An active target for MEG2, a status report G.Cavoto INFN Roma Riunione referee INFN Pisa, 9 settembre 2013

MEG target geometry Emerging positrons into MEG DCH t volume z Muon depth Muon beam ~20 deg Current MEG target total thickness (t) is 205 microm polyethilene photon XEC Muon are stopped along z [effective thickness t/sin(20deg)] Some spill-out in downstream region Positron affected by multiple scattering and energy loss (proportional to t) Positron can annihilate and produce background photons (proportional to t) Spill-out [endplate] 9/9/2013

Active TARget concept 9/9/2013

Performance comparison MEG Upgrade ATAR made of 250 microm thick square scintillating fibers. Fully efficient ATAR is considered Use cylindrical all stereo DCH with full efficiency and best R resolution (100 microm) WITH A PERFECT SPECTROMETER ATAR gives slightly better performances 9/9/2013

Outline Prototypes for ATAR Tests in lab with source (Sr90) Single fiber/Full-scale prototypes, polishing, reflective coating, fiber-SiPM coupling, noise reduction, SiPM choice,… Tests in lab with source (Sr90) 2.3 MeV electron crossing the fiber Test on beam (Pie5) Same positron pathlength as in experiment (stopped muon and emerging positron) Plans A collaboration PSI (A.Papa, P.R.Kettle) and INFN Roma (G.Cavoto, E.Ripiccini) Support from PSI technical services. 9/9/2013

Polishing and reflective coating Polishing with diamond head (at PSI) Baseline ATAR: read only one end deposit Al on the other end Used two techniques for Al deposit Sputtering (50-100 nm) Painting Other deposit tried (Ti02) 9/9/2013

Single fiber study (Sr90) Realized at PSI: tests done in collaboration INFN-PSI External trigger given by energy deposit in BC400 High threshold to select Sr high energy tails 9/9/2013

mip signals on single 0.25mm fiber SiPM Hamamatsu S10362-11-100C With Al x1.8 more light! No Al Detection efficiency ~80% (ext. scint trigger) Similar results with sputtering 9/9/2013

Full scale prototype (Dec 12) Rohacell support Array of 8 250 mm square polished fibers connected to SiPM Hamamatsu S10362-11-100C Other end with Al deposit 9/9/2013

Test beams (Dec 12) MEG muon and positron beams Ext trigger MEG muon and positron beams Setup inserted before BTS(106 m/s) External triggers (scint+PMT) Detect positron emerging from target (MEG Michel positron) Special collimators and degrader Muon (or Positron) beam ATAR Very clean muon signal Too much background from collimator to see Michel positrons 9/9/2013

Test beam optimization (MC) Reduce background (optimize collimator and size of ext trigger) Optimization of collimator and size of ext. trigger (PMT+scint -> SiPM + scint) Prediction S/B ~ 0.4 (signal S is energy deposit from stopping muons) 9/9/2013

Dec 12 prototype investigation Less light wrt to single fibers! Al deposit (different technique used)? SiPM + fiber optical coupling? Effect of optical cement around fibers? Sr90 + ext trigger New prototype constructions: Double readout (to study maximal light production and collection) 9/9/2013

New Prototypes (May 13) SiPM Al deposit Faraday’s cage (in lab) preamp Collimator (Sr90) SiPM 4x SiPM Test with Sr90 before 9/9/2013

Double readout Test with Sr90 and EXT trigger Efficiency (OR) ~ 60% Still lower than single fibers with Al deposit: Al deposit procedure should be made more stable Optical coupling SiPM/fiber seems to be dominant 9/9/2013

Test beam May 13 Use both positron (28 MeV) and muon beams Housing for target and ext. trigger Alignment Ext trigger BTS Beam Collimator Beam Extr trigger with small scint +SiPM (BC400 2x2x2 mm3 coupled to 3x3 mm2 SiPM) Use both positron (28 MeV) and muon beams 9/9/2013

Results with positrons 28 MeV positron beam (change Separator HV) Use downstream external trigger (in synch with RF) Fiber Ext trigger 9/9/2013

Results with muons Surface muon stopped in target Trigger on emerging Michel positron Signal on fiber from emerging positron (muon is microsec away) Ext trigger Fiber Very little time, try a full week later in the year. 9/9/2013

Optimization optical coupling (MC) d = fiber- SiPM pixel distance Best : maximal fill factor (100x100 mm2) 9/9/2013

Plans Full-scale prototype to be improved Al deposit procedure (more reproducible) Optical coupling SiPM/fiber and housing of SiPM on target frame New electronics: less power consumption, more compact (preamp+ bias) [resuse INFN Roma LABE design] Test in Nov 2013 for a week at PSI More stat for Michel positron as in MEG Study of the mechanical integration of an active or passive target with the new chambers. Study of the induced background coming from support (passive/active), fiber,cables into the experiment. 9/9/2013

Back up 9/9/2013

Constraint at the target A point on ATAR can help in reducing such effect k is proportional to the momentum resolution (this effect tends to vanish if momentum resolution improves) 9/9/2013

Beam and target Thin Passive Target need REDUCED Range Straggling Use of SUBSURFACE MUONS PROBLEM is RATE REDUCTION from P3.5 ➪ Longer Measuring Times 9/9/2013

SiPM specs 9/9/2013

mip signal (0.5mm) 9/9/2013

Sketck for target and DCH 9/9/2013