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

Geant4 in Brief

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

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

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

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.

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

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

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.

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…)

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!

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

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.

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

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

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 ...

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: 2457.83 keV 238U 232Th

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.

BambooMC https://github.com/pandax- experiments/BambooMC public version Get it Prepare environment Build it git clone https://github.com/pandax-experiments/BambooMC.git scl enable devtoolset-4 bash . /store/bl2/opt/root/6.08.04_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

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

Detector Hierachy SampleWorldBox: not plotted SampleCylinderDetector inside SampleWorldBox

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

Generator SimpleGPSGenerator A wrapper of the General Particle Source shipped with Geant4. Controlled with /gps/ commands, more powerful than particle gun. http://geant4-userdoc.web.cern.ch/geant4- userdoc/UsersGuides/ForApplicationDeveloper/Backup Versions/V10.3/html/ch02s07.html Particle type, energy, direction, positions can be controlled easily

PandaXAnalysis https://pandax.physics.sjtu.edu.cn/foswiki/Main/BambooMCAvailableAnalysis 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".

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)