1. Simulation in Experiments searching for rare events
Xun Chen (谌勋)
Apr 30, 2019
GEANT4 Hands-on Training Workshop

2. Geant4 in Brief

3. What Does Geant4 Provide?
Basic types for the description of geometry
Shape, size, and materials
You need to write your own geometry
Physics models
EM, Hadronic, photon, decay, optical, shower,…
You need to select the proper physics models according to the energy range.
GUI, visualization, persistence, histograming
Ways to control the program
built-in and extended commands

4. Jargons Run, Event, Track, Step Track – Trajectory
Step – Trajectory Point
Process
Cut
Sensitive Detector, Hit, Hit Collection

5. Run
Run is a collection of events share the same detector and physics conditions
In collider physics
In underground physics
In simulation
Geant4
Run is managed by G4RunManager
Run is represented by G4Run (or its derivative)
Run behavior can be controlled by G4UserRunAction

6. Event Basic unit of simulation in Geant4
Primary tracks are generated at the beginning and pushed into a stack.
A track on top of stack will be popped out for tracking, newly generated tracks will be pushed into the stack.
When the stack is empty, an event is over.
G4Event is a represent of an event
G4EventManager is the manager of event.
G4UserEventAction is used to control the behavior of event.

7. Track Track is a snapshot of particle A track is deleted when
Only the “current” status of a particle
Step is the “delta” information of the track
Step update a track
A track is deleted when
It goes out of the world volume.
It disappears (decay, inelastic scattering)
It loses all kinematic energy and no additional “AtRest” process
It is killed by user
No track object persists at the end of an event
G4Track, G4TrackingManager, G4UserTrackingAction

8. Step
A step has two points (pre-step, post-step) and delta information (energy loss, time-of-flight…)
Each point know its volume (and material). In case of at the boundary, the point logically belongs to the next volume.
G4Step, G4SteppingManager, G4UserSteppingAction

9. Trajectory and Trajectory Point
Track does not persist at the end of an event
G4Trajectory is a copy of information in G4Track
G4TrajectoryPoint is a copy of information in G4Step
G4Tracjectory contains a vector of G4TrajectoryPoint.
Both type of objects persist till the end of an event.
Can be extended to store more information.

10. A Particle’s life – Tracking
A particle with an initial momentum pass through the material - Tracking
Change the status (Snapshot represented by G4Track)
delta information (energy loss, position, represented by G4Step)
Until it is in some state that no tracking is required (decayed, killed, out of world…)

11. Process Physics process need to be selected in tracking A process
All possible processes will propose a step length
The shortest one is taken
A process
Determine the step length
Possible changes of the track property
Generate secondary particles
Change the state of the track
Processes are provided by physics list.
No default physics list in Geant4!

12. Cut
A cut in Geant4 is a production threshold for the generation of secondary particles.
A process has its intrinsic limits to produce secondary particles.
All particles generated and accepted are tracked up to zero range.
Higher cut  Less particles  less time
Lower cut  more particles  more time

13. Get the simulation Information
Geant4 does simulation “silently”.
You need to extract the useful information by yourself.
Two ways
Access information directly in user hooks (Tracking action, stepping action)
Assign G4VSensitiveDetector to a volume
Hits collection is automatically stored in G4Event
Use G4UserEventAction, G4UserRunAction to access the information.

14. Search for Rare Events
Dark matter, neutrinoless double beta decay, 124Xe double electron capture...

15. Low background experiments
Can you find out the rare signal when background is high?

16. Need to suppress the background
Effective shielding
Underground laboratory
Passive shielding with lead, water, PE, copper
Active shielding with veto signal
Clean materials to build the detector
High purity copper and PTFE
High purity target material
Germanium
Xenon
Argon
...

17. MC simulation
Helps to understand the background level in those experiments
Example: search for neutrioless double beta decay of 136Xe, (half life > 1.07×1026 year)
Signal Q-value: keV
238U
232Th

18. How to obtain background level in simulation
Background sources: U238, Th232, Co60, K40...
Material activity (Bq/kg, Bq/piece)
Sample sources in the detector material
Run simulation, get data
Analyze the data, using the material activity to normalize it.
You can obtain effective time of running according to the number of simulated events and input activities.

19. BambooMC
experiments/BambooMC public version
Get it
Prepare environment
Build it
git clone
scl enable devtoolset-4 bash
. /store/bl2/opt/root/ _gcc52/bin/thisroot.sh
. /store/bl2/opt/geant4/10.02p01/bin/geant4.sh
cd BambooMC
mkdir build
cd build
cmake3 -DCMAKE_BUILD_TYPE=Release -DENABLE_DETECTOR_SETS=example_detector ..
make

20. Example XML file BambooMC requires an XML file to start
Detector geometry
PhysicsList
Generator
Analysis

21. Detector Hierachy SampleWorldBox: not plotted
SampleCylinderDetector inside SampleWorldBox

22. Physics List
SimpleUnderGroundPhysics for low energy experiments around keV to MeV
FTFP_BERT for high energy experiments, such as ATLAS, CEPC...

23. Generator SimpleGPSGenerator
A wrapper of the General Particle Source shipped with Geant4.
Controlled with /gps/ commands, more powerful than particle gun.
userdoc/UsersGuides/ForApplicationDeveloper/Backup Versions/V10.3/html/ch02s07.html
Particle type, energy, direction, positions can be controlled easily

24. PandaXAnalysis
EnableEnergyDeposition -- This is used to controll the output of energy deposition in the sensitive detectors. Value "1" means the deposition wil be saved. "0" means not. Default value is "1".
EnableFlatSurfaceFlux -- This is used to controll the output of flux through flat surfaces of box-shaped sensitive detector. Value "1" means that the flux will be saved, otherwise "0". Default: 0.
enable_primary_particle -- This parameter is used to control whether the primary particle in the event will be saved. Default value is "0".
save_null_event -- This parameter is used to specify whether the primary particle of null event will be saved. The null event means no hits generated in the form of energy deposition and flux. So the value of "1" only works when "enable_primary_particle" is set to "1".
enable_decay_chain_splitting -- This parameter is used to break up the long decay chains of unstable nucleus such as U238/Th232. If the chain is break up, the primary particle of the saved entry will be updated according to the decayed nucleus. The default value is "0".

25. Homework
Using the xml provide in page 20, estimate the Rn220 background level in the cylindrical xenon detector in the (1, 10)keV energy window, assuming the activity of Rn220 is 10 μBq/kg.
The background level should be given in the format of DRU, i.e.,number of events/(keV·kg·day)