (PBR) platform, targeting high-energy photons from High-Altitude Horizontal Air Showers to probe the early evolution of extensive air showers. It details a multi-channel scintillator detector (NaI(Tl)/CsI(Tl)) with SiPM readout, Be/Al windows, and a magnet-collimated FOV, integrated with a FPGA-based trigger and a synchronized data acquisition system to enable coincident observations with the Fluorescence and Cherenkov cameras. The instrument’s design supports energy coverage from ~10 keV to several MeV, with anti-coincidence vetoing and a 16°–30° FOV that overlaps with primary imaging instruments for cross-validation. Early laboratory tests show linear response and threshold performance around 10–20 keV, validating the approach for capturing HAHA-related X-ray/gamma signals and enabling multimessenger synthesis to constrain shower development models and improve UHECR/VHE neutrino studies on the path to the POEMMA mission.

Abstract

The POEMMA-Balloon with Radio (PBR) is a NASA mission designed to study Ultra-High-Energy Cosmic Rays and Very-High-Energy Neutrinos from a balloon platform. Serving as a precursor to the planned POEMMA satellite mission, PBR will be launched aboard a NASA Super Pressure Balloon for a flight at of 33 km altitude in Spring 2027 from Wanaka, New Zealand. The unique conditions of low pressure and high altitude will enable in-situ observations of High-Altitude Horizontal Air Showers (HAHAs), a poorly understood class of nearly horizontal Extensive Air Showers induced by cosmic rays skimming the Earth's atmosphere without reaching the ground. Due to the lower atmospheric grammage at these altitudes, HAHAs develop more gradually compared to typical downward-going EASs, with interaction lengths on the order of 100 km. This slow development allows balloon-borne instruments to probe the early stages of cosmic ray shower evolution. At these early stages, high-energy electrons and positrons from the electromagnetic component of the shower can generate X-rays and gamma rays via synchrotron radiation. The X-$γ$ detector onboard PBR is designed to measure these photons across a broad energy range. The instrument consists of four sub-detectors, each optimized for different overlapping energy bandfrom tens of keV to MeV. The current design utilizes CsI(Tl)/NaI(Tl) scintillating crystals coupled with SiPMs for photon detection. To suppress background noise, the detectors are enclosed within an anti-coincidence system to reject charged particle events. The X-$γ$ detector is aligned with PBR's primary instruments, the Fluorescence Camera and the Cherenkov Camera, within a 30$^\circ$ field of view, overlapping with both imaging cameras. It will operate in a triggered mode, with the possibility to receive signals from the other instruments to check for simultaneous events.

The X-$γ$ detector onboard the POEMMA-Balloon with Radio payload

TL;DR

The work presents the X-γ detector designed for the POEMMA-Balloon with Radio (PBR) platform, targeting high-energy photons from High-Altitude Horizontal Air Showers to probe the early evolution of extensive air showers. It details a multi-channel scintillator detector (NaI(Tl)/CsI(Tl)) with SiPM readout, Be/Al windows, and a magnet-collimated FOV, integrated with a FPGA-based trigger and a synchronized data acquisition system to enable coincident observations with the Fluorescence and Cherenkov cameras. The instrument’s design supports energy coverage from ~10 keV to several MeV, with anti-coincidence vetoing and a 16°–30° FOV that overlaps with primary imaging instruments for cross-validation. Early laboratory tests show linear response and threshold performance around 10–20 keV, validating the approach for capturing HAHA-related X-ray/gamma signals and enabling multimessenger synthesis to constrain shower development models and improve UHECR/VHE neutrino studies on the path to the POEMMA mission.

Abstract

The POEMMA-Balloon with Radio (PBR) is a NASA mission designed to study Ultra-High-Energy Cosmic Rays and Very-High-Energy Neutrinos from a balloon platform. Serving as a precursor to the planned POEMMA satellite mission, PBR will be launched aboard a NASA Super Pressure Balloon for a flight at of 33 km altitude in Spring 2027 from Wanaka, New Zealand. The unique conditions of low pressure and high altitude will enable in-situ observations of High-Altitude Horizontal Air Showers (HAHAs), a poorly understood class of nearly horizontal Extensive Air Showers induced by cosmic rays skimming the Earth's atmosphere without reaching the ground. Due to the lower atmospheric grammage at these altitudes, HAHAs develop more gradually compared to typical downward-going EASs, with interaction lengths on the order of 100 km. This slow development allows balloon-borne instruments to probe the early stages of cosmic ray shower evolution. At these early stages, high-energy electrons and positrons from the electromagnetic component of the shower can generate X-rays and gamma rays via synchrotron radiation. The X- detector onboard PBR is designed to measure these photons across a broad energy range. The instrument consists of four sub-detectors, each optimized for different overlapping energy bandfrom tens of keV to MeV. The current design utilizes CsI(Tl)/NaI(Tl) scintillating crystals coupled with SiPMs for photon detection. To suppress background noise, the detectors are enclosed within an anti-coincidence system to reject charged particle events. The X- detector is aligned with PBR's primary instruments, the Fluorescence Camera and the Cherenkov Camera, within a 30 field of view, overlapping with both imaging cameras. It will operate in a triggered mode, with the possibility to receive signals from the other instruments to check for simultaneous events.

Paper Structure

This paper contains 5 sections, 6 figures.

Table of Contents

  1. The PBR payload
  2. X-$\gamma$ emissions from HAHAs
  3. The X-$\gamma$ instrument
  4. Data acquisition and trigger system
  5. Expected performance

Figures (6)

  • Figure 1: PBR sketch along its scientific goals. The X-$\gamma$ detector (light blue box, not in scale) will point in the same direction of the other telescopes (Cherenkov and Fluorescence cameras).
  • Figure 2: Left: Possible trajectories of HAHAs that end up reaching PBR. Picture not on scale. Right: . Energies of HAHAs that can reach the detector as a function of their zenith angle. 90$^\circ$ are horizontal events. Picture adapted from Cherenkov_emission.
  • Figure 3: Left: Electron energy distribution for different shower ages, according to NERLING2006421 for a 10$^{17}$ eV primary. For shower ages $\lesssim$0.2 there is a relevant fraction of electrons and positrons above 1 TeV. Right: Average energy of synchrotron photons emitted by electrons with different energies. Adapted from Galper_2017. Electrons and positrons at 1 TeV should produce $\sim$10 keV photons via bremsstrahlung
  • Figure 4: Left: Exploded view of the detector. The entire instrument fits in a $\sim$30$\times$15$\times$30 cm$^3$ box. Right: Scionix detector for the X-$\gamma$ channel with its schematic.
  • Figure 5: Simplified schematic of a single channel trigger logic. The figures on the right are oscilloscope measurements taken at the points of the electronic chain indicated by the numbers.
  • ...and 1 more figures