1. Electron transparency studies with an exchangeable mesh Micromegas chamber Fabian Kuger 1,2 Paolo Iengo 2, Rui de Oliveira 2, Givi Sekhniaidze 2, Joerg Wotschak 2 1 Julius-Maximilians-Universitä ̈ t Würzburg (Germany), 2 CERN, October 27 th 2014 IWAD & RD51 Collaboration week - Kolkata

2. Motivation Electron transparency T is one of the intrinsic key properties of a Micromegas detector. 2ExMe transparency studies - KolkataOctober 27 th 2014 Depended on the application a not optimized transparency (T < 0.9) reduces the signal strength or may cause impact on the physics result like angular resolution! X-Ray measurements Minimum Ionizing Particles  little importance of single electrons  T effects statistic of measured charge  great importance of single electrons  T effects reconstructed MIP properties

3. Concept of an Exchangeable Mesh MM (ExMe) A new Micromegas prototype with Exchangeable Mesh has been build. -Using the floating mesh concept: The mesh is independent from the readout structure and attaches to the pillars by mechanical positioning and electrostatic force. 3ExMe transparency studies - KolkataOctober 27 th 2014 -Independent mesh frames allow easy mesh exchange: Instead of fixing the mesh on the readout (as e.g. foreseen in ATLAS NSW MM) and additional frame is introduced to hold the mesh only.  Most detector inherent parameters are kept, only the mesh is changed!

4. Realization of an Exchangeable Mesh MM (ExMe)  More technical details will be presented by: Silvia Franchino: ‚Standard GEM and Micromegas‘ RD51 Production-WG6, Wednesday 29 th Oct. 4ExMe transparency studies - KolkataOctober 27 th 2014 Drift panel - with internal gas distribution and HV conduct - mounted on honeycomb + FR4 stiff-back - carrying springs pressing down the mesh frame O-ring placed between external FR4 frame (5mm) and mesh frame (4mm+springs) Mesh frame Mesh glued on lower side, aligned with r/o board via pins in the corner. Ground contact to copper ground on r/o plane. (! Non-flatness of the frame due to mesh tension ~500µm!) Readout panel - copper readout strips routed to Panasonic connectors - Kapton ™ foil with sputtered resistive pattern - coverlay (128µm pyralux) with pillar structure and ‘frame’ to define mesh boarder heigth - glued outer FR4 frame - connectors for HV, r/o (Panasonic) and grounding

5. Features of the Exchangeable Mesh MM (ExMe) A variety of mesh specification details can be studied: - different wire diameter - different openings with same wires - no/ soft / strong calendared meshes - different types of weaving (plain vs. twill weave) - alternative mesh material (metalized synthetics) The ExMe readout is divided in four sectors, covered by different spaced pillar patterns. (Pillar-arrangement in regular triangles with different side-length between 5-10mm) Second ExMe chamber is available, where the sputtered resistive layer is replaced by a screen-printed one. 5ExMe transparency studies - KolkataOctober 27 th 2014 Sector C 8,5 mm Sector D 10 mm Sector A 5 mm Sector B 7 mm * from: K. Nikolopoulos et al.: „Electron transparency of a Micromegas mesh” JINST 2011 *

6. First laboratory measurements First laboratory measurements in the CERN RD51 Lab 6ExMe transparency studies - KolkataOctober 27 th 2014 - Copper X-Ray used to induce 8keV γ- conversion events with rates up to 20kHz/cm 2 - Total charge measured as spectra with 500k events  Identification of the K α -Peak Keeping constant amplification, the position of the K α -Peak (corresponding to 300e - in the drift gap) is proportional to the number of amplified electrons.  relative measurement of the electron transparency T U drift =100V U drift =200V U drift =300V U drift =400V U drift =800V U drift =600V U drift =1000VU drift =1200V U drift =1400V Channel Count

7. First (main) result – transparency comparison 7ExMe transparency studies - KolkataOctober 27 th 2014 First (incomplete) transparency curves from Lab measurements. Comparison between 71/30 and other data rows in difficult (comp. voltage vs. comp. gain?) Further comprehensive measurements to be performed soon.

8. First (side) result – gas impurities 8ExMe transparency studies - KolkataOctober 27 th 2014 Double measurements to ensure repeatability showed a decrease in the number of amplified electrons for low drift voltages within one data series.  First measurements of the day after a night without gas volume exchange  Higher concentration of O 2 leads to attachment of electrons in the drift gap, more pronounced for lower drift voltages (=longer drift time).

9. First (side) result – influence of the pillar distance 9ExMe transparency studies - KolkataOctober 27 th 2014 Transparency is (as expected) not depended on the supporting pillar pattern / distances.  Difference in inactive area are discarded during normalization to T max The mean gain on the contrary is effected: Greater pillar distance leads to higher mean gain (10-15%)  Larger pillar distance leads to higher sagging of the mesh between pillars (<1µm at 5mm few, ~2µm with 10mm)  Yielding a lower effective amplification gap Leading at this working point to higher gain Deviating behavior in one spot  Hint to non-flatness in the r/o or deviation in the pillar height

10. Test-beam measurements 10ExMe transparency studies - KolkataOctober 27 th 2014 Intensive Data taking in test beam periods at T9, T10 (PS) and H6 (SPS) at CERN. Data analysis with respect to efficiency, spatial and angular resolution, using the µTPC mode (data taken in inclined setup) is ongoing.

11. Simulation on electron transparency 11ExMe transparency studies - KolkataOctober 27 th 2014 Simulations on electron transparency based on Magboltz and Garfield++ are currently running. Covering FEM models with different geometries, to test and understand previous results* where best agreements have been reached with flat, cylindrical wire mesh approximation. Now available experimental data for a larger set of geometries could verify simulation as powerful prediction tool! * Presented in : Mesh transparency and gas gain studies in Micromegas detectors” RD51 Zaragoza 2013 *

12. Upcoming Studies -Analysis of MIPs data from several test-beam periods @ CERN  Effects of mesh properties on efficiency and angular resolution -Simulation using FEM models to compare with data for all geometries -Extend Lab measurements to cover wider voltage ranges  reach good comparability of the transparency curves  Enhance understanding of gain changes with different meshes -Include greater variety of meshes (e.g. different weaving types, metalized synthetic meshes, meshes with different tension…) -Study Transparency behavior in different gas mixtures 12ExMe transparency studies - KolkataOctober 27 th 2014 (ongoing) (followed up next) t.b.s. Thank you for your attention!

13. Backup 13ExMe transparency studies - KolkataOctober 27 th 2014

14. Backup - Previous studies on electron transparency 14ExMe transparency studies - KolkataOctober 27 th 2014 Systematic studies of the electron transparency are rare and usually limited on one mesh type: - K. Nikolopoulos et al.: „Electron transparency of a Micromegas mesh” JINST 2011 Comparison off electron transparency in measurements and simulation based on different algorithms (RKF vs. microscopic tracking, FEM vs. neBEM) and different mesh approximations (cylindrical vs. rectangular wires) -F. Kuger.: „ Mesh transparency and gas gain studies in Micromegas detectors” RD51 Zaragoza 2013 -Plenty individual studies to verify camber performance all over the RD51 institutes.  Limited to one mesh geometry and fixed dimensions (e.g.: 45/18  400lpi mesh, standard in many detectors)

15. Backup - Mesh specifications under study 15ExMe transparency studies - KolkataOctober 27 th 2014 So far the studies concentrate on validating mesh specifications to be used in the Micromegas for the ATLAS NSW upgrade, demanding plain weave, stainless steel meshes sufficient T at given working-point (expected to be: E drift =0.6 kV/cm, G=10 4 ) mesh available in large sizes up to 2,5mx3m (on affordable costs) 45/18 – 50% 50/30 – 39% standard for small size MM chambers standard for large size MM prototypes at CERN (MMSW) 71/30 – 49% alternative 30µm wire-mesh with larger openings 60/18 – 59% maximized open area with 18µm thin wires

16. Backup – all layer view of the ExMe layout 16ExMe transparency studies - KolkataOctober 27 th 2014 drift board FR4 & honeycomb stiff-back brass springs O-ring (6mm) mesh frame (4mm) honeycomb stiff-back (below) readout board (with copper strips) FR4 frame (5mm) Kapton™ with resistive layer Pyralux coverlay pattern