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Property of LCP-GEM in Pure Dimethyl Ether under Low Pressure

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### Property of LCP-GEM in Pure Dimethyl Ether under Low Pressure

RIKEN / Tokyo Univ. of Sci.

Yoko Takeuchi

takeuchi@crab.riken.jp

T. TamagawaA, T. KitaguchiA, W. IwakiriA, F. AsamiA,B, A. YoshikawaA.B,

K. KanekoA,B,T. EnotoA,C, A. HayatoA, Y. KohmuraD,

and The GEMS/XACT team

RIKENA，Tokyo Univ. of Sci.B，NASA/GSFCC，KogakuinUniv.D

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MPGD 2013 @Zaragoza

1-1.Cosmic X-ray Polarimetry

Observation of cosmic X-ray objects

image

Polarization

Energy

time variation

？

Few significant detection of X-ray polarization.

History of X-ray polarimetry

year mission type

1972’ Sounding Rocket (Norvick+1972) Bragg

1976’ OSO-8 satellite(Weisskopf+1976, 78) Bragg

==No mission since 1970’s ==

Plan to mount polarimeter

on sounding rocket

- Cosmic X-ray Polarimeter Developed
- Using GEM-TPC technique
- Sensitivity with > 100 times higher than ever before.

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MPGD 2013 @Zaragoza

1-2. XACT rocket Experiment

Mirror

Focal length

2.8 m

- X-ray Advanced Concepts Testbed
- NASA’s sounding rocket program
- Scheduled to be launched in 2015
- Energy range : 1-10 keV
- Observation time ：〜 6 min
- Target : Crab Nebula

XACT rocket

2.8 m

[K. Gendreau + 2012]

GEM-TPC

polarimeter

- One of the brightest X-ray objects
- Only X-ray polarized source detected
- by Bragg polarimeter so far
- Strong X-ray emission around 2 keV

- Using photoelectric effect

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MPGD 2013 @Zaragoza

- Photoelectric Effect

- Direction of X-ray polarization :
- E of the incident X-ray
- distribution of photoE emission :
- Polarization can be measured
- by photoEdistribution.
- Sensitive to only linear polarization

X-ray

E

cos2Φ

Φ

PhotoE Track

θ

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MPGD 2013 @Zaragoza

1-3. GEM-TPC X-ray Polarimeter

Trigger

Drift

Electrode

[ Black +2007 ]

GEM

- Single GEM

- Strip pitch : 121 μm

- Readout ASIC: APV25

(20 MHz sampling)

Photoelectron

2D photoE track image

Readout

strip

Gas Electron Multiplier (LCP-GEM)

y：strip

[Tamagawa+ 2009]

70 um

140 um

- About LCP-GEM,
- See Tamagawa’sTalk (7/3 12:30).

opt

axis

LCP

x：time

Thickness :100 um

developed at RIKEN

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MPGD 2013 @Zaragoza

1-3. GEM-TPC X-ray Polarimeter

Trigger

Drift

Electrode

[ Black +2007 ]

GEM

- Single GEM

- Strip pitch : 121 μm

- Readout ASIC: APV25

(20 MHz sampling)

Photoelectron

2D photoE track image

Readout

strip

requirement for property of target gas

slow drift velocity …… coarse sampling with APV25

small diffusion.…… avoid blurring track image

y：strip

opt

axis

target gas: Pure Dimethyl Ether (DME)

x：time

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MPGD 2013 @Zaragoza

1-4. Requirement of Polarimeter

DME pressure is optimized by calculating track length and count rate.

- As pressure goes down,
- -track length increases
- -count rate decreases
- We estimate optimum
- pressure range by simulation.

Track length (2 keV)[mm]

Count Rate [/sec]

Optimum pressure range

: 50-150 Torr

DME Pressure[Torr]

LCP-GEM has NOTbeen operated under low pressure in DME.

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MPGD 2013 @Zaragoza

1-4. Requirement of Polarimeter

DME pressure is optimized by calculating track length and count rate.

- As pressure goes down,
- -track length increases
- -count rate decreases
- We estimate optimum
- pressure range by simulation.

Track length (2 keV)[mm]

Count Rate [/sec]

Optimum pressure range

: 50-150 Torr

Our study

DME Pressure[Torr]

Measure property of LCP-GEM in Pure DME under low pressure

- Gain curve

sufficient gain without discharge

- Energy scale

check linearity

- Lower pressure limit of normal GEM operation

LCP-GEM has NOTbeen operated under low pressure in DME.

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MPGD 2013 @Zaragoza

2-1. Experimental Setup

・ Single LCP-GEM

( 30 mm x 78 mm x 0.1 mm )

・ Constant drift velocity, 0.24 um/ns

ex.) Ed = 196 V/cm at 190 Torr

・ To avoid electron amplification

in the induction region, Ei is set

suitable value.

・ Signals are read from pad &

GEM anode

・ DME pressure: 10-190 Torr

・ X-ray beam from X-ray generator

- Collimated with Φ200 μm

- Parallel to LCP-GEM

- Energy :

* 6.4 keV (gain curve)

* 4.5, 6.4, 8.0 keV

(Energy scale)

pure DME 10 ~ 190 Torr

Vd

Drift electrode

Vc

20.5 mm

Ed

10 MΩ

X-ray

LCP-GEM

ΔVGEM

10 MΩ

Ei

1 mm

readout pad

R3

preamplifier

ORTEC 109PC

ADC

preamplifier

AMPTEK A225

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MPGD 2013 @Zaragoza

3-1. Gain Curve

- One data : 30000 event
- At 190 Torr
- - Exponential function
- Maximum gain :~ 20000
- (ΔVGEM=560 V)
- w/o discharge rate of 0.1%
- - ΔE/E(FWHM) ~20 %

readout pad

190 Torr

Effective Gain

Next slide, gain curves <190 Torr

Please check the point !!

- function shape

- maximum gain

ΔVGEM

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MPGD 2013 @Zaragoza

3-1. Gain Curve

- One data : 30000 event
- At 110 Torr
- - Non exponential function
- - Maximum gain :~ 20000
- (ΔVGEM=520 V)

readout pad

110 Torr

190 Torr

Effective Gain

ΔVGEM

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MPGD 2013 @Zaragoza

3-1. Gain Curve

- One data : 30000 event
- At 70 Torr
- - Non exponential function
- - Maximum gain :~ 10000
- (ΔVGEM=500 V)

readout pad

110 Torr

190 Torr

70 Torr

Effective Gain

ΔVGEM

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MPGD 2013 @Zaragoza

3-1. Gain Curve

- One data : 30000 event
- At 50 Torr
- - exponential function
- - Maximum gain :~ 5000
- (ΔVGEM=510 V)

readout pad

70 Torr

110 Torr

190 Torr

50 Torr

Effective Gain

ΔVGEM

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MPGD 2013 @Zaragoza

3-1. Gain Curve

- One data : 30000 event
- At 40 Torr
- - Non exponential function
- - Maximum gain :~ 2000
- (ΔVGEM=500 V)

readout pad

70 Torr

110 Torr

190 Torr

50 Torr

Effective Gain

40 Torr

ΔVGEM

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MPGD 2013 @Zaragoza

3-1. Gain Curve

- One data : 30000 event
- At 30 Torr
- - Non exponential function
- - Maximum gain :~ 1000
- (ΔVGEM=490 V)

readout pad

70 Torr

110 Torr

190 Torr

50 Torr

Effective Gain

40 Torr

30 Torr

ΔVGEM

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MPGD 2013 @Zaragoza

3-1. Gain Curve

- One data : 30000 event
- At 20 Torr
- - Non exponential function
- - Maximum gain :~ 300
- (ΔVGEM=470 V)

readout pad

70 Torr

110 Torr

190 Torr

50 Torr

Effective Gain

40 Torr

30 Torr

20 Torr

ΔVGEM

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MPGD 2013 @Zaragoza

3-1. Gain Curve

readout pad

70 Torr

110 Torr

190 Torr

- Difference of each gain curve
- 1) Slope and offset of curve
- 2) Non exponential<110 Torr

50 Torr

Effective Gain

40 Torr

30 Torr

20 Torr

- The pressure dependence of gain curve seems complex.
- To understand this behavior of gain curve,
- we derived the first Townsend coefficient α

ΔVGEM

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MPGD 2013 @Zaragoza

3-2. The First Townsend coef. α/p vs. E/p

definition：

● α[/cm]・・・

※Gain=Gainpad+GainGEMano.

This is why we wanted

to collect all of the amplified

electrons in the GEM channels

● E [V/cm]・・・

ΔVGEM/GEM thickness

(100 μm)

α= ln (Gain)/xGEM

20Torr

xGEM：100 μmfix

(GEM thickness)

50Torr

70 Torr

30Torr

)

(

40Torr

110 Torr

190 Torr

α/ p [ /cm/Torr]

fitting by α/p = A × exp( -B p/E )

The data is well fitted by the empirical formula.

E / p [V/cm/Torr]

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MPGD 2013 @Zaragoza

3-3. Measured αvs. MAGBOLTZ

MAGBOLTZ

Experimental Data

MAGBOLTZ parameters

p=10 - 190 Torr

E=10 - 60 kV/cm

α/p[ /cm/Torr]

[Sharma + 1993]

- Experimentally-measured αgrossly agree with MAGBOLTZ’s result.
- Although the GEM and parallel plate are different each other,
- both results are well fit to the MAGBOLTZ estimation.

E/p[V/cm/Torr]

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MPGD 2013 @Zaragoza

3-4. Linearity of Energy Scale

At 30 Torr

incident Energy

4.5 keV (Ti)

6.4 keV (Fe)

8.0 keV (Cu)

●

gain ~700

●

gain ~500

gain ~400

●

Measured Charge Q [fC]

gain ~300

gain ~200

Incident Energy [keV]

- Charge Q[fC]∝ incident Energy
- Operated in proportional region
- Linear energy scale above 30 Torr.

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MPGD 2013 @Zaragoza

3-5. Odd Pulse Profile at 10 Torr

We challenged to operate LCP-GEM at much lower pressure.

Preamp-out signalcaptured by an oscilloscopefrom readout pad

the signal profiledramatically changed above ΔVGEM= 437 V.

ΔVGEM= 440 V

ΔVGEM= 437 V

ΔVGEM=430 V

10 us

5 sec

4 ms

4 mV

200 mV

Normal signal

200 mV

* duration: 1~10 ms

* pulse height : ~200 mV

* duration : ~10 μs

* pulse height : 4 mV

（gain ~40)

* duration: 2~5 sec

* pulse height: ~200 mV

※ No Signal when X-rays were stopped.

==> Those signals were triggered by X-rays.

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MPGD 2013 @Zaragoza

3-6. GEM operated in Geiger region?

We understand from those signal of oscilloscope,

Proportional

region

Geiger region

Ion chamber

region

1)Change of pulse profile at 10 Torr

- longer duration time

- higher pulse height

2) As ΔVGEMgets higher,

-longer duration

-almost same pulse height

Charge collected

High voltage

and/or

low pressure

E/ P[V/cm/Torr]

Observation results suggest that GEM are probably operated

in the Geiger region.

The operation mode of gas chamber transits from the proportional region to the Geiger region.

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MPGD 2013 @Zaragoza

4. Summary

- We have developed Cosmic X-ray polarimeter using GEM-TPC.
- Polarimeterrequirementof DME pressure : 50 – 150 Torr
- We measured the properties of LCP-GEM in pure DMEat low pressure.
- - gain curve (10-190 Torr) .. The data is well fitted by empirical formula.
- - Energy scale (4.5-8.0 keV) … Linear from 30 to 190 Torr.
- - Odd pulse phase at 10 Torr …GEM are probably operated in the Geiger region.
- LCP-GEM can be operated in proportional region at pressure of 50-150 Torr.

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MPGD 2013 @Zaragoza

LCP-GEM Performance

Gain curve

LCP-GEM (t100 um) is achieved

higher gain than one of t50 um

as same ΔVGEM.

ArCO2(70%:30%) gas flow, 55Fe

single

double

LCP-GEM(d/p/t: 70/140/100 um)

Effective Gain

ArCO2(70%:30%) gas flow, 55Fe

LCP-GEM(d/p/t: 70/140/50 um)

CERN GEM (d/p/t: 70/140/50 um)

500

700

800

600

relative gain

LCP-GEM (d/p/t: 70/140/50 um)

ΔVGEM/100 um (V)

Time variation

Gain of LCP-GEM does not change.

400

1000

800

200

600

elapsed time after HV ON (min)

Drift Velocity of Electron

(MAGBOLTZ Calculation)

Ar-CO2(70%:30%)

DME

CO2

0.24

DME gas is very slow gas !!

Discharge Point

- ＊discharge point :
- the highestΔVGEM
- without discharge rate 0.1 %
- ＊Paschen curve of DME is
- non data.
- ＊Discharge point is similar
- with Paschen curve.

Breakdown Voltage [V]

P・d [Torr cm]

※calculation as d=100 μm(GEM thickness)

Table of Ed & Ei

Vd

Drift electrode

Vc

20.5 mm

Ed

10 MΩ

LCP-GEM

10 MΩ

Ei

1 mm

readout pad

R3

preamplifier

ORTEC 109PC

ADC

preamplifier

AMPTEK A225

Detection Efficiency in the DME gas & Effective Area of X-ray Mirror

X-ray

2D Electron Track Image

Direction of

X-ray Polarization

Strip (121 um pitch)

photoE distribution

- APV25 (20 MHz sampling)

- Drift velocity : 0.24 um/cm

Electron Track Length

(Tabata + 1972 )

Gain Curve at 10 Torr

- One data : 30000 event
- At 10 Torr
- Only one data point
- - Maximum gain :~ 40
- (ΔVGEM=430 V)
- Strange signal appear
- at ΔVGEM> 437 V

readout pad

70 Torr

110 Torr

190 Torr

50 Torr

Effective Gain

40 Torr

30 Torr

20 Torr

10 Torr

ΔVGEM

Energy scale at 190 Torr

incident Energy

4.5 keV (Ti)

6.4 keV (Fe)

8.0 keV (Cu)

●

gain 7500

●

gain 4300

●

gain 2500

gain 600

gain 350

- charge Q[fC]∝ incident Energy
- At 190 Torr, Operated in proportional region

CROSS SECTION of DME

Ionization

Future Works

Linearity of Energy Scale

at 30 Torr

Future plan is measurement of the energy scale.

In MPGD

- below 30 Torr…. check linearity

In XACT mission, X-ray emission of Crab Nebula is strong around 2 keV

- with less than 4.5 keV X-rays at 10-190 Torr.

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