Projectile-target Combinations Research Articles

Theoretical study of the synthesis of superheavy nuclei withZ=119and 120 in heavy-ion reactions with trans-uranium targets

By using a newly developed dinuclear system model with a dynamical potential energy surface---the DNS-DyPES model---hot fusion reactions for synthesizing superheavy nuclei (SHN) with charge numbers $Z$ = 112--120 are studied. The calculated evaporation residue cross sections are in good agreement with available data. In the reaction ${}^{50}$Ti+${}^{249}$Bk ${\ensuremath{\rightarrow}}^{\phantom{\rule{4pt}{0ex}}299\text{--}x}$119 $+\phantom{\rule{4pt}{0ex}}x$$n$, the maximal evaporation-residue (ER) cross section is found to be about 0.11 pb for the 4$n$-emission channel. For projectile-target combinations producing SHN with $Z=120$, the ER cross section increases as the mass asymmetry in the incident channel increases. The maximal ER cross sections for ${}^{58}$Fe+${}^{244}$Pu and ${}^{54}$Cr+${}^{248}$Cm are relatively small (less than 0.01 pb) and those for ${}^{50}$Ti+${}^{249}$Cf and ${}^{50}$Ti+${}^{251}$Cf are about 0.05 and 0.25 pb, respectively.

Experimental and theoretical study of the energy loss of C and O in Zn

We present a combined experimental-theoretical study of the energy loss of C and O ions in Zn in the energy range 50--1000 keV/amu. This contribution has a double purpose, experimental and theoretical. On the experimental side, we present stopping power measurements that fill a gap in the literature for these projectile-target combinations and cover an extended energy range, including the stopping maximum. On the theoretical side, we make a quantitative test on the applicability of various theoretical approaches to calculate the energy loss of heavy swift ions in solids. The description is performed using different models for valence and inner-shell electrons: a nonperturbative scattering calculation based on the transport cross section formalism to describe the Zn valence electron contribution, and two different models for the inner-shell contribution: the shellwise local plasma approximation (SLPA) and the convolution approximation for swift particles (CasP). The experimental results indicate that C is the limit for the applicability of the SLPA approach, which previously was successfully applied to projectiles from H to B. We find that this model clearly overestimates the stopping data for O ions. The origin of these discrepancies is related to the perturbative approximation involved in the SLPA. This shortcoming has been solved by using the nonperturbative CasP results to describe the inner-shell contribution, which yields a very good agreement with the experiments for both C and O ions.

Target dependence for low-Z ion charge state fractions

Equilibrium charge state fractions have been measured for 3–7 MeV lithium, boron and carbon ions passing through thin foils of copper, silver, and gold. The current results are combined with other low-Z ion data from the literature to give the relative influence of different target materials on charge exchange processes. The mean charge of the projectile, the functional form of the charge state distribution, and the charge state distribution width are parameters used to examine the effects of the electronic structure on charge exchange for various target-projectile combinations. Projectile shell structure is found to have a large influence on the widths of the charge state.

Nano-scale modification of 2D surface structures exposed to 6 keV carbon ions: Experiment and modeling

In this work, a Si pitch grating with typical lateral dimensions of 200–250 nm was exposed to 6 keV C + ions at normal incidence and at an angle of 42° both parallel and perpendicular to the grating structure. In contrast to volatile and recycling ions (like Ar + or H +), non-recycling ions are able to modify the surface not only due to sputtering, but also due to implantation of incident ions and the re-deposition of projectile atoms following sputtering or reflection. The target-projectile combination used in this work is an example of such a system forming a mixed Si-C surface. The interaction between the ion beam and the surface has been studied both experimentally and numerically with the focus on validation of the numerical model of the newly developed SDTrimSP-2D code. SDTrimSP-2D is capable of following the evolution of the Si-C system including ion-surface interactions with 2D micro- and nano-structured surfaces. The SDTrimSP-2D code takes the interdependency of surface morphology, sputtering and implantation into account. The simulated surface morphology has been compared to cross-sections of bombarded Si pitch grating obtained by SEM, revealing good agreement between experiment and simulation. The calculations also provide improved insight into the mechanisms of surface modification by sputtering, implantation and material transport by redeposition.

Photon emission from massive projectile impacts on solids.

First evidence of photon emission from individual impacts of massive gold projectiles on solids for a number of projectile-target combinations is reported. Photon emission from individual impacts of massive Au(n) (+q) (1 ≤ n ≤ 400; q = 1-4) projectiles with impact energies in the range of 28-136 keV occurs in less than 10 ns after the projectile impact. Experimental observations show an increase in the photon yield from individual impacts with the projectile size and velocity. Concurrently with the photon emission, electron emission from the impact area has been observed below the kinetic emission threshold and under unlikely conditions for potential electron emission. We interpret the puzzling electron emission and correlated luminescence observation as evidence of the electronic excitation resulting from the high-energy density deposited by massive cluster projectiles during the impact.

Production of neutron-rich Ca, Sn, and Xe isotopes in transfer-type reactions with radioactive beams

The production cross sections of neutron-rich isotopes $^{52,54,56,58,60}\mathrm{Ca}$, $^{136,138,140,142}\mathrm{Sn}$, and $^{146,148,150,152}\mathrm{Xe}$ are predicted for future experiments in the diffusive multinucleon transfer reactions $^{86,90,92,94}\mathrm{Kr}$, $^{124,130,132,134}\mathrm{Sn}$, $^{136,140,142,146}\mathrm{Xe}$, and $^{138,144,146}\mathrm{Ba}$$+$$^{48}\mathrm{Ca}$ with stable and radioactive beams at incident energies close to the Coulomb barrier. Because of the small cross sections, the production of neutron-rich isotopes requires the optimal choice of projectile-target combinations and bombarding energies.

Directional distribution of the ambient neutron dose equivalent from 145-MeV 19F projectiles incident on thick Al target

The directional distribution of the ambient neutron dose equivalent from 145-MeV (19)F projectiles bombarding a thick aluminium target is measured and analysed. The measurements are carried out with a commercially available dose equivalent meter at 0°, 30°, 60° and 90° with respect to the beam direction. The experimental results are compared with calculated doses from EMPIRE nuclear reaction code and different empirical formulations proposed by others. The results are also compared with the measured data obtained from an earlier experiment at a lower projectile energy of 110 MeV for the same target-projectile combination.

Production cross sections of the superheavy nucleus 117 based on the dinuclear system model

Within the framework of the dinuclear system model, the capture of two colliding nuclei, and the formation and de-excitation process of a compound nucleus are described by using an empirical coupled channel model, solving the master equation numerically and the statistical evaporation model, respectively. In the process of heavy-ion capture and fusion to synthesize superheavy nuclei, the barrier distribution function is introduced and averaging collision orientations are considered. Based on this model, the production cross sections of the cold fusion system 76–82Se+209Bi and the hot fusion systems 55Mn+238U, 51V+244Pu, 59Co+232Th,48Ca+247–249 Bk and 45Sc+246–248 Cm are calculated. The isotopic dependence of the largest production cross sections is analyzed briefly, and the optimal projectile-target combination and excitation energy of the 1n-4n evaporation channels are proposed. It is shown that the hot fusion systems 48Ca+247–249 Bk in the 3n evaporation channels and 45Sc+248Cm in the 2n-4n channels are optimal for synthesizing the superheavy element 117.

Entropy and light cluster production in heavy-ion collisions at intermediate energies

The entropy production in medium energy heavy-ion collisions is analyzed in terms of ratio of deuteronlike to protonlike clusters ( d like / p like ) using quantum molecular dynamics (QMD) model. The yield ratios of deuteronlike-to-protonlike clusters calculated as a function of participant proton multiplicity closely agree with experimental trends. Our model predictions indicate that full thermodynamical equilibrium may not be there even for the central geometry. The apparent entropy extracted from the yield ratios of deuteronlike-to-protonlike clusters, however, reflects the universality characteristics i.e. it is governed by the volume of reaction independent of the target-projectile combination. Our calculations for apparent entropy produced in central collisions of Ca + Ca and Nb + Nb at different bombarding energies are in good agreement with 4 π Plastic Ball data.

REACTION DYNAMICS INDUCED FROM LIGHT WEAKLY-BOUND RADIOACTIVE ION BEAMS AT NEAR-BARRIER ENERGIES

This work presents an overview of the latest experiments performed to study the reaction dynamics induced from light weakly-bound Radioactive Ion Beams on medium-mass and heavy targets at near-barrier energies. Production reactions, secondary beam intensities and experimental results on elastic scattering, transfer, breakup and fusion processes are presented and discussed. A comparative analysis of fusion and reaction cross section data for different projectile-target combinations is finally given.

Influence of entrance channels on the formation of superheavy nuclei in massive fusion reactions

Within the framework of the dinuclear system (DNS) model, the production cross sections of superheavy nuclei Hs ( Z = 108 ) and Z = 112 combined with different reaction systems are analyzed systematically. It is found that the mass asymmetries and the reaction Q values of the projectile–target combinations play a very important role on the formation cross sections of the evaporation residues. Both methods to obtain the fusion probability by nucleon transfer by solving a set of microscopically derived master equations along the mass asymmetry degree of freedom (1D) and distinguishing protons and neutrons of fragments (2D) are compared with each other and also with the available experimental data.

Synthesis of superheavy element120viaTi50+CfAhot fusion reactions

Synthesis of superheavy element $120$ in terms of the $^{50}\mathrm{Ti}+^{249\ensuremath{-}252}\mathrm{Cf}$ fusion-evaporation reactions is evaluated and discussed. It is found that the reactions of $^{250,251}\mathrm{Cf}(^{50}\mathrm{Ti},3n)^{297,298}120$ and $^{251,252}\mathrm{Cf}(^{50}\mathrm{Ti},4n)^{297,298}120$ are relatively favorable with the maximum evaporation-residue cross sections of $0.12$, $0.09$, $0.11$, and $0.25$ pb, respectively. However, $^{252}\mathrm{Cf}$ may be difficult to be target because its spontaneous fission will bring about serious background in the experiment. Fusion probabilities for different target-projectile combinations leading to the formation of surperheavy nucleus $^{302}120$ are estimated with the ``fusion-by-diffusion'' model and presented as a function of the Coulomb parameter ${Z}_{1}{Z}_{2}/({A}_{1}^{1/3}+{A}_{2}^{1/3})$. Among the reactions $^{50}\mathrm{Ti}+^{252}\mathrm{Cf}$, $^{54}\mathrm{Cr}+^{248}\mathrm{Cm}$, $^{58}\mathrm{Fe}+^{244}\mathrm{Pu}$, and $^{64}\mathrm{Ni}+^{238}\mathrm{U}$, the reaction $^{50}\mathrm{Ti}+^{252}\mathrm{Cf}$ has the largest fusion probability. Synthesis of superheavy element $120$ is of essential importance for determining whether the magic proton shell should be at $Z=114$ or at higher proton numbers $Z=120--126$. Therefore, the experiment to produce isotopes with $Z=120$ in the fusion reactions $^{50}\mathrm{Ti}+^{250,251}\mathrm{Cf}$ is of great interest.

Single Impacts of C60 on Solids: Emission of Electrons, Ions and Prospects for Surface Mapping

We report on the co-emission of secondary ions and electrons resulting from 15 keV C60+ and 30 keV C602+ impacts on targets of Al, Si, Au, CsI, glycine, and guanine. The study has been performed by the combination of an electron emission microscope and a time-of-flight (ToF) mass spectrometer. The electron emission occurs near the kinetic emission threshold, yet yields are notable (>3) for all investigated targets. A key observation for the projectile-target combinations studied is the absence of correlation between the electron emission and the number and type of co-emitted secondary ions for flat and homogeneous samples. This observation validates a novel concept of “positional mass spectrometry”. In this approach a surface is probed in the event-by-event bombardment detection mode. Impacts of an individual C60 projectile are localized via electron emission. The location combined with the corresponding secondary ion information allows to map the distribution of surface molecules. The unique feature of positional mass spectrometry is the ability to identify co-emitted ions from a single projectile impact. To test the concept an electron emission microscope has been combined with a ToF mass spectrometer; the device operates with synchronized detection of electrons and ions. The spatial resolution of the method depends on the kinetic energy and angular distribution of the secondary electrons and the aberrations of the electron optics. Initial tests of positional mass spectrometry showed a spatial resolution of 1.2 μm. Progress is anticipated with improvements in the electron optics used and application of projectiles generating more prolific electron emission.

Optimal reaction for synthesis of superheavy element 117

Fusion reactions leading to the formation of superheavy element 117 are systematically analyzed. Among the reactions considered, the $^{250}\mathrm{Bk}(^{48}\mathrm{Ca},4n)^{294}\mathrm{117}$ reaction has the largest evaporation residue (ER) cross section of about 2 pb. However, this reaction is hard to realize experimentally because it is difficult to accumulate sufficient amount of target material due to the short lifetime of $^{250}\mathrm{Bk}$ nucleus. For the reaction $^{48}\mathrm{Ca}+^{249}\mathrm{Bk}$, our estimation shows that the ER cross sections in $3n$ and $4n$ channels may be expected to be greater than 1 pb. Therefore, $^{48}\mathrm{Ca}$ and $^{249}\mathrm{Bk}$ should be the optimal projectile-target combination for synthesis of superheavy element 117 in practice. In addition, as a main result of systematic analysis, we find that the ER cross section exponentially depends on the mass difference (in unit of temperature) of fission and neutron emission saddle points. Therefore, it is of essential importance for the successful synthesis of superheavy nuclei to select the isotopic composition of projectile and/or target so as the mass difference of fission and neutron emission saddle points as large as possible. Entrance channel effects are examined by means of a comparison of the reactions $^{48}\mathrm{Ca}+^{245}\mathrm{Bk}$, $^{50}\mathrm{Ti}+^{243}\mathrm{Am}$, and $^{55}\mathrm{Mn}+^{238}\mathrm{U}$ leading to the same compound nucleus $^{293}\mathrm{117}$. The ER cross sections of the reactions $^{50}\mathrm{Ti}+^{243}\mathrm{Am}$ and $^{55}\mathrm{Mn}+^{238}\mathrm{U}$ are much smaller than that of $^{48}\mathrm{Ca}+^{245}\mathrm{Bk}$.

Proton lateral broadening distribution comparisons between GRNTRN, MCNPX, and laboratory beam measurements

Recent developments in NASA’s deterministic High charge (Z) and Energy TRaNsport (HZETRN) code have included lateral broadening of primary ion beams due to small-angle multiple Coulomb scattering, and coupling of the ion-nuclear scattering interactions with energy loss and straggling. This new version of HZETRN is based on Green function methods, called GRNTRN, and is suitable for modeling transport with both space environment and laboratory boundary conditions. Multiple scattering processes are a necessary extension to GRNTRN in order to accurately model ion beam experiments, to simulate the physical and biological-effective radiation dose, and to develop new methods and strategies for light-ion radiation therapy. In this paper we compare GRNTRN simulations of proton lateral broadening distributions with beam measurements taken at Loma Linda University Proton Therapy Facility. The simulated and measured lateral broadening distributions are compared for a 250 MeV proton beam on aluminum, polyethylene, polystyrene, bone substitute, iron, and lead target materials. The GRNTRN results are also compared to simulations from the Monte Carlo MCNPX code for the same projectile-target combinations described above.

Systematics of strong absorption radii and its relevance to the calculation of reaction cross sections

Based on the detailed analysis of the reaction cross sections obtained using the finite range Glauber model, a systematic study of strong absorption radii is carried out. A simple phenomenological expression is proposed to calculate reaction cross sections directly using the average nucleon–nucleon cross section for a given target–projectile combination at given energy. Reliability of the model expression is demonstrated through a variety of illustrative examples.

Role of nuclear dissipation and entrance channel mass asymmetry in pre-scission neutron multiplicity enhancement in fusion-fission reactions

Pre-scission neutron multiplicities are measured for ${}^{12}\mathrm{C} + {}^{204}\mathrm{Pb}$ and ${}^{19}\mathrm{F} + {}^{197}\mathrm{Au}$ reactions at laboratory energies of 75--95 MeV for the $^{12}\mathrm{C}$ beam and 98--118 MeV for the $^{19}\mathrm{F}$ beam. The chosen projectile-target combinations in the present study lie on either side of the Businaro-Gallone mass asymmetry (${\ensuremath{\alpha}}_{\mathrm{BG}}$) and populate the $^{216}\mathrm{Ra}$ compound nucleus. The dissipation strength is deduced after comparing the experimentally measured neutron yield with the statistical model predictions which contains the nuclear viscosity as a free parameter. Present results demonstrate the combined effects of entrance channel mass asymmetry and the dissipative property of nuclear matter on the pre-scission neutron multiplicity in fusion-fission reactions.

Neutron and Proton Diffusion in Fusion Reactions for the Synthesis of Superheavy Nuclei

The restriction of the one dimensional (1D) master equation (ME) with the mass number of the projectile-like fragment as a variable is studied, and a two-dimensional (2D) master equation with the neutron and proton numbers as independent variables is set up, and solved numerically. Our study showed that the 2D ME can describe the fusion process well in all projectile-target combinations. Therefore the possible channels to synthesize super-heavy nuclei can be studied correctly in wider possibilities. The available condition for employing 1D ME is pointed out.

Possible Way to Synthesize Superheavy Element Z = 117

Within the framework of the dinuclear system model, the productionof superheavy element Z = 117 in possible projectile–targetcombinations is analysed systematically. The calculated results showthat the production cross sections are strongly dependent on thereaction systems. Optimal combinations, corresponding excitationenergies and evaporation channels are proposed, suchas the isotopes 248,249Bk in 48Ca induced reactions in 3nevaporation channels and the reactions 45Sc+246,248Cm in 3nand 4n channels, and the system 51V+244Pu in 3n channel.

Collective flow of protons and negative pions in nucleus–nucleus collisions at momentum of [formula omitted

Collective flow of protons and negative pions has been studied within the beam momentum region of 4.2 – 4.5 A GeV / c ( E = 3.4 – 3.7 A GeV ) for different projectile-target combinations involving carbon and, specifically, for He–C, C–C, C–Ne, C–Cu and C–Ta. The data stem from the SKM-200-GIBS streamer chamber and from Propane Bubble Chamber systems utilized at JINR. The directed flow of protons grows dramatically in the carbon region when the counterpart nucleus grows in mass between He and Ta. The elliptic proton flow points out of the reaction plane and also strengthens as system mass increases. Within the reaction plane, the negative pions flow in the same direction as protons for the lighter of the investigated systems, He–C, C–C and C–Ne, and in the opposite direction for the heavier, C–Cu and C–Ta. The Quark–Gluon String Model reproduces observed changes in the flow with system mass.