Post on 23-Jan-2023
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
Beaune 2002
The GEM - gas electron multiplier
Magnitude of the electric field along the center of the GEM channel.See http://gdd.web.cern.ch/GDD/
Beaune 2002
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)
Beaune 2002
Applications
• X-ray imaging• Car key ~5 cm radiography• X-ray energy ~8keV• Xe-10%CO2 at 1bar• absorption length ~3 mm
Beaune 2002
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.
Beaune 2002
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)
Beaune 2002
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
=−
Beaune 2002
• 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);
Beaune 2002
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
Beaune 2002
• 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
EΩ
A∆
),T( λε
λ∆λ).,T(Nc
Beaune 2002
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
Beaune 2002
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’)
Beaune 2002
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.
Beaune 2002
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
Beaune 2002
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
Beaune 2002
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.
Beaune 2002
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
Beaune 2002
Total cross section for dissociation of CF4 into neutrals
Christophorou and Olthoff, J. Phys. Chem. Ref. Data, vol. 28, nº 4, 1999, 967.
Beaune 2002
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.
Beaune 2002
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.
Beaune 2002
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
Beaune 2002
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
Beaune 2002
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
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)
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
Beaune 2002
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
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
Beaune 2002
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%
Beaune 2002
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.