A detailed simulation study of the ASTROSAT-LAXPC

A detailed simulation study of the ASTROSAT-LAXPC

WAPP-2013, Bose Institute, 17-19 December 2013

Biplab [email protected]

[email protected]

High Energy & Cosmic Ray Research CenterUniversity of North BengalSiliguri

North Bengal St. Xavier's CollegeUniversity of North Bengal

Siliguri

mailto:[email protected]

What is ASTROSAT ? ASTROSAT is a multi-wavelength astronomy mission on an IRS-

class 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 X-rays 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 Multi-wavelength 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.


ASTROSAT Detectors


LAXPCASTROSAT

What is LAXPC


LAXPC ConfigurationGas Mixtyre : Xe (90%) + CH4 (10%)Gas Pressure : 2 atmAnode cell size : 3 cm x 3 cm x 100 cmApplied voltage : 3000 volt ( ? )Window configuration : 25 ~ 50 micron (Aluminyzed Mylar)Anode wire thickness : 20 micron


LAXPC ChacteristicsEffective 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) : 90-100 % upto 20 keV, ~40% upto 80 KeV

Energy Resolution (claimed) : 12-13% at 20-60 KeV

Life Time : 3-10 Years, no Consumable


LAXPC : Effective Area (Claimed)

-- B. Paul, Astrophysics with All-Sky X-Ray Observations, Proceedings of the RIKEN Symposium, p.362 (2009)


LAXPC : Energy resolution (Claimed)

25KeV, resolution 8.3%




-- B. Paul, Astrophysics with All-Sky X-Ray Observations, Proceedings of the RIKEN Symposium, p.362 (2009)


22 KeV, resolution 8.7%

25 KeV, resolution 8.3

Why we choose this work ?

• No simulation is done for LAXPC.

• LAXPC is a proportional counter , a particle detector used in Cosmic Ray experiments


• Detailed Garfield (interfaced with MAGBOLTZ, HEED Detailed Garfield (interfaced with MAGBOLTZ, HEED and neBEM) simulation studiesand neBEM) simulation studies have been performed investigating the drift properties of the LAXPC gas (Xe/CH4-90/10, 2 atm), detector response and efficiency in presencein presence of applied electric field.

• The simulation results for energiessimulation results for energies varying from 3 KeV to 3 KeV to 80 KeV80 KeV on X-ray tracks passing vertically in the anode tube were obtained and discussed.

• The reliabilityreliability of the simulation programsimulation 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 .

Outline of our simulation work

-- 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/--http://geant4.cern.ch/ WAPP-2013, Bose Institute, 17-19 December 2013

http://consult.cern.ch/writeup/garfield/

The drift properties of electrons depends ondepends on: gas composition, temperature, pressure variations and Electric field.

We will show this dependence in presence of electric field.

• GarfieldGarfield is a computer program for the detailed simulation of two- and three-dimensional drift chambers.

• MagboltzMagboltz 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 newestnewest Garfield 9Garfield 9 with Magboltz 7Magboltz 7.

Garfield Simulation


Typical muon Events in Garfield(Test)

NONO magnetic field Magnetic field of 3T3T parallel to the wire

Muon Track

Muon Track

Electron drift linesElectron drift lines

Ion drift linesIon drift lines


Drift-lines and a typical X-ray event in Garfield

Temperature 300.15 °K – Pressure 2 atm


Ion-mobility and Drift velocity


Gas coefficients

Diffusion Coefficients Townsend and attachment coefficients


Cluster-size distribution

120 GeV proton 10 KeV X-ray


Typical anode pulse


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.


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


Mean energy deposition and Mean interaction length obtained from GEANT4


Results and DiscussionEFFICIENCY

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 2013WAPP-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.


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


Results and discussion(Continue)Energy Resolution

22 KeV photon

25 KeV photon

60 KeV photon

30 KeV photon26 KeV photon


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.


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.


Results and discussion(Continue)We estimate the energy resolution(∆E/E) by fitting the simulated spectra (for each energy) with Gaussian function (~ exp[-(x-xc)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.


Efficiency RevisitedX-rays of energies above the K-edge 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.


ConclusionOur 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).


My Collaboration

I would like to thank:

• Dr Arunva Bhadra(HECRRC)

• Dr Gobinda Mazumder(TIFR)

and

• Dr Dipankar Bhatacharya(IUCAA)


Acknowledgement

A special thanks to

HECRRC team members


Thank You


A detailed simulation study of the ASTROSAT-LAXPC

Documents

xray sky

bose institute

broad xray

transient xray sources

astrosat detectorshttp

allsky xray observations

xray ccd detector

uv bands