The GEM scintillation in He-CF 4, Ar-CF 4, Ar-TEA and Xe-TEA mixtures

Transcription

1 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, Coimbra, Portugal

2 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

3 The GEM - gas electron multiplier See Magnitude of the electric field along the center of the GEM channel.

4 Applications (with CCD readout of the GEM scintillation) Alpha particle tracking: Triple GEM with Ar+40%CF 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)

5 Applications X-ray imaging Car key ~5 cm radiography X-ray energy ~8keV Xe-10%CO 2 at 1bar absorption length ~3 mm

6 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.

7 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.) E (kev)

8 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 Photodiode TUVP λ (nm) 8 Photomultiplier (56 TUVP) (operating in the pulse mode) σ < 30% N ph / e = fcq Ω M..T.N 4π e

9 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 ( nm);

10 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)

11 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

12 Charge Gains Ar+3%TEA Ar+5%TEA Ar+5%CF4 Ar+10%CF Gain 10 1 Gain 10 1 Xe+4%TEA Xe+3%TEA He+40%Xe+3%TEA He+40%CF4 He+20%CF 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

13 Emission spectrum of Ar+5%CF 4 mixture G = 40; λ = 4 nm (raw data) CF 3 * (1E,2A 2 1A 1 ) Ar I 2p 3s lines I norm (a.u.) OH λ (nm )

14 I norm (a.u.) Ar + 5% CF 4 G= Ar + 10% CF 4 G= Ar + 67% CF 4 G= 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, 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).

15 Emission spectra of He-CF 4 mixtures CF 4 +*, CF 3 +* I norm (nm) He + 20% CF 4 He + 40% CF 4 Raw data I norm (a.u.) CF 3 * λ (nm) He+20% CF 4 : G = 335 He+40% CF 4 : G = 175 λ = 4 nm λ (nm) Emission spectrum of He+40%CF 4, corrected for 2 nd order diffraction effects and the quantum efficiency of the detection system.

16 He-CF 4 mixtures I n (a.u.) % 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.) %CF4 40%CF4 85%CF wavelength (nm) Visible emission spectra of He-CF 4 mixtures, measured with a glass color glass filter (λ cutt-off =435 nm).

17 CF 4 Process Threshold (ev) Direct vibrational excitation ν ν Energy loss (ev) Indirect vibrational excitation Electron attachment Electronic excitation (dissociation into neutral fragments) 12.5 (10) 12.5 (10) Dissociative ionization All electronic excited states of CF 4 and CF 4 + (τ < 10 µs) seem to dissociate or predissociate.

18 Dissociation into neutral fragments in CF 4 e - + CF 4 CF 3 + F + e - Energy threshold: 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

19 Total cross section for dissociation of CF 4 into neutrals Christophorou and Olthoff, J. Phys. Chem. Ref. Data, vol. 28, nº 4, 1999, 967.

20 Pm (%) Vib-CF4 neutral diss. ion exc1 - He exc2 - He Pm (%) 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 E (kv/cm) Mean power lost in collisions in a Ar+40%CF 4 mixture.

21 α exc (cm -1 ) Ar+10%CF4 Ar+40%CF4 He+20%CF4 He+40%CF4 100% CF4 α exc (cm -1 ) ,1 Ar+10%CF4 Ar+40%CF E (kv/cm) 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.

22 Dissociative ionization of CF 4 PES Dipole Ionization Zhang et al, Chem. Phys , 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 (X ~ ) + hν(~ 160 nm) (A ~ ) + hν(~ 240 nm) (B ~ ) + hν(~ 290 nm)

23 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

24 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

25 Ar-TEA (mm) 100 P v = 30 torr 10 T = 293 k 1 0, light intensity/current (a.u.) Ar + 3% TEA λ (nm) λ (nm) Absorption length versus wavelength Emission spectrum ( λ = 4 nm)

26 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

27 Total light yields (PMT) 1,0 0,8 0,6 Nph/e- 0,4 0,2 0, Gain ΤΕΑ mixtures: σ syst < 25% ; He/CF ph/e -

28 0,16 0,12 He+CF 4 0,08 0,04 0, Q.E. 1 0,1 0,01 0,001 Gain (a) 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

29 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 channel # Ar+3%TEA V GEM = 290 V G eff = 640 (R = 53% for G= 460) Channel # Xe+3%TEA V GEM = 320 V G eff = Channel # He+40%CF 4 V GEM = 460 V G eff = 320

30 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.