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, 19-20 nov. 2007

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, 19-20 nov. 2007

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, 19-20 nov. 2007

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-iC4H10 90-10 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, 19-20 nov. 2007

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, 19-20 nov. 2007

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, 19-20 nov 2007

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, 19-20 nov. 2007

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 +/- 0.04 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, 19-20 nov. 2007

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, 19-20 nov. 2007

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 ~1000-2000 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, 19-20 nov. 2007

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, 19-20 nov. 2007

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, 19-20 nov. 2007