Download Polarimeter - Raman Research Institute

Development of a Thomson X-ray Polarimeter for a Small Satellite Mission
Rishin P.V., Gopala Krishna M.R., Biswajit Paul, Duraichelvan R., Chandreyee Maitra, Ateequlla C.M.,
Rajagopala G., Ezhilarasi M.S., Sandhya P., Mamatha T.S., Nagaraja H.N.
Raman Research Institute, Bangalore, India
Abstract
We describe development of a Thomson X-ray polarimeter for a small satellite
mission of the Indian Space Research Organisation (ISRO). A laboratory model
was made and tested successfully and it was redesigned for a flight unit.
Fabrication of an engineering model is in final stages. The instrument consists of
position sensitive proportional counter detectors and works in the 5-30 keV
energy band. This instrument will be suitable for X-ray polarisation
measurement in about 50 hard X-ray sources. The accretion powered X-ray
pulsars, black hole X-ray binaries, rotation powered pulsars and non-thermal
SNR will be the prime targets for this mission. In spite of its moderate
sensitivity, this experiment can give us unique opportunity to expand the field of
X-ray astronomy into a hitherto unexplored dimension. This instrument will
complement the hard X-ray polarisation measurements to be obtained with
Compton polarimeters and allow study of polarisation characteristics of X-ray
sources over a wide energy band. We describe the mission specifications, its
sensitivity, the mechanical and electronic design aspects of the complete system,
the signal processing pipe-line and test results.
Introduction
Instrument Configuration
Figure shows the configuration of the Thomson Polarimeter. Here the
detector elements are flat multi-wire proportional counters placed on four
sides of a disk-like scattering element made of Be or Li. The geometry of the
scatterer is optimized using Geant4 simulation to get highest sensitivity6.
Fabricated Engineering Model Detector
The space X-ray astronomy experiments carried out during the last three
decades have brought a few order of magnitude improvement in sensitivity
for Timing, Spectroscopy and Imaging. However, in the whole history of Xray astronomy, the Crab Nebula is the only X-ray source for which definite
polarisation measurements exist1. Some key scientific issues in high energy
astrophysics require input from polarisation measurement2,3,4. Hence there
is a strong need for X-ray polarisation measurement.
A Thomson polarimeter working in the energy band 5-30 keV is being
developed at Raman Research Institute (RRI)5. A laboratory model has
been made and tested successfully, an engineering model has been designed
and fabrication of the same is in progress. The instrument has been
proposed for an independent small satellite mission of the Indian Space
Research Organisation (ISRO).
Principle of Operation
The instrument is based on anisotropic Thomson scattering of X-ray photons.
X-rays from the source are made to undergo Thomson scattering and the
intensity distribution of the scattered photons is measured as a function of
azimuthal angle. Polarised X-ray will produce an azimuthal modulation in
the count rate. The total configuration will be rotated about the viewing
axis.
We are using position sensitive
proportional counters employing 12
nos. of resistive Nichrome wires split
into two channels. This helps to
improve the modulation factor and
achieve better background rejection
at the edges of the detector. The
position sensitivity is achieved by
charge division method as shown in
figure. The processing electronics
logic is implemented in Field
Programmable Gate Array (FPGA).
FE
Front-end electronics
PE
Processing electronics
Mechanical analysis
Finite Element Analysis (FEA) has been carried out on the above design
based on the Environmental Test Level Specifications (ETLS) specified by the
Indian Space Research Organization (ISRO) of the small satellite platform to
parametrically optimize the design for minimum mass and maximum
stiffness. The natural frequency of the structure and the maximum
stress/displacement developed in the model meets the qualification model
requirements with ample factor of safety.
Fabricated Prototype Collimator
In order to to compensate for
inaccuracy in satellite pointing
and to attain constant effective
area, a collimator with a flattopped response is required. For
the Engineering model, we plan
to use solid aluminum block with
hexagonal
tapered
holes,
machined using wire-cutting
technology to achieve a flattopped response. Figure shows
both faces of the sample piece
machined for the engineering
model collimator.
Results
One of the engineering model detectors has been fabricated and tested.
Figure shows the position determination by charge division for this detector
when shined with radioactive source. Peaks in the figure show the position
of the radioactive source when shined on the detector.
Signal Processing Electronics
Instrument Specifications
The table below describes the specifications of the Thomson X-ray
polarimeter. This instrument will complement the Hard X-ray polarisation
measurements to be obtained with Compton polarimeters and allow study
of polarisation characteristics of X-ray sources over a wide energy band.
Description
Value
Photon collection area
Energy range
Field of view
1018 cm2
5-30 keV
33 degree with ±0.2 degree flat topped response
Proportional counters
~100 kgs.
~ 650 x 650 x 550 mm3
50 Watt
4.2 Gbits per day
Detectors
Total weight
Overall dimension
Power
Data generation
rate
Scattering element
Life time
Modulation factor
Sensitivity
Satellite requirements
Beryllium/Lithium
3-5 years
~40%
2-3% Minimum Detectable Polarisation (MDP) for
50 mCrab sources with 1 mega sec exposure
Equatorial orbit (preferred) of altitude 500-600km,
with pointing accuracy of 0.1 deg, and with spin
along roll axis of about 0.5 rpm
Future Plans
Integration of one engineering model detector and testing with the signal
processing electronics chain is successfully completed. Position
determination using charge division principle is demonstrated. Work
towards fabricating the remaining detectors along with signal processing
electronics is in progress.
References
[1] M. C.Weisskopf et al., “Measurement of the X-ray Polarisation of the
Nebula”, ApJ, 208, L125-L128 (1976)
[2] M. C.Weisskopf et al.,“The prospects for X-ray polarimetry and its potential use for
understanding neutron stars”, astro-ph/0611483, (2006)
[3] Kallman T., “Astrophysical motivation for X-ray polarimetry”, Advances in Space Research
34, 2673-2677 (2004)
[4] Chandreyee Maitra et al., “ Prospect of polarisation measurements from black hole binaries
in their thermal state with a scattering polarimeter”, arXiv:1103.2639 [astro-ph.HE], (2011)
[5] Rishin, P.V. et al., “ Development of a Thomson X-ray Polarimeter”, arXiv:1009.0846v1
[astro-ph.IM], (2009)
[6] Vadawale et al., “Comparative study of different scattering geometries for the proposed
Indian X-ray polarization measurement experiment using Geant4”, Nuclear Instruments and
Methods in Physics Research - A, 618, 182, arXiv:1003.0519v1,(2010)
Acknowledgement
Polarimeter electronics block diagram
We would like to thank Dhiraj Dedhia (TIFR), Parag Shaw (TIFR), Vadawale S.V. (PRL),
Cowsik R. (Washington University, USA), and Seetha S (ISAC, ISRO) for all the timely help
and support. Special thanks to the members of RRI Mechanical Engineering Services.
For details contact : rishinpv/gkrishna/[email protected]