Nuclear Instruments and Methods in Physics Research B 269 (2011) 2850–2856

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Nuclear Instruments and Methods in Physics Research B

journal homepage: www.elsevier .com/locate /n imb

Applications of FLUKA Monte Carlo code for nuclear and accelerator physics

Giuseppe Battistoni a, Francesco Broggi a, Markus Brugger b, Mauro Campanella a, Massimo Carboni e,Anton Empl j, Alberto Fassò c, Ettore Gadioli a, Francesco Cerutti b, Alfredo Ferrari b, Anna Ferrari f,Matthias Lantz m, Andrea Mairani a, M. Margiotta h, Cristina Morone g, Silvia Muraro a, Katia Parodi i,Vincenzo Patera f, Mauricio Pelliccioni f, Larry Pinsky j, Johannes Ranft d, Stefan Roesler b, Sofia Rollet l,Paola R. Sala a, Mario Santana c, Lucia Sarchiapone e, Massimiliano Sioli h, George Smirnov b,Florian Sommerer i, Christian Theis b, Stefania Trovati b, R. Villari f, Heinz Vincke b, Helmut Vincke b,Vasilis Vlachoudis b,⇑, Joachim Vollaire b, Neil Zapp k

a INFN and University of Milano, Milano, Italyb CERN, 1211 Geneva 23, Switzerlandc SLAC, Stanford, USAd University of Siegen, Siegen, Germanye INFN, Legnaro, Italyf INFN, Frascati, Italyg INFN and University of Roma II, Roma, Italyh INFN and University Bologna, Bologna, Italyi HIT, Heidelberg, Germanyj University of Houston, Houston, USAk NASA, Houston, USAl ARC Seibersdorf, Seibersdorf, Austriam Riken Laboratory, Japan

a r t i c l e i n f o a b s t r a c t

Article history:Available online 21 April 2011

Keywords:FLUKASimulationnTOFLHC

0168-583X/$ - see front matter � 2011 Elsevier B.V.doi:10.1016/j.nimb.2011.04.028

⇑ Corresponding author. Tel.: +41 22 7679851; fax:E-mail address: Vasilis.Vlachoudis@cern.ch (V. Vla

FLUKA is a general purpose Monte Carlo code capable of handling all radiation components from thermalenergies (for neutrons) or 1 keV (for all other particles) to cosmic ray energies and can be applied in manydifferent fields. Presently the code is maintained on Linux. The validity of the physical models imple-mented in FLUKA has been benchmarked against a variety of experimental data over a wide energy range,from accelerator data to cosmic ray showers in the Earth atmosphere. FLUKA is widely used for studiesrelated both to basic research and to applications in particle accelerators, radiation protection and dosim-etry, including the specific issue of radiation damage in space missions, radiobiology (including radio-therapy) and cosmic ray calculations.

After a short description of the main features that make FLUKA valuable for these topics, the presentpaper summarizes some of the recent applications of the FLUKA Monte Carlo code in the nuclear as wellhigh energy physics. In particular it addresses such topics as accelerator related applications.

� 2011 Elsevier B.V. All rights reserved.

1. Description of the FLUKA code

FLUKA [1,2] is a multipurpose transport Monte Carlo code, forcalculations of particle transport and interactions with matter,covering an extended range of applications spanning from protonand electron accelerator shielding to target design, calorimetry,activation, dosimetry, detector design, Accelerator driven systems,cosmic rays, neutrino physics, radiotherapy etc. Presently it ismaintained and supported as a joint CERN–INFN project, which is

All rights reserved.

+41 22 7669644.choudis).

continuously undergoing development and benchmarking. FLUKAis able to transport 60 different elementary particles, whicheverheavy ions and can perform hadron–hadron, hadron–nucleus,lepton and electromagnetic interactions from 1 keV up to10000 TeV/A. It is able to perform charged particle transport bothin magnetic and electric field (currently under development)including all relevant processes [1].

The hadronic interaction model is called PEANUT and consistsof a phenomenological description (Dual Parton Model-basedGlauber-Gribov cascade) of high-energy interactions (up to20 TeV), a generalized intra-nuclear cascade, and pre-equilibriumemission models as well as models for evaporation, fragmentation,fission, and de-excitation by gamma emission.

Fig. 1. Thin target example of the angle-integrated cross section for 90Zr(p,xn)reaction at 80.5 MeV. The various histograms show the total and partial crosssections: total (blue), Intranuclear Cascade (green), pre-equilibrium (cyan) andevaporation (magenta). The experimental data are displayed with the red points.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

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For ion–ion interactions FLUKA employs dedicated interfacesthe DPMJET-III [3], rQMD [4] and BME [5] models. FLUKA imple-ments DPMJET-III as event generator to simulate nucleus–nucleusinteractions for energies in excess of 5 GeV/A up to the highest cos-mic ray energies (1018 to 1020 eV). DPMJET is based on the twocomponents Dual Parton Model in connection with the Glauberformalism. FLUKA also has an interface to rQMD-2.4, a relativisticQMD model which has been applied successfully to particle pro-duction in relativistic A–A interactions over a wide energy rangefrom 0.1 GeV/A up to several hundred of GeV/A. Several importantmodifications have been implemented in the rQMD code, in orderto ensure energy–momentum conservation by taking into accountexperimental binding energies, and to provide meaningful excita-tion energies for the residual fragments. For lower energiesE < 100 MeV/A FLUKA makes use of the BME (Boltzmann MasterEquation) model, which is presently under testing for A < 17. Forall generators, de-excitation and evaporation of the excited resid-ual nuclei is performed by calling the standard FLUKA evapora-tion/fission/fragmentation module.

The transport of neutrons with energies below 20 MeV is per-formed by a multi-group algorithm based on evaluated cross sec-tion data (ENDF/B, JEF, JENDL, etc.) binned into 260 energygroups, 31 of which are in the thermal energy region. For a few iso-topes (1H, 6Li, 10B, 14N) point-wise cross sections can be optionallyused during transport. Currently there is work going on the imple-mentation of point wise low energy neutron treatment withemphasis on the correlations between the participants in eachreaction. The detailed implementation of electromagnetic pro-cesses in the energy range between 1 keV and 1 PeV is fully cou-pled with the models for hadronic interactions.

FLUKA features a combinatorial geometry which is constantlyenhanced in order to cope with new demands for more and morecomplex detector descriptions. During the tracking the code hasa double capability to run either fully analogue and/or biasedcalculations.

1.1. Code design

FLUKA is based, as far as possible, on original and well testedmicroscopic models [6,7]. Due to this ‘‘microscopic’’ approach tohadronic interaction modeling (Figs. 1 and 2), each step is self-con-sistent and has solid physical bases. Performances are optimizedcomparing with particle production data at single interaction level:‘‘theory driven, benchmarked with data’’. No tuning whatsoever isperformed on ‘‘integral’’ data, such as calorimeter resolutions, thicktarget yields, etc. Therefore, final predictions are obtained with aminimal set of free parameters, fixed for all energies and for all tar-get/projectile combinations. Results in complex cases as well asscaling laws and properties come forth naturally from the underly-ing physical models and the basic conservation laws which are ful-filled a priori; therefore the code has a predictive power in theregions where no experimental data are directly available.

The philosophy of the FLUKA authors was not to provide a tool-kit for particle physics simulations, but rather a code that providesthe best physics available. For this reason all physical models inFLUKA are fully integrated in the code and the user has limitedmeans of tweaking them. All correlations are fully preserved with-in interactions and among shower components with the onlyexception the low energy neutrons (E < 20 MeV) that presently uti-lizes group cross sections. The authors continue to make a huge ef-fort to ensure self-consistency with full cross-talk between allcomponents (hadronic, electromagnetic, neutrons, leptons, heavyions), and to achieve the same level of accuracy for each compo-nent and for all energies. For example, the transport and interac-tions of leptons and photons are fully coupled to the hadronicsector; in the same simulated event it enables the following of sec-

ondary hadrons from photon–nuclear interactions and c rays fromnuclear de-excitation.

1.2. Continuous evolution

The FLUKA collaboration ensures the constant update of thecode with the most advanced physics and features [8–12]. Someof the latest additions to the code among others are:

- improvements in the Glauber cross section for light nuclei;- improvements in the photo-fission angular momentum

distribution;- new nucleon elastic scattering model;- neutrino event generator;- improvements in the DPMJET-3 and BME heavy ion models;- enhanced transformations in the geometry;- Barkas and Bloch corrections for nuclear stopping power, Mott

corrections for heavy ions;- Non ionizing energy losses (NIEL) and displacement per atom

(DPA) scoring;- Special source distributions;- ICRU predefined materials using the recommended ICRU elec-

tronic and nuclear stopping power.

1.3. Code Interaction with the user

FLUKA is a card driven program based on a text input file; eventhough it offers numerous routines for user customization for mostof the cases there is no need for the user to utilize them. Althoughbeing an excellent tool for particle simulations problems, the de-fault package was offering a limited number of auxiliary programs

(a) (b)

Fig. 2. Double differential cross section for a thin target example (a) p + 90Zr ? p + X (80 MeV) and (b) p + Al ? p- + X (4 GeV/c).

Fig. 3. Scheme of the new spallation target, with the 4 cm thickness for themoderator and the 1 cm thickness for the cooling liquid. The 60 cm diameter, 40 cmlength cylindrical lead core is placed at the center enclosed within a pressurized

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and tools for building the input and post processing the output ofthe code.

Over the last years there was a big effort to make the code moreuser friendly, easier to learn without compromising anything fromits potential. This gave birth to the FLAIR [13] interface. FLAIR is anadvanced user graphical interface for FLUKA, to enable the user tostart and control simulation jobs completely from a GUI environ-ment without the need for command-line interactions. It is an inte-grated development environment (IDE) for FLUKA, provides notonly the means for the post processing of the output but a bigemphasis has been set on the creation and checking of error freeinput files. It contains a fully featured editor for the input files ina human readable way with syntax highlighting, without hidingthe inner functionality of FLUKA from the users. It provides alsomeans for building an executable, debugging the geometry, run-ning the code, monitoring the status of one or many runs, inspec-tion of the output files, post processing of the binary files (datamerging) and interface to plotting utilities like gnuplot [14] andPovRay [15] for high quality plots or photo-realistic images. Theprogram includes also a database of selected properties of allknown nuclides and their known isotopic composition as well areference database of �300 predefined materials together withtheir Sternheimer parameters. One of the latest and importantadditions to the interface is the interactive 2D geometry editorand debugger, permitting the users to inspect and create compli-cated geometries quite easily.

Implementing geometries for particle transport problems is oneof the major time consuming tasks. The common approach of radi-ation transport codes is based on constructive solid geometry(CSG) and requires textual input of the boolean geometry tree. Thismakes the creation of models a tedious and error prone task, whichis especially hard to master for novice users. SimpleGeo [16] is aninteractive solid modeler which allows for flexible and easy crea-tion of the models via drag and drop, as well as on-the-fly inspec-tion. In addition it includes new debugging facilities, based onstochastic as well as deterministic methods, in order to validate

the created geometry with immediate visual feedback of problem-atic regions.

2. Applications in nuclear and high energy physics

The huge potential of FLUKA makes it an excellent tool for manyother applications like target design, beam intercepting devices,windows, dumps, detector optimization, dose estimations,radiological treatment planning, shielding design etc. Therefore,FLUKA is a tool that is constantly employed in the majority of CERNapplications where energy deposition has to be calculated throughbeam-matter interactions, as well as for radiation protection andshielding simulations. The main ingredient in performing such asimulation is to have a good description of the underlyinggeometry, building and construction materials and the source

vessel.

Fig. 4. Neutron fluence at the experimental area as a function of the energy from measurements with the PTB 235U fission chamber, microMegas loaded with 10B, comparedwith the FLUKA predictions together with an optical tracking program to take into account the effect of the capture collimation setup.

Fig. 5. FLUKA simulation of the equivalent neutron path inside the moderator evaluated at the energy of observation inverse proportional to the energy resolution.

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term. Then the user has to ‘‘guide’’ the code in order to extract thebest suited information for the problem in a reasonable CPU-time.In the following sections some examples are illustrated.

2.1. nTOF target design

FLUKA has been used for the optimization of several beam-tar-gets at CERN like the nTOF [17,18] facility. The nTOF target (Fig. 3)produces neutrons through the spallation reaction of 20 GeV/c pro-tons onto a massive lead target, with a cylindrical shape (diameter60 cm with a length of 40 cm) followed by a water moderator of5 cm. The dimensions and material are chosen after a series of sim-ulations to maximize the neutron flux with the best energy resolu-

tion [19]. Therefore the figure of merit used was the output fluxdivided by the square of the neutron energy resolution, a quantitythat cancels the effect of the time-of-flight distance. The end resultwas a target based on a high A material having a length corre-sponding to the maximum of the hadronic cascade of about threeinteraction lengths of the incident proton. The predicted perfor-mances of the nTOF facility were confirmed with an extensiveexperimental program at nTOF (Figs. 4 and 5) [20].

2.2. LHC beam intercepting devices

The LHC will produce proton–proton collisions at a center-of-mass energy of 14 TeV with unprecedented high beam intensi-

Fig. 6. Maximum of energy deposition on a collimator jaw versus mass–length of various candidates collimator materials for the wrong kick scenario at 7 TeV/c.

Fig. 7. Power density [W/cm3] on the first TCSM Phase II SLAC design rotable collimator for the 1 h beam life time scenario. On left a horizontal cut and on the right a photo-realistic image of the collimator with the power density data surface mapped.

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ties. For nominal performance it is planned to fill 2808 bunchesof 1.1 � 1011 protons each. Both LHC beams will store 3 � 1014,7 TeV protons, corresponding to a stored energy of 350 MJ. Thishigh beam power must be handled in a super-conducting envi-ronment, severely constraining the fraction of the beam thatcan be lost in a super-conducting magnet. Even a tiny fractionof order of 1/109 of the total beam power is sufficient to quencha magnet if it is deposited in its cold parts [21]. The LHC colli-mators are designed to restrict the mechanical aperture in themachine and to clean the primary halo so that quenches of mag-nets are avoided. Due to the high beam power LHC will be thefirst machine requiring the collimators to define the mechanicalaperture through the whole cycle of the machine. The conse-quences of beam impact on the collimators were calculated forvarious scenarios and then used as input to the FLUKA studiesof hadronic showers.

Initially an extensive study of candidate materials has been per-formed (Fig. 6), however, the choice of materials for the collimatorsof the LHC was limited to low Z – materials, like Graphite or Beryl-lium. This was not due to the normal operational scenarios but

rather to accidental, rapid impacts of beams, either at injectionor, more severely, during a coast at 7 TeV that could seriously de-stroy the object.

Based on the simulations for Phase I, the energy density on thecold magnets was higher than the quench limit, therefore a twostage collimation system was decided to be used coupled with pas-sive absorbers to protect the warm elements and active absorbersto protect the superconducting magnets. For the Phase II (Fig. 7)there were various alternatives studied and the work is currentlygoing on.

In order to assess the energy deposition in sensitive compo-nents, extensive simulations were performed in IR7. The 1.5 kmlong tunnel section had to be simulated, including the straight sec-tion and part of the ARC for both dispersion suppressors on the leftand right side. For such a challenging simulation work it has beendecided to follow a modular approach in the geometry definitionwith an extensive use of user written programs, thereby allowingthe implementation of all magnets and collimators with high pre-cision, including the modeling of flanges, steel supports and mag-netic fields.

Fig. 8. Neutron background measured as a function of TOF (red histogram) with 3He + polyethylene sphere, compared with the FLUKA simulations (blue histogram). (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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2.3. Shielding applications

Shielding application design is one of the biggest strength ofFLUKA since it was originally created as a tool for this purpose.Among numerous examples of shielding applications one thatdemonstrates the capability of the code not only to estimate thenecessary shielding but to predict rare events was the backgroundproblem at nTOF [22]. The first experimental measurements per-formed at the nTOF [17,18] facility at CERN have unveiled animportant background, 50–100 times higher than expected [23].This background had consequences on capture measurements bothin terms of accuracy and required beam time. Extensive measure-ments have been performed in order to better understand thebackground. However, it was not possible to identify the mecha-nism responsible for producing it, since all ‘‘known’’ backgroundsources have been well studied and shielded. A detailed investiga-tion using FLUKA has lead to the discovery of some ‘‘rare’’ eventswith huge importance, highlighting the origin for the background.These events originated from negative muon capture in the con-crete walls leading to a neutron emission directly into the experi-mental area. The hypothesis has been confirmed with an extensivemeasurement program. The experimental measurements [24] havegiven results consistent with the expectations (Fig. 8). They havedemonstrated that the majority of the neutron background wasnot neutron beam related, and confirmed the presence of a muonfluence with attenuation measurements. Moreover, the experi-ment verified the effectiveness of the iron shielding reducing themuon related background by a factor of 30 rendering the back-ground levels equivalent to the ones estimated in the design [22].

3. Conclusions

FLUKA is a general-purpose particle interaction and transportcode with roots in radiation protection studies at high energyaccelerators. However, with the continuous developments in thecode the field of applications of FLUKA is becoming wider andwider with a growing emphasis towards description and designof physical experiments. The easiness of use and physics capabili-ties of the simulation code in the detailed description of both EM

and hadronic processes from very low energies up to cosmic rayenergies proved to be a key issue in many applications.

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