An Active TARget for MEG-II, a status report G.Cavoto INFN Roma Riunione referee INFN Pisa, 14 gennaio 2014

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] 14/1/2014

Active TARget concept Light collection y Only one end of the fiber instrumented! 14/1/2014

Performance comparison MEG Upgrade ATAR made of 250 mm thick square scintillating fibers. Fully efficient ATAR is considered Use cylindrical all stereo DCH with full efficiency and best R resolution (100 mm) WITH A PERFECT SPECTROMETER ATAR gives slightly better performances 14/1/2014

Use both positron (28 MeV) and muon beams PSI PiE5 test beam May 13 Housing for target and ext. trigger Alignment Ext trigger BTS Beam Collimator Beam Ext. trigger with small scint +SiPM (BC400 2x2x2 mm3 coupled to 3x3 mm2 SiPM) Use both positron (28 MeV) and muon beams for prototype v2 14/1/2014

Results with positrons 28 MeV positron beam (change Separator HV) Use downstream external trigger (in synch with RF) Fiber Ext trigger Prot. V2: Electron (Sr90) and positrons seen with reduced efficiency (~35%) 14/1/2014

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, 10 days forseen in 2014. Start a new prototype (V3) to recover missing efficiency 14/1/2014

Update since last september New full-scale prototype (V3) tested in lab (PSI) Double readout fiber (with/without Al coating): estimate maximal total collected light and detection efficiency Single readout fiber with reflector at the end (sputtered Al and painted Al) Array with coated Al around fiber and sputtered Al on the free end of the fiber (to measure the optical cross-talk) Study of the induced background coming from support (passive/active) into the experiment. First design of the mechanical integration of an active (or passive) target within the new DCH. A collaboration PSI (P.R.Kettle, A.Papa,G.Rutar) and INFN Roma (G.Cavoto, E.Ripiccini) Support from PSI technical services. 14/1/2014

90Sr lab test 14/1/2014

The double-readout prototype Measure the total detected light 14/1/2014

Signal charge spectrum Doubly readout fibers, one SiPM at each end Calibrated spectrum One SIPM only Sum of two SiPM signal Expected 5 p.e. Measured 4.3 14/1/2014

Efficiency estimate At least one p.e. in presence of external trigger Collimator size known with relatively large error. A conservative error of 10% relative on efficiency must be used. A lesson learned: use of optical cement worsens efficiency There is still about 20% efficiency missing (SiPM/fiber mechanical coupling? ) 14/1/2014

Single readout fiber results ATAR MEG-II configuration Test of different Al mirror on not-instrumented end 14/1/2014

Results for single readout fiber Al mirror on one end gives on the other instrumented end 1.5 more light than no mirror (as expected) 14/1/2014

Optical cross-talk Array of 4 fibers: new prototype (V3): fibers with 30 nm sputtered Al coating Prot V2 Prot V3 14/1/2014

XEC background More material (thicker target, service fibers, support) might induced a larger background in XEC (positron Bremsstrahlung, annihilation in flight) AIF + Br. (dotted) AIF background from target itself scales with target thickness Radiative (dashed) What about the other material ? 14/1/2014

MEG-II Geant4 material simulation CDCH with 2pi coverage + passive target 140 mum thickness, 15 deg angle, no holes R[cm] All the photons emitted from Michel positron (no cut on their energy) XEC COBRA endcap CDCH endplate Support Passive target z [cm] 14/1/2014

Different support and service fibers Baseline passive target ATAR with service fibers 14/1/2014

Energy deposited in XEC Number of events with detected photon in XEC per positron Baseline-2pi Baseline (CDCH with missing sector) ATAR-2pi Ee>51MeV only Full EXEC [MeV] EXEC: energy deposited in XEC by photons and converted in scintillation No striking difference between different layouts 14/1/2014

Mechanical integration of an ATAR Discussion started Incoming Muon beam SiPM located on the first endplate All inside DCH inner radius (168 mm) Upstream sector (DCH extension, within COBRA volume) Service fibers (A)TAR need be moved (CEX and CW calibration targets) 14/1/2014

Multiple uses of ATAR Continuous beam monitoring (detecting light from muons) MEG “normalization”: counting muons Y- beam spot measurement If ATAR can be 90deg rotated: X- and Y-beam spot available Muon beam centering (done now with auxiliary device and time consuming) 14/1/2014

Conclusion and plans We have a fiber+SiPM assembly with a 75% efficiency to detector electrons ATAR services not expected to worsen XEC background (besides target material itself does) Discussions of possible solution to integrate ATAR in MEG-II going-on Aim to experimentally detect positrons from stopped muons in the target. Produce a new (V4) version of ATAR prototype Test it in lab (Sr90 setup) (beginning 2014) INFN LNF BTF single tagged electrons and positron in Mar/Apr and later in 2014 PSI PiE5 (positron and MUON): 10days in Fall 2014 Hopefully this device will be considered for the MEG-II detector 14/1/2014

Back up 14/1/2014

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) 14/1/2014

Beam and target Thin Passive Target need REDUCED Range Straggling Use of SUBSURFACE MUONS PROBLEM is RATE REDUCTION from P3.5 ➪ Longer Measuring Times 14/1/2014

SiPM specs 14/1/2014

mip signal (0.5mm) Prot V1 14/1/2014

mip signals on single 0.25mm fiber SiPM Hamamatsu S10362-11-100C Prot V1 Prot V1 With Al x1.8 more light! No Al Detection efficiency ~80% (ext. scint trigger) Similar results with sputtering 14/1/2014

Measurements of collimator 14/1/2014

Distribution of Max E emitted photons ATAR-2pi-service EXEC>40 MeV Ee > 51 MeV generated only Target + CDCH XEC ATAR DCH DCH Outer cylinder 14/1/2014