Igor Mishustin

Multifragment break-up of nuclei by intermediate-energy protons

Abstract A cascade-fragmentation-evaporation (CFE) model of proton-nucleus reactions at intermediate energies is developed. Within the framework of this model a systematic analysis has been performed of a broad set of data on nuclear fragmentation induced by protons with energies E p ≈ 1–100 GeV. This includes: fragment mass and charge yields, angular and energy distribution, fragment yields in correlation with fast particles, etc. A satisfactory agreement between the CFE model predictions and experimental data has been obtained. Unresolved problems and ways of improving the CFE model are discussed.

Multifragmentation of spectators in relativistic heavy ion reactions

Abstract Using the ALADIN forward-spectrometer at SIS we have studied multi-fragment decays of 197 Au projectiles after collisions with C, Al, Cu and Pb targets at a bombarding energy of E A = 600 MeV . The new data presented in this work comprise the measured cross sections of multifragment processes, the N Z ratios of the produced fragments, and the differential distributions of fragment multiplicities and of observables built on the charge correlations within the event. The 197 Au+Cu data are analyzed with the statistical multifragmentation model. It is shown that all observables can be simultaneously reproduced on an absolute scale, apart from an overall normalization constant which relates the number of model events to the measured cross section. A continuous distribution of excited residual nuclei, used as input for the calculations, was derived from the criterion of a best fit to the data. It exhibits a correlation between decreasing mass number A and increasing excitation energy E x A and a saturation of the excitation energy at E x A ≈ 8 MeV .

Distributions of positron-emitting nuclei in proton and carbon-ion therapy studied with GEANT4

Depth distributions of positron-emitting nuclei in PMMA phantoms are calculated within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit (version 8.0). The calculated total production rates of (11)C, (10)C and (15)O nuclei are compared with experimental data and with corresponding results of the FLUKA and POSGEN codes. The distributions of e(+) annihilation points are obtained by simulating radioactive decay of unstable nuclei and transporting positrons in the surrounding medium. A finite spatial resolution of the positron emission tomography (PET) is taken into account in a simplified way. Depth distributions of beta(+)-activity as seen by a PET scanner are calculated and compared to available data for PMMA phantoms. The obtained beta(+)-activity profiles are in good agreement with PET data for proton and (12)C beams at energies suitable for particle therapy. The MCHIT capability to predict the beta(+)-activity and dose distributions in tissue-like materials of different chemical composition is demonstrated.

Neutrons from fragmentation of light nuclei in tissue-like media: a study with the GEANT4 toolkit

We study energy deposition by light nuclei in tissue-like media taking into account nuclear fragmentation reactions, in particular, production of secondary neutrons. The calculations are carried out within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit. Experimental data on depth-dose distributions for 135-400 A MeV (12)C and (18)O beams are described very well without any adjustment of the model parameters. This gives confidence in successful use of the GEANT4 toolkit for MC simulations of cancer therapy with beams of light nuclei. The energy deposition due to secondary neutrons produced by (12)C and (20)Ne beams in a (40-50 cm)(3) water phantom is estimated to be 1-2% of the total dose, that is only slightly above the neutron contribution (approximately 1%) induced by a 200 MeV proton beam.

Mach shocks induced by partonic jets in expanding quark–gluon plasma

We study Mach shocks generated by fast partonic jets propagating through a deconfined strongly-interacting matter. Our main goal is to take into account different types of collective motion during the formation and evolution of this matter. We predict a significant deformation of Mach shocks in central Au + Au collisions at RHIC and LHC energies as compared to the case of jet propagation in a static medium. The observed broadening of the near-side two-particle correlations in pseudorapidity space is explained by the Bjorken-like longitudinal expansion. Three-particle correlation measurements are proposed for a more detailed study of the Mach shock waves.

Ion-induced electron production in tissue-like media and DNA damage mechanisms

This work is the first stage in the development of an inclusive approach to calculation of the DNA damage caused by irradiation of biological tissue by ion/proton beams. The project starts with an analysis of ionization caused by the projectiles and the characteristics of secondary electrons produced in tissue-like media. We consider interactions with the medium on a microscopic level and this allows us to obtain the energy spectrum and abundance of secondary electrons as functions of the projectile’s kinetic energy. The physical information obtained in this analysis is related to biological processes responsible for the DNA damage induced by the projectile. In particular, we consider double strand breaks of DNA caused by secondary electrons and free radicals, and local heating in the ion’s track. The heating may enhance the biological effectiveness of electron/free radical nteractions with the DNA and may even be considered as an independent mechanism of DNA damage. Numerical estimates are performed for the case of carbon-ion beams. The obtained dose-depth curves are compared with results of the MCHIT model based on the GEANT4 toolkit.

Nuclear fragmentation reactions in extended media studied with Geant4 toolkit

Abstract It is well-known from numerous experiments that nuclear multifragmentation is a dominating mechanism for production of intermediate mass fragments in nucleus–nucleus collisions at energies above 100 A MeV . In this paper we investigate the validity and performance of the Fermi break-up model and the statistical multifragmentation model implemented as parts of the Geant4 toolkit. We study the impact of violent nuclear disintegration reactions on the depth-dose profiles and yields of secondary fragments for beams of light and medium-weight nuclei propagating in extended media. Implications for ion-beam cancer therapy and shielding from cosmic radiation are discussed.

Compact stars made of fermionic dark matter

Compact stars consisting of fermions with arbitrary masses and interaction strengths are studied by solving the structure equation of general relativity, the Tolman-Oppenheimer-Volkoff equations. Scaling solutions are derived for a free and an interacting Fermi gas and tested by numerical calculations. We demonstrate that there is a unique mass-radius relation for compact stars made of free fermions which is independent of the fermion mass. For sufficiently strong interactions, the maximum stable mass of compact stars and its radius are controlled by the parameter of the interaction, both increasing linearly with the interaction strength. The mass-radius relation for compact stars made of strongly interacting fermions shows that the radius remains approximately constant for a wide range of compact star masses.

Pion condensation and dynamics of neutron stars

Abstract When the central density of a neutron star reaches the critical value for pion condensation a catastrophic transformation of the star is triggered. In a short time the radius of the superdense core becomes comparable with the stellar radius and performs damped oscillations near the new equilibrium position. Due to the enormous release of energy during this process the envelop may be blown off.

Statistical approach for supernova matter

Abstract We formulate a statistical approach for description of nuclear composition and equation of state of stellar matter at subnuclear densities and temperature up to 20 MeV, which are expected during the collapse and explosion of massive stars. The model includes nuclear, electromagnetic and weak reactions between all constituents of matter, under condition of statistical equilibrium. We construct a realistic ensemble of nuclear species, and demonstrate the differences from the models employing a single-nucleus approximation for heavy nuclei. Thermodynamical characteristics, isentropic trajectories, EOS, and nuclear composition of supernova matter are calculated for different values of the lepton fraction. Special attention is paid to a possible in-medium reduction of the nuclear symmetry energy, as suggested by nuclear multifragmentation experiments. We demonstrate that this effect may lead to significant modifications of the nuclear composition and electron capture rates, which are important for stellar dynamics and nucleosynthesis.