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

2. 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

3. Active TARget concept
9/9/2013

4. 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

5. 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

6. 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 ( nm)
Painting Other deposit tried (Ti02)
9/9/2013

7. 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

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

9. Full scale prototype (Dec 12)
Rohacell support
Array of mm square polished fibers connected to SiPM Hamamatsu S C
Other end with Al deposit
9/9/2013

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

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

17. 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

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

19. 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

20. Back up
9/9/2013

21. 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

22. 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

23. SiPM specs
9/9/2013

24. mip signal (0.5mm)
9/9/2013

25. Sketck for target and DCH
9/9/2013