Energy Spectra Of Fragments Research Articles

Study of dissipative collisions ofNe20(≈7−11MeV/nucleon)+Al27

The inclusive energy distributions of complex fragments ($3\ensuremath{\leqslant}9$) emitted in the reactions $^{20}\mathrm{Ne}$ (145, 158, 200, 218 MeV) $+$ $^{27}\mathrm{Al}$ have been measured in the angular range ${10}^{\ifmmode^\circ\else\textdegree\fi{}}\text{\ensuremath{-}}{50}^{\ifmmode^\circ\else\textdegree\fi{}}$. The fusion-fission and the deep-inelastic components of the fragment yield have been extracted using multiple Gaussian functions from the experimental fragment energy spectra. The elemental yields of the fusion-fission component have been found to be fairly well explained in the framework of the standard statistical model. It is found that strong competition occurs between the fusion-fission and the deep-inelastic processes at these energies. The time scale of the deep-inelastic process was estimated to be typically in the range of $~{10}^{\ensuremath{-}21}$--${10}^{\ensuremath{-}22}$ s, and it was found to decrease with increasing fragment mass. The angular momentum dissipations in the fully energy damped deep-inelastic process have been estimated from the average energies of the deep-inelastic components of the fragment energy spectra. The estimated angular momentum dissipations, for lighter fragments in particular, are found to be greater than those predicted by the empirical sticking limit.

Light charged particle and intermediate mass fragment cross-sections in GeV proton-induced reactions

As an international collaboration PISA (Proton Induced SpAllation) has initiated measurements of total and double differential cross-sections for products of spallation reactions at the proton accelerator COSY in Jülich (Germany) in order to study secondary particle production created in structural, window, and target materials by proton beams up to 2.5 GeV incident kinetic energy. In the framework of Spallation Neutron Sources and accelerator-driven systems, residual nuclei (H, He up to intermediate mass fragrments) production cross-sections are of great importance for estimation of damage in target and structural materials. Reliable and comprehensive experimental data, especially for p-energies beyond 1 GeV will serve as benchmarks for code development and validation. Data taken by Bragg curve spectroscopy and silicon detector telescopes for the reaction 1.9 GeV p+Ni(Au) will be discussed and kinetic energy spectra of light ions and intermediate mass fragments will be presented.

Effects due to multiple scattering of fission fragments in layers of fissioning material

Computer simulation with the SRIM program is used to investigate the passage of fragments from 235U fission by thermal neutrons through 50–400 µg/cm2 thick uranium layers. The calculations are performed in the two-fragment approximation: light 97Mo and heavy 139Ba. The energy spectra of the fission fragments emanating from the layer are calculated and the effects due to multiple scattering of the fragments in thin layers are estimated.

Estimate of average freeze-out volume in multifragmentation events

An estimate of the average freeze-out volume for multifragmentation events is presented. Values of volumes are obtained by means of a simulation using the experimental charged product partitions measured by the 4π multidetector INDRA for 129Xe central collisions on natSn at 32 AMeV incident energy. The input parameters of the simulation are tuned by means of the comparison between the experimental and simulated velocity (or energy) spectra of particles and fragments.

Vibrationally resolved collisions in cold hydrogen plasma

Slow collisions (0.5–100eV) of hydrogen atoms and ions with vibrationally excited hydrogen molecules and molecular ions are studied using fully quantal (below 10eV) and semi-classical approaches. Considered transitions include charge transfer, excitation, dissociation, three-body diatomic association, as well as energy and angular spectra of dissociation fragments. Mutual relations of various elastic and inelastic processes, the relevant physical mechanisms governing vibrational dynamics in inelastic channels and comparison of the fully quantal and semi-classical results are also reported.

Kinetic energy spectra for fragments and break-up density in multifragmentation

We investigate the possibility, in nuclear fragmentation, to extract information on nuclear density at break-up from fragment kinetic energy spectra using a simultaneous scenario for fragment emission. It is found that a decrease of peak centroids for kinetic energy spectra of fragments with increasing excitation energy can be observed at constant low density, which is different from recently published results of Viola et al. [V.E. Viola, K. Kwiatkowski, J.B. Natowitz, S.J. Yennello, Phys. Rev. Lett. 93 (2004) 132701].

A procedure for benchmarking laboratory exposures with 1 A GeV iron ions

A new version of the HZETRN code capable of simulating HZE ions with either laboratory or space boundary conditions is under development. The computational model consists of combinations of physical perturbation expansions based on the scales of atomic interaction, multiple scattering, and nuclear reactive processes with use of asymptotic/Neumann expansions with non-perturbative corrections. The code contains energy loss with straggling, nuclear attenuation, nuclear fragmentation with energy dispersion and downshifts, and off-axis dispersion with multiple scattering under preparation. The present benchmark is for a broad directed beam for 1 A GeV iron ion beams with 2 A MeV width and four targets of polyethylene, polymethyl metachrylate, aluminum, and lead of varying thickness from 5 to 30 g/cm 2. The benchmark quantities will be dose, track averaged LET, dose averaged LET, fraction of iron ion remaining, and fragment energy spectra after 23 g/cm 2 of polymethyl metachrylate.

Intermediate mass fragments and isospin dependence inSn124,Xe124+Sn124,Sn112reactions at28MeV∕nucleon

Intermediate mass fragment (Z>2) emission in Sn124,Xe124+Sn124,Sn112 reactions at 28MeV∕nucleon were studied using neutron ion multidetector for reaction oriented dynamics, a 4π charged particle detection system. A number of observables, such as isotopic yield distributions, energy spectra of light charged particles and intermediate mass fragments, isotopic and isobaric yield ratios, and average neutron to proton ratios are investigated. These observables show significant dependence on the isospin N∕Z of the reacting system. It is observed that the formation of neutron-rich clusters are correlated with the excess neutrons in the composite system and depends on the temperature of the emitting source. The origin of light particles and fragments was studied through observations of rapidity distribution as a function of collision violence. With increasing centrality, the heavier He6 isotope is found to be emitted closer to the midrapidity region than the lighter He3 isotope. The emission of heavy fragments from the midrapidity region becomes increasingly favorable for fragments with higher charge Z. The results suggest that the midrapidity region is not only neutron rich but also a rich source of heavy fragment (cluster) formation.7 MoreReceived 22 July 2003DOI:https://doi.org/10.1103/PhysRevC.68.054605©2003 American Physical Society

Setup for fission and evaporation cross-section measurements in reactions induced by secondary beams

A setup for studying reactions induced by secondary radioactive beams has been constructed. It allows simultaneous measurement of α-particle and fission fragment energy spectra. By measuring the α particles, identification of evaporation residues is achieved. A set of three targets can be used so as to ensure sufficient statistics. Two silicon detectors, located at 90° to the secondary beam direction, face each target, thus covering 30% of the solid angle. This experimental setup is to be used to obtain excitation functions of fusion-fission reactions and of reactions leading to evaporation residue production.

Generalized isoscaling of isotopic distributions

At incident energies in excess of about E/A530 MeV, a rapid collective expansion of the combined system may occur during the later stages of a central collision between heavy nuclei @1,2#. At densities less than about 1 3 of the saturation density, such systems disassemble into a mixture of fragments and light particles; the duration of fragment emission is of the order of 100 fm/c @3,4#. Even though the emission time is short, statistical models such as bulk multifragmentation models, which assume equilibrium at a single breakup density and temperature, have been used successfully to describe many experimental observables such as fragment multiplicities, charge distributions, and energy spectra of the emitted fragments @2,5–7#. These descriptions require careful, though not necessarily obvious, choices for the source size, excitation energy, and collective velocity of expansion @2,6–8#. Many of these statistical models display a phase transition in nuclear matter with subsaturation density @9,10#; such models have been employed to extract the caloric curve, i.e., the relationship between excitation energy and temperature for the nuclear liquid-gas phase transition @5,11–17# and to address whether finite nuclear systems may display negative heat capacities in analogy to those deduced for finite metallic clusters @18#. The success of thermal models raises the fundamental

A microscopic view of organic sample sputtering

Molecular dynamics simulations are performed in order to understand the fragmentation and emission mechanisms of kDa chain segments when a polymer (polystyrene) is bombarded by a keV particle (Ar). The calculated energy spectra of small fingerprint fragments and kDa chain segments are compared to experimental distributions. The results show that the ejection of small fragments is primarily induced by interatomic collisions. In contrast, vibrational excitation of the polymer plays a major role in the emission of kDa chain segments.

Comparative study of multifragmentation of gold nuclei induced by relativistic protons, 4He, and 12C

Multiple emission of intermediate-mass fragments has been studied for the collisions of p, 4He and 12C on Au with the 4π setup FASA. The mean IMF multiplicities (for the events with at least one IMF) are saturating at the value of 2.2±0.2 for the incident energies above 6 GeV. The observed IMF multiplicities cannot be described in a two-stage scenario, a fast cascade followed by a statistical multifragmentation. Agreement with the measured IMF multiplicities is obtained by introducing an intermediate phase and modifying empirically the excitation energies and masses of the remnants.The angular distributions and energy spectra from the p-induced collisions are in agreement with the scenario of “thermal” multifragmentation of a hot and diluted target spectator. In the case of 12C+Au(22.4 GeV) and 4He(14.6 GeV)+Au collisions, deviations from a pure thermal break-up are seen in the energy spectra of the emitted fragments, which are harder than those both from model calculations and from the measured ones for p-induced collisions. This difference is attributed to a collective flow.

Multifragmentation of gold nuclei by light relativistic ions: Thermal breakup versus dynamical disintegration

The multiple emission of intermediate-mass fragments (IMF) is studied for collisions of p, 4He, and 12C on Au with the 4π FASA setup. The mean multiplicities of IMF saturate at a value of around 2 for incident energies above 6 GeV. An attempt at describing the observed IMF multiplicities in the two-stage scenario, a fast cascade followed by a statistical multifragmentation, fails. Agreement with the measured IMF multiplicities is obtained by introducing an intermediate expansion phase and modifying empirically the excitation energies and masses of remnants. The angular distributions and energy spectra from p-induced collisions are in agreement with the scenario of “thermal” multifragmentation of a hot and expanded target spectator. In the case of 12C + Au(22.4 GeV) and 4He (14.6 GeV) + Au collisions, deviations from a pure thermal breakup are seen in the fragment energy spectra, which are harder than those both from model calculations and from the measured ones for p-induced collisions. This difference is attributed to a collective flow with the expansion velocity at the surface of about 0.1c (for 12C + Au collisions).

Transition from thermal to rapid expansion in multifragmentation of gold induced by light relativistic projectiles

Multiple emission of intermediate-mass fragments has been studied for the collisions of p, 4He and 12C on Au with the 4 π setup FASA. In the case of 12C(22.4 GeV) + Au and 4He(14.6 GeV) + Au collisions, the deviations from a pure thermal break-up are seen in the energy spectra of the emitted fragments: the spectra are harder than calculated and than measured in p-induced collisions. This difference is attributed to a collective flow with the expansion velocity on the surface about 0.1 c (for 12C + Au collisions).

Dissociation dynamics of H+2 in intense laser fields: investigation of photofragments from single vibrational levels

We present a photodissociation experiment on H+2 with ultrashort laser pulses ( >/=130 fs) at peak intensities of </=1.5x10(14) W/cm(2). Since in an ion beam setup H+2 is produced in a discharge source spatially separated from the light interaction zone interference with neutral H2 can be excluded in the interpretation. As the beam setup allows a high energy resolution of photofragments, effects predicted by the light induced potential theory can be tested in detail. The one-photon trapping effect was detected and the angular distributions of fragments from single vibrational levels were measured. Fragment energy spectra deviate strongly from those obtained by experiments on H2.

Spectator and participant decay in heavy ion collisions

We analyze the thermodynamical state of nuclear matter in transport calculations of heavy-ion reactions. In particular we determine temperatures and radial flow parameters from an analysis of fragment energy spectra and compare to local microscopic temperatures obtained from an analysis of local momentum space distributions. The analysis shows that the spectator reaches an equilibrated freeze-out configuration which undergoes simultaneous fragmentation. The fragments from the participant region, on the other hand, do not seem to come from a common fragmenting source in thermodynamical equilibrium.

Charm production and fragmentation in charged current DIS

In charged current deep inelastic scattering charm is dominantly produced in scattering events on strange quarks, thereby allowing for an experimental determination of the nucleon's strange sea density. A measurement of the energy spectrum of final state charm fragments (D-mesons) determines the charm fragmentation function at spacelike scales considerably below typical e + e − c.m.s. energies. NLO corrections to the naive s → c parton model production picture are important and well understood.

Present status of the caloric curve of nuclei

Spectator decay was studied for the system Au + Au at an energy of 1000 A·MeV and the decay of the interaction region at energies between 50 and 200 A·MeV. In both cases temperatures were derived from several double-ratios of neighboring isotopes and from the population of excited states in 5Li and 4He. Agreement was found among the different isotope temperatures and also among the two excited state temperatures. The comparison of isotope and excited state temperatures, however, reveals large differences, which cannot be explained by feeding corrections. At incident energies between 600 and 1000 A·MeV the energy spectra of fragments and also neutrons of the decaying projectile spectator were measured. Whereas the slope parameters of the energy spectra and mean energies for fragments with Z ≥ 2 are independent of the incident energy, a strong dependence is found for the lightest particles, so that preequilibrium contributions to the spectator decay should be taken into account.

Multifragment Emission by Nucleation-Evaporation Approach

Nuclear fragmentation events are Monte-Carlo simulated in the nucleation-evaporation picture, with charge and energy conservations. We investigate how the fragment energy spectrum, the charge distribution and the intermediate mass fragment (IMF) multiplicity depend upon the breakup density. The results are compared with some recent experimental data.

Coulomb breakup mechanism of neutron-halo nuclei in a time-dependent method.

The mechanism of the Coulomb breakup reactions of the nuclei with neutron-halo structure is investigated in detail. A time-dependent Schr\"odinger equation for the halo neutron is numerically solved by treating the Coulomb field of a target as an external field. The momentum distribution and the post-acceleration effect of the final fragments are discussed in a fully quantum mechanical way to clarify the limitation of the intuitive picture based on the classical mechanics. The theory is applied to the Coulomb breakup reaction of $^{11}$Be + $^{208}$Pb. The breakup mechanism is found to be different between the channels of $j^{\pi}=\frac{1}{2}^{-}$ and $\frac{3}{2}^{-}$, reflecting the underlying structure of $^{11}$Be. The calculated result reproduces the energy spectrum of the breakup fragments reasonably well, but explains only about a half of the observed longitudinal momentum difference.