x-ray photoelectric polarimetry with the gas pixel detector paolo soffitta iasf-rome/inaf workshop...

X-ray photoelectric polarimetry with the Gas Pixel Detector

Paolo Soffitta IASF-Rome/INAF

Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

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Heitler W.,The Quantum Theory of RadiationThe photo-electric effect is very sensitive to photon polarization

Modern polarimeters dedicated to X-ray Astronomy exploit the photoelectric effect resolving most of the problems connected with Thomson/Bragg polarimeter. The exploitation of the photoelectric effect was tempted very long ago, but only since five-ten years was it possible to devise photoelectric polarimeters mature for a space mission.

An X-ray photon directed along the Z axis with the electric vector along the Y axis, is absorbed by an atom.

The photoelectron is ejected at an angle θ (the polar angle) with respect the incident photon direction and at an azimuthal angle φ with respect to the electric vector.

If the ejected electron is in ‘s’ state (as for the K–shell) the differential cross section depends on cos2 (φ), therefore it is preferentially emitted in the direction of the electric field.

Being the cross section null for φ = 90o the modulation factor µ equals 1 for any polar angle.

β =v/c By measuring the angular distribution of the ejected photelectrons (the modulation curve) it is possible to derive the X-ray polarization.



The cross section with respect to θ shows that the photoelectron is ejected preferentially at 90° with respect to the direction of the incident radiation while at high energy it is slightly bent forward.

Slowing down:

Elastic scattering:E

Z

x

E

222

2

)2

(sin

1

screenE

Z

The information about photoemission direction which brings memory of the X-ray polarization resides therefore in the initial part of the track.

Most of the energy is released at the end of the path.

Stopping power/Scattering 1/Z

Basics of photoelectric effect in materials.Once ejected, the photoelectron interacts with the atoms of the medium 1) It is slowed down by the electrons of the surrounding atoms that ionizes, creating along the path a stream of charges, the ‘track’, without sensitive variation in the direction 2) it is scattered by the atom nuclei with sensitive change in direction. 3) If there is an electric drift fields the track blurs diffusing as √Drift-length.

Most of the slowing down with the creation of secondary charges and elastic scattering happens when the energy of the photoelectron is low, therefore at the end of the path. The tracks start straight and ends as a skein.



The photoelectron range in gas is long enough to be efficiently imaged. In Silicon at 10 keV the range is only 1 m.

Range of photoelectron in gases.


GEM electric field

Polarization information is derived from the angular distribution of the emission direction of the tracks produced by the photoelectrons.

The detector has a very good imaging capability.

pixel

GEM

20 ns

a E

X photon (E)

PCB

conversion gain

collection

The principle of detection

X-ray polarimetry with a Gas Pixel Detector

Costa et al., 2001

A photon crosses a Beryllium window and it is absorbed in the gas gap, the photoelectron produces a track. The track drifts toward the multiplication stage that is the GEM (Gas Electron Multiplier) which is a dielectric foil metallized on both side and perforated by microscopic holes (30 um diameter, 50 um pitch) and it is then collected by the pixellated anode plane that is the upper layer of an ASIC chip.

Costa et al., 2001.

To image the track IASF-Rome/INAF and INFN-Pisa developed the Gas Pixel detector.


The first generation: the PCB approach

The fan-out which connects the segmented anode (collecting the charge) to the front end electronics is the real bottleneck!

Technological constraints limit the maximum number of independent electronics channels (~ 1000 @ ~ 200 m pitch). Crosstalk between adjacent channels (signals traveling close to each other for several cm).Not negligible noise (high input capacitance to the preamplifiers.).

Multilayer metalized kapton foils and vertical vias to fan-out the

signal from each pixel.


The overall detector assembly and read-out electronics

It has been developed a CMOS pixel read-out ASIC

pixel pitch 80 µm in an hexagonal array, comprehensive of preamplifier/shaper, S/H and routing (serial read-out) for each pixel number of pixels: 2101

The pixellated (2101 pixels) top layer of the ASIC chip is the collection plane. The bottom layers (5 layers total ) provide a complete analogue chain separated for each pixel with a preamplifier/shaper/sample and hold and serial readout.


Bellazzini et al., NIMA, 2004


The read-out system

From 2k to 22k pixels


Bellazzini et al. NIMA 2006


ASIC features 105600 pixels 50 μm pitch • Peaking time: 3-10 ms, externally adjustable;

• Full-scale linear range: 30000 electrons;• Pixel noise: 50 electrons ENC;• Read-out mode: asynchronous or synchronous;• Trigger mode: internal, external or self-trigger;• Read-out clock: up to 10MHz;• Self-trigger threshold: 2200 electrons (10% FS);• Frame rate: up to 10 kHz in self-trigger mode (event window);• Parallel analog output buffers: 1, 8 or 16;• Access to pixel content: direct (single pixel) or serial (8-16 clusters, full matrix, region of interest);• Fill fraction (ratio of metal area to active area): 92%)

The chip is self-triggered and low noise. The low noise allows for detect single electron in the track with a small gain of the GEM. Also it is not necessary to readout the entire chip since it is capable to define the sub-frame that surrounds the track. The dead time downloading an average of 1000 pixels is 100 time lower with respect to a download of 105 pixel. The needed power for the chip is 0.5 W. For space application we use a

Peltier to arrive at 2 W. Bellazzini et al., NIMA 2006 (b)

Different filling gas GEM Ne-DME 80-20 1 Bar 1 cm drift 50 μm pitch 50 thick (CERN)He-DME 20-80 1 Bar 1 cm drift 50 μm pitch 50 thick (CERN)DME 100 0.8 Bar1 cm drift 80 μm pitch 100 thick (SciE.)Ar-DME 60-40 2 Bar 2 cm drift 80 μm pitch 100 thick (SciE.)


A sealed X-ray polarimeter (Bellazzini et al., NIMA 2007).

A custom and very compact DAQ system to generate and handle command signals to/from the chip

(implemented on Altera FPGA Cyclone EP1C240), to read and digitally convert the analog data

(ADS5270TI Flash ADC) and to store them, temporarily, on a static RAM, has been developed. By

using the RISC processor NIOS II, embedded on Altera FPGA, and the self-triggering functionality of

the chip, it is possible to acquire the pedestals of the pixels in the same chip-defined event window

(region of interest) immediately after the event is read-out. The readout of the pedestals is user-defined

and can be performed once as well as several times.


Be Window Factor of Safety Verification

XPOL Beryllium Window verification by analysis

• Per GSFC-STD-7000 standard the Beryllium design factors of safety are (for static loads): yield stress ≥ 1.4 ultimate stress ≥ 1.6

• 50μm thick Beryllium window maximum static loads: 1bar

Model parameters:• mesh of 24756 elements (type SHELL181, CONTA174, TARGE170, SURF154) and 12842 nodes• analysis results to count for the membrane behavior of the thin beryllium foil

Analysis results:• maximum beryllium yield stress @ 1.5bar = 249MPa• maximum beryllium strain @ 1.5bar = 7.2 10-4

• yield stress factor of safety > 2

analyses with ANSYS™ software

Beryllium data (as supplied by manufacturer Analytical Oy, Finland):•Beryllium tensile yield strength = 340 MPa•Beryllium tensile ultimate strength = 450 MPa


Be Window Factor of Safety VerificationXPOL Beryllium Window verification by test

Test facility : Mitutoyo BHN506 coordinate measuring machine (CMM), with optical head, 5μm of resolutionLocation: INFN-Pisa 100k class clean room

Be window strain test results vs analysis result

measured deformations along a window vertical mid-plane at various internal differential pressures, 0-1.6 bar

Verification conclusions:

XPOL beryllium window can sustain the on orbit limit loads with a factor of safety that exceeds the GEVS standard


Thermal Tests

The Fe55 source illuminates the whole sensitive area.


Thermo-vacuum test

PT100 on

window

Fe55 source

PT100

on frame

Vibration Tests

The three axes setups are shown. For each axis we have performed a sine sweep between 20 and 2000Hz at 2oct/min and a random test 3dB for 75s over the predicted random vibration environment of the Pegasus rocket. In all the random tests the item was randomly vibrated to an overall 3grms (IXPE proposal). Subsequently a successful vibration test has been performed at 11.4 g

for launcher with ESA rocket. No resonance were founded between 20 and 2000 Hz (the foreseen one is at 3000 Hz).



Survival test with Fe ions.

GPD was exposed to a total dose of 1.7 104 Fe ions corresponding to the total dose in a Low Earth Orbit of 40 years of irradiation.

Bellazzini et al., proceed. of X-ray Polarimetry Workshop, 2010


IASF-Rome facility for the production of polarized X-rays

keV Crystal Line Bragg angle1.65 ADP(101) CONT 45.02.01 PET(002) CONT 45.02.29 Rh(001) Mo Lα 45.32.61 Graphite CONT 45.03.7 Al(111) Ca Kα 45.94.5 CaF2(220) Ti Kα 45.45.9 LiF(002) 55Fe 47.66.4 Si(400) Fe 45.5 8.05 Ge(333) Cu Kα 45.0

9.7 FLi(420) Au Lα 45.1 17.4 FLi(800) Mo Kα 44.8

Facility at IASF-Rome/INAFClose-up view of the polarizer and the Gas Pixel Detector

Capillary plate (3 cm diameter)

Aluminum and Graphite crystals.

Spectrum of the orders of diffraction from the Ti X-ray tube and a PET crystal acquired with a Si-PiN detector by Amptek.A CdTe detector is also avilable

(Muleri et al., SPIE, 2008)

PET

45,5140


In order to characterize completely the GPD as a polarimeter, we devised a mechanical system based on linear and rotary stages connected to a controller which in turn is connected with a PC via ethernet. The linear and rotary stages are manufactured by Newport such as the XPS controller. A lab-view software controls the movements and the acquisition. We move the detector and the beam is fixed.

The stage permit :

• X-Y displacement of the detector for XY mapping.

X-Y displacement of the X-ray beam for alignment of the beam with the rotation axis.

• Rotation of the detector to change polarization direction.

•Inclination of the detector (Large inclination and small inclination).

• Vertical displacement of the detector.

• Rail for manual linear displacement of the X-ray beam for maintenance.


Track reconstruction

1) The track is recorded by the Polarimeter.

2) Baricenter evaluation.

3) Reconstruction of the principal axis of the track: maximization of the second moment of charge distribution.

4) Reconstruction of the conversion point: major second moment (track length) + third moment along the principal axis (asymmetry of charge release).

5) Reconstruction of emission direction: pixels are weighted according to the distance from conversion point. It brings memory of the polarization.

Real track


Position reconstruction capability

Matrix of 0.6 mm holes diam.2 mm apart.


Not only MonteCarlo: Our predictions are based on data

The modulation factor measured 2.6 keV, 3.7 keV and 5.2 keV has been compared with the Monte Carlo previsions. The agreement is very satisfying.

Each photon produces a track. From the track the impact point and the emission angle of the photoelectron is derived. The distribution of the emission angle is the modulation curve.

By rotating the polarization vector the capability to measure the polarization angle is shown by the shift of the modulation curve.

Muleri et al. 2007

Soffitta et al., 2010Present level of absence of systematic effects (5.9 keV).(Bellazzini et al. 2010).

Impact point


We changed the position of the detector in a matrix of 5 x 5 locations with a step of 2.25 mm. We used a Ti X-ray tube and CaF2 crystal at 4.5 keV. At the input we placed a 1/40 capillary plate and 1/100 capillary plate at the beam output adding at the output a diaphragm of 500 m of diameter. We acquired about 30000 events in 2 hour in each position.

Modulation factor as a function ofthe beam position.

Polarization angle at different location of the detector.

More energies, more mixtures

We performed measurement at more different energies and gas mixtures.

Pure DME (CH3)2O

μ = 13.5%

Modulation curve at 2.0 keV

(Muleri et al., 2010).

A detector more tuned on hard X-rays.

The MEP prototype in the IASF-Rome facility.

MEP detector is working apparently well. It is a good Proportional Counter.Unfortunately it broke soon after this testing.Anyway we foresaw further changes. A larger detector for better control of the electric field and to exclude background produced on the walls.

For NHXM multilayer optics, from simulation we devised a 3-cm thick GPD filled with 3-atm of Ar-DME 80-20. The GPD built and tested was, as a first step, 2-cm and 2-atm.


Guard Ring


XPOL-GEM90 Prototype Assembly• The XPOL-GEM90 prototype assembly (INFN-PI-XPOL-01400-001) consists of 3 sub-assemblies:

GEM90-Drift_assemblyINFN-PI-XPOL-01430-001

GEM90-GEM_assemblyINFN-PI-XPOL-01420-001

GEM90-PCB_V2_assemblyINFN-PI-XPOL-01410-001

XY

PCB-Ref-SYS

SciEnergy GEM 50 μm thick 50 μm pitch

First-Light of the Ar-DME polarimeter (70-30) 2-Bar 2-cm (with large case), Fe55 unpolarized source.

Ti polarized Kα line polarized.

Large case He-DME 20-80 GPD below an 55Fe source.


55Fe unpolarized source.

He-DME 20-80 1Bar-1-cm.


New Improvement foreseen for GPD.

A factor of 20 in reduction of the dead time will allow to arrive at a dead time of 10 μs: with 5000 c/s the dead time is 5 %. In the present ASIC chip this level is reached with 250 c/s (approximately the expected counting rate for the Crab in mission like NHXM (˜600 cm2).

- Reduction of ASIC dead time.

-Tiling of ASIC chip.

Costa et al., ExpAst 2010

POLARIX

NHXM

Tagliaferri et al, ExpAst 2010

IXO

Bookbinder, SPIE, 2010

The missions where the GPD was proposed either are waiting after a phase A completed or were not selected or evolved in missions without anymore a polarimeter on-board.

Implementation of X-ray polarimetry with GPD in proposed missions:

- POLARIX (ASI small mission, fasa A completed)

3 Jet-X optics (3,5 m FL, 20 ‘’ HED 450 cm2 @ 2 keV, HEW=(20’’)) 3 GPD (1-cm, 1-Atm, He-DME 20-80) MDP 12 % in 105 s for 1 mCrab source (2-10 keV) 3.8 % in 105 s for 10 mCrab source (2-10 keV)

Costa, et al., Exp Ast 2010- NHXM (Proposed ESA M3 Mission not selected) 1 of 4 Multi-layer optics (Pt-C) (10 m FL, 600 cm2 @ 8 keV) 2 GPD : 1-cm, 1-Atm, He-DME (LEP) (2-10 keV); 3-cm 3-Atm Ar-DME (MEP) (6-35 keV)MDP: LEP 9.7 % in 105 s for 1 mCrab source (2-10 keV) 3.1 % in 105 s for 10 mCrab source (2-10 keV)

MEP 13 % in 105 s for 1 mCrab source (6-35 keV) 4.1 % in 105 for 10 mCrab source (6-35 keV) In study (HEP, Compton scattering) MDP 7.2 % for 10 mCrab in 105 s (20-80 keV)

Tagliaferri et al. , Exp. Ast. 2010; Soffitta et al. SPIE 2010

- IXO (ESA/NASA/JAXA Large Mission Evolved in Athena with no polarimeter on-board)Area= 2.5 m2 FL = 20 m HEW= 5’’ XPOL: MDP 1 % 1 mCrab 105 s.


Measurement in New Prototypes

• Modulation factor for different energies and positions.

• Sensitivity on the polarization angle for different positions.

• Level of absence of systematic effects improved.

• Position resolution.• Energy resolution.


• End

x-ray photoelectric polarimetry with the gas pixel detector paolo soffitta iasf-rome/inaf workshop...

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