CERN Accelerating science


Advanced graphical user interface for particle simulation programs

Fluka Tutorial

author: Vasilis Vlachoudis
version: 3.0
last change: Nov 2019

The present tutorial we will go step-by-step to build from scratch without the use of the Geometry Editor, run and analyze the results of a simple FLUKA simulation.

Basic knowledge of FLUKA is required.

Time needed for this tutorial: XXX h

We will simulate the neutron production and energy deposition of a proton beam hitting a lead target similar to the old n_TOF spallation target at CERN. The target had a rectangular shape of 80x80x60 cm3 and it was submerged in a water container with a ~5 cm layer of water that was used both for cooling and moderation. The neutrons were produced by a 20 GeV proton beam, impacting with 10º angle on the horizontal plane.

The user must make a choice on the coordinate system. The general tendency is to use the Z-axis collinear with the beam axis (usually lying on the horizontal plane), and then select the vertical and horizontal axis. In this example we will use the following convention:

X – horizontal axis, pointing to the left, with respect to beam direction

Y – vertical axis, pointing upwards

Z – beam axis, usually horizontal

Using the Geometry Editor for building the geometry is the strongly recommended way of working but this is covered in another tutorial.
Flair is an evolving project therefore some options, menus, forms or dialog might not be exactly the same or might be replaced by similar functionality
Solution: tutorial.flair
In this chapter we will be starting a new flair project using the FLUKA
  1. Launch flair by typing the following command
    $ flair
    or from the system application menu
    $ flair -h offers a short help page for all available command line arguments of flair. Some can be very useful like:
    • -1 open the first flair project in the current directory.
    • -r open the last opened flair project

    On the flair main window one can find:

    • a tab bar on top for selecting the various pages of flair.
      On the right side of the tab bar there is drop down ▼ icon showing more pages that are available.
      The tab bar is fully customizable by dragging and reordering the location of each tab. Don't do it now
    • a ribbon-bar with buttons for accessing all commands of the current page
      Some of the button groups have a drop-down arrow ▼ offering additional functionality.
    • a status bar at the bottom to display some useful information for the current frame.
      Note that some items in the status bar are clickable and provide some quick access functionality
    • one or more page frames in the center. The frames are fully customizable by the user, by pinning them or dragging them to reorganize
    • The window-title displays the name of the project we are currently are working on with an asterisk if there are modifications.. Since we didn't specify any name it will show the word [Untitled] - flair
  2. Select the Flair tab (if it is not selected).
  3. Click on the button with the new icon to create a new input file based on the basic template.
    User templates are located in the user directory ~/.flair/templates
  4. In the title entry field, type a title like:

    An information dialog box will appear asking if you want to update the TITLE and GEOBEGIN cards inside the FLUKA input using the title that was entered in the project. Click yes and automatically all FLUKA cards that require a title string will be set with the project title.

  5. add also a small note describing the present project
  6. Finally click on the save icon on the left of the tab bar. Save it as tutorial.flair preferably save it under a new directory. Please use this name to be consistent in all the following commands.
    You can create directories directly in the save dialog there is a dedicated button for it.
    Control-S key is a shortcut for save
Now we will edit the input building the nTOF target geometry.
  1. Select the
    The page below will show the input editor. The editor will contain the default basic template, with the TITLE card filled with our Project title. The selected cards are highlighted with a Light Yellow background color, while the active card has a thick black border around it. We start editing the file by going through one by one the input cards:
    Card anatomy and terminalogy:
    To select a card click with the mouse the Tag. If you click on any label or value it will select the card and start editing the value directly
    You can select multiple cards. They will be highlighted with yellow and the active will have a black border.

    warning When you have multiple selected cards any change in the active will be copied to ALL selected cards

    During input editing there are two modes:
    • Card mode where you can manipulate the cards as a single object: i.e. Drag and drop, move, delete, insert, copy, paste...
    • Field editing mode, to modify the contents of a card. .. To start editing the fields of a card, first select the card with the mouse or the arrows. Click Enter to start the field editing mode. To exit editing press Escape-key, and you will return to Card mode.
    Everything in flair is fully customizable, colors, fonts etc.
    Go to the
    , easily accessible at the bottom-right button of the status bar.
  2. Select the BEAM card by clicking on the name of the card.
    Edit the card to look like the following: Start editing either by pressing Enter-key or by clicking with the mouse on the appropriate field. Use the Tab-key to move to the next field:
    • Label Beam select Energy as beam type.
      The next label will change to E:
    • Label E: type 20 at the beam energy, meaning 20 GeV
    • Label Part: select PROTON as particle type
    • Label Δp select Gauss as momentum distribution
    • Label Δp(FWHM) enter 0.082425 as momentum spread (GeV/c)
    • Label Δφ select Gauss as angular distribution
    • Label Δφ(FWHM) type 1.7 as the angular spread in mrad
    You will notice that the card display at the bottom of the screen will start to fill in with the values you typed, highlighting with yellow the changes from the previous state. Flair always converted the numbers into floating point format using the best representation of the number to ensure the maximum accuracy.
    All flair list boxes are searchable. Type-in the starting characters of the item you are searching and the closest match will be selected
    Hovering the mouse over the various fields displays a short help, the default values and the units of each field.
    At any time you can hit F1 to consult the manual for the active card
  3. Fill up the BEAMPOS card with the following values:

    We are defining a beam hitting at the location [-2.2632, -0.5, -10]cm with a direction of 10° on the horizontal plane.

    To use expressions in the fields you need to start with the = character. Look the flair manual for the valid expressions and the limitations. Expressions can be useful for creating more readable input using units, trigonometric expressions etc.
  4. Select the Geometry node on the input tree (Left frame) to de-clatter the display.

    The input editor will show only the cards belonging to the geometry group.

    The basic template generated a default geometry consisting of two concentric huge spheres named blkbody and void, and a cylindrical target named target.

    In the next steps we will replace the target by a right parallelepiped named watercnt and add two parallelepipeds named pbtarget and niche

  5. Select the card RCC target either by clicking on it or using the Up/Down keys
  6. Select from the Ribbon the command
    Change ▼

    Geometry → Body → RPP

    The above command will convert the RCC to RPP

    WARNING: The Change commands will change the type of card, while at the same time it tries to keep as much as possible from the whats/fields. All exceeding whats in the new type will be discarded.
  7. Edit the body to:
    • Change the comment to Water container
    • Change the body name to watercnt
    When changing a body name, region, material, or detector all cards referring to this name will be changed as well.
  8. Create two new RPP bodies.
    Either by right-clicking and selecting

    Geometry → Body → RPP

    or from the Ribbon or by hitting Control-Enter or Ins
    Another way would be to duplicate the current RPP hitting Control-d or with the
    ribbon button.
    To add a comment on a card, right click the card and select Insert Comment or from the Ribbon select the Comment ▼.
    Or with the shortcut Control-M even simply c
    Is always a good practice to avoid touching (co-planar) bodies in FLUKA. Either try to cut the bodies with the use of infinite planes, or slightly overlap the bodies and then performing the appropriate logical operation in the region definition.
  9. Select the REGION TARGET and change the comment and expression as below
    Hitting the +, -, Insert; keys or the icon while editing a REGION's expression shows a list of bodies to select from. Press the Escape key if you are not interested in adding any body. This behavior can change from the configuration panel.
    Lists in flair are search-able. Type the beginning of the item you are looking for and the closest match will be highlighted. Ctrl-N or Ctrl-G repeats the last search
  10. Add a new REGION named WATERCNT and edit it as below
    To re-order the cards you can Drag and Drop them with the mouse, or using the Control-Up/Down keys or from the Ribbon with the
    Move Up
    Move Down
  11. Select the Media item on the Input tree on the left side to edit the material assignments to the two REGIONs TARGET and WATERCNT as follows:
    Materials can be created either manually with the cards MATERIAL and COMPOUND
    or import them from the Material database accessible on the Ribbon as
    Material ▼
  12. Select the Scoring from the Input tree
  13. Add some scoring cards one USRBIN, and two USRTRACKs as shown
    We are selecting as output file BINary since they are the only files we can later process to merge the statistics. The ASCii files cannot be further processed with the standard tools of FLUKA.
  14. Set some primary particles in the START card for a test run. The START card can be found in the Input Tree under the Primary group
  15. Press Control-S key to save the project

We will briefly visualize the geometry and try to navigate in the 3D space using the geometry editor

  1. Click on the
    tab to visualize the geometry

    The geometry editor is extremely powerful tool for building, debugging and visualizing the geometry and data. Please follow the dedicated tutorial to understand how it works.

    At this moment all we want to see is that the geometry looks correct and there is no error or warning displayed that needs to be addressed.

    Using the Middle mouse button and a combination of Shift, Control keys you can navigate very quickly in the geometry.
    • Panning press and hold the Middle mouse button in any viewport and drag the mouse.
    • Set pivot point click once the Middle mouse button the viewport will center to this location and set it as a pivot point for the rotations
    • Orbit press Control and the Middle mouse button the viewport will rotate. Useful only when displaying in 3D. It should be used with Setting the pivot point
    • Zoom In/Out rotate the Wheel of your mouse
    • Pan Backward/Forward press Control and rotate the mouse Wheel
    • Orientation Cube hover the Mouse over the orientation cube on the bottom-left corner of each viewport. The cube will become bigger and interactive. Click at any location, or the colored axis to change the orientation of the viewport and or align the axis
  2. As an exercise go to the green viewport which shows the geometry in 3D. Hover the mouse over the Orientation cube on the bottom-left corner.
  3. Click on the highlighted corner of the cube as in the above image.
    The plot will be oriented facing from 45° azimuthal and 45° polar angle.
  4. Press & hold the Control and rotate the mouse Wheel towards you, until you see the complete geometry. This action moves the viewport position backwards from the screen.
  5. In case you get lost you can can go back to the previous viewport locations using the and arrows on the private toolbar of the viewport.
  6. In case you want to reset the viewport click the Home button in the Orientation cube and/or the Origin button depicted as a circle.
  7. Press Control-S key to save the project
In this chapter we will use the FLUKA plotting capabilities to visualize the geometry as FLUKA will understand it. Flair is using a different geometry kernel from FLUKA, so it is good sometimes to verify that fluka understands correctly as in flair the geometry.
  1. Select the

    If you have selected the geometry tab before the plot window will be populated with 4 geometry plots corresponding to the 4 viewports of the Geometry editor.

  2. Click on the
    plot in the left listbox with the plots.
  3. Click the
    button on the ribbon.

    it will prepare a temporary input file with only the necessary information to plot the geometry, execute FLUKA with this temporary input file and display the plot with the use of Gnuplot as plotting engine.

    Control-Enter shortcut executes the default action on each tab. Run for running, Plot for plotting etc.
    Flair as a "makefile" checks the validity of each file based on the creation time with respect to its dependencies. For example the plot depends on the fluka output which depends on the temporary input file which depends on the input, If something has changed in between it will try to re-calculated only what it is needed. If for some reason it fails, it is a good habit to press the
    button and force recalculation of everything.
  4. Press Control-S key to save the project
We are ready now to launch a FLUKA run and monitor its progress
  1. Select the

    The listbox on the left shows all the runs that are associated with the current input file. The first entry <tutorial> inside the angle-brackets <> corresponds to a run using the input *AS IS*.

    The user can create clones of the run modifying some parameters like the title, primaries, random number seed, operation mode and as well #define variables

    For the moment we are going to run the input as is

    To run on a multicore machine enter the number of CPUs you want to use in the Spawn column of the run list. It will create one separate run for each CPU.
  2. Select the <tutorial> if it is not selected and click
    You should see the progress of the run at the dialog below like in the image. Flair by default will submit 5 cycles as defined with the Prev: No: and To: fields on the Ribbon.
    The refreshing takes place every few seconds but if for some reason it stops you can always press the
    During the execution the “Status” will change, initially to Waiting to attach, followed by Running and finally in Finished OK.
    The run is submitted using the defined submit program (the default is nohup). The program is running decoupled from the flair editor, therefore if you click save on the project and exit the program. The next time you will open the program, flair will try to attach and display the current run status.
    Flair is trying to peek the run information by looking the status of the output files. It doesn't make use of the system process information. This way it increases portability across different platforms, and batch systems (see the example for a substitution of the submit command). Flair will be able to monitor the status only if the run takes place on the same directory. The drawback of this method is that takes some time to attach.
    • Cycle
      will try to make a clean stop of the current cycle by creating a fluka.stop.
    • Run
      will try to make a clean stop of the complete run by creating a rfluka.stop file in the running directory.
    • Kill
      will try to make brutally kill the current run if it is possible, and reset the state of flair.
    • Attach
      tries to re-attach to the running process, in the case that flair lost the connection
  3. Press Control-S key to save the project
  4. Wait until the simulation finishes. It will take a few minutes.
During the FLUKA run or when a cycle is completed, the user can inspect and/or delete the output files generated by FLUKA from the Run:Files sub-page
  1. Selecting the
    tab (which should be already selected and then select the
    button on the ribbon. The page is composed by 3 list-boxes.
    • Run on the left the common listbox with all defined runs
    • Cycles in the middle containing all the cycles that are performed and output files exists for the specific run. There are a few virtual cycles like compile, data, input, plot, temporary containing files generated by other process.
    • Files selecting one or many cycles the last listbox will show all the relevant files belonging to that cycle where you can view, edit, delete, rename etc...
    WARNING: Do not try to open binary files. Could be rather huge for the editor or viewer and in any case incomprehensible.
  2. Click on the file tutorial001.out and click the
    to visualize the output file of FLUKA. Spent some time to familiarize with the output.
    You don't need to wait to finish the run to see the output files. You can inspect them at any time as they are generated.

The next step is to merge the output data files of the run from the various cycles in order to create the files contain the average values and the statistical error to be used for plotting.

A bit of theory on the use of cycles

One would expect that the simulation is equivalent to a counting experiment, therefore the data will follow a Poisson distribution and the error will be the square root of the number of events collected. This is true provided that no biasing is used in the simulation. When importance scoring is involved (quite typical and recommended way of working) to calculate correctly the statistical error, apart from the final value one has to record also the square number of events/hits (second momentum) for every value needed. This doubles the memory and increases the complexity for special estimators. Therefore, FLUKA is making use of the Central Limit Theorem for calculating the mean value of a quantity scored and the error on the determination of the mean. The theorem states:

The distribution of an average tends to be Normal, even when the distribution from which the average is computed is decidedly non-Normal.

This is the main reason we have to perform several cycles, minimum 5 is recommended to simulate correctly a Normal distribution, and then sum-up and average the results. In FLUKA this is done automatically with the us?suw utilities (where ? can be: b=USRBIN, r=RESNUCLEi, t=USRTRACK or USRCOLL, x=USRBDX, y=USRYIELD). These programs expect as input a list of binary files generated from FLUKA with the respective card and using as unit a negative number, and in the end they generate a set of output files both binary, text and tabulated with the results.

Flair makes this process transparent to the user with the Process Data page.

  1. Select the Run:Data tab page
  2. Normally flair should detect all scoring cards and create the appropriate data merging rules.
    To re-scan all detectors: select all Detectors from the top listbox and click
    which will delete everything and force the re-scanning of the input.
  3. Select the detectors to process and click
    It will take a few seconds/minutes to process all files. The progress is visible on the Status bar of flair. A notification will inform on the outcome of the operation.
    The FLUKA utilities usually generate more than one output files. Typically the merge binary data file has the requested name while for a text file is generated with the extension _sum.lis, and a tabulated one with the extension _tab.lis
The last step is to plot the data, we will create 2 plots one for the USRBIN file that contains the energy deposition on the spallation target, one for the USRTRACK estimators with the particle fluences.
  1. Select the
  2. Click on the wizard button
    . It will create one plot for each data file. In our case it will create two plots:
    • tutorial_50_plot
    • tutorial_51_plot
    Normally the plots are ready to be plotted but lets fine tune them

9.1. USRBIN Plotting

  1. Select in the list the
    plot and the frame on the right will be populated with the USRBIN Plot information
  2. Fill in the values as you see in the following image.
    • Title: enter the plot title
    • Title: clicking the title shows additional configuration options like font, color...
    • Detector click on the button and select the tutorial_usrbin_50 file we created in the Data page. Automatically the run and detector information in the file will be filled.
    • Projection & Limits select projection on the Y-axis without supplying any limits. This will generate a projection from -54 to +54cm on the XZ plane.
    • Norm select as normalization the formula “7e12*1.6e-19*1e9”, this way every value will be converted from GeV/cm3/p to J/cm3/pulse where a pulse has 7e12 protons.
    Advanced options are hidden and can be enabled by clicking the relevant LabelButton like the Title: or x:... Once clicked additional options will appear
    You can edit multiple plots at the same time. When you will select more than one plots the common fields will be displayed and will be by default DISABLED (Grayed out). With the mouse Right click on the field and it will enable it. All fields that are enabled will propagate their value as is to all selected plots.
  3. Clicking on the
    button (or Ctrl-Enter), flair will create a temporary input file for FLUKA with the appropriate PLOTGEOM card. It will perform a FLUKA run to create the geometry information, and then process the USRBIN data file with the gplevbin program to create the requested projection. Finally it will call gnuplot to create the plot.
  4. Once the plot is created you can save it as image by clicking the Save ▼ drop down list and select the file type.
    Note that the Save ▼ is located in the Ribbon close to the Plot button.
    The save buttons on the ribbon acts only for the current displayed tab page and not for the flair project.
    Prefer to use the .eps format for higher quality figures.

9.2 Single Differential Quantities Plot

The scoring cards USRBDX, USRCOLL, USRTRACK, USRYIELD after the data merging are producing a single differential quantities that can be plotted with the “USR-1D” plot frame in flair. This frame is using the _tab.lis file and many data can be super imposed one on top of the other.
  1. Select the
  2. Fill in the values as you see in the following image.
  3. Be careful to tick the Log for the X and Y axes
  4. Rename the Detector 1 to Proton and select the Det: 1 Proton ▼
  5. Clone the Proton detector with Ctrl-D or with the button
  6. Rename the cloned detector to Neutron and select Det: 2 Neutron
  7. Select both detectors Proton and Neutron. All the fields will be disabled (grayed out).
  8. Right click on the Value: and select the <X>*Y. You are selecting Iso-lethargic way of plotting
  9. Right click on the Y Norm: and type in 7e12 as the normalization for both detectors
  10. Click on the Proton and on the Neutron to verify that the plotting value and the normalization are properly taken for both
  11. Click on the
    button and you should see the following plot
  12. Optionally you can customize even more the plot using the commands big text box below, issuing directly gnuplot commands
    set xtics ('1meV' 1e-12, '10m' 1e-11, '100m' 1e-10, '1eV' 1e-9, '10' 1e-8,'100' 1e-7,'1keV' 1e-6, '10k' 1e-5, '100k' 1e-4, '1MeV' 1e-3, '10M' 0.01, '100M' 0.1, '1GeV' 1, '10G' 10, '100G' 100, '1TeV' 1000, '10T' 1e4, '100T' 1e5)
    set label 'Evaporation peak' at 5e-6,2e17 font 'Arial,14'
  13. Finally you can "push" the plots to the Project notes page of the
    tab by selecting both plots
    • tutorial_50_plot
    • tutorial_51_plot
    and click the
  14. Select the
    The project note page should look like
  15. Press Control-S key to save the project