arXiv subjects

M. Veselsky

Publications and source records attributed to M. Veselsky.

At least 55 records · Page 3Linked to original sources

arXiv2004

Isoscaling Studies of Fission - a Sensitive Probe into the Dynamics of Scission

The fragment yield ratios were investigated in the fission of 238,233U targets induced by 14 MeV neutrons. The isoscaling behavior was typically observed for the isotopic chains of fragments ranging from the proton-rich to the most neutron-rich ones. The observed high sensitivity of neutron-rich heavy fragments to the target neutron content suggests fission as a source of neutron-rich heavy nuclei for present and future rare ion beam facilities, allowing studies of nuclear properties towards the neutron drip-line and investigations of the conditions for nucleosynthesis of heavy nuclei. The breakdowns of the isoscaling behavior around N=62 and N=80 manifest the effect of two shell closures on the dynamics of scission. The shell closure around N=64 can be explained by the deformed shell. The investigation of isoscaling in the spontaneous fission of 248,244Cm further supports such conclusion. The Z-dependence of the isoscaling parameter exhibits a structure which can be possibly related to details of scission dynamics. The fission isoscaling studies can be a suitable tool for the investigation of possible new pathways to synthesize still heavier nuclei.

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arXiv2004

The influence of shell effects on the fissility of neutron deficient uranium isotopes at N ~ 126

Production cross sections and excitation functions of neutron deficient isotopes of U, Pa and Th have been measured in the reaction of 22Ne + 208Pb. The comparison of experimental production cross sections of uranium isotopes with the neutron number 126 <= N <= 134 to the values calculated using statistical model of nuclear deexcitation has been carried out. There is shown, that none of the commonly used methods of introduction of shell effect influence on the production cross sections of evaporations residues allows to reproduce unambigiously used set of experimental data. Simple semiempirical method of introduction of shell effect influence in fission channel is proposed and good agreement with production cross sections of neutron deficient isotopes in the region from Bi to U is reached.

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arXiv2004

Heavy Residue Isoscaling as a Probe of the Process of N/Z Equilibration

The isotopic and isobaric scaling behavior of the yield ratios of heavy projectile residues from the collisions of 25 MeV/nucleon 86Kr projectiles on 124Sn and 112Sn targets is investigated and shown to provide information on the process of N/Z equilibration occurring between the projectile and the target. The logarithmic slopes $α$ and $β^{'}$ of the residue yield ratios with respect to residue neutron number N and neutron excess N--Z are obtained as a function of the atomic number Z and mass number A, respectively, whereas excitation energies are deduced from velocities. The relation of the isoscaling parameters $α$ and $β^{'}$ with the N/Z of the primary (excited) projectile fragments is employed to gain access to the degree of N/Z equilibration prior to fragmentation as a function of excitation energy. A monotonic relation between the N/Z difference of fragmenting quasiprojectiles and their excitation energy is obtained indicating that N/Z equilibrium is approached at the highest observed excitation energies. Simulations with a deep-inelastic transfer model are in overall agreement with the isoscaling conclusions. The present residue isoscaling approach to N/Z equilibration offers an attractive tool of isospin and reaction dynamics studies in collisions involving beams of stable or rare isotopes.

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arXiv2004

Isoscaling in Peripheral Nuclear Collisions around the Fermi Energy and a Signal of Chemical Separation from its Excitation Energy Dependence

The isoscaling is investigated using the fragment yield data from fully reconstructed quasi-projectiles observed in peripheral collisions of 28Si with 124,112Sn at projectile energies 30 and 50 MeV/nucleon. The excitation energy dependence of the isoscaling parameter beta_prime is observed which is independent of beam energy. For a given quasi-projectile produced in reactions with different targets no isoscaling is observed. The isoscaling thus reflects the level of N/Z-equilibration in reactions with different targets represented by the initial quasi-projectile samples. The excitation energy dependence of the isoscaling parameter beta_prime, corrected for the trivial 1/T temperature dependence, does not follow the trend of the homogeneous system above 4 MeV/nucleon thus possibly signaling the onset of separation into isospin asymmetric dilute and isospin symmetric dense phase.

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arXiv2003

Effect of Shell Structure on Fission Isoscaling

Recently, the fragment yield ratios were investigated in the fission of 238,233U targets induced by 14 MeV neutrons [1]. The effect of the deformed shell structure around N=64 on the dynamics of scission was suggested as an explanation of the observed breakdown of the isoscaling behavior. As an alternative explanation, the effect of binding energy of final fragments, in particular the neutron shell closure of the complementary fragment around N=82, was suggested by Friedman [2]. When applying that concept consistently, the observed two breakdowns of the isoscaling behavior around N=62 and N=80 signal the effect of two shell closures (N=82 and N=64) on the dynamics of scission. To separate the two effects, we examine the isoscaling plots obtained using fission fragment yield data from the spontaneous fission of the heavier nuclei 248,244Cm and find that the breakdown of isoscaling around N=64 persists even if it can not be related to shell closure of the complementary fragment and thus it manifests the effect of the deformed shell structure in the scission configuration.

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arXiv2003

Reaction Dynamics and Multifragmentation in Fermi Energy Heavy Ion Reactions

The reaction systems, 64Zn + 58Ni, 64Zn + 92Mo, 64Zn + 197Au, at 26A, 35A and 47A MeV, have been studied both in experiments with a 4$π$ detector array, NIMROD, and with Antisymmetrized Molecular Dynamics model calculations employing effective interactions corresponding to soft and stiff equations of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained. The velocity distributions of $α$ particles and fragments with Z >= 3 show distinct differences in calculations with the soft EOS and the stiff EOS. The velocity distributions of $α$ particle and Intermediate Mass Fragments (IMF's) are best described by the stiff EOS. Neither of the above direct observables nor the strength of the elliptic flow are sensitive to changes in the in-medium nucleon-nucleon (NN) cross sections. A detailed analysis of the central collision events calculated with the stiff EOS revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with 197Au at 47A MeV a significant radial expansion takes place. For reactions with 58Ni and 92Mo at 47A MeV semi-transparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain.

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arXiv2003

Evidence of Critical Behavior in the Disassembly of Nuclei with A ~ 36

A wide variety of observables indicate that maximal fluctuations in the disassembly of hot nuclei with A ~ 36 occur at an excitation energy of 5.6 +- 0.5 MeV/u and temperature of 8.3 +- 0.5 MeV. Associated with this point of maximal fluctuations are a number of quantitative indicators of apparent critical behavior. The associated caloric curve does not appear to show a flattening such as that seen for heavier systems. This suggests that, in contrast to similar signals seen for liquid-gas transitions in heavier nuclei, the observed behavior in these very light nuclei is associated with a transition much closer to the critical point.

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arXiv2003

Isotopic Scaling of Heavy Projectile Residues from the collisions of 25 MeV/nucleon 86Kr with 124Sn, 112Sn and 64Ni, 58Ni

The scaling of the yields of heavy projectile residues from the reactions of 25 MeV/nucleon 86Kr projectiles with 124Sn,112Sn and 64Ni, 58Nitargets is studied. Isotopically resolved yield distributions of projectile fragments in the range Z=10-36 from these reaction pairs were measured with the MARS recoil separator in the angular range 2.7-5.3 degrees. The velocities of the residues, monotonically decreasing with Z down to Z~26-28, are employed to characterize the excitation energy. The yield ratios R21(N,Z) for each pair of systems are found to exhibit isotopic scaling (isoscaling), namely, an exponential dependence on the fragment atomic number Z and neutron number N. The isoscaling is found to occur in the residue Z range corresponding to the maximum observed excitation energies. The corresponding isoscaling parameters are alpha=0.43 and beta=-0.50 for the Kr+Sn system and alpha=0.27 and beta=-0.34 for the Kr+Ni system. For the Kr+Sn system, for which the experimental angular acceptance range lies inside the grazing angle, isoscaling was found to occur for Z<26 and N<34. For heavier fragments from Kr+Sn, the parameters vary monotonically, alpha decreasing with Z and beta increasing with N. This variation is found to be related to the evolution towards isospin equilibration and, as such, it can serve as a tracer of the N/Z equilibration process. The present heavy-residue data extend the observation of isotopic scaling from the intermediate mass fragment region to the heavy-residue region. Such high-resolution mass spectrometric data can provide important information on the role of isospin in peripheral and mid-peripheral collisions, complementary to that accessible from modern large-acceptance multidetector devices.

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arXiv2003

Heavy Residues with A<90 in the Asymmetric Reaction of 20 AMeV 124Sn+27Al as a Sensitive Probe of the Onset of Multifragmentation

The cross sections and velocity distributions of heavy residues from the reaction of 20 AMeV 124Sn + 27Al have been measured at forward angles using the MARS recoil separator at Texas A&M in a wide mass range. A consistent overall description of the measured cross sections and velocity distributions was achieved using a model calculation employing the concept of deep-inelastic transfer for the primary stage of peripheral collisions, pre-equilibrium emission and incomplete fusion for the primary stage of more violent central collisions and the statistical model of multifragmentation (SMM code) for the deexcitation stage. An alternative calculation employing the sequential binary decay (GEMINI code) could not reproduce the observed yields of the residues from violent collisions (A<90) due to different kinematic properties. The success of SMM demonstrates that the heavy residues originate from events where a competition of thermally equilibrated fragment partitions takes place rather than a sequence of binary decays.

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arXiv2003

Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral Collisions at Fermi Energies

A large enhancement in the production of neutron-rich projectile residues is observed in the reactions of a 25 MeV/nucleon 86Kr beam with the neutron rich 124Sn and 64Ni targets relative to the predictions of the EPAX parametrization of high-energy fragmentation, as well as relative to the reaction with the less neutron-rich 112Sn target. The data demonstrate the significant effect of the target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi energies. A hybrid model based on a deep-inelastic transfer code (DIT) followed by a statistical de-excitation code appears to account for part of the observed large cross sections. The DIT simulation indicates that the production of the neutron-rich nuclides in these reactions is associated with peripheral nucleon exchange. In such peripheral encounters, the neutron skins of the neutron-rich 124Sn and 64Ni target nuclei may play an important role. From a practical viewpoint, such reactions between massive neutron-rich nuclei offer a novel and attractive synthetic avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.

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arXiv2003

Competition of fusion and quasi-fission in the reactions leading to production of the superheavy elements

The mechanism of fusion hindrance, an effect observed in the reactions of cold, warm and hot fusion leading to production of the superheavy elements, is investigated. A systematics of transfermium production cross sections is used to determine fusion probabilities. Mechanism of fusion hindrance is described as a competition of fusion and quasi-fission. Available evaporation residue cross sections in the superheavy region are reproduced satisfactorily. Analysis of the measured capture cross sections is performed and a sudden disappearance of the capture cross sections is observed at low fusion probabilities. A dependence of the fusion hindrance on the asymmetry of the projectile-target system is investigated using the available data. The most promising pathways for further experiments are suggested.

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arXiv2002

Enhanced Production of Neutron-Rich Rare Isotopes in the Reaction of 25 MeV/nucleon Kr-86 on Ni-64

The cross sections and velocity distributions of projectile-like fragments from the reaction of 25 MeV/nucleon 86Kr + 64Ni have been measured using the MARS recoil separator at Texas A&M, with special emphasis on the neutron rich isotopes. Proton-removal and neutron pick-up isotopes have been observed with large cross sections. A model of deep-inelastic transfer (DIT) for the primary interaction stage and the statistical evaporation code GEMINI for the deexcitation stage have been used to describe the properties of the product distributions. The results have also been compared with the EPAX parametrization of high-energy fragmentation yields. The experimental data show an enhancement in the production of neutron-rich isotopes close to the projectile, relative to the predictions of DIT/GEMINI and the expectations of EPAX. We attribute this enhancement mainly to the effect of the extended neutron distribution (neutron ``skin'') of the 64Ni target in peripheral interactions of 86Kr with 64Ni. The large cross sections of such reactions near the Fermi energy, involving peripheral nucleon exchange, suggest that, not only the N/Z of the projectile and the target, but also the N/Z distribution at the nuclear surface may properly be exploited in the production of neutron-rich rare isotopes. This synthesis approach may offer a fruitful pathway to extremely neutron-rich nuclei, towards the neutron-drip line.

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arXiv2002

Production mechanism of hot nuclei in violent collisions in the Fermi energy domain

A production mechanism of highly excited nuclei formed in violent collisions in the Fermi energy domain is investigated. The collision of two nuclei is decomposed into several stages which are treated separately. Simplified exciton concept is used for the description of pre-equilibrium emission. A modified spectator-participant scenario is used where motion along classical Coulomb trajectories is assumed. The participant and one of the spectator zones undergo incomplete fusion. Excitation energies of both cold and hot fragment are determined. Results of the calculation are compared to recent experimental data in the Fermi energy domain. Data on hot projectile-like, mid-velocity and fusion-like sources are described consistently. Geometric aspects of pre-equilibrium emission are revealed. Explanations to previously unexplained experimental phenomena are given. Energy deposited into non-thermal degrees of freedom is estimated.

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arXiv2001

Studies of reaction dynamics in the Fermi energy domain

An overview of recent results on reaction dynamics in the energy region 20 - 50 A.MeV is given. The results of the study of projectile multifragmentation using the detector array FAUST are presented. Reaction mechanism is determined and thermodynamical properties of the hot quasiprojectile are investigated. Preliminary results on fragment isospin asymmetry obtained using the 4$π$ detector array NIMROD are given. Procedure for selecting centrality in two-dimensional multiplicity histograms is described. Possibility to extract thermodynamical temperature from systematics of isotope ratios is investigated. Reaction mechanism leading to production of hot sources is discussed. Furthermore, the possibilities for production of rare isotopes are discussed and recent experimental results obtained using recoil separator MARS are presented.

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arXiv2001

Caloric curve of 8 GeV/c negative pion and antiproton + Au reactions

The relationship between nuclear temperature and excitation energy of hot nuclei formed by 8 GeV/c negative pion and antiproton beams incident on 197Au has been investigated with the ISiS 4-pidetector array at the BNL AGS accelerator. The double-isotope-ratio technique was used to calculate the temperature of the hot system. The two thermometers used (p/d-3He/4He) and (d/t-3He/4He) are in agreement below E*/A ~ 7 MeV when corrected for secondary decay. Comparison of these caloric curves to those from other experiments shows some differences that may be attributable to instrumentation and analysis procedures. The caloric curves from this experiment are also compared with the predictions from the SMM multifragmentation model.

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arXiv2000

Isospin dependence of isobaric ratio Y(3H)/Y(3He) and its relation to temperature

A dependence of the isobaric ratio Y(3H)/Y(3He) on the N/Z ratio of the reconstructed quasiprojectile for the reaction of 28Si beam with 112,124Sn targets at two different projectile energies 30 and 50 MeV/nucleon is presented. We demonstrate a linear dependence of ln(Y(3H)/Y(3He)) on the quasiprojectile N/Z ratio and show the dependence of the slope on reconstructed excitation energy of the quasiprojectile. We relate this slope dependence at a given excitation energy to the temperature of the fragmenting system. Using the model assumptions of the statistical multifragmentation model, a method of temperature determination is proposed. A caloric curve is constructed and compared to the result of double isotope ratio method for the same set of data and to the results of other studies.

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arXiv2000

Production of hot nuclei in Fermi energy domain: from peripheral to central collisions

The production mechanism of highly excited nuclei in the Fermi energy domain is investigated. A phenomenological approach, based on the exciton model, is used for the description of pre-equilibrium emission. A model of deep inelastic transfer is employed for the peripheral collisions in the post-pre-equilibrium stage. An approach to describe more central collisions is proposed. A geometric overlap formula is employed in a way suitable for given energy domain. A simple geometric approach describing the interaction of participant and spectator zones is used to determine the incomplete fusion channel. Excitation energies of both fragments are determined. Results of the calculation are compared to available experimental data and an overall satisfactorily agreement is obtained. The model's ability to describe the production of the hot nuclei can be employed in the study of multifragmentation and/or in the production of rare beams.

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arXiv2000

Inhomogeneous isospin distribution in the reactions of 28Si + 112Sn and 124Sn at 30 and 50 MeV/nucleon

We have created quasiprojectiles of varying isospin via peripheral reactions of 28Si + 112Sn and 124Sn at 30 and 50 MeV/nucleon. The quasiprojectiles have been reconstructed from completely isotopically identified fragments. The difference in N/Z of the reconstructed quasiprojectiles allows the investigation of the disassembly as a function of the isospin of the fragmenting system. The isobaric yield ratio 3H/3He depends strongly on N/Z ratio of quasiprojectiles. The dependences of mean fragment multiplicity and mean N/Z ratio of the fragments on N/Z ratio of the quasiprojectile are different for light charged particles and intermediate mass fragments. Observation of a different N/Z ratio of light charged particles and intermediate mass fragments is consistent with an inhomogeneous distribution of isospin in the fragmenting system.

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