[Paper Review] Gamma Polari-Calorimetry with SOI pixels for proposals at Extreme Light Infrastructure (ELI-NP)

Kensuke Homma, Y. Nakamiya|arXiv (Cornell University)|Jul 22, 2015

Radiation Detection and Scintillator Technologies3 citations

TL;DR

This paper proposes a Gamma Polari-Calorimeter (GPC) using thin SOI pixel sensors for measuring the polarization and energy of high-energy gamma rays at the ELI-NP facility. The system uses photon conversion in a thin high-Z foil followed by tracking of electron-positron pairs in a uniform magnetic field, achieving 7% momentum resolution and 50% analyzing power, demonstrating feasibility for nonperturbative QED studies.

ABSTRACT

We introduce the concept of Gamma Polari-Calorimetry (GPC) dedicated for proposals at Extreme Light Infrastructure in the Romanian site (ELI-NP). A simulation study shows that an assembly of thin SOI pixel sensors can satisfy our requirements to GPC.

Motivation & Objective

• To develop a detection system capable of measuring the linear polarization and energy of high-energy gamma rays (0.1–few GeV) at the ELI-NP facility.
• To address the challenge of detecting vacuum birefringence and nonlinear QED effects via polarized gamma-ray probes in intense laser fields.
• To achieve high momentum resolution (<20 MeV) and multiple incident gamma-ray resolution (up to ~10 per shot) in a compact, movable design.
• To enable precise measurement of the reaction plane anisotropy relative to the incident gamma-ray polarization plane for polarization analysis.
• To validate the feasibility of using INTPIX4 SOI pixel sensors for simultaneous polarimetry and calorimetry under ELI-NP conditions.

Proposed method

• Utilizes photon conversion in a 20 µm thick gold foil to produce electron-positron pairs from incident gamma rays, enabling energy and polarization measurement.
• Employs a uniform dipole magnetic field to separate and track the charged pairs, with momentum reconstruction from curvature in the magnetic field.
• Uses thin SOI pixel sensors (17×17 µm² pixels) in a layered geometry to minimize multiple scattering and preserve angular resolution.
• Applies a compact Halbach-type permanent magnet to provide a uniform magnetic field in a space-efficient, movable configuration.
• Employs a simulation framework based on Geant4 to model particle interactions, sensor response, and track reconstruction.
• Uses the analyzing power of the polarimeter, defined as the ratio of signal to noise in polarization measurement, to quantify performance.

Experimental results

Research questions

• RQ1 Can a SOI pixel-based detector system achieve the required momentum resolution (<20 MeV) for gamma-ray energy and polarization measurements at ELI-NP?
• RQ2 To what extent can the polarization state of incident gamma rays be reconstructed from the angular anisotropy of e+e− pair production planes?
• RQ3 What thickness of high-Z foil (e.g., gold) optimizes conversion efficiency while preserving angular and momentum resolution?
• RQ4 Can the system resolve up to ten gamma-ray interactions per shot without significant cross-talk or resolution degradation?
• RQ5 Is the INTPIX4 SOI sensor chip suitable for real-time readout at 10 Hz repetition rate under ELI-NP conditions?

Key findings

• The momentum resolution for electron and positron tracks above 0.2 GeV is achieved at the 7% level, meeting the <20 MeV requirement.
• The analyzing power of the polarimeter is approximately 50%, indicating strong sensitivity to polarization states.
• A 20 µm thick gold foil provides sufficient conversion efficiency while minimizing multiple scattering and angular resolution degradation.
• The use of SOI pixel sensors with 17×17 µm² pixels enables precise track reconstruction and accurate extrapolation to the first layer hit points.
• The compact Halbach magnet design satisfies the requirement for a movable, space-efficient detection system.
• The simulation results confirm that the INTPIX4-based sensor assembly is capable of simultaneous high-resolution polarimetry and calorimetry at ELI-NP.