Xin Zhang

The influences of monovalent ions on the stability of scintillation properties and radiation hardness of PbWO4:Y3+ crystals

Abstract Some Y 3+ -doped PbWO 4 crystals grown by modified Bridgmann method showed exceptional behaviors, namely, light yield increased after low dose rate irradiation, and the radiation-induced absorption coefficient was negative around 430xa0nm. In this paper, the influences of monovalent ion impurities (Na + and K + ) on the stability of scintillation properties and radiation hardness were studied. The experimental results show that Na + ion enhances the absorption band at cut-off edge and at 430xa0nm, while K + ion only makes optical transmission unstable around 430xa0nm when the crystals were annealed at high temperatures. The contamination by Na + and K + ions makes scintillation properties and radiation hardness more unstable in the temperature range from 50°C to 350°C. The radiation-induced absorption coefficient is also negative around 430xa0nm. The charge compensation mode of K + ion along the crystal does not change, but those of Na + ion are different along the crystal. The concentrated Na + , K + and Y 3+ ions, which occupied Pb sublattice, lead to the shortage of Pb vacancies ( V Pb ″). Thus instead of the formation of [2(Y 3+ Pb ) — V Pb ″] ((Y 3+ Pb ) stands for the Y 3+ ion occupying the Pb sublattice), Na + and K + ions compel the defect (Y 3+ Pb ) to form cluster [2(Y 3+ Pb ) —O i ″] or [(Y 3+ Pb ) —O i ″+hole] in which the interstitial oxygen (O i ″) was considered to be the origin of light yield increase after low dose rate irradiation.

The study of light yield increase after low dose rate irradiation in Y3+ doping PbWO4 crystals

Abstract Although trivalent ions (such as La 3+ , Lu 3+ and Y 3+ ) doping can significantly improve the radiation hardness of lead tungstate crystals, some Y 3+ doping PbWO 4 (PWO) crystals showed exceptional irradiation behavior, namely, light yield increase after low dose rate irradiation. In addition the radiation hardness was sensitive to annealing temperature. In this study, we chose typical Y 3+ doping crystal samples and investigated the relationship between transmission, light yield and radiation hardness. The experimental results show that high temperature annealing can enhance the absorption band around 430xa0nm, while annealing at 50°C produces different results and can suppress this exceptional behavior. Light output increase not only exists in the top side of the boule, but also in the seed side, and is accompanied with optical transmission change in the wavelengths from 380xa0nm to about 500xa0nm simultaneously. The pre-existing 430xa0nm color centers in as-grown PWO:Y 3+ or annealing induced ones are unstable and can be “bleached” by low dose rate irradiation. The results of annealing and irradiation experiments of the samples show that the radiation hardness phenomenon was sensitive to annealing temperature, and thus a proper annealing temperature for PWO crystals was determined.

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.

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

Gamma-ray polarimetry is a new powerful tool to study the processes responsible for the emission from astrophysical sources and the environments in which this emission takes place. Few successful polarimetric measurements have however been performed thus far in the gamma-ray energy band due to the difficulties involved. POLAR is a dedicated polarimeter designed to perform high precision measurements of the polarization of the emission from gamma-ray burst in the 50-500 keV energy range. This new polarimeter is expected to detect approximately 50 gamma-ray bursts per year while performing high precision polarization measurements on approximately 10 bursts per year. The instrument was launched into lower earth orbit as part of the second Chinese space lab, the Tiangong-2, on September 15th 2016 and has been taking data successfully since being switched on one week after. The instrument uses a segmented scintillator array consisting of 1600 plastic scintillator bars, read out by 25 flat-panel multi-anode photomultipliers, to measure the Compton scattering angles of incoming photons. The small segmentation and relatively large uniform effective area allow the instrument to measure the polarization of a large number of transient events, such as gamma-ray bursts, with an unprecedented precision during its two year life-time. The final flight model underwent detailed calibration prior to launch as well as intensive space qualification tests, a summary of which will be presented in this paper. The instrument design will be discussed first followed by an overview of the on-ground tests, finally the in-orbit behavior as measured during the first weeks of the mission will be presented.

Calibration of gamma-ray burst polarimeter POLAR

Gamma Ray Bursts (GRBs) are the strongest explosions in the universe which might be associated with creation of black holes. Magnetic field structure and burst dynamics may influence polarization of the emitted gamma-rays. Precise polarization detection can be an ultimate tool to unveil the true GRB mechanism. POLAR is a space-borne Compton scattering detector for precise measurements of the GRB polarization. It consists of a 40×40 array of plastic scintillator bars read out by 25 multi-anode PMTs (MaPMTs). It is scheduled to be launched into space in 2016 onboard of the Chinese space laboratory TG2. We present a dedicated methodology for POLAR calibration and some calibration results based on the combined use of the laboratory radioactive sources and polarized X-ray beams from the European Synchrotron Radiation Facility. They include calibration of the energy response, computation of the energy conversion factor vs. high voltage as well as determination of the threshold values, crosstalk contributions and polarization modulation factors.

Low dose rate irradiation behaviors of Y3+-doped PbWO4 crystals

Abstract Light yield of some Y 3+ : PWO crystals increases after irradiation, and is accompanied with transmission change in the wavelengths from 380 to 500xa0nm. The pre-existing 430xa0nm color centers or annealing induced ones are unstable during irradiation. These results interpret the temperature sensitivity of light yield and radiation hardness. A proper annealing temperature and the way to eliminate the instability are found out.