Speaker
Description
Polarization measurements are essential in astrophysics and nuclear physics, offering profound insights into fundamental physical processes. In astrophysics, the polarization of gamma rays provides critical information about high-energy astrophysical phenomena such as black holes, neutron stars, and gamma-ray bursts, helping to explore the radiation mechanisms and the structure of the universe[1-2]. The development of polarimeters with high sensitivity in the MeV region has been proposed[1], and it is expected that these detectors will be flown in space in the future.
Highly polarized MeV gamma rays can be generated in the laboratory by inverse Compton scattering (ICS) of a polarized laser with an electron beam. ICS gamma rays possess characteristics such as energy tunability, quasi-monochromaticty, and low divergence (<1 mrad). Therefore, ICS gamma rays are ideal as a light source for measurements of directional sensitivity, polarization, and energy for astronomical gamma-ray detectors.
In UVSOR synchrotron facility, 6.6 MeV gamma rays can be generated by a 90-degree collisional ICS between a 750 MeV electron beam and an 800 nm laser. The gamma rays are used for user applications such as positron annihilation spectroscopy and evaluation of gamma-ray detectors. As the electron beam energy, laser wavelength and laser injection angle are fixed, it is necessary to use gamma rays with different divergence angles to change the ICS gamma-ray energy. Specifically, the gamma-ray energy can be changed from 3 to 6.6 MeV by placing a lead collimator on the beam axis of the gamma rays and scanning its position. As the polarization characteristics of ICS gamma rays vary with the position of the beam cross section, understanding their spatial polarization distribution is important for gamma-ray applications, including the evaluation of astronomical gamma-ray detectors. However, detailed two-dimensional measurements of this distribution have not yet been conducted. To address this, a polarimeter was developed at UVSOR, enabling precise measurement of the spatial polarization distribution of MeV polarized gamma rays. The polarimeter is based on asymmetry measurements of the Compton scattering cross section. The ICS gamma rays are irradiated onto an iron target, and the azimuthal intensity distribution of the Compton scattered gamma rays is measured by seven NaI detectors to determine the polarization axis. Moreover, the spatial polarization distribution of the ICS gamma rays can be measured by scanning a 1-mm diameter collimator in two dimensions. In this conference, we will report on measurement results of the spatial polarization distribution of linearly and circularly polarized ICS gamma-rays. The polarization axis of the polarized gamma rays was clearly measured to vary with scattering and azimuth angle. We use the developed polarimeter to also measure the spatial polarization distribution of gamma rays generated by an axially symmetric polarized laser[3].
References
[1] C.Ilie,PASP 131, 111001 (2019).
[2]Y.H.Wang, J.C.Sun et.al, NIMA 988, 164866 (2021).
[3]Y. Taira.Phys. Rev. A 110, 043525 (2024).
