1. EuroTag Workshop, Glasgow, 1/9/08 A General Geant4 Tagger Simulation David J. Hamilton University of Glasgow (Email: d.hamilton@physics.gla.ac.uk) EuroTag Workshop Glasgow, 1 st September 2008

2. EuroTag Workshop, Glasgow, 1/9/08 Introduction A generic tagger simulation within the CERN Geant4 framework could be utilised at several experimental facilities, including Mainz, Lund, Bonn, Jlab Halls B and D. The motivation is to gain a greater understanding of the tagged photon beam and the behaviour of the detector system, with a primary focus on the tagging efficiency. The simulation involves a modular approach including:  Electron/bremsstrahlung event generator;  Magnet yoke, beamline and collimator geometry definition;  Magnetic field map;  Data output and analysis in Root format. Implementation for the Glasgow Tagger at MAMI has been partially realised.

3. EuroTag Workshop, Glasgow, 1/9/08 Geometry Definition Magnet yoke and beamline geometry can be defined by hand in Geant4 or converted from AutoCAD where available (by utilising the STEP and GDML formats). 3D magnetic field map read from ASCII file, which can be produced by TOSCA. The event generator, physics and output classes are generic in nature allowing for maximum flexibilty. Glasgow Tagger at MAMI:  AutoCAD model by A. Clarkson.  TOSCA calculations by I. Anthony.

4. Electromagnetic Interactions in Geant4 Standard or polarised electromagnetic interactions are used in the simulation. Geant4 utilises the Berger and Saltzer bremsstrahlung parametrisation, which includes polarisation observables (Olsen and Maximon). Currently, a simple electron event generator with a standard von Neumann rejection method is used. In the future, a dedicated event generator involving both circularly and linearly polarised photons will be implemented.

5. Sample Output Before CollimatorAfter CollimatorAt Lead-Glass photon electron positron

6. Summary A framework for a generic Geant4 tagger simulation has been developed. The Glasgow tagger at MAMI has been simulated by making use of already available AutoCAD and TOSCA material. Extension to other experimental facilities is a relatively trivial undertaking. Possible applications include:  Understanding EM background in the beam and detectors;  Determining tagging efficiency;  Helping to better understand energy calibration;  Providing photon energy distribution for physics event generators;  Determining optimal focal plane design for new designs/upgrades. The next step is development of a dedicated electron/bremsstrahlung event generator, including circular and linear polarisation degrees of freedom.