D-2 M. Fraga

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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. Fsica, 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

  • 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 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)

  • Beaune 2002

    Detection systems Total light yields:4 Planar-diffused silicon

    photodiode (UDT PIN-25DP)

    < 15%

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

    < 30%

    00.10.20.30.40.50.60.70.80.9

    200 300 400 500 600 700 800 900

    (nm)

    Q.E

    .

    Photodiode56 TUVP

    =

    4.T..Q.I

    Iph

    phe/Ne

    e

    cph

    N.T.4

    .M

    qfe/N

    =

  • 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 -20C, operating in single photon counting mode.

    8 Spectral sensitivity of the detection system is measured with:

    ! Standard tungsten strip lamp (Osram WI 17/G) ( >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

    L1IFOsram

    WI 17/G

    NDF

    E

    VR)(S)(N)(Q 0

    =

    Calibration factor:

    ss

    M

    S1

    Io

  • Beaune 2002

    N of photons entering the monochromator per second:

    is the effective solid angle;

    effective emitting area of the tungsten ribbon;

    emissivity of tungsten;

    n of photons emitted per unit time, per unit area and per steroradian, by a blackbody at a temperature T, in the wavelength range between and +

    )(f...A).,T(N).,T()(S Ec =

    E

    A

    ),T(

    ).,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

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    Emission spectrum of Ar+5%CF4 mixtureG = 40; = 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

    .)

    (n m )

    Ar I2p3s lines

    OH-

    CF3* (1E,2A21A1)

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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

    .)

    (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

    N 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

    .) (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

    )

    (nm)

    Raw dataCF3*

    CF4+*, CF3+*

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

    = 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 = 620 nm,as a function of CF4 concentration.

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

    He/CF4Ar/CF4

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    CF4Process Threshold

    (eV)Energy loss

    (eV)Direct vibrational excitation 4 3

    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+ ( < 10 s) 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,2A21A1(repulsive state)) visible 620 nm

    CF3* (2A11A2 (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, n 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. Sgur, 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

    ex

    c (cm

    -1)

    E (kV/cm)0 25 50 75 100

    0,1

    1

    10

    100

    1000

    Ar+10%CF4 Ar+40%CF4

    ex

    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~

  • 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 ()

    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 ()

    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

    .) (nm)

    0,1

    1

    10

    100

    120 140 160 180 200 220 240 260 280

    (nm)

    (mm

    )

    Pv = 30 torrT = 293 k

    Absorption length versus wavelength Emission spectrum ( = 4 nm)