Compton Polarimeter Research Articles

High Voltage Monolithic Active Pixel Sensors for the MOLLER experiment

The MOLLER experiment [1] aims to measure the parity-violating asymmetry (APV) in electron–electron scattering at 11 GeV, and thus determine the weak charge of the electron to a high precision (2.4%). Two of the key sub-systems in the experimental apparatus are the main detector array, consisting of 224 fused silica detector modules and an electron beam Compton polarimeter. The main detector array can be operated in either pulsed mode for particle tracking, or integration mode for the asymmetry measurement at high rate detection. A subset of these detector modules will be equipped with an array of High Voltage Monolithic Active Pixel Sensors (HVMAPS), to be used in particle tracking and allowing for real-time monitoring of the event profile in the main Møller region. HVMAPS are also used for the Compton polarimeter, consisting of four-plane planes of HVMAPS chips, housed in a vacuum chamber upstream of the interaction point. An overview of the implementation of the HVMAPS for the main detectors and Compton polarimeter is provided.

Kinematic analysis of multiple Compton scattering in quantum-entangled two-photon systems

The Stokes–Mueller method is used to analyze the scattering of entangled photon pairs in a two-photon system. This study examines the scenario where one of the photons, part of a pair of maximally entangled annihilation photons, undergoes intermediate Compton scattering before both photons are detected using Compton polarimeters. The method also accounts for potential quantum-decoherence effects resulting from Compton scattering. The analysis investigates the scattering behavior in both parallel and perpendicular planes, identifying variations in the modulation factor that affect azimuthal correlations. These variations include increases, decreases, sign changes, or disappearances at certain intermediate scattering angles. This work aims to provide theoretical results that support the testing and verification of predictions made by quantum field theory.

Investigation of the spatial resolution of PET imaging system measuring polarization-correlated Compton events

Recent studies of positron emission tomography (PET) devices have shown that the detection of polarization-correlated annihilation quanta can potentially reduce the background and creation of false lines of response (LORs) leading to improved image quality. We developed a novel PET demonstrator system, capable of measuring correlated gamma photons with single-layer Compton polarimeters to explore the potential of the method. We tested the system using sources with clinically relevant activities at the University Hospital Centre Zagreb. Here we present, for the first time, the images of two Ge-68 line sources, reconstructed solely from the correlated annihilation events. The spatial resolution at two different diameters is determined and compared to the one obtained from events with photoelectric interaction.

Experimental demonstration of polarization control in Thomson scattering X-ray sources

Thomson scattering sources are known for generating quasi-monochromatic, high-brightness, and polarization-tunable X-rays, which are widely applied in fields such as fluorescence imaging, magnetic circular dichroism, and nuclear physics. In this paper, we present an experimental demonstration of polarization control at the Tsinghua Thomson scattering X-ray source (TTX) by adjusting the laser polarization. A specially designed Compton polarimeter is employed to effectively analyze the Degree of Linear Polarization (DoLP) of the X-rays produced at the TTX. Our results indicate that the DoLP of X-rays within a collecting angle of 1/(3γ), where γ is the Lorentz factor of the relativistic electron, closely matches the DoLP of the laser. Furthermore, we demonstrate that the X-ray polarization can be continuously modulated from linear polarization to circular polarization by adjusting the laser polarization, which can significantly enhance the functionality and applicability of Thomson Scattering sources for scientific research.

Systematic effects on a Compton polarimeter at the focus of an X-ray mirror

XL-Calibur is a balloon-borne Compton polarimeter for X-rays in the ∼15–80 keV range. Using an X-ray mirror with a 12 m focal length for collecting photons onto a beryllium scattering rod surrounded by CZT detectors, a minimum-detectable polarization as low as ∼3% is expected during a 24-hour on-target observation of a 1 Crab source at 45° elevation.Systematic effects alter the reconstructed polarization as the mirror focal spot moves across the beryllium scatterer, due to pointing offsets, mechanical misalignment or deformation of the carbon-fiber truss supporting the mirror and the polarimeter. Unaddressed, this can give rise to a spurious polarization signal for an unpolarized flux, or a change in reconstructed polarization fraction and angle for a polarized flux. Using bench-marked Monte-Carlo simulations and an accurate mirror point-spread function characterized at synchrotron beam-lines, systematic effects are quantified, and mitigation strategies discussed. By recalculating the scattering site for a shifted beam, systematic errors can be reduced from several tens of percent to the few-percent level for any shift within the scattering element. The treatment of these systematic effects will be important for any polarimetric instrument where a focused X-ray beam is impinging on a scattering element surrounded by counting detectors.

Directional-polarization correlations using EXILL

Eight EXOGAM-type, Ge-clover detectors of the EXILL array, arranged on an octagon plan and acting as Compton polarimeters, were used to measure directional-polarization correlations of γ rays populated in various processes. Measurements of γ radiation following neutron-induced fission of 235U and 241Pu targets; β --decay following fission; (n,γ) reactions on various stable targets and measurements using radioactive sources of 60Co, 133Ba and 152Eu were performed at the PF1B cold-neutron facility of the Institut Laue-Langevin in Grenoble. Digital electronics and a trigger-less acquisition system allowed the collection of signals from Ge detectors down to about 20 keV, providing measurements of linear polarization down to 120 keV. The precise sensitivity calibration was determined for the set of eight EXILL clover polarimeters in a range from 100 keV to 10000 keV. A new formula for calculating the directional-polarization correlations for the upper transition in a γγ cascade is derived from first principles. Directional-polarization correlations were measured for a few dozens of transitions in a number of nuclei, including values for twenty transitions in the 152Sm nucleus measured for the first time, and compared against the calculated values, to illustrate the quality of the technique. The combined analysis of angular and directional-polarization correlations is presented, which helps the determination of spin-parity assignments to excited levels.

Conceptual study of a Compton polarimeter for the upgrade of the SuperKEKB collider with a polarized electron beam

The physics scope of the Belle II experiment currently acquiring data at the SuperKEKB collider will expand with a polarized electron beam upgrade, as recently proposed. Among the required elements for this upgrade, a real time diagnosis of the polarization is necessary to ensure it is large for all bunches in the accelerator during its regular operation. This will be realized by inserting a Compton polarimeter in the accelerator. Its conceptual design is described and no show-stopper for its integration has been identified. An estimation of the sensitivity of the polarimeter is made by means of toy Monte-Carlo studies. The proposed design accounts for the constraint to preserve the performance of the SuperKEKB accelerator and to cope with the short time separation of successive bunches. We show that the polarimeter will measure for each bunch the polarization within five minutes with a statistical precision below 1% and systematic uncertainties below 0.5%. It has the capability of providing this information online on a similar timescale. This work paves the way towards future implementation of real-time Compton polarimetry in several future projects.

Proton irradiation of plastic scintillator bars for POLAR-2

POLAR-2, a plastic scintillator based Compton polarimeter, is currently under development and planned for a launch to the China Space Station in 2025. It is intended to shed a new light on our understanding of Gamma-Ray Bursts by performing high precision polarization measurements of their prompt emission. The instrument will be orbiting at an average altitude of 383 km with an inclination of 42° and will be subject to background radiation from cosmic rays and solar events. In this work, we tested the performance of plastic scintillation bars, EJ-200 and EJ-248M from Eljen Technology, under space-like conditions, that were chosen as possible candidates for POLAR-2. Both scintillator types were irradiated with 58 MeV protons at several doses from 1.89 Gy(corresponding to about 13 years in space for POLAR-2) up to 18.7 Gy, that goes far beyond the expected POLAR-2 life time. Their respective properties, expressed in terms of light yield, emission and absorption spectra, and activation analysis due to proton irradiation are discussed. Scintillators activation analyses showed a dominant contribution of β + decay with a typical for this process gamma-ray energy line of 511 keV.

Testing entanglement of annihilation photons

We present a new experimental study of the quantum entanglement of photon pairs produced in positron-electron annihilation at rest. Each annihilation photon has an energy that is five orders of magnitude higher than the energy of photons in optical experiments. It provides a unique opportunity for controlled Compton pre-scattering of initial photons before the polarization measurements. The experimental setup includes a system of Compton polarimeters to measure the angular correlations of annihilation photons in initial and thus prepared pre-scattered states. For the first time, a direct comparison of the polarization correlations of initial and pre-scattered annihilation photons has been carried out. The angular distributions of scattered in polarimeters photons turned out to be the same for both types of events. Moreover, the correlation function in the Bell’s inequality is also the same for both states. We discuss the implications of our results for quantum measurement theory and for the quantum-entangled positron emission tomography.

Compact Polarized X-Ray Source Based on All-Optical Inverse Compton Scattering

Polarized x-ray source is a useful probe for many fields, such as fluorescence imaging, magnetic microscopy, and nuclear physics research. All-optical inverse Compton scattering source (AOCS) based on a laser wakefield accelerator (LWFA) has drawn great attention in recent years due to its compact scale and high performance, especially its potential to generate polarized x rays. Here, polarization-tunable AOCS x rays are generated by a plasma-mirror-based scheme. The linearly and circularly polarized AOCS x rays are measured to have the mean photon energy of $60(\ifmmode\pm\else\textpm\fi{}5)/64(\ifmmode\pm\else\textpm\fi{}3)$ keV and the single-shot photon yield of approximately $1.1/1.3\ifmmode\times\else\texttimes\fi{}{10}^{7}$. A Compton polarimeter is designed to diagnose the x-ray polarization states, demonstrating the polarization tunability of AOCS, and indicating that the average polarization degree of the linearly polarized AOCS is $75(\ifmmode\pm\else\textpm\fi{}3)\mathrm{%}$ within 18.2 mrad x-ray divergence.

Electron beam polarimeter and energy spectrometer

The backscattering of laser radiation by a relativistic electron beam is a widely used method for measuring the spin polarization of electrons. We consider again the properties of the scattering process paying special attention to recoil electrons. Based on this consideration we propose the concept of the Compton polarimeter in which, in addition to all the polarization components, it becomes possible to accurately measure the energy and other parameters of the electron beam. To demonstrate the capabilities of the method we conduct a Monte Carlo simulations of the polarimeter developed for the FCC-ee project.

Setup of Compton polarimeters for measuring entangled annihilation photons

An experimental setup for studying the Compton scattering of annihilation photons in various (entangled and decoherent) quantum states is presented. Two entangled γ-quanta with an energy of 511 keV and mutually orthogonal polarizations are produced by positron-electron annihilation in a thin aluminum plate and are emitted in opposite directions. To measure both photons, the setup provides two equivalent arms of Compton polarimeters. A Compton polarimeter consists of a plastic scintillation scatterer and an array of NaI(Tl) detectors for measuring photons deflected at an angle of 90°. The intermediate scatterer of the GAGG scintillator with SiPM readout is inserted into one of the arms to create a tagged decoherence process prior to the measurement of annihilation photons in polarimeters. The performance of Compton scatterers and NaI(Tl) counters is discussed. The polarization modulation factor and the analyzing power of Compton polarimeters are evaluated from the angular distributions of scattered gammas. The Compton scattering of photons in entangled and decoherent states is compared reliably for the first time.

Polarization measurement of L -shell radiative recombination x rays from highly charged bismuth ions

We report a polarization measurement of $L$-shell radiative recombination (RR) x rays emitted from He- to F-like bismuth ions. The polarization feature was obtained from the azimuthal angular distribution of the Compton-scattered RR x rays detected with a recently developed state-of-the-art Compton polarimeter installed on an electron beam ion trap. This is one of a very few polarization measurements of RR x rays ever conducted and the first one with bismuth ions. We derived the degree of linear polarization averaged over the charge states to be $0.666\ifmmode\pm\else\textpm\fi{}0.014$. By contrast, a theoretical estimation of the degree of polarization with the Flexible Atomic Code (fac) yields $0.716\ifmmode\pm\else\textpm\fi{}0.023$. The discrepancy between the observed and theoretical values is likely to be due to the electron correlation effects.

The polarization sensitivity of GRETINA

Compton polarimeters have played an important role in the study of nuclear structure physics, but have often been limited in their applications because of relatively low γ-ray detection efficiency. With the advent of γ-ray tracking detector arrays, which feature nearly 4π solid angle coverage and the ability to identify the location of Compton-scattering events to within a few millimeters, this limitation can be overcome. Here we present a characterization of the performance of the Gamma Ray Energy Tracking In-beam Nuclear Array (GRETINA) as a Compton polarimeter using the 24Mg(p,p′) reaction at 2.45 MeV proton energy. We also discuss a new capability added to the simulation package UCGretina to simulate the emission of polarized photons, and compare it to the measured data. Finally, we use these simulations to predict the performance of the Gamma Ray Energy Tracking Array (GRETA).

Possible Polarization Measurements in Elastic Scattering at the Gamma Factory Utilizing a 2D Sensitive Strip Detector as Dedicated Compton Polarimeter

AbstractFor photon energies from several 10 keV up to a few MeV Compton polarimetry is an indispensable tool to gain insight into subtle details of fundamental atomic radiative processes. Within the SPARC collaboration several segmented semiconductor detectors are developed that are well suited for application as efficient Compton polarimeters. In this report, these recent developments are reviewed and it is discussed how Compton polarimetry can be employed at the upcoming Gamma Factory.

First in-beam experiment with the ELIADE detectors: a spectroscopic study of 130La

The new facility, Extreme Light Infrastructure – Nuclear Physics (ELI-NP), is a combined laser-gamma nuclear physics research facility currently undergoing its final implementation stages in Măgurele near Bucharest, Romania. It already hosts two fully-operational 10 PW laser arms and, by 2023, it will also house a γ-beam system based on laser Compton backscattering, capable of delivering a high-brilliance, low-energy beam at E γ ≲ 19.5 MeV. Owing to this unique laser-gamma instrumentation combination, several types of experiments will be possible at ELI-NP, including high precision nuclear resonance fluorescence (NRF) experiments. In this case, the main γ-beam detection system for performing NRF studies at ELI-NP is represented by the ELI Array of DEtectors (ELIADE), featuring eight high-purity germanium (HPGe) segmented clover detectors. The current work presents the characteristics of two of the ELIADE detectors, including their photopeak detection efficiency, energy resolution, and peak-to-total ratio measured using γ-ray sources, as well as the timing performance obtained via in-beam measurements. For these latter detector tests, 130La was populated via the fusion evaporation reaction 121Sb(12C,3n)130La using a beam energy of 53 MeV at the Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), also located in Măgurele. Herein, we report on the results of the ^130La linear polarization measurements taken using the ELIADE detectors as Compton polarimeters. The results obtained from the in-beam experiment were compared to several already published works and we present new information on the transition multipolarity in 130La.

Redesigning the Jefferson Lab Hall A beam line for high precision parity experiments

The Continuous Electron Beam Accelerator Facility (CEBAF) was built with a thermionic electron source and the three original experimental hall lines reflected this. A few years after beam delivery began a parity violation experiment was approved and two polarimeters were installed in the Hall A beam line. The beam raster system was placed after the new Compton polarimeter, before one accelerator quadrupole and four quadrupoles in the new Moller polarimeter. It was very difficult to meet experimental requirements on envelope functions and raster shape with this arrangement so a sixth quadrupole was installed downstream of the Moller polarimeter to provide an additional degree of freedome. All of the parity experiments in Hall A have been run with this still-unsatisfactory configuration. The MOLLER experiment is predicated on achieving a 2% error on a 32 ppb asymmetry. Beam line changes are required to meet the systematic error budget. This paper documents the existing beam line, an interim change which can be accomplished during a annual maintenance down, and the final configuration for MOLLER and subsequent experiments.

An application of a Si/CdTe Compton camera for the polarization measurement of hard x rays from highly charged heavy ions.

Methods to measure the polarization of x rays from highly charged heavy ions with a significantly higher accuracy than that of the existing technology are needed to explore relativistic and quantum electrodynamics effects, including the Breit interaction. We developed an Electron Beam Ion Trap Compton Camera (EBIT-CC), a new Compton polarimeter with pixelated multi-layer silicon, and cadmium telluride counters. The EBIT-CC detects the three-dimensional position of Compton scattering and photoelectric absorption, and thus, the degree of polarization of incoming x rays can be evaluated. We attached the EBIT-CC on the Tokyo Electron Beam Ion Trap (Tokyo-EBIT) in the University of Electro-Communications. An experiment was performed to evaluate its polarimetric capability through an observation of radiative recombination x rays emitted from highly charged krypton ions, which were generated by the Tokyo-EBIT. The CC of the EBIT-CC was calibrated for the ∼75keV x rays. We developed event reconstruction and selection procedures and applied them to every registered event. As a result, we successfully obtained the polarization degree with an absolute uncertainty of 0.02. This uncertainty is small enough to probe the difference between the zero-frequency approximation and full-frequency-dependent calculation for the Breit interaction, which is expected for dielectronic recombination x rays of highly charged heavy ions.

Feasibility of Observing Gamma-Ray Polarization from Cygnus X-1 Using a CubeSat

Abstract Instruments flown on CubeSats are small. Meaningful applications of CubeSats in astronomical observations rely on the choice of a particular subject that is feasible for CubeSats. Here we report the result of a feasibility study for observing gamma-ray polarization from Cygnus X-1 using a small Compton polarimeter on board a 3U CubeSat. Silicon detectors and cerium bromide scintillators were employed in the instrument models that we discuss in this study. Through Monte Carlo simulations with a Geant4-based MEGAlib package, we found that, with a 10 Ms on-axis, zenith-direction observation in a low-inclination, low-altitude, Earth-orbit radiation background environment, the minimum detectable polarization degree can be down to about 10% in 160–250 keV, 20% in 250–400 keV, and 65% in 400–2000 keV. A 3U CubeSat dedicated to observing Cygnus X-1 can therefore yield useful information on the polarization state of gamma-ray emissions from the brightest persistent X-ray black hole binary in the sky.

A fully automated data center for the space-borne hard X-ray Compton polarimeter POLAR developed at PSI

Abstract Constant data inflow from the hard X-ray polarimeter POLAR onboard of the Chinese Space Laboratory TG-2 requires fully automated and safe guarded data processing. For this purpose, a dedicated data center was established at the Paul Scherrer Institute (PSI). This paper presents its design concept and structure, and describes the data flow and data products, data processing pipelines, quick-look utilities and web-based applications. Because the amount of POLAR telemetry data can reach up to 100 GB daily, specialized software and hardware with dedicated architecture are required to store and process this large amount of data. POLAR is a space-borne detector to precisely measure the linear polarization of hard X-rays emitted from solar flares and gamma-ray bursts in the energy range of 50–500 keV. It was launched into a low earth orbit on September 15th, 2016.