12.2} Handling optical photons

 In order to help the user to understand how to deal with optical photons,
 in the following we describe two input files respectively concerning the
 production in Liquid Argon of Cherenkov (section 12.2.3}) and Scintillation
 light (section 12.2.4}). A specific user routine, giving the refraction index
 of Liquid Argon as a function of wave-length is also shown (section 12.2.2}).

 It is a very simple case, in which muons are generated inside a box
 filled with Liquid Argon. Notice that at present it is not yet
 possible to request optical photons as primary particles via the
 BEAM card. Therefore light must be generated starting from ordinary particles,
 or by a special user-written SOURCE routine, where optical photons are
 loaded into their dedicated stack (OPPHST) instead of that of ordinary
 particles (FLKSTK). An example of such SOURCE is shown in 1.2.1}.

 The examples presented here consider 0.5 GeV muons in a box of 4 x 4 x 4 m^3.
 In order to avoid unnecessary complications in the example, secondary particle
 production by muons is switched off. Of course this is not required in real
 problems.

 As far as the output is concerned, the following example proposes a
 standard energy spectrum scoring at a boundary (option USRBDX)
 applied to optical photons, together with a user-specific output built
 via the MGDRAW user routine (see Chap. 13}), where a dump of optical photon
 tracking is inserted. At the end of this section (in section 12.2.5}) we shall
 propose the relevant code lines to be inserted in MGDRAW (activated by the
 USERDUMP card), together with an example of readout (section 12.2.6}).

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