DRIFTIIa Simulations using GEANT4

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DRIFTIIa Simulations using GEANT4

• Chamkaur Ghag

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Geant4

• Geant4 is a toolkit for the simulation of the
  passage of particles through matter.
• Its application areas include high energy physics
  and nuclear experiments, medical, accelerator and
  space physics studies
• Using this toolkit, it is possible to simulate
  detectors, or designs of detectors that could be
  used in current or future Dark Matter searches.
• G4 allows a great deal of flexibility in detector
  and environment design, physics processes, event
  and run processing, tracking particles, etc
• For more info see the Geant4 homepage
  http//wwwasd.web.cern.ch/wwwasd/geant4/geant4.htm
  l

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The Physics

• The physics list asks for definitions of
  particles (eg bosons, leptons, ions, etc),
  processes (eg EM, Optical, Hadronic, neutron,
  etc) and models to be used from the various
  options provided by Geant4 (eg Low Energy, High
  Energy, Neutron High Precision Models, etc)
• Cuts on particles (individually or as groups)
  where interactions occurring within the defined
  cut-length are ignored, are also defined here
• For this simulation, most particles and processes
  are defined and appropriate models chosen based
  on previous/current Geant4 dark matter
  simulations (ZepII, ZepIII, ZepMax) by UKDM G4
  users
• Cuts changeable through messenger whereas models
  are hard-coded

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Geometry

• LAB
• - 30m x 26m x 26m (XYZ) rock-salt world
• - 30m x 4.5m x 6.5m cavern of air _at_ 1.2atm
• - 30m x 3.24m x 5.06m JIF lab suspended 45cm
  above the floor (81cm from roof), 72cm from walls
• Beneath some of the floor of the lab is CH2
  shielding (10m x 0.45m x 3.3m).

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Geometry

• VESSEL
• Steel, internal dimensions 1.5 m3, central in X,
  25cm above floor, 35cm from rear wall
• 7mm thick walls, 12.5mm thick door
• Filled with CS2 _at_ 40 Torr room temp
• Stands on three legs (24cm tall) each on 1cm
  thick steel plates
• Inside vessel sits a Steel skate-plate and
  Perspex base-plate

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Geometry

• Inner Vessel
• MWPC strong-backs (Perspex) with 30cm diameter
  hole, left right sides
• Separated by Perspex corner posts and central
  cathode support frame
• 100micron steel wire plane (512 wires, 2mm pitch)
  1cm from CC facing strong-back surface another
  orthogonal plane 3 cm from surface
• 1m3 volume CS2 gas between MWPCs is fiducial
  volume
• 62 hollow Copper field-rings (6mm diameter, 1mm
  thick) surround FV, held by 8 Perspex combs
• Perspex side-panels on all sides of FV
• 6mm thick HHV-Shielding surround field rings
• Further 12mm thick Perspex top bottom of the
  HHV shielding

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Sensitive Detectors

• Geant4 allows specific volumes to be allocated as
  sensitive to allow particle tracking
  interaction recording
• Allows tracking of every particle within these
  volumes individually, until they leave the volume
  or their energy drops below the cut value for
  that particle
• Certain values and details are recorded for not
  only the entire run, but also for each step (a
  pre-determined user-defined increment) within the
  run
• Sensitive Detectors are
• Steel Vessel
• MWPC Strong-backs (left right together)
• Grid wire planes (inner outer, and left right
  together)
• CS2 gas that is not part of the fiducial volume
• Fiducial volume CS2 gas

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Primary Generators Macros

• Macros have been created that vary the primary
  particle generator
• For example, combinations of the following
• - Firing a beam from a given position in a given
  direction
• - Randomly varying either(both) position or(and)
  direction of the
• incoming particles on an event-by-event basis
• - Specifying mono-energetic or extended sources
  through biasing
• for single or multiple particles
• - Allowing particles to be generated from within
  specific volumes or
• surrounding environment to mimic radioactivity
  from detector
• components and background radiation

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Cf-252 neutron source
Spectrum from Sources 4C (Eirini Tziaferi) Peak
1.0MeV Activity 13000 neutrons/second
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Messengers

• Many properties of the simulation from detector
  components to physics processes, and primary
  event generation to visualisation properties, are
  all easily adjustable without affecting the rest
  of the simulation from the Geant4 command prompt
  due to hard-coded messenger code
• This allows the end-user to edit the simulation
  without having to recompile code or risk damaging
  protected functions user friendly!

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Action!

• The running of this simulation is broken up into
  3 sections
• (a) Run Action controls the overall running of
  a simulation from start to finish, calling all
  appropriate functions and classes to do their own
  jobs
• (b) Event Action controls what happens during
  each event of a Run, recording data that has been
  flagged and ensuring all particles are
  appropriately dealt with
• (c) Stepping Action controls what happens with
  each particles along each step of the simulation.
  
• Of the 3 Actions, the Event Action
  paramount for data recording and extraction. In
  this simulation, I utilise a program called
  G4UIRoot which allows the integration of Geant4
  with ROOT, the object-oriented data analysis
  framework
• (see http//root.cern.ch/)

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Output Data

• The simulation presently provides the following
  information for all sensitive volumes
• - Number of particles that enter volume and
  deposit energy
• - Number of multiple nuclear recoils from a given
  event
• - Initial energy of ionising particle
• - Identification of recoil particle
• - Physics Process involved in interaction
• - Energy deposited by particle
• - Time of interaction
• - Co-ordinates of interaction
• - Track length of ionisation
• Fiducial Volume Hits are branched two
  wayssplitting the whole volume into left and
  right halves

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G4UIRoot

• G4UIRoot has allowed the simulation to output
  recorded data straight into ROOT Trees and
  Ntuples (as well as ASCIIs)
• Multiple window GUI to separate output, errors,
  commands, etc

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Runs

• Initial runs representing 38.5s exposure (5x105
  neutrons) were conducted with the source in 4
  positions, firing neutrons isotropically
• - Runs 1-4 Bare source is under the vessel
• - Runs 9 Collimated source on top of left FV
• - Runs 12-13 Collimated source in front of left
  FV
• - Runs 14-16 Bare source 2m from left vessel
  face
• Multiple runs with source in the same position
  where either
• all particles were allowed to deposit energy in
  FV, only a
• nucleus was allowed, or only nuclear elastic
  recoils

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Run 1

• Un-collimated source 18cm below centre of vessel
  
• Energy deposition hits from all particles no
  cuts on electron recoils
• 1267 Hits Total
• Mean E Dep. 27.9keV
• Mean Track Length 65cm
• Hits Left 50.51
• Hits Right 49.49

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Run 2

• Un-collimated source 18cm below centre of vessel
  
• Electron Recoils CUT
• 110 Hits Total
• Mean E Dep. 40.4keV
• Mean Track Length 2.8mm
• Hits Left 46.4
• Hits Right 53.6

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Run 3

• Un-collimated source 18cm below centre of vessel
  
• Electron Recoils CUT non-Elastic Scatter CUT
• 105 Hits Total
• Mean E Dep. 26.9keV
• Mean Track Length 2.3mm
• Hits Left 45.7
• Hits Right 54.3
• Rate 2.73Hz
• (8 0.22Hz)

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Track Length V Energy Deposition
All particles (0-50cm)
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Track Length V Energy Deposition
All Particles (0-2cm)
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Track Length V Energy Deposition
Carbon Sulphur Recoils
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Track Length V Energy Deposition
CARBON
SULPHUR
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Run 9

• Collimated source on TOP of left FV (central)
• Electron Recoils CUT
• 24 Hits Total
• Mean E Dep. 30.3keV (left 3keV gt right)
• Mean Track Length 2.4mm
• Hits Left 50
• Hits Right 50
• Rate 0.62Hz

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Run 13

• Collimated source in FRONT of left FV (central)
• Electron Recoils CUT non-Elastic CUT
• 4 Hits Total
• Mean E Dep. 9.66keV (left 6keV gt right)
• Mean Track Length 1.4mm
• Hits Left 50
• Hits Right 50

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Run 14

• Un-collimated source 2m left of vessel
• All particles
• 149 Hits Total
• Mean E Dep. 26.7keV
• Mean Track Length 61.6cm
• Hits Left 55.7
• Hits Right 44.3
• Long range of e-s

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Run 16

• Un-collimated source 2m left of vessel
• Electron Recoils CUT non-Elastic Scatter CUT
• 19 Hits Total
• Mean E Dep. 11.9keV
• Mean Track Length 1.7mm
• Hits Left 68.4
• Hits Right 31.6
• Rate 0.49Hz
• Better collimation!

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Next

• Long Run 3 repeated with Cf-252 source carry case
  (wax) and lead pot, beneath it - lid may block
  some neutrons
• Missing stub for rock neutrons
• Track neutrons in collimated runs to check paths
  taken to enter FV
• Upon entering FV, record neutron energy,
  direction position into ntuples as well as
  tracking display
• Record direction of ionisation tracks from
  scatters in FV into ntuples

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Summary

• Simulation is complete for external calibration
  sources, giving energy spectrums and number of
  events in FV, regardless of source
• Rates agree well with calculated actual rates
  although better stats required
• Simulation ready to use for rock neutron
  simulations
• Addition of recoil angles directions to be
  implemented shortly for directional simulations