Swiss Physical Society Meeting Geneva, March 2008

POLAR: Design of a novel X-ray polarimeter based on plastic scintillators and MAPMTs.

Simulation Laboratory Source position

dependence 10-20 % 10-15 %

Best results with wrapping

Aluminum + Air gap

3M® reflective foil + Air gap

Scintillator surface Crucial point High quality

- The Monte Carlo simulations show POLAR capabilities to measure GRB polarization (typical µ100 ≈ 37%. MDP(3σ)≈10% for a GRB of E=10-5 erg cm-2 s-1 ).

- Plastic bar light collection ~45%. Signal dependence on source position ~15%. Scintillator surface quality is crucial. Results obtained in laboratory confirm the simulations.

- Present status & Future tests: Demo model (2 modules 8x8 bars each) is being tested in the laboratory. It will be exposed to polarized photons from PSI Synchrotron Light Source. The Engineering-Qualification Model finished by 2010.

- Launch opportunities for the Flight Model: Chinese Space Agency (≈ 2012) International Space Station ISS

Description of POLAR - Polarimeter based on Compton Scattering - Incoming photon energy range: 50 keV – 500 keV - Array of 40x40 plastic scintillators (6x6x200 mm3) - Light, fast and low-Z plastic: BC400 - 25 MAPMT, H8500 Hamamatsu (each 8x8 anodes) - Readout: ASIC + FPGA. - Volume: 30x30x30 cm3; mass: 30 kg - Mean power Consumption below 30 W - Large field of view: more than 1/3 of the sky - Large modulation factor: 40% (at 200 keV) - Large effective area: 400 cm2 (at 200 keV)

1Centre de Physique des Particules de Marseille (CPPM, France) 2Département de physique Nucléaire et Corpusculaire, Université de

Genève (DPNC, Switzerland) 3Institute of High Energy Physics (IHEP, China)

[1] Produit, N. et al., 2005. NIM in Physics Research A, 550: 616-625. [2] Hajdas, W. et al., 2006. SPIE Conf. Proc. 6266: 84.

Estela Suarez-Garcia2,4, T.J.-L. Courvoisier4, M. Gierlik7, D. Haas4, W. Hajdas6, M. Jaworski6, G. Lamanna5, C. Leluc2, R. Marcinkowski7, A. Mtchedlishvili6, S.Orsi2, M. Pohl2, N. Produit4, D. Rapin2, Ch. Tao1 , J.-P. Vialle5, R. Walter4, G. Wrochna7, B. Wu3 , S. Xiang3, S.N. Zhang3

4INTEGRAL Science Data Centre (ISDC, Switzerland) 5Laboratoire d'Annecy de Physique des Particules (LAPP, France)

6Paul Scherrer Institut (PSI, Switzerland), 7The Andrzej Soltan Institute for Nuclear Studies (IPJ, Poland)

POLAR[1],[2] is a space-borne hard X-ray polarimeter designed to measure the linear polarization of the gamma-ray bursts (GRB) in the energy range 50-500 keV. In POLAR, the GRB photons undergo Compton scattering in a target made out of 1600 plastic scintillator bars. The azimuthal distribution of the scattered photons inside the target provides the information on the GRB polarization. To be able to measure polarization of photons with energy as low as 50keV, an energy threshold for each single channel of maximum 5keV is required. This introduces strong constrains in the photon collection efficiency. To improve it, detailed studies of the scintillator bar surfaces and the available wrapping materials have been performed using both Monte Carlo simulations and laboratory measurements. At present, a POLAR demonstration model (1/10 of the final design) is being tested in the laboratory. POLAR engineering-qualification model will be ready in 2010.

Single bar Incoming Photon

Aluminum Box

Mass model (Geant4) - Exact representation of the instrument - Physical processes included: Low Energy (LE)

Polarized Compton, Gamma Conversion, Photoelectric, eIonisation, eBremsstrahlung

Light collection studies

  Results from mass-model: -  Compton scattering dominates -  Orbital Studies: Low Earth Orbit good orbit -  Background studies:

Cosmic Ray Protons: very low rate Spectrum can be used for calibration

-  Modulation factor (µ100) dependence on energy, photomultiplier uniformity, gamma-ray incoming angle (θ) -  BATSE Catalog simulations:

Minimum Detectable Polarization (MDP) of 10% (3σ) for about 12 GRB/year, with energy fluence of 10-5 erg cm-2 s-1

Abstract

Laboratory Setup

Simulation Setup

Summary

  Simulations with GEANT4 Optical Photons): - 1 scintillator bar: 6x6x200 mm3 - Different wrapping materials:

Al-mirror (50 µm thick) Al-foil with air gap Teflon (50µm) Refractive paint No wrapping (air)

- Results: ∼ 45% of optical photons are collected Scintillator surface smoothness is very important

References:

  Laboratory Several scintillator bars were tested sequentially, wrapped with different materials (Al-foil, Teflon, no wrapping, 3M® reflective foil)

  Method -  Large angle Compton scattering: azimuthal (ξ) distribution strongly modulated around photon polarization direction. -  Observe for each interacting photon: recoil e- from Compton scattering and scattered photon (by a second Compton scattering or photo electric effect) the 2 most energetic hits -  The line connecting them is well correlated with ξ. -  From ξ modulation curve POLARIZATION

Large Angle Compton scattering

Amplitude = µ100= 37%