Beaune 2002 The GEM scintillation in He-CF 4 , Ar-CF 4 , 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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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
Beaune 2002
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
=λ
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
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).
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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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)