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Time of Flight (ToF): basics
Start counter
Stop counter
• TOF – General consideration
- early developments combining particle identifiers with TOF
• TOF for Beam Detectors or mass identification
- TOF Constituents - based on the use of SEE effect:
- Thin Foils (SE generation)
- SE transport
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2. part
- SE detection ( mainly MCP – some basic set-up )
• Fast electronics
- Fast preamplifiers and discriminators LE; CFD; ARC-CFD
- Time walk and jitter –basic consideration
Timing measurements
• Pulse height measurements discussed up to now
emphasize accurate measurement of signal charge
• Timing measurements optimize determination of time of
occurrence  timing output signal ( “time stamp” )
• For timing, the figure of merit is not the Signal / Noise ratio
but the Slope / Noise ratio
Fast Timing
a) Timing measurements  Detectors for
Timing and their FEE
- Scintillator & Photomultiplier assembly
- MCPs & Fast Preamplifiers
- Semiconductor detectors & Preamplifiers
( CSP vs. Current )
b) Ultra-fast Timing Circuits and Signal
- Time - stamp
- Time - walk and Time - jitter
~300 ps
2nd layer
239Pu
244Cm 241Am
( ~200 keV energy loss )
IKP - TOF & BPM Preliminary results
- 250 +/- 50 ps
- coincidence with energy
measurements (SC + DGF-4C-rev.F)
- transparent beam detector and
tracking with 32x SC matrix as
Stop detector 
(real beam test is requested!)
Counts
[ns]
~350 ps
1st layer
Counts
Time of Flight
~250 ps
3rd layer
239Pu
241Am
244Cm
5.155
5.486
5.804
[MeV]
a) Timing measurements  Detectors for
Timing and their FEE
- Scintillator & Photomultiplier assembly
- MCPs & MCP-PMT and Fast Preamplifiers
 and very briefly about other ultra-fast detectors
- Semiconductor detectors (Si, Diamond) & their Preamplifiers
( CSP vs. Current )
Scintillators & Photomultiplier tubes (PMT)
Detector
Photomultiplier tube
-
(PMT)
Gain ~ 106  sec. secondary electrons / photo-electron
Different geometries of PMT
Circular-cage type PMT
Box-and-grid type PMT
and the typical electron trajectories
Linear-focused type PMT
Transmittance (%)
The transparent window material
commonly used in PMT:
(nm)
Basic Photocathode
commonly used in PMT:
- Cs-I  100 M
- Cs-Te  200 S;M
- Bialkali (Sb-Rb-Cs, Sb-K-Cs)  400 U;S;K
- Multialkali (Sb-Na-K-Cs  500 K;U;S
- Ag-O-Cs  700K;S-1
- GaAsP(Cs)
Photocathode Radiant sensitivity
Wavelength
(mA/W)
- MgF2 crystal ; - Sapphire ; - Synthetic silica;
- UV glass; - Borosilicate glass
Wavelength
(nm)
Photons
• transport
• WLS
WLS
Detector
Wavelength shifter
BriLanCe Crystals - Properties (1)
BriLanCe Crystals - Properties (3)
s/n!
BriLanCe Crystals - Properties (2)
Signal output problem
and the solution
Countermeasures for very fast response circuits
(the “miraculous” (small) series resistor and not parallel capacitor)
The effect of damping
resistor on ringing
(
remember the influence
of resistor in the quality
factor of an oscillator or
larger capacitor value in a
low-pass frequency filter :)
C - filter-change the frequency !
Rs –oscillation damper !
The importance of Poles and Zeros
Pole-zero diagram
 Ideal oscillator
 real oscillator -
R series
e.g. 10 pF * 10 nH
 500 MHz
Step 1
Step 2
Step 3
Going from PMT ( Photomultiplier Tube)
to MCP (Microchannel Plate)
• from a discrete dynode structure to a
continuous distributed dynode structure but also
• more than 8 orders of magnitude scaled down
design in volume
( 102 in length and > 103 in diameter )
Multi-channel Plate Detectors
Electroding
(on both face)
Channels
- e initial velocity ( ~1eV)
- channel length/diameter ratio
Kc
- constant
Metallized ++
Metallized+
• Much stable operation vs. external high magnetic fields in comparison with PMT
• lower gain but in chevron configuration the gain ~106
• lower power consumption (gain vs. HV)
MCP assembly in chevron configuration
MCPs in Single, vs. Chevron and Z-stack configurations
Gain:
103-104
106
108
MCP gain dependence vs.
 - parameter and
stage configuration
Comparison of gain characteristics of
various single and multi stage MCPs
MCP gain dependence vs.
channel diameter and technology
Comparison of gain characteristics of
three different types of 2-stage MCPs
Parameter
Rise time
[ ps]
6 µm
Channel
12 µm
Channel
167
245
Fall time
[ps]
721
716
Transit time
[ps]
406
650
67
81
Transit time spread [ps]
Comparison of timing characteristics of
chevron 2-stage MCP-PMT, one with
6 µm and another with 12 µm pore diameter
Time
x10 -8
s
Mesh form anode
( e.g. X,Y delay lines  signal pulse
amplitude only 15-20% compared
to the solid anode standard version)
?
Hamamatsu
R-3809 U-50
• Photocathode
diam. ~10mm
• Price -  !
Standard operating circuit for an MCP-PMT
HV ~ 3 kV
Rise time ~150ps
Fall time ~350 ps
FWHM ~ 300 ps
Anode Return Path Problem
Current out of MCP is inherently fast- but return path depends on where
in the tube the signal is, and it can be long and so rise-time is variable
Incoming Particle Trajectory
Signal
Would like to have return path be short, and located right next
to signal current crossing MCP-OUT to Anode Gap
10cm wire; 0.2mm diam  150 nH
Impedance @ 1Ghz ~ 1 kOhm
10 pF ~ impedance
@ 1GHz ~ 1.5 kOhm
Signal & Return
Load
The Signal
is a current
and not a
potential
Detector Signal Collection
High Z
Circuit development
Low Z
+
Rp
-
•
•
•
•
Voltage source
Zo
Z
Low Z
Impedance adaptation
Amplitude resolution
Time resolution
Noise cut
T
• Low Z output voltage source circuit can drive any load
• Output signal shape adapted to subsequent stage (ADC)
• Signal shaping is used to reduce noise (unwanted fluctuations) vs. signal
Front-end electronics – overview
Detector as fast signal generator  electron-hole pairs collection
 only electrons (or particles)
if Z is high
charge is kept on capacitor nodes and voltage
builds up (until capacitor is discharged)
+
Rp
•
Z ~ Ci
- excellent E resolution
- friendly pulse shape analysis
•
Detector
FEE
(Input stage)
Advantages:
Disadvantages:
- channel-to-channel crosstalk
- pile up above 40 k c.p.s.
- sensitivity to e.m.c.
Front-end electronics – overview
Detector as fast signal generator  electron-hole pairs collection
 only electrons (or particles)
if Z is low
charge flows as a current through the impedance
in a short time.
+
Rp
•
Z ~ Ri
- limited signal pile up
- limited channel-to-channel crosstalk
- low sensitivity to parasitic signals
- good timing resolution
-
•
Detector
FEE
(Input stage)
Advantages:
Disadvantages:
- pour signal/noise ratio
- sensitive on return GND loop !
Capacitive Return Path Solution
Return Current from anode
Current from MCP-OUT
Ultra-fast detectors, extremely user-friendly solutions, the only
disadvantages: - small area of photocathode and extremely expensive
?
CERN - LHC experiment
Chemical
Vapour
Deposition
techniques
CVD-Diamond
the CVD - Diamond Detectors
Two “optical grade” CVD and
~ 100µm thickness
• a 30 x 30 mm2 detector with 9 strips
with a pitch of 3.1 mm and
• a 20 x 20 mm2 pixel detector with a
pixel size of 4.5 x 4.5mm2 
the first large-area CVD diamond detectors
Installed at SIS
E. Berdermann et al, CVD-Diamond Detectors…
Nucl. Phys. B 78 (1999), 533
The largest diamond detector of 60 x 40 mm2
and ~200µm thickness <0> in the focal plane
detector of a magnetic spectrometer
E. Berdermann et al, The use of CVD-Diamond for heavy ions…
Diamond and Related Materials 10(2010),1770
Charge Sensitive Preamplifier
•
•
•
•
•
Active Integrator (“Charge Sensitive Pre-Amplifier”)
Input impedance very high ( i.e. NO signal current flows into amplifier)
Cf (Rf ) feedback capacitor (resistor) between output and input
very large equivalent dynamic capacitance
sensitivity A(∆qi) ~ q / Cf
large open loop gain Ao ~ 10,000 - 150,000
• very fast active integrator
• tr < 1ns (sub-nanosecond CSP)
∆Qi
• A0 ~ 1,000-10,000
• Transconductance amplifier
Ci
• ASIC integrated structure
E. Berdermann et al, The use of CVD-Diamond for heavy ions…
Diamond and Related Materials 10(2010),1770
Ultra-fast branch of a CSP
Standard current amplifier solution
G1
G2
G1 > G2  to minimize S/N ratio
HSMP 3862 series
tr ~ 1.2 ns
(10 to 90%)
Simulation results of the amplifiers with
THS 3201 ultra-fast current feedback amplifier
Imax (1µs)~ 1A
Peak Inverse Voltage ~50V
Tj –Max. Junction Temperature ~ 150°C
(OK to be used in vacuum)
Signal Output 
Noise Output

A1. A2. A3 . e
A1.A2.A3. e1 + A2.A3. e2 + A3.e3
the gain of the first block of amplification must be kept as high
as possible, in order to reduce the importance of the noise contributions
coming from the following blocks
i.e. the preamplifier gain has to be as large as possible  ! Ao >>10 4

b) Ultra-fast Timing Circuits and Signal
-Time-stamp
- Time-walk & Time-jitter as perturbation effects
* Timing – time stamp but actually timing means
measurement of time intervals (from fs to ms)
Walk effect - variation of time stamp (timing) caused
by signal variation in amplitude and/or rise time
Jitter effect - timing fluctuations caused by noise and/or
statistical fluctuations in the detector (intrinsic noise)
 two identical signal will not always trigger at the
same point (time stamp) time variation dependent
on the amplitude of fluctuation – slope/noise ratio
Fast Pulse Shaping
“MVP “
in fast time domain
tra ~ tc
The noise bandwidth
approaches the
signal  bandwidth
 the timing jitter
- the Ortec 579 – to slow for fast timing
New fast amplifiers:
- Ortec 9327 (1 GHz Amplifier
and Timing Discriminator)
- Ortec 9309-4 ( Quad Ultra-Fast
Amplifier)
- Ortec 9306 (1-GHz Preamplifier)
• this is the reason while only
1-2 amplifier stages *
* this can be implemented only
if the product [gain x bandwidth]
of the amplifier is large enough  !
10 cm wire; 0.2 mm diam  150 nH
Impedance @ 1Ghz ~ 1 kOhm
Simulation results of the amplifiers with
THS 3201 ultra-fast current feedback amplifier
1 pF ~ impedance
@ 1GHz ~ 150 Ohm
Gain ~10
(th. 20)
Gain ~7
(th. 10)
Current Feedback Amplifier
THS-3201 Main features:
tr ~ 1.2 ns
(10 to 90%)
- 1.8 GHz;
- 6700 V/µs @ G= 2V/V;
RL =100 Ohm
- 18mA @ +/- 3.3V
(120mW  vacuum)
Simulation results of the amplifiers with
THS 3201 ultra-fast current feedback amplifier
Wire impedance  skin effect
(i.e. skin depth calculator)
R0 = 1 /πro2 σ
(DC & low frequencies)
- σ bulk conductivity
- r0 wire radius
L0 = μ / 8π
- μ permeability
(μ0 = 4π.10-7 Henry/Meter)
Rs = 1/ (σδ);
q = √2 r0 / δ
δ is the “skin depth”  (πfμσ)-1/2
* - this “calculator” only cover the range q < 8
Which roughly correspond to r0/δ < 5 … above
this value the Bessel functions become hard to evaluate…
* to remember about skin effect:
- Material dependence
(e.g. Ni vs. Cu ~ skin effect
depth one order of magnitude)
- Frequency dependence
Time walk
Time walk for a fixed trigger
level  time stamp (time of
threshold crossing) depends
on pulse amplitude
Accuracy of timing
measurement is limited
by two factors:
- time jitter ( ~ to the slope/noise)
- time walk *)
(due to dependency on signal
amplitude and rise time variations)
*) - if the rise time is known and constant,
the “time walk” can be compensated in
software event-by-event by measuring the
pulse height and correcting the timing
- if rise time vary (e.g. HP-Ge Det.) this
technique fails!  PSA required
Hardware: - threshold lowest practical level (i.e. > noise)
or compensation technique (e.g. CFD)
LE – method
• timing occurrence function of:
- amplitude
- rise time
- noise
Time Walk in LE discriminator due to:
- amplitude and rise time variations
- charge sensitivity
Time jitter in LE discriminator due to:
- noise on the Input Signal
- pulse high variation
going from LE to CFD
Constant Fraction Timing
+
--
Ideal
comparator
• Implementing an “active threshold”, namely
the threshold is derived from the signal passing
it through an attenuator Vt = f Vs ; (f < 1)
• The signal applied to the comparator input  the
transition occurs after the threshold signal reached
Its maximum value: VT = f V0
tr
The circuit compensates for amplitudes and
rise time if pulses have a sufficiently large
range that extrapolates to the same origin
delayed input
signal
attenuated input
signal
Timing occurrence at the output
• The condition for the delay must
be met for the minimum rise time
and in this mode the fractional
threshold VT / V0 varies with rise time
For all amplitudes and rise times the
compensation range the comparator
fires at the time  time stamp
t
Another view of CFD, namely
the CFD can be analyzed as a
special pulse shaper
Op.
Amp
+/- 1.
Pulse Shaper, comprising the
- delay (td)
- attenuator (f)
- subtraction
followed by a zero cross trigger
The new timing jitter
depends on:
- the slope at the zerocrossing (depends on
choice of f and td)
- the noise at the output
of the shaper (which
increases the noise
bandwidth)
Signal formation in a CFD & ARC-CFD
Ortec AN 42 – Principles and Applications
of Timing Spectroscopy
T.J. Paulus - Timing Electronics and Fast Timing Methods with
scintillation detectors; IEEE Trans. NS NS-32, (1985), 1242
Constant-Fraction Discrimination
for TFC Bipolar Signals
vs.
Constant-Fraction Discrimination
for or ARC Timing
T.J. Paulus, Timing Electronics and Fast Timing Methods
with Scintillation Detectors, EG&G Ortec,
IEEE Trans. on NS, Vol.NS-32; No-3 (1985), 1242
r.m.s. value of the input noise
CFD attenuation factor
mean-square value of the input noise
autocorrelation function of the input noise
CFD shaping delay
-for uncorrelated noise / signals:
Timing uncertainty due to noiseinduced jitter for TFF timing
signal
noise
For ARC timing with linear input
signal the slope of the CFD signal
at zero crossing is
Combining former equations, we get
the expressions for noise-induced jitter
with linear input signals:
- for TCF timing
- for ARC timing
CFD a realistic approximation
In the case of MCP real signals
i.e. non-linear rise times
The development of MSCD method for
picosecond lifetime measurement.
J.-M. Regis- PhD work 2010
Mirror Sensitive Centroid Method
• the prompt curve determination
 energy dependent walk in CFD
• the prompt curve has to be
calibrated for each branch but the
timing asymmetry in the branch timing
characteristic is canceled when a new
physical quantity is defined, namely
the Centroid Difference:
(a) CFPHT
(b) ARC-Timing
M.A. El-Wahab et al, CFT with scintillation detectors,
IEEE Trans. On NS, Vol.36, No.1,(1989) 401-406
CFD
ARCCFD (a)
ARCCFD (b)
• Variation of resolving time (W*1/2) with
the attenuation factor for three cases of
CFD timing:
(1) - CFPHT, ~equivalent to LE timing
(2) - ARC timing where ts =tr
td and tm from numerical solution
(3) – ARC timing where F(tm) =A;
F(tm-td)=A² and ts calculated
from Eq.10
Attenuation factor A
• Variation of resolving time (W*1/2) with
attenuation factor for different delay times
LE
CFD
Attenuation factor A
M.A. El-Wahab et al, CFT with scintillation detectors,
IEEE Trans. On NS, Vol.36, No.1,(1989) 401-406
LE
Walk
CFD
Ortec 583B , Ortec 584, Ortec 935, ESN-4000
Different design for walk adjustment, i.e. “monitor-inspect out.”
Anode
Anode
Dynode
Dynode
Ortec 113
Preamplifier
Ortec 113
Preamplifier
T1
Ortec 572
Filter Amplifier
E1
T2
E2
Ortec 572
Filter Amplifier
• Anode vs Dynode
as timing signal is still
an open dispute  ?!
Typical Fast / Slow Timing system for gamma-gamma coincidence
measurements with scintillators and photomultiplier tubes
Timing MCA
a)
Classical approach TPHC (TAC) – ADC
b)
TDC
- direct Time-Digitizer (TDC)
- Time - Expansion (Time-to-Charge)
- direct Digital Interpolation TDC
Principle of TPHC (TAC)
 ADC (13-14 bit)
Dead Time 1-4 µs
CC interface
Principle of Direct Time Digitizer
Time expanding (multihit) TDC
Principle of Interpolating in Direct Time Digitizers
Measurement of:
5.0 mm
5.1 mm
5.5 mm
Waveform diagram “vernier like scale”- TDC
An interpolating Time-to-Digital converter implemented on an FPGA structure