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
Motivation Experimental set-up Summary Emission spectra of Ar/CF 4 and He/CF 4. Excitation and de-excitation processes in CF 4 and CF 4 mixtures Emission spectra of Ar/TEA Total light yields Conclusions
The GEM - gas electron multiplier See http://gdd.web.cern.ch/gdd/ Magnitude of the electric field along the center of the GEM channel.
Applications (with CCD readout of the GEM scintillation) Alpha particle tracking: Triple GEM with Ar+40%CF 4 241 Am α particles E = 5.48 Mev Range in Ar = 3.42 cm (F. Fraga et al., IEEE Trans. Nuc. Sci. 49 (2002) 281) Thermal neutron detection: Proton and triton tracks in 3 He- 400 mbar CF 4 Triple GEM AmBe source with Polyethylene shielding (F. Fraga et al., NIM A 478 (2002) 357)
Applications X-ray imaging Car key ~5 cm radiography X-ray energy ~8keV Xe-10%CO 2 at 1bar absorption length ~3 mm
Objectives: measure the emission spectra of Ar- and He-CF 4 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.
Experimental set-up GEM Detection system Glass window X rays from: Fe-55 source (5.9 kev); X-ray generator Quartz window Front electrode -Vdrift Back electrode I (a.u.) 12 10 8 6 4 2 0 2 4 6 8 10 E (kev)
Detection systems Total light yields: 4 Planar-diffused silicon photodiode (UDT PIN-25DP) N ph / e I = ph Ω I.Q. e.t. 4π σ < 15% Q.E. 0.9 0.8 0.7 0.6 0.5 0.4 0.3 Photodiode 0.2 56 TUVP 0.1 0 200 300 400 500 600 700 800 900 λ (nm) 8 Photomultiplier (56 TUVP) (operating in the pulse mode) σ < 30% N ph / e = fcq Ω M..T.N 4π e
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);
Spectroradiometric calibration with the tungsten strip lamp s s Ω E M Osram WI 17/G NDF L 1 IF Calibration factor: S 1 I o R V N ( ) Q( ) 0 λ λ = S( λ)
Nº of photons entering the monochromator per second: S( λ ) = ε(t, λ). N c (T, λ). A. Ω E. λ. f ( λ) Ω E A ε( T, λ) N c (T,λ). λ» 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
Charge Gains 1000 1000 Ar+3%TEA Ar+5%TEA Ar+5%CF4 Ar+10%CF4 100 100 Gain 10 1 Gain 10 1 Xe+4%TEA Xe+3%TEA He+40%Xe+3%TEA He+40%CF4 He+20%CF4 150 200 250 300 350 400 100 150 200 250 300 350 400 450 500 V GEM (V) V GEM (V) He Ar CF 4 Xe TEA I. P. (ev) 24,58 15,7 15,9 12,12 7,51 E exc (ev) 19,82 11,54 12,5 8,4 4,77
Emission spectrum of Ar+5%CF 4 mixture G = 40; λ = 4 nm (raw data) 1200 1000 800 CF 3 * (1E,2A 2 1A 1 ) Ar I 2p 3s lines I norm (a.u.) 600 400 OH - 200 0 200 300 400 500 600 700 800 900 λ (nm )
I norm (a.u.) 3 2 1 0 3 2 1 0 3 2 1 0 Ar + 5% CF 4 G=40 500 550 600 650 700 750 800 850 Ar + 10% CF 4 G=90 500 550 600 650 700 750 800 850 Ar + 67% CF 4 G=40 500 550 600 650 700 750 800 850 λ (nm) Visible and NIR emission spectra of Ar- CF 4 mixtures, normalized to the light intensity at 620 nm. Nph/e- 0,7 0,6 0,5 0,4 0,3 0,2 0,1 1 10 Gain 5% CF 4 10% CF 4 67% CF 4 Nº of photons emitted, between 400 and 1000 nm, per secondary electron, as a function of the effective gain, in Ar-CF 4 mixtures. (Measurements performed with the photodiode).
Emission spectra of He-CF 4 mixtures CF 4 +*, CF 3 +* I norm (nm) 500 400 300 200 He + 20% CF 4 He + 40% CF 4 Raw data I norm (a.u.) 800 600 400 CF 3 * 100 200 0 200 300 400 500 600 700 800 λ (nm) He+20% CF 4 : G = 335 He+40% CF 4 : G = 175 λ = 4 nm 0 200 300 400 500 600 700 800 900 λ (nm) Emission spectrum of He+40%CF 4, corrected for 2 nd order diffraction effects and the quantum efficiency of the detection system.
He-CF 4 mixtures 1200 600 I n (a.u.) 1000 800 600 400 200 0 0 20 40 60 80 100 % CF 4 He/CF Ar/CF 4 Ar/CF He/CF 44 Light intensity, normalized to the current, measured for λ = 620 nm, as a function of CF 4 concentration. Light intensity/electron current (a.u.) 500 400 300 200 100 20%CF4 40%CF4 85%CF4 0 400 500 600 700 800 wavelength (nm) Visible emission spectra of He-CF 4 mixtures, measured with a glass color glass filter (λ cutt-off =435 nm).
CF 4 Process Threshold (ev) Direct vibrational excitation ν 4 0.078 ν 3 0.159 Energy loss (ev) 0.078 0.159 Indirect vibrational excitation 4.0 0.4 Electron attachment 4.3 4.3 Electronic excitation (dissociation into neutral fragments) 12.5 (10) 12.5 (10) Dissociative ionization 15.9 15.9 All electronic excited states of CF 4 and CF 4 + (τ < 10 µs) seem to dissociate or predissociate.
Dissociation into neutral fragments in CF 4 e - + CF 4 CF 3 + F + e - Energy threshold: 10-12.5 ev (?) Dissociation energy: D(CF 3 F) = 5.25 ev Electronic excited states of CF 3* : 1E (7.95 ev), 2A 2 (7.68 ev), 2A 1 (8.40 ev). Observed electronic transitions: CF * 3 (1E,2A 2 1A 1 (repulsive state)) visible 620 nm CF * 3 (2A 1 1A 2 (ground state)) UV 265 nm
Total cross section for dissociation of CF 4 into neutrals Christophorou and Olthoff, J. Phys. Chem. Ref. Data, vol. 28, nº 4, 1999, 967.
100 40 80 30 Pm (%) 60 40 20 Vib-CF4 neutral diss. ion exc1 - He exc2 - He Pm (%) 20 10 0 25 50 75 100 125 150 E (kv/cm) Mean power lost in collisions in a He+40%CF 4 mixture. Penning ionization is not considered. Calculations based on the numerical solution of Boltzmann equation for a uniform electric field configuration. The code used was developed by the group of P. Ségur, CPAT, Toulouse, France 0 25 50 75 100 125 150 E (kv/cm) Mean power lost in collisions in a Ar+40%CF 4 mixture.
1000 1000 100 α exc (cm -1 ) 100 10 Ar+10%CF4 Ar+40%CF4 He+20%CF4 He+40%CF4 100% CF4 α exc (cm -1 ) 10 1 0,1 Ar+10%CF4 Ar+40%CF4 0 25 50 75 100 E (kv/cm) 0 25 50 75 100 E (kv/cm) Number of collisions per cm leading to dissociation of CF 4 into neutral fragments, as a function of the electric field. Number of collisions per cm leading to excitation of Ar* (2p+high lying forbidden) levels, as a function of the electric field.
Dissociative ionization of CF 4 PES Dipole Ionization Zhang et al, Chem. Phys. 137 1989, 391 Fragmentation (%) (e,e+ion) Ion Products 3µ s < τc ~ < 10µ s A) CF +* 4 dissociates before emitting: D(CF 3 -F) = 5.25 ev IP (CF 3 ) = 9.25 ev CF +* 3 CF 3+ + hν (UV) B) CF 4 +* emits before dissociating CF + * 4 (C ~ ) CF CF CF + 4 + * 4 + * 4 (X ~ ) + hν(~ 160 nm) (A ~ ) + hν(~ 240 nm) (B ~ ) + hν(~ 290 nm)
CF 4 + Excitation and de-excitation mechanisms CF 4 * CF 4 dissociation Ar + X Ar** products IR X Ar* (2p) products CF 4 (ν ) CF 3 + F NIR Ar* (3s) X products Ar X = Ar, CF 4, H 2 O,... CF 4 Ar
Excitation and de-excitation mechanisms CF He + 4 products (CF 4+ ) * He** CF 4 He* products dissociation CF 4 * CF 4 + CF 4 X X = He, H 2 O,... CF 3 + F CF 4 (ν ) CF 4 He
Ar-TEA (mm) 100 P v = 30 torr 10 T = 293 k 1 0,1 120 140 160 180 200 220 240 260 280 light intensity/current (a.u.) 120 100 80 60 40 20 0 Ar + 3% TEA λ (nm) 240 260 280 300 320 340 360 380 λ (nm) Absorption length versus wavelength Emission spectrum ( λ = 4 nm)
Excitation and de-excitation mechanisms Ar** X products Ar + X = TEA, Ar, H 2 O,... Ar* (2p) X products TEA * Ar* (3s) X products Ar TEA + Ar Ar 2 * UV VUV TEA Ar
Total light yields (PMT) 1,0 0,8 0,6 Nph/e- 0,4 0,2 0,0 0 100 200 300 400 500 600 700 800 Gain ΤΕΑ mixtures: σ syst < 25% ; He/CF 4-0.1-0.3 ph/e -
0,16 0,12 He+CF 4 0,08 0,04 0,00 10 100 1000 Q.E. 1 0,1 0,01 0,001 Gain (a) 200 300 400 500 600 700 800 900 λ (nm) Quantum efficiency of the photodiode and PMT. Nph/e- Nph/e- 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 Ar+10%CF 4 (b) 1 10 Gain photodiode PMT + filter Photodiode 56 TUVP Nº of photons (λ > 400 nm) emitted per secondary electron as a function of the effective gain in (a) He+20%CF 4 and He+40% CF 4 ; (b) Ar+10%CF 4
Pulse height spectra from 5.9 kev X-rays 50 G A =50 R = 41% 40 G A =80 R = 58% 10 8 G A =320 R = 46% Nº counts/s 40 30 20 30 20 6 4 10 10 2 0 0 0 40 80 120 160 channel # Ar+3%TEA V GEM = 290 V G eff = 640 (R = 53% for G= 460) 0 20 40 60 80 100 120 Channel # Xe+3%TEA V GEM = 320 V G eff = 370 0 0 40 80 120 160 Channel # He+40%CF 4 V GEM = 460 V G eff = 320
Conclusions Ar-CF 4 mixtures emit mainly in the visible (CF 3* ) and in the NIR (Ar I 2p-3s atomic lines, which can be detected even for high CF 4 concentrations). He-CF 4 emit both in the UV and visible (CF 3* ) Visible emissions are more important in Ar/CF 4 than in He/CF 4. 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.