The Fluorescence Camera for the PBR mission

TL;DR

The paper addresses the challenge of probing the origins of Ultra-High-Energy Cosmic Rays (UHECRs) by increasing exposure at the highest energies and validating fluorescence-based detection techniques suitable for space-based observatories. It presents the POEMMA-Balloon with Radio (PBR) as a long-duration suborbital pathfinder featuring a tiltable $1.1~\mathrm{m}$ Schmidt telescope that hosts a hybrid focal surface with a Fluorescence Camera (FC) and Cherenkov Camera (CC), plus a Radio Instrument. A key focus is the FC's modular Photo Detection Modules (PDMs) built from MAPMTs and SPACIROC-3 ASICs, capable of single-photon counting and fast charge integration over a $1.05~\mu\mathrm{s}$ gate, achieving a $24^{\circ}\times24^{\circ}$ field of view with ground sampling near $115~\mathrm{m}$. The work details the FC architecture, read-out electronics, and data processing chain, and outlines the flight plan (up to $50$ days) and current construction status. This pathfinder approach aims to validate detection strategies for future space missions like POEMMA and to advance balloon-based multi-messenger capabilities for UHECR research.

Abstract

The Probe Of Extreme Multi-Messenger Astrophysics (POEMMA) Balloon with Radio (PBR) is an instrument designed to be borne by a NASA suborbital Super Pressure Balloon (SPB), in a mission planned to last as long as 50 days. The PBR instrument consists of a 1.1 m aperture Schmidt telescope, similar to the POEMMA design, with two cameras in its hybrid focal surface: a Fluorescence Camera (FC) and a Cherenkov Camera (CC), both mounted on a frame that can be tilted to point from nadir up to 13 degrees above the horizon. The FC camera is designed to detect the fluorescence emission of Extensive Air Showers produced by Ultra-High Energy Cosmic Rays from sub-orbital altitudes. This measurement will validate the detection strategy for future space-based missions, such as POEMMA. The FC will be made of 4 Photo Detection Modules (PDMs), each consisting of a 6x6 matrix of 64-channel Multi Anode PhotoMulTipliers (MAPMT), for a grand total of 2304 pixels for each PDM. Custom-designed SPACIROC-3 ASICs perform single photoelectron counting on each pixel as well as charge integration on groups of 8 pixels to measure extremely bright or fast signals, reaching a double pulse resolution in the order of 10 ns for a 1 microsecond acquisition gate. A field flattener lens and a BG3 filter, to match the wavelength range of interest (300-400 nm), are mounted in front of the PDM. The camera will be able to detect showers in a field of view of 24x24 square degrees, with a pixel size on ground corresponding to 115 m. Details on the camera design and implementation will be given, along with the expected performance and the state of the construction.

Figures (4)

• Figure 1: (\ref{['fig:pbr']}) The PBR payload without the containment boxes, ancillary detectors and electronic boxes. (\ref{['fig:fc']}) The drawing of the FC camera. Part of the mechanical frame is shaded along with some component of the top right Photo Detection Module (PDM).
• Figure 2: A PDM prototype under test. Only three Elementary Cells (ECs) are mounted. The read-out electronics is attached on the back of the 3D printed mechanical frame.
• Figure 3: (\ref{['fig:ec_nopot']}) An Elementary Cell before potting. The ASICs are visible mounted on the lateral boards that are integrated with kapton flat cables on both board sides. (\ref{['fig:ec_potted']}) An Elementary Cell after the potting, ready to be integrated in the PDM frame.
• Figure 4: (\ref{['fig:zynq']}) The AMD/Xilinx Zynq based read-out board of a PDM. (\ref{['fig:dp']}) A partial view of the FC Data Processor. The second CPU is not shown in the picture.