1. Timing Counter analysis
C.Voena - INFN Roma
for the TC group
MEG Review Meeting
Feb 17th 2010

2. TC before insertion in COBRA

3. TC configuration in 2009 run
Trigger thresholds lower than 2008:
20mV on single PMT, 70mV on sum of PMT(40/100mV in 2008)
PMT gains re-equalized by adjusting HV
Waveform digitization:
- DRS3 for NIM pulse from Double Threshold Discriminator
(DTD) for time measurement (like 2008)
- DRS4 for PMT pulse
Double-Threshold Discriminator thresholds revisited
to optimize efficiency and time resolution
2009: 25/600 mV :25/800mV

4. PMT gain equalization Use mean of log(QPMT1)/QPMT0) in cosmic ray data
TC bar
Use mean of
log(QPMT1)/QPMT0)
in cosmic ray data
at “center” of the bar
Change HV to have
PMTs (inner vs outer)
and bar equalization
within 30%
(changed 10 PMT HV
with respect to 2008)
PMT0
(inner)
PMT1
(outer)

5. DTD thresholds scan
Low threshold: as low as possible (first photoelectron)
=> Can improve time resolution
Limited by noise
No improvement seen lowering this threshold (tried mV)
=> left at 25mV which was already optimal value!
High threshold: select good pulses (tracks producing enough p.e., Landau peak)
- Lowered to 600mV (was 800mV)
to have more acceptance on
positron with lower pulses
800-25
400-25
250-25
Eloss (a.u.)

6. TC time measurement amplitude of PMT signal effective velocity e+z
PMT1 L
amplitude of PMT signal
effective velocity
t0,1= extracted with waveform template fits to NIM pulses
from Double Threshold Discriminator for PMT0,1
T : time of positron at the impact point on first hit bar (connected to the positron track from DCH)
z : impact point along bar length

7. TC calibrations Time walk correction for each PMT Improve single bar
time resolution
Michel nmult=3
Time offset between PMT of the same bar (z offset calibration)
Michel
(Cosmic in 2008)
Effective velocity for each bar
(z scale)
DCH matched Michel positrons
Relative time offset between bars
(Boron in 2008)
Absolute time offset between positron and photon _
Dalitz 0

8. Time Walk calibration
As in 2008 TW corrections from triple-bars events
in Michel Data
On events with three adjacent hit bars (triples) minimize the
differences (for all the bars) of:
bar # e+ TC TA
1st bar TB
|z|
no TW
with TW
Control sample: double-bars events TA TB
TA-TB (ns)

9. TC Time Resolution from double-bars events: upper limit on:
TC intrinsic resolution + DRS resolution (~10ps)
Upper limit on time
resolution () in
ps range
in 2009 (except
bar 21)
Slightly worse than
2008: under study.
Still adequate for
MEG performances
2009 resolution
2008 resolution

10. z-offset calibration Difference of PMTs electronic offsets
- Needed for z measurement and to combine time
measurement of adjacent hits
- Use Michel matched positrons instead of previously used cosmic rays
Obtained by aligning mean
of ztrack-zTC
ztrack= z predicted extrapolating
track at TC
zTC = z measured by TC
The procedure assumes
φ-symmetry

11. TC hit-map with z-offset calibration
Good alignment of bars as can be seen from hit-map
trigger “MEG” data
before calibration
trigger “MEG” data
after calibration

12. z-scale:effective velocity
Temporary calibration
- Michel matched positrons
Use ztrack as estimate of z at TC
- Eventually use fibers for z mesurement
veff = 14.8 cm/ns
ztrack –zcenter (cm)
t0-t1(ns)

13. Inter-bar time offset calibration
Double-bar events in Michel data Tj
LIB Ti
LIB = Inter-Bar path,
taken from MC
LIB/c ~200ps
Offsets for
DS bars
Offsets for
US bars
After that, Downstream bars are aligned with bar #0 and Upstream bars are aligned with bar#15 (there are not double-bar events which connect US and DS)

14. The Boron sample Boron events before calibration
two photons: 4.4MeV (XEC) and 11.7MeV(TC)
Cuts for cosmic ray rejection
before calibration
RMS of residual offsets
(after calibration) is~70ps
after calibration
Mean of Tγγ (ns)
Systematic effect of
assumption on LIB/c
(path between two
bars) under study
TC bar #

15. Monitoring TC stability
Single bar time resolution
- different colors correspond to
different weeks
Inter-bar time offset
- Boron sample
different colors correspond
to Oct/Nov/Dec
Mean of Tγγ (ns)
Stability over time
TC bar #

16. Absolute XEC-TC time offsets
Dalitz 0 events
Same topology as signal
Worse resolution due to LH2 target Do not look at time resolution!
Center of blinding
window
(for pre-selection)
μ=24.9ns
Teγ for
reference bar
- Dalitz data suffer from
hardware problem on
DCH side (DRS) that
may affect resolution
Teγ (ns)

17. TC-DCH match and Te+ algorithm
DCH-TC match
Extrapolated track at bar surface
Reject bars with multiple hits
Reject pairs TC-DCH with
bad ztrack-zTC , rtrack-rTC and bad χ2
of match (multiple turns taken into
account)
Positron time :
- If more than 1 TC hit in matched cluster: combine
time measurement taking into account track length between bars
Correct ad-hoc for Te+ correlation with ztrack-zTC
Plans: do systematic studies of the algorithms
(Monte Carlo and Dalitz sample)

18. Check on radiative decay peak
Clearly see radiative decay peak
in Eγ sideband
Inter-bar calibration working well
- Pre-selection window not well
centered at this stage
Temporary track and photon
selection, kinematic cut (m2νν>0)
Example : radiative
peak for bar 19
Position of radiative decay
peak vs TC bar#
Mean of Teγ (ns)
Teγ (ns)
TC bar #

19. Summary TC bars very stable during 2009 run
- Ready for the incoming long data-taking
Calibration strategy applied successfully to 2009
data
TC intrinsic time resolution < ps
(a little worse than 2008, investigating)
- one of the best Timing Counter detector
Calibration methods and analysis algorithm
constantly improving

20. Backup

21. Possible causes of worse resolution
Electronic
We checked the electronic
contribution to the resolution
by splitting PMT signal in two
different electronic channels
=>Same as 2008
Noise
Same noise level as 2008
at DRS input for PMT signal
(not sure of situation
at DTD input)
tin-tout
PMT#

22. Possible causes of worse resolution
Other possible causes:
deterioration of PMT-bar coupling?
Less scintillation light?
1) (relative) width of Landau peak: done but
not conclusive, dominated by Eloss fluctuation
2) change of Λeff. Underway but may be not
conclusive since we do not have precise
measurement of veff
3) Tests in labs foreseen

23. Width of CR landau distr.
2008
2009

24. Applying z-calibration to doubles
before calibration
z-offset calibration from Michel data
z-offset calibration from CR data z1
Mean of DZ=z2-z1
in double bar events (cm) z2
Second hit bar number

25. Boron: High energy photon
High energy photon in XEC
Low energy photon in XEC

26. CR backgound in Boron Sample
Tγγ (ns)
Black: boron data
Red: CR data taken with same Boron trg

27. Resolution in Boron Sample
Tγγ resolution (ns) vs TC bar:
2009, 2008 (July processing)
Tγγ resolution vs Time

28. Multiple hits of TC bars
Positron track
TC hit position: dz = |z2|-|z1|
Bar 1
Bar 2 * *
1st bar with multiple hit
|z|
dz>0 * *
|z|
dz< 0

29. Teγ vs ztrack-zTC on Dalitz
No constraint for the track to come from muon target
TC US
Correlation is visible
Correction of 20ps/cm
(DZCorrection)
Same as in 2008