1 Micromegas for sampling calorimetry Chronology & people  Initiated by LAPP LC-group in 2006 (C. Adloff, M. Chefdeville, Y. Karyotakis, I. Koletsou) Imaging calorimetry for Particle Flow application at a future LC (within CALICE) → R&D focus: granularity, large area, compactness, MIP efficiency & low noise (self-trigger)  Collaboration with CMS labs in 2013 * NCSR Demokritoss (G. Fanourakis, T. Geralis, A. Kalamaris, C. Kitsaki, D. Nikas, Ch. Vassou) * University of Athens (N. Saoulidou) * IRFU Saclay (M. Titov) Proposal for a HGCAL tail catcher for CMS upgrades, R&D focus: High rate capability, sparking, time resolution, rad. hardness Why using Micromegas for calorimetry?  Gas is cheap (ILC/SiD HCAL area ~ 3000 m^2)  It is nicely linear (proportional mode, not the case with RPCs)  Does not care about rates: no dead time (except when sparking)  Fast: signal (from avalanche electrons) rise time of a few ns  Compact design (3 mm of Ar gas sufficient for full efficiency)

2 Large area prototypes for an ILC Calorimeters will be highly segmented: (30.10^6 channels for SiD/HCAL, 1x1 cm^2 pads)  Limit power consumption with simple 2-bit readout 3 thresholds to restore linearity at high energy  Integrated ASIC on PCB, Micromegas & ASICs on same board (Fig. 1) Detectors will be power-pulsed and self-triggered (low noise + memory + time- stamp)  Several chips developed by IN2P3/Omega group for CALICE We use the MICROROC (Fig. 1) 1x1 m^2 = 6 boards in 1 vessel (Fig.2, dead zones < 2%) Total thickness once chamber closed ~ 1 cm (Fig. 3) Published in NIMA, 729 (2013) 90 Fig. 1 Fig. 2 Fig. 3

3 Measured performance (1/2) Standalone test of 4 prototypes (SPS H4, Nov. 2012)  High efficiency (>95%), low hit multiplicity (<1.1), good signal uniformity (~5%)  Shower response constant from 1-30 kHz pion beam, spark proba. ~ 10^-5 / shower Published in NIMA, 763 (2014) 221 Iron block → shower set-up with 4 layers1 shower event in first layer50 k shower events

4 Measured performance (2/2) Test inside the RPC-SDHCAL (46 RPC+ 4 Micromegas, SPS H2, Nov. 2012)  Common readout system → tag shower start layer z0 → N_hit in Micromegas for different z0  Build longitudinal pion shower profile Get response by integration of the profile (test Geant4 models...) SPSProfiles in MicromegasExtrapolated response

5 R&D on resistive Micromegas Spark threads are two-fold  Can damage ASICs if not diode-protected & Induce non-negligible dead time (recharge of mesh) Resistive layers are known to quench sparks at early stage  “Horizontal” evacuation of charge (à la LC-RPC) Tried in 2013 on small prototypes (PoS (TIPP2014) 054) Segmented R-layer to limit physical crosstalk (Fig. 1)  Sparks are suppressed (Fig. 2) but might be too slow for large area: Evacuation time seen as efficiency loss versus beam rate (Fig. 3) Fig. 1 Fig. 3 Fig. 2

6 Optimisation for high currents When current is flowing, resistive elements introduce a voltage drop across the gap At “high” current, the voltage drop is sufficient to lower the gas gain by several %. We loose the nice proportionality of Micromegas ! This can happen in the following cases:  Tracking at very high rates (low dE/dx);  Calorimetry at ILC (large dE/dx but low rates);  Or calorimetry at high rates (CMS-like), worse case! Optimisation means reducing resistivity and evacuation time but still suppress sparking  “Vertical” evacuation of charge using buried resistors (Fig. 1), proposed by Rui de Oliveira  On-going program: Vary the RC (Fig. 2), measure the linearity (rate & dE/dx scans), check sparking R = 400 kOhmR = 4 MOhmR = 40 MOhm Fig. 1 Fig. 2

7 Latest test results Loss of linearity with rate (Fig. 1) easily seen with Cu X-gun  Depending on resistivity, deviations from linearity = 10 MHz / cm^2 for X-rays Measurement of dE/dx-related loss more tricky:  Vary primary charge with GEM injector (Fig. 2)  Vary beam energy or material upstream of detectors (tesbeam, Fig. 3) Fig. 1 Xgun rate scan Fig. 2: GEM injector: dE/dx scan RD51 testbeam 2 std. + 4 resistive Centaure Labview, Subatech Nantes Bulk onlyBulk x GEM

8 Sparking during rate scan with showers Absorber is ~ 2 interaction length Detectors ~ shower max Vary beam rate from 2 to 400 kHz Beam spot on absorber is below 1x1 cm^2 Sparking is suppressed in all resistive configurations! → Next batch with lower resistivity → Next RD51 test beam in summer this year