R. Marcinkowski

/sup 6/LiI(Eu) in neutron and /spl gamma/-ray spectrometry-a highly sensitive thermal neutron detector

Europium doped 6LiI crystals (enriched to 96%6 Li) have been studied in neutron detection and gamma-ray spectrometry. Two crystals, 50 mmtimes5 mm and 30 mmtimes3 mm in size were coupled to a calibrated Photonis XP5200 photomultiplier, were tested. The response of 6LiI(Eu) to neutrons emitted from a paraffin moderated Pu-Be source has been investigated and the thermal neutron peak has been found at a Gamma Equivalent Energy (GEE) of about 3.5 MeV. The high sensitivity of the 6LiI(Eu) crystal is demonstrated by the observation of a neutron peak at a dose equivalent as low as 0.05 muSv/h. Apart from the neutron response, the light output, energy resolution and nonproportionality of the 6LiI(Eu) crystals versus gamma-ray energies have been measured. The light yield of 1.5times104 ph/MeV (about 40% of NaI(Tl)) was obtained and the energy resolution at 662 keV (137 Cs) of 7.5plusmn0.1% was found for the large crystal. The linearity of the light output with energy has been measured and is better compared to NaI(Tl). Due to the high sensitivity to thermal neutrons and good proportionality against gamma-ray energy, the 6 LiI(Eu) crystal was tested with a few samples of nuclear materials. Shielded samples can be recognized via detection of neutrons following spontaneous fission

A method to localize gamma-ray bursts using POLAR

The hard X-ray polarimeter POLAR aims to measure the linear polarization of the 50–500 keV photons arriving from the prompt emission of γ-ray bursts (GRBs). The position in the sky of the detected GRBs is needed to determine their level of polarization. We present here a method by which, despite of the polarimeter incapability of taking images, GRBs can be roughly localized using POLAR alone. For this purpose scalers are attached to the output of the 25 multi-anode photomultipliers (MAPMs) that collect the light from the POLAR scintillator target. Each scaler measures how many GRB photons produce at least one energy deposition above 50 keV in the corresponding MAPM. Simulations show that the relative outputs of the 25 scalers depend on the GRB position. A database of very strong GRBs simulated at 10 201 positions has been produced. When a GRB is detected, its location is calculated searching the minimum of the χ2χ2 obtained in the comparison between the measured scaler pattern and the database. This GRB localization technique brings enough accuracy so that the error transmitted to the 100% modulation factor is kept below 10% for GRBs with fluence Ftot≥10−5ergcm−2. The POLAR localization capability will be useful for those cases where no other instruments are simultaneously observing the same field of view.

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.

Shielding an MCP Detector for a Space-Borne Mass Spectrometer Against the Harsh Radiation Environment in Jupiter’s Magnetosphere

Detectors of scientific instruments on spacecraft flying through Jupiter radiation belts need to be protected from high fluxes of penetrating radiation by means of radiation shields. Electrons constitute the most difficult component of Jupiter’s magnetosphere to shield from, because of their abundance, penetration depth in matter, and intensity of bremsstrahlung radiation generated upon interaction with the shielding material. For the Neutral and Ion Mass spectrometer (NIM) of the Particle Environment Package (PEP) instrument suite on board the European Space Agency’s mission JUpiter Icy moons Explorer (JUICE), we devised a shielding design made of an aluminum and tantalum stack to reduce the radiation-induced noise on its Micro-Channel Plate (MCP) detector. To predict the expected radiation background in the mass spectra in space, we manufactured a flight-like shielded detector and submitted it to radiation testing at the Paul Scherrer Institut with an electron beam in the energy range ~ 30 to ~ 345 MeV. The results of this test provide a verification of the NIM capability to fulfill its science requirements in the mission’s worst-case scenario (the Europa flyby), and give insights into new directions of optimization of shielding elements’ design for NIM and similar instrument bound to operate in a harsh radiation environment.

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.

POLAR trigger — Experimental verification

POLAR is a space-borne instrument designed for measurements of the polarization of the prompt hard X- and gamma-ray emission from the Gamma Ray Bursts (GRB). POLAR consists of 25 identical Detection Modules equipped with Front-End Electronics (FEE) units. This paper describes: design, strategy and verification process of the POLAR trigger mechanism.

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.

Development of the Central Task Processing Unit for space-borne Gamma-Ray Burst polarimeter, POLAR

POLAR, a joint European-Chinese experiment, is a novel compact space-borne Compton polarimeter conceived and optimized for detection of the prompt emission of Gamma-Ray Bursts (GRB) and precise measurements of polarization in the hard X-ray energy range 50-500 keV. The complete instrument consists of two parts: internal one, placed inside spacelab and the detector itself, placed outside spacelab, called respectively IBOX and OBOX. The OBOX constitutes of 25 frontend electronic modules (FEE), high voltage and low voltage power supplies and the Central Task Processing Unit. The main functions of Central Task Processing Unit system are defined as follows: communication and transfer of data to IBOX, communication with all frontends, analysis of trigger signals and generation of global trigger signals, data acquisition, synchronizing of all frontends and control of power supplies. The functional requirements are fulfilled by three individual FPGA chips named respectively to their functions: Concentrator, Trigger and CPU. This article presents description of the Central Task Processing Unit hardware design and brief introduction to main components of the firmware developed for this device. Ongoing integration activities of the device with the complete POLAR instrument proved that all basic functions are working correctly. The qualification model of the instrument has been constructed and currently undergoes verification and validation tests in view of planned flight onboard the Chinese spacelab TG-2 scheduled for 2015.