Xin Wu


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

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


POLAR GRB light curves


Département de Physique Nucléaire et Corpusculaire | Impressum.