Timing Counter Status
Timing Counter 2008 configuration
Monitoring and Calibrations
Timing resolution (TC and MEG)
G.Cavoto
INFN Roma
Feb 18th 2009
G. Cavoto
1
Timing Counter (TC)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Two sectors, DownStream and UpStream, each with
– 15 scintillator bars readout by PMTs (TICP)
» Trigger
» Measure time, and z
– 256 Scint. Fibers readout by APDs (TICZ)
» (Meant for) geometrical trigger selection
» Measure z
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Hardware upgrades
New EVAL bag for N2
Reduced APD noise
Very tight!
He concentration at
few times atm. bkg level
Light tight
Tedlar foil added
Some maintenance work (fix connectors,cables…)
Both US and DS TC remounted in late spring
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TICZ readout tests
After installation, test with ‘pass-thru’ cosmics
• All analog channels (8 APD in one ch.)
show good signals!
• US TICZ digital readout working!
• DS TICZ worsened
by higher environmental noise
US TICZ
fiber #
• 3 (out of 8) DS TICZ readout sections failed during the 2008 run
• “on-board” hardware problems: fixing deferred to 2009
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Laser system
– Additional timing calibration device
» Providing 532 and 266 nm light at 50 Hz
» Optical fibers distribution to TC bars (and XEC)
– Problems in operation:
» environment temperature became too high 
cavity unstable  drift in the feedback signal
 electronic saturation, damages of optical
components and electronics.
256 nm
50Hz @ 532nm
50 Hz
2 stages pulse amplifier
Acustic-optical
pulse selectors
-For 2009 run:
- Proper box with
actively stabilized temperature
- Retuning of cavity
- New feedback electronics
3m cavity 48 MHz,
Diode pumped Nd:YVO 1064nm
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PMT gain equalization
• Select Michel positron
hitting bar center
(magnetic field ON)
• Change HV to have
same average charge
for PMTs of the same bar
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TC measurements
e+
PMT0
PMT1
h
L
t0  T 
h
c
 b0  0
v eff
A0
t1  T 
Lh
c
 b1  1 .
v eff
A1
amplitude of PMT signal
 velocity
effective
T : time of positron at the impact point on first hit bar
(connected to the positron track from DCH)
h : impact point along bar length (z direction)
DRS3 digitization for Double Threshold Discriminator NIM pulses
t0 and t1 extracted with waveform template fit to NIM pulses.
Checked with cross-correlation method (optimal filtering).
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TC calibrations
Timing with DTD NIM signals
Time walk correction
for each PMT
Time offset between
PMT of the same bar
(z offset calibration)
Relative time offset
between bars


Effective velocity for
each bar
(z measurement)


Absolute time offset
between positron

and gamma
c0
c1
Michel nmult=3
Cosmics
b0  b1
L
b0  b1

2v eff
2
v eff
_
Boron
B( p,  )C
Pass-thru cosmics

Dalitz 0
 0  e  (e )
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bar #
‘Double’ and ‘triple’ hit events
e+
On events with three adjacent hit bars
(triples)
minimize the differences (for all the bars)
TC
TA
1st bar
1  c
t 0C  t 0B
c 0B
0C
 t 0A   


2
A0B

2  A0C
TB
 c 
0A

 A .

0A 

|z|

On sample of two hit adjacent
bars (‘doubles’) test time walk
correction (c0 and c1)
T  TA  TB
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ns
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TC time resolution
 T
2
Estimate of single bar time resolution
Assuming the two bars to have
the same intrinsic time resolution
Upper limit on average time
resolution () in 60-90ps
range

Includes effect of DRS
digitization (~10 ps)
(estimated sending same signal
to two DTD inputs)
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TC time resolution stability
Runs 24xxx
Runs 25xxx
Runs 26xxx
Runs 27xxx
Runs 29xxx
Runs 30xxx
Runs 31xxx
G. Cavoto
• Same TW calibration
constants
• Stable over time
no need of different
sets of constants
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Inter-PMT offsets
Dedicated cosmics runs taken in different down-time periods
– Select event with 1 or 2 hit bar
Assume symmetric distribution along the bars
Most of the
bars have
this flat
distribution
t1 - t0 [ns]
Data taken in different periods very consistent (TICP very stable!!!)
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MEG physics runs TC hitmap
After
Before
US
DS
t1-t0 [ns]
t1-t0 [ns]
Distribution nicely “justifies”
Need veff to have the correct z scale
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Effective velocity with TICZ
Pass-thru cosmics, 2 hit fibers (clusters) expected
Single-cluster
inefficiency: 27%
due to cosmics geometrical
inefficiency + dead/hot fibers (5%)
Bar16
veff (cm/ns)
1
Using z fiber  v eff  (t1  t 0 )
2

Syst err.1.5%
zbar-zfiber (cm)
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Inter-bar offset extraction
Boron events
4.4MeV (XEC) and 11.7MeV(TC)
TC 



LXEC
L
t

t
T  TXEC     0 1   
c   2
c 

Bar14
-signal
-bkg (rescaled)
Bar17
-signal
-bkg (rescaled)
T[ns]
T[ns]
• Gaussian fits.
– Cosmic background evaluated in runs with no-beam and
same trigger requirements (TC & XEC)
• Syst error on calibration constants at 50ps level
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Inter-bar offsets monitoring
Bar15: change in DRS board
T
mean
[ns]
relative to bar 17
Sep
08/10
13/10
20/10
25/10
10/11
24/11
01/12
Weekly
monitoring
(periodic DB
updates)
bar#
- No clear trend vs time
- Relative offsets stable
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DCH-TC match hitmap
Extrapolate tracks from DCH to TC bars
z  zDCH  zbar
[cm]
Given a track and a TC bar hit matching efficiency is 91%
Some data/MC discrepancy

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MEG absolute time offset
Dalitz
0
events
 0  e  (e )
– Same topology as signal !
– Gamma/positron energy range (can be chosen) same as for signal
– Worse resolution due to LH2 target

Comparison
with signal is not exact

  LTC 
LXEC

Te  TXEC 
 T  e 
c  
c 

Centre of signal window
Bar17
μ = 26.06±0.01ns
 = 267±10ps
Control sample
(flight length correction)
MC validation
ns
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Timing systematics check
Dalitz
before calibration
Dalitz
after calibration
with Boron
Boron
(E=4.4MeV)
Dalitz
(E>45MeV)
Relative
to bar 17
bar#
Residual time walk effect
on XEC timing
bar#
Residual inter-bar differences
(different target for 0 Dalitz)
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Summary
• TICP very stable over the 2008 run.
• TICZ: 13 out of 16 sectors working.
• Calibration strategy developed and
successfully applied to data.
• Data monitored during data-taking.
• Some fine-tuning still needed
– Residual bar differences
– Detailed data-MC comparison
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Outlook for 2009
– Detector activity
• New APD electronics
– Shorter shaping time to fit trigger timing
– Currently prototyping new el. boards
• LASER
• Upgrade of TC slow control
– On track for a 2009 data-taking period
– Data analysis
• Calibrated TC data and TC simulation available for MEG
analysis
• MEG timing measurements well underway
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Back up
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New APD electronics
To insert APD into trigger
• Requirements:
–
–
–
–
-
fast signal ( 3-5 times faster)  15ns achievable.
better EMI immunity
Implement redundant schemes for the command transmission (I2C)
Improve over all settings capability ( on board test /calibration signal,…)
Bread-boarding is started, final design of the prototype board will be
sent for the production by Dec 08.
Test pulse response
G. Cavoto
Move Amplifier to
the APD board and/or
differential input
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