SR radiation photon tracking fails inside dipole

[Stewart Boogert]

SR photons cause an infinite loop at a certain critical angle (approx 0.035*rad for the magnet configuration explained below), there are no warnings or exceptions.

For a

10 m dipole 10 cm beam pipe radius 0.5 mm beam pipe thickness

• < 0.035*rad the SR tracks
• Above the tracking does not finish
• Not a problem with the large number of low energy photons. Verified by placing a collimator after the bend.

The problem is the normal/old and modular physics lists.

To recreate the problem examples/features/processes/3_sr

[Laurie Nevay]

This was caused by two problems together: 1) poor tracking or looping scenarios for high transverse momentum, low energy particles by the dipole stepper and 2) the continuous SR process proposing successively smaller and smaller step sizes leading to an infinite loop condition.

This was resolved by removing the contSR process from the physics lists when SR is enabled (previously made of discrete and continuous SR processes) and secondly, by having a back up Geant4 runge kutte stepper for high pt particles. With both of these modifications, stuck particles were not observed on several tens of thousands of events, where as before it was observed immediately.