User Control

The User controls the generator primarily through 4 MCxx banks banks:-

MCMA

The MAster bank. The bank number is 1. The MCMA controls a number aspects of the Monte Carlo, but the only word concerning event generation is MCTYPE. The MCTYPE (Monte Carlo run type) word gives the bank number of the selected MCPI bank. If the MCTYPE is set to 0, processing of the MCPI is disabled and the user routine MCG_USER (q.v.) is called instead.

MCPI

Is the Particle Information bank, although strictly speaking, what is specified are interactions rather than particles. One or more can be defined and for each the user can sample a number of random distributions of:-

• Position.
• Energy.
• Direction.
• Time.
• Cerenkov Photon Polarisation

The code is distributed with one MCPI. The User can either control the contents of the default MCPI with SET BANK or symbolic commands or can create his/her own MCPI and alter the MCTYPE word of the MCMA bank to point to it. For a list of all the distribution codes and parameters see:-

Companion/Id Codes/*_types

If the interaction code describes a decay scheme, then the bank number of the MCDS bank is given by the interaction code

MCDS

is the standard Decay Scheme bank. Unless you know what you are doing, it is better to leave these banks alone!

MCPL

These banks contain lists of particle properties (particle types, positions, directions, energies, times, and polarisations where appropriate). When MCTYPE (in the MCMA bank) is set equal to minus the MCPL bank number that bank is read and can be used to define the initial state of the event. This can be used to feed the results of an external MC generator into SNOMAN. For each event generated, a particle block, consisting of one or more particles is written in the MCPL. The number of particles can vary from event to event. To feed the external generator output, unmodified, into SNOMAN use the commands:-

$mc_interaction_type   $pick_particle_list
$mc_position           $pos_mcpl
$mc_direction          $dir_mcpl
$mc_time               $tim_mcpl
$mc_energy             $en_mcpl
$mc_miscellaneous      $misc_mcpl_pol

which tells SNOMAN to take the next particle block and create as many tracks as it contains. Each track is given the ``start particle'' interaction code and all particle properties are drawn from the current particle block. If required, more than one of these processes can be started per event, simply by specifying more than one particle (using the $mc_num_seed_vx command) and using the same distributions for each `particle'. Once all the data in the MCPL has been used the system starts again at the start of the bank.

Its possible to have a hybrid system, taking some properties from MCPL and others from a built-in generator. To do this simply specify the required property, e.g.:-

$mc_energy             $en_mono 2.5

It is even possible to change the interaction type, e.g.:-

$mc_interaction_type   $start$$e_minus$

although this has to be done with caution. First it could result in changing the particle type, which may not be a sensible thing to do. Second, as such an interaction only specifies a single particle, it only uses the first particle of the current particle block. If there is more than one particle in the block the others will not be used and as this is almost certainly going to give a skewed distribution, a ZTELL warning is recorded each time data is ignored in this way.

Feeding in very large amounts of data from an external Monte Carlo can place an excessive demand on the titles memory. To bypass this problem:-

• Prepare a file, say my_mc_data.dat, that contains the contents of the MCPL. Note this file is going to be read directly by FORTRAN, it cannot be a titles file, it must only contain data acceptable to FORTRAN. Each track will be read with a single I/O statement so track records must always start at the beginning of a line, although each may span several lines. Any of the example MCPL banks in the mcpl_data.dat has a legal format once the user data has been isolated.

• Prepare an MCPL titles bank that is big enough to take the largest event in my_mc_data.dat. Failure to make this big enough will result in SNOMAN quitting with an error when it reaches an event that exceeds it.

• Add the following line to the command file:-

  file mco 2 my_mc_data.dat

Now when the program runs, the FILE command signals that the MCPL titles bank is just a buffer into which should be loaded one event at a time from the file. The wrapping after the last event now becomes a file rewind. So only a small amount of titles memory is used although this comes at two costs:-

• A slight drop in performance due to the extra I/O. However, as this only needs to be employed with very large Monte Carlos, this should be lost in the noise.

• Only one MPCI particle can be used as the system can only deal with one event at at time i.e.$mc_num_seed_vx = 1. Attempting to do more will cause the program to terminate with an error message. This too is not much of a drawback, if feeding in a large external Monte Carlo, its very unlikely that there will be a need to fire off several external events within a single SNOMAN event.