Time Projection Chamber Simulation

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Time Projection Chamber Simulation
Michael Sitwell
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Simulation of TPC
Add picture of TPC
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Simulation of TPC

• Track of incident particle
• Ionization of gas
• Energy loss
• Gas normally ionized in clusters
• Recombination of electrons and ions

dE/dx
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Simulation of TPC

• Drift of electrons produced in ionization to
  readout pad
• Diffusion in gas
• Multiplication of electrons
• Attachment in gas
• Avalanche area

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Simulation of TPC

• Readout pads and wire grids
• Number and layout
• Electronic response
• Induced signals

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Enriched Xenon Observatory

• Goal Measure the rate of neutrinoless double
  beta decay to find mass scale of neutrinos
• Need good energy resolution (lt1)

Add energy spectrum picture
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EXO

• No avalanche area or multiplication
• One large readout pad
• Measures analog signal when electrons are
  collected on readout pad
• Alpha source inside chamber
• 5.5 MeV alpha particles produced
• Electrons from ionization produced in a constant
  stream
• More electrons produced at end of track

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EXO
Ar-CH4 90-10 _at_ 2 atm
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Simulation for EXO

• Alpha source mounted on adjustable platform
• Drift length can be set in 1 cm increments from 5
  cm to 20 cm
• Currently filled with 90-10 Argon methane
• Will be replaced by Xenon

Add picture
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Geant4
Add X

• Complex geometry easy to construct
• Includes physics tables and particle interactions
• Easy event generation
• Continuously tracks all particles
• Including ions from ionization
• No physics tables for low energy ? particles
• Can't drift electrons to readout pad

Recombination
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Garfield

• Able to simulate the drift of electrons
• Used in conjunction with Geant4 simulation
• Can only simulate electron drift
• Is unaware of ions created by incident particle
• Problem for simulating recombination

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Geant4 GarfieldSimulation of alpha track21
electrons produced in ionization
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Geant4 Garfield

• Currently simulates drift of each individual
  electron produced in ionization
• Approximately ¼ million electrons produced by
  ionizing alpha particle
• Too many electrons for current simulation
• A 'cluster' class is needed as a higher level
  representation of electrons
• Requires a large portion of Geant4 to be
  rewritten

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Stand Alone Simulation

• Takes energy steps of 4400 eV
• Subtracts step from current energy
• Finds change in range the alpha particle
• ?R Range(Energy) Range(Energy step)
• Assumes ionization accounts for all energy loss

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Stand Alone Simulation

• ? within ?R of current position
• 4400 eV/step 22 eV/e- 200 e-/step
• are created

Add graph

• Electrons created with a flat probability
  distribution within ?R
• Step size is small enough that electron
  distribution does not change between runs

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Stand Alone Simulation
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Stand Alone SimulationAlpha particle track 10o
below horizontal
Electron Density
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Stand Alone Simulation

• e- pass wire grid induced signal on readout pad

• Integrate arrival times
• Convolute with function

f(t) t / Trise
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Stand Alone Simulation

• e- pass hits readout pad signal from
  electronics

g(t) exp(-t/tf) x ( 1-exp(-t/tr) )

• Convolute with function

Signal Decay
Rise More Gradual
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Stand Alone SimulationAlpha particle track
parallel to readout pad
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Stand Alone SimulationAlpha particle track
parallel to readout pad
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Stand Alone SimulationAlpha particle track
traveling towards readout pad
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Stand Alone SimulationAlpha particle track
traveling towards readout pad
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Stand Alone Simulation
rise times 0 550 ns 90 250 ns
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Comparison with Experiment

• A set of time signals in 1 increments from 0 to
  90 was generated
• Stored for comparison with experimental data
• A chi² fit was used to match an experimental time
  signal to a signal in the generated set

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Comparison with Experiment
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Comparison with Experiment
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Comparison with Experiment
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Comparison with Experiment
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Comparison with Experiment

• Many signals identified with angles lt 40 with a
  high degree of error
• Caused by high levels of noise
• Noise amplitude ? 7 of signal amplitude

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Comparison with Experiment
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Comparison with Experiment

• With signals identified with angles lt 40, the
  rise time of the signal was compared to the
  simulation's rise times to achieve a better
  estimate of its angle

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Comparison with Experiment
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Stand Alone Simulation

• Does not account for ions produced by alpha
  particle
• No attachment is seen
• Results in constant number of electrons produced

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2? mode a conventional 2nd order process
in nuclear physics
0? mode a hypothetical process can happen
only if M? ? 0 ? ?
Since helicity has to flip
Several new particles can take the place of the
virtual ? But 0?ßß decay always implies new
physics
Lepton-Photon 03
G. Gratta
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2??? spectrum (normalized to 1)
0??? peak (5 FWHM) (normalized to 10-2)
Summed electron energy in units of the kinematic
endpoint (Q)
from S.R. Elliott and P. Vogel,
Ann.Rev.Nucl.Part.Sci. 52 (2002) 115.
The only effective tool here is energy resolution
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