property of lcp-gem in pure dimethyl ether under low pressure

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

RIKEN / Tokyo Univ. of Sci.Yoko Takeuchi

[email protected]

　 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 ， Kogakuin Univ.D

MPGD 2013 @Zaragoza 1

1-1.Cosmic X-ray Polarimetry

image Polarization

？time variation Energy

History of X-ray polarimetry

Observation of cosmic X-ray objects

MPGD 2013 @Zaragoza

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

year mission type1972’ Sounding Rocket (Norvick+1972) Bragg1976’ OSO-8 satellite(Weisskopf+1976, 78) Bragg

== 　 No mission since 1970’s ==

Few significant detection of X-ray polarization.

2

Plan to mount polarimeter on sounding rocket

1-2. XACT rocket Experiment

MPGD 2013 @Zaragoza

XACT rocket • 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

[K. Gendreau + 2012]

3

- 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

2.8 m

Mirror

GEM-TPC polarimeter

Focal length 2.8 m

Using photoelectric effect

• Direction of X-ray polarization : E of the incident X-ray

• distribution of photoE emission :

• Polarization can be measured by photoE distribution.

• Sensitive to only linear polarization

1-3. Photoelectric Effect

X-ray

MPGD 2013 @Zaragoza

PhotoE Track

Photoelectric Effect

E

4

cos2Φ

θΦ

optaxis

x ： time

y ： strip[ Black +2007 ]GEM

Drift Electrode

Trigger

Readout strip

Photoelectron

1-3. GEM-TPC X-ray Polarimeter

MPGD 2013 @Zaragoza

- Single GEM- Strip pitch : 121 μm - Readout ASIC: APV25 (20 MHz sampling)

5

Gas Electron Multiplier (LCP-GEM)

developed at RIKEN

140 um70 um

LCP

Thickness :100 um

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

[Tamagawa + 2009]

2D photoE track image

optaxis

x ： time

y ： strip[ Black +2007 ]GEM

Drift Electrode

Trigger

Readout strip

Photoelectron

1-3. GEM-TPC X-ray Polarimeter

MPGD 2013 @Zaragoza

- Single GEM- Strip pitch : 121 μm - Readout ASIC: APV25 (20 MHz sampling)

2D photoE track image

target gas: Pure Dimethyl Ether (DME)

requirement for property of target gasslow drift velocity …… coarse sampling with APV25small diffusion .…… avoid blurring track image

5

1-4. Requirement of Polarimeter

MPGD 2013 @Zaragoza

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

6

Trac

k le

ngth

(2 k

eV)　

[mm

]

DME Pressure[Torr]

Coun

t Ra

te [

/sec

]　

Optimum pressure range: 50-150 Torr

LCP-GEM has NOT been operated under low pressure in DME.

As pressure goes down, -track length increases

-count rate decreases

We estimate optimum pressure range by simulation.

1-4. Requirement of Polarimeter

MPGD 2013 @Zaragoza

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

6

Trac

k le

ngth

(2 k

eV)　

[mm

]

DME Pressure[Torr]

Coun

t Ra

te [

/sec

]　

Optimum pressure range: 50-150 Torr

LCP-GEM has NOT been operated under low pressure in DME.

As pressure goes down, -track length increases

-count rate decreases

We estimate optimum pressure range by simulation.

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

Our study

2-1. Experimental Setup

Drift electrode

readout pad

preamplifierAMPTEK A225

20.5 mm

1 mm

R3

Ed

Ei

pure DME 10 ~ 190 TorrVd

ADC

MPGD 2013 @Zaragoza

Vc

preamplifierORTEC 109PC

ΔVGEM

LCP-GEM

・ Single LCP-GEM ( 30 mm x 78 mm x 0.1 mm )

・ Constant drift velocity, 0.24 um/nsex.) 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)

X-ray

7

10 M

Ω10

MΩ

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 % 19

0 Torr

ΔVGEM

Effec

tive

Gain

readout pad

Next slide, gain curves <190 Torr Please check the point !! - function shape

- maximum gain

3-1. Gain Curve


190 T

orr

ΔVGEM

Effec

tive

Gain

readout pad


At 110 Torr

- Non exponential function


110 Torr

3-1. Gain Curve


190 T

orr

ΔVGEM

Effec

tive

Gain

readout pad


At 70 Torr



110 Torr

70 Torr

3-1. Gain Curve


190 T

orr

ΔVGEM

Effec

tive

Gain

readout pad


At 50 Torr

- exponential function


110 Torr 70 To

rr

50 Torr

3-1. Gain Curve


190 T

orr

ΔVGEM

Effec

tive

Gain

readout pad


At 40 Torr



110 Torr 70 To

rr

50 Torr

40 Torr

3-1. Gain Curve


190 T

orr

ΔVGEM

Effec

tive

Gain

readout pad


At 30 Torr



110 Torr 70 To

rr

50 Torr

40 Torr

30 Torr

3-1. Gain Curve


190 T

orr

ΔVGEM

Effec

tive

Gain

readout pad


At 20 Torr



110 Torr 70 To

rr

50 Torr

40 Torr

30 Torr

20 Torr

3-1. Gain Curve


190 T

orr

ΔVGEM

Effec

tive

Gain

readout pad

110 Torr 70 To

rr

50 Torr

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

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

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


190 Torr110 Torr

70 Torr50 Torr

20 Torr

α / p

[

/cm

/Tor

r]

40 Torr30 Torr

E / p [V/cm/Torr]

definition ：　

α 　 = ln (Gain)/xGEM

xGEM ： 100 μm fix (GEM thickness)

)(

The data is well fitted by the empirical formula.

fitting by α/p = A × exp( -B p/E )

● α [/cm] ・・・

※ Gain=Gainpad+GainGEM ano. This is why we wanted to collect all of the amplified electrons in the GEM channels

● E [V/cm] ・・・ΔVGEM /GEM thickness

(100 μm)

3-3. Measured α vs. MAGBOLTZ

MPGD 2013 @Zaragoza

α/p

　[ /c

m/T

orr]

E/p [V/cm/Torr]

MAGBOLTZExperimental Data

11

[Sharma + 1993]

- Experimentally-measured α grossly agree with MAGBOLTZ’s result.- Although the GEM and parallel plate are different each other,

both results are well fit to the MAGBOLTZ estimation.

MAGBOLTZ parametersp=10 - 190 TorrE=10 - 60 kV/cm

3-4. Linearity of Energy Scale

gain ~200

gain ~300gain ~400gain ~500gain ~700●

●

●

MPGD 2013 @Zaragoza

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

incident Energy 4.5 keV (Ti)6.4 keV (Fe)8.0 keV (Cu)

At 30 Torr

12

Incident Energy [keV]

Mea

sure

d Ch

arge

Q [f

C]

ΔVGEM = 440 V

5 sec

200 mV

3-5. Odd Pulse Profile at 10 Torr

4 ms

200 mV

MPGD 2013 @Zaragoza

the signal profile dramatically changed above ΔVGEM= 437 V.

ΔVGEM = 430 V

* duration : ~10 μs* pulse height : 4 mV （ gain ~40)

* duration: 1~10 ms* pulse height : ~200 mV

※ No Signal when X-rays were stopped. ==> Those signals were triggered by X-rays.

10 us

4 mV

ΔVGEM = 437 V

Preamp-out signal captured by an oscilloscope from readout pad

13

* duration: 2~5 sec* pulse height: ~200 mV

Normal signal

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

E/ P[V/cm/Torr] Ch

arge

col

lect

ed

Geiger region

Proportional region

Ion chamberregion

High voltage and/or

low pressure

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

MPGD 2013 @Zaragoza

1) Change of pulse profile at 10 Torr- longer duration time- higher pulse height

2) As ΔVGEM gets higher,-longer duration-almost same pulse

height

14

3-6. GEM operated in Geiger region?

Observation results suggest that GEM are probably operated in the Geiger region.

We understand from those signal of oscilloscope,

4. Summary

MPGD 2013 @Zaragoza

We have developed Cosmic X-ray polarimeter using GEM-TPC. Polarimeter requirement of DME pressure : 50 – 150 Torr 　

We measured the properties of LCP-GEM in pure DME at 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.

15

Thank you for your attention !!

Backup side!

LCP-G

EM(d/p/t: 70/140/50 um)

double

LCP-G

EM(d/p/t:

70/140/1

00 um) sin

gle

ΔVGEM/100 um (V)

Effec

tive

Gain

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

LCP-GEM Performance

500 600 700 800

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

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

elapsed time after HV ON (min)

rela

tive

gain

Gain curveLCP-GEM (t100 um) is achieved higher gain than one of t50 um as same ΔVGEM.

Time variation Gain of LCP-GEM does not change.

200 400 600 800 1000

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

Drift Velocity of Electron

DMECO2

Ar-CO2(70%:30%)

0.24

(MAGBOLTZ Calculation)

DME gas is very slow gas !!

Drift velocity of electron and ion(Magboltz Calculation)

DMEDME

Ar-CO2

electronAr-CO2

Ion

＊ discharge point : 　 the highest ΔVGEM without discharge rate 0.1 %

＊ Paschen curve of DME is non data.

＊ Discharge point is similar with Paschen curve.

※calculation as d=100 μm(GEM thickness)

Discharge Point Br

eakd

own

Vol

tage

[V

]

P ・ d [Torr cm]

Energy Resolution

Table of Ed & Ei

readout pad

preamplifierAMPTEK A225

20.5 mm

1 mm

10 M

Ω10

MΩ

R3

Ed

Ei

ADC

Vc

preamplifierORTEC 109PC

LCP-GEM

Ed [V/cm] Ei [V/cm]190 Torr 196 4000-5500110 Torr 113 3500-530070 Torr 72.2 300-55050 Torr 51.6 300-55040 Torr 41.3 300-55030 Torr 30.9 300-55020 Torr 20.6 400-52010 Torr 10.3 ~150

Drift electrode

Vd

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

X-ray

Directi

on of

X-ray P

olarization

Strip

(12

1 um

pitc

h)2D Electron Track Image

- APV25 (20 MHz sampling) - Drift velocity : 0.24 um/cm

photoE dist

ribution

Electron Track Length

(Tabata + 1972 )

Polarimeter Design for XACT

Effective area : 31.6 cm

Close chamber

With MXS (Modulated X-ray Source)

Gain Curve at 10 Torr- One data : 30000 event

At 10 Torr

- Only one data point


Strange signal appear at ΔVGEM > 437 V

190 T

orr

ΔVGEM

Effec

tive

Gain

readout pad

110 Torr 70 To

rr

50 Torr

40 Torr

30 Torr

20 Torr

10 Torr

Energy scale at 190 Torr

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

gain 600

gain 2500gain 4300

gain 7500 ●

●

●

incident Energy 4.5 keV (Ti)6.4 keV (Fe)8.0 keV (Cu)

gain 350

Townsend coef. vs Ei at 190 Torr[Y. Takeuchi 2011]

Ei [V/cm]

α [/

mm

]

CROSS SECTION of DME

Ionization

ADC Spectrum

Incident energy: 6.4 keV At 70 TorrdVGEM= 460 V

Future Works

Future plan is measurement of the energy scale.In MPGD

- below 30 Torr …. check linearityIn 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. 　

Linearity of Energy Scale at 30 Torr

property of lcp-gem in pure dimethyl ether under low pressure

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