Measure Tracks decay from heavy flavor mesons

Primary tracks

From D0 decays

kD 0mc 9.122

XJB /

mc 500400

• Semiconductor with moderate bandgap (1.12 eV)

• Thermal energy = 1/40 evo Little cooling required

• Energy to create e/h pair (signal quanta) = 3.6 eV3.6 eV c.f Argon gas = 15 eV15 eV– Scintillator: 100-200ev

o High carrier yield

• Good energy resolution• Fano factor: for Si, F = 0.1

Why semiconductor?Why semiconductor?

N

NF

Valence band

forbidden gap

conduction band

• Cost of Area coveredo Detector material could be cheap – Standard

Sio Most cost in readout channels

• Material budget o Radiation length can be significant

Tracking due to multiple scattering

• Radiation damageo Replace often or design very well

Disadvantages?Disadvantages?

P-N JunctionP-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".

Energy Loss in the Medium

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 metal–oxide–semiconductor (CMOS) , active pixel sensor

n

p+ p+ p+ p+ p+

p+ p+ p+ p+ p+

N

P

+++++++++++++++++++

---------------------------------

+++++++++++++++++++

---------------------------------

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.

• 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

RHIC STAR Experiment

STAR

Input for the simulator

• For the charged particles: The number of ionized electrons/hole pairs. – Energy deposition in the detector volume.

• StMcTrack->dE().– 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

• Urb´an model;• 1 PAI model;• 2 ASHO model for 1

– Will use the Bichsel distribution as input since it’s 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.

• 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.

Boundary conditions questions fo hardware experts.

• what’s the gap between pWell and nWell?

•Are they fully depleted?

•If the gap is zero, what are the depletion thickness?

•What’s the thickness of p-epi layer? (14 microns?)

•What’s the substrate thichness, i.e. 50um-p_epi?

•What’s the size of the p-well and n-well?

•What’s the shape? Is the cubic shape reasonable approximation?

N Wellp Well p Well

p-epi layer

p+ substrate

• 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.

p Well

• 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.

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

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

N Wellp Well

p-epi layer

p+ substrate

p Well

Question on interface between pixels.

N Wellp Well

p-epi layer

p+ substrate

p Well N Wellp Well

p-epi layer

p+ substrate

p Well

Will the boundary be total transparent to the electrons going across the pixel boundaries?

p Well

• 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 Shengdong’s 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 shengdong’s thesis).

Other simulation details• Diffusion simulation: see page 17-page21 of Shendong’s thesis.

– The time increment Δt 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 model[48]. Using an electron diffusion coefficient (Dn) ~ 35 cm2/s, the step size σ 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 there’re 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 Shengdong’s 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.