The Low Energy Polaimeter based on the Gas Pixel Detector

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27 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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The Low Energy
Polaimeter

based on
the Gas Pixel Detector

Paolo
Soffitta

IAPS/Roma Italy

GEM electric field

pixel

GEM

20 ns

a


E

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䅓䥃

conversion

gain

collection




2
cos


The Gas Pixel Detector


A beryllium window


A conversion/drift gap with parallel electric field. The gas is
diluited so that photoelectrons leave a ionization track. The
electrons are drifted by the electric field


A Gas Electron Multiplier (GEM) amplifies the track without
changing the shape


The electrons are collected from a multi
-
pad plane with pads
of 50 µm pitch on an hexagonal pattern.


The pad
-
plane, acting as the anode, is the top layer of a VLSI
chip. Each pad is independently read

A new design for GPD
-

SPIE2012
-

Bellazzini, Costa et al.

s
-
photoelectrons are
ideal analyzers of
polarization

A new design for GPD
-

SPIE2012
-

Bellazzini, Costa et al.

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
n

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

the conversion
point.

Tracks

reconstruction

A new design for GPD
-

SPIE2012
-

Bellazzini, Costa et al.



Peaking time: 3
-
10
m
s, 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 core: an ASIC performing the functions of anode,
FEE, event pre
-
selection

A new prototype with an extended GEM for better drift field
uniformity

Mixture filling

He 20% + DME 80% 1 bar

Gas cell
thickness

1 cm

GEM

50 um pitch, 50 um thick,
88 x 88 mm

OLD

NEW

A new design for GPD
-

SPIE2012
-

Bellazzini, Costa et al.

Same window, same ASIC but a
much larger GEM, with the
addition of a large Guard Ring and
field forming frames.

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


keV

Crystal Line Bragg angle

1.65 ADP(101) CONT

45.0

2.01 PET(002) CONT

45.0

2.29 Rh(001) Mo L
α


45.3

2.61 Graphite CONT

45.0

3.7 Al(111)
Ca

K
α


45.9

4.5
CaF
2
(220) Ti K
α

45.4

5.9
LiF
(002)
55
Fe
47.6

6.4 Si(400) Fe 45.5

8.05
Ge
(333) Cu
K
α

45.0


9.7
FLi
(420)
Au

L
α

45.1

17.4
Fli
(800)
Mo

K
α

44.8

Facility at IASF
-
Rome/INAF

Close
-
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

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

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
.


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

Modulation factor

Energy

Modulation factor

2.6

26.68 +/
-

0.42

4.5

42.75 +/
-

0.24

6.4

52.44 +/
-

0.31

2.6 keV

with a cut on low energy tail of Pulse
Height

A new design for GPD - SPIE2012 - Bellazzini, Costa et al.
Spurious modulation @ 5.9 keV

125 kcounts:

Modulation factor:




~50%

Spurious modulation measured:

~0.54%

Spurious polarization measured:

~1%

MDP 99% with
m
=50% and 125 kc:

~ 2.3%

A new design for GPD
-

SPIE2012
-

Bellazzini, Costa et al.

Energy resolution

6.4
keV

Energy (
keV
)

D
E/E

2.6

25.0%

4.5

19.8%

5.9

19.0%

6.4

17.0%

A new design for GPD
-

SPIE2012
-

Bellazzini, Costa et al.

FWHM 82
m
m

Position resolution

4.5
keV

MEASUREMENT WITH THE GPD

Core and wings

Core : 83.5 % (from simulations)

The core is 84 % of the total counts from simulations.

True interaction point

Reconstructed
position

MONTE CARLO SIMULATIONS OF THE GPD

Quantum Efficiency included the Beryllium window for the
gas mixture filling He
-
DME (20
-
80)

We are limited by the transmission through the USB the limit is 20
-
30 c/s.

Resources :



Weight GPD + Electronics (total)= 1850 g




Dimensions of the electronic BOX (with foot) :


142 mm (+ 110 for extensions for V.D. and connectors)

192 mm (+ 40 for sealed gas filling tube)

80 mm (H)




Geometrical active area GPD = 15 mm x 15 mm



POWER :


HV = (400 V , 870 V, 2650 V negative) CAEN N470



LV = 5 Volt, 1A




Control & Data Transmission : USB connector to PC Windows


Center
GPD

Possible
meaurements

at
Panter



Measurements of the overall position resolution of the GPD + JET
-
X
optics.




Measurement of the overall modulation factor.




Measurement (upper limits) on the residual modulation

Methods :


1)
Alignment detector optical axis by inclining in two direction and taking the beam with
the maximum symmetry.


2
)
Monochromatise

the X
-
ray tube with suitable crystals (with known R



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歮潷n
B
r慧朠慮杬g⸠


3)
Collect fluorescence photons excited by fluorescence X
-
ray lines with larger energies

(and excluding the reflected lines by means of the spectral capability of the GPD


END