Tianwei Bao

A crosstalk and non-uniformity correction method for the space-borne Compton polarimeter POLAR

Abstract In spite of extensive observations and numerous theoretical studies in the past decades several key questions related with Gamma-Ray Bursts (GRB) emission mechanisms are still to be answered. Precise detection of the GRB polarization carried out by dedicated instruments can provide new data and be an ultimate tool to unveil their real nature. A novel space-borne Compton polarimeter POLAR onboard the Chinese space station TG2 is designed to measure linear polarization of gamma-rays arriving from GRB prompt emissions. POLAR uses plastics scintillator bars (PS) as gamma-ray detectors and multi-anode photomultipliers (MAPMTs) for readout of the scintillation light. Inherent properties of such detection systems are crosstalk and non-uniformity. The crosstalk smears recorded energy over multiple channels making both non-uniformity corrections and energy calibration more difficult. Rigorous extraction of polarization observables requires to take such effects properly into account. We studied influence of the crosstalk on energy depositions during laboratory measurements with X-ray beams. A relation between genuine and recorded energy was deduced using an introduced model of data analysis. It postulates that both the crosstalk and non-uniformities can be described with a single matrix obtained in calibrations with mono-energetic X- and gamma-rays. Necessary corrections are introduced using matrix based equations allowing for proper evaluation of the measured GRB spectra. Validity of the method was established during dedicated experimental tests. The same approach can be also applied in space utilizing POLAR internal calibration sources. The introduced model is general and with some adjustments well suitable for data analysis from other MAPMT-based instruments.

Design and construction of the POLAR detector

Abstract The POLAR detector is a space based Gamma Ray Burst (GRB) polarimeter with a wide field of view, which covers almost half the sky. The instrument uses Compton scattering of gamma rays on a plastic scintillator hodoscope to measure the polarization of the incoming photons. The instrument has been successfully launched on board of the Chinese space laboratory Tiangong 2 on September 15, 2016. The construction of the instrument components is described in this article. Details are provided on problems encountered during the construction phase and their solutions. Initial performance of the instrument in orbit is as expected from ground tests and Monte Carlo simulation.

The POLAR gamma-ray burst polarimeter onboard the Chinese Spacelab

POLAR is a joint European-Chinese experiment aimed at a precise measurement of hard X-ray polarization (50-500 keV) of the prompt emission of Gamma-Ray Bursts. The main aim is a better understanding of the geometry of astrophysical sources and of the X-ray emission mechanisms. POLAR is a compact Compton polarimeter characterized by a large modulation factor, effective area, and field of view. It consists of 1600 low-Z plastic scintillator bars read out by 25 at-panel multi-anode photomultipliers. The incoming X-rays undergo Compton scattering in the bars and produce a modulation pattern; experiments with polarized synchrotron radiation and GEANT4 Monte Carlo simulations have shown that the polarization degree and angle can be retrieved from this pattern with the accuracy necessary for identifying the GRB mechanism. The flight model of POLAR is currently under construction in Geneva. The POLAR instrument will be placed onboard the Chinese spacelab TG-2, scheduled for launch in low Earth orbit in 2015. The main milestones of the space qualification campaign will be described in the paper.

A method to measure the transition energy γ t of the isochronously tuned storage ring

Abstract The Isochronous Mass Spectrometry (IMS) is a powerful technique developed in heavy-ion storage rings for measuring masses of very short-lived exotic nuclei. The IMS is based on the isochronous setting of the ring. One of the main parameters of this setting is the transition energy γ t . It has been a challenge to determine the γ t and especially to monitor the variation of γ t during experiments. In this paper we introduce a method to measure the γ t online during IMS experiments by using the acquired experimental data. Furthermore, since the storage ring has (in our context) a relatively large momentum acceptance, the variation of the γ t across the ring acceptance is a source of systematic uncertainty of measured masses. With the installation of two time-of-flight (TOF) detectors, the velocity of each stored ion and its revolution time are simultaneously available for the analysis. These quantities enabled us to determine the γ t as a function of orbital length in the ring. The presented method is especially important for future IMS experiments planned at the new-generation storage ring facilities FAIR in Germany and HIAF in China.

In-flight energy calibration of the space-borne Compton polarimeter POLAR

POLAR is a compact wide-field space-borne detector for precise measurements of the linear polarisation of hard X-rays emitted from gamma-ray burst and solar flares in the energy range of 50 keV to 500 keV. It consists of a 40 x 40 array of plastic scintillator bars used as a detection material. POLAR was launched into a low Earth orbit on-board the Chinese space-lab TG-2 on September 15, 2016. To achieve high accuracies in polarisation measurements it is essential to perform a precise energy calibration both before and during the flight. Such calibrations are performed with four low activity Na-22 radioactive sources placed inside the instrument. Energy conversion factors are related to Compton edge positions from the collinear annihilation photons from the sources. This paper presents the main principles of the in-flight calibration, describes studies of the method based on Monte Carlo simulations and its laboratory verification, and provides some observation results based on the in-flight data analysis

Instrument performance and simulation verification of the POLAR detector

Abstract POLAR is a new satellite-born detector aiming to measure the polarization of an unprecedented number of Gamma-Ray Bursts in the 50–500 keV energy range. The instrument, launched on-board the Tiangong-2 Chinese Space lab on the 15th of September 2016, is designed to measure the polarization of the hard X-ray flux by measuring the distribution of the azimuthal scattering angles of the incoming photons. A detailed understanding of the polarimeter and specifically of the systematic effects induced by the instrument’s non-uniformity are required for this purpose. In order to study the instrument’s response to polarization, POLAR underwent a beam test at the European Synchrotron Radiation Facility in France. In this paper both the beam test and the instrument performance will be described. This is followed by an overview of the Monte Carlo simulation tools developed for the instrument. Finally a comparison of the measured and simulated instrument performance will be provided and the instrument response to polarization will be presented.

Masses of the Tz = −3/2 nuclei 27P and 29S

Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the

POLAR: Final calibration and in-flight performance of a dedicated GRB polarimeter

T_z=-3/2

POLAR trigger — Experimental verification

nuclei

Calibration of gamma-ray burst polarimeter POLAR

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