X-ray polarimetry promises to give qualitatively new information about
high-energy astrophysical sources, such as binary black hole systems,
micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built
and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of
the InFOCuS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z
Compton scatterer with a CZT detector assembly to measure the polarization of
10-80 keV X-rays making use of the fact that polarized photons Compton scatter
preferentially perpendicular to the electric field orientation. X-Calibur
achieves a high detection efficiency of order unity.Comment: 9 pages, 5 figures, conference proceedings: SPIE 2011 (San Diego

Baring, M. G.

Barthelmy, S.

Beilicke, M.

Binns, W. R.

Buckley, J.

Cowsik, R.

Dowkontt, P.

Garson, A.

Guo, Q.

Haba, Y.

Israel, M. H.

Krawczynski, H.

Kunieda, H.

Lee, K.

Matsumoto, H.

Miyazawa, T.

Okajima, T.

Schnittman, J.

Tamura, K.

Tueller, J.

English

arXiv

X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOC(mu)S grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 10 - 80 keY X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity

NASA Technical Reports Server

Design and tests of the hard X-ray polarimeter X-Calibur 
M. Beilickea, M.G. Baringb, S. BarthelmyC, W.R. Binnsa, J. Buckleya, R. Cowsika, 
P. Dowkontta, A. Garsona, Q. Guoa, Y. Habad, M.H. Israela, H. Kuniedad, K. Leea, 
H. Matsumotod, T. l\1iyazawad, T. OkajimaC, J. SchnittmanC, K. Tamurad, J. Tuellerc, and 
H. Krawczynskia 
aDepartment of Physics and McDonnell Center for the Space Sciences, Washington University, 
St. Louis, MO, USA; bRice University, TX, USA; cGoddard Space Flight Center, MD, USA; 
dNagoya University, Japan; 
ABSTRACT 
X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such 
as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and 
tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOCpS grazing incidence hard 
X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the 
polarization of 10 - 80 keY X-rays making use of the fact that polarized photons Compton scatter preferentially 
perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity. 
Keywords: X-rays, polarization, black hole, InFOCuS, X-Calibur 
https://ntrs.nasa.gov/search.jsp?R=20120001985 2019-08-30T19:05:10+00:00Z


Design and Tests of the Hard X-Ray Polarimeter X-Calibur

M. Beilicke

M. G. Baring

S. Barthelmy

W. R. Binns

J. Buckley

R. Cowsik

P. Dowkontt

A. Garson

Q. Guo

Y. Haba

M. H. Israel

H. Kunieda

K. Lee

H. Matsumoto

T. Miyazawa

T. Okajima

J. Schnittman

K. Tamura

J. Tueller

H. Krawczynski

Crossref

Design and tests of the hard x-ray polarimeter X-Calibur

X-ray polarimetry promises to give new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOC(mu)S grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 10-80 keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity

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2011 Fermi Symposium, Roma., May. 9-12 1
Design and Tests of the Hard X-ray Polarimeter X-Calibur
M. Beilicke∗, W.R. Binns, J. Buckley, R. Cowsik, P. Dowkontt,
A. Garson, Q. Guo, M.H. Israel, K. Lee, H. Krawczynski
Department of Physics and McDonnell Center for the Space Sciences,
Washington University in St. Louis, MO, USA (∗E-mail: beilicke@physics.wustl.edu)
M.G. Baring
Rice University, TX, USA
S. Barthelmy, T. Okajima, J. Schnittman, J. Tueller
Goddard Space Flight Center, MD, USA
Y. Haba, H. Kunieda, H. Matsumoto, T. Miyazawa, K. Tamura
Nagoya University, Japan
X-ray polarimetry promises to give new information about high-energy astrophysical sources,
such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We
designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the
InFOCµS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer
with a CZT detector assembly to measure the polarization of 10 − 80 keV X-rays making use of
the fact that polarized photons Compton scatter preferentially perpendicular to the electric field
orientation. X-Calibur achieves a high detection efficiency of order unity.
I. INTRODUCTION
Motivation. Spectro-polarimetric X-ray observa-
tions are capable of providing important information
to study the non-thermal emission processes of various
astrophysical sources – namely the fraction and orien-
tation of linearly polarized photons [1]. So far, only a
few missions have successfully measured polarization
in the soft (OSO-8 [2]) and hard (Integral [3]) X-ray
energy regime. The Crab nebula is the only source
for which the polarization of the X-ray emission has
been established with a high level of confidence [2, 3].
Integral observations of the X-ray binary CygnusX-
1 indicate a high fraction of polarization in the hard
X-ray/gamma-ray bands [4].
Future missions. There are currently no mis-
sions in orbit that are capable of making sensitive
X-ray polarimetric observations. This will change by
the launch of the satellite-borne Gravity and Extreme
Magnetism SMEX (GEMS) mission [5] scheduled for
2014 (2 − 10 keV). The Soft Gamma-Ray Imager on
ASTRO-H [6] will also have capabilities of measur-
ing polarization, but the results may be plagued by
systematic uncertainties. The hard X-ray polarimeter
X-Calibur discussed in this paper has the potential to
cover the energy range above 10 keV, combining a high
detection efficiency with a low level of background and
well-controlled systematic errors.
Scientific potential. Synchrotron emission results
in linearly polarized photons (electric field oriented
perpendicular to the magnetic field lines). The po-
larized synchrotron photons can be inverse-Compton
(IC) scattered by relativistic electrons – weakening the
fraction of polarization and imprinting a scattering
angle dependence [7] to the observed fraction of po-
larization. Other important mechanism for polarizing
photons are Thomson scattering and curvature radia-
tion. The scientific potentials of spectro-polarimetric
hard X-ray observations are (see Krawczynski et al.
(2011) and references therein [1]):
1) Binary black hole (BH) systems. Relativis-
tic aberration and beaming, gravitational lensing,
and gravitomagnetic frame-dragging will result in an
energy-dependent fraction of X-ray polarization from
a Newtonian accretion disk [8] since photons with
higher energies originate closer to the BH than the
lower-energy photons. Schnittman and Krolik [9, 10]
calculate the expected polarization signature includ-
ing the effects of deflection of photons emitted in the
disk by the strong gravitational forces in the regions
surrounding the black hole and of the re-scattering
of these photons by the accretion disk. Spectro-
polarimetric observations can constrain the mass and
spin of the BH [9], as well as the inclination of the
inner accretion disk and the shape of the corona [10].
2) Pulsars and pulsar wind nebulae. High-energy
particles in pulsar magnetospheres are expected to
emit synchrotron and/or curvature radiation which
are difficult to distinguish from one another. However,
since the orbital planes for accelerating charges that
govern these two radiation processes are orthogonal to
each other, their polarized emission will exhibit differ-
ent behavior in position angle and polarization frac-
tion as functions of energy and the phase of the pul-
sar [3]. In magnetars, the highly-magnetized cousins
of pulsars, polarization-dependent resonant Compton
up-scattering is a leading candidate for generating the
observed hard X-ray tails. In both these classes of neu-
eConf C110509
https://ntrs.nasa.gov/search.jsp?R=20120002600 2019-08-30T19:18:15+00:00Z
2 2011 Fermi Symposium, Roma., May. 9-12
tron stars, phase-dependent spectro-polarimetry can
probe the emission mechanism, and provide insights
into the magnetospheric locale of the emission region.
3) Relativistic jets in active galactic nuclei (AGN).
Relativistic electrons in jets of AGN emit polarized
synchrotron radiation at radio/optical wavelengths.
The same electron population is believed to produce
hard X-rays by inverse-Compton scattering of a pho-
ton field. Simultaneous measurements of the polariza-
tion angle and the fraction of polarization in the radio
to hard X-ray band could help to study (i) synchrotron
self-Compton models (X-ray polarization tracks polar-
ization at radio/optical wavelengths in fraction and
orientation angle) [7] vs. (ii) external-Compton mod-
els for which the hard X-rays will have a relatively
small (<10%) fraction of polarization [11]. Polariza-
tion also allows one to test the structure (i.e. helical)
of the magnetic field of the jet.
4) Gamma-ray bursts (GRBs). GRBs are believed
to be connected to hyper-nova explosions and the for-
mation of relativistic jets. As in AGN jets, the struc-
ture and particle distribution responsible for GRBs
can be revealed by X-ray polarization measurements.
5) Lorentz invariance. X-ray polarimetric observa-
tions can be used to test theories violating Lorentz in-
variance [12] with unprecedented accuracies by prob-
ing the helicity dependence of the speed of light.
II. DESIGN OF X-CALIBUR
Figure 1 illustrates the conceptual design of the
X-Calibur polarimeter. A low-Z scintillator is used
as Compton-scatterer. For sufficiently energetic pho-
tons, the Compton interaction produces a measurable
scintillator signal which is read by a PMT. The scat-
tered X-rays are photo-absorbed in surrounding rings
of high-Z Cadmium Zinc Telluride (CZT) detectors.
This combination of scatterer/absorber leads to a high
fraction of unambiguously detected Compton events.
Linearly polarized X-rays will preferably Compton-
scatter perpendicular to their E field vector – resulting
in a modulation of the azimuthal event distribution.
Each CZT detector (2× 2 cm2) is contacted with a
64-pixel anode grid and a monolithic cathode facing
the scintillator rod. Two detector thicknesses (0.2 cm
and 0.5 cm) are being tested in the current setup.
Each CZT detector is permanently bonded (anode
side) to a ceramic chip carrier which is plugged into
the readout board. Figure 2 (left) shows a single CZT
detector unit as well as the readout electronics. Each
CZT detector is read out by two digitizer boards (32
channel ASIC developed by G. De Geronimo (BNL)
and E. Wulf (NRL) [13] and a 12-bit ADC). 16 dig-
itizer boards (8 CZT detectors) are read out by one
harvester board transmitting the data to a PC-104
computer. Four CZT detector units form a ring sur-
rounding the scintillator slab. The scintillator EJ-200
FIG. 1: Left: Conceptual design of X-Calibur: Incom-
ing X-rays are Compton-scattered in the scintillator rod
(read by a PMT) and are subsequently photo-absorbed in
one of the CZT detectors surrounding the rod. Right:
X-Calibur design with read-out electronics, shielding and
azimuthal rotation bearing.
is read by a Hamamatsu R7600U-200 PMT. The PMT
trigger information allows to effectively select scintil-
lator/CZT events from the data which represent likely
Compton-scattering candidates. The polarimeter and
the front-end readout electronics will be located inside
an active CsI(Na) anti-coincidence shield with 5 cm
thickness and a passive lead shield/collimator at the
top (Fig. 1) to suppress charged and neutral particle
backgrounds.
We plan to use the X-Calibur polarimeter in the fo-
cal plane of the InFOCµS experiment [14]. A Wolter
grazing incidence mirror focuses the X-rays on the X-
Calibur polarimeter. In order to reduce the systematic
uncertainties (including biases generated by the active
shield, a possible pitch of the polarimeter with respect
to the X-ray telescope, etc.), the polarimeter and the
active shield will be rotated around the optical axis
using a ring bearing (see Fig. 1). Counter-rotating
masses will be used to cancel the net-angular mo-
mentum of the polarimeter assembly during the flight.
The advantages of the X-Calibur/InFOCµS design are
(i) a high detection efficiency by using more than 80%
of photons impinging on the polarimeter, (ii) low back-
ground due to the usage of a focusing optics instead
of large detector volumes, and (iii) minimization and
control of systematic effects.
eConf C110509
2011 Fermi Symposium, Roma., May. 9-12 3
FIG. 2: Left: Top: 2×2×0.2 cm3 CZT detector (64 anode
pixels). Bottom: 2 × 2 × 0.5 cm3 detector bonded to a
ceramic chip carrier, plugged into 2 ASIC readout boards.
The high-voltage cable is glued to the detector cathode
(red wire). Right: Laboratory version of X-Calibur. The
scintillator (blueish glow) is surrounded by two detector
rings – each consisting of four 64 pixel CZT detectors.
III. SIMULATIONS
Simulations of the X-Calibur polarimeter were per-
formed using the Geant4 package [15] with the Liv-
ermore low-energy electromagnetic model list. A bal-
loon flight in the focal plane of the InFOCµS mirror
assembly was assumed. We accounted for atmospheric
absorption at a floating altitude of 130, 000 feet us-
ing the NIST XCOM attenuation coefficients [16] and
an atmospheric depth of 2.9 g/cm
2
(observations per-
formed at zenith). A Crab-like source was simulated.
The X-Calibur modulation factor is µ = 0.52 for a
100% polarized beam. The minimum detectable po-
larization [1] in the 10−80 keV energy range will be 4%
assuming 5.6 hr of on-source observations of a Crab-
like source combined with a 1.4 hr background obser-
vation of an adjacent empty field. More details about
the simulations can be found in Guo et al. (2010) [17].
IV. FIRST MEASUREMENTS
Using funding from Washington University’s Mc-
Donnell Center for the Space Sciences, a flight-ready
version of the X-Calibur polarimeter was assembled
and tested in the laboratory. First measurements
were performed with 3 detector rings installed (0.5 cm
thickness). Before installation, IV-curves were taken
for all detector pixels, followed by a calibration run
using a Eu152 source (lines at 39.9, 45.7, 121.8 and
344.3 keV). After calibration, a collimated Eu152
source was used to determine the azimuthal X-Calibur
acceptance for an unpolarized beam. Another data
run was taken without a source to determine the back-
ground induced by cosmic rays secondaries hitting the
detector assembly. Only CZT events with a simulta-
neous (30µs) scintillator trigger are used.
A polarized beam was generated by scattering a
strong Cs137 source (line at 662 keV) off a lead brick.
A lead collimator allowed only X-rays with a scatter-
ing angle of ∼ 90 deg to enter the polarimeter. The X-
ray beam of the scattered photons has a mean energy
of 288 keV and was polarized to ∼ 55% (modulation
factor of µ = 0.4). The expected relative amplitude
in the normalized Φ distribution is 0.55× 0.4 = 0.22.
Figure 3 (left) shows the raw spectrum of the first
CZT polarimeter ring measured from (i) the polarized
beam, (ii) a background spectrum measured without a
source and (iii) the excess spectrum corresponding to
the energy spectrum of the scattered/polarized beam.
As expected, the excess spectrum drops off for ener-
gies higher than 288 keV (vertical line) – the energy
of the 90 deg-scattered Cs137 photons entering the po-
larimeter. The continuum below this energy is the re-
sult of 288 keV photons being Compton-scattered at
different depths in the scintillator rod and therefore
being reflected to the first CZT ring under different
scattering angles and corresponding different Comp-
ton energy losses. The little bump in the spectrum
around 288 keV may originate from direct CZT hits
without a Compton-scattering in the scintillator.
Figure 3 (right) shows the azimuthal scattering dis-
tribution of the polarized and unpolarized beam for
the second installed CZT detector ring. Only events
with a simultaneous scintillator trigger and with a de-
posited CZT energy between 100 − 330 keV are used
(see spectrum in Fig. 3, left). The data of the polar-
ized beam are corrected for the acceptance of the po-
larimeter (derived from the unpolarized X-ray beam).
As expected for a polarized beam, a 180 deg modula-
tion can be seen with a maximum of azimuthal scat-
tering angle perpendicular (Φ + 90 deg) to the plane
of the E field vector of the polarized beam (indicated
by the gray arrows). A sine function was fit to the
Φ-distribution of the polarized beam resulting in a
relative amplitude of 0.22. The data are in excellent
agreement with expectations.
V. SUMMARY AND CONCLUSION
We designed a hard X-ray polarimeter X-Calibur
and studied its projected performance and sensitivity
for a 1-day balloon flight with the InFOCµS X-ray
telescope. X-Calibur combines a detection efficiency
of close to 100% with a high modulation factor of µ ≈
0.5, as well as a good control over systematic effects.
X-Calibur was successfully tested/calibrated in the
laboratory with a polarized beam of 288 keV photons.
We applied for a 1-day X-Calibur/InFOCµS balloon
flight. Our tentative observation program includes
galactic sources (Crab, HerX-1, CygX-1, GRS 1915,
EXO0331) and one extragalactic source (Mrk 421) for
which sensitive polarization measurements would be
eConf C110509
4 2011 Fermi Symposium, Roma., May. 9-12
/s
/k
eV
2
ct
s/
cm
-510
-410
-310
Energy [keV]
210
Compton candidates (PMT)
ring 1
BG (no source)
excess (ON-OFF)
 [deg]Φazimuth angle 
0 100 200 300
Φδ
n
o
rm
. e
n
tr
ie
s 
/ 
0.6
0.8
1
1.2
1.4
Ring 2 Compton candidates (PMT))157unpolarized (Eu
, raw)137polarized (Cs
, corr.)137polarized (Cs
FIG. 3: Left: Energy spectrum of 288 keV X-rays (partially polarized) after being scattered in the scintillator (different
scattering angles and corresponding energy transfer). The background measurement is done without a source (cosmic
rays). The vertical line (288 keV) indicates the energy of the incoming X-ray beam. Right: Azimuthal event distribution
for the second installed CZT detector ring. Shown are raw events of the polarized beam (red), an unpolarized beam
(black), and the acceptance corrected polarized events (blue). A sine function (180 deg modulation) was fit to the
corrected data of the polarized beam. The vertical arrows indicate the plane of the electric field vector. The 90 deg
modulation of the unpolarized beam is a result of the 4-fold geometry of the CZT detector assembly.
carried through. We envision follow-up longer dura-
tion balloon flights (from the northern and southern
hemisphere), possibly using a mirror with increased
area. In the ideal case these flights would be per-
formed while the GEMS mission is in orbit to achieve
simultaneous coverage in the 0.5−80 keV regime. Suc-
cessful balloon flights would motivate a satellite-borne
hard X-ray polarimetry mission.
Acknowledgments
We are grateful for NASA funding from grant
NNX10AJ56G and discretionary founding from the
McDonnell Center for the Space Sciences to build
the X-Calibur polarimeter. Q.Guo thanks the Chi-
nese Scholarship Council for the financial support
(NO.2009629064, stay at Washington University).
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eConf C110509


Design and tests of the hard X-ray polarimeter X-Calibur

Design and tests of the hard X-ray polarimeter X-Calibur

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