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 particleand hence the ratio E/M. If a separate E measurement is made, the massof 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:

Mass resolution as a function of E/M for various time resolution ( t) in TOF

e.g. we see that 16OIons with E/M of 6 MeV/amu require atiming resolution of ~9 ps/cm of flight pathif they are to be resolvedfrom other isotopes of mass 17O ( ~ 6 %) i.e. for 1% required timing resolution ~1.5-2 ps/cm !!

Timing vs. flight path

• 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.

• 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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3keV electron speed 3.24 cm/ns

5.5 MeV alpha particle speed 1.63 cm/ns

from time dilatation

L

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)

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)

TOF - Thin Foil

[ E.J. Sternglass, Phys. Rev. 90 (1953),380 & H.P. Garnir et al, Nucl. Instr. Meth. 202 (1982), 182-192 ]

Total SEE yield as a function of the target’s tilt angleTotal SEE yield as a function of the targettemperature for Ar+ ions at different energies

( 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)

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

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.

Sketch of a detector using electron emission from a carbon foil into a channel multiplier.

- Left a fast timing detector - Right a system using a position-sensitive using a chevron MCP. detector to provide an image of the emission pattern from 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

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Energy

TOF spectrum of:

fission fragments from 252 Cf

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Alpha particles from 252Cf

Carbon foil:

10-100 µg/cm2

Other foils: - Al - Au - magnesium-oxide coated C(emissive power is5-6 time > as C butdue inhomogeneityof the MgO layerstraggling of theTOF pulses up to150-200ps)

~ 117ps

~ 87.5ps

J. Girard, M. Bolore, Saclay, NIM 140 (1977), 279-282

Comparison of emissive power for different elements and different carbon thickness

Schematic diagram: voltage distribution and distances foil / grid / MCP

ToF spectrum of: - fission fragment (~ 87.5ps)-Alpha particle ( ~117 ps)

J. Girard, M. Bolore, Saclay, NIM 140 (1977), 279-282

D. Shapira et al. / Factors affecting the performance of SED, thin foil… Nucl. Instr. Meth. A 449 (2000) 396-407

3-4 mm

3-4 mm

3-4 mm

45 mm

45 mm

Foils:C ~ 30µg/cm2

Mylar ~ 290 µg/cm2 Foil at 30° (relative to the beam direction )

( 2x to balance theelectrostatic forces)

(position sensitive read-out)

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 …

Measured TOF spectra at different accelerating voltages

(calibration peak at 0, 10, 25, 35, 50 ns, respectively)

• 5.8 MeV alpha particle and • 30 MeV 16O passing 2µm thick,• Mylar foil (~500µg/cm2), tilted 30° *Mylar PET (PolyethyleneTerephthalate)

( set ofbatteries

* SE kinetic energy, i.e. velocity distribution much higher spectrum as expected

Secondary electrons path ion path

W. Starezecki at al./ LNL-Padova NIM 193 (1982) 499-505

Cu-Be 20µm diam. @ 1mm 98% transparence

1cm

see Michael PfeifferSIMION simulations

(a)

(b)

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)

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

C-foil

10-20 µg/cm2

+ ~3.5 µg/cm2

• LiF evaporated onto the C-foil to enhance theSE emission

~157 ps

~280 ps

W. Starezecki at al./ LNL-Padova NIM 193 (1982) 499-505

- from foil to mirror front side (field free drift) region- inside electrostatic mirror (~homogeneous field ??)- electrostatic mirror out and MCP in (-out ?)

d ~ 270 mmFoil 1-to-Foil 2

Apparent (intrinsic) MCP detectors resolution ~ 170 ps

K. Kosev et al, FZ Dresden-Rosendorf NIM A 594 (2008) 178-183

Simulation – SIMEON 3D( http://www.simion.com )

40Cl ~ 40 MeV

Alpha-particle ~ 5.8MeV

SEEyield

Michael Pfeiffer

SIMION3D simulation for TOF, BPM for the HISPEC-DESPEC @ FAIR

239Pu 241Am 244Cm

Counts

5.155 5.486 5.804 [MeV]

Cou

nts

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!)

244 C

m 2

41A

m 23

9 Pu

( ~200 keV energy loss )

Tim

e of

Flig

ht

[ns

]

Heavy Ion Magnetic Spectrometer @ PRISMA (LNL-Padova )

• 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!)

beam test 40Ca

~ 400 ps

~ 350 ps

Lab. test alpha-particles

G. Montagnoli et al. NIM A 547 (2005) 455-463

Fragment Separator (~72m)

SchwerIonenSynchrotron~216m

ExperimentaStorageRing(~108m)

LAND

(Pygmy Dipole Resonance)

Michael PfeifferMario Cappellazzo

GSI- FAIRHISPEC- DESPEC

Beam detectors

The TOF detector subject is worldwide still a hot business at the present and for future, not only @ AMS but also @ FAIR, CERN, RIA …