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[ <--- prev -- ] [ HOME ] [ -- next ---> ] 2 A FLUKA beginner's guideThis Chapter is intended to provide a minimal set of instructions to install and run FLUKA for a beginner user, going through the different steps required to run a simple example. References to the relevant chapters of the manual are given to help the user in finding more detailed information on the different topics. After describing the installation procedure, instructions are given on how to build the input file for a simple case. Instructions for running and accessing the results are also reported. As a further step, the user is addressed to Chapter (13) to learn how to tailor FLUKA to specific needs by means of user routines. 2.1 Installing FLUKA
A full description of the installation procedure is given in Chapter (3).
The FLUKA package for LINUX and UNIX platforms is distributed as a tar file
which can be downloaded from the FLUKA Website http://www.fluka.org or other
authorised mirror site.
mkdir fluka cd fluka tar zxvf ../fluka2011-linuxAA.tar.gz An alternative way to expand the tar file is: gunzip -dc ../fluka2011-linuxAA.tar.gz | tar xvf At this stage, the user must define an environmental variable FLUPRO pointing to the directory where the distribution tar file has been opened. This directory is made available as follows: Bash shell: use the export command C or tc shell: use the setenv command ... ... cd fluka cd fluka export FLUPRO=$PWD setenv FLUPRO $PWD Of course the definition of FLUPRO can be placed once for ever in the login
script.
... cd $FLUPRO $FLUPRO/flutil/lfluka -m fluka The default executable is called flukahp. Even better, the command make will produce, in addition to the default FLUKA executable, all the executables of the postprocessing tools available in the $FLUPRO/flutil subdirectory. 2.2 Building a FLUKA input
2.2.1 Generalities about FLUKA inputFLUKA reads user input from an ASCII "standard input" file with extension .inp.
The general characteristics and rules of FLUKA input are described in
Chapter (6).
The input consists of a variable number of "commands" (called also öptions"),
each consisting of one or more "lines" (called also "cards" for historical
reasons).
In addition, special commands are available in FLUKA for more advanced problems
involving magnetic fields, time-dependent calculations, writing of history
files (so-called "collision tapes"), transport of optical photons,
event-by-event scoring, calling user-written routines, etc. These options are
expected to be requested only by users having some previous experience with the
more common commands: therefore they will be mostly ignored in this beginner's
guide.
Some WHATs represent numerical quantities (e.g. energy, coordinates), while
others, converted to integers, are indices corresponding to a material, a
type of particle, a region etc. In this latter case, it is possible to replace
the number by the corresponding name (a character string).
Not necessarily all WHATs and SDUMs are used. In some cases, a command
line can be followed by a line of text (for instance a filename path or a
title). Any line having an asterisk (*) in the first position is treated as a
comment. All lines (commands, text strings and comments) are echoed on
the standard output (the file with extension .out). In case of problems, it is
a good idea to check how every line has been printed in the standard output.
Often, output reveals typing or format errors by showing how the program has
misinterpreted them.
2.2.2 Input alignmentBe careful to properly align keywords and numbers. Format is not free, unless a
command is issued at the beginning of input: see option GLOBAL or FREE.
Even in the free format for the input file, the part of the input describing the
geometry can still be written in fixed format (which is different from the
general FLUKA input format, see Chap. (6)). There is the possibility of having
free format also for the geometry part: this can also be activated using the
GLOBAL command.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 In numerical fields, blanks are treated as zero. Therefore, numbers written with a decimal period and without an exponent (e.g. 1.2345) can be placed anywhere inside their respective format fields, For instance, the following two input lines are equally acceptable: BEAM 200. 0.2 1.5 1.2 0.7 1.0 PROTON *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 BEAM 200. 0.2 1.5 1.2 0.7 1.0PROTON If a number is written in exponential notation (e.g. 2.3E5) or in integer form (without decimal point), it must be aligned to the right of the field. Depending on the platform and the compiler, sometimes the number is correctly interpreted even if the alignment rule is not respected. However THIS IS NOT GUARANTEED AND THE RIGHT ALIGNMENT RULE SHOULD ALWAYS BE FOLLOWED. For instance in: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 BEAM 2.E2 0.2 1.5 1.2 0.7 1. PROTON the first value might be interpreted as 2.E200. Another case is the following: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 BEAM 200. 0.2 1.5 1.2 0.7 1 PROTON here the last numerical field would be interpreted as 1000000000. To avoid mistakes due to this kind of input errors, the present FLUKA versions now recognise such potential problems and the program is forced to stop. At the same time a message is printed on the standard output file, as shown here for the above example: *** The 6th field 1 of the following input card *** BEAM 200. 0.2 1.5 1.2 0.7 1 PROTON *** does not contain a valid formatted fortran real number!!! *** *** It is ambiguous and it could be read differently on different compilers *** *** depending how strictly the blank=0 formatted input rule is implemented *** Keywords (character strings such as BEAM and PROTON) must be aligned to the left of their field and must be in upper case. An exception is the continuation character & used in some commands, which can be placed anywhere within its 10-characters field. Non-numerical values of WHATs ("names") can be aligned anywhere within the corresponding fixed-format fields. Sometimes a special option requires a region or a particle number to be entered with a negative sign: in this case the equivalent name must also be entered preceded by a minus sign. 2.2.3 A simple exampleLet us now consider a simple starting application. We want to calculate the
charged pion fluence produced by a monochromatic proton beam of momentum
50 GeV/c impinging on a 5 cm thick beryllium target of simple shape: a small
parallelepiped (20 cm x 20 cm x 5 cm). A further simplification will be made
for the purpose of this example, neglecting all the surrounding environment and
substituting it with ideal vacuum.
2.2.4 The titleTypically, an input file begins with a TITLE card followed by one line of text describing the problem, or identifying that particular run, or providing some kind of generic information. In our case, for example: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 TITLE Charged pion fluence inside and around a proton-irradiated Be target Further information can be provided by comment cards, but the title, because it
is printed at the top of the standard output and of each estimator output, is a
useful reminder of what the corresponding file is about. In addition, the title
is printed in all separate output files (error file, estimator files etc.)
together with the date and time of the FLUKA run, allowing to keep track of
their origin.
2.2.5 Definition of the primary particlesAll "events" or "histories" are initiated by primary particles, which in the
simplest case are monoenergetic, monodirectional and start from a single point
in space (pencil beam). The card BEAM defines the particle energy (or momentum)
while the card BEAMPOS controls their starting position and direction.
These two commands can be used also to define particle beams having a simple
angular or momentum distribution (Gaussian or rectangular), or a simple
transverse profile (Gaussian, rectangular or annular), or a simple space
distribution of starting points (spherical, cartesian or cylindrical shell).
Isotropic or semi-isotropic angular emission can be described as a special case
of an angular rectangular distribution.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 BEAM 50.E+00 PROTON In our example, the beam starting point is given by: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 BEAMPOS 0.0 0.0 -50.0 In the cartesian geometry used by FLUKA, the previous card means that the beam is injected at x, y, z cordinates: 0, 0, -50 cm and is directed along the positive z axis. Of course, the choice of the point of injection, the direction, etc., must be coherent with the geometrical description of the set-up, as discussed in the following section. 2.2.6 The geometryThe input for the Combinatorial Geometry (bodies, regions and optional region
volumes) must be immediately preceded by a GEOBEGIN card and immediately
followed by a GEOEND card. These two cards follow the normal FLUKA format. It
is recalled that the format for the geometry has its own special rules,
described in Chap. (8).
z ^ ----------------------------------- | / ______________________ /| | / / / / | | ----------------------------------- | | | / vacuum (region 2) / | | | | | ------------------------ | | | | | | ___________ | | | | | | | / Be /| | | | | | | | ------------ | | | | | | | | | (reg. 4) |/| | | | | | | | ----------- | | | | | | | | | (reg. 3) |/ | | | | +-- | | ------------ | | | |--------> / | | ^ | / | | y / | | | Beam |/ | | / | ------------------------ | / / | Blackhole (region 1) |/ / ----------------------------------- x Only four bodies are used here: an RPP body (Rectangular Parallelepiped, body
no. 3) to define a volume which will be the Be target region, inside another
larger RPP body (no. 2), which will be filled with ideal vacuum and in turn is
contained inside another larger RPP body (no. 1), to define the blackhole
region. The fourth body, an XYP half-space (defined by a plane perpendicular to
the z axis), will be used to divide the target into 2 different regions: the
upstream half will be defined as the portion of body 3 contained inside the
half-space, and the downstream half as the portion outside it. Therefore,
region "3" (the upstream half of the target) is the part of body no. 3 which is
also inside body 4, while region "4" (downstream half of the target) is the
part of body no. 3 which is not inside body 4. Region "2" (the vacuum around
the target) is defined as the inside of body no. 2 from which body no. 3 is
subtracted. Region "1" is simply the interior of body no. 1 from which body
no. 2 is subtracted.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 GEOBEGIN COMBINAT 0 0 A simple Be target inside vacuum RPP 1-5000000.0+5000000.0-5000000.0+5000000.0-5000000.0+5000000.0 RPP 2-1000000.0+1000000.0-1000000.0+1000000.0 -100.0+1000000.0 RPP 3 -10.0 +10.0 -10.0 +10.0 0.0 +5.0 * plane to separate the upstream and downstream part of the target XYP 4 2.5 END * black hole BH1 5 +1 -2 * vacuum around VA2 5 +2 -3 * Be target 1st half BE3 5 +3 +4 * Be target 2nd half BE4 5 +3 -4 END GEOEND The same geometry can be described in name-based free format as follows: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 GEOBEGIN COMBNAME 0 0 A simple Be target inside vacuum RPP blakhole -5000000.0 +5000000.0 -5000000.0 +5000000.0 -5000000.0 +5000000.0 RPP vacumbox -1000000.0 +1000000.0 -1000000.0 +1000000.0 -100.0 +1000000.0 RPP betarget -10.0 +10.0 -10.0 +10.0 0.0 +5.0 * plane to separate the upstream and downstream part of the target XYP cutplane 2.5 END * black hole Blckhole 5 +blakhole -vacumbox * vacuum around Vacarund 5 +vacumbox -betarget * Be target 1st half UpstrBe 5 +betarget +cutplane * Be target 2nd half DwnstrBe 5 +betarget -cutplane END GEOEND 2.2.7 MaterialsEach geometry region is supposed to be filled with a homogeneous material, or
with vacuum, or with "blackhole". The latter is a fictitious material used to
terminate particle trajectories: any particle is discarded when reaching a
blackhole boundary. Materials can be simple elements or compounds, where an
element can have either natural composition or consist of a single nuclide,
and compound indicates a chemical compound or a mixture or an alloy (or an
isotopic mixture) of known composition.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 MATERIAL 4.0 9.0122 1.848 5.0 BERYLLIU Notice that the chosen name is BERYLLIU and not BERYLLIUM, in order to match
the name in the list of "low-energy cross section materials" for low
energy neutrons (see Chap. (10)), where material names have a maximum length of
8 characters.
2.2.8 Assigning materials to regionsA material must be associated to each of the geometry regions, except to those
defined as blackhole. This is done in a very straightforward way by command
ASSIGNMAt. Assigning explicitly blackhole to a region is allowed, but is not
necessary (except for region 2) because a region is blackhole by default unless
another material has been associated to it. (Region 2, if not assigned a
material, is COPPER by default).
The table entitled "Regions: materials and fields", in the standard output, can
be consulted to check that material assignment has been done as desired.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * Be target, 1st and 2nd half ASSIGNMAT 5.0 3.0 4.0 * External Black Hole ASSIGNMAT 1.0 1.0 * Vacuum ASSIGNMAT 2.0 2.0 The same material assignments in a name-based input would be the following (BLCKHOLE and VACUUM are the predefined names of material 1 and 2): *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * Be target, 1st and 2nd half ASSIGNMAT BERYLLIU UpstrBe DwnstrBe * External Black Hole ASSIGNMAT BLCKHOLE Blckhole * Vacuum ASSIGNMAT VACUUM Vacarund 2.2.9 Production thresholdsThe implicit NEW-DEFA default setting adopted in the example, sets, among other things, the production and transport threshold of heavy particles to 10 MeV. Production thresholds for e+e- and photons must be explicitly set using the EMFCUT command for all materials in the problem. Let us choose also in this case a 10 MeV threshold for the single material of the example (previously marked with material index 5 and name BERYLLIU). Following the instructions about the EMFCUT option, the card can be written as: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * e+e- and gamma production threshold set at 10 MeV EMFCUT -0.010 0.010 1.0 5.0 PROD-CUT where the first numerical field is the threshold for e+e- (the minus sign meaning kinetic energy) and the second is for photons. The material number is given in the fourth numerical field. For details on all other parameters, and for other possibilities (for example how to introduce a transport cutoff different from production threshold) the user should accurately consult the Notes to EMFCUT. In a name-based input, the above card could be: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 EMFCUT -0.010 0.010 1.0 BERYLLIU PROD-CUT Production and transport threshold for all other particles can be overwritten using the PART-THR command. 2.2.10 Estimators and DetectorsEven though, for setting-up purposes, it is conceivable that no estimator be
requested in a preliminary run, in most cases FLUKA is used to predict the
expectation value of one or more quantities, as determined by the radiation
field and by the material geometry described by the user: for such a task
several different estimators are available. The quantities which are most
commonly scored are dose and fluence, but others are available. Dose equivalent
is generally calculated from differential fluence using conversion
coefficients.
region region region first second third fourth number name volume quantity quantity quantity quantity 1 ...... ...... ........ ........ ........ ........ 2 ...... ...... ........ ........ ........ ........ etc. on a line for each geometry region. The region volumes (in cm3) have the value
1.0, or values optionally supplied by the user at the end of the geometry
description. All other columns are normalised per region volume and per primary
particle.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * score in each region energy deposition and stars produced by primaries SCORE 208.0 210.0 The same, in a name-based input: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 SCORE ENERGY BEAMPART Other estimators are more flexible: the corresponding detectors can be
requested practically in any number, can be written as unformatted or text
files, and in most cases can provide differential distributions with respect to
one or more variables. On the other hand, their output is presented in a very
compact array form and must generally be post-processed by the user. For this
purpose several utility programs are available: but output in text format can
even be exported to a spreadsheet for post-processing.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * Boundary crossing fluence in the middle of the target (log intervals, one-way) USRBDX 99.0 209.0 -47.0 3.0 4.0 400. piFluenUD USRBDX +50.0 +50.0 0.0 10.0 & * * Boundary crossing current in the middle of the target (log intervals, one-way) USRBDX -1.0 209.0 -47.0 3.0 4.0 400. piCurrUD USRBDX +50.00 +50.0 0.0 10.0 & The same, in a name-based input: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 USRBDX 99.0 PIONS+- -47.0 UpstrBe DwnstrBe 400. piFluenUD USRBDX +50.0 +50.0 0.0 10.0 & * USRBDX -1.0 PIONS+- -47.0 UpstrBe DwnstrBe 400. piCurrUD USRBDX +50.00 +50.0 0.0 10.0 & According to the instructions reported in the description of the USRBDX
command, it can be seen that the combined fluence of pi+ and pi- is requested
only when particles exit region "3" ("UpstrBe", the upstream half of the target)
to enter into region "4" ("DwnstrBe", the downstream half). There is no interest
in the reverse, therefore öne-way scoring" is selected. The scoring of the
first detector will be inverse cosine-weighted, in order to define correctly the
fluence. Results will be written unformatted on unit 47 for both quantities (so
there will be two "Detectors" on the same output unit, but this is not
mandatory). The energy distribution is going to be binned in 50 logarithmic
intervals, from 0.001 GeV (the default minimum) up to 50 GeV. The angular
distribution will be binned into 10 linear solid angle intervals from 0. to 2pi
(having chosen the one-way estimator). The results will be normalised dividing
by the area of the boundary (separation surface between the two regions, in
this case the transverse section of the target), and will provide a double-
differential fluence or current averaged over that surface in cm-2 GeV-1 sr-1.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * Tracklength fluence inside the target, Upstream part and Downstream part * Logarithmic energy intervals USRTRACK -1.0 209.0 -48.0 3.0 1000.0 20. piFluenU USRTRACK 50.0 0.001 & USRTRACK -1.0 209.0 -49.0 4.0 1000.0 20. piFluenD USRTRACK 50.0 0.001 & The volume input is 20 x 20 x 2.5 = 1000 cm3. We are requesting an energy
spectrum in 20 logarithmic intervals between 0.001 and 50 GeV. In this case, we
ask that the corresponding output be printed, unformatted, on two different
files.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 USRTRACK -1.0 PIONS+- -48.0 UpstrBe 1000.0 20. piFluenU USRTRACK 50.0 0.001 & USRTRACK -1.0 PIONS+- -49.0 DwnstrBe 1000.0 20. piFluenD USRTRACK 50.0 0.001 & Option USRBIN provides detailed space distributions of energy deposition, star density or integrated fluence (not energy fluence, unless by writing a special user routine). Using some suitable graphics package, USRBIN output can be presented in the form of colour maps. Programs for this purpose are available in the $FLUPRO/flutil directory (pawlevbin.f and various kumac files), and on the FLUKA website www.fluka.org. USRBIN results are normalised to bin volumes calculated automatically by the program (except in the case of region binning and special 3-variable binning which are only seldom used). The binning structure does not need to coincide with any geometry region. In our example we propose to ask for two Cartesian space distributions, one of charged pion fluence and one of total energy deposited. The first will extend over a volume larger than the target, because fluence can be calculated even in vacuum. Energy deposition, on the other hand, will be limited to the target volume. *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * Cartesian binning of the pion fluence inside and around the target USRBIN 10.0 209.0 -50.0 50.0 50.0 50. piFluBin USRBIN -50.0 -50.0 -10.0 100.0 100.0 60.0 & * Cartesian binning of the deposited energy inside the target USRBIN 10.0 208.0 -51.0 10.0 10.0 5. Edeposit USRBIN -10.0 -10.0 0.0 20.0 20.0 5.0 & Also in this case, the request is for output on two separate files.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 USRBIN 10.0 PIONS+- -50.0 50.0 50.0 50. piFluBin USRBIN -50.0 -50.0 -10.0 100.0 100.0 60.0 & USRBIN 10.0 ENERGY -51.0 10.0 10.0 5. Edeposit USRBIN -10.0 -10.0 0.0 20.0 20.0 5.0 & Angular yields around a fixed direction of particles exiting a given surface
can be calculated using option USRYIELD. The results are double-differential
distributions with respect to a pair of variables, one of which is generally
energy-like (kinetic energy, momentum, etc.) and the other one angle-like (polar
angle, rapidity, Feynman-x, etc.) Distributions in LET (Linear Energy Transfer)
can also be requested by this option. An arbitrary normalisation factor can be
input.
2.2.11 Initialisation of the random number sequenceThe random number sequence used in a run is initialised by default by the seeds contained in file random.dat provided with the code. To calculate the statistical error of the results, it is necessary to perform other independent runs (at least 4 or 5), each with a different independent initialisation, using the seeds written by the program at the end of each run. The rfluka script provided with the code on UNIX and LINUX platforms takes care of this task, provided the following card is issued in the input file: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 RANDOMIZE 1.0 0.0 The seeds of the random number generator are printed on a special file in
hexadecimal form at the end of each group of histories (the size of
a group depends on the number of histories requested in the START card).
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 RANDOMIZE 1.0 1198. 2.2.12 Starting signal and number of requested historiesAt the end of the input file, a START card is mandatory in order to actually start the calculation. That card must indicate also the number of particle histories requested. The run, however, may be completed before all the histories have been handled in two cases: if a time limit has been met (on some systems) or if a "stop file" is created by the user (see instructions in Note 2 to option START). The START card is optionally followed by a STOP card. For example, if the user wants to generate 100000 histories, the input file can be closed with the following two cards: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 START 100000.0 STOP 2.2.13 The sample input fileIn summary, the input file for our basic example (example.inp), name-based and written in fixed format, could be the following: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 TITLE Charged pion fluence inside and around a proton-irradiated Be target *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 BEAM 50.E+00 PROTON BEAMPOS 0.0 0.0 -50.0 * GEOBEGIN COMBNAME A simple Be target inside vacuum RPP blakhole -5000000.0 +5000000.0 -5000000.0 +5000000.0 -5000000.0 +5000000.0 RPP vacumbox -1000000.0 +1000000.0 -1000000.0 +1000000.0 -100.0 +1000000.0 RPP betarget -10.0 +10.0 -10.0 +10.0 0.0 +5.0 * plane to separate the upstream and downstream part of the target XYP cutplane 2.5 END * black hole Blckhole 5 +blakhole -vacumbox * vacuum around Vacarund 5 +vacumbox -betarget * Be target 1st half UpstrBe 5 +betarget +cutplane * Be target 2nd half DwnstrBe 5 +betarget -cutplane END GEOEND *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 MATERIAL 4.0 9.0122 1.848 5.0 BERYLLIU *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * Be target, 1st and 2nd half ASSIGNMAT BERYLLIU UpstrBe DwnstrBe * External Black Hole ASSIGNMAT BLCKHOLE Blckhole * Vacuum ASSIGNMAT VACUUM Vacarund *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 * e+e- and gamma production threshold set at 10 MeV EMFCUT -0.010 0.010 1.0 BERYLLIU PROD-CUT * score in each region energy deposition and stars produced by primaries SCORE ENERGY BEAMPART * Boundary crossing fluence in the middle of the target (log intervals, one-way) USRBDX 99.0 PIONS+- -47.0 UpstrBe DwnstrBe 400. piFluenUD USRBDX +50.0 +50.0 0.0 10.0 & * Boundary crossing current in the middle of the target (log intervals, one-way) USRBDX -1.0 PIONS+- -47.0 UpstrBe DwnstrBe 400. piCurrUD USRBDX +50.00 +50.0 0.0 10.0 & * Tracklength fluence inside the target, Upstream part and Downstream part * Logarithmic energy intervals USRTRACK -1.0 PIONS+- -48.0 UpstrBe 1000.0 20. piFluenU USRTRACK 50.0 0.001 & USRTRACK -1.0 PIONS+- -49.0 DwnstrBe 1000.0 20. piFluenD USRTRACK 50.0 0.001 & * Cartesian binning of the pion fluence inside and around the target USRBIN 10.0 PIONS+- -50.0 50.0 50.0 50. piFluBin USRBIN -50.0 -50.0 -10.0 100.0 100.0 60.0 & * Cartesian binning of the deposited energy inside the target USRBIN 10.0 ENERGY -51.0 10.0 10.0 5. Edeposit USRBIN -10.0 -10.0 0.0 20.0 20.0 5.0 & *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 RANDOMIZE 1.0 *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 START 100000.0 STOP The same input file, number-based and using the free format option for the FLUKA commands, but not for the geometry, can instead be written as follows: *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 TITLE Charged pion fluence inside and around a proton-irradiated Be target *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 GLOBAL 2.0 BEAM 50.E+00 0. 0. 0. 0. 0. PROTON BEAMPOS 0. 0. -50.0 0. 0. 0. ' ' *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8 GEOBEGIN COMBINAT A simple Be target inside vacuum RPP 1-5000000.0+5000000.0-5000000.0+5000000.0-5000000.0+5000000.0 RPP 2-1000000.0+1000000.0-1000000.0+1000000.0 -100.0+1000000.0 RPP 3 -10.0 +10.0 -10.0 +10.0 0.0 +5.0 XYP 4 2.5 END * black hole BH1 5 +1 -2 * vacuum around VA2 5 +2 -3 * Be target 1st half BE3 5 +3 +4 * Be target 2nd half BE4 5 +3 -4 END GEOEND MATERIAL 4.0 9.0122 1.848 5.0 0. 0. BERYLLIU * Be target, 1st and 2nd half ASSIGNMAT 5.0 3.0 4.0 0. 0. 0. * External Black Hole ASSIGNMAT 1.0 1.0 0. 0. 0. 0. * Vacuum ASSIGNMAT 2.0 2.0 0. 0. 0. 0. * e+e- and gamma production threshold set at 10 MeV EMFCUT -0.010 0.010 1.0 5.0, , , PROD-CUT * score in each region energy deposition and stars produced by primaries SCORE 208.0 210. 0. 0. 0. 0. * Boundary crossing fluence in the middle of the target (log intervals, one-way) USRBDX 99.0 +209.0 -47.0 3.0 4.0 +400.0 piFluenUD USRBDX +50.00 0. +50.0 0. 0. 10.0 & * Boundary crossing current in the middle of the target (log intervals, one-way) USRBDX -1.0 +209.0 -47.0 3.0 4.0 +400.0 piCurrUD USRBDX +50.00 0. +50.0 0. 0. 10.0 & * Tracklength fluence inside the target, Upstream part and Downstream part * Logarithmic energy intervals USRTRACK -1.0 209.0 -48.0 3.0 1000.0 20. piFluenU USRTRACK 50.0 0.001 0. 0. 0. 0. & USRTRACK -1.0 209.0 -49.0 4.0 1000.0 20. piFluenD USRTRACK 50.0 0.001 0. 0. 0. 0. & * Cartesian binning of the pion fluence inside and around the target USRBIN 10.0 209.0 -50.0 50.0 50.0 50. piFluBin USRBIN -50.0 -50.0 -10.0 100.0 100.0 60.0 & * Cartesian binning of the deposited energy inside the target USRBIN 10.0 208.0 -51.0 10.0 10.0 5. Edeposit USRBIN -10.0 -10.0 0.0 20.0 20.0 5.0 & RANDOMIZE 1.0 0. 0. 0. 0. 0. START 100000.0 0. 0. 0. 0. 0. STOP Other possible combinations are name-based free format and number-based fixed format. 2.2.14 Running FLUKAIt is advisable, but not mandatory, to keep separate the $FLUPRO directory from that or those where calculations are run and input files are kept. For instance, flukawork: cd /home/user/flukawork As mentioned above, the rfluka script in $FLUPRO/flutil should be used to drive the FLUKA run. In the following it is supposed that the user is going to ask for five statistically independent runs, each one made of 100000 histories, of the proposed basic example. The command is: $FLUPRO/flutil/rfluka -N0 -M5 example & (on LINUX/UNIX, the & character allows to run the program in the background
without "freezing" the terminal window).
$TARGET_MACHINE = Linux $FLUPRO = /home/user/flupro 2789: old priority 0, new priority 10 Initial seed already existing Running fluka in /home/user/flukawork/fluka_2789 ================================ Running FLUKA for cycle # 1 =================== At the end of each cycle the output files will be copied onto the running
directory, the temporary directory will be erased and a new one will be created
where the next run will take place. The names of the output files from each run
are built by appending to the input file name the run number and an extension
depending on their content: .out for the standard output, .err for the error
file, .log for the log file and _fort.nn for the estimator files (with
nn = absolute value of the selected output unit). The file containing the
random number seeds will be called "ran<input file name><run number>".
================================ Running FLUKA for cycle # 1 ==================== Removing links Removing temporary files Saving output and random number seed Saving additional files generated Moving fort.47 to /home/fasso/Fluka/test/example01_fort.47 Moving fort.48 to /home/fasso/Fluka/test/example01_fort.48 Moving fort.49 to /home/fasso/Fluka/test/example01_fort.49 Moving fort.50 to /home/fasso/Fluka/test/example01_fort.50 Moving fort.51 to /home/fasso/Fluka/test/example01_fort.51 ================================ Running FLUKA for cyle # 2 ==================== Removing links Removing temporary files Saving output and random number seed Saving additional files generated Moving fort.47 to /home/fasso/Fluka/test/example002_fort.47 Moving fort.48 to /home/fasso/Fluka/test/example002_fort.48 Moving fort.49 to /home/fasso/Fluka/test/example002_fort.49 Moving fort.50 to /home/fasso/Fluka/test/example002_fort.50 Moving fort.51 to /home/fasso/Fluka/test/example002_fort.51 ================================ Running FLUKA for cycle # 3 =================== ..... ================================ Running FLUKA for cycle # 4 =================== ..... ================================ Running FLUKA for cycle # 5 =================== Removing links Removing temporary files Saving output and random number seed Saving additional files generated Moving fort.47 to /home/fasso/Fluka/test/example005_fort.47 Moving fort.48 to /home/fasso/Fluka/test/example005_fort.48 Moving fort.49 to /home/fasso/Fluka/test/example005_fort.49 Moving fort.50 to /home/fasso/Fluka/test/example005_fort.50 Moving fort.51 to /home/fasso/Fluka/test/example005_fort.51 End of FLUKA run At this time, in the working directory, the following new files exist: example001_fort.47 example002_fort.47 .... example005_fort.47 example001_fort.48 example002_fort.48 .... example005_fort.48 example001_fort.49 example002_fort.49 .... example005_fort.49 example001_fort.50 example002_fort.50 .... example005_fort.50 example001_fort.51 example002_fort.51 .... example005_fort.51 example001.out example002.out .... example005.out example001.err example002.err .... example005.err In Chapter (9) the user can find a comprehensive description of the content of the FLUKA standard output. For the purpose of this beginner's guide, it can just be pointed out that, according to the content of the USRBDX command, the files with extension fort.47 contain, in binary form, the boundary crossing estimator output for the required pion fluence and current detectors (for more details see Chap. (9)). These files must be combined together to produce a table of values with their statistical errors which can be easily interfaced by the user to some analysis codes and/or graphic visualisation tools. Similarly, the files with extension fort.48 and fort.49 will contain the tracklength estimator output, and those with extension fort.50 and fort.51 the output from USRBIN. 2.2.15 Accessing resultsBoundary crossing estimatorBinary files from the USRBDX estimator can be accessed by means of the usxsuw.f
readout code, which is located in the $FLUPRO/flutil directory.
cd $FLUPRO/flutil ./lfluka usxsuw.f -o usxsuw The simplest way, however, is to use the makefile which is available in the $FLUPRO/flutil directory. In that directory, just type: make and all the postprocessing utilities will be compiled and linked.
cd /home/user/flukawork $FLUPRO/flutil/usxsuw The readout code will ask for the first FLUKA detector file name: Type the input file: For each detector file the program will show the content of the TITLE card of the FLUKA input file, the date and time of the FLUKA run and the number of histories for the given run. The request will be iterated until a blank line is given. This will be interpreted as the end of the list of files, and then a name for the output file prefix will be requested. Let's use, for example, the name "pionbdx": Type the input file: example001_fort.47 Charged pion fluence inside and around a proton-irradiated Be target DATE: 7/15/ 5, TIME: 16:22:11 100000. 100000 Type the input file: example002_fort.47 Charged pion fluence inside and around a proton-irradiated Be target DATE: 7/15/ 5, TIME: 16:23: 3 100000. 100000 Type the input file: example003_fort.47 Charged pion fluence inside and around a proton-irradiated Be target DATE: 7/15/ 5, TIME: 16:23:54 100000. 100000 Type the input file: example004_fort.47 Charged pion fluence inside and around a proton-irradiated Be target DATE: 7/15/ 5, TIME: 16:24:51 100000. 100000 Type the input file: example005_fort.47 Charged pion fluence inside and around a proton-irradiated Be target DATE: 7/15/ 5, TIME: 16:25:45 100000. 100000 Type the input file: Type the output file name: pionbdx At this point the following 3 new files are produced: pionbdx pionbdx_sum.lis pionbdx_tab.lis The first one (pionbdx) is again a binary file that can be read out at any time
by usxsuw. The content of this file is statistically equivalent to that of the
sum of the files used to obtain it, and it can replace them to be combined with
further output files if desired (the usxsuw program takes care of giving it the
appropriate weight).
The other two files are ASCII text files.
Charged pion fluence inside and around a proton-irradiated Be target Total primaries run: 500000 Total weight of the primaries run: 500000. Detector n: 1( 1) piFluenUD (Area: 400. cmq, distr. scored: 209 , from reg. 3 to 4, one way scoring, fluence scoring) Tot. resp. (Part/cmq/pr) 8.6904905E-04 +/- 0.6976866 % ( --> (Part/pr) 0.3476196 +/- 0.6976866 % ) The total (summed) number of primaries (histories) is reported at first, then
the main features of USRBDX request are summarised. The following numbers
represent the energy and angle integrated fluence ("total response").
Here and later, the statistical error is always expressed in percentage.
**** Different. Fluxes as a function of energy **** **** (integrated over solid angle) **** Energy boundaries (GeV): 49.99992 40.27077 32.43475 26.12349 21.04029 16.94620 13.64875 10.99293 8.853892 7.131072 5.743484 4.625898 3.725775 3.000802 2.416896 1.946608 1.567831 1.262757 1.017045 0.8191454 0.6597533 0.5313764 0.4279793 0.3447017 0.2776285 0.2236066 0.1800965 0.1450527 0.1168279 9.4095118E-02 7.5785778E-02 6.1039131E-02 4.9161937E-02 3.9595842E-02 3.1891152E-02 2.5685664E-02 2.0687662E-02 1.6662188E-02 1.3420003E-02 1.0808695E-02 8.7055033E-03 7.0115575E-03 5.6472253E-03 4.5483690E-03 3.6633324E-03 2.9505091E-03 2.3763892E-03 1.9139835E-03 1.5415541E-03 1.2415934E-03 Lowest boundary (GeV): 1.0000000E-03 Flux (Part/GeV/cmq/pr): 1.5418744E-09 +/- 99.00000 % 4.8503271E-08 +/- 6.709127 % 2.3456116E-07 +/- 6.506497 % 5.9040013E-07 +/- 3.466331 % 1.2585346E-06 +/- 4.051404 % 2.5295039E-06 +/- 2.039807 % 4.6113087E-06 +/- 2.195296 % 7.6260553E-06 +/- 1.939942 % 1.2214471E-05 +/- 0.8310503 % 1.8394410E-05 +/- 0.6178440 % 2.6636921E-05 +/- 1.128397 % 3.6855919E-05 +/- 1.204921 % 5.1703457E-05 +/- 1.100655 % 6.9101960E-05 +/- 0.7564522 % 9.0419722E-05 +/- 1.799108 % 1.1945122E-04 +/- 1.256268 % 1.5757892E-04 +/- 0.8898824 % 1.9452766E-04 +/- 1.332425 % 2.4165030E-04 +/- 1.521364 % 3.0573772E-04 +/- 2.473622 % 3.6900895E-04 +/- 1.399170 % 4.4734811E-04 +/- 0.9543594 % 5.2953843E-04 +/- 1.964312 % 6.1596523E-04 +/- 1.349476 % 6.4003764E-04 +/- 3.323846 % 6.8828161E-04 +/- 0.9288639 % 6.8151421E-04 +/- 2.018673 % 7.0822553E-04 +/- 4.401796 % 7.4972271E-04 +/- 2.600316 % 6.9859857E-04 +/- 3.693749 % 6.8915845E-04 +/- 4.332464 % 6.6514849E-04 +/- 8.753220 % 6.4636284E-04 +/- 11.30834 % 5.5008888E-04 +/- 7.691558 % 4.3721433E-04 +/- 11.36630 % 3.2056248E-04 +/- 8.380781 % 4.2511927E-04 +/- 12.24571 % 2.2697043E-04 +/- 12.99932 % 2.0069227E-04 +/- 13.10813 % 1.7180138E-04 +/- 16.90801 % 9.9383309E-05 +/- 21.15753 % 2.9268101E-04 +/- 39.29378 % 1.5672133E-04 +/- 44.01294 % 2.1093644E-04 +/- 34.72458 % 7.4201569E-05 +/- 33.68359 % 7.2452240E-05 +/- 33.54827 % 8.6934262E-05 +/- 62.03180 % 1.0245090E-04 +/- 99.00000 % 1.6312006E-04 +/- 82.06016 % 1.3002084E-04 +/- 52.15991 % Soon after, the cumulative fluence distribution as a function of energy is also given: **** Cumulative Fluxes as a function of energy **** **** (integrated over solid angle) **** Energy boundaries (GeV): 49.99992 40.27077 32.43475 26.12349 21.04029 16.94620 13.64875 10.99293 8.853892 7.131072 5.743484 4.625898 3.725775 3.000802 2.416896 1.946608 1.567831 1.262757 1.017045 0.8191454 0.6597533 0.5313764 0.4279793 0.3447017 0.2776285 0.2236066 0.1800965 0.1450527 0.1168279 9.4095118E-02 7.5785778E-02 6.1039131E-02 4.9161937E-02 3.9595842E-02 3.1891152E-02 2.5685664E-02 2.0687662E-02 1.6662188E-02 1.3420003E-02 1.0808695E-02 8.7055033E-03 7.0115575E-03 5.6472253E-03 4.5483690E-03 3.6633324E-03 2.9505091E-03 2.3763892E-03 1.9139835E-03 1.5415541E-03 1.2415934E-03 Lowest boundary (GeV): 1.0000000E-03 Cumul. Flux (Part/cmq/pr): 1.5001119E-08 +/- 99.00000 % 3.9507350E-07 +/- 7.326498 % 1.8754495E-06 +/- 5.464718 % 4.8765669E-06 +/- 1.819896 % 1.0029117E-05 +/- 1.898280 % 1.8370021E-05 +/- 1.277005 % 3.0616819E-05 +/- 0.6900454 % 4.6929261E-05 +/- 0.9553517 % 6.7972585E-05 +/- 0.7029299 % 9.3496434E-05 +/- 0.6531623 % 1.2326548E-04 +/- 0.5382378 % 1.5644032E-04 +/- 0.6154544 % 1.9392396E-04 +/- 0.6043725 % 2.3427299E-04 +/- 0.5368618 % 2.7679623E-04 +/- 0.5548110 % 3.2204165E-04 +/- 0.6000980 % 3.7011484E-04 +/- 0.6263003 % 4.1791250E-04 +/- 0.6480659 % 4.6573509E-04 +/- 0.7125404 % 5.1446725E-04 +/- 0.7778813 % 5.6183949E-04 +/- 0.8066853 % 6.0809392E-04 +/- 0.7142704 % 6.5219263E-04 +/- 0.7654761 % 6.9350738E-04 +/- 0.7260005 % 7.2808337E-04 +/- 0.8159186 % 7.5803063E-04 +/- 0.7573094 % 7.8191340E-04 +/- 0.7549785 % 8.0190296E-04 +/- 0.7531289 % 8.1894622E-04 +/- 0.7366922 % 8.3173712E-04 +/- 0.6872664 % 8.4189989E-04 +/- 0.6799491 % 8.4980001E-04 +/- 0.6579692 % 8.5598318E-04 +/- 0.6862395 % 8.6022145E-04 +/- 0.6667165 % 8.6293457E-04 +/- 0.6859071 % 8.6453673E-04 +/- 0.6995495 % 8.6624804E-04 +/- 0.6864265 % 8.6698390E-04 +/- 0.6886846 % 8.6750800E-04 +/- 0.6864119 % 8.6786930E-04 +/- 0.6882262 % 8.6803763E-04 +/- 0.6885374 % 8.6843700E-04 +/- 0.6933275 % 8.6860918E-04 +/- 0.6915213 % 8.6879585E-04 +/- 0.6911866 % 8.6884876E-04 +/- 0.6931223 % 8.6889038E-04 +/- 0.6942393 % 8.6893054E-04 +/- 0.6953420 % 8.6896872E-04 +/- 0.6967193 % 8.6901762E-04 +/- 0.6981055 % 8.6904905E-04 +/- 0.6976866 % The numbers for the cumulative distribution have been obtained by multiplying
each value of the differential distribution by the corresponding energy bin
width (variable if the distribution is logarithmic as in our example). The
integral fluence in any given energy interval can be obtained as the difference
between the values of the cumulative distribution at the two bounds of that
interval.
**** Double diff. Fluxes as a function of energy **** Solid angle minimum value (sr): 0.000000 Solid angle upper boundaries (sr): 0.6283185 1.256637 1.884956 2.513274 3.141593 3.769911 4.398230 5.026548 5.654867 6.283185 Angular minimum value (deg.): 0.000000 Angular upper boundaries (deg.): 25.84193 36.86990 45.57299 53.13010 60.00000 66.42182 72.54239 78.46304 84.26083 90.00000 Let us take for instance the energy bin between 0.345 GeV and 0.278 GeV: Energy interval (GeV): 0.3447016 0.2776284 Flux (Part/sr/GeV/cmq/pr): 2.2090337E-04 +/- 2.271138 % 1.6099877E-04 +/- 2.023665 % 1.2373505E-04 +/- 3.802638 % 9.4749055E-05 +/- 2.419357 % 7.0389280E-05 +/- 5.640523 % 6.6853667E-05 +/- 9.292711 % 6.8042267E-05 +/- 5.421218 % 6.8482914E-05 +/- 11.91976 % 5.8157104E-05 +/- 2.943847 % 4.8027632E-05 +/- 39.71496 % Flux (Part/deg/GeV/cmq/pr): 5.3710260E-06 +/- 2.271138 % 9.1729089E-06 +/- 2.023665 % 8.9330297E-06 +/- 3.802638 % 7.8776966E-06 +/- 2.419357 % 6.4377805E-06 +/- 5.640523 % 6.5410413E-06 +/- 9.292711 % 6.9850003E-06 +/- 5.421218 % 7.2676362E-06 +/- 11.91976 % 6.3026077E-06 +/- 2.943847 % 5.2580162E-06 +/- 39.71496 % The same structure is then replicated for Detector no. 2: Detector n: 2( 2) piCurrUD (Area: 400. cmq, distr. scored: 209 , from reg. 3 to 4, one way scoring, current scoring) Tot. resp. (Part/cmq/pr) 7.1694393E-04 +/- 0.7243900 % ( --> (Part/pr) 0.2867776 +/- 0.7243900 % ) and so on. Note that in this case the ratio between the calculated fluence (8.690E-04) and the corresponding current (7.169E-04) is about 1.2. The ratio between the numerical values of the two quantities would be 1 if the pions were all crossing the boundary at a right angle, 2 in the case of an isotropic distribution, and could even tend to infinity if the particle direction were mainly parallel to the boundary: FLUENCE AND CURRENT ARE VERY DIFFERENT QUANTITIES AND SHOULD NOT BE CONFUSED! Note also that the above output reports also the current value not normalised
per unit area. This is equivalent to a simple count of crossing particles, so
we see that in our example about 0.287 charged pions per primary proton cross
the middle plane of the target.
# Detector n: 1 piFluenUD (integrated over solid angle) # N. of energy intervals 50 1.000E-03 1.242E-03 1.300E-04 5.216E+01 1.242E-03 1.542E-03 1.631E-04 8.206E+01 1.542E-03 1.914E-03 1.025E-04 9.900E+01 1.914E-03 2.376E-03 8.693E-05 6.203E+01 2.376E-03 2.951E-03 7.245E-05 3.355E+01 2.951E-03 3.663E-03 7.420E-05 3.368E+01 3.663E-03 4.548E-03 2.109E-04 3.472E+01 4.548E-03 5.647E-03 1.567E-04 4.401E+01 5.647E-03 7.012E-03 2.927E-04 3.929E+01 ..... By convention, when in a given bin the statistics is not sufficient to
calculate a standard deviation, the statistical error is printed as 99%. For a
null fluence the statistical error is also null.
# double differential distributions # number of solid angle intervals 10 # 0.000E+00 6.283E-01 6.283E-01 1.257E+00 1.257E+00 1.885E+00 ... # .... 2.069E-02 2.569E-02 4.013E-05 2.472E+01 4.509E-05 2.068E+01 ... 2.569E-02 3.189E-02 5.408E-05 1.907E+01 4.657E-05 2.200E+01 ... 3.189E-02 3.960E-02 5.150E-05 7.137E+00 5.355E-05 1.587E+01 ... .... Track length estimatorThe program to analyse USRTRACK binary output is called ustsuw.f and can also be found in $FLUPRO/flutil. Its use is very similar to that of usxsuw.f described above. Applying it to the example00*_fort.48 files (output of the first USRTRACK detector in our example), we obtain for the average fluence of charged pions in the upstream half of the beryllium target: Tot. response (p/cmq/pr) 5.4765277E-04 +/- 0.6965669 % and from the example00*_fort.49 files (pion fluence in the downstream half): Tot. response (p/cmq/pr) 1.3474772E-03 +/- 0.5352812 % As it was to be expected, the average fluence obtained above by the boundary crossing estimator on the middle surface (8.69E-04 cm-2) has a value which is intermediate between these two. Binning estimatorTo analyse the binary output from USRBIN, two programs are needed, both available in $FLUPRO/flutil. The first, usbsuw.f, performs a statistical analysis of the results and produces a new unformatted file, with a name chosen by the user. The second program, usbrea.f, reads the latter file and writes on a formatted file two arrays, namely the content of each bin, averaged over the given number of runs, followed by the corresponding errors in percent. The second USRBIN detector defined in example.inp gives the following values of energy deposition (in GeV/cm3): 1 Cartesian binning n. 1 "Edeposit " , generalized particle n. 208 X coordinate: from -1.0000E+01 to 1.0000E+01 cm, 20 bins ( 1.0000E+00 cm wide) Y coordinate: from -1.0000E+01 to 1.0000E+01 cm, 20 bins ( 1.0000E+00 cm wide) Z coordinate: from 0.0000E+00 to 5.0000E+00 cm, 5 bins ( 1.0000E+00 cm wide) Data follow in a matrix A(ix,iy,iz), format (1(5x,1p,10(1x,e11.4))) accurate deposition along the tracks requested 1.7164E-07 3.4587E-07 2.1976E-07 3.0997E-07 1.4963E-07 3.5431E-07 ..... 5.6597E-07 7.5792E-07 3.6563E-07 2.7822E-07 2.6084E-07 2.8645E-07 ..... 2.6191E-07 1.6716E-07 3.8680E-07 2.4925E-07 4.2334E-07 3.5025E-07 ..... ............................................................................. and the following corresponding percentage errors: Percentage errors follow in a matrix A(ix,iy,iz), format (1(5x,1p,10(1x,e11.4))) 1.3936E+01 4.3211E+01 3.0601E+01 2.2874E+01 1.7783E+01 2.7942E+01 ..... 1.6548E+01 1.2291E+01 1.4539E+01 2.4576E+01 2.7828E+01 1.7247E+01 ..... 2.2423E+01 1.7258E+01 2.0349E+01 3.7997E+01 2.6855E+01 2.9230E+01 ..... ............................................................................. 2.2.16 Readout of other FLUKA estimatorsThe $FLUPRO/flutil directory contains other similar programs to average the outputs from other FLUKA estimators (not used in the present example): 2.2.17 Various settingsAccuracy and computer speed depend on several parameters that can be freely
tuned by the user: but in general an increase of either one results in a
decrease of the other. The proper balance must be based both on physical and on
practical considerations. The present defaults have been designed to give
satisfactory results in the most common cases: but other sets of defaults can
be implemented using option DEFAULTS (to be placed at the beginning of the
input file, just after the title).
2.2.18 BiasingAlthough FLUKA is able to perform fully analogue particle transport calculations (i.e. to reproduce faithfully actual particle histories), in many cases of very non-uniform radiation fields, such as those encountered in shielding design, only a very small fraction of all the histories contributes to the desired response (dose, fluence) in the regions of interest, for instance behind thick shielding. In these cases, the user's concern is not to simulate exactly what occurs in reality, but to estimate in the most efficient way the desired response. This can be obtained by replacing the actual physical problem with a mathematically equivalent one, i.e. having the same solution but faster statistical convergence. A rigorous mathematical treatment of such variance-reduction techniques can be found in several textbooks (see for instance those of Lux and Koblinger [Lux91] or Carter and Cashwell [Car75]). In the present practical introduction we will only issue a few important warnings:
The simplest (and safest) biasing option offered by FLUKA is importance
biasing, which can be requested by option BIASING. Each geometry region is
assigned an "importance", namely a number between 1.0E-4 and 1.0E+4,
proportional to the contribution that particles in that region are expected to
give to the desired result. The ratio of importances in any two adjacent
regions is used to drive a classical biasing algorithm ("Splitting" and
"Russian Roulette"). In a simple, monodimensional attenuation problem, the
importance is often set equal to the inverse of the expected fluence
attenuation factor for each region.
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