MICE Beamline and Upstream PID in MICE Simulations

1. MICE Beamline and Upstream PID in MICE Simulations Tom Roberts Illinois Institute of Technology March 31, 2004

2. Problem Statement Now that TOF0, TOF1, and Cherenkov1 are interspersed within the Quads, we can no longer separate the beamline from the MICE experiment. To accurately model the apparatus, G4MICE must model the DESY quadrupole magnets. We no longer have an obvious demarcation between beamline and detector/cooling-channel.

3. Comments on modeling quads • The DESY quads have a “rounded +” aperture. To model it, first you have to thread through the bugs and limitations of Geant4. • There are undocumented restrictions on Boolean Solids in Geant4 – no error message, you just don’t get the solid you expected . • Fortunately, the aperture can be modeled by making it a daughter volume of the magnet (the only real disadvantage of this is you cannot see through it when visualizing the detector). • Note that Geant4 does not have a suitable solid for modeling hyperbolic pole tips (not even as polygons in XY extruded along Z) – g4beamline models the pole tips as circles, not hyperbolas • But for the DESY quads the hyperbola misses the known points on the pole tip by ~6mm, so circles through the known points are probably better. • At present, we have no fringe field map of the quads. • g4beamline uses a COSY-style fringe-field computation • This is probably not a major issue, as the quad fields are rather small (<½T), fall off rather quickly, and particles have only small angles relative to the centerline. • This could change for the better, as there exists a TOSCA model of the DESY quads, and it could be used to generate a field map.

4. Beam into the Detector / Cooling Channel • The MICE beamline does not generate a Gaussian beam into Tracker1. • To accurately simulate the experiment, we should generate pions from the target, track them through the beamline, detectors, and cooling channel, and generate output files corresponding to the detector models (it is utterly infeasible to start from the proton beam). • Starting from the target, fewer than 1% of the pions generate a track that reaches TOF0, and fewer than 0.1% generate a track that reaches TOF2. So we probably want to track Target-to-Q4 in a separate program (not to mention G4MICE does not model bends) • Since TOF0 is included in the output files, the demarcation between beamline and detector must be ahead of TOF0.

5. Proposal • Transplant ~2 pages of C++ code from g4beamline into G4MICE to model the quads: • 1 page creates the geometry • 1 page generates the field • Add whatever glue code is necessary for G4MICE • g4beamline includes a global bounding box for fields that greatly speeds up tracking; we might want to transplant that into G4MICE also (speedup for tracking Target→Calorimeter was ~15x). • Select a plane between B2 and Q4 to be the demarcation between g4beamline and G4MICE. • Avoid both magnets’ fringe fields in selecting the plane’s position • Use g4beamline to generate tracks at that plane from the target • Interface g4beamline → G4MICE via an ASCII file interface (e.g. g4beamline’s BLTrackFile, which is dirt simple). • This could also be either HistoScope or ROOT.