Measure Tracks decay from heavy flavor mesons

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Measure Tracks decay from heavy flavor mesons
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Primary tracks
From D0 decays
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• Why semiconductor?

• Semiconductor with moderate bandgap (1.12 eV)
• Thermal energy 1/40 ev
• Little cooling required
• Energy to create e/h pair (signal quanta) 3.6
  eV c.f Argon gas 15 eV
• Scintillator 100-200ev
• High carrier yield
• Good energy resolution
• Fano factor for Si, F 0.1

conduction band
forbidden gap
Valence band
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• Disadvantages?

• Cost of Area covered
• Detector material could be cheap Standard Si
• Most cost in readout channels
• Material budget
• Radiation length can be significant
• Tracking due to multiple scattering
• Radiation damage
• Replace often or design very well

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• P-N Junction

• One of the crucial keys to solid state
  electronics is the nature of the P-N junction.
  When p-type and n-type materials are placed in
  contact with each other, the junction behaves
  very differently than either type of material
  alone. Specifically, current will flow readily in
  one direction (forward bias), creating the basic
  diode.
• Near the junction, electrons diffuse across to
  combine with holes, creating a "depletion
  region".

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Energy Loss in the Medium
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Different kind of Silicon detectors

• Charge coupled devices (CCD)
• Silicon pixel, strip detector ( a few hundreds of
  micros thick)
• Silicon drift detector
• CMOS APS detector Complementary
  metaloxidesemiconductor (CMOS) , active pixel
  sensor

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CMOS APS
Epitaxy is a kind of interface between a thin
film and a substrate. The term epitaxy (Greek
epi "above" and taxis "in ordered manner")
describes an ordered crystalline growth on a
monocrystalline substrate.
Can use the standard Integrate Circuit production
process for the production. Rely on charge
diffusion instead of drifting.
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• From Equation 1.4 and 1.6, the depletion width is
  about 2 µm under normal reset condition.
• the bulk of the p-epi region is free of electric
  field and the minority carriers diffuse rather
  than drift in this region

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RHIC STAR Experiment
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Input for the simulator

• For the charged particles The number of ionized
  electrons/hole pairs.
• Energy deposition in the detector volume.
• StMcTrack-gtdE().
• H. Matis for thin layer of material better use
  the Bichsel distribution,
• H. Bichsel, "Straggling in Thin Silicon
  Detectors," Review in Modern Physics, vol. 60,
  pp. 663, 1988
• GEANT has implement other models for the thin
  layers energy fluctuations under different
  limitation of applications
• Urban model
• 1 PAI model
• 2 ASHO model for 1
• Will use the Bichsel distribution as input since
  its tested with the experiment.
• For neutral particles including photons, the
  energy deposition in StMcTrack will be used to
  generate the number of ionizing electron/hole
  pairs unless we have other models for these
  particles.

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• For charged track,
• the ionized electron/hole pairs originate
  randomly alone the track.
• Number of ionized electrons is E/3.6eV where E is
  the energy deposition from the Bichsel
  distribution.
• For photons and neutral particles
• No good reference to my knowledge.
• Si detector should have very low efficiency for
  high energy gamma-ray due to pair production.
• Two possible ways to deal with it
• Will use the GEANT to get energy deposition from
  the gamma-ray or neutral particles and assume all
  ionized electrons are from the point where gamma
  and silicon interact. (not quite reasonable)
• Use Bischsel distribution if we the pair
  production vertex for high energy photons and the
  electron pair tracks (I think we should know).
• For low energy gamma-rays, use the energy
  deposition from GEANT.

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Boundary conditions questions fo hardware
experts.

• whats the gap between pWell and nWell?
• Are they fully depleted?
• If the gap is zero, what are the depletion
  thickness?
• Whats the thickness of p-epi layer? (14
  microns?)
• Whats the substrate thichness, i.e. 50um-p_epi?
• Whats the size of the p-well and n-well?
• Whats the shape? Is the cubic shape reasonable
  approximation?

N Well
p Well
p Well
p-epi layer
p substrate
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• when the p substrate electron hit p-epi/p
  substrate interface, the interface is totally
  transparent.

• When the electron fall into the depletion region
  between N-Well and P-Well or the N_well region,
  it will be fully collected into the readout
  electronics.
• Electrons in the p-well region will be neglected.

p Well

• When electrons hit the n-well/p-epi depletion
  region, has very little chance to be reflected
  but pass through. Consequently, the n-well/p-epi
  interface can be recognized as a boundary with
  total absorption
• When electron hits the p-epi and p-well
  interface, the p-well/p-epi interface can be
  recognized as a boundary with total reflection
  for electrons in the epitaxial silicon because
  pWell are more heavily doped and field in the
  depletion region will reflect the electron away.

• when the p-epi electron hit p-epi/p substrate,
  because p is more heavy doped, interface is
  recognized as a
• boundary with total reflection for electrons in
  the epitaxial silicon

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Question on interface between pixels.
Will the boundary be total transparent to the
electrons going across the pixel boundaries?
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• electron will recombine with the lattice during
  the diffusion and can not reach the electronics.
• We used 10 µs as the electron lifetime in the
  epitaxial silicon after going over a number of
  references 37, 43, 45, 46 . Plugging it into
  Equation 2.48, we obtain a recombination rate on
  the order of 10-7. As the precise lifetime
  depends on material properties that is only
  available through experimental measurement, these
  estimated values serves only as the starting
  point for simulation and they need to be refined
  by comparing the simulation with the measurements
  (see page 40 of the Shengdongs thesis).
• in the p-substrate region and p-well region,
  much higher doping density than p-epi and the
  quality is also lower, the electron lifetime in
  p-substrate is much shorter. Similar to the
  method in dealing with the epitaxial silicon, a
  lifetime of 10 ns is estimated for electrons in
  the bulk substrate and the corresponding
  recombination rate is on the order of 10-4.
  (see page 41 of shengdongs thesis).

p Well
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Other simulation details

• Diffusion simulation see page 17-page21 of
  Shendongs thesis.
• The time increment ?t used is on the order of
  10-12 s, similar to those reported in references
  22, 28, 42, 47, quite close to the average
  collision time in Drude model48. Using an
  electron diffusion coefficient (Dn) 35 cm2/s,
  the step size s from Equation 2.47 is about 80
  nm, once again close to the mean-free-path of
  electrons in silicon estimated by Drude model
  (page42)
• The charge recombination simulation in page 46
• Total integration time is 200us.
• This is the end of diffusion?
• If therere still electron diffusion after this
  time
• We will keep it and add it to the simulation for
  the next track hit the same pixel. This might
  have an impact for pileup
• But according to Shengdongs thesis, this effect
  is very small.
• A computationally convenient depth is usually
  selected as a total absorption boundary where
  electrons crossing it will not be counted anymore
  due to recombination. This topic will be treated
  systematically in Chapter 4 and Chapter 5.