Some physical effects are automatically activated, but only when certain default sets are in force (see option DEFAULTS), and can be switched on or off with appropriate commands. The command to simulate fluorescence is EMFFLUO, that for Rayleigh scattering and Compton binding corrections and Doppler broadening is EMFRAY, while for multiple scattering there are MULSOPT and MCSTHRESh which we have already introduced above. High-energy effects such as production of bremsstrahlung and electron pairs by heavy charged particles (in particular muons) are regulated by option PAIRBREM. A few physical effects need to be requested explicitly, whatever the defaults. These are photon polarisation (command POLARIZA), polarisation of pion, kaon and muon decays (command PHYSICS), photonuclear reactions (PHOTONUC), and muon hadronic interactions via virtual photons (MUPHOTON). In some cases, it is also possible to switch off some important effects to study the relative importance of different processes. Command THRESHOLd allows to set a lower energy limit for hadron elastic scattering and inelastic reactions, and EMFCUT does the same with various kinds of electron and photon interactions. The user must bear in mind, however, that results obtained suppressing effects which are critical for the development of the electromagnetic or hadronic cascade are unlikely to be physically correct.
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