Monte Carlo simulation of liquid scintillation neutron detectors: BC501 vs. BC537
The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… ·...
Transcript of The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… ·...
![Page 1: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/1.jpg)
Ralf Averbeck
ExtreMe Matter Institute EMMI and Research Division
GSI Helmholtzzentrum für Schwerionenforschung
Darmstadt, Germany
Summer Term 2017
Ruprecht-Karls-University, Heidelberg
The Physics of
Particle Detectors
![Page 2: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/2.jpg)
Ralf Averbeck
ExtreMe Matter Institute EMMI and Research Division
GSI Helmholtzzentrum für Schwerionenforschung
Darmstadt, Germany
Summer Term 2017
Ruprecht-Karls-University, Heidelberg
Scintillation
Detectors
![Page 3: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/3.jpg)
R.Averbeck, 3 Heidelberg, 21.06.2017
Scintillation detectors in general principle of scintillation detector
dE/dx converted into visible light
detection via photo sensor (photomultiplier, human eye)
detect radiation via scintillation
one of the oldest methods of particle detection
example: particle hits ZnS screen flash of light
main features
sensitivity to energy deposit
fast time response
pulse shape discrimination PID
![Page 4: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/4.jpg)
R.Averbeck, 4 Heidelberg, 21.06.2017
Scintillation detectors in general requirements
high efficiency for conversion of deposited energy to
fluorescent radiation
transparency to its fluorescent radiation to allow the
transmission of light
emission of light in a spectral range detectable for
photosensors
short decay time to allow fast response
scintillating materials
inorganic crystals
organic crystals
polymers (plastic scintillators) TAPS BaF2 crystals
![Page 5: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/5.jpg)
R.Averbeck, 5 Heidelberg, 21.06.2017
Basic Detector Setup
Particle
Output
signal topics
scintillators
propagation of light
photon detection
applications
![Page 6: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/6.jpg)
R.Averbeck, 6 Heidelberg, 21.06.2017
Inorganic crystals
PbWO4 crystals from
the CMS electromagnetic
calorimeter
![Page 7: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/7.jpg)
R.Averbeck, 7 Heidelberg, 21.06.2017
Inorganic crystals crystals (electric insulators)
with impurities (sometimes
introduced via doping,
e.g. NaI(Tl))
sodium iodide (NaI)
cesium iodide (CsI)
barium fluoride (BaF2)
lead tungstate (PbWO4)
working principle
energy deposit via ionization
promote electrons into higher bands
energy transfer to impurities
emission of scinillation photons
![Page 8: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/8.jpg)
R.Averbeck, 8 Heidelberg, 21.06.2017
Inorganic crystals: time constants time constants
fast: recombination from activation centers [ns - ms]
slow: recombination from traps [ms – s]
![Page 9: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/9.jpg)
R.Averbeck, 9 Heidelberg, 21.06.2017
Inorganic crystals: light output
scintillation spectra of
undoped and doped
CsI crystals
strong temperature
dependence of light
output (in contrast to
organic scintillators)
![Page 10: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/10.jpg)
R.Averbeck, 10 Heidelberg, 21.06.2017
Light output & PMT sensitivity spectral sensitivities of PMTs
compared to emission spectra
![Page 11: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/11.jpg)
R.Averbeck, 11 Heidelberg, 21.06.2017
Scintillation in (noble) gases many gases exhibit some degree of scintillation
response to 4.7 MeV a particles
careful in Cherenkov detectors!
cryogenic (liquid) noble
gas scintillators
fast and slow decay time
constants PID
![Page 12: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/12.jpg)
R.Averbeck, 12 Heidelberg, 21.06.2017
Inorganic scintillator properties
![Page 13: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/13.jpg)
R.Averbeck, 13 Heidelberg, 21.06.2017
Inorganic scintillator properties a numerical example
NaI(TI):
PbWO4:
light yield eSC: fraction of energy loss going into photons
NaI(TI):
PbWO4:
![Page 14: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/14.jpg)
R.Averbeck, 14 Heidelberg, 21.06.2017
Application in large experiments BaBar at SLAC
6580 CsI(TI) crystals
depth: 17 X0
volume: 5.9 m3
read-out: Si-photodiode (gain = 1)
read-out noise: 0.15 MeV
dynamic range: 104
CMS at LHC 76150 PbWO4 crystals
depth: 26 X0
volume: 11 m3
read-out: APD (gain = 50)
read-out noise: 30 MeV
dynamic range: 105
![Page 15: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/15.jpg)
R.Averbeck, 15 Heidelberg, 21.06.2017
Organic scintillators aromatic hydrocarbon compounds
naphtalene: C10H8
antracene: C14H10
scintillation
based on electrons of
the C=C bond
light arises from delocalized electrons in p orbitals
transitions of ‘free’ electrons
![Page 16: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/16.jpg)
R.Averbeck, 16 Heidelberg, 21.06.2017
Organic scintillators principle of operation of organic scintillators
valence electrons
pair wise in p states
molecular level scheme
singlet states
triplet states
energy absorption
excitation of p electrons
fluorescence: S1 S0
(in UV range wavelength shifters!)
ionization followed by recombination populates T states
phosphorescence: T S0
excitation of d electrons thermal deexcitation
other ionization radiation damage!
![Page 17: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/17.jpg)
R.Averbeck, 17 Heidelberg, 21.06.2017
Organic scintillators material should be
transparent for radiation
shift of emission and
absorption spectra
Stokes shift due to Franck-
Condon principle
excitation into higher
vibrational states on
a time scale of 10-14 s
vibrational time scale: 10-12 s
deexcitation from lowest
vibrational state
(S1 lifetime: 10-8 s)
![Page 18: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/18.jpg)
R.Averbeck, 18 Heidelberg, 21.06.2017
Plastic or liquid scintillators
organic scintillator material
in practice
solution of organic scintillators
(solved in plastic or liquid)
+ large concentration
of primary ‘fluor’
+ smaller concentration of
secondary ‘fluor’ (wavelength
shifter)
![Page 19: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/19.jpg)
R.Averbeck, 19 Heidelberg, 21.06.2017
Plastic or liquid scintillators
![Page 20: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/20.jpg)
R.Averbeck, 20 Heidelberg, 21.06.2017
Plastic or liquid scintillators
![Page 21: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/21.jpg)
R.Averbeck, 21 Heidelberg, 21.06.2017
Wavelength shifting
working principle
absorption of primary
scintillation light
re-emission at longer
wavelength
adapts light to
spectral sensitivity
of photo sensor
requirement:
good transparency
for emitted light
![Page 22: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/22.jpg)
R.Averbeck, 22 Heidelberg, 21.06.2017
Organic scintillator properties
![Page 23: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/23.jpg)
R.Averbeck, 23 Heidelberg, 21.06.2017
Organic scintillator properties
light yield
without quenching
with quenching (non-
linear response due to
saturation of available
states): Birk’s law
where kB is a
parameter that needs
to be measured
![Page 24: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/24.jpg)
R.Averbeck, 24 Heidelberg, 21.06.2017
Scintillators: comparison
![Page 25: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/25.jpg)
R.Averbeck, 25 Heidelberg, 21.06.2017
Propagation of light in the scintillator itself
absorption
reflection at the edge, total reflection for q > qB = sin-1(ne/ns)
typical scintillator: ns~1.4, qB ~ 45o
careful shaping needed in order
not to lose too much light
alternative: light collection
via 2nd wavelength shifter
along the edge of the
scintillator
![Page 26: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/26.jpg)
R.Averbeck, 26 Heidelberg, 21.06.2017
Light guides scintillator light to be guided to the photo sensor
light guide
(plexiglas, optical fibers)
light transfer by total
internal reflection (maybe
wavelength shifter in addition)
Liouville’s theorem
Dx Dqx = const. keep area
constant since divergence is
constant when guiding light
use small curvatures to
maintain total reflection
for photons captured once
adiabatic light guides (e.g. ‘fish tail’)
![Page 27: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/27.jpg)
R.Averbeck, 27 Heidelberg, 21.06.2017
Photon detection goal: convert light into detectable electronic signal
principle: use photo-electric effect to convert
photons into photo-electrons (p.e.)
what is needed?
high photon detection efficiency (PDE) or
quantum efficiency: QE = Np.e./Nphotons
(some) available devices
photomultipliers (PMT)
silicon photomultipliers (SiPM)
micro channel plates (MCP)
photo diodes (PD)
….
![Page 28: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/28.jpg)
R.Averbeck, 28 Heidelberg, 21.06.2017
Photomultipliers (PMT)
principle:
electron emission from
photo cathode
secondary emission from
dynodes (gain: 3-50)
typical PMT gain: > 106
PMTs can count single photons
![Page 29: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/29.jpg)
R.Averbeck, 29 Heidelberg, 21.06.2017
PMTs: photocathode g conversion via
photo effect
3-step process electron generation via
ionization
propagation through
cathode
escape of electron into
vacuum
Q.E. ~10-30% (with specifically developed alloys)
Bialkali: SbRbCs, SbK2Cs
![Page 30: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/30.jpg)
R.Averbeck, 30 Heidelberg, 21.06.2017
PMTs: dynode chain
multiplication process
e- accelerated towards dynode
further e- produced avalanche
secondary emission coeff.
d = #(e-produced)/ #(e-
incoming)
typical values
d = 2-10; n = 8-15
gain G = dn = 106 – 108
gain fluctuations
d = kUD; G = a0(kUD)n
dG/G = n dUD/UD = n dUB/UB
![Page 31: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/31.jpg)
R.Averbeck, 31 Heidelberg, 21.06.2017
PMTs: energy resolution energy resolution is influenced by
linearity of PMT: high dynode currents can lead to saturation by
space charge effects; IA a ng possible over 3 orders of magnitude
photo electron statistics as given by Poisson statistics:
secondary electron fluctuations:
![Page 32: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/32.jpg)
R.Averbeck, 32 Heidelberg, 21.06.2017
PMTs: energy resolution
![Page 33: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/33.jpg)
R.Averbeck, 33 Heidelberg, 21.06.2017
PMTs: dynode chain optimization of
PMT gain
anode isolation
linearity
transit time
PMTs in general very
sensitive to magnetic
fields
shielding is even required
for earth field (30-60 mT)
![Page 34: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/34.jpg)
R.Averbeck, 34 Heidelberg, 21.06.2017
PMT in magnetic field B field disturbs focusing of photo electrons and
secondary electrons
(typical kinetic energies ≤ 200 eV)
requirement in region of dynodes: B ≤ 10-4 T
typical effect: current decreases by factor 2 if B
field increases from 0 to 0.15 x 10-4 T
possible solutions
shield small field by so-called m-metal
use of ‘mesh type’ dynodes
use of micro channel plate or photo diodes
![Page 35: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/35.jpg)
R.Averbeck, 35 Heidelberg, 21.06.2017
Micro Channel Plate
![Page 36: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/36.jpg)
R.Averbeck, 36 Heidelberg, 21.06.2017
Photodiodes photo diode: gain = 1 (see Chapter
on semiconductor detectors)
avalanche photo diode silicon PMT
pixelized photo diodes operated
in Geiger mode
energy: #photons seen in all pixels
granularity: 103 pixels/mm2
gain: 106
bias voltage: < 100 V
efficiency: ~30 %
insensitive to magnetic fields
works at room temperature
![Page 37: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/37.jpg)
R.Averbeck, 37 Heidelberg, 21.06.2017
Silicon photodiodes
![Page 38: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/38.jpg)
R.Averbeck, 38 Heidelberg, 21.06.2017
Applications fast signals time of flight (TOF) measurements
energy measurements
crystal calorimeter
precise electron/photon energy
sampling calorimeter: alternating layers of
absorber (Fe, W, U) and scintillator with
wavelength shifters and PMTs
electromagnetic & hadronic calorimetry
fast tracking/vertexing
scintillating fibre hodoscopes
![Page 39: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/39.jpg)
R.Averbeck, 39 Heidelberg, 21.06.2017
v
xT 0 v
xLT
0
L
L – x x
PMT1 PMT2
(T0,x0)
T1 T2
2
/)( 21 vLTT
vTT
2
21
TOF
Y position
Cartoon
T1, T2 : Timing measured by PMT1,2
L : slat length
v : light velocity in scintillator
TOF (and position) with scintillators
![Page 40: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/40.jpg)
R.Averbeck, 40 Heidelberg, 21.06.2017
TOF example TOF measurement of
charged particles with
scintillators
example: PHENIX
experiment at RHIC
time resolution ~ 100 ps
pion/kaon separation up to p = 2.4 GeV/c
p
mp
c
L
c
Lt
22
1112
L
t
mp
![Page 41: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/41.jpg)
R.Averbeck, 41 Heidelberg, 21.06.2017
CMS: crystal calorimeter (ECAL)
![Page 42: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/42.jpg)
R.Averbeck, 42 Heidelberg, 21.06.2017
H1: spaghetti calorimeter
![Page 43: The Physics of Particle Detectors - Physikalisches Institutsma/teaching/ParticleDetector… · Scintillation detectors in general principle of scintillation detector dE/dx converted](https://reader034.fdocuments.net/reader034/viewer/2022042212/5eb5771982f19470392e276d/html5/thumbnails/43.jpg)
R.Averbeck, 43 Heidelberg, 21.06.2017
Scintillating fibre hodoscopes scintillating fibres: diameter ~1 mm (or less)
fast vertexing