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, 1st September 2008

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.

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.

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.

Sample Output

Before Collimator After Collimator At Lead-Glass

photonelectronpositron

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.