Time of Flight (ToF): basics
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• 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
- SE detection ( mainly MCP – some basic set-up )
• Fast electronics
- Fast preamplifiers and discriminators LE; CFD; ARC-CFD
- Time walk and jitter –basic consideration
TOF – General Considerations
• A Time-of-Flight (TOF) measurement determines the velocity of a particle
and hence the ratio E/M. If a separate E measurement is made, the mass
of a particle can be determined .
• Sometimes this is adequate identification, sometimes is used to subtract
the background of other components.
• When combined with a dE/E identifier, which determines both E and
qeff.2, TOF provides a complete determination of M, E and qeff.2
dE/E  -dE/dx = (aZ2c2/v2) x In [bv2/(c2-v2 )]  (Bethe-Bloch equation),
the rate of energy loss actually depends on the rms charge state qeff of the
moving ion, which may not be fully stripped of atomic electrons (i,e, qeff < Z),
• Unfortunately, the basic simplicity of TOF methods is not matched by the
hardware required to achieve the required timing performance.
Either ultra-fast timing or long flight path necessarily involve serious
problems, namely: - timing resolution and/or - poor collection geometry.
The velocity of an ion as function of E/M 
Ions velocity vs. E/M ratio:
• If small statistical fluctuations dE, dt and dd occur in measuring E, t and d
the resulting fluctuation (dM) in mass determination is given by:
• Where dE/E is much less than 1% in most experiments, and dd/d is usually
very small. Therefore dt/t is commonly the most important measurement
error. In this case, we have:
• While dE/E is much less than 1% in most experiments, and dd/d is usually
very small  dt/t is commonly the most important measurement error.
Timing vs. flight path
Mass resolution as a function of E/M
for various time resolution ( t) in TOF
e.g. we see that 16O
Ions with E/M of
6 MeV/amu require a
timing resolution of ~
9 ps/cm of flight path
if they are to be resolved
from other isotopes of
mass 17O (
~ 6 %)
 i.e. for 1% required timing
resolution ~1.5-2 ps/cm !!
• Combining TOF measurement with a dE/E particle identification (MZ2)  a very useful
two-dimensional result that is more tolerant of fluctuations in both the mass and MZ2
determinations than in a single-parameter experiment.
• Allowing reasonable spreads both in particle identifier output and mass determination
(due to timing errors) and assuming that particles have E ~ 100MeV  well separated
regions. (* Note that: 9C and 14B and 15B depends on mass identifying capability of TOF)
(MZ2)
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L
from time
dilatation
3keV electron speed
3.24 cm/ns
5.5 MeV alpha particle speed
1.63 cm/ns
Start and Stop Transparent Detectors
• Thin Foils (conductive and/or coated metallized)
some non-transparent (or only cvasi-transparent)
but thin dE detectors:
• Diamond
• Si
fast and ultra-fast scintillators
• Scintillators ( organic; inorganic)
TOF - Thin Foil
The SEE (Secondary Electron Emission) yield from a foil depends on:
- the type of foil and the work function at its surface,
- the type, velocity and angle of incidence of the incident ion.
• Although low work-function surface coatings increase the electron
yield, difficulties in preserving these surfaces have led to the use
of uncoated plastic or carbon foils as thin as about 10 µg /cm2
if no conductive then metallized (~ 20 to 30 nm)
• Electron yields from these foils range from ~ 10 for natural α particles
to about 100 for fission fragments. (Much of the early work using
these foils was in the field of fission studies, but it has recently
been extended to nuclear reaction product analysis & beam detectors).
• Several types of electron detectors have been used to detect the
secondary electrons. Early systems employed scintillation or
semiconductor detectors. These are insensitive to electrons having
energies below ~ 5-10 keV, so secondary electrons from the foil must
be accelerated to this potential. However, positive ions in the
detector region are also accelerated, striking surfaces in the vicinity
and releasing electrons that are then attracted to the detector to
produce spurious signals  Very careful design is therefore
required to avoid very high background counting rates with this
type of detector (see Shapira)
The total yield of Secondary Electrons emitted when ions pass through a
thin carbon foil  function of beam current and tilt angle of the target
- Sternglass theory and latter the Modified-Sternglass theory, namely a
two steps mechanism:
- formation of internal secondary electrons in material as a result of
excitation and ionization processes
- SEE (secondary electron emission) - escape from the target
( the beam current is interpreted as a
temperature effect, i.e. the rise of temperature 
increased vibrations of the atoms  mean free
path of electrons is shortened  yield decreases)
Total SEE yield as a function of the target
temperature for Ar+ ions at different energies
[ E.J. Sternglass, Phys. Rev. 90 (1953),380
Total SEE yield as a function of the target’s tilt angle
& H.P. Garnir et al, Nucl. Instr. Meth. 202 (1982), 182-192 ]
A better situation prevails when the secondary electrons from the foil are
accelerated directly onto an electron multiplier structure.
Only a low accelerating voltage of less then ~1 kV is required to
achieve the full secondary emission ratio from the initial multiplying
stage, and background is a much less serious problem.
• The first work using open electron-multiplier structures employed a
multiplier with Cu-Be-(BeO) dynodes (e.g. 56P17-2) that exhibit a time
spread near to ~1 ns. Besides this limitation, an open-ended electron
multipliers are sensitive to contamination that degrades their gain,
and are affected by magnetic fields.
• Latter on, the channel electron multipliers have been employed.
Time resolutions in the 400 - 700 ps range were obtained using these
devices, which are also much less sensitive to contamination than the
conventional multiplier surfaces.
• Finally, micro channel plates (MCP) have now been used as the
electron detector in particle-timing experiments. These plates, about
0.3-1 mm thick and up to 7-8 inches in diameter, contain closely
spaced micro-channels of only 5-50 μm in diameter in which electron
multiplication occurs.
Because of the short distance traveled by the electron cloud advancing
down a channel, only a very small time spread ~100 ps is introduced
by the electron-multiplying process. Furthermore, the plates are
rugged and rather insensitive to contamination problems.
• The electron gain of a single channel plate is limited by ion feedback
effects to about 104, but higher gains are realized by using two plates
in series, one with holes biased at a small angle. This chevron plate
provides an electron gain of 107, with a time spread below ~100 ps.
• Channel plates actually image the impact point of electrons on their
front face  if parallel field geometry is retained in the acceleration
structure from the foil to the multiplier, the position of bursts of
electrons emitted from the multiplier output side directly reflects the
point of passage of the detected particle through the foil.
Accelerator grid
Channel plate
Channel plate
Post accelerator grid
Coaxial anode
Position sensitive
detector
• timing spread below
~150 ps for particles
passing through a
10 μg / cm2 C foil
Sketch of a detector using electron emission from a carbon foil into a
channel multiplier.
- Left a fast timing detector
using a chevron MCP.
- Right a system using a position-sensitive
detector to provide an image of the
emission pattern from the foil.
Carbon foil:
10-100 µg/cm2
Other foils:
- Al
- Au
- magnesiumoxide coated C
Stop
Start
Energy
TOF spectrum of:
~ 87.5ps
 fission fragments
from 252 Cf
------------------------------------
Alpha particles
from 252Cf 
~ 117ps
J. Girard, M. Bolore, Saclay,
NIM 140 (1977), 279-282
(emissive power is
5-6 time > as C but
due inhomogeneity
of the MgO layer
straggling of the
TOF pulses up to
150-200ps)
ToF spectrum of:
- fission fragment
(~ 87.5ps)
-Alpha particle
( ~117 ps)
Schematic diagram: voltage distribution
and distances foil / grid / MCP
J. Girard, M. Bolore, Saclay,
NIM 140 (1977), 279-282
Comparison of emissive power for different
elements and different carbon thickness
(position sensitive
read-out)
( 2x to balance the
electrostatic forces)
Foils:
C ~ 30µg/cm2
Mylar ~ 290 µg/cm2
3-4 mm
3-4 mm
3-4 mm
Foil at 30°
(relative to the
beam direction )
D. Shapira et al. / Factors affecting the performance of SED, thin foil…
Nucl. Instr. Meth. A 449 (2000) 396-407
Secondary electron emission is a surface phenomenon  thin foil can be deployed
• Some foils as C, Ni and Al can sustain high radiation doses with minimum damage
- most common SEE materials: BeO; MgO; GaP, GaAsP, PbO, CsI etc.
• Foil inclined 30° or 45°, with symmetric arrangement of accelerating grids balances the
electrostatic forces that could otherwise deform the foil
• Second accelerating grids in front of the MCP (!?)
• Multiple scattering of the ions passing through the foil (code SRIM /TRIM)
(http://www.research.ibm.com/ion/beams/#SRIM)
• Secondary electron transport from the vicinity of the foil to the front of the MCPs 
- second accelerating grid (?) and electrostatic shielding (as we did in the IKP-ES mirror)
 influence on position resolution …
( set of
batteries
• 5.8 MeV alpha particle and
• 30 MeV 16O passing 2µm thick,
• Mylar foil (~500µg/cm2), tilted 30°
*Mylar  PET (PolyethyleneTerephthalate)
Measured TOF spectra at
different accelerating voltages
* SE kinetic energy, i.e. velocity
(calibration peak at 0, 10, 25, 35, 50 ns, respectively) distribution  much higher
spectrum as expected
Secondary
electrons path
ion path
1cm
Cu-Be 20µm diam. @ 1mm
 98% transparence
see Michael Pfeiffer
SIMION simulations
W. Starezecki at al./ LNL-Padova
NIM 193 (1982) 499-505
Experimental set-up to determine the time spread of
an electrostatic mirror.
The electrons emitted from both side of the C foil are accelerated
by a harp and directed to the MCPs by bending through:
- a mirror (a), or - directly (b)
~280 ps
(a)
C-foil
10-20 µg/cm2
+ ~3.5 µg/cm2
• LiF evaporated
onto the C-foil to
enhance the
SE emission
(b)
Time of flight spectrum of:
~6 MeV α particle; both start & stop from MCP detectors
- 213 MeV 58Ni elastically scattered at 4 °
from a 20 μg/cm(*2) 12C foil-target
W. Starezecki at al./ LNL-Padova
NIM 193 (1982) 499-505
~157 ps
Apparent (intrinsic) MCP detectors
resolution ~ 170 ps
d ~ 270 mm
Foil 1-to-Foil 2
Simulation – SIMEON 3D
( http://www.simion.com )
40Cl ~ 40 MeV
- from foil to mirror front side (field free drift) region
- inside electrostatic mirror (~homogeneous field ??)
- electrostatic mirror out and MCP in (-out ?)
SEE
yield
Alpha-particle ~ 5.8MeV
K. Kosev et al, FZ Dresden-Rosendorf
NIM A 594 (2008) 178-183
Michael Pfeiffer
SIMION3D simulation
for TOF, BPM for the
HISPEC-DESPEC @ FAIR
244Cm 241Am 239Pu
[ns]
Time of Flight
( ~200 keV energy loss )
Counts
- 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
IKP - TOF & BPM Preliminary results
239Pu
241Am
244Cm
5.155
5.486
5.804
[MeV]
Heavy Ion Magnetic Spectrometer @ PRISMA (LNL-Padova )
Lab. test
alphaparticles
~ 350 ps
beam test
40Ca
• Large MCP - 80x100 mm (as @IKP-FAIR)
• C-foil ~20µg/cm*2
• Grids at 4mm and only 300eV (see Shapira et al.)
(20µm gold-plated Tungsten@ 1mm)
• SE drift path ~ 10 cm
• External Magnetic field ~120 Gauss
(important for position resolution!)
~ 400 ps
G. Montagnoli et al. NIM A 547 (2005) 455-463
(Pygmy Dipole Resonance)
SchwerIonen
Synchrotron
~216m
Fragment
Separator
(~72m)
Experimenta
Storage
Ring
(~108m)
LAND
The TOF detector subject is worldwide still a hot business at the
present and for future, not only @ AMS but also @ FAIR, CERN, RIA …
GSI- FAIR
HISPEC- DESPEC
Beam detectors
Michael Pfeiffer
Mario Cappellazzo