Radiation quality of cosmic ray nuclei studied with Geant4-based simulations

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1 FAIRNESS 0: FAIR NExt generation o ScientistS 0 Journal o Phsics: Conerence Series 50 (0) 00 doi:.88/-6596/50//00 Radiation qualit o cosmic ra nuclei studied with Geant-based simulations Lucas N. Burigo,, Igor A. Pshenichnov,, Igor N. Mishustin, and Marcus Bleicher, Frankurt Institute or Advanced Studies, Johann Wolgang Goethe Universit, 608 Frankurt am Main, German Institut ür Theoretische Phsik, Johann Wolgang Goethe Universit, 608 Frankurt am Main, German Institute or Nuclear Research, Russian Academ o Sciences, Moscow, Russia Kurchatov Institute, Russian Research Center, 8 Moscow, Russia burigo@ias.uni-rankurt.de Abstract. In uture missions in deep space a space crat will be exposed to a non-negligible lux o high charge and energ (HZE) particles present in the galactic cosmic ras (GCR). One o the major concerns o manned missions is the impact on humans o complex radiation ields which result rom the interactions o HZE particles with the spacecrat materials. The radiation qualit o several ions representing GCR is investigated b calculating microdosimetr spectra. A Geant-based Monte Carlo model or Heav Ion Therap (MCHIT) is used to simulate microdosimetr data or HZE particles in extended media where ragmentation reactions pla a certain role. Our model is able to reproduce measured microdosimetr spectra or H, He, Li, C and Si in the energ range o MeV/u. The eect o nuclear ragmentation on the relative biological eectiveness (RBE) o He, Li and C is estimated and ound to be below %.. Introduction Future space travels shall take humans to the Moon or extended periods and even beond in interplanetar missions [, ]. Not onl urther technological developments will be necessar, but also a better understanding o the health risks to crew members exposed to galactic cosmic ras (GCR). The GCR spectrum is composed mainl b protons and helium nuclei with onl a raction o about % due to heavier ions showing a broad energ spectra peaked between 0 00 MeV/u. Although the lux o high energ and charge (HZE) particles is low, a high biological eectiveness o such particles increases their contribution to the received biological dose. Since HZE particles can not be eectivel shielded during the journe ollowing chronic radiation eects are expected to be signiicant [] Shielding o crew members is indispensable or their protection rom protons o solar particle events and rom the proton component o GCR. However, the linear energ transer (LET) o a HZE particle increases due to the energ loss in spacecrat materials, and a complex radiation ield is created due to nuclear ragmentation reactions. Dose monitors or such radiation ields shall be able to cope with a broad LET spectrum. In particular, a Tissue Equivalent Proportional Counter (TEPC) has been applied or microdosimetr measurements aboard Space Shuttle []. TEPC emulates a micrometer volume o human tissue and measures lineal energ,, o stochastic Content rom this work ma be used under the terms o the Creative Commons Attribution.0 licence. An urther distribution o this work must maintain attribution to the author(s) and the title o the work, journal citation and DOI. Published under licence b Ltd

2 FAIRNESS 0: FAIR NExt generation o ScientistS 0 Journal o Phsics: Conerence Series 50 (0) 00 doi:.88/-6596/50//00 events. Lineal energ is deined as the energ deposited to a sensitive volume o TEPC divided b its mean chord length [5]. The probabilit distributions o (microdosimetr spectra) measured b TEPC can be used in estimating LET and RBE o complex radiation ields [6, ]. Our previous investigations o the response o TEPC to radiation ields relevant to space research [8] are extended to detailed simulations o microdosimetr spectra. In the present work such spectra resulting rom irradiation o extended media with protons, helium and HZE particles in the energ range o MeV/u are compared to experimental data. The contributions o secondar ragments created in ragmentation o beam nuclei are calculated. The FAIR acilit will allow microdosimetr measurements and radiobiological experiments with HZE particles at higher energies. This will help to reduce the uncertainties in estimating the health risks o astronauts due to expose to GCR.. Materials and Methods The radiation environment inside a spacecrat due to GCR irradiation can be studied b Monte Carlo method. The interaction o GCR with various materials can be simulated with the Geant toolkit [9, ]. Originall being developed or experiments in high energ phsics, the code is now widel used or modeling in space research. It comprises sotware libraries with a broad set o unctionalities or the simulation and analsis o particle propagation in various media. A user should implement its own sotware application tailored to the particular research task. The Monte Carlo model or Heav Ion Therap (MCHIT) is a Geant-based application intended to stud phsics processes relevant to ion-beam cancer therap [, ]. MCHIT has been used to describe a wide set o experimental data including depth-dose proiles or protons and carbon nuclei in tissue-like materials, ields o secondar nuclear ragments, energ spectra and angular distributions o secondar neutrons. Microdosimetr spectra or neutrons, protons and light ions measured with TEPCs were also calculated [, ]. MCHIT is currentl built with Geant o version 9.5 with patch 0. The electromagnetic processes are simulated b means o GEmPenelope including models or ionization process o gas media b ions. The ast stage o nucleus-nucleus collision is modeled b Light Ion Binar Cascade model or helium and lithium projectiles and b Quantum Molecular Dnamics model or carbon and silicon projectiles. Further details on the phsics list and respective models are given elsewhere []. In this stud the MCHIT model is used or investigating the eect o nuclear ragmentation reactions on microdosimetr spectra and RBE or protons, helium and HZE particles. Microdosimetr spectra measured at NIRS, Japan or 60 MeV H, 50 MeV/u He and 90 MeV/u 8 Si [5] as well as at GSI, German or 85 MeV/u Li and 00 MeV/u C [6] were taken or benchmarking. The measurements at NIRS were perormed with a wall-less TEPC emulating a clindrical tissue volume o 0. µm in diameter while at GSI a was applied emulating a spherical volume o. µm in diameter. At NIRS the wall-less TEPC was placed behind range shiters made o PMMA, while the was placed inside a water phantom at GSI. RBE is estimated b MCHIT coupled with the Microdosimetric-Kinetic (MK) model developed b Hawkins [6] and extended b Kase et. al []. The impact o nuclear ragmentation reactions on the relative biological eectiveness (RBE) o radiation ields b helium, lithium and carbon at dierent water-equivalent depths is evaluated. Details o the calculational procedure can be ound elsewhere [].. Results and Discussions Microdosimetr spectra were simulated or various ions showing a general agreement with experimental data as presented in igure. Panel (a) shows the spectrum or homogeneous proton irradiation o a wall-less TEPC behind a 6 mm-we (water-equivalent) range shiter. The spectrum is peaked at lineal energ = kev/µm extending up to 0 kev/µm. In panel

3 FAIRNESS 0: FAIR NExt generation o ScientistS 0 Journal o Phsics: Conerence Series 50 (0) 00 doi:.88/-6596/50//00 (b) the spectrum or helium beam with same wall-less TEPC but behind a range shiter o 5. mm-we is presented. In this case the spectrum is peaked at 5 kev/µm due to primar ions while secondar protons give the main contribution or events below kev/µm. Lineal energ events due to helium projectiles extend up to 00 kev/µm. Panels (c) and (d) show the spectra measured with a irradiated b a pencil-like lithium beam inside a water phantom at the plateau and Bragg peak o the depth-dose distribution, respectivel. The disagreement between experimental data and simulation results in panel (c) is likel related to pile-up o events during data acquisition in experimental set-up as discussed elsewhere []. The change o radiation qualit with depth in water is clearl seen. Not onl the contribution o lithium changes due to the reduction o kinetic energ, but also the role plaed b secondar particles evolves. At the plateau position, the contribution o projectile-like helium ragments is seen between 0. 8 kev/µm, and o target-like ragments between 8 0 kev/µm. This is explained b the act that projectile-like ragments have velocities similar to the primar ion but smaller stopping power due to a smaller nuclear charge. The contribution o primar ions is peaked at 5 kev/µm and extends up to kev/µm. When TEPC is moved deeper into phantom, the peak is shited to 60 kev/µm and a shoulder in the spectrum at lower values is clearl visible due to the contribution o helium and hdrogen ragments. Panels (e) and () show similar measurements with a pencil-like carbon beam at plateau and peak positions, respectivel. Also in this case a clear change o radiation qualit is observed with depth where a variet o secondar particles along with the primar carbon ions contribute to the spectra. The maxima are observed at 6 kev/µm and 0 kev/µm at plateau and peak, respectivel. The underestimation o the satellite peak observed in the spectrum at the peak position indicates that the Geant models or nuclear ragmentation ma need urther improvements. The panels (g) and (h) present the spectra measured with the wall-less TEPC behind two range shiters o dierent thickness irradiated b silicon ions. As seen, the shape o the calculated spectra is deined b light (Z < ) and heav (Z > 6) nuclei, which contribute to low ( < 0 kev/µm) and high ( > kev/µm) linear energ events. Behind a range shiter o 5 mm-we the spectrum is peaked at 0 kev/µm, while the peak is shited to 00 kev/µm when the thickness o the range shiter is increased to 59.6 mm-we. Microdosimetr spectra or 5 MeV/u He, 6 MeV/u Li and 90 MeV/u C ions corresponding to the same range in water were calculated with MCHIT at various depths inside a water phantom. The resulting spectra were used as an input to MK model in order to estimate the RBE corresponding to % survival o Human Salivar Gland (HSG) cells ater irradiation. The obtained RBE as a unction o depth is shown in igure. Microdosimetr spectra were also calculated at same positions or simulations when nuclear ragmentation reactions are neglected. The corresponding RBE is also presented in igure or comparison. One can see that nuclear ragmentation causes a reduction o RBE in case the radiation ield would be composed onl b primar ions. The RBE or helium and lithium is onl signiicantl decreased at the ar end o the ion range while or carbon ions the RBE values at all investigated positions are decreased b less than %. This shows that the loss o a primar carbon ion and ield o secondar ragments shall result in less biological eect or this particular end-point.. Conclusions Microdosimetr spectra or hdrogen, helium, lithium, carbon and silicon ions in the energ range relevant to GCR can be successull calculated with Geant/MCHIT. The measurements o microdosimetr spectra behind shielding open another possibilit to validate Monte Carlo transport codes. Nuclear ragmentation reactions that happen in the shielding o a spacecrat decrease the RBE or survival o HSG cells with respect to primar nuclei. Thereore, not onl the phsical dose, but also biological eects o radiation can be reduced b such shielding.

4 FAIRNESS 0: FAIR NExt generation o ScientistS 0 Journal o Phsics: Conerence Series 50 (0) 00 doi:.88/-6596/50//00 F (a) H, 60 MeV (b) a range shiter o 5. mm we He, 50 MeV/u Z= Z= a range shiter o 6 mm we (c) at plateau Li, 85 MeV/u with pile up Z= Z= Z= (d) at Bragg peak Li, 85 MeV/u Z= Z= Z= (e) at plateau C, 00 MeV/u Z=6 Z=5 Z= Z= Z= Z= () C, 00 MeV/u Z=6 Z=5 Z= Z= Z= Z= at Bragg peak (g) a range shiter o 5.0 mm we 8 Si, 90 MeV/u Z>= Z<=6 (h) a range shiter o 59.6 mm we 8 Si, 90 MeV/u Z>= Z<=6 Figure. Microdosimetr spectra or (a) H, (b) He, (c-d) Li, (e-) C and (g-h) 8 Si at dierent water-equivalent depths. Experimental data rom [5, 6] are shown.

5 FAIRNESS 0: FAIR NExt generation o ScientistS 0 Journal o Phsics: Conerence Series 50 (0) 00 doi:.88/-6596/50//00 RBE.5.5 +MK C 90 MeV/u Li 6 MeV/u He 5 MeV/u Depth in water (mm) Figure. RBE or HSG cells as a unction o depth in water ater irradiation b light nuclei. Solid line present expected results calculated b MCHIT+MK models when all interaction processes are taken into account in simulations. Dashed lines present hpothetical RBE in the absence o nuclear ragmentation o the considered beams. Acknowledgments The presented results were obtained in the ramework o NanoBIC-NanoL project. L.N.B. is grateul to the Beilstein Institute or support. This work was also partiall supported b HIC or FAIR within the Hessian LOEWE-Initiative. Our calculations were perormed at the Center or Scientiic Computing (CSC) o the Goethe Universit Frankurt. We are grateul to the sta o the Center or support. Reerences [] NASA The NASA Vision or Space exploration URL [] ESA Aurora s roadmap to Mars URL Activities/Human Spacelight/Exploration/Aurora s roadmap to Mars [] Durante M and Cucinotta F A 008 Nat Rev Cancer 8 65 [] Badhwar G D, Cucinotta F A, Brab L A and Konradi A 99 Radiat. Res. 9 5 [5] ICRU 98 Report No. 6, Microdosimetr (Bethesda, MD: ICRU) [6] Hawkins R B 00 Radiation Research [] Kase Y et al 006 Radiat. Res [8] Burigo L, Pshenichnov I, Mishustin I and Bleicher M 0 Journal o Phsics: Conerence Series [9] Agostinelli S et al 00 Nucl. Instrum. Methods A [] Allison J et al 006 IEEE T. Nucl. Sci [] Mishustin I, Pshenichnov I and Greiner W 0 Eur. Phs. J. D 60 9 [] Pshenichnov I, Botvina A, Mishustin I and Greiner W 0 Nucl. Instrum. Methods B [] Burigo L, Pshenichnov I, Mishustin I and Bleicher M 0 Nucl. Instrum. Methods B 5 [] Burigo L, Pshenichnov I, Mishustin I and Bleicher M 0 Nucl. Instrum. Methods B [5] Tsuda S et al 0 J. Radiat. Res. 5 6 [6] Martino G, Durante M and Schardt D 0 Phs. Med. Biol