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[ <--- prev -- ] [ HOME ] [ -- next ---> ] 7 Description of FLUKA input optionsThere are more than 80 option keywords available for input in FLUKA. A summary
is given in the next section, where the commands will be shortly introduced and
grouped by type of functionality. Some of the commands, which can provide
several different services, will be mentioned in the context of more than one
group.
Introduction to the FLUKA input options
Summary of the available options.
ASSIGNMAt defines the correspondence between region and material indices and defines regions where a magnetic field exists AUXSCORE allows to filter scoring detectors of given estimator type with auxiliary (generalized) particle distributions and dose equivalent conversion factors, and with isotope ranges BEAM defines most of the beam characteristics (energy, profile, divergence, particle type) BEAMAXES defines the axes used for a beam reference frame different from the geometry frame BEAMPOS defines the starting point of beam particles and the beam direction BIASING sets importance sampling (Russian Roulette/splitting) at boundary crossings and at high-energy hadronic collisions on a region by region basis COMPOUND defines a compound or a mixture or a mixture of isotopes CORRFACT allows to alter material density for dE/dx and nuclear processes on a region-by-region basis DCYSCORE associates selected scoring detectors of given estimator type with user-defined decay times DCYTIMES defines decay times for radioactive product scoring DEFAULTS sets FLUKA defaults for specified kinds of problems DELTARAY activates delta-ray production by heavy charged particles and controls energy loss and deposition DETECT scores energy deposition in coincidence or anti-coincidence with a trigger, on an event by event basis DISCARD defines the particles which must not be transported ELCFIELD sets the tracking conditions for transport in electric fields and possibly defines an homogeneous electric field (not yet implemented) EMF requests detailed transport of electrons, positrons and photons EMF-BIAS defines electron/photon leading particle biasing or biases electron/photon interaction length EMFCUT sets energy cutoffs for electrons, positrons and photons, for transport and production, or for switching off some physical interactions EMFFIX sets the size of electron steps corresponding to a fixed fraction loss of the total energy EMFFLUO activates production of fluorescence X rays in selected materials EMFRAY activates Rayleigh (coherent) scattering in selected regions EVENTBIN scores energy or star densities in a binning structure independent from the geometry, and prints the binning output after each "event" (primary history) EVENTDAT prints event by event the scored star production and/or energy deposition in each region, and the total energy balance EVENTYPE defines the hadron particle production model to be used EXPTRANS requests exponential transformation ("path stretching") (not yet implemented) FLUKAFIX sets the size of the step of muons and charged hadrons to a fixed fraction loss of the kinetic energy FREE switches to free-format input (geometry excluded) GCR-SPE initialises Galactic Cosmic Ray calculations GEOBEGIN starts the geometry description GEOEND ends the geometry description; can also be used to activate the geometry debugger GLOBAL issues global declarations about the class of the problem (analogue or weighted) and about the complexity of the geometry. It also allows to use free format input (geometry included) HI-PROPE defines the properties of a heavy ion primary IONFLUCT calculates ionisation energy losses with fluctuations IRRPROFI defines an irradiation profile for radioactive decay calculations LAM-BIAS biases decay length and interaction length LOW-BIAS requests non-analogue absorption and defines the energy cutoff for low-energy neutron transport on a region by region basis LOW-DOWN biases the downscattering probability in low energy neutron transport on a region by region basis LOW-MAT sets the correspondence between FLUKA materials and low-energy neutron cross section data LOW-NEUT requests low-energy neutron transport MATERIAL defines a material and its properties MAT-PROP supplies extra information about gaseous materials and materials with fictitious or inhomogeneous density and defines other material properties MCSTHRES defines energy thresholds for applying the multiple Coulomb scattering algorithm to the transport of muons and charged hadrons MGNFIELD sets the tracking conditions for transport in magnetic fields and possibly defines a homogeneous magnetic field MULSOPT controls optimisation of multiple Coulomb scattering treatment. It can also request transport with single scattering MUPHOTON controls photonuclear interactions of high-energy heavy charged particles (mediated by virtual photons) MYRQMD defines some I/O parameters relevant to the new heavy ion event generator RQMD OPEN defines input/output files without pre-connecting OPT-PROP defines optical properties of materials OPT-PROD controls Cherenkov and Transition Radiation photon production PAIRBREM controls simulation of pair production and bremsstrahlung by high-energy heavy charged particles PART-THR sets different energy cutoffs for selected particles PHOTONUC activates photon interactions with nuclei PHYSICS controls some physical processes for selected particles PLOTGEOM calls the PLOTGEOM package to draw a slice of the geometry POLARIZA defines polarised beams (only for photons at present) RADDECAY requests simulation of radioactive decays and sets the corresponding biasing and transport conditions RANDOMIZe sets the seeds and selects a sequence for the random number generator RESNUCLEi scores residual nuclei after inelastic hadronic interactions ROT-DEFIni defines rotations/translations to be applied to user-defined binnings ROTPRBIN sets the storage precision (single or double) and assigns possible rotations/translations for a given user-defined binning (USRBIN or EVENTBIN) RQMD defines some I/O parameters relevant to the heavy ion event generator RQMD SCORE defines the energy deposited or the stars to be scored by region SOURCE tells FLUKA to call a user-defined source routine SPECSOUR calls special pre-defined source routines (particles created by colliding beams, or cosmic ray sources) START defines the number of primary particles to follow, gets a primary particle from a beam or from a source, starts the transport and repeats until the predetermined number of primaries is reached STEPSIZE sets the maximum step size in cm (by region) for transport of charged particles STERNHEIme allows users to input their own values of the density effect parameters STOP stops input reading TCQUENCH sets scoring time cutoffs and/or Birks quenching parameters THRESHOLd defines the energy threshold for star density scoring, and sets thresholds for elastic and inelastic hadron reactions TIME-CUT sets transport time cutoffs TITLE gives the title of the run USERDUMP requests a collision file and defines the events to be written USERWEIG defines extra weighting to be applied to scored yields, fluences, doses, residual nuclei or star densities (at scoring time) USRBDX defines a detector for a boundary crossing fluence or current estimator USRBIN scores energy, star density or particle fluence in a binning structure independent from the geometry USRCOLL defines a detector for a collision fluence estimator USRGCALL calls user-dependent global initialisation USRICALL calls user-dependent initialisation USROCALL calls user-dependent output USRTRACK defines a detector for a track-length fluence estimator USRYIELD defines a detector for scoring particle yield around a given direction WW-FACTOr defines weight windows in selected regions WW-PROFIle defines energy group-dependent extra factors ("profiles") to modify the basic setting of the low-energy neutron weight windows in selected sets of regions, or the low-energy neutron importances in each region WW-THRESh defines the energy limits for a RR/splitting weight window 7.1 Basic commands
Most FLUKA commands are optional, and if anyone of them is not used an
appropriate set of defaults is provided. A few commands, however, are nearly
always needed in order to provide a meaningful definition of the problem to be
studied.
Defaults are provided in FLUKA for all the above features, but those for
items 1), 2) and 3) are unlikely to be useful: therefore the few commands used
to define source, geometry and materials are practically always present in the
input file.
7.2 Definition of the radiation source
The simplest particle source is pointlike, monoenergetic and monodirectional,
that is, a "particle beam". Option BEAM, fully described later, is used to
define the particle type and momentum (or energy). If desired, this option can
also define an energy spread, a beam profile shape and an angular divergence.
However, the two latter distributions are restricted to a beam is directed in
the positive z direction: to describe divergence and beam profile for an
arbitrary beam direction it is necessary to define a beam reference frame by
means of option BEAMAXES.
7.3 Description of the geometry
The Combinatorial Geometry used by FLUKA is based on two important concepts:
bodies and regions. The first ones are closed solid bodies (spheres,
parallelepipeds, etc.) or semi-infinite portions of space (half-spaces,
infinite cylinders) delimited by surfaces of first or second degree. The user
must combine bodies by boolean operations (addition, intersection and
subtraction) to perform a complete partition of the space of interest into
regions, namely cells of uniform material composition. One important rule to
remember is that inside the space of interest, defined by means of an external
closed body, every point must belong to one and only one region.
7.4 Materials
Materials in FLUKA are identified by a name (an 8-character string) and by a
number, or material index. Both are used to create correspondences, for
instance between region number and material number, or between material name
and neutron cross section name.
7.5 Setting options
Many FLUKA input options are not used to describe the radiation transport
problem but to issue directives to the program about how to do the
calculations. Other options are used just to select a preferred input format.
We refer to these options as "setting options".
7.6 Format setting
The default, fixed input format can be replaced by a free format using option FREE or better GLOBAL. The latter allows to choose free format for both the normal input and the geometry input separately, and serves also a few other purposes: it can be used to increase the maximum allowed number of geometry regions, and to force a calculation to be fully analogue (i.e., simulating physical reality as directly as possible, without any biasing to accelerate statistical convergence. A more esoteric capability of GLOBAL, used mainly for debugging, is to ensure that the random number sequence be exactly reproduced even in cases where the geometry tracking algorithm has the possibility to follow different logical paths to achieve the same result. 7.7 General setting options
The difficult task of choosing the best settings for a calculation problem is
made much easier by the existence of several "pre-packaged" sets of defaults,
each of which is optimised for a particular type of application. Each set is
chosen by option DEFAULTS, which has to be placed at the beginning of the input
file, possibly preceded only by TITLE or GLOBAL. Several possibilities include
hadrotherapy, calorimetry, pure electromagnetic runs without photonuclear
reactions, low-energy neutron runs without gamma production, and others. One
set of defaults is tuned for maximum precision (but not necessarily great time
efficiency). Reasonable defaults, acceptable for most generic routine
calculations, are provided in case DEFAULTS is missing. In most cases, the user
has the possibility to use some of the other setting options described below,
to override one or more of the defaults provided by the chosen set.
7.8 Multiple Coulomb scattering
The concept of multiple scattering is an approximation to physical reality
(condensed history approximation [Ber63], where charged particles undergo a
very large number of single collisions with the atomic electrons, too many to
be simulated in detail except in very special cases. All the theoretical
treatments which have been developed are valid only within certain limits, and
none of them gives rules on how to handle material boundaries and magnetic
fields. FLUKA uses an original approach [Fer91a, based on Molière's theory
[Mol47,Mol48,Bet53,Mol55], which gives very good results for all charged
particles in all circumstances (even in backscattering problems), preserving
various angular and space correlations and freeing the user from the need to
control the particle step length.
7.9 Step length
Another aspect of the condensed history approximation is that charged particle
transport is performed in steps. The finite fraction of the particle energy
which is lost and deposited in matter in each step is an approximation for the
sum of innumerable tiny amounts of energy lost by the particle in elastic and
inelastic collisions.
7.10 Energy cutoffs
Setting energy cutoffs, for both transport and production, is an important
responsibility of the user, who is interested in choosing the best compromise
between accuracy and time efficiency. Each of the parameter sets available via
option DEFAULTS, including the basic defaults set which exists when that option
has not been explicitly requested, offers a well-optimised choice for the
corresponding class of applications, with only one exception. But even so, it
is often convenient to override some of the default cutoffs in order to
improve performance. The exception concerns the default particle production
cutoffs for electrons, positrons and photons, which are dependent on other
settings (see EMFCUT below).
7.11 Time cutoffs
For time-dependent calculations, two time cutoff options are available: one for particle transport, TIME-CUT, and one for scoring, TCQUENCH. While option TIME--CUT sets a particle-dependent time limit after which the corresponding particle history is terminated, the limits set by TCQUENCH are assigned to selected binnings. Scoring contributions to a binning by particles having exceeded the corresponding time limit are ignored, but particle transport continues, possibly contributing to other detector scores. 7.12 Ionisation energy loss
Transport of charged particles can be done in many ways: without delta ray
production and ionisation fluctuations (continuous slowing down approximation),
with ionisation fluctuations and no delta rays, with delta ray production above
a chosen energy threshold and no ionisation fluctuations below the threshold,
and with both: delta rays above the threshold and ionisation fluctuations below
it. Depending on the application type chosen with option DEFAULTS, different
defaults and thresholds apply, which can be modified by the user by means of
options IONFLUCT, DELTARAY and EMFCUT. Option IONFLUCT is used to request
(restricted) ionisation fluctuations on a material basis. In FLUKA, these
fluctuations are not simulated according to Landau or Vavilov theory but
according to an original statistical approach [Fas97a]. They can be requested
separately for electrons and positrons and for muons and charged hadrons. Delta
ray production thresholds are instead set for the two particle families by two
separate options, which have already been mentioned above in the context of
production cutoffs: EMFCUT and DELTARAY. DELTARAY can be used also to define
(and print) the mesh width of the stopping power tabulations used by the program.
7.13 Special radiation components or effects
In FLUKA, an effort has been made to implement a full cross-talk between
different radiation components (hadronic, muonic, electromagnetic, low-energy
neutrons, heavy ions, optical photons). However, some components are not
activated by default, and others are only activated in some of the available
default settings. Input options are provided to switch them on and off.
7.13.1 Radiation componentsHigh-energy hadrons and muons are always generated and transported, except with
defaults settings EM-CASCA and NEUTRONS (however, they cannot be requested
overriding these two defaults). To suppress them, one can use command DISCARD.
7.13.2 Physics effectsSome 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.
7.14 Scoring options
Any result in a Monte Carlo calculation is obtained by adding up the
contributions to the "score", or "tally" of a detector defined by the user.
A detector is the Monte Carlo equivalent of a measurement instrument. Each
"estimator" (detector type) is designed to estimate one or more radiometric
quantities, and the final score is a statistical estimation of the average
value of the corresponding population. As in experimental measurements, it is
possible to calculate a standard deviation by running several independent
calculations.
7.15 Event by event scoring options
Typical particle physics applications, in particular calorimetry, require separate scoring event by event (that is, results are printed after each primary particle history). Two commands, EVENTBIN and EVENTDAT, are respectively the event-equivalent of USRBIN and SCORE which have been introduced before. A third command, DETECT, allows to score event by event energy deposition simulating a detector trigger, defining coincidences and anticoincidences. All these options are incompatible with any biasing. It is suggested to use command GLOBAL to make sure that the run will be completely analogue. 7.16 Scoring modifying options
There are a few commands which are used to modify some of the scoring options already described. TCQUENCH, which has already been shown to define a time cutoff, can be used also to apply a quenching factor (Birks factor) to energy deposition scored with USRBIN or EVENTBIN. ROT-DEFI and ROTPRBIN allow to define roto-translation transformations for binnings not aligned with the coordinate axes. ROTPRBIN can be used also to set the binning storage precision: a space saving feature, which is useful mainly when scoring event by event with EVENTBIN. 7.17 Options to handle radioactive decay
It is possible to transport and score in the same run also the beta and gamma radiation emitted in the decay of radioactive nuclei produced in the hadronic or electromagnetic cascade. Several options are available for this purpose: RADDECAY is used to request the simulation of radioactive decays, IRRPROFIle defines a time profile for the intensity of the primary particles, DCYTIMES requests one or more decay times at which the desired scoring shall occur, and DCYSCORE associates selected scoring detectors to the decay times so requested. 7.18 Biasing options
When run in fully analogue mode, FLUKA allows the user to study fluctuations
and correlations, and to set up a direct simulation of physical reality where
all moments of phase space distributions are faithfully reproduced. On the
other hand, in the many applications where only quantities averaged over many
events are of interest, it is convenient to use calculation techniques
converging to the correct expectation values but reducing the variance (or the
CPU time, or both) by sampling from biased distributions. This is especially
useful in deep penetration calculations, or when the results of interest are
driven by rare physical interactions or cover a small domain of phase space.
7.18.1 Simple biasing optionsThe easiest biasing command is fittingly called BIASING. It provides two
different kinds of variance reduction: Multiplicity Reduction and Importance
Biasing, which is based on the two complementary techniques Geometry Splitting
and Russian Roulette (RR).
7.18.2 Weight window optionsThe weight window is a very powerful biasing technique, not based on relative importances, but on the absolute value of particle weight. The user sets an upper and a lower limit for the particle weight in each geometry region, possibly tuned per type of particle and energy. Splitting and RR will be applied so that the weight of all relevant particles will have a value between the two limits. In addition to controlling the particle population, this technique helps also to "damp" excessive weight fluctuations due to other biasing options. Its use is not as easy as that of importance biasing, because it is necessary to have at least a rough idea of what are the average weights in different regions. Special splitting and RR counters can be printed on request to help setting the window parameters setting SDUM = PRINT in command BIASING. An explanation about the meaning of the counters can be found in Chap. (9). Weight window setting is done in FLUKA by three input commands: WW-FACTOr, WW-THRESh and WW-PROFIle. The first two commands must be used together: WW-FACTOr sets the upper and lower weight limits per region, while WW-THRESh defines energy limits within which the weight window must be applied, and the particles to which it is to be applied. The third option is reserved to low-energy neutrons, whose transport characteristics often require a more detailed biasing pattern: WW-PROFIle allows indeed to tune the weight window by neutron energy group. 7.18.3 Biasing options for low-energy neutronsThe special multigroup transport structure used by FLUKA for low-energy neutrons calls for some biasing options specific to these particles. We have just introduced the weight window command WW-PROFIle. Two more options are LOW-BIAS, which has already been mentioned before in the context of energy cutoffs, but which is used also to set a user-defined non-analogue absorption probability, and LOW-DOWN, by which it is possible to bias neutron thermalisation (downscattering). The latter, however, is an option recommended only to users with a good knowledge and experience of neutronics. 7.19 Calls to user routines
The purpose of several FLUKA input options is to trigger calls to user routines
(user routines are described in Chap. (13)). One of the most important ones is
SOURCE, which makes FLUKA get the characteristics of its primary particles from
subroutine SOURCE instead of from options BEAM and BEAMPOS. This option allows
to pass to the subroutine several parameters, thus allowing to drive it from
input without the need to re-compile it. Note that even when using a
user-written source, it is still necessary to have in input a BEAM card
indicating the maximum expected energy of a primary particle, so that the
program can prepare appropriate cross section tables. If command SOURCE is
present, but no SOURCE routine has been linked, the default one in the FLUKA
library will be called, which leaves unchanged the particle type, energy,
position etc. as defined by BEAM and BEAMPOS.
Complex magnetic fields can be defined or read from a map by a user routine
MAGFLD. Calls to the routine are activated by command MGNFIELD.
7.20 Miscellaneous
Command RANDOMIZe starts a new independent random number sequence. It can be
omitted only in a first run, but it is compulsory if a sequence of independent
runs is desired in order to calculate statistical errors.
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