1. 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
ANT Sunnyside-Tahoe City May 2013

2. 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)

3. NEXT – Neutrino Experiment with a Xenon TPC
Very promissing experiment under development:
Expected energy resolution R ≤ 1% 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.

4. 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.

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

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

7. 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

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

9. 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).

10. 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

11. 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).

12. 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.

13. 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)= E/p p(Torr), E/p (Vcm-1Torr-1)
Y/N(10-17 cm2) = E/N N(cm-3), E/N (Td)
C.M.B.Monteiro et al., JINST 2(2007)P05001.
Y/N (10-17 cm2) = 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),

14. 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

15. 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.

16. 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).

17. 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

18. 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…

19. Electron scattering cross sections in Xe and CH4
sion
sexc

20. 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).

21. Electron scattering cross sections in Xe and CF4

22. 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).

23. 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).

24. 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

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

26. 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.

27. 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)

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

29. 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

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

31. 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 nm - wavelength shifter
may produce EL in alternative to xenon
symmetric molecule (non-electronegative)

32. Xe exc 8.32 eV
TMA IP 7.85 eV
TMA IP

33. 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 .....

34. 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 .....

37. 14/21

38. 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

39. Electroluminescence fluctuations in doped xenon
Monte Carlo
15/21

40. Electron energy in Xe and Xe-10%CF4

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

43. 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

44. 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

45. 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

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

47. 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

48. Dopagem de Xe com CH4 e CF4 - Efeito em H e em Qz 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

49. 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%