X-ray polarimetry with balloon borne gas proportional counters

1
X-ray polarimetry with balloon borne gas
proportional counters

• K.Hayashida, H. Tsunemi, T. Horikawa, Y.
  Nakashima (Osaka University, Japan), F. Makino
  (NASDA, Japan),
• B. Paul (TIFR, India)
• Contact hayasida_at_ess.sci.osaka-u.ac.jp

2
ABSTRACT

• X-ray polarimetry has been an unexploited
  field in X-ray astronomy. Detection of the X-ray
  polarization was succeeded only in a few sources
  including Crab nebulae at 2.6keV and 5.2keV. In
  this paper, we present our plan of a balloon
  experiment for X-ray polarimetry. We will employ
  conventional Xe gas proportional counters with
  collimators. We will measure the signal rise time
  of each event, which reflects polarization
  direction of the incident X-rays relative to the
  anode wire direction. Ground experiments revealed
  that the modulation contrast (M) of this method
  amounts to about 0.1 at 20keV and 0.35 at 40keV
  (K. Hayashida et al., 1999, NIMA421, p.241).
  Different from scattering methods, the large
  effective area of gas proportional counters is
  exploited as it is in this method. We show that a
  pair of 300 cm2 PCs is enough for detecting X-ray
  polarization of Crab nebulae at 20-60keV range,
  where no polarization measurement has been done
  so far.

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Anisotropic Emission of Photo-electrons

• K-shell photo-electrons are emitted
  preferentially toward the direction parallel to
  the electric vector of incident photons.
• We have been developing polarization detectors
  employing this principle.
• CCD (Tsunemi et al.,1992 Hayashida et al.,
  1999).
• Gas proportional counter risetime (Hayashida et
  al., 1999).

Electric vector of incident photon
Emission direction of photo-electron
4
PC risetime polarimeter
Polarization and Anode Direction
5
PC risetime polarimeter Experiment
on the Ground

• Polarized X-ray beam was irradiated to the Xe gas
  proportional counter of which anode direction was
  set at angle Q from the E vector of the incident
  beam.
• Recently, we have developed a facility for
  polarized X-ray beam in our laboratory, in which
  electron impact type X-ray source and double
  crystal spectrometer are employed. (Tanaka et
  al., 1997, Koike et al. 2000).

Hayashida et al., 1999
6
Sampling the rise time of the output pulse from
proportional counters

• Risetime was sampled from the digitized signal of
  the pre-amplifier output of the proportional
  counters.
• We have started developing an equivalent digital
  circuit using an ADC and an FPGA instead of using
  the oscilloscope and the computer.

Hayashida et al., 1999
7
PC risetime polarimeter Results
from the Ground Experiment

• Average of the risetime shows the modulation
  depending on the angle Q between the beam E
  vector and the anode (top panel).
• Modulation contrast (M) of this PC risetime
  polarimeter for 10-40keV incidence is shown in
  the bottom panel.

Hayashida et al., 1999
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Polarization Detector for a Balloon Experiment

• Merit of the PC risetime polarimeter for a
  balloon experiment
• The system will be realized with conventional gas
  proportional counters by adding some electronics
  which enable the risetime measurement.
• Large effective area of gas proportional counters
  is utilized easily without mirror system. On this
  point, CCD method and Thomson-scattering method
  are more suitable for the focal-plane
  polarization detector.

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Plan of the observation

• A pair of Xe-gas proportional counters (PC) with
  risetime measurement will be used. The counters
  will be set orthogonally so that the anode
  direction of one PC is parallel to the E vector
  of the target and another perpendicular.
• Target is Crab nebulae, in which
  P19.2-1.0(2.6keV) 19.5-2.8(5.2keV) were
  detected (Weisskopf et al., 1978), but no
  positive measurements have been done above 10keV.
  

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Feasibility Study Assumptions

• Exposure Time20ksec (5.6hr)
• Gas proportional counters
• Xe 1.5atm, Depth 144mm
• Geometrical Area of one counter in the pair
• Plan-A 1200 cm2 Plan-B 300cm2
• M measured values are used for 20-40keV, the
  value 0.35 is used above 40keV.
• BGD counts, escape events, air attenuation are
  taken into account.

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Feasibility Study Pmin

• Feasibility of the observation is evaluated in
  terms of Pmin (minimum delectable polarization
  degree) for each 5 keV band.
• Since Crab nebulae has P19-20 below 10keV, we
  expect to get positive detection for each band
  even with plan-B(a pair of 300cm2 PCs)

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Calibration Facility at Osaka

• Polarized X-ray Beam-Line at Osaka

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Polarization Degree of the X-ray Beam with
various Ex/HV

• We had measured the polarization degree of the
  X-ray beam in Koike et al., 2000. We did further
  experiment with higher accuracy. When we set
  Ex/HV95, we obtain P0.4-0.5 as shown in the
  figure.

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Experiments at TIFR

• Polarized X-ray beam sources were developed at
  TIFR, in which X-rays with high Ex/HV are
  selected with absorption filters. Another system
  using scatters was also constructed.
• We have been testing a large area Xe PC counter
  on its polarization sensitivity through the rise
  time method. Small Xe PC specialized for the
  similar experiment is developed.

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References

• A precision measurement of the X-ray polarization
  of the Crab Nebula without pulsar contamination,
  Weisskopf, M.C., Silver, E.H., Kestenbaum, H.L.,
  Long, K.S., Novick, R., 1978, ApJ, Vol.220, L117.
• Detection of X-ray Polarization with a Charge
  Coupled Device, H.Tsunemi, K.Hayashida,
  K.Tamura, S.Nomoto, M.Wada, A.Hirano and
  E.Miyata,1992, Nuclear Instruments Methods, A
  Vol.321(3), p.629-p.631.
• How Linearly Polarized is the X-ray Beam from the
  X-ray Generator ?, S. Tanaka, H. Tsunemi, K.
  Hayashida, 1997, Jpn. J. Appl. Phys., Vol.36,
  p.5770-5773.
• X-ray Polarimetory with a conventional gas
  proportional counter through rise-time analysis,
  K. Hayashida, N. Miura, H. Tsunemi, K. Torii, K.,
  H. Murakami, Y. Ohno, K. Tamura, 1999,
  Nucl.Instr, and Methods A, Vol. 421, p.85-90.
• Optimization of polarimetry sensitivity for X-ray
  CCD, K. Hayashida, S. Tanaka, H. Tsunemi. Y.
  Hashimoto, M. Ohtani, 1999, Nucl.Instr, and
  Methods A, Vol. 436, p.96-101.
• Polarized Monochromatic X-ray Beam Extracted from
  Laboratory Electron Impact Source, T. Koike, K.
  Hayashida, Y. Hashimoto, D. Akutsu, M. Ohtani, H.
  Tsunemi, 2000, Proc. of SPIE, Vol.4012,
  p.414-421.