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Polarimeter: Activities

B. Wojtsekhowski and B. Vlahovic already developed a high energy photon polarimeter which was successfully implemented at Jefferson National Laboratory nuclear experiments. That polarimeter has the highest analyzing power ever achieved for the energies of few MeV up to few GeV.i The concept of the polarimeter and the theoretical work is shown in our NIM paper.

This seed project has the goal to expand the application of the polarimeter developed for nuclear physics for astrophysics measurements. Since 1958 when the very first cosmic gamma-rays were detected from the class II solar flare, much progress has been made in high-energy astrophysics due to advancements in detector technology. It has been possible to observe the energy spectrum and time variability of stellar objects in a wide energy range from below 1 keV to several hundred GeV. However, for detailed research of radiation mechanisms from high-energy pulsars and magnetars (PSR 1509-58), black hole binaries (Cyg X-1), active galactic nuclei and blazars (Mkn 421, 3C273), supernova remnants (Crab Nebula), and gamma-ray bursters, observations of polarization are extremely important. The polarization measurements are anticipated to play a significant role in major discoveries addressing fundamental questions of thermal and relativistic astrophysics in the universe.

The CREST Center supported calculations needed for development a prototype that will be adequate for NASA mission. Our task was focused on the concept development and the test of the models that could work for NASA and will allow us to submit NASA proposal which will further facilitate this research. The models of the polarimeter considered in ours calculations are explained bellow and in the finding section of the report. The NASA proposal was submitted and awarded.

We have to consider several options for the photon polarimeters based on QED processes which have a calculable asymmetry and large cross section. They are the photo-effect at keV energy range, Compton scattering up to 10 MeV, and a pair production at higher photon energy (three sub-options here are: coherent production from a crystal, pair from a nuclei and pair from an electron). All these methods were successfully used for the photon beam polarimetry which doesn't require a high efficiency of the polarimeter but benefits from a large analyzing power. Secondary photon polarimetry was used in few nuclear physics experiments including parity-non-conservation measurements in the polarized neutron on proton reaction np  d and neutrino helicity determination.

At the medium energy range 0.5-30 MeV under construction is the Advanced Compton Telescope (ACT) which will consist of 16 cm x 16 cm three dimensional track imagining detectors. The recently launched AGILE and GLAST telescope that use silicon-strip detectors interleaved with thin metal foils to track electron-positron pairs from photons with energies above ~20 to 100 MeV consists of 16 towers, each containing 18 layers of silicon strip detectors with thin tungsten plates in between. For high-energy region 50 MeV -100 GeV, the Advanced Pair Telescope (APT) is under development.

All previous and current instruments for energies > 30 MeV have been based on pair production in thin metal foils that has been recognized for some time but is limited by the effects of multiple Coulomb scattering, which makes it difficult to define the plane of pair production. Efforts to measure polarization both with COS-B and with CGRO/EGRET have been unsuccessful, largely for this reason. It also appears that both GLAST and AGILE will suffer from similar difficulties, making polarization measurements with those instruments unlikely. One recent design for an effective pair production polarimeter involves the use of gas micro-well detectors for tracking the electron-positron pair with minimal scattering and 3-Dimensional Track Imagining a large production chamber (TPC) with a two dimensional gas micro-well detector (MWD) readout.

We performed the following specific tasks:

  • Considered possibilities for the improvement of the existing polarimeters, data acquisition and data analysis; How to recalculate the existing data collected by several NASA missions, to reduce the errors and improve the signal to noise ratio. The motivation for this work is based on our new theoretical approach and our numerical results that indicate that our theoretical model has much higher analayzing power and that the existing data may be re organized to be suitable for our approach.
  • Calculations for the pair production based microstrip polarimeter for linearly polarized medium energy photons. In this kind of polarimeter the electron positron pair which contains information related to the photon polarization is produced on nuclei. We considered a prototype of a polarimeter made of many layers of Si micro strip detectors.
  • Calculations foe a polarimeter prototype based on triplet pair production. In this kind of the polarimeter pairs are produced from interaction with atomic electrons.