Polarimetria X di sorgenti astronomiche con rivelatori “particellari”:

una nuova finestra osservativa dell’Universo

Ronaldo BellazziniINFN - Pisa

Polarimetry: The Missing Piece of the Puzzle Polarimetry: The Missing Piece of the Puzzle

Imaging: Chandra

Spectroscopy: AstroE2, Constellation-X, Chandra

Timing: RXTE

Polarimetry: AXP

Polarization from celestial sources may derive from:

•Emission processes themselves:

cyclotron, synchrotron, non-thermal bremmstrahlung(Westfold, 1959; Gnedin & Sunyaev, 1974; Rees, 1975

•Scattering on aspherical accreting plasmas: disks, blobs, columns.

(Rees, 1975; Sunyaev & Titarchuk, 1985; Mészáros, P. et al. 1988)

•Vacuum polarization and birefringence through extreme magnetic fields

(Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura, 1979)

WhyWhy XX--rayray AstrophysicalAstrophysical PolarimetryPolarimetry??

Importance of Astronomical XImportance of Astronomical X--rayray polarimetrypolarimetry

• Photon scattering and emission processes impart unique polarization signatures:- polarization probes geometry of X-ray sources and physical properties of emission sites (e.g. magnetic field strength and direction)

• Polarization studies address important astrophysical questions in a new, unique way:- cosmic ray acceleration in supernova remnants- accretion geometry (disk or spherical) in supermassive black holes (AGN)

• Polarimetry is an unexplored discovery space.• Interpretations based on spectral and timing data are often

ambiguous: polarization measurements will resolve the ambiguities.• Polarization studies will place strong constraints on the possibility to

detect quantum gravity effects.

Polarimetry would add to energy and time two furtherobservable quantities, the amount and the angle of polarization, constraining any model and interpretation: a theoretical/observational breakthrough.”P. MeszarosP. Meszaros etet al. 1988al. 1988

Polarization & Quantum GravityPolarization & Quantum Gravity

Test Quantum Gravity effects seems to be out of reach of any foreseeable experiment. The natural scale of the theory, i.e. the Planck energy scale of 1019 GeV (EQG), is infact 16 orders of magnitude beyond the highest-energy ground-based particles accelerators .Astrophysical phenomena can provide a way to constrain such effects.

The linear polarization of high-energy photons that propagate over cosmological distances (i.e. AGN,GRB...) allows to test possible effect of quantum space-time.

In the loop representation of QG, space-time exhibits the property of birefringence. When a linearly polarized e.m. wave, that can be represented as superposition of two circularly polarized waves, propagates inside a birifringence substance, the plane of polarization rotates along the path because of the difference in the group velocity betwen the two circular components (v± = c(1 ± χ (E/EQG )n) ).

Polarization & Quantum GravityPolarization & Quantum GravityIn the linear case n=1, the plane of linear polarization ϕ changes along a distance D (in light years) as:

∆ϕ(E) ≈ χ(D/hc)E2/EQG ≈ 104 χ(E/0.1MeV)2 D

p.e. two photons of ∼0.1 MeV with an energy difference of ∼0.01% after 1 yr propagation in space with birefringence will have a phase difference of ∼ χ (factor nearly 1 in the QG loop representation).For cosmological sources coming from distances D>> 1 light year, the planes of linear polarization of photons with different energies should be completely random.

The best constraint to QG comes from mesurement of the polarization of X-rays.The only measurement of the X rays polarization from the Crab nebula made by Novick and collaborators in 1972 and confirmed by a different instrument four years later, is in strong conflict (χ<10-4) with the predictions of loop quantum gravity, if the effects of quantum gravity depend linearly on photon energy.

Polarization from Supernova Remnants:Polarization from Supernova Remnants: the Crab casethe Crab case

Crab-Nebula shows the same degree and angle of polarization from radio to X-rays and this is a signature of synchrotron emission.

Radio

(VLA)

Optical

(Palomar)

Infrared

(Keck)

X-rays

(Chandra)

Photoelectric cross sectionPhotoelectric cross section

( ) ( )( )( )4

222

4

5

2

cos124

137cossin2

7

θβϕθ

ν∂∂σ

−

=

Ω hmcZ

ro

The photoelectric effect is very sensitive to photon polarization!

Simple analytical expression for photoemission differential cross

section (k-shell photoelectron in non-relativistic limit):

If we project on the plane orthogonal to the propagation direction…

φ∂∂σ 2cos∝Ω

Photoelectron emission angular distributions

Auger emission directions

Photoelectron and AugerPhotoelectron and Auger angularangular distributionsdistributions

( )( )( )4

3

cos1sin

θβθ

∂θ∂σ

−∝

φ∂φ∂σ 2cos∝

Photoelectron emission directions

Dependence of polar angle of photoDependence of polar angle of photo--electron in Neelectron in Ne

Slowing down:

Elastic scattering:

EZ

xE∝

∂∂

222

2

)2

(sin

1

screenEZ

αθσ

+∝

Ω∂∂

Elastic scattering is responsible of a progressive randomization of photoelectron direction; most of the information about photoemission

direction resides 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 photoelectricBasics of photoelectric effect in gaseseffect in gases

5.0 keV photoelectrons tracks in Ne (100% linearly polarized,

collimated photons beam).

Basics of photoelectric effect in gas IIBasics of photoelectric effect in gas II

5.0 keV photoelectrons tracks in Ne (100% linearly polarized, collimated photons beam).

Modulation factor, as evaluated from charge released within a certain distance from conversion point.

Basics of photoelectric effect in gas IIIBasics of photoelectric effect in gas III

GEM electric field

Polarization information is derived from the tracks of the photoelectron, imaged by a finely subdivided gas detector.

A “modern cloud chamber”: the Micro Pattern Gas DetectorA “modern cloud chamber”: the Micro Pattern Gas Detector

pixel

GEM

20 nsα E

X photon (E)

PCB

conversion gain

collection

trigger

A precursor of full track imagingA precursor of full track imaging

P. Auger, 1926

30 keV photons in Ar

30 keV photons in H/N 90/10

45 keV photons in H/Ar 95/5

Read out plane

Gas electron Multiplier

• GEM pitch: 90 µm• GEM holes diameters: 45 µm, 60 µm• Read out pitch: 260 µm• Absorption gap thickness: 6 mm

512 electronic channels from a few mm2 active areaare individually read out by means of a multi-layer PCB fan out

A first prototype: the PCB approachA first prototype: the PCB approach

5.9 KeV unpolarized source 5.4 KeV polarized source

Modulation factor = (Cmax – Cmin)/ (Cmax + Cmin) ˜ 50% at 6 KeV

( )ATBS

SnnMDP +

⋅⋅=ε

εµσ

σ 21 MDP scales as: ( )εµ1−

for bright sourcesfor faint sources( )µε 1−

( ) ( )02cos φφφ −⋅+= BAC

PolarimetricPolarimetric sensitivitysensitivity

The limits of the PCB approachThe limits of 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 µm pitch).Crosstalk between adjacent channels (signals traveling close to each other for several cm).

Not negligible noise (high input capacitance to the preamplifiers).

A further technological step: the CMOS VLSI approachA further technological step: the CMOS VLSI approach

If the pixel size is small (below 100µm) and the number of pixels is large (above 1000) it is virtually impossible to bring the signal charge from the individual pixel to a chain of external read-out electronics even by using the advanced, fine-line, multi-layer, PCB technology.

When it is not possible to bring out the signal charge to external, peripheral electronics than it is the electronics that has to be brought in to the individual pixel!

A CMOS full custom pixel array used directly as the charge collecting anodeof the GEM has been designed, produced and it is currently under test.

Advantages: asynchronous, fast, low noise, honeycomb array design, no problems in the realization of the fan out to front-end electronics.

For the first time the full electronics chain and the detector are completely integrated; the read-out chip itself is, at the same time, the charge collecting anode.

First prototype of pixelized readFirst prototype of pixelized read--out: 2101 channels and 80 out: 2101 channels and 80 µµm pitch.m pitch.

Each microscopic pixel is fully covered by a hexagonal metal electrode realized using the top layer of a 6 layers, 0.35 µm CMOS technology. This charge collecting pad is individually connected to a fullchain of “nuclear type” electronics (preamplifier, shaper amplifier, sample and hold, multiplexer) which is built immediately below it making use of the remaining 5 active layers.

The collecting anode/readThe collecting anode/read--out chipout chip

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

Electronics conceptual designElectronics conceptual design

• ~3.5 microseconds shaping time• 100 e- ENC ( very small detector capacitance) • dynamic range: 0.2-20 fC • power consumption: around 100 µwatt/pixel• input sensitivity 100 mv/fC

• external trigger (from the GEM) for parallel S/H on all the channels • ADC after S/H: external , flash • 400 µs read-out time (with 5 MHz system clock)

PIXIE: the PIXel Imager ExperimentPIXIE: the PIXel Imager Experiment

Detector and associated electronics are the same thing!

VLSI chip (collecting anode)

absorbtion gap

transfer gap

GEM

entrance window

The analog signalThe analog signalAnalog output(60000 electrons)

Write signal(no Maxhold)

3.5 µs

Automatic search of the maximum of the signal within a 10 µsec window after an asyncronous external trigger (from the TOP GEM)

S/H analog output

Maxhold signal

Shaper output

Analog output(60000 electrons)

Analog output(6000 electrons)

Analog output(1000 electrons)

Internal calibration system and addressing capabilityInternal calibration system and addressing capability

Detector response to 20 mV calibration signal(~1000 electrons)injected in a subset of pixels to create the experiment Logo.

Excellent response uniformity even before any attempt of calibration.

TracksTracks reconstructionreconstruction

1) The track is recorded by the PIXel Imager

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.

Real track

Auger ElectronAuger Electron

Bragg PeakBragg Peak

Tracks “morphology”Tracks “morphology”

Raw data: less than 40000 electrons subdivided on 46 pixels!

Some events…Some events…

An efficient photoelectric X-ray polarimeter for the study of black holes and neutron starsE. Costa, P. Soffitta, R. Bellazzini, A. Brez, N. Lumb, G. SpandreNature, Vol. 411 (2001) 662.

A new chip implementation in 0.35 A new chip implementation in 0.35 µµm CMOSm CMOSSquare matrix of maximum area :12x12 mm2

Active area : 11x11 mm2 (22080 pixels)8 sectors of 2.8k hexagonal pixels each 80 µm pitch4 µs shaper peaking time

8 parallel analog buses10 MHz read-out frequency280 µs (read-out time) frame rate 1 kHz source rate possible

Global self-triggering function with adjustable thresholds for each clusterClock drivers, bias circuitry, trigger output & analogbuffer placed on the left- and right-hand sides

From 2k to 22k pixelsFrom 2k to 22k pixels

Real tracks: 5.9keV photon energy (Real tracks: 5.9keV photon energy (5555Fe source) Fe source)

Ne50/ DME 50 – 3cm – 1 atm 80 µm R.O. Pitch with 22K ASIC chip

∆VGEM = 500V, ET = 4kV/cm

2 keV polarized photons in CF4-DME 20-80, 0.5atm(same events, with different pixel pitch)

µ = 9%pitch = 80µm

µ =15%pitch = 50µm

What matters ? The ReadWhat matters ? The Read--out samplingout sampling

The pixel pitch size is critical at low energies!

A new 50 mm pitch GEMA new 50 mm pitch GEM

pitch – 50 µm

Φhole – 30 µm

thickness – 25 µm

Effective Gain ~ Gain · εcollection

Ne(80)/DME(20)

The choice of gas mixtureThe choice of gas mixtureCriteria in the choice of the gas mixture:

• Low K-edge (low Z materials)• Low electromagnetic scattering, favorable slowing down/scattering ratio

(low Z materials)• Favorable range/pixel size ratio• High detection efficiency (high Z materials)• Small tranverse diffusion in the charge drift (organic quenchers)

• Generation (photoelectron + Auger)

• Propagation (SS_MOTT)

• Creation and diffusion of primary ionization (Maxwell, Garfield, Magboltz)

• Gas multiplication

• Digitization

• Pixel Representation

Full detector simulationFull detector simulation

Photoelectron transport code originally developed by D. Joy for electron microscopy (particularly accurate at low energies) adapted for the transport in gas mixtures

MonteCarlo simulation: 3keV photon energy MonteCarlo simulation: 3keV photon energy

Ar40/ DME 60 – 1cm – 1atm50 µm R.O. pitch

MonteCarlo simulation: 12keV photon energy MonteCarlo simulation: 12keV photon energy

Ar40/ DME 60 – 3cm50 µm R.O. pitch

1 atm

3 atm

Real dataunpolarised 5.9 keV photons

MC data100% polarised 12 keV photons

Cumulative distributionsCumulative distributions

MDP HerMDP Her--X1 & Crab : High EnergyX1 & Crab : High Energy

Ar40/ DME 60 – 3cm 3atm SimbolPlus mirror

MDP HerMDP Her--X1 & Crab Low Energy (II)X1 & Crab Low Energy (II)

With this value, observation time can be reduced by ~ 3 orders of magnitude respect to the Weisskopf measurement (AstroPh Journ. 220 (1978) L117)

Ne40/ DME 60 2cm 1atm

SimbolPlus mirror

Ne40/ DME 60 2cm 1atm

SimbolPlus mirror

MDP HerMDP Her--X1 & Crab Low Energy (III)X1 & Crab Low Energy (III)

CO2 – 1cm 1atm SimbolPlus mirror

MDP for different source fluxes & spectral indexes MDP for different source fluxes & spectral indexes

•Observations will measure energy fluxes and polarization of the flux as functions of time/energy and will image the source.

•The arrival time, the energy and the conversion point of eachphoton are determined.

•The direction in which each photon kicks out an electron isdetermined.

•From the ensemble of electron tracks in a time interval the average polarization direction and its fractional amplitude are determined.

•For pulsars the data are folded on a pulse period to determine the polarization as a function of angle around the pulsar.

Science MeasurementsScience Measurements

The performance of the tested prototypes looks like a significant step forward, compared with traditional X-raypolarimeters and promises a large increase in sensitivity.In its final configuration the MPGD target performance is the detection of ~1% polarization for few mCrabs sources (in the XEUS focal plane, for example). This sensitivity will likely allow polarimetry measurements to be made on thousands of galactic and extragalactic sources: a real breakthrough in X-ray astronomy.

Conclusions IConclusions I

A system in which the GEM foil, the absorption gap and the entrance window are assembled directly over a CMOS chip die has been developed. The ASIC itself becomes at the same time, the charge collecting anode and the pixelized read-out of a MicroPattern Gas Detector.

The full electronics chain and the detector are now completely integrated.

At a gain of 1000 non negligible sensitivity to single primary electron detection is reached. No problems found up to now in operating the system under HV and in gas environment.

An astronomical X-ray Polarimeter application has been presented, but, depending on pixel and die size, electronics shaping time, analog vs. digital read-out, counting vs. integrating mode, many others applications can be envisaged.

Final design will have around 100 k channels and 50 microns pixel size.This would open new directions in gas detector read-out, bringing the field to the same level of integration of solid state detectors.

Conclusions II