1. réponse d’un détecteur Micromegas
à un électron unique  
développement d’une source ponctuelle d’électrons
présenté par VL à la conférence IEEE-NSS, Hawaii, 29 oct. 2007
B. Genolini , J. Peyré , J. Pouthas , T. Zerguerras
service détecteurs, IPN Orsay, France
V. Lepeltier, LAL Orsay
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

2. single electron response and energy resolution
of a Micromegas detector  
outline
● introduction
● experimental device: a point-like electron source
● results on:
- spatial resolution
- energy resolution single electron response
● conclusion and prospects
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

3. single electron response and energy resolution
of a Micromegas detector  
introduction
why a point-like electron source?
 in order to characterize and optimize gaseous detectors one needs generally
electrons.
 usual sources are:
- radioactive sources: they produce a well known number of electrons, but
without precise spatial and time origin
- particle beams: not easy to operate and adjust, and sometimes with large
fluctuations (Landau for example)
 so a pulsed laser photoelectron source seems to be well suited, since it is
potentially:
- small in size
- adjustable in location,
- “ “ time,
- “ “ intensity 3
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

4. single electron response and energy resolution
of a Micromegas detector  
experimental device
developed at IPN, Orsay, France
intensity tuning
beam tuning
beam focusing
3D positioning
attenuation
laminae
cylindrical
lenses
spherical
lenses
focusing
triplet X
UV N2 Laser
337nm (~3eV) Z X Y
Micromegas
electron source
● point-like and adjustable in 3D
(accuracy ~1m)
● very small transverse size <100 µm
due to the strong focusing and multiphotonic
ionization:
~no electrons produced on the micromesh
● adjustable electron population from a few
to more than 104 (limitation by discharges)
● very accurate (ns) time synchronization.
amplification and detection device
quartz lamina with a
Ni-Cr 5Å thick deposit
(for photo-electron production)
d=3 mm
E=3kV/cm
125m
Ne-iC4H
laser
338m anode strips
Fe X-ray
source
golden Mylar foil
(for 55Fe calibration)
drift
500 lpi Ni mesh 4
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

5. single electron response and energy resolution
of a Micromegas detector  
registered signals
3D Detector position
mesh
Laser
strip 1
ion collection on the mesh ~100ns
T. Zerguerras et al., Xth Vienna Conf. on Instrumentation,
Feb 2007, Nucl Instr. and Meth. A581,258(2007)
strip 2
strip 3
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

6. single electron response and energy resolution
of a Micromegas detector  
spatial resolution
● after diffusion photoelectrons are spread over ~3 adjacent pads
(pad width/transverse diffusion ~1.5 for the experimental conditions).
● the X transverse position of the source is extracted from the three amplitudes measured on
the 3 pads after multiplication of electrons in the Micromegas device (Gaussian fit)
● in principle, for a large enough number of electrons, the spatial resolution is given by:
σX2 = σ02 + σdiff2 /Neeff where:
σdiff is the total spread of the electron cloud
(electron spot size  transverse diffusion experimented by electrons),
Neeff the “effective number” of electrons contributing to the signal,
σ02 a constant term containing some systematic (electronics, noise etc.)
Neeff is expected to be smaller than the total number of electrons Ne, due to fluctuations:
- statistical from the production (for example Landau for a MIP)
- gain fluctuation from the amplification device
● as an example for a MIP producing ~60 primary electrons multiplied in a Micromegas device, and collected by a pad device, the effective number of electrons is equal to ~25 only (measurements).
● this number may fluctuate depending on the set-up (geometry, electric field, gas mixture, gain, etc.) so it is interesting and important to study the response for a given number of electrons and of course especially for a single electron. 6
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov 2007

7. single electron response and energy resolution
of a Micromegas detector  
spatial resolution
- the origin σ0=-1.27+/-0.25 µm is
consistent with a zero resolution for an
infinite number of electrons.
- the slope value 222+/-8µm is consistent
with the size of the electron cloud at the anode, calculated from the convolution between:
 the source size (<100µm rms)
 the transverse diffusion 201+/-2µm,
(calculated from Magboltz).
 conclusion: no visible degradation of the
resolution from fluctuations.
spatial resolution (µm)
Micromegas
gap thickness: 125 µm
drift gap: 3 mm
strip pitch: 333 µm
gas: Ne/Isobutane 90/10
1/√Ne-
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

8. single electron response and energy resolution
of a Micromegas detector  
energy resolution
example of ampitude spectrum
registered with 1000 laser shots with ~2300 electrons
width dominated by the
laser fluctuation
energy resolution
the experimental energy resolution σE/E has been fitted by a quadratic function:
(σE/E)2 = a2 + b2/Ne
the fitted values of a and b are:
- a=7.9 +/- 0.1 %
this value could be explained by the fluctuations
of the laser intensity and the photoelectron
production process,
- b=1.17 +/ is close to 1,
indicating again that the gain fluctuations are small.
Micromegas
thickness: 125 µm
drift gap: 3 mm
strip pitch: 333 µm
gas: Ne/Isobutane 90/10
1/√Ne- 8
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

9. single electron response and energy resolution
of a Micromegas detector  
single electron response: method
σ ~ 220 m
strip width = 333 m σ X0 X
method:
from a cloud of photoelectrons
- well localized in position (Xo)
- with a well known population (Ne~200),
- and with a well known gaussian spread (σ)
we choose a strip located at such a distance from Xo (a few σ) that the mean
number of photoelectrons collected on it is less than 1 (mean value < 0.1)
and we register the amplitude spectrum.
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

10. single electron response and energy resolution
of a Micromegas detector  
single electron response: an example
amplification gap: 50 µm
drift gap: 3 mm
strip pitch: 338 µm
gas: Ne-Isobutane 90-10
gain:40000 (noise ~ e)
S.E.R
Noise
(0 electron)
preliminary
previous measurements at Saclay, J.Derré et al.
Nucl Instr. and Meth. A449,314(2004)
limitations of the method:
1. laser rate: a few Hz
2. if you want to be sure that you have only 1 electron and no more, in most cases you will have 0 electron
 it takes a long time for spectrum registration
for typically 104 electrons: ~one hour or more
3. “edge” effect if an electron is amplified near the frontier with the neighbor strip 10
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

11. single electron response and energy resolution
of a Micromegas detector  
prospects
improvements on the experimental device (done):
1. laser: less fluctuation 8%  4%
2. photomultiplier  time information available (ns)
3. more reliable intensity tuning (precise optical attenuators)
4. new multianode readout pad plane, specially designed:
single electron signals from a very attenuated laser beam,
will be collected on the center of a “large” anode pad
(~1mm wide, more than 5 times the electron cloud size),
without any edge effect, as previously.
5 cm
statistical distribution of single electrons (2D)
“large” pad
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007

12. single electron response and energy resolution
of a Micromegas detector  
a few words to conclude…
 we have developed a point-like photoelectron source,
accurately adjustable in position and intensity.
 this source has been successfully experimented for
energy and position resolution studies of a Micromegas.
 with this source it is possible to study the single
electron response of a(ny?) MPGD device.
 in the next future, after recent improvements,
this electron source will be used for more systematic
detector characterization tests, especially on SER,
with different gas mixtures and pressures, gain ,etc.
Vincent Lepeltier, LAL, Orsay, réunion SOCLE, Clermont-Ferrand, nov. 2007