Deep Inelastic Collisions Research Articles

Nuclear Parton Distributions - a DGLAP Analysis

Nuclear parton distributions fA(x, Q2) are studied within a framework of the DGLAP evolution. Measurements of FA2/FD2 in deep inelastic lA collisions, and Drell-Yan dilepton cross sections measured in pA collisions are used as constraints. Also conservation of momentum and baryon number is required. It is shown that the calculated Q2 evolution of FSn2/FC2 agrees very well with the recent NMC data, and that the ratios RAf = fA/f are only moderately sensitive to the choice of a specific modern set of free parton distributions. For general use, we offer a numerical parametrization of RAf(x, Q2) for all parton flavours f in A > 2, and at 10−6 ≤ x ≤ 1 and 2.25 GeV2 ≤ Q2 ≤ 104 GeV2.

Multifragmentation in Xe(50 AMeV) + Sn: Confrontation of theory and data

We compare in detail central collisions Xe(50 AMeV) + Sn, recently measured by the INDRA collaboration, with the Quantum Molecular Dynamics (QMD) model in order to identify the reaction mechanism which leads to multifragmentation. We find that QMD describes most of the data quite well, in the projectile/target region as well as in the midrapidity zone where also statistical models can be and have been employed. The agreement between QMD and data allows us to use this dynamical model to investigate the reaction in detail. We arrive at the following observation: (a) the in-medium nucleon-nucleon cross section is not significantly different from the free cross section, (b) even the central collisions have a binary character, (c) most of the fragments are produced in the central collisions, (d) the simulations as well as the data show a strong attractive in-plane flow resembling deep inelastic collisions, and (e) at midrapidity the results from QMD and those from statistical model calculations agree for almost all of the observables with the exception of d 2σ dZ dE . This renders it difficult to extract the reaction mechanism from midrapidity fragments only. According to the simulations the reaction shows a very early formation of fragments, even in central collisions, which pass through the reaction zone without being destroyed. The final transverse momentum of the fragments is very close to the initial one and due to the Fermi motion. A heating up of the systems is not observed and hence a thermal origin of the spectra cannot be confirmed. The disagreement between the simulations and the data for some observables is presumably due to a force range which is too large as compared to reality but necessary to keep the nuclei stable in these semiclassical approach.

(νg9/2−2)8+isomersin82Se48and80Ge48populated by deep-inelastic collisions

New ${8}^{+}$ isomers in ${}^{82}\mathrm{Se}$ and ${}^{80}\mathrm{Ge}$ and their decays were studied by observing $\ensuremath{\gamma}$ rays from the projectilelike fragments produced by the deep-inelastic collision ${}^{198}\mathrm{Pt}+743 \mathrm{MeV}{}^{82}\mathrm{Se}$. The lifetimes of the ${8}^{+}$ isomers have been measured to be ${T}_{1/2}=6.6(4) \mathrm{ns}$ and ${T}_{1/2}>0.4 \mathrm{ns}$ ${\mathrm{for}\mathrm{}}^{82}\mathrm{Se}$ and ${}^{80}\mathrm{Ge}$, respectively. The $N=48$ isotones from ${}_{32}^{80}\mathrm{Ge}$ to ${}_{46}^{94}\mathrm{Pd}$ turned out to show quite different energy spectra of the yrast states up to ${I}^{\ensuremath{\pi}}{=8}^{+}$, depending on the orbital occupied by valence protons.

Scale evolution of nuclear parton distributions

Using the NMC and E665 nuclear structure function ratios F 2 A / F 2 D and F 2 A / F 2 C from deep inelastic lepton-nucleus collisions, and the E772 Drell-Yan dilepton cross sections from proton-nucleus collisions, and incorporating baryon number and momentum sum rules, we determine nuclear parton distributions at an initial scale Q 0 2. With these distributions, we study QCD scale evolution of nuclear parton densities. The emphasis is on small values of x, especially on scale dependence of nuclear shadowing. As the main result, we show that a consistent picture can be obtained within the leading twist DGLAP evolution, and in particular, that the calculated Q 2 dependence of F 2 Sn/ F 2 C agrees very well with the recent NMC data.

Lifetime measurement of theπg9/2isomer in79As

The lifetime of the 773 keV $\ensuremath{\pi}{g}_{9/2}$ isomer in ${}^{79}\mathrm{As}$ has been measured to be ${T}_{1/2}=0.87(6) \ensuremath{\mu}\mathrm{s}$ by taking coincidences between $\ensuremath{\gamma}$ rays and projectilelike fragments produced in deep-inelastic collisions of ${}^{198}\mathrm{Pt}{+}^{76}\mathrm{Ge}$ (635 MeV). The ${B(M2,9/2}^{+}\ensuremath{\rightarrow}{5/2}^{\ensuremath{-}})$ value for the 542 keV transition in ${}^{79}\mathrm{As}$ is found to be 0.042(3) Weisskopf units (W.u.). An identification method of nuclei produced in deep-inelastic collisions is also described.

Dynamical deformation of nuclei participating in deep-inelastic collisions

TheN/Z equilibration process taking place in deep-inelastic heavy ion collisions above the Coulomb barrier was studied by means of thick target gamma spectroscopy method. The analysis of gamma coincidence data obtained for four heavy ion systems gave complete distributions of primary reaction products which allowed to extract the most probableN/Z ratios as a function of mass. The comparison of experimentalN/Z values with expectations based on potential energy minimization leads to conclusion that during the crucial interaction time the involved nuclei are dynamically deformed.

γ Spectroscopy of 209Pb with deep inelastic reactions

High spin states in nuclei around 208Pb were populated in deep inelastic collisions of 76Ge, 136Xe, and 208Pb projectiles with 208Pb targets at beam energies about 12% above the Coulomb barrier. New states in 209Pb were found by measuring γ-γ-coincidences. They are interpreted as the yrast states that originate from the coupling of one neutron to the lowest excitations of the 208Pb core. The results are discussed in the frame of the shell model.

Production of nuclei in 32,34,36S-induced reactions in the energy range 6–75 MeV/ A

Some regularities in the production of isotopes with 6 ⩽ Z ⩽ 14 are investigated in reactions induced by 32,34,36S beams at 6 < E < 75 MeV/ A on targets of nat C, 181Ta and 197Au. the isotope yields and the most probable fragment mass A (for a given Z) are studied as a function of target, projectile mass and energy. A comparison is made of the experimental data at low energies with calculations in the framework of the dynamical model of deep inelastic collisions.

Theory of Nuclear Reactions

1. Quantum theory of potential scattering 2. Semiclassical scattering 3. The wave-optical description of potential scattering 4. The formal theory of potential scattering 5. The formal theory of reactions 6. The optical model 7. Single-step approximations 8. The coupled-channel description of direct reactions 9. Fusion 10. Compound nucleus reactions 11. Deep-inelastic collisions. Phenomena and theory 12. Analysis of deep-inelastic collisions, fusion and heavy ion induced fission.

High spin states in 210Pb

High spin states in nuclei in the 208Pb region were populated in deep inelastic collisions of 136Xe and 208Pb projectiles with thick 208Pb targets at about 50 MeV above the Coulomb barrier. Prompt and prompt-delayed γ-ray coincidences were measured. Four new states above the 8+ isomer in 210Pb were found and spins and parities assigned from theoretical expectations. The results are discussed in the frame of shell model theory.

Correlated oscillations in the excitation functions of deep-inelastic collisions: evidence for nuclear pulsars?

Excitation functions for the strongly dissipative collision 19F(Elab = 135–140 MeV)+89Y with an energy step of 250 keV in the lab. system have been measured. The data are consistent with previous measurements and exhibit oscillations, which are interpreted as an indication of quantum chaotic phenomena in dissipative heavy-ion collisions. The Fourier component of the energy autocorrelation of the total projectile-like angle-integrated dissipative yield has been calculated and is found to have a pulsing behaviour with a time interval between sequential pulses equal to the period of the coherent nuclear rotation. This provides another possibility of experimentally detecting quantum chaos in dissipative heavy-ion collisions.

Isomer-scope: A new instrument for in-beam γ-ray spectroscopy through deep inelastic collisions

A new instrument, isomer-scope, has been constructed to observe the ns-μs isomers produced by deep inelastic collisions. It consists of a Si detector, a γ-absorber and Ge detectors. The Si detector provides the energy and timing signals of projectile-like fragments(PLF) for the coincidence measurement and also stores the isomers of PLF for the Ge detectors. The γ-absorber shields the Ge detectors from the strong γ-radiation generated in the target. Thus, the isomer-scope greatly intensifies the γ-rays from the isomers of PLF in the PLF-γ(-γ) coincidences. In the reaction 635 MeV 76Ge + 198Pt , a new isomer of 69Cu was found to be excited at 2741 keV. It decayed with T 1 2 = 0.36(3) μ s via transition cascade 190-680-1871 and via cascade 73-486-471-1711. The cascade 486-471-1711 was fed in part from another state; either the 486 keV γ-ray or a side-feeding γ-ray unobserved was an isomeric transition of T 1 2 = 25(5) ns .

Reaction mechanism coexistence in the 123 MeV 19F+56Fe reaction.

Mass and charge identified ejectiles, spanning from $^{11}\mathrm{B}$ to $^{22}\mathrm{Ne}$, have been detected in the 123 MeV $^{19}\mathrm{F}$${+}^{56}$Fe reaction. The coexistence of deep inelastic collision (DIC) and incomplete fusion (IF) mechanisms has been observed. The shape of the energy spectra and their behavior with angle allowed us to identify two components: The less dissipative one was dominating near the grazing angle. For both components experimental optimum Q values were derived. Two approaches based on the sum rule (SR) model of Wilczy\ifmmode \acute{n}\else \'{n}\fi{}ski were used to calculate DIC and IF contributions to the complex fragment cross sections. Both prescriptions fit reasonably well experimental ejectile cross sections and Q optimum values. Results of the present investigation support the idea that the DIC can be treated on the same footing as IF in the SR model once the first process is confined in an inner angular momentum window, starting from the maximum fusion angular momentum, with respect to quasielastic processes. \textcopyright{} 1996 The American Physical Society.

Effects of shell structure and N/Z ratio of a projectile on the excitation energy distribution between interacting nuclei in deep-inelastic collisions.

Deep inelastic collisions of stable and radioactive projectiles with heavy targets are considered. The effects of shell structure and the N/Z ratio of a projectile on the excitation energy distribution between interacting nuclei, and on the centroid position and variance of the charge (mass) distribution of binary reaction products are explored. The role of nucleon exchange and particle-hole excitation mechanisms in the transformation of relative motion kinetic energy into the internal excitation energy of nuclei is studied. It is shown that a change in the mass number of the projectile nucleus causes a sufficient change both in the sharing of the excitation energy between fragments and in the charge (mass) distribution of reaction products. The nucleon exchange process between interacting nuclei is the main mechanism of dissipation.

Nucleon structure functions at moderate Q2: Relativistic constituent quarks and spectator mass spectrum

We present a model description of the nucleon valence structure function applicable over the entire region of the Bjorken variable x, and above moderate values of Q 2 (∼ 1 GeV 2). We stress the importance of describing the complete spectrum of intermediate states which are spectator to the deep-inelastic collision. At a scale of 1 GeV 2 the relevant degrees of freedom are constituent quarks and pions. The large- x region is then described in terms of scattering from constituent quarks in the nucleon, while the dressing of constituent quarks by pions plays an important role at intermediate x values. The correct small- x behavior, which is necessary for the proper normalization of the valence distributions, is guaranteed by modeling the asymptotic spectator mass spectrum according to Regge phenomenology.

Multi-Neutron and Proton Transfer Reactions in Deep Inelastic Heavy-Ion Collisions

Multi-Neutron and Proton Transfer Reactions in Deep Inelastic Heavy-Ion Collisions

Dissipation, polarization and fluctuation in grazing heavy-ion collisions and the boundary to the chaotic regime

The boundary between the grazing deep inelastic collisions and the quasi-fission or fusion reactions is studied though classical, semiclassical and WKB approximations. Although transfer reactions are responsible for the main part of the large energy loss in grazing collisions that come close to the barrier, the excitation energy is more than compensated by an additional attraction, due to polarization, so that the projectile reaches the barrier almost as if no reactions had taken place. At some distance outside the Coulomb barrier an instability occurs towards a merging of the surfaces, which also appears if no Coulomb barrier exists. Fluctuations in energy, partly due to transfer reactions, but mainly to the zero-point motion of the low-lying surface modes give an important spread in the capture angular momentum.

Binary dissipative processes and formation of hot nuclei in reactions from 55 to 95 MeV/u

A 4π multidetector array with a high efficiency has been used to detect the charged products from reactions induced by 36 Ar on 27 Al at energies ranging from 55 to 95 MeV/u. Well-characterized events were selected and sorted as a function of the impact parameter. Fusion or full stopping events have a very small cross section. Dissipative binary collisions accompanied by pre-equilibrium (mid-rapidity) emission, and followed by de-excitation of the quasi-projectile and quasi-target, were found to dominate for all impact parameters. The mass of the quasi-projectile has been reconstructed from its products and found to be slighty below the projectile mass. Isotropic emission in its rest frame has been observed at all energies and impact parameters, indicating that equilibrated nuclei may have been formed. The excitation energy of the quasi-projectile, determined via calorimetry, and the slope of kinetic energy spectra of α-particles were found to increase with incident energy and with decreasing impact parameter. Excitation energies per nucleon above 11 MeV are reached in a sizable cross section (70 mb). The mass distribution and kinetic energy spectra of emitted particles and fragments are essentially governed by the excitation energy, with a weak dependence on the impact parameter. The evolution of dissipative processes from deep inelastic collisions to the participant-spectator regime is discussed.

Mean field instabilities in dissipative heavy ion collisions

Abstract We discuss new reaction mechanisms that may occur in semi-peripheral heavy ion collisions at intermediate energies. In particular we focus on the dynamics of the overlapping zone, showing the development of neck instabilities, coupled with the possibility of an increasing amount of dynamical fluctuations. In a very selected beam energy range between 40 and 70 MeV/u we observe an important interplay between stochastic nucleon exchange and the random nature of nucleo-nucleo collisions. Expected consequences are intermediate mass fragment emissions from the neck region and large variances in the projectile-like and target-like observables. The crucial importance of a time matching between the growth of mean field instabilities and the separation of the interacting system is stressed. Some hints towards the observation of relatively large instability effects in deep inelastic collisions at lower energy are finally suggested.

Deep inelastic collisions of32S +27Al reaction at 130 MeV bombarding energy

The32S +27Al reaction was studied to investigate the deep inelastic collisions at a bombarding energy of 130 MeV which is well above the Coulomb barrier. The energy distributions of the binary decay products of 6⩽Z⩽10 were determined using a large area position sensitive ionization chamber. The average kinetic energies of the reaction products indicate that the exit shapes correspond to highly stretched scission configurations in the deep-inelastic processes.