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Page 1: D-2 M. Fraga

Beaune 2002

The GEM scintillation in He-CF4, Ar-CF4, Ar-TEA and Xe-TEA mixtures

M. M. Fraga, F. A. F. Fraga, S. T. G. Fetal, L. M. S. Margato, R. Ferreira Marques and A. J. P. L. Policarpo

LIP- Coimbra, Dep. Física, Univ. Coimbra, 3004-516 Coimbra, Portugal

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Summary• Motivation • Experimental set-up• Emission spectra of Ar/CF4 and He/CF4.• Excitation and de-excitation processes in CF4

and CF4 mixtures• Emission spectra of Ar/TEA• Total light yields• Conclusions

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The GEM - gas electron multiplier

Magnitude of the electric field along the center of the GEM channel.See http://gdd.web.cern.ch/GDD/

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Applications (with CCD readout of the GEM scintillation)

• Alpha particle tracking:Triple GEM with Ar+40%CF4

241Am α particles E = 5.48 MevRange in Ar = 3.42 cm(F. Fraga et al., IEEE Trans. Nuc. Sci. 49 (2002) 281)

• Thermal neutron detection:Proton and triton tracks in 3He- 400 mbar CF4Triple GEMAmBe source with Polyethylene shielding(F. Fraga et al., NIM A 478 (2002) 357)

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Applications

• X-ray imaging• Car key ~5 cm radiography• X-ray energy ~8keV• Xe-10%CO2 at 1bar• absorption length ~3 mm

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Objectives:• measure the emission spectra of Ar- and He-CF4

mixtures and identify the main emitting channels.• quantify the total light yields;• study other gas mixtures leading to a stable

operation of the GEM detector and exhibiting large photon yields either in the visible and near infrared (NIR) or in the UV region.

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Experimental set-up

-Vdrift

Front electrode Back electrode

GEM

Glass window

Quartz window

Detection system

X rays from:

• Fe-55 source (5.9 keV);• X-ray generator

2 4 6 8 1002468

1012

I (a.

u.)

E (keV)

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Detection systems• Total light yields:4 Planar-diffused silicon

photodiode (UDT PIN-25DP)

σ < 15%

8 Photomultiplier (56 TUVP)(operating in the pulse mode)

σ < 30%

00.10.20.30.40.50.60.70.80.9

200 300 400 500 600 700 800 900

λ (nm)

Q.E

.

Photodiode56 TUVP

πΩ

− =

4.T..Q.I

Iph

phe/Ne

e

cph

N.T.4

.M

qfe/N

πΩ

=−

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• Emission spectra:8 Monochromator (Applied Photophysics m. 7300, with a

1200 g/mm grating blazed at 500 nm) + photomultiplier(RCA C31034), cooled to -20ºC, operating in single photon counting mode.

8 Spectral sensitivity of the detection system is measured with:

! Standard tungsten strip lamp (Osram WI 17/G) (λ >310 nm), previously calibrated against a black body (in Institute of Physics, Belgrade)

! Deuterium lamp (200 - 370 nm);

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Spectroradiometric calibration with the tungsten strip lamp

L1

IFOsram WI 17/G

NDF

ΩE

VR)(S)(N)(Q 0

λλ

Calibration factor:

s’s

M

S1

Io

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• Nº of photons entering the monochromator per second:

» is the effective solid angle;

» effective emitting area of the tungsten ribbon;

» emissivity of tungsten;

» nº of photons emitted per unit time, per unit area and per steroradian, by a blackbody at a temperature T, in the wavelength range between λ and λ+∆λ

)(f...A).,T(N).,T()(S Ec λλ∆Ω∆λλε=λ

A∆

),T( λε

λ∆λ).,T(Nc

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Charge Gains

100 150 200 250 300 350 400 450 500

1

10

100

1000

Xe+4%TEA Xe+3%TEA He+40%Xe+3%TEA He+40%CF4 He+20%CF4

Gai

nV

GEM(V)

150 200 250 300 350 400

1

10

100

1000 Ar+3%TEA Ar+5%TEA Ar+5%CF4 Ar+10%CF4

Gai

n

VGEM

(V)

He Ar CF4 Xe TEAI. P. (eV) 24,58 15,7 15,9 12,12 7,51Eexc (eV) 19,82 11,54 12,5 8,4 4,77

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Emission spectrum of Ar+5%CF4 mixtureG = 40; ∆λ = 4 nm (raw data)

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 00

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

I norm

(a.u

.)

λ (n m )

Ar I2p→3s lines

OH-

CF3*

(1E’,2A2”→1A1’)

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500 550 600 650 700 750 800 8500

1

2

3

Ar + 67% C F 4

G =40

I norm

(a.u

.)

λ (nm )

500 550 600 650 700 750 800 8500

1

2

3

Ar + 10% C F 4

G =90

500 550 600 650 700 750 800 8500

1

2

3Ar + 5% C F 4

G =40

1 10

0,1

0,2

0,3

0,4

0,5

0,6

0,7

5% CF4 10% CF4 67% CF4

Nph

/e-

Gain

Nº of photons emitted, between400 and 1000 nm, per secondaryelectron, as a function of the effective gain, in Ar-CF4 mixtures.(Measurements performed withthe photodiode).

Visible and NIR emission spectra of Ar-CF4 mixtures, normalized to the light intensity at 620 nm.

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Emission spectra of He-CF4 mixtures

200 300 400 500 600 700 800 9000

200

400

600

800

I norm

(a.u

.)λ (nm)

Emission spectrum of He+40%CF4 , corrected for 2nd order diffraction effects and the quantum efficiencyof the detection system.

200 300 400 500 600 700 8000

100

200

300

400

500 He + 20% CF4 He + 40% CF4

I norm

(nm

)

λ (nm)

Raw dataCF3

*

CF4+*, CF3

+*

He+20% CF4 : G = 335He+40% CF4: G = 175

∆λ = 4 nm

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He-CF4 mixtures

400 500 600 700 8000

100

200

300

400

500

600

20%CF4 40%CF4 85%CF4

Ligh

t int

ensi

ty/e

lect

ron

curre

nt (a

.u.)

wavelength (nm)

0 20 40 60 80 1000

200

400

600

800

1000

1200

Ar/CF4 He/CF4

I n (a.

u.)

% CF4

Light intensity, normalized to the current, measured for λ = 620 nm,as a function of CF4 concentration.

Visible emission spectra of He-CF4mixtures, measured with a glass color glass filter (λcutt-off=435 nm).

He/CF4Ar/CF4

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CF4

Process Threshold(eV)

Energy loss(eV)

Direct vibrational excitation ν4

ν3

0.0780.159

0.0780.159

Indirect vibrational excitation 4.0 0.4

Electron attachment 4.3 4.3

Electronic excitation (dissociationinto neutral fragments)†

12.5(10)

12.5(10)

Dissociative ionization† 15.9 15.9

†All electronic excited states of CF4 and CF4+ (τ < 10 µs) seem

to dissociate or predissociate.

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Dissociation into neutral fragments in CF4

e- + CF4 → CF3 + F + e-

Energy threshold: 10 - 12.5 eV (?)

Dissociation energy: D(CF3 — F) = 5.25 eV

Electronic excited states of CF3*:

1E’ (7.95 eV), 2A2” (7.68 eV) , 2A1’ (8.40 eV).

Observed electronic transitions:

CF3* (1E’,2A2”→1A1’(repulsive state)) visible ∼ 620 nm

CF3* (2A1’→1A2” (ground state)) UV ∼ 265 nm

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Total cross section for dissociation of CF4 into neutrals

Christophorou and Olthoff, J. Phys. Chem. Ref. Data, vol. 28, nº 4, 1999, 967.

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0 25 50 75 100 125 150

20

40

60

80

100

Vib-CF4 neutral diss. ion exc1 - He exc2 - He

Pm (%

)

E (kV/cm)25 50 75 100 125 150

0

10

20

30

40

Pm (%

)

E (kV/cm)

Mean power lost in collisions in a He+40%CF4 mixture. Penning ionization is not considered.

Calculations based on the numerical solution ofBoltzmann equation for a uniform electric field configuration . The code used was developed by the group of P. Ségur, CPAT, Toulouse, France

Mean power lost in collisions in a Ar+40%CF4 mixture.

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0 25 50 75 100

10

100

1000

Ar+10%CF4 Ar+40%CF4 He+20%CF4 He+40%CF4 100% CF4

αex

c (cm

-1)

E (kV/cm)0 25 50 75 100

0,1

1

10

100

1000

Ar+10%CF4 Ar+40%CF4

αex

c (cm

-1)

E (kV/cm)

Number of collisions per cm leading to excitation of Ar* (2p+high lying forbidden) levels, as a function of the electric field.

Number of collisions per cm leading to dissociation of CF4 into neutral fragments, as a function of the electric field.

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Dissociative ionization of CF4

IonProducts

Fragmentation (%)(e,e+ion)

PES DipoleIonization Zhang et al, Chem. Phys. 137

1989, 391

s10s3 C~ µ<τ<µ

A) CF4+* dissociates before emitting:

D(CF3-F) = 5.25 eV

IP (CF3) = 9.25 eV

CF3+*→ CF3

+ + hν (UV) )nm290(~h)B~(CF

)nm240(~h)A~(CF

)nm160(~h)X~(CF)C~(CF

*4

*4

4*

4

ν+→

ν+→

ν+→

+

+

++B) CF4

+* emits before dissociating

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Excitation and de-excitation mechanisms

Ar* (2p)

Ar* (3s)

Ar**

X

X

IR

NIRX

Ar

X = Ar, CF4, H2O, ...

Ar

Ar+CF4+

CF4products

dissociation

CF4* products

products

CF4 (ν’)

CF3+ F

CF4

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Excitation and de-excitation mechanisms

X

CF4*

CF4+ CF4

He

X = He, H2O, ...

CF3+ F

CF4CF4

He+ productsHe**(CF4

+)*

He* products

dissociation

CF4 (ν’)

CF4

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Beaune 2002

Ar-TEA

240 260 280 300 320 340 360 380

0

20

40

60

80

100

120

Ar + 3% TEA

light

inte

nsity

/cur

rent

(a.u

.)λ (nm)

0,1

1

10

100

120 140 160 180 200 220 240 260 280

λ (nm)

(mm

)

Pv = 30 torrT = 293 k

Absorption length versus wavelength Emission spectrum (∆λ = 4 nm)

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Beaune 2002

Excitation and de-excitation mechanisms

Ar* (2p)

Ar* (3s)

Ar**X

X

Xproducts

Ar

TEA+ Ar Ar2*Ar

VUV

Ar+

products

X = TEA, Ar, H2O, ... products

TEA*

UV

TEA

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Total light yields (PMT)

ΤΕΑ mixtures: σsyst < 25% ; He/CF4 - 0.1-0.3 ph/e-

0 100 200 300 400 500 600 700 8000,0

0,2

0,4

0,6

0,8

1,0N

ph/e

-

Gain

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Beaune 2002

He+CF4

10 100 10000,00

0,04

0,08

0,12

0,16 N

ph/e

-

Gain

(a)

1 100,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

photodiode PMT + filter

Nph

/e-

Gain

Ar+10%CF4

(b)

Quantum efficiency of the photodiodeand PMT.

0,001

0,01

0,1

1

200 300 400 500 600 700 800 900

λ (nm)

Q.E

.

Photodiode56 TUVP Nº of photons (λ > 400 nm) emitted per

secondary electron as a function of the effective gain in (a) He+20%CF4 and He+40% CF4; (b) Ar+10%CF4

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Pulse height spectra from 5.9 keV X-rays

Ar+3%TEAVGEM = 290 VGeff = 640(R = 53% for G= 460)

0 40 80 120 1600

10

20

30

40

50

channel #

Nº c

ount

s/s

0 20 40 60 80 100 1200

10

20

30

40

Channel #0 40 80 120 160

0

2

4

6

8

10

Channel #

GA=50R = 41%

Xe+3%TEAVGEM = 320 VGeff = 370

He+40%CF4VGEM = 460 VGeff = 320

GA=80R = 58%

GA=320R = 46%

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Conclusions• Ar-CF4 mixtures emit mainly in the visible (CF3

*) and in the NIR (Ar I 2p-3s atomic lines, which can be detected even for high CF4 concentrations).

• He-CF4 emit both in the UV and visible (CF3*)

• Visible emissions are more important in Ar/CF4 than in He/CF4. • The GEM detector has been operated with Ar- and Xe-TEA

mixtures but, for TEA concentrations above 5%, large leakage currents arise and the detector becomes unstable.

• No visible or NIR light was detected in Ar-TEA mixtures• Light emitted in the gas (single GEM) was detected with a PMT

operating in the pulse mode. The energy resolution is limited by statistics associated with charge multiplication process.


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