A. M. F. Trindade, A. N. Garcia, P. J. B. M. Rachinhas Electroluminescence in noble gases : simulation and experimental results Filomena P. Santos, C. A. N. Conde, A. D. Stauffer, T. H. V. T. Dias, F. I. G. M. Borges, J. M. Escada, S. J. C. do Carmo, A. M. F. Trindade, A. N. Garcia, P. J. B. M. Rachinhas LIP Coimbra - Portugal ANT2013 - Sunnyside-Tahoe City 10-12 May 2013

may be identical to its own anti-particle (LQN violation) decays Rate 2-0v No backgrounds above Q-value Energy Q value Neutrinos are puzzling elementary particle with unique properties: very low (but non-zero) mass no charge Two neutrinos are emitted in standard double beta decay (2-2) already observed for 11 isotopes (from the 35 allowed) with T1/2 from 1018 to 1021 yr). If neutrino is its own anti-particle, a neutrinoless version (2-0) may occur, a most sensitive method to assess neutrino nature and mass. Experimental signature of 2-0 is a line at Q may be identical to its own anti-particle (LQN violation)

NEXT – Neutrino Experiment with a Xenon TPC Very promissing experiment under development: Expected energy resolution R ≤ 1% FWHM @ Q (~2.460 MeV) Background reduction from event topology (2 blobs at the ends) Electroluminescence layer Readout plane - energy (PMT) Readout plane - Position (SiPM) TPC HP Xe – gas detection medium proportional scintillation - amplification mechanism.

Why HP Xe? Why proportional scintillation? Several experiments (different detection media and techniques) currently search for 2-0. A sole claim (from Heidelberg-Moscow group) is still unconfirmed Xe gas detection medium and source (136Xe isotope) 136Xe enrichment easy and inexpensive 136Xe is the only long-lived Xe isotope very good energy resolution efficient background rejection from event topology HP Xe scalable, pressure/size better energy resolution than LXe easier handling/purification procedures Proportional Scintillation much lower fluctuations than charge multiplication.

Noble gas scintillation Excimer formation and decay: ( , ) ( , ); (VUV scintillation)

Noble gas scintillation - from recombination Radiative recombination: electron impact ionization dimer ion formation recombination excimer decay / scintillation

Xe emission continua Two VUV continua: 1st continuum (peaked at ~150 nm) from vibrationally excited molecular states, disappears at few hundred Torr 2nd continuum (peaked at ~170 nm) from vibrationally relaxed molecular states

Gas Proportional Scintillation Counter - GPSC Amplification stage: scintillation produced in the deexcitation of electron impact excited atoms of the medium

Xenon scintillation in GPSC/TPC Two types of scintillation Primary scintillation, electric field independent Secondary/proportional scintillation - Electroluminescence, @ reduced electric field E/P from 1 to ~6 V cm-1 Torr-1 Proportional* scintillation, also called electroluminescence (EL) is produced while electrons drift for a distance D under an uniform electric field which allows excitation but not ionization of the gas atoms. *proportional to number of e-, drift distance D (also ~ to electric field).

Primary scintillation Source - when/why? Xe excitation by electron impact in the detector absorption region; recombination. Amplitude Weaker than secondary scintillation (EL) because ionization wins over excitation above ionization threshold; solid angle is smaller for primary scintillation detection than for EL

Experimental w-value for primary scintillation Average energy to produce a primary scintillation photon ws=111  16 eV 5.9 keV x-rays in Xe measured from primary pulse GPSC, ~1 atm. (we-~22 eV ) secondary primary Primary pulse: - measured by triggering the osciloscope with EL pulse at low threshold; - averaged over 128 pulses. The technique averages out noise level to ~zero. Experimental pulse shapes (note different scales).

Electroluminescence (EL) EL is produced under appropriate uniform electric field Field is such that electrons excite, but do not ionize, the atoms of detector gas filling. EL efficiency very high in noble gases. High purity noble gas required.

Electroluminescence in pure xenon Reduced EL yield (photons electron-1cm-1Torr-1) Monte Carlo simulation and experimental results Excitation & EL efficiencies Monte Carlo simulation Q exc QEL F.P.Santos et al., J. Phys. D 27(1994)42. Y/p (cm-1Torr-1)= - 0.1325 + 0.1389 E/p p(Torr), E/p (Vcm-1Torr-1) Y/N(10-17 cm2) = - 0.4020 + 0.1389 E/N N(cm-3), E/N (Td) C.M.B.Monteiro et al., JINST 2(2007)P05001. Y/N (10-17 cm2) = - 0.474 + 0.140 E/N A. Bolozdynya et al, NIM A 385 (1997) 225 Y/p (cm-1 bar-1)= 70*(E/p-1) p(bar), E/p(kV cm-1, bar-1),

Electroluminescence simulation – Monte Carlo flowchart Electron from sample energy Initial direction Initial position Electron path final direction final position final energy time  Elapsed time  Collision type Real collision Scattered electron direction Count number of excitations Secondary electron position time Direction n. of electrons gas density electric field ionization elastic excitation Electron from sample Next electron Drift and scintillation parameters End of simulation No more electrons in sample No more electrons null

Why EL efficiency is high in Xe elastic total ion exc Absence of inelastic energy losses for electrons below electronic excitation threshold; ionization and excitation thresholds are well separated.

Electrons in Xe Energy of one electron drifting across EL region. Arrows indicate Xe electronic excitation collisions. 8.32 eV E/p = 5 Vcm-1Torr-1 (E/N = 15 Td).

EL amplification high gain: a single primary electron produces ~ 500 EL VUV photons in Xe along D=1 cm EL region at ~1 atm. and E/p ~ 5 V cm-1Torr-1, low fluctuation: GPSC energy resolution approaches intrinsic limit H n = number of primary e- per absorbed event H - number of EL photons per electron F = (Fano factor) relative variance in n J= / H - relative variance in H

Pure xenon / xenon mixtures Best energy resolution, low drift velocities high diffusion coefficients These may be severe drawbacks in high dimension detectors when tracking capabilities required. Molecular additives may be a solution to increase drift velocities to decrease diffusion coefficients BUT EL yield is reduced and fluctuations increased The best balance will determine the choice of additive Candidates are CH4, CF4, TMA…

Electron scattering cross sections in Xe and CH4 sion sexc

Electron energy in Xe and Xe-0.5%CH4 Energy of one electron drifting across EL region. Arrows indicate Xe electronic excitation collisions. 8.32 eV E/p = 5 Vcm-1Torr-1 (E/N = 15 Td).

Electron scattering cross sections in Xe and CF4

Electron energy in Xe and Xe-0.5%CF4 Energy of one electron drifting across EL region. Arrows indicate Xe electronic excitation collisions. 8.32 eV E/p = 5 Vcm-1Torr-1 (E/N = 15 Td).

Mean electron energy and excitation efficiency Monte Carlo Gas medium em Qexc Xe 3.715 92.0% Xe-0.1%CH4 3.7 87.5% Xe-0.5%CH4 3.65 74.0% Xe-1%CH4 3.575 58.1% Xe-10%CH4 2.498 0.1% Xe-0.1%CF4 3.723 80.3% Xe-0.5%CF4 3.717 45.6% Xe-1%CF4 3.648 20.8% Xe-10%CF4 2.178 0.0% m mean electron energy Qexc excitation efficiency p = 760 Torr E/p = 5 Vcm-1Torr-1 (E/N=15 Td).

Electron drift velocities in Xe, Xe-CH4 and Xe-CF4 Monte Carlo Addition of CH4 or CF4 to Xe increases electron drift velocity 11/21

Electron diffusion in Xe, Xe-CH4 and Xe-CF4 Monte Carlo Addition of CH4 or CF4 to Xe decreases electron diffusion with

Monte Carlo simulation results: EL Yield D=0.5 cm, p = 1o atm T=293 K Rint2  (1/n ) (F + Q) { F = sn 2/n Q = J / H J = sH 2/H EL yield H (UV photons /electron) produced under uniform reduced electric fields E/N, when one electron drifts across the EL region in Xe and in Xe-CH4 and Xe-CF4 mixtures with the indicated CH4 and CF4 concentrations.

Monte Carlo simulation results: EL fluctuations Fluctuations parameter Q=J /H of the EL yield H , where J=sH 2/H is the relative variance of H. The bar FXe marks the Xe Fano factor. Fraction z of electrons that become attached to CH4 or CF4 molecules in the EL region. Rint2  (1/n ) (F + Q)

Experimental system GPIC GPSC Noble gas purifier Molecular gas purifier HP gas container Pressure gauge

Experimental spectra for Xe and Xe-1.5% CH4 GPIC p= 800 Torr 5.9 keV x rays R G G Xe – 1.5% CH4 and 100% Xe

Experimental spectra for Xe and Xe-1.5% CH4 GPSC Y p= 800 Torr 5.9 keV x rays R Y Centroid - 27 FWHM - 12,2 R - 44,7% Amplification - 22 Acq. Time - 500s

TMA - N(CH3)3 TMA - next molecular additive to be tested mildly toxic, foul smell… Expectations: improves electron drift parameters Penning ionization (decreases Fano factor, not crucial) Xe VUV emission quenched TMA fluoresces in 275-330 nm - wavelength shifter may produce EL in alternative to xenon symmetric molecule (non-electronegative)

Xe exc 8.32 eV TMA IP 7.85 eV TMA IP

Part of the work presented here has been funded by FEDER, through the Programa Operacional Factores de Competitividade- COMPETE and by National funds through FCT- Fundação para a Ciência e Tecnologia in the frame of project .....

Part of the work presented here has been funded by FEDER, through the Programa Operacional Factores de Competitividade- COMPETE and by National funds through FCT- Fundação para a Ciência e Tecnologia in the frame of project .....

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Electroluminescence fluctuations in doped xenon The addition of CH4 / CF4 to Xe Monte Carlo decreases EL (n. sc.photons, produced per electron in sc. gap) increases EL fluctuations (CF4 has catastrophic effect …) 5 cm drift, 1 atm ↔ 5 mm, 10 atm 5 cm drift, 1 atm ↔ 5 mm, 10 atm

Electroluminescence fluctuations in doped xenon Monte Carlo 15/21

Electron energy in Xe and Xe-10%CF4

Drift velocities for electrons in Xe and Xe-CH4 Monte Carlo Addition of CH4 or CF4 to Xe increases drift velocity

Noble gas scintillation - from recombination Recombination with participation of a neutral (high pressure effect) At high pressure a 3rd partner is likely to take away the energy released and no scintillation will occur

Electron diffusion in Xe, Xe-CH4 and Xe-CF4 Monte Carlo Addition of CH4 or CF4 to Xe increases drift velocity decreases longitudinal and transverse electron diffusion where 12/21

Electron diffusion in Xe, Xe-CH4 and Xe-CF4 Monte Carlo Addition of CH4 or CF4 to Xe increases drift velocity decreases longitudinal and transverse electron diffusion where 13/21

Xe doped with CH4 e CF4 - w and ekT, ekL

Electroluminescence: cylindrical versus parallel geometry H number of scint. photons per electron fluctuations J= /H cylindrical geometry  parallel geometry ∥ Calculations were made for a gap distance yielding the same H as cylindrical geometry

Dopagem de Xe com CH4 e CF4 - Efeito em H e em Q z E O D = 0.5 cm p = 10 atm - Rendimento de eletroluminescência (EL) - Variância relativa no nº de fotões H - Variância relativa no nº n de eletrões primários [23] T.H.V.T. Dias et al. 1993 [24] C.M.B. Monteiro et al. 2007

Dopagem de Xe com CH4 e CF4 - Discussão Xe puro 1 exc → 1 fotão VUV Xe *(1s5; 1s4) + 2Xe → Xe2* + Xe; Xe2* → 2Xe + hn (~172 nm) Y – CH4 ou CF4 Xe*(1s5; 1s4) + Y → produtos Xe*(1s5; 1s4) + Xe + Y → produtos fração x de eletrões que são capturados por moléculas i) ii) < pXe iii) exc. vib. → < em → < exc. Xe iv) capt. e- < exc. Xe Contribuições para a perda de H, para E/N = 15 Td Meio gasoso (i) (ii) (iii) (iv) Total   Xe-0.1%CH4 22.0% 0.1% 3.6% 1.2% 26.9% Xe-0.1%CF4 0.0% 7.0% 75.6% 82.7%