A detailed simulation study of
the
ASTROSAT-LAXPC
Biplab Bijay
biplabbijay@rediff.com
biplabbijay2009@gmail.com
High Energy & Cosmic Ray Research Center
University of North Bengal
Siliguri
North Bengal St. Xavier's College
University of North Bengal
Siliguri
WAPP-2013, Bose Institute, 17-19 December 2013
What is ASTROSAT ?
ASTROSAT is a multi-wavelength astronomy mission on an IRSclass satellite in a 650-km, near-equatorial orbit. It is currently
scheduled to be launched by the Indian launch vehicle PSLV
from the Sriharikota launch centre in 2013. The expected
operating life time of the satellite will be five years. ASTROSAT
will carry five astronomy payloads for simultaneous multi-band
observations:
•
Twin 40-cm Ultraviolet Imaging Telescopes (UVIT) covering Far-UV to
optical bands.
•
Three units of Large Area Xenon Proportional Counters (LAXPC)
covering medium energy X-rays from 3 to 80 keV with an effective area
of 6000 sq.cm. at 10 keV.
•
A Soft X-ray Telescope (SXT) with conical foil mirrors and X-ray CCD
detector, covering the energy range 0.3-8 keV. The effective area will be
about 200 sq.cm. at 1 keV.
•
A Cadmium-Zinc-Telluride coded-mask imager (CZTI), covering hard Xrays from 10 to 150 keV, ith about 10 deg field of view and 1000 sq.cm.
effective area.
•
A Scanning Sky Monitor (SSM) consisting of three one-dimensional
position-sensitive proportional counters with coded masks. The
assembly will be placed on a rotating platform to scan the available sky
once every six hours in order to locate transient X-ray sources.
http://astrosat.iucaa.in
WAPP-2013, Bose Institute, 17-19 December 2013
Objectives of ASTROSAT
• ASTROSAT will be a powerful mission for Multiwavelength studies of various types of sources using 5
co-aligned telescopes covering broad X-ray , near- UV ,
far- UV and Optical bands.
•
Simultaneous multi-wavelength monitoring of intensity
variations in a broad range of cosmic sources.
•
Monitoring the X-ray sky for new transients.
•
Sky surveys in the hard X-ray and UV bands.
•
Broadband spectroscopic studies of X-ray binaries,
AGN, SNRs, clusters of galaxies and stellar coronae.
•
Studies of periodic and non-periodic variability of X-ray
sources.
WAPP-2013, Bose Institute, 17-19 December 2013
ASTROSAT Detectors
http://astrosat.iucaa.in
WAPP-2013, Bose Institute, 17-19 December 2013
What is LAXPC
ASTROSAT
http://astrosat.iucaa.in
LAXPC
WAPP-2013, Bose Institute, 17-19 December 2013
LAXPC Configuration
Gas Mixtyre :
Gas Pressure :
Anode cell size :
Applied voltage :
Window configuration :
Anode wire thickness :
Xe (90%) + CH4 (10%)
2 atm
3 cm x 3 cm x 100 cm
3000 volt ( ? )
25 ~ 50 micron (Aluminyzed Mylar)
20 micron
WAPP-2013, Bose Institute, 17-19 December 2013
LAXPC Chacteristics
Effective Area
:
6000-7,500 cm2
Mass
:
390 kg
Time Resolution
: 10 µs, dead time 10-35 µs
Absolute Timing Accuracy
:
10 µs
Front-end and Processing Electronics : separate electronics for each detector
Data Storage
: Onboard Memory 570 MB per Orbit
Detection Efficiency (Claimed)
:
Energy Resolution (claimed)
: 12-13% at 20-60 KeV
Life Time
: 3-10 Years, no Consumable
90-100 % upto 20 keV, ~40% upto 80 KeV
WAPP-2013, Bose Institute, 17-19 December 2013
LAXPC : Effective Area (Claimed)
-- B. Paul, Astrophysics with All-Sky X-Ray Observations,
Proceedings of the RIKEN Symposium, p.362 (2009)
WAPP-2013, Bose Institute, 17-19 December 2013
LAXPC : Energy resolution (Claimed)
22 KeV, resolution 8.7%
25KeV, resolution 8.3%
25 KeV, resolution 8.3
26KeV, resolution 10.2%
30KeV, resolution 8.8%
60KeV, resolution 11.4%
-- B. Paul, Astrophysics with All-Sky X-Ray Observations,
Proceedings of the RIKEN Symposium, p.362 (2009)
WAPP-2013, Bose Institute, 17-19 December 2013
Why we choose this work ?
• No simulation is done for LAXPC.
• LAXPC is a proportional counter , a particle
detector used in Cosmic Ray experiments
WAPP-2013, Bose Institute, 17-19 December 2013
Outline of our simulation work
• Detailed Garfield (interfaced with MAGBOLTZ, HEED
and neBEM) simulation studies have been performed
investigating the drift properties of the LAXPC gas (Xe/CH490/10, 2 atm), detector response and efficiency in presence
of applied electric field.
• The simulation results for energies varying from 3 KeV to
80 KeV on X-ray tracks passing vertically in the anode tube
were obtained and discussed.
• The reliability of the simulation program -Garfield with
Magboltz, HEED and neBEM- has been verified by cross
checking with GEANT4 for the same inputs. Also both
Garfield and GEANT4 results were compared with the
experimental data .
-- GARFIELD, a computer program for simulation of gaseous detectors,
http://consult.cern.ch/writeup/garfield/
--S. F. Biagi, Nucl. Instr. and Meth. A421, (1999) 234
--http://heed.web.cern.ch/heed/
--http://nebem.web.cern.ch/nebem/
WAPP-2013, Bose Institute, 17-19 December 2013
--http://geant4.cern.ch/
Garfield Simulation
The drift properties of electrons depends on:
gas composition,
temperature,
pressure variations and
Electric field.
We will show this dependence in presence of electric field.
• Garfield is a computer program for the detailed simulation of two- and
three-dimensional drift chambers.
• Magboltz provides the computation of electron transport properties in
gas mixtures under the influence of electric and magnetic fields.
• HEED provides cluster statistics, produced when an ionising radiation
pass through a gaseous medium.
• nEBEAM provides the computation of electric field.
We have used the newest Garfield 9 with Magboltz 7.
WAPP-2013, Bose Institute, 17-19 December 2013
Typical muon Events in Garfield(Test)
Muon
Track
Muon
Track
Electron drift lines
Electron drift lines
Ion drift lines
NO magnetic field
Ion drift lines
Magnetic field of 3T parallel to the wire
WAPP-2013, Bose Institute, 17-19 December 2013
Drift-lines and a typical X-ray event in Garfield
Temperature 300.15 °K – Pressure 2 atm
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Ion-mobility and Drift velocity
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Gas coefficients
Diffusion Coefficients
Townsend and attachment coefficients
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Cluster-size distribution
120 GeV proton
10 KeV X-ray
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Typical anode pulse
WAPP-2013, Bose Institute, 17-19 December 2013
GEANT4 simulation
For simulation using GEANT4 one needs to select the
required physics processes. Since our primary concern is to
examine the response of the detector when x-rays /electrons
of KeV energy range are passing through it, we have
considered all the electromagnetic process which include
photoelectric including Auger and fluorescence effects,
Compton scattering, Bremsstrahlung, ionization, multiple
scattering etc. (the list of physics processes are given in
next slide). Another important requirement is the
construction of geometry of the detector. In this aspect we
construct the exact geometry of LAXPC. We placed
Aluminized Mylar windows of thickness 25/ 50 microns at the
top but have not considered the collimators.
WAPP-2013, Bose Institute, 17-19 December 2013
GEANT4 Physics List
#physics processes
#/testem/phys/addPhysics emstandardFLUO
/testem/phys/addPhysics emstandard_opt2 : Photoelectric,
compton, pair production
/process/em/fluo true : fluorescence effect
/process/em/auger true : Auger effect included
#/testem/phys/addPhysics emlivermore
#/testem/phys/addPhysics empenelope
WAPP-2013, Bose Institute, 17-19 December 2013
Mean energy deposition and Mean
interaction length obtained from GEANT4
WAPP-2013, Bose Institute, 17-19 December 2013
Results and Discussion
EFFICIENCY
In GEANT4 an event is considered as
detected if it deposits non-zero
energy.
Detection
efficiency
initially
increases rapidly upto 10 KeV (100 %),
remains nearly constant till 20 KeV (
Effect of Mylar Window).
 After that it stars decreasing to ~
50% upto 34.5 keV (Cross-section
decreases).
At the 34.6 KeV efficiency jumps
suddenly (nearly 100%) and decreases
thereafter
steadily reaching about
35% and 20% at 80 keV and 100 keV
respectively (fluorescwnce edge at
34.5KeV).
WAPP-2013, Bose Institute, 17-19 December 2013
Results and discussion(Continue)
Effective Area
The effective (detection) area of
the LAXPC instrument, which is
the
product
of
effective
Geometrical Area of the detector
with the detection efficiency, for
x-ray photons in the energy
range 2-100 KeV has been
estimated for two thickness (25
and 50 micron) of Mylar window
using GEANT4
(considering
effective geometrical area is 6000
cm2). It is interesting to note that
even at 100 KeV the effective
detection area of LAXPC is quite
significant, ~1500 cm2.
WAPP-2013, Bose Institute, 17-19 December 2013
Results and discussion(Continue)
Electron Efficiency
The efficiency of LAXPC when
incident particles are electrons has
also been examined.

It is found that the Aluminized
Mylar windows of thickness 25 and
50 micron block the incoming
electrons up to 40 and 60 keV
respectively

WAPP-2013, Bose Institute, 17-19 December 2013
Results and discussion(Continue)
Energy Resolution
22 KeV photon
25 KeV photon
30 KeV photon
26 KeV photon
60 KeV photon
WAPP-2013, Bose Institute, 17-19 December 2013
Results and discussion(Continue)
 We plot the total charge distribution for two incident photon
energies 22 keV and 25 keV and compared with the result
obtained in the laboratory using LAXPC.
 It appears that the energy resolution of LAXPC in practice is
slightly poor than the prediction of GARFIELD simulation
(several factors those come into play in a real detector like
anode wire non-uniformity, attachment of electrons to
impurities etc are not included in simulation).
 But it is clear that the LAXPC can easily discriminate
photons of 22 keV and 25 keV.
WAPP-2013, Bose Institute, 17-19 December 2013
Results and discussion(Continue)
We plot the simulated total charge distribution for incident photons of energies 5, 7, 15, 17,
25, 27, 35, 37, 45, 47, 55, 57, 65, 67, 75, 77 keV. One may see that the photo-electron peaks
are placed according to the increasing energy of the incident photons which is expected as
the energy of photo-electrons will be equal to energy of the incident photon minus 34.6 keV.
WAPP-2013, Bose Institute, 17-19 December 2013
Results and discussion(Continue)
We
estimate
the
energy
resolution(∆E/E)
by
fitting
the
simulated spectra (for each energy)
with Gaussian function (~ exp[-(xxc)2/2µ2) and ∆E is taken equal to µ.
The simulation results give better
energy resolution than we has been
observed (10-12% across the 20-80
keV). However, this is not unexpected
as the energy resolution depends upon
many other technical factors, like
anode wire non-uniformity, attachment
of electrons to impurities, electronic
(amplifier) noise etc. which are mot
incorporated in the simulation results.
WAPP-2013, Bose Institute, 17-19 December 2013
Efficiency Revisited
X-rays of energies above the Kedge energy produce a pulse at
much
lower energy channel
(equivalent to of 25.4 keV electron
starts deep in the detector
volume).

It is important to estimate the
detection efficiency of these X-ray
photons at its appropriate channel.

We put a cut ~ 3 sigma for a
particular energy. Any event of that
energy is considered as detected if
the total charge produced is within
the corresponding limit.

WAPP-2013, Bose Institute, 17-19 December 2013
Conclusion
Our results suggest that while extracting spectra of a x-ray
source/background from LAXPC observations over the wide energy range
up to 80 keV or so, one needs to consider the contribution from
fluorescence photon, delta electron due to higher energy photons at
lower energy channels(Particularly, in the energy range above 20 keV).

We should check the detector response by varying pressure, size and
applied voltage so that we can optimize it in terms of resolution and
efficiency(The simulation work is under progress).

WAPP-2013, Bose Institute, 17-19 December 2013
My Collaboration
I would like to thank:
• Dr Arunva Bhadra(HECRRC)
• Dr Gobinda Mazumder(TIFR)
and
• Dr Dipankar Bhatacharya(IUCAA)
WAPP-2013, Bose Institute, 17-19 December 2013
Acknowledgement
A special thanks to
HECRRC team members
WAPP-2013, Bose Institute, 17-19 December 2013
Thank You
WAPP-2013, Bose Institute, 17-19 December 2013