POLAR

A compact detector for Gamma Ray Bursts photon polarization measurements

56 years after their discovery (1960 Vela satellites), gamma ray bursts (GRBs) are still full of mysteries. The production mechanism of these very intense explosions in the universe is still unknown. To validate or exclude existing models about their creation, a precise measurement of the polarization of the GRB is an important input. But, measuring convincingly the polarization of GRBs is a difficult experimental challenge, because GRBs are rare events that happen at random locations and times in the universe. Thus, existing general purpose detectors cannot properly measure the polarization and existing results are often unprecise and controversial. 
The POLAR detector design is a dedicated polarimeter:

• The Compton Scattering Effect is used to measure the polarization of incoming photons.
• POLAR makes no attempt to do a precise localization of the GRB in the sky. This greatly simplifies the design of the detector. Currently, special purpose satellites like SWIFT can provide this information immediately. Many future satellites will have similar capabilities. To complement this, POLAR can roughly localize strong GRBs independently and with sufficient precision for a stand-alone measurement.

Artistic view of the POLAR instrument on the Tiangong-2 chinese spacelab

POLAR flight spare passing the thermal balance tests in Shanghai

POLAR FMS in display at Beijing science museum

POLAR flight model just before beeing definitively closed in DPNC clean room

POLAR attached mounting to Tiangong2

POLAR attached to Tiangong2 just before closing the fairing

Control room, POLAR can be seen flying in space on the screen to the right

Launch of Tiangong2

First declassified image of Tiangong2, POLAR is bolted on top of Tg2

RAW photon rate of POLAR

RAW cosmic rate of POLAR

Paused

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The POLAR detector

The POLAR Detector Design is based on the following requirements:

• A simple and compact instrument that:
  • relies on a given burst position and spectrum
  • or provides a crude estimate in case of stand-alone observation
• Dedicated for GRB observations only, thus needs:
  • a large effective area,
  • a large modulation factor
  • a large field-of-view.
• The energy range for the incoming photons needs to cover 50 keV to 500 keV, a region, where the Compton Scattering Effect is dominant.

These requirements lead to the basic design of POLAR:

• 1600 scintillating bars form a uniform array of 40x40
• Light, fast and low Z plastic scintillators are used, optimised for compton scattering
• The scintillator size of 6x6x200 mm matches the chosen readout photomultiplier MAPM8500 from Hamamatsu
• passive shielding will be used to protect POLAR against low energetic cosmic rays
• the main readout electronics will be housed under the detection target
• the detection target achieves:
  • an effective area of 400 cm2,
  • a modulation factor of about 35 percent
  • and a field-of-view covering about one third of the sky

The POLAR collaboration

• INSTITUTE OF HIGH ENERGY PHYSICS (IHEP), CHINESE ACADEMY OF SCIENCES

  Tianwei bao, Junying Chai, Yongwei Dong, Minnan Kong, Siwei Kong, Lu Li, Jiangtao Liu, Xin Liu, Haoli Shi, Liming Song, Jianchao Sun, Ruijie Wang, Xing Wen, Bobing Wu, Hanhui Xu, Laiyu Zhang, Li Zhang, Shuangnan Zhang, Xiaofeng Zhang, Yongjie Zhang
  Address: Yuquan Rd. 19B, Shijingshan district, Beijing
  Phone: +86-10-88236415
  Email: wubb(at)ihep.ac.cn

• DPNC, UNIVERSITY OF GENEVA, GENEVA, SWITZERLAND

  F. Cadoux, N. De Angelis, M. Kole, X. Wu Email: Merlin.Kole(at)unige.ch

• PAUL SCHERRER INSTITUT, VILLIGEN, SWITZERLAND

  Hualin Xiao, R. Marcinkowski, W. Hajdas Email: Wojtek.Hajdas(at)psi.ch

• ISDC DATA CENTER FOR ASTROPHYSICS, UNIVERSITY OF GENEVA, GENEVA, SWITZERLAND

  H.C. Li, N. Produit Email: Nicolas.Produit(at)unige.ch

• NATIONAL CENTRE FOR NUCLEAR RESEARCH, SWIERK, POLAND

  T. Batsch, A. Zwolinska, J. Szabelski, D. Rybka Email: dominik.rybka(at)ncbj.gov.pl

Publications

POLAR GRB light curves