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A simple problem like the one chosen for this example can be managed by means of a simple data card file like the following one, that we shall name as seamu.inp. It can be noticed that the NEW-DEFA set of defaults has been chosen, the main reason being a reasonably low value of particle transport threshold, set at 10 MeV, except for neutrons (19.6 MeV), antineutrons (50 MeV), and (anti)neutrinos (0, but they are discarded by default). However other features are implied, as here summarized:

• EMF on, with electron and photon transport thresholds to be set using the EMFCUT command.
• Inelastic form factor corrections to Compton scattering activated (no need for EMFRAY).
• Low energy neutron transport on (no need for LOW-NEUT). The neutron high energy threshold is set at 19.6 MeV.
• Non analogue absorption for low energy neutrons with probability 0.95 for the last (thermal) group.
• Multiple scattering threshold for secondary charged particles lowered to 20 MeV (equal to that of the primary ones)
• Delta ray production on with threshold 1 MeV (see option DELTARAY)
• Restricted ionisation fluctuations on both for hadrons/muons and EM particles (see option IONFLUCT)
• Heavy particle e⁺/e^– pair production activated with full explicit production (with the minimum threshold = 2 m_e )
• Heavy particle bremsstrahlung activated with explicit photon production above 1 MeV
• Muon photonuclear interactions activated with explicit generation of secondaries

We do not need all these features, and some of them must be over-written. The critical parts of this file will be examined in detail in the next sections.

Here is the full listing of seamu.inp:

TITLE
	 Test of Muon Propagation in sea water
*
* 	Sets the standards defaults of FLUKA (see user manual)
DEFAULTS                                                              NEW-DEFA
*
* 	Here sets muon energy. (1 TeV momentum in this example)
BEAM          1000.0                                                  MUON+
*
* 	Here sets initial coordinates of muons. They are assumed to go
* 	along the +z axis. Muons are injected just before the water layer
*       in a vacuum layer (see description of geometry below)
BEAMPOS          0.0       0.0     -50.0
*
* 	Switching off Electromagnetic Showering
EMF                                                                   EMF-OFF
*
*
DISCARD     ELECTRON  POSITRON    PHOTON  HAD-CHAR  HAD-NEUT
*
* 	Neutrinos are discarded by default
*
* 	Starts description of geometry: a large box of water delimited
*       by vacuum layers at the top and bottom. Here depth is fixed
*       at 1 km. Muons are injected in the initial vacuum.
GEOBEGIN                                                              COMBNAME
    0    0               water layer
SPH LARGESPH   0.0 0.0 0.0 5000000.0
RPP WORLD      -1000000.0 1000000.0 -1000000.0 1000000.0 -100.0 100100.0
XYP SEATOP     0.0
XYP SEABOT     100000.0
END
* black hole
BLACKHOL     5  +LARGESPH -WORLD
* vacuum at the beginning
VACTOP       5  +WORLD +SEATOP
* water layer
SEA          5  +WORLD +SEABOT -SEATOP
* vacuum at the end
VACBOT       5  +WORLD -SEABOT
END
GEOEND
*
* 	Here defines materials for the sea water compound (Antares 1)
*
* 1) Hydrogen
MATERIAL         1.0    1.0079  8.99D-05       3.0                 1.0HYDROGEN
* 2) Oxygen
MATERIAL         8.0    15.999  0.001429       8.0                    OXYGEN
* 3) Magnesium
MATERIAL        12.0    24.305     1.738       9.0                    MAGNESIU
* 4) Sodium
MATERIAL        11.0     22.99  0.038349      19.0                    SODIUM
* 5) Calcium
MATERIAL        20.0     40.08      1.54      21.0                    CALCIUM
* 6) Potassium
MATERIAL        19.0    39.102  0.031165      26.0                    POTASSIU
* 7) Chlorine
MATERIAL        17.0   35.4529 0.0029947      27.0                    CHLORINE
* 8) Sulphur
MATERIAL        16.0    32.066      2.07      28.0                    SULFUR
* 9) Sea water compound (Antares 1) using atom relative contents
MATERIAL                          1.0341      29.0                    SEAWATER
COMPOUND         2.0  HYDROGEN    1.0088    OXYGEN   0.00943    SODIUMSEAWATER
COMPOUND    0.000209  POTASSIU  0.001087  MAGNESIU  0.000209   CALCIUMSEAWATER
COMPOUND     0.01106  CHLORINE   0.00582    SULFUR                    SEAWATER
*
* Here assigns materials to regions
* Sea Water in region 3
ASSIGNMA    SEAWATER       SEA
*  External Black Hole in region 1
ASSIGNMA    BLCKHOLE  BLACKHOL
*  Vacuum at beginning (region 2)
ASSIGNMA      VACUUM    VACTOP
*  Vacuum at end (region 4)
ASSIGNMA      VACUUM    VACBOT
*
* 	Setting SCORE and Ouptut levels
SCORE         ENERGY
*
*       Activates the routine  MGDRAW (what(1) = or > 100.)
*       with what(3)=2 the entry BXDRAW in MGDRAW routine is activated
USERDUMP       101.0                 2.0                              SEAMU
*
* 	Activates Random Number initialization
RANDOMIZ         1.0
*
* 	Activates user dependent initialization
USRICALL
*
* 	Starts Job asking for a given number of events
START         1000.0
*
* 	Activates user dependent output
USROCALL
STOP

We draw the attention on the presence of a USERDUMP card with what(3) = 2. As remarked in the comments, this enables the call to the entry BXDRAW in the MGDRAW routine, where we introduce our specific code lines. The entry BXDRAW is called also for what(3) = 3,4,5,6 or what(3) ≤ 0, but with what(3) = 2 the number of calls to the other MGDRAW entries (here not necessary) is minimized.

Giuseppe Battistoni; INFN, Milano
Francesco Cerutti; CERN, Geneva

Last updated: 10th of December, 2008