Gammaray Polarimetry Astrophysics Application

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Gamma-ray Polarimetry Astrophysics Application

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Outline

• Introduction.
• Concepts.
• Key features (requirements.)
• Instrument description.
• Silicon Strip Detector System.
• Double-sided SSD.
• Low noise front-end VLSI.
• Results.
• Noise performance.
• Energy resolution.
• Conclusions and future prospects.

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Polarization in Astrophysics

• Gamma-ray is excellent probe to study particle
  acceleration mechanism in BHs and pulsars.
• Polarization is important parameter to understand
  gamma-ray production mechanism.
• Inverse Compton Scattering.
• Polarization depends on scattering angle.
• Geometrical information.
• Synchrotron Radiation.
• Polarization is perpendicular to magnetic field
  direction.

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Soft Gamma-ray (X-ray) Imaging
X-ray mirror
Well-type Phoswich
Compton telescope
Coded mask
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Polarization Measurement

• Photoelectric absorption
• Compton Scattering
• Polarization depends on scattering angle, photon
  energy.

Eg
Polarization vector
Eg
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Polarimetry with Micro Pattern Gas Detectors
R. Bellazzini, INFN Pisa
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Analyzing Power
5.9 keV unpolarized
5.4 keV polarized
Cmax
Cmin
Analyzing power (Cmax- Cmax)/(Cmax Cmax)
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Multiple Compton Technique
Proposed by T. Kamae et al. 1987
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Key Features (Requirements)

• High energy resolution 1 keV (FWHM).
• High angular resolution 1o (FWHM).
• High background rejection.
• Constraints from Compton kinematics
• Polarimetry.
• Large Field-of-View gt 2 sr.
• Wide energy band 0.0520 MeV.
• Low weight.
• No heavy calorimeter.

120 e (RMS)
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Angular Resolution
Doppler broadening will limit ultimate angular
resolution.
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Source Localization
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With 10X Better Resolution
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Semiconductor Multiple-Compton Telescope
Si Detector
Doppler broadening is much smaller in Si.
CdTe Detector
photo- absorption
Si
Compton Scattering
pair- creation
CdTe
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Collaboration
Hiroyasu Tajima, T. Kamae Stanford Linear
Accelerator Center T. Takahashi, K.
Nakazawa Institute of Space and Astronautical
Science Y. Fukazawa Hiroshima University M.
Nomachi Osaka University
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Performance

• Si is essential for wide energy band.

25cm
80 layers
80 layers of0.5 mm thick Si and CdTe
Si Si
Si CdTe
CdTe CdTe
Takahashi (ISAS)
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Compton Kinematics

• Background suppression

EGS4.4 MC
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Polarization Sensitivity
EGS4.4 MC
100 polarized, 100 keV cosq lt 0.6 Analyzing
power 75
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Recent Development of CdTe Detector

• Tremendous amount of progress has been made.
• Highly uniform, fully active sensors can be
  produced.

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CdTe Gamma-ray Imager
Takahashi (ISAS)
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Si Strip Detector System

• Why Double-sided Si Strip Detector?
• No self-trigger for CCD.
• Hybrid-pixel detector increases non-active
  material.
• Monolithic-pixel detector cannot be produced in
  large volume.
• System configuration.

Front-end VLSI VA32TA
Double-sided Si Strip Detector
R
V
RC chip
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Double-sided Si Strip Detector

• 2.56cm x 2.56 cm
• 1/2 length of full size sensor.
• 300 µm thick, 400 µm pitch, 100 µm gap.
• Strip capacitance 5.5 pF (measured.)

Junction side
Ohmic side
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VA32TA Low Noise Front-end VLSI

• Low noise
• Front-end MOSFET geometry optimized for small
  capacitance load in 1.2 µm process.
• Internal DAC (4-bit trim DAC, bias).
• Fabricated in AMS 0.35 µm process
• Radiation hard to 20 MRad.
• SEU (single event upset) tolerant.

Ideas ASA
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Prototype System

• AC configuration
• DC configuration

RC chip
VA32TA
Singled-sided SSD
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Noise Analysis

• Amplifier noise capacitance load.
• Shot noise leakage current.
• Thermal noise detector bias resistance.

120 e (RMS)
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Noise Performance Summary
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241Am Spectrum
DC configuration, 0 oC
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241Am Fit
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57Co Spectrum
Compton edge
DC configuration, 0 oC
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57Co Fit
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Energy Resolution Summary
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Conclusions and Future Prospects

• Energy resolution measured using prototypes.
• 1.3 keV _at_0 oC for 60 and 122 keV X-rays.
• Noise sources are well understood.
• Imaging test with stacked DSSD prototypes.
• Polarization measurements.
• Further improvements are required for larger
  full-sized sensors.
• Optimization of front-end MOSFET in 0.35 µm
  process.
• Reduction of detector capacitance.
• Sensor thickness 300 µm 400 µm.
• Pitch adapter.

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Sensitivity Gap

• No approved mission to cover 0.120 MeV energy
  band.
• Sensitivity is mostly limited by BG.

COMPTEL
OSSE
EGRET
Sensitivity (MeV/cm2/s)
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Liquid Xenon TPC
LXeGRIT collaboration
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Germanium Compton Telescope
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