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

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.